Mid-Infrared Fluoride and Chalcogenide Glasses and Fibers 9811679401, 9789811679407

Table of contents :
Preface
Contents
1 Introduction to Mid-infrared Fluoride and Chalcogenide Glasses and Fibers
1.1 Aim and Scope
1.2 A Brief History of Fluoride Glasses and Fibers
1.3 A Brief History of the Chalcogenide Glasses and Fibers
1.4 Unique Contribution of This Book
1.5 Target Audience for This Book
References
2 Fluoride Glass Composition, Processing and Structure Characterization
2.1 Introduction
2.2 Compositions and Structures of Fluoride Glasses
2.2.1 Fluorozirconate Glasses
2.2.2 Fluoroaluminate Glasses
2.2.3 Fluoroindate Glasses
2.3 Glass Synthesis and Processing
2.3.1 Starting Materials
2.3.2 Melting and Fining
2.3.3 Casting, Cooling and Annealing
2.4 Future Prospects
References
3 Fluoride Glass Optical Fibers
3.1 Introduction
3.2 Fiber Loss
3.2.1 Absorption Loss
3.2.2 Scattering Loss
3.2.3 Radiation Losses
3.3 Fiber Parameters
3.4 Preform Fabrication
3.4.1 Hot-Jointing
3.4.2 Build-in Casting
3.4.3 Rod-in-Tube
3.4.4 Extrusion
3.5 Fiber Drawing
3.5.1 Fiber Drawing Equipment
3.5.2 Fiber Drawing Techniques
3.6 Structures of Fluoride Fiber
3.6.1 Single-Clad Fluoride Fiber
3.6.2 Double-Clad Fluoride Fiber
3.7 Applications of Fluoride Fiber
3.7.1 Low-loss Mid-infrared Transmission Fiber
3.7.2 Fiber Lasers
3.7.3 Fluoride Fiber Based Optical Fiber Amplifiers
3.7.4 Supercontinuum Source
References
4 Chalcogenide Glass Composition, Processing and Structure Characterization
4.1 ChG Glasses Thermal Properties
4.1.1 As–S
4.1.2 As–Se
4.1.3 Te–As–Se
4.1.4 Ge–As–S
4.1.5 Ge–As–Se–Te
4.1.6 Ge–Te–AgI
4.1.7 Ga Contained Chalcogenide/Chalcohalide Glasses
4.2 Transparent Windows and Phonon Energy
4.3 Viscosity and Glass Reformation
4.3.1 Fundamental Theory
4.3.2 Techniques
4.3.3 Applications
4.4 Conclusion
References
5 Chalcogenide Glass Preparation, Purification and Fiber Fabrication
5.1 Loss Mechanisms in Fiber Optics
5.1.1 Intrinsic Losses
5.1.2 Extrinsic Losses
5.1.3 Distillation Purification with Subsequent Vacuum Distillation
5.1.4 Distillation Purification with Subsequent Static Distillation
5.1.5 Glass Preparation Using Volatile Compounds
5.1.6 Chemical Vapor Transport Reactions Technique
5.1.7 Other Methods
5.2 Chalcogenide Glass Fiber Fabrication
5.2.1 Overview
5.2.2 Double-Crucible Method
5.2.3 Rod-In-Tube Method
5.2.4 Extrusion Method
5.2.5 Stack and Draw Method
5.2.6 Other Methods
References
6 Chalcogenide Fiber Structures: Design and Performance Analysis
6.1 Overview
6.2 Traditional Step-Index Fiber
6.2.1 Standard Chalcogenide Single-Mode Fiber Design
6.2.2 Multi-mode Fiber
6.2.3 Damage Threshold for Chalcogenide Fibers
6.3 Multi-cladding Fiber
6.3.1 W-type Fiber
6.3.2 M-type Fiber
6.3.3 Others
6.4 Suspended Fiber
6.5 Tapered Fiber
6.6 PCF
6.7 Conclusion
References
7 Mid-Infrared Spectral Properties of Rare Earth Ion Doped Chalcogenide Glasses and Fibers
7.1 RE Ion Species and MIR Energy Level Transition Mechanism
7.1.1 The Electronic Layer Configuration of RE Elements
7.1.2 RE Ions and Energy Level Transitions that Produce MIR Transitions
7.2 Local Field Characteristics of RE Ions in the Chalcogenide Glass Structure
7.2.1 Multi-phonon Relaxation
7.2.2 Extended X-Ray Absorption Fine Structure Spectrum
7.3 MIR Luminescence Characteristics of RE Doped Chalcogenide Glasses
7.3.1 MIR Luminescence of Dy3+ Doped Chalcogenide Glass
7.3.2 MIR Luminescence of Pr3+ Doped Chalcogenide Glasses
7.3.3 MIR Luminescence of Tm3+ Doped Chalcogenide Glass
7.3.4 MIR Luminescence of Er3+ Doped Chalcogenide Glass
7.3.5 Mid-Infrared Luminescence of Ho3+ Doped Chalcogenide Glass
7.3.6 MIR Luminescence of Tb3+ Doped Chalcogenide Glass
7.4 Problems and Prospects
References
8 Supercontinuum Generation in Mid-Infrared Glass Fibers
8.1 Overview of SC Sources
8.1.1 A Brief History
8.1.2 SC Generation Mechanism
8.2 MIR SC Generation in Fluoride Fibers
8.2.1 Fluoride Glass Fiber Properties
8.2.2 SC Generation
8.2.3 Fluorotellurite Fiber for SC Generation
8.3 MIR SC Generation in Chalcogenide Fibers
8.3.1 ChG Glass Fiber Characteristics
8.3.2 SC Generation
8.3.3 Novel ChG Fibers for MIR SC Generation
8.4 Cascading SC Sources
8.4.1 Two-Stage Cascading
8.4.2 Three-Stage Cascading
8.5 Applications of MIR SC Sources
8.5.1 Gas Sensing
8.5.2 Solid Detection
8.5.3 Spectral Imaging
8.6 Summary
References
9 Industrial, Medical and Military Applications of Fluoride and Chalcogenide Glass Fibers
9.1 Laser Power Delivery
9.1.1 Step-Index Fiber
9.1.2 Microstructured Fiber
9.2 Infrared Optical Fiber Imaging Bundles
9.3 Fiber Lasers
9.3.1 Direct Mid-Infrared Laser Generation Based on RE Doped Fluoride Fibers
9.3.2 Mid-Infrared Laser Generation by Stimulated Raman Scattering in Chalcogenide Fiber
9.4 Optical Fiber Couplers
9.5 Optical Fiber Gratings
9.5.1 Fiber Grating in Rare-Earth Doped Fluoride Fibers
9.5.2 Fiber Gratings in Chalcogenide Fibers
9.6 Optical Fiber Sensor
9.6.1 Biological Sensor
9.6.2 Temperature Sensor
9.6.3 Solution Concentration Sensor
9.6.4 Gas Sensor
References
10 Conclusion

Citation preview

Progress in Optical Science and Photonics

Pengfei Wang · Xunsi Wang · Haitao Guo · Peiqing Zhang · Shunbin Wang · Shijie Jia · Gerald Farrell · Shixun Dai

Mid-Infrared Fluoride and Chalcogenide Glasses and Fibers

Progress in Optical Science and Photonics Volume 18

Series Editors Javid Atai, Sydney, NSW, Australia Rongguang Liang, College of Optical Sciences, University of Arizona, Tucson, AZ, USA U. S. Dinish, Singapore Bioimaging Consortium (SBIC), Biomedical Sciences Institutes, A*STAR, Singapore, Singapore

The purpose of the series Progress in Optical Science and Photonics is to provide a forum to disseminate the latest research findings in various areas of Optics and its applications. The intended audience are physicists, electrical and electronic engineers, applied mathematicians, biomedical engineers, and advanced graduate students.

More information about this series at https://link.springer.com/bookseries/10091

Pengfei Wang · Xunsi Wang · Haitao Guo · Peiqing Zhang · Shunbin Wang · Shijie Jia · Gerald Farrell · Shixun Dai

Mid-Infrared Fluoride and Chalcogenide Glasses and Fibers

Pengfei Wang College of Physics and Optoelectronic Engineering Harbin Engineering University Harbin, Heilongjiang, China

Xunsi Wang Laboratory of Infrared Materials and Devices Ningbo University Ningbo, Zhejiang, China

Haitao Guo Xi’an Institute of Optics and Fine Mechanics Chinese Academy of Sciences Xi’an, Shaanxi, China

Peiqing Zhang Laboratory of Infrared Materials and Devices Ningbo University Ningbo, Zhejiang, China

Shunbin Wang College of Physics and Optoelectronic Engineering Harbin Engineering University Harbin, Heilongjiang, China

Shijie Jia College of Physics and Optoelectronic Engineering Harbin Engineering University Harbin, Heilongjiang, China

Gerald Farrell Photonics Research Centre Technological University Dublin Dublin, Ireland

Shixun Dai Laboratory of Infrared Materials and Devices Ningbo University Ningbo, Zhejiang, China

ISSN 2363-5096 ISSN 2363-510X (electronic) Progress in Optical Science and Photonics ISBN 978-981-16-7940-7 ISBN 978-981-16-7941-4 (eBook) https://doi.org/10.1007/978-981-16-7941-4 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

Glass has been in existence for millennia, mainly for glass vessels, decorative art, jewelery and of course in windows. The use of silica-based glass fibers as light guides began in the early twentieth century, initially in fiber imaging bundles for medical and industrial applications. While silica is the most common glass material, it is not the only material which can be used to fabricate glass. Silica is easy to manufacture and is an excellent choice as a glass for visible light wavelengths but is opaque at mid-IR wavelengths. The growth in a wide array of technologies operating in the IR and mid-IR region drove the growth of alternative glass types in the latter half of the twentieth century, with the development of chalcogenide glasses in the 1950s and then fluoride glasses in the 1970s. The history of guided light is well documented, beginning in 1854 when Irishman John Tyndall demonstrated that sunlight can be guided by a curved flow of water. This demonstration not only illustrated the principle of total internal reflection but also the basic operating principle underpinning modern optical fiber communications. Optical fiber communications became a practical reality with the first demonstration of silica optical fiber transmission in 1977, for live telephone traffic operating at 6 Mbit/s. Since then, the entire field of information communication has undergone unprecedented change. The growth of the Internet has seen a near-exponential rise in the demand for transmission bandwidth and with it the global deployment of optical fiber. Optical fiber systems now span the globe and literally provide the backbone for all global communications. Currently, submarine cables provide longhaul transmission of over 99% of voice, media and data communications worldwide. At present, there are 1.3 million kms of submarine cables in service and as each cable contains at least four fiber pairs or more, this means that there are more than 10 million kms of fiber in service in the global submarine network alone. In addition, national and metro transmission systems also utilize optical fiber as the primary backbone for transmission and furthermore fiber to the home or cabinet is being rapidly deployed in many urban and rural areas, to meet the demands of home working and media streaming and online gaming services.

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While optical fiber is a transmission medium with seemingly limitless bandwidth, it is becoming clear that the assertion that it has “limitless bandwidth” is only justifiable relative to the demand for bandwidth. Our world is experiencing an exponential growth in the amount of data being generated and stored, and in turn, the demand for bandwidth for data communications is growing at a similar rate. At the beginning of the digital communications era, human speech transmission was the main generator of digital data, through voice telephony over the new digital systems that emerged in the early 1970s. Today, it is no longer voice traffic but instead it is the Internet, entertainment media, gaming and interconnected technologies which is driving the growth in data. Unlike the 1970s where the growth in digital data was related to the relatively slow expansion in the voice telephone network, growth is now driven in many other ways, for example in 2020, there were 20.4 billion Internet of Things (IoT) devices online in applications such as home control, healthcare etc. and by 2025 this number is expected to rise to 75 billion IoT devices, a rate of increase which far surpasses human population growth rates. Moreover because digital technology and connectivity are embedded in almost all new technological advances, this in turn drives further increases in the rate at which data is being generated. A good example is that it is estimated that a single autonomous-driving car could generate up to 300 Tbs of data per year. Put simply, for the future the challenge is to continue to provide seamless global communications at a time when the amount of bandwidth needed is doubling every few years. The present spectral bandwidth available for transmission over silica fiber is less than circa 400 nm, and demonstrated transmission rates have been in excess of 150 Tbits/s over a single fiber. Improvements in modulation and multiplexing technology and the adoption of space-division multiplexing can all provide increased capacity over silica fiber. Simpler strategies such as increasing the number of fiber pairs in long-haul submarine cables can also significantly extend the capacity of silica optical fiber technology, for example prior to 2001, older submarine cables typically had four pairs of fibers, but this has grown so that the 2021 Durant north Atlantic cable has twelve fiber pairs. However, assuming the demand for bandwidth continues to grow (the current growth rate of global data is 25% per year), then the use of a broader transmission window based on the use of fluoride or chalcogenide fibers working at longer IR wavelengths is an option which must be seriously considered. Achieving reliable transmission over long distances demands firstly that optical fiber can be repeatably manufactured to very demanding structural and material specifications at high speed (up to 2 km of fiber per minute for silica fiber) and in very long lengths in the order of thousands of kms. As an example, typical singlemode silica fiber with a cladding diameter of 125 µm has a physical specification that requires a diameter tolerance within +/– 0.7 µm and a core/cladding concentricity better than 0.5 µm. Secondly, and of equal importance, the manufactured optical fiber must achieve very low attenuation in the order of a small fraction of a dB/km, provide very low dispersion to maximize bandwidth and possess a wide spectral window in order to maximize transmission capacity in multi-channel systems. Generations of scientists and engineers have worked to achieve the highest performance for silica fiber, for example the attenuation for silica fiber which is now routinely 0.2 dB/km

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or better, which is very close to the theoretical loss limit of 0.18 dB/km at circa 1550 nm. In meeting these requirements, silica-based fibers have set the bar for excellence in repeatable manufacturing and transmission performance. In addition since the development and deployment of optical fiber for communications in the 1970s, the application of optical fibers as sensors for a wide variety of measurands has been the subject of extensive research. This has resulted in the development of a wide variety of sensor types for a range of application scales from the macroscale, for example structural sensing for bridges and other civil structures, to the nanoscale for bio and chemical sensing. While many of the advances in optical fiber sensing have taken place in silica fiber, the major limitation for silica fiber is that it cannot be used in the mid-IR 3–5 µm wavelength range. This mid-IR range is of great importance for sensing as it corresponds to the characteristic molecular absorption or vibration energy bands of most gas, liquid and solid phases and it is located within the atmospheric transmission window, which is of importance in many applications, for example for sensors to detect pollutants in air. Biological tissues also have typical fingerprint spectra in the mid-IR spectrum and mid-IR light sources can be used as the basis of many medical diagnosis and treatment techniques. In this book in Chap. 1, the authors provide an introduction and context for the central themes in the book. A short but comprehensive history of fluoride and chalcogenide glasses is presented, showing how the technology developed from early demonstrations in the 1950s and then on through the last few decades. The authors describe the key discoveries which have underpinned the development and evolution of the different glass systems which have been explored by researchers seeking to deliver glass compositions with good chemical stability, the widest infrared transmission range and which are also more resistant to devitrification and moisture erosion. All of these characteristics are critically important to the large-scale production of passive optical fibers for long-haul transmission but also of equal importance in the production of doped fluoride and chalcogenide glass fibers which can be pumped. This is an important application of these glasses as the basis for fiber lasers operating at a wide range of IR wavelengths. The next two chapters focus on fluoride glasses and optical fibers. Fundamentally achieving very loss in an optical fiber demands a glass composition which can deliver such a low loss, but is also suitable for large-scale production of fiber, socalled fiber drawing. Chapter 2 considers several fluoride glass compositions, starting with a brief overview of glass compositions and characteristics before moving on to several specific glass compositions. Fluorozirconate glass is the first composition to be considered. The refractive index, dispersion, Rayleigh scattering and intrinsic absorption of fluorozirconate glass are very low, and this glass is suitable for various optical components and as a material for mid-infrared optical fiber. The main disadvantages of fluorozirconate are its low glass transition temperature, poor water resistance and low mechanical strength. Fluoroaluminate glass is then discussed. While this glass also has a low refractive index, low dispersion, low nonlinear refractive index and high optical transparency from UV to IR, the advantage of fluoroaluminate glass is that its chemical stability is three orders of magnitude higher than that of fluorozirconate-based glass. Fluoroindate glasses are then considered. This

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glass is transparent from the UV region up to 7–8 µm in the IR region, which is a significantly broader transmission range than fluorozirconate and fluoroaluminate glasses, making this fluoride glass type a particularly suitable candidate for long-haul optical fiber communications systems. This chapter continues with consideration of the practical but very important topics of material selection, melting and fining, all of which are critical to the achievement of the lowest possible loss, which can only be achieved by minimizing impairments such as optical scattering and hydroxyl ion content. Finally, the chapter concludes with a brief description of the other important processing steps which are needed and which include casting, cooling and annealing of the homogenous glass. Having considered fluoride glass composition and processing in detail, Chap. 3 deals with fluoride glass-based optical fibers. This chapter opens with a comprehensive overview of the sources of loss in a glass fiber covering all the forms of absorption and scattering loss which can occur, with a specific focus on these losses for fluoride fibers. While the development of fluoride fibers has advanced considerably in recent years, the attenuation achieved for manufactured fibers falls far short of the theoretical limit. For example the most common fluoride-based fiber, so-called ZBLAN (ZrF4 -BaF2 -LaF3 -AlF3 -NaF) fiber has a theoretical loss in the order of 10-3 dB/km, but in practice, losses for manufactured ZBLAN fibers are much larger, in the order of 0.7–1 dB/km. This chapter continues with a detailed description of the steps and processes involved in moving from bulk glass material to an actual optical fiber. The topics covered include preform fabrication, probably the most critical step involved, since the macroscale preform forms the structural and material “template” for the manufactured microscale glass fiber and any shortcomings in the preform will directly impact the performance of the optical fiber. A range of preform manufacturing techniques for fluoride fibers is presented from the earliest to more recent techniques such as rod-in-tube. Once the preform is fabricated, fiber drawing from the preform takes place, using a fiber drawing tower. Several different fiber drawing techniques suitable for fluoride glass fiber drawing are presented in this chapter. The chapter concludes with an overview of the different fluoride fibers structures, such as single-mode fiber and double clad fiber, as a foundation for the last part of this chapter which considers the applications of fluoride fiber, not only as a passive fiber for transmission but very importantly as a rare earth-doped fiber which can be pumped so as to provide for fiber lasers and amplifiers operating over a very wide range of IR wavelengths. The three chapters which follow concentrate on chalcogenide glass and fiber. Chapter 4 provides an introduction to chalcogenide glass composition, processing and structural characterization. Chalcogenide glass is a glass which contains one or more chalcogens (sulfur, selenium and tellurium). Chalcogenide glasses can typically transmit light from 2 to 12 µm, and depending on composition, the upper limit for transmission can even be as high as 20 µm, but unlike other IR glasses, chalcogenide glasses do not transmit in the visible light spectrum. Chalcogenide glasses have been traditionally used in IR optics as lenses and optical fibers due to their high refractive index and low phonon energy. In addition, many chalcogenide glasses exhibit nonlinear optical effects, making them valuable for a variety of photonic

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applications. This chapter provides a detailed description of the thermal properties of various chalcogenide glass compositions, an understanding of which is critical if chalcogenide glass is to be used as the basis for optical fiber. An equally important issue discussed is the width of the transmission window, which is also dependent on the chalcogenide glass composition used. In addition, chalcogenide glass can be readily used as a host material for rare earth ion doping and thus light emission in the mid-infrared region is possible, making chalcogenide fibers suitable for active optical applications such as lasers. In active applications, some chalcogenide glasses possess low phonon energies which results in high quantum efficiency for wavelength conversion and mid-IR laser applications. Chapter 4 also provides a very useful overview of the viscosity and glass reformation properties of chalcogenide glasses, which are critical in both the formation of bulk glass and also optical fiber drawing. This section covers a range of topics which include fundamental viscosity theory, along with a description of the various techniques used to characterize viscosity. The chapter ends with a brief treatment of topics such as extrusion and fiber drawing as a foundation for the chapters which follow. Chapter 5 opens with a comprehensive overview of the sources of absorption and scattering loss found in chalcogenide glass fiber. As is the case with silica-based optical fiber, impurities in the glass will increase loss and may even render parts of the transmission spectrum unusable. This chapter therefore describes a variety of methods which have been developed to remove impurities from the glass. The methods employed can be classified as either chemical methods, for example the addition of specific compounds to the glass mix or physical methods such as vacuum distillation. In some cases, both methods are used in order to reduce impurity levels to the greatest extent possible. As was the case for fluoride-based optical fiber, the process of drawing chalcogenide optical fiber is challenging. The two main methods used to fabricate chalcogenide optical fibers are described and compared: the doublecrucible method, which can directly provide chalcogenide fibers and the preform method, which starts with the fabrication of an optical fiber preform which is then drawn into a chalcogenide fiber. The preform method has evolved into a number of variations that include the rod-in-tube, extrusion and stack and draw methods. Having dealt with the topics of chalcogenide glass composition and the processes involved in fabricating low loss chalcogenide optical fiber in the previous two chapters, Chapter 6 focuses on a variety of chalcogenide optical fiber structures and their design and performance. Initially, this chapter considers the fundamentals of propagation in an optical fiber, using a combination of a geometrical optics approach and EM wave propagation theory based on Maxwell’s equations. In addition, this opening section also provides a brief overview of the simulation of propagation, focusing on two typical methods, the finite difference time domain (FDTD) method and the finite element method (FEM). Having laid down the foundations of propagation, this chapter continues by considering propagation in some specific basic optical fiber structures, starting with step-index fiber and focusing on the key milestone developments over the last few decades for single-mode and multimode chalcogenide fiber types. In addition, a short but very useful overview of laser power damage thresholds for chalcogenide fibers is provided along with some of the methods by which the

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threshold can be raised, an important topic given the demand for ever higher optical power delivery by chalcogenide fibers. The limitations of simple single clad fibers have meant that multi-clad fibers, with their more complex refractive index profiles have received a lot of attention from researchers and application developers. Fibers with W-type and M-type refractive index profiles are considered in some detail, along with other profiles, which can allow for example for dispersion tuning for chalcogenide fibers. Subsequently, more sophisticated fiber designs are introduced such as chalcogenide suspended core fiber in which a small diameter core is suspended at the center of the fiber. As the core is surrounded by air, tuning of the zero dispersion wavelength is possible by altering the core diameter of the fiber. The chapter concludes by introducing two other very important fiber structure types which have been fabricated from chalcogenide glass: tapered fiber and photonic crystal fiber (PCF). Tapered fiber is a form of microstructure fiber which dramatically increases the potential for interaction between the evanescent field in the fiber and the local environment. Depending on the application, this greatly enhances tunability for devices such as couplers and wavelength division multiplexers or the sensitivity of tapered fiber sensors for specific measurands. The chapter concludes with an overview of the operation of PCF and the characteristics of several different chalcogenide-based PCFs. While the focus of the previous three chapters is on passive chalcogenide fibers, rare earth (RE) doping of chalcogenide glasses and fibers is increasingly an important area of endeavor as a means to fabricate active chalcogenide fibers, which are a key component of fiber lasers and amplifiers operating in the mid-IR region. Chalcogenide fiber has inherent advantages in this context, firstly it possesses an exceptionally wide IR transmission window, and secondly the inherent low phonon energy of the glass means that high efficiency optical sources for the mid-IR region can be developed. Thus, Chap. 7 begins with some fundamentals, specifically the different types of RE ion species and their characteristics and also the associated mid-IR energy level transition mechanism for RE-doped chalcogenide glass. This is followed by a discussion of the local field characteristics of RE ions in a chalcogenide glass structure. RE-doped chalcogenide glasses have the potential to deliver mid-IR light sources operating over the range 3–5 µm, an important spectral region which overlaps the characteristic spectra of many environmental and biological processes. For this reason, this chapter considers in some detail the mid-IR luminescence characteristics of RE-doped chalcogenide glasses, illustrated with detailed examples such as a dysprosium ion (Dy3+ )-doped chalcogenide fibers, praseodymium (Pr3+ ) ion-doped chalcogenide fibers and thulium (Tm3+ ) ion-doped chalcogenide fibers. Chapter 8 deals with supercontinuum (SC) generation in mid-IR glass fibers. Significant progress in SC generation based on silica fibers has been made, but silica fiber SC sources are limited to operation below 2.5 µm due to the strong material absorption of silica glass. Mid-IR sources have attracted much attention because of their potential for use in various applications such as spectroscopy, sensing and optical coherence tomography. Chalcogenide glass is particularly attractive in this context given its very wide IR spectral transmission range. This chapter begins with

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an overview and introduction to SC sources, starting with a brief history of the development of SC generation, since the first examples of SC generation were reported in the 1960s and 1970s. The physical mechanisms for SC generation are then considered starting with the underlying principles of SC generation, which originates from the interaction of ultrashort pump pulses in nonlinear media with dispersion and nonlinear effects within the media. Having considered the physical mechanisms, SC generation in the mid-IR region in fluoride fibers is then considered. One of the keys goals in research in this area is increasing the output power of SC sources, and thus a number of approaches to achieving useful increases are described. Another key goal examined is increasing SC spectral coverage to 5 µm and beyond. The remainder of this chapter then focuses on mid-IR SC generation in chalcogenide fibers, which offer the potential for SC generation into IR regions up to 20 µm, far beyond the reach of fluoride fibers. SC generation was first reported in step-index fibers, but more recently, SC generation has depended heavily on the use of chalcogenidebased microstructured fibers and tapered chalcogenide fibers. Mid-IR SC generation is considered for all three of these fibers types, in each case explaining the challenges faced by researchers and the successes achieved. The chapter then considers some novel chalcogenide-based fibers, specifically designed with SC generation in mind in an effort to extend the spectral coverage and the increased the output power. Continuing on the theme of extending the spectral coverage, the use of multistage cascading of SC sources as a means to further extend the spectral coverage is then discussed. The chapter closes with a useful overview of the wide variety of applications of mid-IR SC sources which have developed and matured in recent years. The penultimate chapter, Chapter 9, provides a broad coverage of the diverse range of applications which have emerged for mid-infrared glasses and fibers. The first application considered is fiber-based laser power delivery. Since the demonstration of the first laser in 1960, a vast variety of applications in areas such as industrial processing and medicine are evolved. A key advantage of a laser, and the source of its attractiveness for many applications, is that a laser beam has excellent spatial coherence, which allows a high level of optical energy to be focused on a very small area. Delivering this spatially coherent energy can be a challenge in many applications, for example in laser surgery where the beam must be delivered to a very precise spot with great physical flexibility and compactness. Optical fiber is an obvious choice as a delivery technology, but this will often involve mid-IR laser power delivery. For example, laser surgery is often performed at 2.94 µm, as optical radiation absorption at this wavelength is very strong since the wavelength is very close to the maximum absorption band of cellular water. Chalcogenide fibers are a popular choice for laser power delivery, and a variety of impressive achievements is described over the last decade and more in the level of power delivery and the spectral range possible. While great progress has been made in power delivery by chalcogenide step-index fibers, with the increase in transmission power demanded by some applications have meant that the optical power handling limits of step-index fibers have been reached. Hollow-core microstructured fibers and large mode area PCFs are therefore considered as these fibers have proved very useful in efforts to increase the power handling and power delivery capability of chalcogenide fibers.

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This chapter continues with consideration of one of the oldest and most established fiber technologies, the use of fibers in fiber bundles for imaging. Visible range fiber bundles for imaging are a well-established technology, but IR fiber bundle imaging is gaining increased importance in medical, industrial and aerospace applications. A variety of chalcogenide fiber-based bundles are described and consideration is also given to the challenges which remain to be overcome, for example reducing the impact of the poorer mechanical properties of chalcogenide fibers on the fabrication of fiber bundles. Fiber lasers operating in the IR region operate at wavelengths which overlap a wide variety of absorption peaks associated with remote sensing, atmospheric monitoring, bio-chemical sensing and general spectral analysis. This chapter considers a range of approaches to the implementation of mid-IR lasers based on fluoride and chalcogenide fibers. The chapter concludes with a discussion of a range of other applications, including optical fiber gratings in fluoride and chalcogenide fibers and a variety of optical fiber sensors. Finally, in Chap. 10, the authors present some concluding remarks and insights. The forces which are continually driving research and development in fluoride and chalcogenide glasses and fibers are discussed. For example for the next generation of optical transmission systems, it is clear that given that the utilization of transmission bandwidth is doubling every few years, the present spectral bandwidth of 400 nm for silica fiber will ultimately prove to be bottleneck for global communications. Improvements in modulation and multiplexing technology can significantly extend the useful life of silica fiber technology, but ultimately transmission in a broader IR window over non-silica glasses is likely to become a necessity and for this to happen, significant advances in glass and fiber production are needed. In conclusion, this book provides a comprehensive overview of the fundamentals and technologies associated with fluoride and chalcogenide glass and fibers, along with up-to-date descriptions of the key research efforts and accomplishments of many scientists and engineers. The book provides an excellent foundation for those embarking on research and development in the area as well as providing the detailed knowledge and insights needed by researchers to contribute to the further developments and innovations that are expected in the coming years as fluoride and chalcogenide glass and fiber technology matures. Dublin, Ireland

Prof. Gerald Farrell

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Introduction to Mid-infrared Fluoride and Chalcogenide Glasses and Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pengfei Wang, Jibo Yu, and Gerald Farrell 1.1 Aim and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 A Brief History of Fluoride Glasses and Fibers . . . . . . . . . . . . . . . . 1.3 A Brief History of the Chalcogenide Glasses and Fibers . . . . . . . . . 1.4 Unique Contribution of This Book . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Target Audience for This Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fluoride Glass Composition, Processing and Structure Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shijie Jia, Pengfei Wang, and Gerald Farrell 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Compositions and Structures of Fluoride Glasses . . . . . . . . . . . . . . . 2.2.1 Fluorozirconate Glasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Fluoroaluminate Glasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Fluoroindate Glasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Glass Synthesis and Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Starting Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Melting and Fining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Casting, Cooling and Annealing . . . . . . . . . . . . . . . . . . . . . . . 2.4 Future Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fluoride Glass Optical Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pengfei Wang, Jiquan Zhang, Yuxuan Jiang, Jibo Yu, Shunbin Wang, and Gerald Farrell 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Fiber Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Absorption Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Scattering Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 4 7 9 9 9 13 13 13 15 17 20 21 21 23 23 24 25 29

29 30 30 32 xiii

xiv

Contents

3.2.3 Radiation Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Fiber Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Preform Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Hot-Jointing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2 Build-in Casting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.3 Rod-in-Tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.4 Extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Fiber Drawing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Fiber Drawing Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2 Fiber Drawing Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Structures of Fluoride Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.1 Single-Clad Fluoride Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.2 Double-Clad Fluoride Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Applications of Fluoride Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.1 Low-loss Mid-infrared Transmission Fiber . . . . . . . . . . . . . 3.7.2 Fiber Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.3 Fluoride Fiber Based Optical Fiber Amplifiers . . . . . . . . . . 3.7.4 Supercontinuum Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

5

Chalcogenide Glass Composition, Processing and Structure Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Xunsi Wang, Gerald Farrell, and Zheming Zhao 4.1 ChG Glasses Thermal Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 As–S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 As–Se . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3 Te–As–Se . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.4 Ge–As–S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.5 Ge–As–Se–Te . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.6 Ge–Te–AgI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.7 Ga Contained Chalcogenide/Chalcohalide Glasses . . . . . . . 4.2 Transparent Windows and Phonon Energy . . . . . . . . . . . . . . . . . . . . 4.3 Viscosity and Glass Reformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Fundamental Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

34 37 38 38 39 44 45 46 47 50 55 55 56 56 57 57 60 61 62 67 70 71 72 73 77 78 79 81 84 85 86 88 91 94 94

Chalcogenide Glass Preparation, Purification and Fiber Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Xiange Wang, Kai Jiao, Gerald Farrell, and Xunsi Wang 5.1 Loss Mechanisms in Fiber Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 5.1.1 Intrinsic Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 5.1.2 Extrinsic Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

Contents

5.1.3 Distillation Purification with Subsequent Vacuum Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.4 Distillation Purification with Subsequent Static Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.5 Glass Preparation Using Volatile Compounds . . . . . . . . . . . 5.1.6 Chemical Vapor Transport Reactions Technique . . . . . . . . . 5.1.7 Other Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Chalcogenide Glass Fiber Fabrication . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Double-Crucible Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Rod-In-Tube Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.4 Extrusion Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.5 Stack and Draw Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6 Other Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

7

Chalcogenide Fiber Structures: Design and Performance Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jinjing Wang, Zan Feng, Xiaolin Liang, Guolin Wu, Jun Wang, Gerald Farrell, and Xunsi Wang 6.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Traditional Step-Index Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Standard Chalcogenide Single-Mode Fiber Design . . . . . . . 6.2.2 Multi-mode Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3 Damage Threshold for Chalcogenide Fibers . . . . . . . . . . . . 6.3 Multi-cladding Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.1 W-type Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 M-type Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.3 Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Suspended Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Tapered Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 PCF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mid-Infrared Spectral Properties of Rare Earth Ion Doped Chalcogenide Glasses and Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Haitao Guo, Jian Cui, Chenyu Xu, Yantao Xu, and Gerald Farrell 7.1 RE Ion Species and MIR Energy Level Transition Mechanism . . . 7.1.1 The Electronic Layer Configuration of RE Elements . . . . . 7.1.2 RE Ions and Energy Level Transitions that Produce MIR Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Local Field Characteristics of RE Ions in the Chalcogenide Glass Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 Multi-phonon Relaxation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 Extended X-Ray Absorption Fine Structure Spectrum . . . .

xv

107 113 119 122 127 129 129 130 138 143 157 160 165 173

173 182 183 185 187 189 190 193 196 199 200 204 210 210 217 218 218 219 223 223 224

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7.3 MIR Luminescence Characteristics of RE Doped Chalcogenide Glasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 MIR Luminescence of Dy3+ Doped Chalcogenide Glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.2 MIR Luminescence of Pr3+ Doped Chalcogenide Glasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.3 MIR Luminescence of Tm3+ Doped Chalcogenide Glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.4 MIR Luminescence of Er3+ Doped Chalcogenide Glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.5 Mid-Infrared Luminescence of Ho3+ Doped Chalcogenide Glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.6 MIR Luminescence of Tb3+ Doped Chalcogenide Glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Problems and Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

9

Supercontinuum Generation in Mid-Infrared Glass Fibers . . . . . . . . Shixun Dai, Yingying Wang, Gerald Farrell, and Peiqing Zhang 8.1 Overview of SC Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.1 A Brief History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.2 SC Generation Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 MIR SC Generation in Fluoride Fibers . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Fluoride Glass Fiber Properties . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 SC Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.3 Fluorotellurite Fiber for SC Generation . . . . . . . . . . . . . . . . 8.3 MIR SC Generation in Chalcogenide Fibers . . . . . . . . . . . . . . . . . . . 8.3.1 ChG Glass Fiber Characteristics . . . . . . . . . . . . . . . . . . . . . . 8.3.2 SC Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.3 Novel ChG Fibers for MIR SC Generation . . . . . . . . . . . . . . 8.4 Cascading SC Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.1 Two-Stage Cascading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.2 Three-Stage Cascading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 Applications of MIR SC Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.1 Gas Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.2 Solid Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.3 Spectral Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

225 226 250 262 266 269 272 278 279 285 285 285 287 289 289 290 298 300 300 302 307 311 313 315 316 317 318 320 322 322

Industrial, Medical and Military Applications of Fluoride and Chalcogenide Glass Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 Haitao Guo, Hao Zhang, Lutao Liu, Xusheng Xiao, and Gerald Farrell 9.1 Laser Power Delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328 9.1.1 Step-Index Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328

Contents

9.1.2 Microstructured Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Infrared Optical Fiber Imaging Bundles . . . . . . . . . . . . . . . . . . . . . . . 9.3 Fiber Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.1 Direct Mid-Infrared Laser Generation Based on RE Doped Fluoride Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.2 Mid-Infrared Laser Generation by Stimulated Raman Scattering in Chalcogenide Fiber . . . . . . . . . . . . . . . 9.4 Optical Fiber Couplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5 Optical Fiber Gratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5.1 Fiber Grating in Rare-Earth Doped Fluoride Fibers . . . . . . 9.5.2 Fiber Gratings in Chalcogenide Fibers . . . . . . . . . . . . . . . . . 9.6 Optical Fiber Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6.1 Biological Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6.2 Temperature Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6.3 Solution Concentration Sensor . . . . . . . . . . . . . . . . . . . . . . . . 9.6.4 Gas Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xvii

335 341 345 345 349 351 353 353 355 358 358 359 361 364 365

10 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 Pengfei Wang, Jibo Yu, and Gerald Farrell

Chapter 1

Introduction to Mid-infrared Fluoride and Chalcogenide Glasses and Fibers Pengfei Wang, Jibo Yu, and Gerald Farrell

1.1 Aim and Scope The “middle infrared” region covers the electromagnetic spectral range of from 2.5 to 25 µm. This range is of particular interest for many applications, especially for spectroscopy, since the electromagnetic frequencies involved coincide with the frequencies of the internal vibrational motion of most molecules (the “molecular fingerprint” region). In all, the mid-infrared (IR) spectral range is of particular interest for two main reasons: many molecules exhibit signature optical absorptions in this wavelength range and specific transmission windows within these wavelengths are available in the Earth’s atmosphere. Options for compact, reliable, high power mid-IR optical sources are currently rather limited by the difficulty of finding host materials that are both transparent in the mid-IR wavelength range and sufficiently stable, robust and easy to fabricate. The development of mid-IR optical lasers and spectrometers have been delayed by the lack of compact, reliable, high power mid-IR optical sources, either narrowband or broadband. This limitation results from the difficulty of finding glass or crystal host materials that are both sufficiently transparent in the mid-IR wavelengths range and sufficiently stable, robust and easy to fabricate. A promising option to engineer such laser sources relies on the use of mid-IR transmitting glass, in the form of an optical fiber. This solution has some considerable advantages such as simplified thermal management, light weight, minimum occupied volume and a distributed system architecture. The optical confinement reduces the need for free space optics which are sensitive to misalignment; in addition, light in fibers is totally confined within the core-cladding structure and thus completely shielded from environmental contaminants and distubances like dust, vibration, and moisture. The choice of a suitable glass composition for a mid-IR application based on a fiber or a planar structure involves the evaluation of series of parameters related to the glass: the maximum phonon energy, the transparency widow, the chemical durability, the fiber drawing ability and the purity of the starting materials. In Table © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Wang et al., Mid-Infrared Fluoride and Chalcogenide Glasses and Fibers, Progress in Optical Science and Photonics 18, https://doi.org/10.1007/978-981-16-7941-4_1

1

2

1 Introduction to Mid-infrared Fluoride …

1.1, a comparison between the typical values of selected properties of main glass systems is reported. These values can adjusted by changing the glass compositions. There are two typical cases, the first is that the high transparency of fluoride glasses up to about 6 µm along with a chemical stability comparable with a sodium silicate glass, make this glass an excellent candidate for fiber lasers in the visible and midIR regions where emissions are hard to obtain from silicate and phosphate fibers. The second is that the low phonon energy of chalcogenide glasses activates many mid-IR transitions for rare-earth ions that are normally not present in materials with higher phonon energies. The chalcogenide glasses have, by contrast, been doped with a number of rare-earth ions including Ho3+ , Tm3+ , Tb3+ , Dy3+ , Pr3+ , and Er3+ for studies on > 3 µm mid-IR luminescence. Another important feature of chalcogenides is their high optical non-linearity that makes these glasses promising candidates for all-optical switching applications. For the application in fiber spectroscopy, there are also two cases: active and passive devices. Active devices, such as fiber amplifiers and lasers, rely on the presence of rare earth (RE) dopants in the glass. This places further requirements on glass compositions and properties, such as a homogeneous distribution of RE in the glass matrix and a stable oxidation state of the optically active ion. Here, typical RE trivalent ions (e.g. Er3+ , Ho3+ , Tm3+ , Dy3+ ) possess fluorescence emissions in the mid-IR spectral range which can potentially be made to lase in mid-IR fluoride, tellurite and chalcogenide glasses. Inactive devices, such as a fiber Supercontinuum (SC) source, rely on the optical nonlinearity of the fibers. As is well known, spectroscopy in the molecular fingerprint regions of the mid-IR spectral range requires an optical source of sufficient brightness whose wavelength range extends from near-IR into the mid-IR region. Traditional laser sources provide brightness and coherence, but have very limited spectral bandwidth. Other sources, such as thermal sources, have very wide bandwidths but low coherence and low brightness. Fiber generated SC light provides a useful balance of brightness, coherence and bandwidth, making it a promising optical source for investigations in this spectral region. An alternative technology relies on microstructured mid-IR transmitting glass optical fiber in order to generate a very broadband SC light emission spanning the optical range from near to mid-IR wavelength (1.2–16 µm). This fiber generated SC light possesses the characteristics of coherence, bandwidth and brightness required by spectroscopic applications in the mid-IR region. In this book the fluoride and chalcogenide glass host materials suitable for the development of mid-IR coherent sources based on RE doping and optical third nonlinearity are presented in the mid-IR glasses section. In addition, the current state of the art in mid-IR fiber laser and SC sources is also presented in the section on fiber spectroscopy.

1.38

0.55

2.2

70

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68

Thermal conductivity [W/(mK)]

Expansion coefficient [10−6 /K]

Density [g/cm3 ]

Young’s modulus [GPa]

Refractive index [@µm]

Abbe number

0.4–2.4

SC range [µm]

10–21

0.947–3.934

10 mW@ 1.56 µm

20 W@ ~ 1 µm –

1021

[email protected] µm

1021

0.4–6.28

41.6 [email protected] µm

1021

[email protected]–9 µm

~ [email protected] µm

2.5 × ~ 16.4

76 10–19

[email protected]

58.3

4.33

17.2

0.628

260

600

0.2–6

Fluoride (ZBLAN)

10–20

[email protected]

33.6

5.5

12–17

1.25

350

800

0.4–5.0

Mid-IR

Tellurite

Typical values of the properties are reported for each glass family: these may vary with changing the glass compositions

50 kW @ ~ 1 µm

Laser output power [W]

RE solubility

1019

1.5 × 103 @1.05 µm

[email protected] µm

Fiber loss [dB/Km]

[ions/cm3 ]

~ 4.7

[email protected] µm

10–20

68

[email protected]

47

2.59

13.4

0.57

461

1200

Thermo-optic coefficient [10−6 / K]

Nonlinear index

[m2 /W]

10–20

1100

1000

Glass transition temperature [o C]

Maximum phonon energy

0.2–4

0.2–2.5

Transmission range [µm]

[cm−1 ]

Visible and near IR

Application

Phosphate

Silica

Glass property

Table 1.1 A comparison of selected properties of main glass systems

1.2–16

[email protected] µm

0.1 mol%

12@3 µm

[email protected] µm

10–18

NA

[email protected]

21.5

4.51

14

0.2

300

350

0.8–20

Chalcogenide

1.1 Aim and Scope 3

4

1 Introduction to Mid-infrared Fluoride …

1.2 A Brief History of Fluoride Glasses and Fibers Halide glass has an ultra-low transmission loss of ~ 10–3 dB/km based on theoretical calculations, which is one-thousandth of the value in silica optical fiber. However the actual test loss is much higher than the theoretical loss due to the influence of impurities. If there are breakthrough achievements in technological innovation and halide glass fibers with a loss close to the theoretical loss limit can be prepared, then long-distance (10,000 km) communication without amplification or regeneration will become a reality. Halide glasses are mainly divided into two categories: chloride glass and fluoride glass. The maximum transmission wavelength of chloride glass in the mid-IR region is 14 µm. Since chloride glass dissolves easily and has poor chemical stability, it has not attracted significant attention. In contrast, most research has been focused on fluoride glass. The major fluoride glass types are fluorozirconate (ZrF4 -based), fluoroaluminate (AlF3 -based) and fluoroindate (InF3 -based) glasses. In 1949 and 1950, fluoride glasses based on BeF2 and AlF3 were reported [1, 2], and then in 1975, a systematic study of glass formation in heavy metal fluoride glass was undertaken by M. Poulain, J. Lucas et al. at the University of Rennes in France [3]. The ZrF4 –BaF2 –LaF3 system is one of the earliest and most important ternary systems. From the early 1980s, AlF3 -based fluoride glasses began to attract the attention of researchers. The potential for useful applications for such glasses was credible as the glasses had a higher glass transition temperature (Tg ) and a better chemical stability [4], with the expectation that AlF3 -based fluoride glass would replace ZrF4 -based fluoride glass for the development of practical optical fiber devices. However it was also realised that a disadvantage of the AlF3 -based fluoride glass was that it has a greater tendency to crystallize, and the range of glass formation is smaller compared with ZrF4 -based fluoride glass. In recent years, many studies on the components of AlF3 -based glasses have been undertaken. Table 1.2 lists the components of several representative aluminum fluoride-based glasses and their transition temperatures and T values. InF3 -based fluoride glass was also widely studied [5–8]. In 1997, A. Boutarfaia et al. first studied the ternary system of InF3 -based fluoride glass (InF3 –BaF2 –YF3 ) [6], it was found that compared with ZrF4 -based and AlF3 -based fluoride glass, it has lower photon energy (~ 510 cm−1 ) and wider IR transmission region (cutoff Table 1.2 Representative composition of AlF3 -based glass Glass component (mol%)

Tg (o C) T (o C)

40AlF3 –16YF3 –22BaF2 –22CaF2

430

105

35AlF3 –15YF3 –10BaF2 –20CaF2 –10SrF2 –10MgF2

438

82

30AlF3 –20YF3 –15BaF2 –25PbF2 –10MgF2

367

138

30.2AlF3 –8.3YF3 –10.2BaF2 –20.2CaF2 –13.2SrF2 –3.5MgF2 –3.8NaF–10.2ZrF4 388

94

1.2 a Brief History of Fluoride Glasses and Fibers

5

wavelength ~ 12 µm), which is a beneficial quality in seeking to achieve higher efficiency for fluorescence emission and lasing. In 1998, C. Ribeir et al. obtained a strong luminescence around a wavelength of 2.7 µm in Er3+ /Yb3+ ions co-doped InF3 -based fluoride glass [9]. Later in 2015, L. Gomes et al. systematically studied the luminescence properties of Ho3+ ions in InF3 -based fluoride around the wavelength of 3.9 µm [10]. More recently in 2019, M Kochanowicz et al. used Er3+ /Tm3+ co-doped InF3 -based fluoride glass to obtain a strong luminescence around the wavelength of 2.7 µm [11]. These research results show that InF3 -based fluoride glass is a better choice for a glass material that can be used to achieve mid-IR luminescence. The optical transmission loss of the fluoride optical fiber can be divided into intrinsic and extrinsic loss, and the intrinsic loss can be further divided into three parts: ultraviolet (UV) absorption, Rayleigh scattering and multiphonon absorption. The UV absorption edge is caused by electronic transitions of ions at very short wavelengths (UV to visible). However, the phenomenon of UV absorption decreases rapidly as the wavelength increases. In the wavelength range from the visible to infrared, Rayleigh scattering is the main source of intrinsic loss caused by uneven glass density and composition. It is widely known that the Rayleigh scattering is inversely proportional to the 4th power of the wavelength, so the multiphonon absorption is the main limiting factor in the IR wavelength band. The extrinsic loss is caused by absorption due to impurities in the material and the scattering induced by the crystallites in the glass. Most transition metal elements and some RE elements have material absorption loss at mid-IR wavelengths range, so the extrinsic loss could be reduced below the value of the inherent loss, and the crystal scattering is usually the main source of extrinsic loss. It is worth noting that glass devitrification is more likely to happen in fabrication of the fluoride glass compared to silicate glass, so it is difficult to avoid the formation of the internal crystallites. Currently scientific research is focused on the production of the fluoride glass compositions that are stable against devitrification and moisture erosion, and furthermore researchers continue to search glass compositions that are more stable and have lower transmission loss in the IR band. However, one of the significant problems in this field of research is that it is difficult to accurately estimate the stability of any given fluoride glass composition. Some laboratories researching fluoride optical fibers have selected different glass compositions, and studied the composition and strictly eliminated the nucleation of glass to form the crystallites, so that the fabricated fluoride optical fibers have the required scattering level to achieve the expected ultra-low optical transmission loss in mid-IR band. In 1981, Shibata et al. first predicted that the theoretical ultimate optical transmission loss in fluoride glass fiber at 3.5 µm would be 10–3 dB/km [12]. And in 1985 and 1987, Uitert et al. [13] and France et al. [14] estimated that the fluoride glass optical fiber attenuation was about 10–2 dB/km at 2.5 µm based on experimental data. In 1990, L. E. Busse et al. of the Naval Research Laboratory tested the scattering level of short fluoride fibers (~ 5 cm) to be 0.025 dB/km [15]. The above results show the tremendous progress in the transmission loss of the fluoride fiber in the past number of years. At present, the background loss of the commercial ZBLAN fiber is less than 50 dB/km in the mid-IR band below 3 µm.

6

1 Introduction to Mid-infrared Fluoride …

Compared with silicate glass, fluoride glass with its lower phonon energy has a longer cut-off IR wavelength, and the optical transmission loss of fluoride glass in the IR band is extremely low. Therefore, fluoride glass fiber lasers can be manufactured by doping RE ions based on the above advantages. The laser output has a wavelength range of 0.5–4 µm in fluoride fiber laser, which is useful for communication applications and IR spectroscopy. Furthermore RE doped fibers can also be used as the basis of fiber amplifiers in optical fiber communication systems, which are preferable to unreliable and expensive analog-to-digital signal converters and repeaters. In addition, a higher laser efficiency can be achieved since the doping concentration of RE ions in fluoride glass is higher and the phonon energy is lower than that in silicate glass. As early as 1988, M. Brierley et al. used an argon ion laser as a pump source to obtain a mid-IR continuous wave (CW) laser with a wavelength of 2.7 µm in an Er3+ -doped ZBLAN glass fiber [16]. In 1990, R. Allen et al. used a 792 nm laser as the pump source and realized CW lasing with center wavelengths at 2.71, 2.75 and 2.78 µm in an Er3+ -doped ZBLAN glass fiber [17]. In 1995, M. Pollnau et al. of the University of Berlin used an Er3+ /Pr3+ co-doped ZrF4 -based glass fiber as the gain medium to obtain the dual laser output with the wavelength of 1.72 and 2.71 µm based on the method of cascaded laser outputs, and the laser output power of 2.71 µm was 158 mW [18]. With the continuous improvement of the output power of the semiconductor lasers, the output power of Er3+ doped fiber lasers was increased progressively to values in the order of watts. In 1999, S. Jackson et al. used a semiconductor laser with a wavelength of 790 nm to pump an Er3+ /Pr3+ co-doped fluoride fibers, the obtained laser output power in the 2.7 µm band was 1.7 W [19]. In 2007, X. Zhu et al. used a wavelength of 975 nm diode laser with an output power of 100 W as the pump source, and the laser output power of 2.8 µm was increased to 9 W by using a ZBLAN fiber with highly doped Er3+ ions as the gain medium [20]. In 2009, S. Tokita et al. used a laser pump source wavelength of 975 nm to achieve a laser output power of 24 W in an Er3+ -doped ZBLAN fiber immersed in a fluorocarbon liquid refrigerant [21]. In 2015, V. Fortin and others of Laval University in Canada increased the laser output power at ~ 3 µm in Er3+ -doped ZBLAN fiber laser to 30.5 W by writing Bragg gratings in fluoride fibers and using passive cooling method [22]. In 2018, Y. Aydin et al. used a laser with a wavelength of 980 nm as the pump source in the laboratory of Laval University, and wrote Bragg gratings on both ends of the ZBLAN fiber doped with Er3+ ions to reduce the number of fusion points in the fiber cavity. A laser output power of 41.6 W was achieved in the 3 µm wavelength band [23]. In the same year, F. Maes et al. obtained the longest output wavelength for a fiber laser in IR band at room temperature based on Ho3+ -doped InF3 -based glass fiber [24].

1.3 A Brief History of the Chalcogenide Glasses and Fibers

7

1.3 A Brief History of the Chalcogenide Glasses and Fibers Chalcogenide glass is the multi-element compound glass composed of three elements of sulfur (S), selenium (Se) and tellurium (Te) in group VIA elements and other glass network bodies, such as arsenic (As), antimony (Sb), germanium (Ge), etc. The nonlinear refractive index n2 of the chalcogenide glass is larger than that of ordinary oxide glass. As is well known, the nonlinear refractive index coefficient n2 of glass mainly depends on the refractive index and the composition of the glass. In chalcogenide glass, the S, Se, As and other elements can easily produce anharmonic distortion under the influence of strong light. On the other hand, chalcogenide glass is easier to polarize compared with oxide glass because of the structural variability and the strong interaction between the electronic lattices, resulting in a larger nonlinear refractive index coefficient n2 . In addition, chalcogenide glass also has good chemical stability, thermal stability, unique photosensitivity and wide mid-to-far IR transparent range, so it has broad application potential in areas such as light storage, IR sensing, IR imaging, inorganic lithography, antireflection coatings, optical switching and night vision systems [25]. In 1953, R. Frerics first studied the binary chalcogenide glass As2 S3 and proposed a new optical glass with a maximum IR cut-off wavelength of 12 µm [26]. Researchers began to pay more attention to the exploration and development of the selenide and telluride glasses inspired by the excellent IR transmission properties of the sulfide glasses and the research on chalcogenide glasses has been flourishing since then. In 1957, F. Glaze et al. proposed the first industrial batch method for preparing As2 S3 glass by improving the raw material purification and preparation process of the As2 S3 glass [27]. The thermal stability and crystallization resistance is improved by adding As to the chalcogenide glass. Thus a chalcogenide glass based on an As–S system has better glass forming ability and thermal stability than other chalcogenide glasses [28]. Furthermore the Tg of As–S glass is lower than the softening point of the optical fiber, which can avoid devitrification during optical fiber drawing. However development of the glass materials using this system was limited due to the strong toxicity of the glass, so that researchers began to focus on slightly toxic or non-toxic heavy metals such as germanium (Ge), gallium (Ga) and other chalcogenide systems. The chalcogenide glass of the Ge-La-S system gradually attracted the attention of researchers due to its non-toxicity and good glass properties. However, the temperature difference between the Tg and Tx of the glass in this system is very small, only about 46 °C, and the thermal stability of the glass is therefore not very good, which hinders the further development of the chalcogenide glass based on this system. In 1988, X. Zhang et al. discovered and prepared a tellurium-halogen (Te-X) based glass [29]. The glass has low transmission loss in the wavelength range of 8–14 µm, which can be used for the transmission of the 10.6 µm high power laser. However, the glass of this system contains halogen elements, and its anti-deliquefaction ability

8

1 Introduction to Mid-infrared Fluoride …

is poor. The prepared glass will be deliquescent due to water absorption when placed in the air for a long time, so it is not suitable for preparing optical glass fibers. With the increasing demand for the application of mid- to far- IR materials, the IR transmission range of the chalcogenide glass has increased, ensuring that Te-based chalcogenide glass with a wide IR transmission window become a distinct focus of the work of many researchers in the IR transmission materials. In 2006, S. Danto reported that in the Ga–Ge–Te system glass with a high light transmission in the 2–21 µm wavelength band [30]. However the glass of this system has a strong Ga-O absorption peak in the 15–20 µm band due to the presence of O ions, and its thermal stability is not very good. In addition, the metal Te nanoparticles in the glass may display a directional crystallization phenomenon due to the strong metallicity of the Te element when the temperature is lightly higher than the glass Tg , which will affect the transmission of the light waves and limit its application. There are many types of chalcogenide glasses, which are divided into single element (S, Se, Te based glass), binary, ternary and multi-element glasses according to the number of constituent elements. Using the chalcogen elements as a nomenclature, chalcogenide glasses can be divided into three categories. S-based glass: Ge–S, As– S, Ge–As–S, Ge–P–S, etc.; Se-based glass: Ge–Se, As–Se, Ge–As–Se, Ge–Sb–Se, Ge–P–Se, etc.; Te-based glass: Ge–Sb–Te, Ge–Se–Te, As–Se–Te, etc. In 1965, J. A. Savage et al. reported a chalcogenide glass optical fiber for the first time [31]. But this optical fiber could not be used in practical applications due to the high optical loss induced by the impurities of the materials. In order to improve its optical transmission performance, many researchers began to work on reducing loss in chalcogenide glass to reduce loss in optical fibers based on the glass. In 1980, Miyashita et al. confirmed that the absorption of impurities is the main factor limiting the high transparency of As–S glass in the mid-IR band [32]. In 1976, researchers at Hitachi, Japan, produced a chalcogenide glass optical fiber using the Ge-Se-Sb system, with a loss of as low as 20 dB/km. However the high proportion of the Se element in the glass system limits its application in the far-IR range. The Te element with a wider IR transmission region is used to replace part of the Se. The minimum loss of the glass fiber of the Ge–Se–Te system at a wavelength of 10.6 µm is 1.5 dB/km, and the optical fiber prepared from it has been used for the transmission fiber for a CO2 high-energy laser. In 1992, M. Asobe et al. used As2 S3 fiber to make Kerr light switch for the first time [33]. In 2000, G. Lenz et al. found that there is a big Kerr nonlinear effect in Se-based chalcogenide glass fibers [34]. In 2003, P. A. Thielen et al. obtained a Raman gain greater than 20 dB in a 1.1 cm long As–Se based fiber [35]. The Raman gain coefficient of the fiber is about 2.3 × 10–11 m/W, which is 300 times that of silica glass. In 2004, R. E. Slusher et al. found that the Kerr nonlinear effect in As2 Se3 fiber is more than 1000 times that of ordinary silica fiber [36], and its Raman gain is almost 800 times that of ordinary silica fiber. In 2012, C. Clement et al. prepared a unclad (GeTe4 )90 (AgI)10 bare fiber with the maximum wavelength transmission edge of 16 µm, but that an optical fiber made of Ge15 Ga20 Te75 matrix glass still has a high transmission loss of up to 60 dB/km at 9 µm [37]. Currently research on low-loss chalcogenide glass fibers is still in the

1.3 A Brief History of the Chalcogenide Glasses and Fibers

9

exploratory stage, and the optical transmission loss is one of the difficult problems restricting the development of the chalcogenide glass fibers.

1.4 Unique Contribution of This Book There are a number of books published which focus on IR optics, such as: “MidIR Coherent Sources and Applications”, “Solid-State, mid-Infrared Laser Sources”, “Infrared and Raman Spectroscopy in Forensic Science”, “Basic Principles of Fluorescence Spectroscopy, Wiley–VCH”. However most books restrict their coverage to the short wave region of the mid-IR, such as 3–5 µm, such as “Solid-State midInfrared Laser Sources, Springer”, in this book, we define the “mid-infrared” more loosely to cover a broader range starting from ~ 2 µm wavelength. At the longer wavelength end, near-IR laser sources (0.8–1.6 µm) achieved significant market penetration during the past decade, but the technology for mid-IR lasers at a 16 µm wavelength, especially those which operate at room temperature, needs considerable improvement, despite the fact that mid-IR lasers have existed since the beginning of the laser era.

1.5 Target Audience for This Book We believe that this book will be useful for academics, researchers and engineers in a variety of disciplines who require a broad introduction to the subject and would like to learn more about the state-of-the-art and upcoming trends in the development of mid-IR fiber and fiber based laser sources. The book is also particularly suitable for researchers or students who are focused on the photoluminescence performance of RE doped soft glasses, spectroscopy lasers, or who are exploring optical nonlinearity and its applications.

References 1. K.H. Sun, US Patent. 2,466,509 (1949) 2. D.M. Roy, R. Roy, E.F. Osborn, Phase relations and structural phenomena in the fluoride-model systems LiF–BeF2 and NaF–BeF2 . J. Am. Ceram. Soc. 33, 85–90 (1950) 3. M. Poulain, M. Poulain, J. Lucas, P. Brun, Fluorine-containing glass with ZrF4 -optical properties of a glass doped with Nd3+ . Mater. Res. Bull. 10, 243–246 (1975) 4. T. Kanamori, The crystallization kinetics of 33SrF2 –16MgF2 –16YF3 –35AlF3 glass for infrared transmission. J. Non. Cryst. Solids 57, 443–446 (1983) 5. A. Boutarfaia, M. Poulain, New stable fluoroindate glasses. Solid State Ionics 144, 117–121 (2001) 6. A. Boutarfaia, M.A. Poulain, M.J. Poulain, S.E. Bouaoud, Fluoroindate glasses based on the InF3 –BaF2 –YF3 system. J. Non. Cryst. Solids 213–214, 36–39 (1997)

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1 Introduction to Mid-infrared Fluoride …

7. M. Poulain, M. Poulain, Multicomponent fluoride glasses. J. Non. Cryst. Solids 213–214, 40–43 (1997) 8. G. Zhang, B. Friot, M. Poulain, New gallium and indium based fluoride glasses. J. Non. Cryst. Solids 213–214, 6–10 (1997) 9. C.T.M. Ribeiro, A.R. Zanatta, L.A.O. Nunes, Y. Messaddeq, M.A. Aegerter, Optical spectroscopy of Er3+ and Yb3+ co-doped fluoroindate glasses. J. Appl. Phys. 83, 2256–2260 (1998) 10. A. Berrou, C. Kieleck, M. Eichhorn, Mid-infrared lasing from Ho3+ in bulk InF3 glass. Opt. Lett. 40, 1699–1701 (2015) 11. M. Kochanowicz, J. Zmojda, A. Baranowska, P. Miluski, M. Lesniak, M. Kuwik, J. Pisarska, W.A. Pisarski, J. Dorosz, D. Dorosz, Near-IR and mid-IR luminescence and energy transfer in fluoroindate glasses co-doped with Er3+ /Tm3+ . Opt. Mater. Express 9, 4772–4781 (2019) 12. S. Shibata, M. Horiguchi, K. Jinguji, S. Mitachi, T. Kanamori, T. Manabe, Prediction of loss minima in infra-red optical fibres. Electron. Lett. 17, 775–777 (1981) 13. L.G. Van Uitert, A.J. Bruce, W.H. Grodkiewicz, D.L. Wood, Minimum loss projections for oxide and halide glasses. Mater. Sci. Forum 5, 591–605 (1985) 14. P.W. France, S.F. Carter, M.W. Moore, C.R. Day, Progress in fluoride fibres for optical communications. Br. Telecom Technol. J. 5, 28–44 (1987) 15. L.E. Busse, G.H. McCabe, I.D. Aggarwal, Wavelength dependence of the scattering loss in fluoride optical fibers. Opt. Lett. 15, 423–424 (1990) 16. M. Brierley, P.W. France, Continuous wave lasing at 2.7 µm in an erbium-doped fluorozirconate fibre. Electron. Lett. 24, 935–937 (1988) 17. R. Allen, L. Esterowitz, R.J. Ginther, Diode-pumped single-mode fluorozirconate fiber laser from the 4I11/2 →4I13/2 transition in erbium. Appl. Phys. Lett. 56, 1635–1637 (1990) 18. M. Pollnau, C. Ghisler, G. Bunea, M. Bunea, W. Lüthy, H.P. Weber, 150 mW unsaturated output power at 3 µm from a single-mode-fiber erbium cascade laser. Appl. Phys. Lett. 66, 3564–3566 (1995) 19. S.D. Jackson, T.A. King, M. Pollnau, Diode-pumped 1.7-W erbium 3 µm fiber laser. Opt. Lett. 24, 1133–1135 (1999) 20. X. Zhu, R. Jain, 10-W-level diode-pumped compact 2.78 µm ZBLAN fiber laser. Opt. Lett. 32, 26–28 (2007) 21. S. Tokita, M. Murakami, S. Shimizu, M. Hashida, S. Sakabe, Liquid-cooled 24 W mid-infrared Er:ZBLAN fiber laser. Opt. Lett. 34, 3062–3064 (2009) 22. V. Fortin, M. Bernier, S.T. Bah, R. Vallée, 30 W fluoride glass all-fiber laser at 2.94 µm. Opt. Lett. 40, 2882–2885 (2015) 23. F. Maes, V. Fortin, S. Poulain, M. Poulain, J.-Y. Carrée, M. Bernier, R. Vallée, Roomtemperature fiber laser at 3.92 µm. Optica 5, 761–764 (2018) 24. Y.O. Aydin, V. Fortin, R. Vallée, M. Bernier, Towards power scaling of 2.8 µm fiber lasers. Opt. Lett. 43, 4542–4545 (2018) 25. X. Zhang, H. Ma, J. Lucas, Applications of chalcogenide glass bulks and fibres. J. Optoelectron. Adv. Mater. 5, 1327–1333 (2003) 26. R. Frerichs, New optical glasses with good transparency in the infrared. J. Opt. Soc. Am. 43, 1153 (1953) 27. F.W. Glaze, D.H. Blackburn, J.S. Osmalov, D. Hubbard, M.H. Black, Properties of arsenic sulfide glass. J. Res. Natl. Bur. Stand. 59, 83–92 (1957) 28. M. Asobe, T. Kanamori, K. Naganuma, H. Itoh, T. Kaino, Third-order nonlinear spectroscopy in As2 S3 chalcogenide glass fibers. J. Appl. Phys. 77, 5518–5523 (1995) 29. X.H. Zhang, G. Fonteneau, J. Lucas, Tellurium halide glasses. New materials for transmission in the 8–12 µm range. J. Non. Cryst. Solids 104, 38–44 (1988) 30. S. Danto, P. Houizot, C. Boussard-Pledel, X.-H. Zhang, F. Smektala, J. Lucas, A family of far-infrared-transmitting glasses in the Ga–Ge–Te system for space applications. Adv. Funct. Mater. 16, 1847–1852 (2006) 31. J.A. Savage, S. Nielsen, Chalcogenide glasses transmitting in the infrared between 1 and 20 µm—a state of the art review. Infrared Phys. 5, 195–204 (1965)

References

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32. T. Miyashita, Y. Terunuma, Optical transmission loss of As–S glass fiber in 1.0-5.5 µm wavelength region. Jpn. J. Appl. Phys. 21, L75–L76 (1982) 33. M. Asobe, T. Kanamori, K. Kubodera, Ultrafast all-optical switching utilizing a highly nonlinear chatcogenide glass fiber, in Quantum Electronics and Laser Science Conference, JWA6 (1992) 34. G. Lenz, J. Zimmermann, T. Katsufuji, M.E. Lines, H.Y. Hwang, S. Spälter, R.E. Slusher, S.-W. Cheong, J.S. Sanghera, I.D. Aggarwal, Large Kerr effect in bulk Se-based chalcogenide glasses. Opt. Lett. 25, 254–256 (2000) 35. P.A. Thielen, L.B. Shaw, P.C. Pureza, V.Q. Nguyen, J.S. Sanghera, I.D. Aggarwal, Small-core As–Se fiber for Raman amplification. Opt. Lett. 28, 1406–1408 (2003) 36. R.E. Slusher, G. Lenz, J. Hodelin, J. Sanghera, L.B. Shaw, I.D. Aggarwal, Large Raman gain and nonlinear phase shifts in high-purity As2 Se3 chalcogenide fibers. J. Opt. Soc. Am. B 21, 1146–1155 (2004) 37. C. Conseil, J.-C. Bastien, C. Boussard-Plédel, X.-H. Zhang, P. Lucas, S. Dai, J. Lucas, B. Bureau, Te-based chalcohalide glasses for far-infrared optical fiber. Opt. Mater. Express 2, 1470–1477 (2012)

Chapter 2

Fluoride Glass Composition, Processing and Structure Characterization Shijie Jia, Pengfei Wang, and Gerald Farrell

2.1 Introduction Over the past few decades, much attention has been paid to the fluoride glass family because of their potential in the development of a second generation of optical fibres for long-distance telecommunication links [1]. To date, the ultimate predicted ultralow optical loss of 0.01 dB/km has not been reached, nevertheless, with an attenuation less than 0.1 dB/m having been achieved and an optical window extending from 0.3 to 5 μm, this new family of optical fibres offers unrivalled advantages for ultralong distance transmission in the mid-IR spectral range [1, 2]. Moreover, fluoride glasses exhibit the lowest refractive index among IR glasses, consequently, the lowest optical nonlinearity [3–5], which makes them particularly well suited for high-power delivery and lasing applications where nonlinear effects are undesirable. The optimization of glass composition as well as structural investigations have shown that rare earth (RE) ions Ln3+ play an vital role in the stabilization of the glass framework. Such ions occupy quite well-defined sites and can be inserted in large amounts into the glass matrix where the phonon energy is governed by the bond of the metal and fluorine ions. This highly transparent fluoride glass matrix allows RE excitation and emission over a wide spectral range and it is also very favorable to the excited-state absorption and energy transfer process.

2.2 Compositions and Structures of Fluoride Glasses The first heavy metal fluoride glass was discovered by accident [6]. Since then, a large number of investigations have been carried out in order to find new fluoride glasses or more stable glass compositions. There are some observed correlations between composition and ability to vitrify. They may be summarized as follows:

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Wang et al., Mid-Infrared Fluoride and Chalcogenide Glasses and Fibers, Progress in Optical Science and Photonics 18, https://doi.org/10.1007/978-981-16-7941-4_2

13

14

(1) (2)

(3) (4) (5) (6)

2 Fluoride Glass Composition, Processing …

In multicomponent systems, glass formation usually occurs near a eutectic composition. Some fluorides enhance vitrification at medium or high concentration, such as ZrF4 , AlF3 , InF3 and some others. Generally, these fluorides display a rather high binding energy. The more stable glass compositions usually lie near the center of the vitreous area. The mixing of two fluoride glasses usually results in a vitreous composition. Large-size divalent cations (e.g., Ba2+ , Pb2+ , Sr2+ ) and medium-size monovalent cations (Na+ , Li+ ) can be found in most fluoride glasses. To some extent, chemical substitutions may be carried out with elements belonging to the same column of the periodic chart, e.g., F/Cl, Zr/Hf, Zn/Cd, Al/Ga, Ga/In.

One generally used guideline for glass composition research is the so-called confusion principle which may be expressed as follows: glass stability increases with number of glass components. Put another way, starting from a glass containing several chemical elements, it is possible to obtain a glass with lower devitrification rate in a multicomponent system by adding one supplementary element. Numerous multicomponent fluoride compositions have been claimed as being capable of forming glasses, even though only a very limited number of them can be considered as stable enough to be suitable for optical applications. Being solids which are in thermodynamic non-equilibrium, fluoride glasses have a considerable tendency to nucleate and crystallize during the melt cooling operation or after reheating above the glass transition temperature Tg . Generally, fluoride glasses can be divided into four types: fluoroaluminate glasses based on AlF3 , fluorozirconate glasses based on ZrF4 , fluoroindate glasses based on InF3 , and fluoride glasses based on divalent fluorides. As the energy of stretching vibration between the metal and fluorine ions decreases in the order AlF3 > ZrF4 > InF3 > MF2 (e.g., Zn, Mn), the MIR transmission edge is shifted to longer wavelengths in the same order, see Fig. 2.1 [7]. The most-established and widely used fluoride glasses are the fluorozirconate glasses. Within this glass family, the so-called “ZBLAN” glass with composition (in mol%) 53ZrF4 –20BaF2 –4LaF3 –3AlF3 –20NaF has been the most widely used. ZBLAN glass exhibits high stability against crystallization, enabling low-loss fiber fabrication [4, 8]. By contrast, no fibers have been reported for divalent-fluoridebased glasses, a fact which is ascribed to their low crystallization stability. Recently, fluoroindate glasses have drawn increasing interest due to their extended transmission compared with ZBLAN glass [9, 10]. Meanwhile, fluoroaluminate glasses have gained comparable interest for MIR applications due to their high Tg and chemical stability [11, 12].

2.2 Compositions and Structures of Fluoride Glasses

15

Fig. 2.1 MIR transmission spectra of typical AlF3 -based, ZrF4 -based, InF3 -based and ZnF2 -based fluoride glasses bulk samples of 1 mm thickness

2.2.1 Fluorozirconate Glasses Fluorozirconate glass is a type of glass with ZrF4 as the main component. It has a phonon energy of about 580 cm−1 , which reduces the probability of multi-phonon relaxation of rare earth ions and thus enhances the luminescence efficiency. The refractive index, dispersion, Rayleigh scattering and intrinsic absorption of fluorozirconate glass are very low and therefore they can be used for applications such as a laser output window, prism, light filter and as mid-infrared fiber material. The main disadvantages of fluorozirconate are low glass transition temperature, poor water resistance and low mechanical strength. Unlike BeF2 , zirconium fluoride does not exist in a vitreous form, but it has the best glass forming ability in fluoride glass systems with the exception of BeF2 glass. It can be used for glasses with binary combinations such as ZrF4 –BaF2 or ZrF4 –ThF4 [13, 14]. However ternary combinations are needed in practice to obtain samples thick enough for physical and optical characterization. Indeed, the first fluorozirconate glasses were observed in the ZrF4 –BaF2 –NaF ternary system [15]. Nonetheless, it is possible to obtain these glasses when the cooling rate is fast enough. More stable glasses can be synthesized in the ZrF4 –BaF2 –ThF4 and ZrF4 –BaF2 –LaF3 systems [16, 17]. Early observations showed that zirconium could be replaced by hafnium [18]. However, the development of HfF4 -based glass was hampered by the limited purity and the high price of hafnium compounds. In general, a direct Hf/Zr substitution may be carried out in most cases and does not significantly change the physical properties, except for density and refractive index, which are slightly decreased. Glass compositions allowing slower cooling rates may be found in ZrF4 – BaF2 –LaF3 –NaF quaternary systems [19]. The most efficient stabilizing agent is AlF3 . The demonstration that the addition of a few percent of AlF3 improves glass forming ability resulted in the development of the standard compositions that are now commonly used [20], the most common of which is ZBLAN glass. Table 2.1 shows the typical compositions of fluorozirconate glasses, all of which have been extensively studied.

16

2 Fluoride Glass Composition, Processing …

Table 2.1 Typical compositions of fluorozirconate glasses Glass composition

Tg ± 2/°C

T ± 2/°C

nD ± 0.001

References

30ZrF4 –10ZnF2 –10AlF3 –5YF3 –10BaF2 –35LiF

232

104

1.480

[21]

30ZrF4 –5ZnF2 –15AlF3 –5YF3 –10BaF2 –35LiF

245

111

1.474

[21]

25ZrF4 –10ZnF2 –15AlF3 –5YF3 –10BaF2 –35LiF

237

108

1.466

[21]

20ZrF4 –10ZnF2 –20AlF3 –5YF3 –10BaF2 –35LiF

246

125

1.458

[21]

20ZrF4 –10ZnF2 –20AlF3 –5YF3 –10BaF2 –30LiF–5KF

250

140

1.456

[21]

40ZrF4 –5YF3 –10BaF2 –20NaF–25LiF

200

121

1.475

[21]

35ZrF4 –5YF3 –10BaF2 –20NaF–30LiF

192

81

1.468

[21]

57ZrF4 –34BaF2 –5LaF3 –4AlF3

316

74

1.519

[22]

57ZrF4 –33BaF2 –5LaF3 –4AlF3 –1KF

316

82

1.519

[22]

57ZrF4 –20BaF2 –5LaF3 –4AlF3 –14KF

287

81

1.500

[22]

54ZrF4 –24BaF2 –4LaF3 –4AlF3 –14NaF

276

98

1.500

[22]

54ZrF4 –24BaF2 –4LaF3 –4AlF3 –10NaF–4KF

276

84

1.503

[22]

54ZrF4 –24BaF2 –4LaF3 –4AlF3 –2NaF–12KF

280

82

1.500

[22]

52ZrF4 –20BaF2 –4LaF3 –4AlF3 –20NaF

268

80

1.501

[22]

52ZrF4 –16BaF2 –4LaF3 –4AlF3 –20NaF–4PbF2

261

85

1.511

[22]

52ZrF4 –12BaF2 –4LaF3 –4AlF3 –20NaF–8PbF2

256

97

1.520

[22]

20ZrF4 –20ZnF2 –20BaF2 –20AlF3 –20YF3

349

122

1.488

50ZrF4 –33BaF2 –10YF3 –7AlF3

340

75

[23] [24]

The early research on the structure of fluorozirconate glasses was carried out by Almeida et al. [25] using infrared and Raman spectroscopy techniques. They conducted in-depth research on fluoride glasses with compositions of 50ZrF4 – 50BaF2 and 73ZrF4 –27BaF2 (mol%). They believed that a Zr ion is associated with six F ions, and put forward the idea that the [ZrF6 ] octahedron forms the glass framework with a zigzag chain like structure. Ba ions cross link the chains and two F ions in the octahedron are bridged with other [ZrF6 ] octahedrons in the chain, and the other four F ions are non-bridged, see Fig. 2.2 [25]. Lucas et al. [26] obtained the radial distribution function from X-ray scattering of BaF2 -2ZrF4 glass. They also performed simulations of the molecular dynamics of radial distribution functions for all possible atoms in the glass to confirm this coordination number. They put forward a dimer model that one Zr ion is coordinated with 8 F ions and the other with 7 F ions in the dimer. In this model, each Zr ion is bonded with five bridged F ions, three of which are bridged with three other Zr ions, and the other two bridged F ions are linked with another Zr ion in the dimer. In this configuration, there are three non-bridged F ions left in one Zr ion and two non-bridged F ions left in the other. The bridged F ions linked with other Zr ions form a common angle, while the two bridged F ions linked with Zr ions are of common edge in the same dimer. Inoue et al. [27] measured the radial distribution functions of 2BaF2 –3ZrF4 and BaF2 –3ZrF4 glasses and compared these with functions of different coordination numbers. They believed that the most suitable coordination is one Zr ion with 8 F ions. Kawamoto et al. [28] studied BaF2 –ZrF4 , PbF2 –ZrF4 and SrF2 -ZrF4 glasses by Raman spectra, X-ray scattering and molecular dynamics.

2.2 Compositions and Structures of Fluoride Glasses

17

Fig. 2.2 a Structure of 2ZrF4 –BaF2 glasses. b Structure of 3ZrF4 –BaF2 glasses

It was found that a glass and a crystal with the same composition have similar Raman spectra, so it is considered that Zr ion in the glass is eight-coordinate. In addition, Ko and Doremus [29] have studied the infrared and Raman spectra of multi-component fluorozirconate glasses and crystals, and they also proposed an 8coordination structure model. They also pointed out that the number of co-edge or co-angle Zr–F–Zr chains is affected by Ba ions and other ions in the glass structure.

2.2.2 Fluoroaluminate Glasses The earliest demonstration of AlF3 -based fluoride glass was in Sun Guanghan’s patent in 1949 [30]. This patent states that AlF3 can be used as a glass forming body to obtain multi-component fluoride glass. However, it was not until the early 1980s that the value of AlF3 -based fluoride glass was recognised given its application prospects. A typical AlF3 -based fluoride glass is the AlF3 –MF2 (M is alkaline earth metal) glass, the properties of which have been reported extensively [31]. Fluoroaluminate glass has a low refractive index, low dispersion, low nonlinear refractive index and high optical transparency from UV to IR. The refractive index of fluoroaluminate glass is 1.4–1.5 (Na–D), which is lower than that of ZBLAN glass. The chemical stability of fluoroaluminate glass is three orders of magnitude higher than that of ZrF4 based glass [32]. Given this it is hoped to replace fluorozirconate glass for various optical applications. Compared with fluorozirconate glass, it has the disadvantage of relatively low glass forming ability, which limits its applications in fiber laser, energy transfer and so on. Table 2.2 shows the typical compositions of fluoroaluminate glasses which have been extensively studied.

430 425

40AlF3 –20YF3 –20CaF2 –20BaF2

35AlF3 –15YF3 –12.5CaF2 –12.5BaF2 –12.5SrF2 –12.5MgF2 438 430 432 430 426 379 369 322 337 334 338 347 367 349

35AlF3 –15YF3 –20CaF2 –10BaF2 –10SrF2 –10MgF2

37AlF3 –15YF3 –18CaF2 –10BaF2 –10SrF2 –10MgF2

40AlF3 –15YF3 –15CaF2 –10BaF2 –10SrF2 –10MgF2

37AlF3 –15YF3 –15CaF2 –13BaF2 –10SrF2 –10MgF2

37AlF3 –15YF3 –15CaF2 –12BaF2 –9SrF2 –12MgF2

30.2AlF3 –8.3YF3 –20.2CaF2 –14.1BaF2 –13.2SrF2 –3.8NaF–10.2 ZrF4

30AlF3 –20YF3 –50CdF2

30AlF3 –20YF3 –50PbF2

30AlF3 –10YF3 –30CdF2 –30PbF2

20AlF3 –10YF3 –35CdF2 –35PbF2

10AlF3 –10YF3 –40CdF2 –40PbF2

30AlF3 –10YF3 –5MgF2 –35CdF2 –20PbF2

30AlF3 –20YF3 –10MgF2 –15BaF2 –25PbF2

35AlF3 –15YF3 –5MgF2 –10BaF2 –20PbF2 –15CaF2

40AlF3 –5YF3 –15CaF2 –15BaF2 –10SrF2 –10MgF2 –5BeF2

Tg ± 2/°C

Glass composition

Table 2.2 Typical compositions of fluoroaluminate glasses

122

138

109

140

138

139

110

113

161

126

117

108

115

104

110

105

T ± 2/°C

1.520

1.430

1.427

1.440

nD ± 0.001

References

[38]

[38]

[37]

[37]

[37]

[37]

[37]

[37]

[36]

[35]

[35]

[35]

[35]

[35]

[34]

[33]

[23]

18 2 Fluoride Glass Composition, Processing …

2.2 Compositions and Structures of Fluoride Glasses

19

Table 2.3 Compositions of AMCSBY glasses and their thermal properties Number

Composition (mol%)

Properties (°C)

AlF3

MgF2

CaF2

SrF2

BaF2

YF3

Tg

Tc

T

AMSY-1

34.8

8.7

26.1

8.7

8.7

13

438

543

105

AMSY-2

35

10

20

10

10

15

438

542

104

AMSY-3

37

10

18

10

10

15

430

545

115

AMSY-4

40

10

15

10

10

15

432

540

108

AMSY-5

37

10

15

10

13

11

430

547

117

AMSY-6

37

12

15

9

12

15

436

550

126

Kanamori [39] prepared quaternary fluoroaluminate glass by a traditional quenching method. The composition of the glass is 35AlF3 –50(MgF2 + SrF2 )– 15YF3 , which is named as AMSY. Hu Hefang et al. [40] found that the stability of the glass can be improved by using four different alkaline earth metal fluorides, the modified composition is AlF3 –MgF2 –SrF2 –CaF2 –BaF2 –YF3 (AMCSBY). However, the glass still has a crystallization tendency. This group [41] also studied the effect of rare earth fluorides YF3 , LaF3 and YbF3 on the crystallization of AlF3 –MgF2 – SrF2 –CaF2 –BaF2 glass. The results show that the main crystalline phases of 40AlF3 – 60MF2 glass are CaSrAlF7 and β–CaAlF5 . The introduction of appropriate amount of YF3 and YbF3 inhibits the formation of these crystalline phases, reduces the crystallization activation energy, frequency factor and critical cooling rate of the system glass, and consequently improves the glass stability. On this basis, Seddon et al. [42] further optimized the glass composition of this glass system. Table 2.3 shows a series of glass compositions and their thermal properties. Messaddeq and Poulain [43] have studied the stabilization of Al, Y and Zr in divalent metal fluoride glasses. Based on the ternary system of ZnF2 –SrF2 –BaF2 , adding AlF3 and YF3 into the glass can increase the stability of the glass. Compared with fluorozirconate glass, the structure of fluoroaluminate glass has been less studied. Videam [44] and Kawamoto [45] studied the Raman spectra of AlF3 -CaF2 -BaF2 glass, but they did not agree on whether these glass structures were composed of an [AlF6 ] octahedron and [AlF4 ] tetrahedron or only an [AlF6 ] octahedron. Chen Haiyan and Gan Fuxi [46] studied the infrared and Raman vibration spectra of AlF3 –YF3 –MF2 (M = Ca + Mg + Sr + Ba) glass. They investigated the influence of different AlF3 /YF3 ratio on the glass structure. They also compared the vibration bands observed in the glass with those observed in molten fluoroaluminate glass, AlF3 and YF3 crystals. The results show that both the [AlF6 ] octahedron and [AlF4 ] tetrahedron exist in the structure of 50AlF3 –50MF2 glass. When AlF3 is replaced by YF3 , the number of [AlF4 ] units decreases and YFn polyhedron appears. Moreover, almost all [AlF4 ] units are replaced by YFn polyhedron in the structure of 20AlF3 –30YF3 –50MF2 glass. Nanba et al. [47–49] studied the formation and basic structure of AlF3 –BaF2 –CaF2 (ABC) glasses prepared by a rapid quenching method. Based on the results of a simulation molecular dynamics, they believe that the branched chains consisting of [AlF6 ] octahedron make up the glass

20

2 Fluoride Glass Composition, Processing …

framework. Akasaka and Nanba [50] studied the structure of 40AlF3 –20YF3 –40CaF2 glass (AYC) by X-ray, neutron diffraction and molecular dynamics simulation, and compared it with 40AlF3 –20BaF2 –40CaF2 (ABC) glass. The results show that their network structure is similar, which is a branched chain formed by the co-angle link of AlF6 octahedron. Although AlF7 polyhedron is not found in ABC glass, it exists in AYC glass where these polyhedrons are deformed.

2.2.3 Fluoroindate Glasses Intensive studies of fluoride glasses began in the mid-1970s. At that time, fluoroberyllate and fluoroaluminate glasses were well known. An unexpected discovery of fluorozirconate glasses with high values of coordination numbers of glass-forming atoms stimulated the development of studies of glass formation in multicomponent fluoride systems and revealed other fluorides as ‘progenitors of glass’. In 1983, Videau first studied fluoride glass based on InF3 –BaF2 –YF3 system, but it did not attact significant attention at the start. However subsequently due to the development of optical fiber, fiber lasers and fiber amplifiers, fluoroindate glass has been paid more and more attention by researchers. Extensive research on fluorinate glass began since the early 1990s. The study of ternary and multicomponent fluoride glasses using InF3 as the main component has gradually revealed the characteristics and advantages of fluoroindate glasses, which resulted in rapid development of fluoroindate glasses, but so far there are few studies on its structure. It is generally considered that the structure of the fluoroindate glass is similar to that of fluoroaluminate glass, and it is also composed of [InF6 ] octahedral units. GaF3 is commonly added into fluoroindate glass to improve the stability of the glass and form the InF3 –GaF3 based glass, resulting in the so-called BIG glass [51]. BIG glass is almost as stable as ZBLAN glass. In recent years, a series of relatively stable glass systems such as InF3 –BaF2 –ZnF2 – SrF2 , PbF2 –InF3 –GaF3 [52–55] have been developed. In recent years, investigators have shown a significant interest in glasses with high content of InF3 . These glasses are characterized by lower mean values of phonon energy (~ 510 cm−1 ) compared to conventional fluorozirconate glasses (~ 580 cm−1 ), see Fig. 2.3. The range of transparency extends from the UV region to ~ 7–8 μm in the IR region (for fluorozirconate and fluoroaluminate glasses, this extends from the UV region to ~ 5.5 μm). Consequently, the calculated values of the minimum optical losses determined by the sum of Rayleigh scattering and the multi-phonon absorption are substantially smaller in fluoroindate glasses than in fluorozirconate glasses. Therefore, fluoroindate glasses may be regarded as promising IR materials, in particular in optical fibre communication systems for the transmission of information in the IR region. Table 2.4 shows the typical compositions of fluoroindate glasses that have been extensively studied. Messaddeq et al. [64] studied the glass formation of the InF3 –SrF2 –BaF2 –ZnF2 – X(X = PbF2 , CdF2 , NaF, CaF2 ) system. In these systems, the larger glass forming region is the glass system containing CaF2 and PbF2 , which can be prepared in some

2.2 Compositions and Structures of Fluoride Glasses

21

Fig. 2.3 Raman spectra of InF3 -based, ZrF4 -based and AlF3 -based glasses

cases with a thickness of 15 mm. Bulk samples can be prepared by adding a small amount of GdF3 into the glass, and the most stable glass contains GdF3 less than 4%. Soufiane et al. [65] added GdF3 , NaF and GaF3 to the In–Zn–Ba–Sr fluoride glass to increase the thermal stability. Boutarfaia et al. [66] studied the glass formation in the InF3 –SrF2 –BaF2 –YF3 quaternary system, the optimal concentration of YF3 is 5–10%. It was determined for the first time that in fluoride system, YF3 may possess a glass forming ability in addition to stabilization. Later, Boutarfaia et al. [67] studied the formation and crystallization kinetics of multicomponent In/Ga based glasses. They used a base glass with a composition of 30InF3 –10YF3 –25BaF2 –5SrF2 –10NaF–10ZnF2 – 10GaF3 . According to the “interference principle”, LiF, KF and CsF were introduced into the glass to replace NaF. When NaF is replaced by LiF, the glass forming ability increases, while KF and CsF decrease the glass forming ability. The Tg and Tx of these new type fluoroindate glasses are higher than those of fluorozirconate glass.

2.3 Glass Synthesis and Processing 2.3.1 Starting Materials The first step of fluoride glass processing is the selection of starting materials, in which purity is the greatest importance. Anionic impurities, such as sulphates, nitrates and carbonates, must be avoided because they are a source of anionic oxygen in the glass even when a fluorinating step is carried out. Generally, adsorbed gaseous species do not appear in supplier data sheets, but they may form a group of less obvious impurities. Lanthanum oxide may contain fairly large amounts of carbon dioxide.

Tg ± 2/°C 323 328 339 327 332 333 301 265 228 217 285 309 280 305 332 279 265 340 262 259

Glass composition

50InF3 –3YF3 –33BaF2 –14SrF2

50InF3 –5YF3 –31.5BaF2 –13.5SrF2

45InF3 –10YF3 –31.5BaF2 –13.5SrF2

45InF3 –5YF3 –35BaF2 –15SrF2

50InF3 –5YF3 –13.5BaF2 –31.5SrF2

50InF3 –40YF3 –10BaF2

40InF3 –20ZnF2 –20BaF2 –20SrF2

28InF3 –20ZnF2 –17BaF2 –9PbF2 –12GaF3 –6NaF–3CaF2 –5SrF2

35InF3 –20ZnF2 –10BaF2 –35PbF2

35InF3 –45PbF2 –20CdF2

45InF3 –5YF3 –35BaF2 –10NaF–5SrF2

30InF3 –10YF3 –17ZnF2 –25BaF2 –10GaF3 –10NaF–5SrF2

38InF3 –20ZnF2 –17BaF2 –9PbF2 –12GaF3 –6NaF–3CaF2 –5SrF2

47InF3 –2YF3 –20ZnF2 –16BaF2 –4GaF3 –20SrF2

18InF3 –4ZrF4 –6ThF4 –10YbF3 –20ZnF2 –30BaF2 –12GaF3

17InF3 –5YF3 –2AlF3 –15ZnF2 –29PbF2 –22GaF3 –9CaF2 –2SrF2

15InF3 –15ZnF2 –30PbF2 –20GaF3 –20CaF2

6InF3 –47ZnF2 –2NaF–4GaF3 –2LaF3 –24SrF2 –10BaF2 –5CdF2

25.5InF3 –11.5GaF3 –15ZnF2 –18BaF2 –8SrF2 –5YF3 –5LiF–12PbF2

25.5InF3 –11.5GaF3 –14ZnF2 –19BaF2 –8SrF2 –5YF3 –7LiF–10NaF

Table 2.4 Typical compositions of fluoroindate glasses

96

84

75

180

115

128

118

82

108

84

58

68

105

87

83

86

58

51

84

72

T ± 2/°C

[63]

[63]

[62]

[61]

[60]

[59]

[58]

[58]

[55]

[55]

[57]

[57]

[53]

[23]

[56]

[56]

[56]

[56]

[56]

[56]

References

22 2 Fluoride Glass Composition, Processing …

2.3 Glass Synthesis and Processing

23

Water is the most damaging impurity because it is commonly found in many socalled anhydrous fluorides. Depending on the process, the final level of hydroxyl and anionic oxygen are largely determined by the degree of water contamination. For instance, HF is commonly contaminated by H2 O, and most pressured gases contain traces of water. To control the water level during processing, careful measures should be built into the selection, storage and handling of starting materials. For example alkali and alkaline earth fluorides should be reheated in an oven for dehydration. However other fluorides, such as aluminium, zirconium and rare-earth, can not be easily dehydrated. Therefore, the ammonium bifluoride process has been widely used for remove the residual water in hydrated materials.

2.3.2 Melting and Fining After the raw materials are well mixed, the batch is heated to the melting temperature. The selected crucible for melting can be made from platinum, gold or vitreous carbon. The heating rate may be fast if there is no fluorination step. The dryness of the atmosphere enclosing the melting process is the key point of this step. As the water can remain adsorbed on the walls of the oven, it is not always sufficient to flow dry gas into the melting atmosphere. After the initial melting process, a raw glass is obtained. This raw glass may be grey or black and exhibit a rather high devitrification rate. Consequently, large glass batches cooled within the crucible may be crystallized. The optical scattering and hydroxyl content may be fairly high. To remove most of these defects, the fining process is employed which consists of heating the melt above the liquidus temperature in an oxidizing atmosphere. The melt is homogenized and volatile species are eliminated and the reduced phases are then oxidized and dissolved. The majority of the hydroxyls decompose into HF gas and anionic oxygen. Depending and glass composition and batch size, the fining time and temperature may need to be adjusted.

2.3.3 Casting, Cooling and Annealing At the end of the fining process, homogenous glass is obtained after cooling. One traditional cooling method includes two steps: first, the melt is cooled just above the liquidus temperature; then, the melt is cast into a mold. When a mould is used, the cooling rate is relevant to mould temperature, its geometry, and the weights of both the sample and the mould. Since the solidification interface starts at the mould surface and then moves into the glass sample, the cooling rate is not uniform through the sample. Because the thermal diffusivity of the glass is lower than that of the metallic mould, the heat flow is reduced in these conditions. In fact, the mould temperature is usually kept below the glass transition temperature to avoid glass-metal adherence.

24

2 Fluoride Glass Composition, Processing …

Brass is commonly used as the mould material for its good thermal diffusivity and low cost. The casting method offers several merits: it is fast, flexible, and allows for variations in sample sizes and shapes. Compared to the oxide glasses, the low melt viscosity of fluoride glasses makes it possible to fill moulds of small size or complex shape. There are also some limitations and problems related to the casting method. First, atmosphere contamination is enhanced as the melt surface is increased during casting process. Moreover, the liquid motion may generate small bubbles that cannot always reach the surface before the glass is solidified. Another way of preparing homogeneous glass samples is the “mold-crucible” method: the glass is cooled inside the crucible in which it was melted. This method can be easily carried out by using a gold or platinum crucible and a heating furnace. The samples have limited interaction with the surrounding atmosphere and defectfree samples may be prepared by this way. However, this method requires lower cooling rates and the bubble formation is almost inevitable on the walls. Therefore, the outside of the glass sample must be polished to remove the bubbles. For less stable glasses, fast quenching is required. It may be easily achieved by squeezing the melt between two metal plates or by using cooled rollers or splat quenching. Finally a thermal annealing process is usually carried out before cutting or polishing. Heating time and temperature can be adjusted based on experience. The annealing temperature is commonly set around the glass transition temperature to remove the thermal stresses generating in the cooling process.

2.4 Future Prospects In recent decades, fluoride glass has been widely studied because of its excellent properties and potential application value. Fluorozirconate glass, fluoroaluminate glass and fluoroindate glass have been extensively investigated and their stability and durability have been progressively improved through in-depth research and development. With the improvement of glass preparation technology, related disciplines and technologies in various fields, many fluoride glass materials have been applied to related optical devices, including infrared window materials and rare earth doped fiber laser. With further advances in fluoride glass research, some new research directions will gradually emerge. The following aspects may be the focus of future research. Firstly, fluoroaluminate glasses have the potential for use in high power ~ 3 μm fiber laser due to their high Tg and good chemical stability until the fiber loss is comparable to that of fluorozirconate glass fibers. Secondly, the demand in fiber lasers beyond 3.5 μm will propel the application of fluoroindate glass fibers due to their lower phonon energy. However novel fluoride glass with low phonon energy, high thermal stability and high chemical stability remains a challenge for investigators.

References

25

References 1. J. Lucas, Fluoride glasses for modern optics. J. Fluorine Chem. 72, 177–181 (1995) 2. G. Tao, H. Ebendorff-Heidepriem, A.M. Stolyarov, S. Danto, J.V. Badding, Y. Fink, J. Ballato, A.F. Abouraddy, Infrared fibers. Adv. Opt. Photon. 7, 379–458 (2015) 3. D. Tran, G. Sigel, B. Bendow, Heavy metal fluoride glasses and fibers: a review. J. Lightwave Technol. 2, 566–586 (1984) 4. M. Saad, Fluoride glass fiber: state of the art. Proc. SPIE 7316, 73160N (2009) 5. T.M. Monro, H. Ebendorff-Heidepriem, Progress in Microstructured Optical Fibers. Annu. Rev. Mater. Res. 36, 467–495 (2006) 6. M. Poulain, M. Poulain, J. Lucas, Verres fluores au tetrafluorure de zirconium proprietes optiques d’un verre dope au Nd3+ . Mater. Res. Bull. 10, 243–246 (1975) 7. S. Jia, C. Li, Z. Zhao, C. Yao, Z. Jia, G. Qin, Y. Ohishi, W. Qin, Er3+ -doped ZnF2 –BaF2 –SrF2 – YF3 fluoride glasses for 2.7 μm laser applications. Mater. Lett. 227, 97–99 (2018) 8. P. W. France, Fluoride Glass Optical Fibres (Blackie, 1990) 9. J. Bei, T.M. Monro, A. Hemming, H. Ebendorff-Heidepriem, Reduction of scattering loss in fluoroindate glass fibers. Opt. Mater. Express 3, 1285–1301 (2013) 10. www.thorlabs.com. Retrieved December 31, 2020 11. S. Wang, J. Zhang, N. Xu, S. Jia, G. Brambilla, P. Wang, 2.9 μm lasing from a Ho3+ /Pr3+ co-doped AlF3 -based glass fiber pumped by a 1150 nm laser. Opt. Lett. 45, 1216–1219 (2020) 12. S. Jia, Z. Jia, C. Yao, S. Wang, H. Jiang, L. Zhang, Y. Feng, G. Qin, Y. Ohishi, W. Qin, Ho3+ doped fluoroaluminate glass fibers for 2.9 μm lasing. Laser Phys. 28, 015802 (2018) 13. Mi. Poulain, Ma. Poulain, J. Lucas, Etude comparee de verres fluores dans les diagrammes ternaires ZrF4 -BaF2 -MFN (M=Na, Ca, Ln, Th; N=1, 2, 3, 4). Rev. Chim. Min. 16, 267–279 (1979) 14. M. Matecki, Mi. Poulain, Ma. Poulain, J. Lucas, Nouveaux verres au tetrafluorure de zirconiuine contenant pas d’element modifieur. Mat. Res. Bull. 13, 1039–1046 (1978) 15. Mi. Poulain, Ma. Poulain, J. Lucas, P. Brun, Verres fluores au tetrafluorure de zirconium proprietes optiques d’un verre dope au Nd3+ . Mat. Res. Bull. 10, 243–246 (1975) 16. M. Poulain, M. Chanthanasinh, J. Lucas, Nouveaux verres fluores. Mat. Res. Bull. 12, 151–156 (1977) 17. A. Lecoq, M. Poulain, Lanthanum fluorozirconate glasses. J. Non-Cryst. Solids. 34, 101–110 (1979) 18. M. Poulain, J. Lucas, Une nouvelle classe de materiaux: les verres fluores au tetrafluorure de zirconium. Verres Refract. 32, 505–513 (1978) 19. K. Ohsawa, T. Shibata, Preparation and Characterization of ZrF4 –BaF2 –LaF3 –NaF–AIF3 Glass Optical Fibers. J. Lightwave Technol. LT2, 602–606 (1984) 20. A. Lecoq, M. Poulain, Fluoride glasses in the ZrF4 –BaF2 –YF3 –AIF3 quaternary system. J. Non-Cryst. Solids 41, 209–217 (1980) 21. C. Le Deit, M. Poulain, Alkali fluorozirconate glasses. J. Non-Cryst. Solids 213–214, 49–54 (1997) 22. R. Lebullenger, S. BenjabaUah, C. Le Deit, M. Poulain, Systematic substitutions in ZBLA and ZBLAN glasses. J. Non-Cryst. Solids 161, 217–221 (1993) 23. Mi. Poulain, Ma. Poulain, Multicomponent fluoride glasses. J. Non-Cryst. Solids 213–214, 40–43 (1997) 24. V. Lavin, V.D. Rodriguez, I.R. Martin, U.R. Rodriguez-Mendoza, Site selective study in Eu3+ doped fluorozirconate glasses and glass-ceramics. J. Lumin. 72–74, 437–438 (1997) 25. R.M. Almeida, J.D. Mackenzie, Vibrational spectra and structure of fluorozirconate glasses. J. Chem. Phys. 74, 5954–5961 (1981) 26. J. Lucas, D. Louer, C.A. Angell, A structural model for fluorozirconate glass. Mater. Sci. Forum 5–6, 449–455 (1985) 27. H. Inoue, H. Asegawa, I. Yasui, A study of the structure of glasses based on ZrF4 . Phys. Chem. Glasses. 26, 74–81 (1985)

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28. Y. Kawamoto, T. Horisaka, K. Hirao, N. Soga, A molecular dynamics study of barium metafluorozirconate glass. J. Chem. Phys. 83, 2398–2404 (1985) 29. H.S. Ko, H. Doremus, Infrared spectra and structure of fluorozirconate glasses. Phys. Chem. Glasses. 32, 136–139 (1991) 30. K.N. Sun, Fluoride glass, US. Patent. 2466509, 1–4 (1949) 31. M.R. Shahriari, T. Iqbal, G.H. Sigel, G. Merberg, Synthesis and characterization of aluminum fluoride-based glasses and optical fibers. Mater. Res. Forum 32–33, 99–105 (1991) 32. T. Yano, J. Mizuno, S. Inoue, S. Shibata, Y. Onoda, M. Yamane, NMR study on glass structure of chlorine-doped AlF3 -based glasses with various glass-forming abilities. J. Non-Cryst. Solids. 213–214, 345–352 (1997) 33. H. Hu, F. Lin, M. Li, D. Gu, Study on fluoride glass of RF2 –AIF3 –YF3 system. J. Silicate 13, 402–407 (1985) 34. S. Kanamori. Japan’s Public Charter Gazette, 135152 (1985) 35. G. Cao, Improvement of fluoroaluminate glass and study of new fluorogermanate glass. Ph. D. thesis. Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, 2003 36. R.M. Zakalyukin, P.P. Fedorov, Classification of fluoroaluminate glasses. Inorganic Mater. 39, 756–760 (2003) 37. C. Benhamidèche, A. Boutarfaia, M. Poulain, Fluoroaluminate glasses. J. Alloy. Comput. 366, 233–240 (2004) 38. Y. Wang, N. Sawanobori, S. Nagahama, Formation of fluoride glasses based on AlF3 –YF3 – PbF2 system. J. Non-Cryst. Solids 128, 322–325 (1991) 39. T. Kanamori, The crystallization kinetics of 33SrF2 –16MgF2 –16YF3 –35AlF3 glasses for infrared transmission. J. Non-Cryst. Solids 57, 443–446 (1983) 40. H. Hu, F. Lin, Y. Yuan, J. Feng, Effect of rare earth fluoride on properties of fluoroaluminate glass. Mater. Sci. Forum. 67–68, 239–244 (1991) 41. H. Hu, F. Lin, J. Feng, Effect of rare earth fluoride on the properties of fluoroaluminate glass. J. Silicate 18, 501–505 (1990) 42. A.V. Cardoso, P. Korgul, A.B. Seddon, Identification of crystalline phases found in multicomponent AlF3 -based glasses. J. Non-Cryst. Solids 161, 56–59 (1993) 43. Y. Messaddeq, M. Poulain, Stabilizing effect of aluminum, Yttrium and zirconium in divalent fluoride glasses. J. Non-Cryst. Solids 140, 41–46 (1992) 44. J.J. Videan, J. Portier, B. Piriou, Raman spectroscopic studies of fluorophosphates glasses. J. Non-Cryst. Solids 48, 385–392 (1982) 45. Y. Kawamoto, A. Kono, Raman spectroscopic study of AlF3 –BaF2 –CaF2 glasses. J. Non-Cryst. Solids 85, 335–345 (1986) 46. H.Y. Chen, F.X. Gan, Viberational spectra and structure of AlF3 –YF3 fluoride glasses. J. NonCryst. Solids 112, 272–276 (1989) 47. H. Inoue, T. Nanba, H. Hagihara, T. Kanazawa, I. Yasui, Computer simulation of Raman spectra of fluoride glasses. Mater. Sci. Forum 32–33, 403–408 (1991) 48. T. Nanba, H. Inoue, Y. Arai, H. Hasegawa, M. Misawa, I. Yasui, Diffraction Studies of AlF3 – BaF2 –CaF2 Glasses. Mater. Sci. Forum 32–33, 385–390 (1991) 49. I. Yasui, H. Hagihara, Y. Arai, Glass formation in the system of AlF3 –BaF2 –CaF2 and properties of these glasses. Mater. Sci. Forum 32–33, 173–178 (1991) 50. Y. Akasaka, T. Nanba, H. Inoue, T. Osuka, I. Yasui, Structural analysis of AlF3 -CaF2 -YF3 glass by diffraction methods. J. Non-Cryst. Solids 140, 249–254 (1992) 51. J. Lucas, J. Chiaruttini, G. Fonteneau, P. Christensen, S. Mitachi, New multicomponent fluoride glasses with low critical cooling rates for optical fibers. Proc. SPIE 1228, 56–62 (1990) 52. A. Boutarfaiaa, M. Poulain, Fluoride glasses in the InF3 –GaF3 –YF3 –PbF2 –CaF2 –ZnF2 system. J. Phys. Chem. Solids 63, 2129–2133 (2002) 53. A. Akella, E.A. Downing, L. Hesselink, New fluoroindate glass composition. J. Non-Cryst. Solids 213–214, 1–5 (1997) 54. J. Qiu, K. Maeda, R. Terai, H. Wakabayashi, Properties and structure of fluoroindate glasses containing various divalent cations. J. Non-Cryst. Solids 213–214, 363–368 (1997)

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55. A. Boutarfaia, M. Poulain, New stable fluoroindate glasses. Solid. State. Ion. 114, 117–121 (2001) 56. A. Boutarfaia, M.A. Poulain, M.J. Poulain, S.E. Bouaoud, Fluoroindate glasses based on the InF3 –BaF2 –YF3 system. J. Non-Cryst. Solids 213–214, 36–39 (1997) 57. G. Zhang, B. Friot, M. Poulain, New gallium and indium based fluoride glasses. J. Non-Cryst. Solids 213–214, 6–10 (1997) 58. W.A. Pisarski, J. Pisarska, W. Ryba-Romanowski, Effect of erbium concentration on physical properties of fluoroindate glass. Chem. Phys. Lett. 380, 604–608 (2003) 59. G. Rault, J.L. Adam, F. Smektala, J. Lucas, Fluoride glass compositions for waveguide applications. J. Fluor. Chem. 110, 165–173 (2001) 60. C. Duverger-Arfuso, B. Boulard, Y. Jestin, M. Ferrari, A. Chiasera, Influence of PrCl3 /PrF3 on the optical and spectroscopic properties of fluorogallate and fluoro-gallo-indate glasses. Opt. Mater. 28, 441–447 (2006) 61. R. Lebullenger, L.A.O. Nunes, A.C. Ernandes, Properties of glasses from fluoride to phosphate composition. J. Non-Cryst. Solids 284, 55–60 (2001) 62. J.L. Adam, Fluoride glass research in France: fundamentals and applications. J. Fluor. Chem. 107, 265–270 (2001) 63. S. Jia, Z. Jia, C. Yao, L. Zhang, Y. Feng, G. Qin, Y. Ohishi, W. Qin, 2875 nm lasing from Ho3+ -doped fluoroindate glass fibers. IEEE Photon. Technol. Lett. 30, 323–326 (2018) 64. Y. Messaddeq, A. Delben, M. Bosolo, M.A. Aegerter, A. Soufiane, M. Poulain, New fluoroindate glass compositions. J. Non-Cryst. Solids 161, 210–212 (1993) 65. A. Soufiane, Y. Messaddeq, M. Poulain, Optimization of thorium-free indium fluoride glass compositions and application to optical fibers, in Proceedings of 9th International Symposium on Non-Oxide Glasses, Hangzhou, China, 459–464 (1994) 66. A. Boutarfaia, M. Poulain, M. Poulain, S.E. Bouaoud, Glass formation and crystallization kinetics, in Proceedings of 10th International Symposium on Non-Oxide Glasses, Corning, NY. USA, 82–90 (1996) 67. A. Boutrafaia, M. Legouera, M. Poulain, Glass formation and crystallization kinetics in a multicomponent fluoride glass. J. Non-Cryst. Solids 291, 176–180 (2001)

Chapter 3

Fluoride Glass Optical Fibers Pengfei Wang, Jiquan Zhang, Yuxuan Jiang, Jibo Yu, Shunbin Wang, and Gerald Farrell

3.1 Introduction In the early 1840s, Jean Daniel Colladon and Jacques Babinet proposed a theory that light can be guided by light reflection. In 1854, the Irishman John Tyndall, using light trapped in the flow of a simple cascade of water, demonstrated the phenomenon of total internal reflection for the case when light enters air from water in a specific angle range (> 48°). In 1880, Alexandra Graham Bell showed that it was possible to carry a voice signal over a line-of-sight light beam. Looked at in retrospect, the weakness of Bell’s experiment was that communication was limited to a straight line between two points, but Tyndall’s experiment showed that light could be guided on a curved path. Taken together the Tyndall and Bell experiments foreshadowed the possibility of communications using light guided in an optical fiber, over a century later. The notion of guided communications for light was left unexplored for many years following Bell’s experiment. In the following decades, the applications of uncoated glass rods or fibers were mainly in image transmission, such as endoscopes, flexible periscopes and even in early televisions. However transmission in such a glass fiber is difficult to use in practice, for example, when fibers are butted together with each other, or the interface was damaged, it was found that the light beam was severely attenuated. In 1950s, Brian O’ Brien discovered that the key to effectively transmit light in optical fibers was the use of low-index coatings enclosing the core of the glass fiber, effectively implementing light containment in a fibre core over a distance. Later, A. C. S. Van Heel provided a detailed report on the technology of fiber cladding [1]. Simultaneously, Harold Hopkins and Narinder S. Kapany independently implemented the first fiber optic system for viewing the inside of a patient’s stomach during an endoscopy, using high-quality image transmission in glass fiber bundles for the first time [2]. Since 1960s, with the invention of the first laser by Theodore Maiman and the development of optical fiber fabrication technology, optical fiber communication has © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Wang et al., Mid-Infrared Fluoride and Chalcogenide Glasses and Fibers, Progress in Optical Science and Photonics 18, https://doi.org/10.1007/978-981-16-7941-4_3

29

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3 Fluoride Glass Optical Fibers

become one of the most dominant communication methods, underpinning communications systems that range from short local distance systems in buildings to global systems that span the planet. For ultra-long distance communications, there is a need for very low-loss optical fiber, with attenuation values per km which are much lower than that offered by silica, the current glass type used in commercial optical fiber. For this, research on fluoride glass fiber focused on the ultra-low loss optical fiber for communication systems has become very important. In the next section the mechanisms which result in fiber loss are considered.

3.2 Fiber Loss The main source of loss or attenuation of light in an optical fiber include absorption, scattering and radiation loss.

3.2.1 Absorption Loss 3.2.1.1

Intrinsic Electronic Absorption

Intrinsic electronic absorption refers to the absorption effect introduced by the materials used in the optical fiber, manifesting in the ultraviolet and infrared bands. The absorption caused by electronic transitions in optical fiber materials is usually in the ultraviolet region and is called the Urbach tail. Electron absorption induced by defects such as dangling bonds and vacancies is called the weak absorption tail. The absorption losses caused by Urbach tail and weak absorption tail will decrease with an increase of wavelength. In the infrared band, when the light wave propagates in the material, part of the light wave energy will transfer to molecular vibration due to the multiphonon absorption effects. Intrinsic absorption is attributed to the optical fiber material itself, which usually involves a low loss transmission range for a range of wavelengths, with steep increases in attenuation at the upper and lower wavelength bounds on either side of the loss-loss transmission range.

3.2.1.2

Extrinsic Electronic Absorption

Extrinsic electronic absorption, also known as impurity electronic absorption, originates from impurity ions mainly including transition metal ions (Fe2+ , Co2+ , Ni2+ , Cu2+ , Mn2+ , etc.) and rare-earth ions. These impurities are either present in very low quantities in the raw material used for fiber fabrication or are introduced during fiber manufacture. These ions absorb lights at specific wavelengths, an ion can show many absorption peaks (so called overtones) from the ultraviolet to mid-infrared. With the

3.2 Fiber Loss

31

progress of techniques in material purification and fabrication, these impurities can be controlled at a very low level.

3.2.1.3

Intrinsic Vibration Absorption

Intrinsic vibration absorption, includes atomic defect absorption and multi-phonon vibration absorption. Atomic defect absorption is due to the incompleteness of the atomic structure of the optical fiber material, such as defects due to oxygen atoms, high aggregations of atomic groups and molecular defects. In the fabrication process, when the optical fiber material receives thermal excitation or light radiation, certain covalent bonds will be broken, causing atomic defects. The material thus vibrates and absorbs light energy under the action of the light field. With modern-day fabrication techniques, the loss caused by these factors can be ignored compared with other factors. Multi-phonon vibration absorption depends on a complex function of the effective charge, mass and size of the atoms that make up the glass, which is the intrinsic loss of the material, and increases as the wavelength increases.

3.2.1.4

Extrinsic Vibration Absorption

Extrinsic vibration absorption comes from impurities and materials such as OH− , 2− 2− 3− − − NH+ 4 , CO3 , SO4 , PO4 , NO3 , CO2 and CO in the materials. Absorption of OH appears at ~ 0.95, ~ 1.24, ~ 1.38, ~ 2.9 and 6 µm, these absorption peaks and intrinsic absorption mainly affect the three communication windows used for the silica fiber at 0.85, 1.31 and 1.55 µm. The peak at 1.38 µm is of particular concern as it reduces the ability of a silica fiber to carry very wideband dense wavelength division multiplexed signals, which can span the wavelength region from just under 1.3 µm to just over 1.6 µm. For fluoride fibers, OH absorption peaks at 2.9 and 6 µm are the equivalent undesirable peaks [3]. Therefore, OH− must be eliminated to obtain low loss fluoride fibers. There are many sources of OH− in optical fibers including moisture and hydroxide in the material used to fabricate the optical fiber. These hydroxides are not easily removed during the raw material purification process, and remain in the fabricated optical fiber in the form of OH− ions; the second is a small amount of water in the hydrate in the manufacturing materials; third, water molecules are generated due to the chemical reactions during the manufacturing process and the fourth is the water vapor in the preparation environment. With the increase of materials purities and strict controls to water molecules in the fabrication environment, the OH− content has been reduced to a sufficiently low level.

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3 Fluoride Glass Optical Fibers

3.2.2 Scattering Loss 3.2.2.1

Linear Scattering Loss

Any optical fiber waveguide cannot be perfect. Its material, size, shape and refractive index distribution, etc., may be uneven or otherwise non-ideal, causing scattering losses of the fiber propagation modes. Since the power loss caused by the scattering has a linear relationship with the power of the propagation mode, it is called linear scattering loss.

Rayleigh Scattering Loss When the incident light quantum collides with particles much smaller than the incident light wavelength, elastic scattering will occur, the energy of the incident light is absorbed and converted into the energy of the particles, and the particles re-radiate the energy in the form of light energy in all directions, while the energy and frequency of the light quantum remain unchanged. This is called Rayleigh scattering. The requirement of Rayleigh scattering is that the diameters of the particles must be much smaller than the wavelength of the incident wave, and the upper limit is usually about 1/10 of the wavelength. Rayleigh scattering in an optical fiber is mainly caused by the uneven molecular density and other fluctuations of the material composition. Thermal disturbance during the heating and manufacturing process of optical fiber will result in unevenness of the material composition and density, which in turn can lead to a slight refractive index fluctuation (random variation on a scale smaller than the light wavelength) and thus loss caused by Rayleigh scattering in the optical fiber. Compared to silica fibers, since the level of Rayleigh scattering loss is inversely proportional to the 4th power of the light wavelength, the influence of Rayleigh scattering loss can be greatly reduced for fluoride glass fibers working in the long wavelength region. The Rayleigh scattering coefficient also depends on the glass refractive index and glass transition temperature. Fluoride glass fiber with its low refractive index and glass transition temperature should exhibit low Rayleigh scattering. Rayleigh scattering is the most basic scattering process, which belongs to intrinsic light scattering. Rayleigh scattering loss is also one type of intrinsic loss. Together with intrinsic absorption loss, it sets the theoretical lower limit value of fiber loss.

Waveguide Scattering Loss Waveguide scattering loss is typically caused by external defects. These can be attributed to limitations in fiber drawing and other random factors in the optical fiber manufacturing process, which may cause defects in the optical fiber structure. For example, in the process of manufacturing optical fibers, due to the different

3.2 Fiber Loss

33

physical and chemical properties of the core and cladding materials, especially the different viscosities and melting points, imperfections at the core-cladding interface will arise, such as cracks, geometry imperfections or other fluctuations. In the process of preform fabrication and fiber drawing, poor control of temperature, speed, flow, etc. will also produce imperfections in the core-cladding interface and unevenness of the core diameter along the axial direction, resulting in energy coupling between the guided wave mode and the radiation mode. Thus the energy is transferred from the guided wave mode to the radiation mode, resulting in waveguide scattering loss.

Other Scattering Losses Due to the presence of micro-bubbles, crystallites, and infusible impurities during the optical fiber manufacturing process, external defect losses can occur. These defects sometimes give rise to a lot of external losses. When the diameters of these particles are equivalent to the radiation wavelength, Mie scattering occurs. The scattering intensity of Mie scattering is proportional to the second power of the frequency, the scattering is stronger in the forward light direction than in the backward light direction, and so the directivity is more obvious than Rayleigh scattering. When the particles diameters are large, the Mie scattering can be approximated as Fraunhofer diffraction.

3.2.2.2

Nonlinear Scattering Loss

Spontaneous Brillouin scattering and spontaneous Raman scattering occur with lowpower transmission in optical fibers. As the light wavelength increases, the scattering loss decreases rapidly. When the light energy in the optical fiber increases above a certain level, stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) will occur in the optical fiber, resulting in energy loss.

Brillouin Scattering When the incident light intensity is relatively small, the Brillouin scattering generated in the fiber is spontaneous Brillouin scattering. From the perspective of traditional physics, the Brownian motion of particles in the optical fiber will result in elastic mechanical vibration. This vibration will have a periodic effect on the fiber density, and then generate an acoustic wave field that is not affected by external forces. The refractive index of the optical fiber is periodically modulated under the influence of this acoustic wave field, so that the scattered light generated in the optical fiber has a Doppler effect, this is called spontaneous Brillouin scattering. When the incident light with a certain angular frequency enters the optical fiber, the periodically changing sound field can be regarded as a moving grating, and the incident light is reflected by

34

3 Fluoride Glass Optical Fibers

Bragg diffraction. Affected by the Doppler effect, when the moving direction of the grating is opposite to the incident light, the scattered light generated is the Brillouin Stokes scattering light with a lower frequency; when the incident light moves in the same direction as the grating, the generated scattered light is Brillouin antiStokes scattering light with a higher frequency. When the light wave is strong, the light propagating in the optical fiber will create electrostriction. Elastic wave formed by electrostriction and the light wave will cause significant stimulated Brillouin scattering.

Raman Scattering Raman scattering is an inelastic scattering. When a pump light is incident into a nonlinear medium, two new light waves with different frequencies from the excitation light will be observed in the spectrum. The wave that has lower frequency than that of the pump light is called Stokes wave, while wave that has higher frequency than pump light is called the anti-Stokes wave. Under the effect of strong pump light, the conversion efficiency of this nonlinear process will be greatly increased. The molecular vibration and rotation in the optical fiber material make the optical fiber polarization different and therefore it appears nonlinear, the induced scattering is called stimulated Raman scattering.

3.2.3 Radiation Losses The transmission characteristics of an optical fiber will change when the fiber is bent, a portion of the energy in a guided wave mode will be converted into a radiation mode and enters into the cladding, where it is absorbed by the coating or cladding, causing additional optical fiber loss. There are two forms of fiber bending: one is that the radius of curvature is much larger than the diameter of the fiber, by convention called macrobending; the other is micron-level bending along the axis of the optical fiber, commonly known as microbending.

3.2.3.1

Macrobending Loss

The additional loss caused by fiber bending that has much larger radius of curvature than the diameter. The main reasons are: 1. 2.

The fiber path is bent during laying of the cable structure protecting the fiber; Coiling in splice box, equipment, racks and other enclosures etc.

When the radius of curvature decreases, the loss will increase exponentially. Higher-order modes are more prone to macro-bending loss than lower-order modes, so sometimes the higher-order modes can be filtered out by bending.

3.2 Fiber Loss

3.2.3.2

35

Microbending Loss

The additional loss caused by the fiber micrometer-level bending in the axial direction. The main reasons are: 1.

2. 3. 4. 5.

Within the protective cable structure, slight irregularities in the supporting surface causes uneven stress of each part of the optical fiber, forming random microbending. The uneven tension during fiber fabrication and the uneven interface between the core and cladding, forming microbending. When drawing optical fiber cables in ducts, microbending can result from uneven drawing tension. Microbending caused by the external pressure on the cable, for example cable supports/retainers which are tied too tight, putting pressure on the cable jacket. Microbending due to the thermal expansion and contraction. When the optical fiber encounters temperature changes, the differences in the thermal expansion coefficients of the fiber core, cladding and coating will cause microbending.

In common with macrobending loss, the mechanism of microbending loss is also a result for mode conversion. Microbending causes repetitive energy coupling between guided wave modes and leaky or non-guided wave modes. One way to reduce microbending is to add an elastic protective sleeve to the bare glass fiber. In summary, the transmission loss in an optical fiber is composed of intrinsic and extrinsic factors. If the preparation process is sufficiently ideal, impurities and defects can be sufficiently minimized, only the intrinsic loss will remain. Therefore, the minimum achievable loss can be estimated from the following intrinsic factors: Urbach tail, multiphonon absorption and Rayleigh scattering. Shibata et al. calculated the minimum theoretical loss of ZrF4 –BaF2 –GdF3 glass and obtained a value of about 10–3 dB/km at 3.4 µm [4]. France et al. estimated that the minimum loss of ZBLAN glass at 2.45 µm is 0.022 dB/km [5]. In terms of loss in actual fibers, Kanamori et al. measured losses on two fluoride glass fibers, obtaining losses of 0.7 and 0.9 dB/km [6]. Tran et al. measured a loss of 0.9 dB/km [7], France et al. reported a loss of 3.9 dB/km on a 100 m fiber, 6.5 dB/km on a 200 m fiber, and 9.5 dB/km on a 400 m fiber [8]. With the development of optical fiber preparation technology, low-loss fluoride glass optical fiber preparation technology has gradually become mature, and ZrF4 based, InF3 -based and AlF3 -based glass optical fibers have seen substantial improvement. Among them, the most common example of ZrF4 -based glass fiber is ZBLAN (ZrF4 –BaF2 –LaF3 –AlF3 –NaF) fiber, which has a wide transmission window and low phonon energy. InF3 -based glass fiber has more potential than that in the longwavelength field, because of its wider transmission window and lower phonon energy. For AlF3 -based glass fiber, its mechanical strength is stronger than the above two types, and it has better ability to resist deliquescence. As a consequence, AlF3 -based glass fiber has more important value in obtaining long-term stable light output in a relative high humidity environment. The achievements on non-rare-earth-doped fluoride glass fibers are listed in Table 3.1, from the commercial companies FiberLabs

FiberLabs

ZBLAN

SMF

< 0.1

1.5

Inc

Material

Type

Loss dB/m

Loss@ µm

2.5

< 0.1

MMF

2.9

< 0.1

MMF

AlF3

2.3–3.6

< 0.2

SMF

ZBLAN

Thorlabs

3.2–4.6

< 0.45

≤ 0.2 2.0–3.6

MMF

SMF

InF3

Table 3.1 Recent achievements on non-rare-earth-doped fluoride optical fibers

2.0–4.6

≤ 0.3

SMF 2.5

< 0.01

MMF

ZBLAN

2.2

< 0.01

SMF

Le Verre Fluoré InF3

3.5

< 0.01

MMF

3.5

< 0.035

SMF

AlF3

2.2

< 0.06

MMF

36 3 Fluoride Glass Optical Fibers

3.2 Fiber Loss

37

(Japan), Thorlabs (United States) and Le Verre Fluoré (France). Although there are still many shortcomings in the research of fluoride glass materials and the preparation technology of fluoride glass fibers, the fiber minimum loss depends on the glass composition. Compared with silica material, fluoride glass material has the potential to deliver significantly improved low-loss performance. Accordingly, it is inevitable further development of ultra-low transmission loss fluoride glass fibers will occur in line with continuous advancements in technology.

3.3 Fiber Parameters When preparing fluoride glass fiber, it is necessary to consider the following important parameters, which are also the parameters of fluoride glass. Generally, the research of fluoride fiber starts from basic glass. In terms of thermal properties, whether the fluoride glass fiber can be drawn into a thin fiber depends mainly on the thermal stability of the base glass, which is usually characterized by T =Tx − Tg

(3.1)

where T x is the onset of crystallization temperature, and T g is the glass transition temperature. A larger value of T results in a better glass-forming ability as well as the stronger ability against crystallization. When the material composition is affected by impurities, the glass forming ability will be weakened; the rare-earth solubility has a great influence on the preparation of high-rare-earth-doped optical fibers; Toxicity of the fiber materials also restricts the production and use of certain optical fibers. For example, BeF2 -based glass is no longer researched because of its high toxicity. With regard to mechanical properties, the main parameter that has a major influence on the fiber is the thermal expansion coefficient of the glass itself. When the thermal expansion coefficient of the core and the cladding materials are poorly matched, cracks are easily generated at the core-clad interface, affecting the transmission of light and the mechanical strength of the fiber; In fiber drawing, the core and cladding materials need to have a matching softening temperature; Optical fibers with good thermal conductivity also make for high-power light transmission; Fiber density, Knoop hardness, fracture toughness, Poisson ratio, Young’s modulus, Shear’s modulus are usually parameters that need to be considered when designing and installing optical cables, especially fibers used in long-distance transmission. The primary function of optical fiber cables is to protect the fiber from damage caused by excessive strain, moisture ingress etc., particularly when the cable is being physically installed as this is generally the time during which the fiber is most vulnerable to damage. In fibers where optical gain is to arranged, specific optical properties are important for mid-infrared fibers, one of the main factors affecting the luminescence efficiency

38

3 Fluoride Glass Optical Fibers

of rare earths is the phonon energy of the host material. The lower maximum phonon energy result in the lower non-radiative relaxation rate and the higher luminescence efficiency. In consequence, the maximum phonon energy of the host material is an important indicator for evaluating the rare earth luminescence efficiency; the spectral transmission range indicates the light transmission ability of the fiber at different wavelengths. The mid-infrared fluoride glass fiber with a wide transmission window has attracted more and more attention in recent years; the refractive index is usually used to characterize the numerical aperture (NA) of light in a specific wavelength band and to analyze the light guiding ability of the fiber. A greater refractive index difference between the core and cladding lead to the stronger ability to guide light; The Abbe number is an index used to express the dispersion capability of an optical fiber. Generally speaking, the larger the refractive index, the more serious the dispersion and the smaller the Abbe number; the thermo-optic coefficient is the gradient of the fiber refractive index with temperature. It needs to be studied for fiber grating fabrication.

3.4 Preform Fabrication Preform fabrication is an essential technology underpinning low-loss optical fiber manufacturing. During the development of the silica fiber manufacturing technology, reseachers have explored many methods in order to obtain excellent fibers, such as Interfacial-gel Polymerization [9], Direct melting [10], Glass Phase-Separation technology [11], Sol–gel method [12], Direct Nanoparticle Deposition [13] and Mechanical Extrusion [14, 15]. Currently, the most widely used silica fiber is usually prepared by Chemical Vapor Deposition (CVD). In 1972, Corning invented Outside Vapor Deposition (OVD) [16]. In 1974, the first commercial optical fiber preform [17] was produced by Modified Chemical Vapor Deposition (MCVD) by AT&T Bell Laboratories in America [17]. Subsequently, the Japanese NTT Company and the Dutch Philips company developed the Vapor Axial Deposition (VAD) [18] and Plasma Chemical Vapor Deposition (PCVD) [19, 20], respectively. To date, these methods remain as important techniques for silica fibers. However, for fluoride glass fiber, a very suitable vapor deposition method has not been developed to obtain an excellent preform. The current techniques for preparing fluoride glass fiber preforms includes the following:

3.4.1 Hot-Jointing In 1981, Mitachi et al. prepared the first fluoride glass optical fiber preform using hot-jointing, casting a cladding glass melt into a metal mold in which there was a polished core glass rod [21]. The hot-jointing method usually requires a prepared core rod with suitable diameter, its surface needs to be polished and cleaned. Then

3.4 Preform Fabrication

39

Fig. 3.1 Hot-Jointing

the cladding glass melt is poured into a specially designed semi-cylindrical mold with a groove in the middle to prepare a semi-cylindrical cladding rod. The core rod is place in the groove of the semi-cylindrical cladding rod and the combination is placed together into a mold, then heated to the glass transition temperature for sufficient preheating. Then the other half of the cladding glass melt is slowly poured into the mold, so that the molten liquid fills the other half of the semi-cylinder to become a complete core-cladding preform after annealing. It is worth noting that during the pouring process, the mold should be carefully inclined at an angle of 45° to avoid introducing bubbles. The disadvantage of using the semi-cylindrical hotjointing method is that it is easy for defects to arise at the core-cladding interface, which are caused by the surface tension of the liquid and insufficient bonding. The ideal hot-jointing method is to fix the core rod and preheated in the mold, then fill the molten cladding around it (Fig. 3.1).

3.4.2 Build-in Casting The preform fabrication by Build-in Casting method generally requires pouring core and cladding melt into a preheated mold and annealing together. Due to the poor mechanical properties of fluoride materials, annealing must follow a carefully setout procedure. For instance, there is a need for a long annealing time to ensure the full release of stress, along with precise temperature control and a relatively slow cooling rate to avoid chipping or adhesion between preform and mold. The benefit of Build-in Casting is that the preform prepared has a relatively good core-clad interface. The drawback is that if the inner surface of the mold is not completely smooth and clean,

40

3 Fluoride Glass Optical Fibers

Fig. 3.2 Traditional build-in casting

it will contaminate the surface of the preform, and may even result in impurities in the glass melt during pouring, inducing the appearance of crystallites, cracks, bubbles or inclusions in the optical fiber, all of which will cause the increased loss in the optical fiber and reduced mechanical strength. There are many Build-in Casting processes. The most commons methods are listed below.

3.4.2.1

Traditional Build-in Casting

In 1981, Mitachi et al. used Build-in Casting to improve the preform manufacturing technique by pouring cladding molt into a brass cylindrical mold preheated near the glass transition temperature. Then, before the cladding melt cools and solidifies, the mold is quickly turned upside down to allow the melt in the center of the mold to flow out, forming a hollow cladding tube. Subsequently, the core glass melt is poured into the center of the cladding tube to be annealed together [22]. The preform made by Build-in Casting has a smooth core-cladding interface due to the surface tension between the melts, avoids contamination and obtains a tighter bonding between the core and cladding. Later in 1986, Kanamori et al. used this method to prepare an optical fiber with a transmission loss less than 1 dB/km [6]. The difficulty and drawback of the method is that due to the relationship between viscosity and temperature, the diameter of the center hole is difficult to control. If the cladding melt is poured into the mold for too long, it is difficult to pour it out after turning over. If the interval is too short, it will easily cause the center hole to be too large. It is also necessary to consider that different glass melts have different viscosity-temperature curves, so a lot of experiments are needed to determine the precise parameters needed for the preparation process for different glasses. In addition, during the pouring process, it has been found that the pouring speed needs to be carefully controlled to avoid the formation of bubbles and other undesirable artifacts (Fig. 3.2).

3.4.2.2

Rotational Casting

In order to obtain a preform with higher radial and longitudinal uniformity, in turn to improve the uniformity of a drawn fiber, Tran et al. developed the Rotational Casting

3.4 Preform Fabrication

41

Fig. 3.3 Rotational casting

method in 1982, also called Centrifugal Casting [23]. Firstly, a gold-plated mold is fully preheated to near the glass transition temperature, then the molten cladding glass melt is poured into the mold and the mold is then quickly spun at a speed greater than 3000 rpm. The centrifugal effect ensures the melt uniformly adheres onto the inner surface of the mold, forming a tubular space in the center, thereby obtaining a highly concentric fluoride glass tube whose inner diameter can be precisely controlled by the initial volume of the poured melt. In this way fluoride glass preforms with different core/cladding diameter ratios can be repeatably prepared. Another method is to melt the cladding glass in a rotating mold to obtain a cladding tube, after which the core melt is poured into the cladding tube. The preform is completed after annealing and cooling. In 1987, Lu et al. tested a fiber prepared by this method and reported a transmission loss of 1 dB/km [24] (Fig. 3.3).

3.4.2.3

Jacketing

In order to obtain a single-mode fluoride glass fiber with the necessary smaller core diameter, Ohishi et al. developed the Jacketing Method in [25]. This method is essentially an improvement on the traditional Build-in Casting method. The method uses traditional Build-in Casting to prepare a fiber preform with a core-cladding structure firstly. Then, using the same method, the cladding melt is poured into a preheated mold with a larger diameter, turned over and the center melt is poured out, forming a hollow jacket tube after directly annealing. Finally, the preform is put into a jacketing tube to draw fiber or a new preform. During the drawing process, the space between the jacketing tube and the preform is filled with dry nitrogen in order to eliminate moisture and avoid hydration and dehydration reactions on the glass surface. For the preform prepared by this method, the cladding/core diameter ratio must be greater than five, since when the preform cladding is too thin, there is

42

3 Fluoride Glass Optical Fibers

Fig. 3.4 Jacketing

still a large amount of light energy outside the cladding, while defects between the jacket tube and the cladding can increase extrinsic losses (Fig. 3.4).

3.4.2.4

Modified Build-in Casting

In order to reduce the tapering of the core and cladding, and to accurately control the size of the cladding, Sakaguchi and Takahashi proposed a Modified Build-in Casting method in [26]. The method is to drill the mold substrate, and after pouring the cladding melt into a preheated gold-plated cylindrical mold, before the central part of the cladding melt solidifies, the core melt with a precisely controlled volume is poured on the cladding melt. Then the cylindrical mold is quickly transferred to the perforated substrate, so that the unhardened cladding glass melt flows into the center hole in the bottom plate. At the same time, the central part of the core glass melt flows into the hole, thus being introduced into the cladding center and thereby forming a preform with a core/cladding structure (Fig. 3.5).

3.4.2.5

Lifting

In principle, lifting is as same as the Modified build-in casting to let the cladding melt flow out, then introducing the core glass melt into the cladding. The preform mold is a cylindrical mold that has a funnel-shaped top. First, the cladding melt crucible is tilted and the melt is slowly poured into the preheated mold until the mold is nearly. The core glass melt is then quickly poured onto the top of the cladding to cover

3.4 Preform Fabrication

43

Fig. 3.5 Modified build-in casting

the cladding to a defined height. The cladding melt, which was poured first, adheres on the inner wall of the mold and solidifies to form a tube with a certain thickness. After pouring, the cylindrical mold is lifted vertically and steadily. The unsolidified cladding glass melt in the center of the mold will leak out, at the same time, the core melt in the top mold under the influence of gravity will flow into the center of the tube. In this way, the core glass and the cladding glass attached to the inner wall of the cylindrical mold form a fiber preform with a core-cladding structure. This technique requires a lot of experimentation to accurately control the pouring time. If the time is too short, a large amount of cladding melt will leak at a high flow velocity, introducing too much core melt into the cladding tube, so the core diameter of the obtained preform may be too large; if the mold lift is too slow, the center of the cladding melt in the mold will solidify, so that the glass melt can no longer flow out, thus the core-cladding structure will fail (Fig. 3.6).

Fig. 3.6 Lifting

44

3.4.2.6

3 Fluoride Glass Optical Fibers

Suction Casting

Suction Casting is a preform fabrication technique proposed by Ohishi et al. in 1986, similar to the improved Build-in Casting method. However, in this method, it is the principle of thermal expansion and contraction that helps the cladding center subside to form a core-cladding structure [27]. This technique needs a cylindrical mold with an accumulator (reservoir) at the bottom. Firstly the cladding melt is poured into the mold, and before it is completely solidified, core glass melt is poured onto the cladding glass. During the cooling process, the entire cladding melt undergoes volume shrinkage, forming a cylindrical cladding tube along the long axis, thereby producing a suction effect on the core glass melt. By correctly choosing the volume of the accumulator and the diameter of the mold, the core/cladding diameter ratio and length of preform can be effectively controlled. However, the disadvantage is that when preparing preforms with different sizes, it is necessary to prepare a variety of mold specifications. In addition for this method it is crucial control the temperature and speed of the cladding glass melt during pouring to control the volume shrinkage (Fig. 3.7).

3.4.3 Rod-in-Tube Rod-in-tube is one of the most common methods for preparing optical fiber preforms. It involves pouring a glass melt into a preheated mold. After a precise annealing

Fig. 3.7 Suction casting

3.4 Preform Fabrication

45

operation, a core or cladding glasses rod or block is obtained, depending on the mold size. Through cutting, polishing, perforating and other mechanical processes, the glass rod or glass block is made into a core rod and a cladding tube with the required size. Finally, the core rod is inserted into the cladding tube to form a complete fiber preform. It should be noted that the outer diameter of the core rod and the inner diameter of the cladding tube need to be closely matched. The advantage of this technique is that it is easy to precisely control the diameter ratio of the core and cladding. The process itself and processing equipment required are relatively simple, furthermore, it is easy to prepare a variety of optical fiber structures without being limited by the shapes of the molds. However there are some shortcomings, mainly in the following aspects: 1.

2.

3. 4.

5.

6.

The mechanical strength properties of fluoride material are worse than that of silica material. The prepared glass rod or glass block must have no bubbles and cracks. However during processing, fluoride glass may crack or even break, damage, etc., which increases the difficulty of the process and reduces yield. The surface of the core rod and cladding tube can be easily contaminated during the preparation and processing. For example, the surface can easily be contaminated by dust caused by polishing and drilling. Additionally, fluoride material has poor resistance to deliquescence, increasing the difficulty of cleaning any surface contamination. The contact surfaces of the rod and tube exposed to the air for a long time are prone to deliquesce, which increase the fiber loss. The roughness of the contact surface between the rod and tube will affect the quality of the interface them. When it is too rough, core and cladding interface may be incomplete or have small local fluctuations which cause scattering centres to develop, especially bubbles. As a result the drawn fiber can suffer from high loss and degraded mechanical properties. Due to the relatively large viscosity of the glass during fiber drawing, it is difficult to fully eliminate the gap between the core rod and cladding tube. As a result air holes can form which creates bubbles in the fiber. Due to the need for precise diameter control for the inner wall of the cladding tube and the outer wall of the core rod, drilling and polishing processes need to be strictly controlled (Fig. 3.8).

3.4.4 Extrusion In 1970, E. Roeder et al. developed an extrusion technique and firstly applied it to preform fabrication [14, 28]. Preform fabrication using this technique usually operates under high viscosity (108 −109 P). Firstly the core and cladding glass blocks are prepared, then placed into the press cylinder of an extrusion setup and heated to the deformation temperature of the glass to soften it in a dry atmosphere. Compared to other preparation methods, the operating temperature for extrusion is relatively low, which has considerable advantages for volatile materials or fluoride

46

3 Fluoride Glass Optical Fibers

Fig. 3.8 Rod-in-tube

materials with a lower crystallization temperature and a narrower stability range. The extrusion process is simple with a high production efficiency. Depending on the mold design, preforms with various core-cladding diameter ratios and microstructure preforms with various structures can be extruded. The low temperature and high pressure extrusion conditions ensure no additional transmission loss during the preparation of the glass preform. In 1991, Kuniaki Miura et al. achieved a loss of 0.9 dB/m at 2.94 µm for the first time in a fluoride fiber manufactured by extrusion [29] (Fig. 3.9).

3.5 Fiber Drawing Compared with silica materials, fluoride glass fiber has poor mechanical properties, and is susceptible to crystallization when affected by temperature and other environmental influences Furthermore, the viscosity of fluoride glass shows a strong temperature dependence. Therefore, optimizing the fiber drawing parameters is an essential issue for fluoride glass fiber, including parameters such as feeding speed, heating temperature, drawing speed, drawing tension, pressure and humidity of the local environment and so on.

3.5 Fiber Drawing

47

Fig. 3.9 Extrusion

3.5.1 Fiber Drawing Equipment Fiber drawing is carried using a fiber drawing tower, illustrated in a simplified form in Fig. 3.10. Its main structure consists of a feeding mechanism, heating device, ventilation and air extraction device, diameter measuring system, coating and curing system, tension meter, drawing system and winding system, all controlled by a central computer system. The consistency of the fiber geometry (before a coating is applied) is a key indicator of the quality of a fiber drawing system. For comparison for commercial silica SMF-28 singlemode fiber, which is manufactured in vast quantities, a typical specification demands that the manufactured fiber has a cladding diameter of 125 µm with a diameter tolerance within ± 0.7 µm and a core/cladding concentricity better than 0.5 µm. Given these very tight physical tolerances and the fact that drawing speeds can reach up over 2000 m of fiber per minute, means that drawing towers demand very sophisticated engineering and control technology. The functions of the specific system components are: 1.

The feeding mechanism is a precise arrangement to control the delivery of a certain amount of fiber raw materials such as preforms or glass blocks into the heating device. The feeding mechanism needs to run smoothly, feeding a precise

48

3 Fluoride Glass Optical Fibers

Fig. 3.10 Structure of fiber drawing tower

2.

amount of raw material, otherwise, it will affect the thickness of the fiber and cause fluctuations on the outer diameter. The heating device generally uses a resistively-heated furnace, its temperature field distribution is gradually increased from the upper and lower sides to the middle part. The drawing temperature is in the high temperature zone. When the furnace reaches a certain temperature, the material is softened and the optical fiber is drawn out under tension. In the fiber drawing process, the temperature fluctuation in the furnace can easily cause the fiber diameter to vary unacceptably. Generally, the temperature fluctuation in the furnace should be less than 0.2 °C, especially for fluoride glass system, its viscosity changes rapidly with temperature, the operation temperature range for fiber drawing is very narrow. A resistively-heated furnace that transfers temperature through a thermal radiation field may have large temperature fluctuations, which may cause the material surface to overheat and softened glass to flow too easily, resulting in surface pits and uneven diameters and even more serious defects such as grooves. Fluoride glass fiber has a narrow glass stability range, therefore the fiber drawing temperature needs to be strictly controlled so that the operating temperature does not enter the crystallization temperature region, otherwise the crystallization is likely to occur, causing the fiber quality to decrease. An improved heating technique is to use hot air convection to heat the preform, which makes

3.5 Fiber Drawing

3.

4.

5.

49

the temperature in the fiber drawing furnace more stable, so that the fiber has a more uniform diameter along the length [30]. For a glass system such as fluoride glass which is easily affected by moisture in the air, the glass it must be kept in a dry and clean environment during the fiber drawing process. Otherwise, the fiber will absorb moisture or adhere to dust particles and cause fiber performance degradation. Therefore, it is generally necessary to pass a dry inert gas, such as argon, nitrogen, etc., into the drawing environment during the fiber drawing process to protect the fiber. The gas flow rate should also be accurately controlled. If the flow rate is too small, it will not provide good protection. If the flow rate is too large, it will increase the temperature fluctuations in the furnace, causing fiber jitter and fiber diameter fluctuations. For fiber drawn by the tube-rod method, the attachment of core rod and cladding tube to each other needs to be as complete and uniform as possible. When the negative pressure is too large, it is easy to produce excessive fiber deformation or even prevent the fiber being drawn. When it is too small, it is easy to produce an air gap between the core and cladding which affects the fiber quality. Therefore, in the fiber drawing process, an air extraction system is needed to precisely control the air pressure. The diameter measuring gauge is an instrument used to continuously monitor the fiber diameter very precisely in real time. A diameter measuring gauge is always used to test the cladding diameter, but a second gauge may be used to test the coated fiber diameter. The computer control system used the information from these gauges to precisely control the feeding mechanism and the overall drawing system to ensure that the drawn optical fiber is manufactured with a precise and uniform diameter, within the tolerance specification. Because the bare optical fiber has poor mechanical properties and is easily affected by the environment, a coating and curing system is required to protect the surface of the optical fiber. During coating, the optical fiber passes through the coating cup at a uniform speed, so that a layer of coating material is evenly coated on the surface of the optical fiber. After passing through the subsequent curing furnace, the coating is cured and tightly bonded to the fiber surface. Depending on the optical fiber material and operating requirements, different coatings can be used to protect the optical fiber. The coating must not only be suitable for the drawing technique and be a good match mechanical match to the fiber glass, for example the thermal expansion properties of the coating and fiber glass must be a good match to avoid thermal stress, which can lead to microbending which increases fiber loss. It is therefore necessary to select a suitable coating according to the strength requirements. Coatings can be divided into two categories depending on the curing technique: thermal curing and UV curing. At present, UV curing coating is widely used in communication optical fiber applications, generally including these three coating types: polyurethane acrylate, silicone and modified epoxy acrylate. Depending on the fiber drawing tower, the fiber can also be coated with multiple layers to enhance the fiber quality and protection to abrasion.

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3 Fluoride Glass Optical Fibers

6.

The tension meter is used to detect the axial tension of the fiber in the drawing process. Different optical fibers have different tension bearing ranges. When the tension meter detects high tension, it indicates that the material is too hard, the heating device needs to increase temperature, otherwise the stress on the optical fiber will very likely lead to fiber breakage; When the tension is low, it indicates that the viscosity of the material is low, the surface of the fiber is likely to cause flow, or even fuse directly in the high temperature area. For this reason the tension meter is a critical component in the overall drawing system. The optical fiber delivered by the heating device needs to be drawn by the drawing system composed of one or more pairs of capstan rollers. Under ideal conditions such as constant temperature without crystallization, the fiber diameter is determined by the drawing-system-controlled drawing speed and the feeding-system-controlled feeding speed. The fiber diameter will increase with high feeding speed and low drawing speed. Together with the feeding mechanism, the drawing system should be at a stable speed for drawing. The last step in fiber drawing process is a means to take up and store the drawn fiber on a rotating fiber takeup spool or reel. The speed of this takeup spool is automatically controlled by a tension meter monitoring the passage of the drawn fiber between the takeup winding system and the drawing system so as to avoid winding failure caused by speed differential which could stress and break the fiber.

7.

8.

3.5.2 Fiber Drawing Techniques There are two main methods for preparing optical fibers: the Preform Fiber Drawing method and the Crucible Fiber Drawing method.

3.5.2.1

Preform Fiber Drawing

The operation of Preform Fiber Drawing method involves fixing the prepared fiber preform on the feeding mechanism, the fiber drawing tower heats and draws the larger-diameter preform into a smaller-diameter optical fiber, where the fiber cladding/core diameter ratio is determined by geometry of the prepared preform. If the diameter ratio is relatively large and meets the technical requirements, the optical fiber can be drawn in one go. If the diameter ratio is small and cannot be drawn into an optical fiber directly, it is necessary to draw a so-called precursor preform in the tower into an appropriate thickness preform, then polish and clean the preform surface and insert it into the outer tube to undergo a second final drawing process. The Preform Fiber Drawing method is usually for glass systems with good mechanical properties, such as silicate, phosphate, tellurite glass, etc. However, because of easy crystallization, it is difficult for fluoride glass to use Crucible Fiber Drawing method, so the Preform Fiber Drawing method is often applied.

3.5 Fiber Drawing

3.5.2.2

51

Crucible Fiber Drawing

The Crucible Fiber Drawing method is a method with an improved feeding mechanism, heating device along with ventilation and air extraction systems. The clean glass blocks, cullet or powder are directly packed in the crucible, the crucible is heated by the heating system, and the glass softening environment is controlled by the ventilation and air extraction system, finally resulting in softened glass fibers. The Crucible Fiber Drawing method was originally an improvement proposed to replace the Preform Fiber Drawing method. Compared with the Preform Fiber Drawing method, the Crucible Fiber Drawing method has the following main advantages: 1.

2.

3.

4.

5.

When manufacturing preforms, especially rods and tubes, the operating procedures for the Preform Fiber Drawing method are complex as there is a need to go through cutting, rough grinding, fine grinding, polishing and other processes. The result is a lot of raw material waste as well as a high cost and the production efficiency is too low. The Crucible Fiber Drawing method avoids such a complex process. The preform surface or the core and cladding interface can be easily contaminated which affects the quality of the optical fiber. The Crucible Fiber Drawing method avoids such contamination by melting the powder directly. Crucible Fiber Drawing method can realize a continuous production of fiber instead of batches. For Preform Fiber Drawing method, the ultra-long fiber is susceptible to the preform size limitation, it usually needs splicing techniques to connect shorter fibers to create long lengths, whereas the Crucible Fiber Drawing method is more flexible in fiber length. The Crucible Fiber Drawing method can produce low-loss optical fiber with step index or graded index and this method is also capable of producing a small core fiber with low NA or a larger core fiber with a high NA. Compared with Rod-in-tube prepared preform or fiber, the mechanical properties of infrared soft glass fiber by Double Crucible Fiber Drawing method are significantly improved, additionally, the structure and size of core or cladding are adjustable.

For glass systems that are easy to crystallize, such as fluoride glass fibers, it is essential that the dimensions and temperature of the fiber drawing nozzle are very precisely set, to avoid the glass melt staying at the crystal nucleation or crystal growth temperature for an excessive time.

Double Crucible Fiber Drawing The Double Crucible Fiber Drawing method was successfully developed by the British Telecom. In the late 1970s, the British General Electric Company used the Double Crucible Fiber Drawing method to produce commercial optical fiber products for the first time. The method, is in addition a common preparation method for multi-component glass optical fibers, especially for some glass systems with poor

52

3 Fluoride Glass Optical Fibers

Fig. 3.11 Double crucible fiber drawing

mechanical processing performance like fluoride glass, which can also avoid the glass cooling influence on fiber loss found in the Preform Fiber Drawing technique. The feeding mechanism of the fiber drawing tower is divided into two crucibles, called inner crucible and outer crucible. The inner one is filled with core materials, and the other filled with cladding materials, the crucibles are generally made of platinum. A cylindrical sealing cover is placed above the crucible, into which high-purity, dry inert gas is introduced to ensure a dry and inert environment. As distinct from the Preform Fiber Drawing method, the Double Crucible Fiber Drawing method requires precise temperature control at the nozzle, therefore, a thermocouple is installed near the fiber drawing nozzle to monitor the temperature (Fig. 3.11). The fiber drawing process is a complex fluid dynamic process of viscous flow, affected by instability factors such as temperature changes, differences in viscosity distribution, vibration and dynamic fluctuations when drawing, as well as tension and surface tension changes. Due to the relatively complicated equipment structure needed for the technique, steady state control is more complicated and relatively difficult compared to the Preform Fiber Drawing technique. The difficulties are as follows: 1.

2.

Concentricity: The core and cladding do not automatically have the same central axes, the position of the core center axis can be adjusted by altering the the position of the inner crucible. While a non-concentric core might occasionally be required in theory, in practice the vast majority of fibers demand a concentric core and cladding and the fact that the core center can be adjusted is of little value and is another variable to control, increasing processing complexity. Cladding thickness: The cladding should be evenly coated on the core with a suitable thickness. Achieving very low loss for fiber connectors and fiber fusion splicing depends on ensuring that the cladding diameter has very precise dimensional accuracy, typically the cladding diameter tolerance is substantially

3.5 Fiber Drawing

53

Fig. 3.12 Bubble formation in double crucible fiber drawing

3.

better than 1%. The cladding thickness can be controlled by controlling the flow rate of core and cladding melt. The cladding thickness decreases as the core melt flow rate gets larger. Ways to adjust the flow rate include selecting the appropriate nozzle diameters under the inner and outer crucibles, or adjusting the pressure of the inner and outer crucibles. Bubbles: In this technique, bubbles are easily created at the core and cladding interface which in turn cause fiber loss. To solve this problem, when determining the glass composition, best practice is to ensure that the core and the cladding glass have the same alkali ion concentration and that fiber drawing takes place in an oxygen-free atmosphere. Bubble formation is due to the difference in the alkali ion concentration between the core and cladding separated by platinum. Referring to Fig. 3.12 when the electrode I is inserted into the glass with a higher alkali ion concentration, O2− is converted into O2 and electrons; when the electrode II is inserted into the glass with a lower alkali ion concentration, the oxygen in the atmosphere will be dissolved into the glass melt and converted into O2− , the chemistry can be expressed by the following process equations: 2O2− → O2 + 4e− O2 + 4e− → 2O2−

(3.2)

When using viscosity to control fiber drawing, if the viscosity of the cladding is higher than that of the core, the viscous resistance will increase, in order to maintain the correct flow, the annular gap between cladding and core needs to be increased accordingly. At this stage, the eccentricity of the cladding and core has a small effect on the optical fiber, with the benefit of reducing unwanted deviations in the cladding thickness.

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3 Fluoride Glass Optical Fibers

Fig. 3.13 Single crucible fiber drawing

Single Crucible Fiber Drawing The Single Crucible Fiber Drawing method is a combination of the Preform Fiber Drawing and the Crucible Fiber Drawing method. In this method the cladding tube with a core rod in the center is placed into the crucible, the heating device and crucible are filled with dry inert gas then heat is applied to the base of the crucible so as to reach the deformation temperature, to ensure that the bottom of the cladding tube evenly adheres to the crucible inner surface. At this stage also a negative pressure is maintained between the core and cladding along with the introduction of inert gas into the crucible. Under the action of gravity and pressure, the optical fiber is drawn out from the nozzle. Compared with the Double Crucible Fiber Drawing method, the Single Crucible Fiber Drawing method combines the advantages of the above two methods. This method has a lower drawing temperature, better fiber uniformity and lower fiber loss, which can be attributed to the fact that the Single Crucible Fiber Drawing method heats the glass for a short time and that the core and cladding are adhered in a vacuum state to obtain a better interface. With regard to the Double Crucible Fiber Drawing method, the glasses are heated to form melts throughout the whole fiber drawing process. In a similar manner to the Preform Fiber Drawing method, performs may have some defects which can cause other fiber quality problems (Fig. 3.13).

3.5 Fiber Drawing

3.5.2.3

55

Extrusion

Unlike the Crucible Fiber Drawing method, Extrusion can achieve fiber drawing with relatively high viscosity. The feeding speed depends on the pushing pressure and pushing volume per unit time of the extrusion rod. Extrusion shows a unique advantage to further reduce the fiber drawing temperature, avoiding the fiber quality degradation that could occur especially in fluoride glass systems whose the drawing temperature is close to the glass crystallization temperature region. The core/cladding diameter ratio of the fabricated fiber can be controlled by the feeding speed and the mold diameters. Several variations on traditional Extrusion have been developed including Peeled Extrusion and Double Peeled Extrusion, moreover, they have already been applied in chalcogenide glass fiber. Although there is no report of low loss for Extrusion prepared fluoride glass fiber, it is expected that the fluoride glass fiber has the potential to make progress in achieving low loss when using this method.

3.6 Structures of Fluoride Fiber The advancement of fluoride preform and optical fiber fabrication techniques has resulted in the increasing availability and applications of fluoride glass fibers. In turn a variety of fiber structures have been gradually developed, including single-mode fibers (SMF) and multi-mode fibers (MMF).

3.6.1 Single-Clad Fluoride Fiber The core/cladding diameter of a single-mode fiber is usually 2–14 µm/125–250 µm, and the core/cladding diameter of multi-mode fiber is 85–600 µm/118–710 µm. Single-clad fiber is mainly used for low-power optical transmission in the midinfrared band. The common single-cladding fluoride glass fiber has a non-doped core, it can be doped by Er3+ , Ho3+ , Dy3+ and other rare earth ions as a gain fiber as needed. When the polarization state needs to be selectively transmitted, a polarizing (PZ) optical fiber with a pair of air holes around the core can be used; when the polarization needs to be maintained, a polarization-maintaining (PM) single-mode fiber with an elliptical core can be used. It is important to note that PZ fibers are not the same as PM fibers. While PM fibers maintain the linear polarization state when the polarization direction is aligned with the birefringence axis, they are also capable of propagating any polarization direction. Unlike PM fibers, PZ fibers suffer no polarization crosstalk, which makes them ideal for polarization-sensitive applications (Fig. 3.14).

56

3 Fluoride Glass Optical Fibers

Fig. 3.14 Single-clad fluoride fiber

3.6.2 Double-Clad Fluoride Fiber On the outer surface of a single-clad fiber (core refractive index n0 , first cladding n1 ), another cladding (second cladding) with a lower refractive index (n2 ) than the first cladding is added. This forms a Double-clad fiber (DCF), having a refractive index distribution n0 > n1 > n2 . The second cladding material of the existing doubleclad optical fiber can be fluoride glass, or other materials such as fluororesin with lower refractive index. The typical diameter range of the core/first cladding/second cladding is approximately 2–30 µm/12–300 µm/180–490 µm. Compared with ordinary single-mode fiber, in addition to satisfying the conditions for single-mode transmission for the core and first cladding, the added second cladding with lower refractive index has a larger transverse dimension and high numerical aperture, forming another wave guide with the first cladding. It is thus relatively easy to couple high-power multi-mode semiconductor lasers into the fiber and propagate strictly within the first cladding. Along with the light propagated in the core, a portion of the light energy can also be transmitted in the first cladding resulting in a larger energy distribution across the fiber cross section. At the same time, since the diameter of the DCF is thicker than that of the single-clad fiber, thermal diffusion is more effective for this fiber type, making it easier to maintain stable high-power light transmission. For double-clad fluoride fiber, in addition to doping rare earth ions in the core, the first cladding can also be doped with rare earth ions to achieve a higher optical gain. For some rare-earth-doped fibers, in order to make the light coupled into the fiber pass through the core and be absorbed by rare-earth ions effectively, with the first cladding formed into polygonal structures, such as square, rectangular, octagonal, etc., to eliminate vortex light (Fig. 3.15).

3.7 Applications of Fluoride Fiber The original intention in the development of fluoride glass fiber was to replace silica fiber and achieve low-loss information transmission over long distances. With the expansion of optical fiber applications into new areas, fluoride glass fiber with

3.7 Applications of Fluoride Fiber

57

Fig. 3.15 Double-clad fluoride fiber

its unique advantages has stimulated further development of related materials and fluoride-glass-fiber-based devices with a wider transmission range from ultraviolet to mid-infrared, lower phonon energy, and higher rare earth solubility compared with silica glass fiber; lower dispersion and lower refractive index (thus lower return loss and lower Fresnel reflection can be achieved) compared with chalcogenide glass fiber. It has been widely used in mid-infrared signal transmission, medical diagnosis, spectroscopy (such as environmental monitoring), industrial processing (laser marking, laser cutting), military countermeasures and other fields. Fluoride-glass-fiber-based optical devices are also gradually increasing market share [31].

3.7.1 Low-loss Mid-infrared Transmission Fiber At present, the silica-fiber-based ultra-long-distance non-relay transmission has been successfully used in production and forms the backbone of global communications and the Internet. With a theoretical limit loss of 0.001 dB/km for fluoride glass fiber is less than one hundredth of silica fiber, accordingly, fluoride glass fiber has the potential to achieve transmission over 10,000 km without regeneration, with a particular focus on applications in submarine optical cable systems, many of which span thousands of kilometers. However, up to now, due to the limitations of fluoride glass fiber fabrication, ultra-long-distance optical transmission has not been realized, whereas in the field of short-distance transmission, especially in the mid-infrared band, low-loss fluoride glass fiber has gradually replaced silica fiber.

3.7.2 Fiber Lasers Fiber laser, a new laser type that has been successfully developed in recent decades, is a product of the fiber technology development that generate or transmit laser in a core with micrometer-level dimensions. Compared with other types of lasers, a fiber laser has a series of advantages such as excellent beam quality, good heat dissipation, compact structure, high conversion efficiency, etc., and has attracted

58

3 Fluoride Glass Optical Fibers

Table 3.2 Transparent and operation wavelengths in fluoride glass fibers

Materials

ZrF4 -based InF3 -based AlF3 -based

Transparent WLs (µm) 0.2–9.3

0.25–11

0.3–8.9

Operation WLs (µm)

0.3–5.5

0.3–4

0.3–4.5

extensive attention from many application areas. Fluoride glass fiber has become a hot spot in laser research due to its excellent optical properties which include: 1.

Wide transmission range: fluoride glass is transparent from ultraviolet to midinfrared (Table 3.2).

2.

High rare-earth solubility: rare earth ions as activator can be easily doped into the fluoride glass matrix resulting in a higher rare-earth solubility than silica fiber. Low phonon energy: Compared with silica glass, fluoride glass has lower phonon energy. In silica glass, due to the high phonon energy, the probabilities of nonradiative transitions of rare earth ions are increased, as the energy level lifetimes are reduced. In order to obtain radiative transitions, the energy level gap is generally larger than 4000 cm−1 , while in fluoride glass, this gap is reduced to 2500–3000 cm−1 . In consequence, rare earth ions have longer energy level lifetimes in fluoride glasses and form more metastable energy levels to achieve abundant laser transitions.

3.

4.

Based on the above advantages, fluoride glass fiber doped with trivalent rare earth ions (RE3+ ) has attracted a lot of interest in laser research. Using the principles of up-conversion and down-conversion, in the rare-earth-doped fluoride glass fiber, not only is it possible to implement ultraviolet and visible lasers, but also near infrared and mid-infrared lasers can be achieved (Fig. 3.16).

Fig. 3.16 Fiber laser structure

3.7 Applications of Fluoride Fiber

3.7.2.1

59

Up-conversion Lasers

Up-conversion luminescence uses low-energy pump light to excite ions onto a higher energy level than the pump photon energy, then transferring to a relative lower energy level, thus emissions with shorter wavelengths than those of the pump lights are produced. The main principle now used in fiber lasers is to use the excited state absorption of rare earth ions. The rare earth ions used in up-conversion lasers typically include: Tm3+ (0.28, 0.45, 0.48, 0.81 µm), Pr3+ (0.491, 0.492, 0.520, 0.615, 0.635 µm), Er3+ (0.546, 0.544 µm), Ho3+ (0.55 µm) and Nd3+ (0.381 µm) [32].

3.7.2.2

Down-Conversion Lasers

As distinct from up-conversion luminescence, when the generated light wavelength is longer than that of the pump light, it is called down-conversion luminescence. By this means fluoride glass fiber can implement lasers in the near-infrared and mid-infrared band. Near-infrared includes the important 1–2 µm communications band, which is of great importance in recent years given the global deployment of fibercommunication-based high-speed networks and ultra-fast real-time information processing which rely on this band. In some initial studies, researchers have realized near-infrared lasers in rare earth-doped fluoride glass fibers, such as Er3+ (1.5, 1.7 µm), Pr3+ (1.31 µm), Tm3+ (1.47 µm) and Nd3+ (1.05, 1.3, 1.35 µm). However, laser devices based on silica fiber show good transmission characteristics, especially in high power operation, therefore fluoride glass fiber lasers have been less studied in the near-infrared wavelength range. In the mid-infrared wavelength region of 2–5 µm, many organic and inorganic molecules possess strong optical absorptions. These absorptions can constitute a “molecular fingerprint” to identify their respective “identities” for molecular detection. In this range, the wavelength at 3–5 µm also belongs to the atmospheric transmission window. In this window, low-loss light transmission can be realized due to the fact that water vapor and carbon dioxide in the atmosphere have low absorptions [33, 34]. Consequently, the concentration of many gases (methane, ethane, nitrogen dioxide, nitrous oxide, hydrogen chloride, ammonia, ethylene, methanol, ethanol, ammonia, etc.) can be monitored by mid-infrared laser detector using the respective absorption wavelengths of these gases. In addition water molecules do have important strong absorption peaks at ~ 2 and 3 µm. When lasers with the above wavelengths are used in medical procedures on tissue, more efficient and safer tissue ablation or cutting can be achieved owing to the shallow penetration depth and less damage to the surrounding tissues, especially for some soft tissues with rich water content such as skin, cornea and brain tissue and so on. There are a range of other application areas, for example polymers containing carbon-hydrogen bonds can also be cut and marked by mid-infrared lasers thanks to

60

3 Fluoride Glass Optical Fibers

Table 3.3 Some significant achievements in fluoride glass fiber lasers Year

RE

Material

Wavelength/µm

Maximum power/W

References

2007

Er

ZBLAN

2.78

>9

[35]

2009

ZBLAN

3

24

[36]

2014

ZBLAN

3.604

0.26

[37]

2015

ZBLAN

2.94

30.5

[38]

2017

ZBLAN

3.55

5.6

[39]

2018

ZBLAN

2.82

41.6

[40]

2019

ZBLAN

2.84

35

[41] [42]

2018

ZBLAN

3.15

1.06

2019

Dy

ZBLAN

3.24

10.1

[43]

2019 (pulsed)

ZBLAN

3.1

4.2 k (peak power)

[44]

ZBLAN

2.97–3.23

39 (peak power)

[45]

ZBLAN

3.95

0.011

[46]

ZBLAN

3.002

0.77

[47]

2019 (pulsed) 1997

Ho

2011 2018 2004 2014

Ho/Pr

InF3 -based

3.92

0.197

[48]

ZBLAN

2.86

2.5

[49]

ZBLAN

2.9

3.38

[50]

the presence of appropriate absorption peaks. As a final example in military applications, a fluoride glass fiber laser can be used in optoelectronic countermeasures. The response wavelength of most military detectors is in the mid-infrared band, thus a mid-infrared lasers can directly overload and thus blind or even destroy the sensitive detector in the target seeking system of infrared guided missiles so that the missiles lose their tracking ability. In short, it can be seen that the development of 2–5 µm mid-infrared fiber lasers plays an extremely important role in range of civil, medical and military applications. Lasers using rare-earth-doped fluoride glass fiber as the gain medium show superior performance, such as Er3+ (2.7, 3.5 µm), Ho3+ (2.0, 2.85, 3.9 µm) and Dy3+ (2.9 µm). Table 3.3 lists recent research developments in fluoride glass fiber lasers in recent years.

3.7.3 Fluoride Fiber Based Optical Fiber Amplifiers In the initial research of optical fiber amplifier, researchers have achieved effective signal amplification in rare-earth-doped fluoride glass fibers, such as Er3+ (1.58 µm) [51], Pr3+ (1.31 µm) [52], Nd3+ (1.3 µm) [53] and Tm3+ (1.44 µm) [54]. In recent years, the same as near-infrared fluoride glass fiber lasers, research on fluorideglass-fiber-based amplifiers is gradually decreasing, replaced by excellent silicafiber-based amplifiers in near-infrared region. However, it is undeniable that there

3.7 Applications of Fluoride Fiber

61

is still potential for development in the mid-infrared field [55, 56]. A significant achievement was demonstrated in 1993 when a net gain of 20 dB at 2722 nm was obtained in an Er3+ /Pr3+ co-doped fluoride fiber pumped at 800 nm [57].

3.7.4 Supercontinuum Source Supercontinuum (SC) refers to the phenomenon that the emitted laser spectrum is greatly broadened under the combined action of various nonlinear effects and dispersion when the output of a narrow-band laser is incident into a nonlinear medium. The nonlinear effects include self-phase modulation (SPM) [58], cross-phase modulation (XPM) [59], modulation instability (MI) [60], four-wave mixing (FWM) [61] and stimulated Raman scattering (SRS) [62], etc. In 1970, Alfano R et al. reported SC generation for the first time, and achieved a broad spectral output of 400–700 nm in BK7 glass [63]. In 1976, Lin C and his collaborators realized SC in optical fiber for the first time, laying the foundation for optical-fiber-based SC sources [64]. Compared with non-linear media such as solids, gases, and liquids, the nonlinear coefficient of an optical fiber can be very high since the waveguide structure of the optical fiber can confine the beam inside a micrometer-level core, significantly enhancing the potential for non-linear effects. In addition, the cylindrical nature of a fiber waveguide means that it has an extremely good longitudinal extension performance, so that a long non-linear working distance can be obtained, a natural advantage of fiber SC generation. SC generation is the result of the combination of several fiber characteristics (linear characteristics: dispersion and loss; nonlinear characteristics: nonlinear coefficients) and those of the pump light pulse (wavelength, peak power, pulse width, etc.). Moreover, when the pump wavelength is in the anomalous dispersion region or close to the zero-dispersion wavelength (ZDW) of the fiber, it has the best spectral broadening effect. Generally, the ZDW of ordinary silica fiber is around 1.3 µm and the ZDW of fluoride glass fiber is usually 1.6–2.1 µm. Given the strong development of light source technology in recent years, pump light sources are no longer limited to short wavelengths so that SC sources are gradually expanding into the mid-infrared band. Although an silica-fiber-based SC source can achieve an average power output greater than 200 W [65], due to the limitations resulting from the spectral characteristics of the silica loss, when the spectrum of the silica-fiber-based SC source extends to ~ 2.5–2.6 µm, strong absorption will prevent further SC spectrum expansion. The method of continuously expanding the spectrum of the SC source to the mid-infrared band mainly relies on the soft glass fibers with low phonon energy and low loss beyond 2.4 µm. Various soft glass fibers such as fluoride glass fiber, tellurite fiber [66], chalcogenide glass fiber [67], selenide glass [68], etc. have been extensively studied. Some reports indicate that in terms of the development potential for supercontinuum light sources, although chalcogenide or other materials have the ability to achieve wider and longer spectral ranges than fluoride, it is worthy of

62

3 Fluoride Glass Optical Fibers

Table 3.4 Supercontinuum generation in ZBLAN and InF3 -based fibers Year

Material

Wavelength

Average power

References

2006

ZBLAN

1.8–3.4

5 mW

[69]

2006

0.8–4.5

23 mW

[70]

2009

0.8–4

10.5 W

[71]

2009

0.35–6.28

–

[72]

2011

~ 1.9–4.5

2.6 W

[73]

2012

1.9–3.6

1.08 W

[74]

2012

1.75–4.4

550 mW

[75]

2013

1.9–3.9

7.11 W

[76]

2014

~ 0.9–4

5.24 W

[77]

2014

1.9–4.3

13 W

[78]

2014

1.9–3.8

21.8 W

[79]

2014

1.9–3.3

24.3 W

[80]

2016

0.8–4.5

550.8 mW

[81]

2017

1.9–4.2

15.2 W

[82]

2019

1.85–3.41

30.2 W

[83]

2020

2–3.8

5.89

[84]

2013

2.7–4.7

–

[85]

2014

InF3 -based

1–3.05

2.09 W

[86]

2016

2.4–5.4

8 mW

[87]

2019

0.8–4.7

11.3 W

[88]

2020

1.9–4.9

11.8 W

[89]

recognition that fluoride glass fiber is currently the most mature and commercialized optical fiber in soft glass fiber used in mid-infrared supercontinuum light sources. Among them, ZBLAN and InF3 -based fibers are the main representatives of fluoride glass fiber SC sources, in which high-performing supercontinuum generation has been achieved. Table 3.4 lists some SC progress in these two fibers.

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56. J.-C. Gauthier, L.-R. Robichaud, V. Fortin, R. Vallée, M. Bernier, Mid-infrared supercontinuum generation in fluoride fiber amplifiers: current status and future perspectives. Appl. Phys. B 124, 122 (2018) 57. H. Ibrahim, D. Ronarch, M. Guibert, H. Poignant, J.Y. Allain, Erbium-praseodymium co-doped 2.7 µm fluoride amplifier. J. Non-Cryst. Solids 161, 290–293 (1993) 58. E.P. Ippen, C.V. Shank, T.K. Gustafson, Self-phase modulation of picosecond pulses in optical fibers. Appl. Phys. Lett. 24, 190–192 (1974) 59. G.P. Agrawal, P.L. Baldeck, R.R. Alfano, Modulation instability induced by cross-phase modulation in optical fibers. Phys. Rev. A 39, 3406–3413 (1989) 60. A. Hasegawa, Generation of a train of soliton pulses by induced modulational instability in optical fibers. Opt. Lett. 9, 288–290 (1984) 61. A. Kudlinski, V. Pureur, G. Bouwmans, A. Mussot, Experimental investigation of combined four-wave mixing and Raman effect in the normal dispersion regime of a photonic crystal fiber. Opt. Lett. 33, 2488–2490 (2008) 62. A.K. Abeeluck, C. Headley, C.G. Jørgensen, High-power supercontinuum generation in highly nonlinear, dispersion-shifted fibers by use of a continuous-wave Raman fiber laser. Opt. Lett. 29, 2163–2165 (2004) 63. R.R. Alfano, S.L. Shapiro, Emission in the region 4000 to 7000 Å via four-photon coupling in glass. Phys. Rev. Lett. 24, 584–587 (1970) 64. C. Lin, R.H. Stolen, New nanosecond continuum for excited-state spectroscopy. Appl. Phys. Lett. 28, 216–218 (1976) 65. K. Yin, R. Zhu, B. Zhang, T. Jiang, S. Chen, J. Hou, Ultrahigh-brightness, spectrally-flat, shortwave infrared supercontinuum source for long-range atmospheric applications. Opt. Express 24, 20010–20020 (2016) 66. G.S. Qin, X. Yan, M. Liao, A. Mori, T. Suzuki, Y. Ohishi, Wideband supercontinuum generation in tapered tellurite microstructured fibers. Laser Phys. 21, 1115 (2011) 67. K. Jiao, J. Yao, X.-G. Wang, X. Wang, Z. Zhao, B. Zhang, N. Si, J. Liu, X. Shen, P. Zhang, 1.2–15.2 µm supercontinuum generation in a low-loss chalcohalide fiber pumped at a deep anomalous-dispersion region. Opt. Lett. 44, 5545–5548 (2019) 68. L.-R. Robichaud, V. Fortin, J.-C. Gauthier, S. Châtigny, J.-F. Couillard, J.-L. Delarosbil, R. Vallée, M. Bernier, Compact 3–8 µm supercontinuum generation in a low-loss As2 Se3 stepindex fiber. Opt. Lett. 41, 4605–4608 (2016) 69. C.L. Hagen, J.W. Walewski, S.T. Sanders, Generation of a continuum extending to the midinfrared by pumping ZBLAN fiber with an ultrafast 1550-nm source. IEEE Photonics Technol. Lett. 18, 91–93 (2006) 70. C. Xia, M. Kumar, O.P. Kulkarni, M.N. Islam, F.L. Terry, M.J. Freeman, M. Poulain, G. Mazé, Mid-infrared supercontinuum generation to 4.5 µm in ZBLAN fluoride fibers by nanosecond diode pumping. Opt. Lett. 31, 2553–2555 (2006) 71. C. Xia, Z. Xu, M.N. Islam, F.L. Terry, M.J. Freeman, A. Zakel, J. Mauricio, 10.5 W timeaveraged power mid-IR supercontinuum generation extending beyond 4 µm with direct pulse pattern modulation. IEEE J. Sel. Top. Quantum Electron. 15, 422–434 (2009) 72. G. Qin, X. Yan, C. Kito, M. Liao, C. Chaudhari, T. Suzuki, Y. Ohishi, Ultra-broadband supercontinuum generation from ultraviolet to 6.28 µm in a fluoride fiber, in Optical Fiber Communication Conference, OSA Technical Digest (CD) 2010), OTuJ6 73. O.P. Kulkarni, V.V. Alexander, M. Kumar, M.J. Freeman, M.N. Islam, F.L. Terry, M. Neelakandan, A. Chan, Supercontinuum generation from ~1.9 to 4.5 µmin ZBLAN fiber with high average power generation beyond 3.8 µm using a thulium-doped fiber amplifier. J. Opt. Soc. Am. B 28, 2486–2498 (2011) ´ 74. M. Eckerle, C. Kieleck, J. Swiderski, S.D. Jackson, G. Mazé, M. Eichhorn, Actively Q-switched and mode-locked Tm3+ -doped silicate 2 µm fiber laser for supercontinuum generation in fluoride fiber. Opt. Lett. 37, 512–514 (2012) 75. P.M. Moselund, C. Petersen, S. Dupont, C. Agger, O. Bang, S.R. Keiding, Supercontinuum: broad as a lamp, bright as a laser, now in the mid-infrared, in Laser Technology for Defense and Security VIII, (International Society for Optics and Photonics, 2012), 83811A.

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76. W. Yang, B. Zhang, K. Yin, X. Zhou, J. Hou, High power all fiber mid-IR supercontinuum generation in a ZBLAN fiber pumped by a 2 µm MOPA system. Opt. Express 21, 19732–19742 (2013) 77. J. Swiderski, M. Michalska, High-power supercontinuum generation in a ZBLAN fiber with very efficient power distribution toward the mid-infrared. Opt. Lett. 39, 910–913 (2014) 78. W. Yang, B. Zhang, G. Xue, J. H. Ke Yin, Thirteen watt all-fiber mid-infrared supercontinuum generation in a single mode ZBLAN fiber pumped by a 2 µm MOPA system. Opt. Lett. 39, 1849–1852 (2014) 79. K. Liu, J. Liu, H. Shi, F. Tan, P. Wang, High power mid-infrared supercontinuum generation in a single-mode ZBLAN fiber with up to 21.8 W average output power. Opt. Express 22, 24384–24391 (2014) 80. K. Liu, J. Liu, H. Shi, F. Tan, P. Wang, 24.3 W Mid-infrared supercontinuum generation from a single-mode ZBLAN fiber pumped by thulium-doped fiber amplifier, in Advanced Solid State Lasers, OSA Technical Digest (online) 2014), AM3A.6 81. K. Yin, B. Zhang, J. Yao, L. Yang, S. Chen, J. Hou, Highly stable, monolithic, single-mode mid-infrared supercontinuum source based on low-loss fusion spliced silica and fluoride fibers. Opt. Lett. 41, 946–949 (2016) 82. Y. Ke, Z. Bin, Y. Linyong, H. Jing, 15.2W spectrally flat all-fiber supercontinuum laser source with >1W power beyond 3.8 µm. Opt. Lett. 42, 2334–2337 (2017) 83. L. Yang, T. Wu, B. Zhang, Y. Li, Y. Zhao, J. Hou, 30-W Supercontinuum Genaration in ZBLAN Fiber, in 2019 18th International Conference on Optical Communications and Networks (ICOCN), 2019, 1–3 84. L. Yang, B. Yan, R. Zhao, D. Wu, T. Xu, P. Yang, Q. Nie, S. Dai, Ultra-low fusion splicing loss between silica and ZBLAN fiber for all-fiber structured high-power mid-infrared supercontinuum generation. Infrared Phys. Technol. 103576 (2020) 85. F. Théberge, J.-F. Daigle, D. Vincent, P. Mathieu, J. Fortin, B.E. Schmidt, N. Thiré, F. Légaré, Mid-infrared supercontinuum generation in fluoroindate fiber. Opt. Lett. 38, 4683–4685 (2013) 86. J. Swiderski, F. Théberge, M. Michalska, P. Mathieu, D. Vincent, High average power supercontinuum generation in a fluoroindate fiber. Laser Phys. Lett. 11, 015106 (2013) 87. J.-C. Gauthier, V. Fortin, J.-Y. Carrée, S. Poulain, M. Poulain, R. Vallée, M. Bernier, Mid-IR supercontinuum from 2.4 to 5.4 µm in a low-loss fluoroindate fiber. Opt. Lett. 41, 1756–1759 (2016) 88. T. Wu, L. Yang, Z. Dou, K. Yin, X. He, B. Zhang, J. Hou, Ultra-efficient, 10-W-level midinfrared supercontinuum generation in fluoroindate fiber. Opt. Lett. 44, 2378–2381 (2019) 89. L. Yang, B. Zhang, X. He, K. Deng, S. Liu, J. Hou, High-power mid-infrared supercontinuum generation in a fluoroindate fiber with over 2 W power beyond 3.8 µm. Opt. Express 28, 14973–14979 (2020)

Chapter 4

Chalcogenide Glass Composition, Processing and Structure Characterization Xunsi Wang, Gerald Farrell, and Zheming Zhao

In the mid-infrared optical region, oxide glasses are generally opaque at long wavelengths of about 5 μm while chalcogenide (ChG) glasses made from sulfur (S), selenium (Se) and tellurium (Te) have good transparency up to 20 μm. ChGs are well-known materials which exhibit unique properties in the infrared (IR) region and their technological developments and applications are widespread. They have been used as lenses and optical fibers because of their high linear refractive index and low phonon energy in IR [1]. Many ChGs show nonlinear optical effects, making them valuable in various IR photonic applications [2]. ChGs are also used as X-ray photoconductors in various image-sensor applications, most notably in digital X-ray imaging with applications in security, medical and industrial imaging [3]. Some ChGs exhibit thermally-driven amorphous-crystalline phase changes with a high level of distinction between the crystalline and amorphous states based on their reflectivity or electrical conductivity. These phenomena could be utilized in optical rewritable memory discs and non-volatile PCRAM memory devices [4]. ChG fibers are also investigated for their potential applications in medicine and bio-optical sensors, as they make it possible to monitor the response of living cells to toxins by detecting trace change in their IR spectra [5]. The most important applications of ChGs are presented in Fig. 4.1 [6]. The upper part of the Fig. 4.1 schematically shows the timeline of the development of diverse ChG applications along with the cumulative number of relevant published papers (based on Web of Science since 1945; topic: chalcogenide glass). The publication years reporting the viscosity data for the three most reported ChG materials [6] is shown in the lower part of Fig. 4.1. Among all IR materials, ChG is a special optical material, as it possesses low phonon energy, high third nonlinearity, high stability in the presence of atmospheric moisture and is resistant to crystallization during fiber fabrication, etc. [7] ChG materials are generally transparent in the IR spectral region beyond 8 μm (Sulfide), 10 μm (Selenide) or 12 μm (Telluride), as the absorption coefficient is only 1 cm−1 at the whole wavelength range of 0.62–11.5 μm for As2 S3 , 0.85–17.5 μm for As2 Se3 © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Wang et al., Mid-Infrared Fluoride and Chalcogenide Glasses and Fibers, Progress in Optical Science and Photonics 18, https://doi.org/10.1007/978-981-16-7941-4_4

67

68

4 Chalcogenide Glass Composition, Processing and Structure Characterization

Fig. 4.1 Applications of ChG materials (Adapted from Ref. [6], with permission from the Maney publishing)

and 0.75–12.25 μm for As2 S1.5 Se1.5 , respectively. The minimum optical losses of As2 S3 and As2 Se3 glasses were calculated to be (6–7) × 10–2 dB/km between 4 and 6 μm. For this reason, ChGs can be applied to the manufacture of IR fibers and other IR optic components [8]. Optical fibers drawn from ChGs were first reported in the 1970s [9], when these glass fibers based on As–Se or Ge–Se and tellurides were found to be useful in the delivery of CO2 laser (λ ~ 10 μm) radiation. ChG fiber drawing has been reviewed by several authors [10, 11]. IR optical fibers have drawn increased attention because of their potential ultra-low loss in mid-IR region, which is much lower than that of silica fibers and also their ability of high-power laser delivering, such as that from COx (x = 1, 2) lasers [12]. This chapter focuses on the fundamental principle of glass forming and basic ChG glass compositions of fiber drawing. The details of the fiber and its fabrication will be reviewed in the next chapter. As to the detailed compositions, ChG glasses, formed by individual chalcogenides of the III–V group elements or their compounds have been known for more than 50 years [13], as shown in Fig. 4.2. The first report of their application in fiber optics was reported in 1965 [14, 15]. Active investigations of ChG as a material for optical fiber in the mid-IR range have been carried out for the past 20–25 years [16]. A number of glasses with ChGs of III–V group elements were investigated for the production of optical fibers. The most important results were obtained for glasses based on chalcogenide compositions of arsenic and germanium contained. These glasses have interesting properties, e.g., a wide transparency range, low optical losses, high nonlinearity, a good stability to atmospheric moisture, etc. Many technical problems in IR optics and optoelectronics can be solved efficiently by using low-loss ChG optical

4 Chalcogenide Glass Composition, Processing and Structure Characterization Fig. 4.2 Elements distribution of ChG glass composition in the periodic table

IIIA

IVA

VA

69

VIA

fibers. The main efforts of researchers have and are being directed to increasing the chemical and phase purity of ChG in order to improve the quality of optical fibers [17]. ChG glass systems of As–S, As–Se, As–S–Se, Ge–As–S, As–Se–Te, Ge–As– Se, Ge–As–Se–Te, Ga–La–S, and Ge–Te–AgI were investigated as host materials for optics fiber. A variety of glass formations and structures have been investigated and described in details in a number of papers. Much attention was given to ChGs of arsenic and germanium, i.e., As2 S3 , As2 Se3 , and GeTe. The structure of vitreous As2 S3 or As2 Se3 is described as a layered two-dimensional network mainly consisting of structural units [AsS3/2 ] or [AsSe3/2 ] in a trigonal pyramid shape. Inside the layers, the atoms (structural units) are connected with strong covalent bonds while only van der Waals bonds exist between the layers. The structure of vitreous GeTe2 or GeSe2 is described as a spatial three-dimensional network consisting of structural units [GeTe4/2 ] or [GeSe4/2 ] in a tetragon shape. Structural skeletons of more complex systems (Ge–As–S, Ge–As–Se, As–Se–Te, Ge–As–Se– Te) can contain different groups of short-range ordering (SeSe2/2 , AsAs3/3 , GeGe4/4 , AsSe3/2 , GeSe4/2 , As2Se4/2 , GeSe2/2 ), and the mixed structural units ([AsSe3 –xTex ], [GeSe4 –xTex ]). For Ge–Sb–S system, the structural units contain [GeS4/2 ] and [SbS3/2 ]. Glasses suitable for mid-IR fiber optics need to meet a number of conditions including a wide spectral transmission range, strong resistance to crystallization, high mechanical strength, and a relative broad range of operating temperatures.

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4 Chalcogenide Glass Composition, Processing and Structure Characterization

There is no ChG glass which can meet all of these conditions. Glasses systems of As–S/Ge–As–S, As–Se/Ge–As–Se, and As–Se–Te/Ge–Te–AgI have a relatively low softening temperature (usually smaller than 185 °C) and high chemical stability during heat reformation. Glasses systems of Ge–S, Ge–Se, Ge–As–Se and telluriumcontained ChG glasses can crystallize in the fiber drawing process, and thus more attention needs to be paid in fiber preform fabrication. Arsenic-contained glasses are always considered to be toxic as the presence of arsenic, although showing well in glass stability. Sulfur-contained fibers have a limited transparency in the 0.6–7 μm wavelength range. Glass fibers from selenide and selenium-telluride are suitable for transmission across the entire range of 2–12 μm wavelengths. Optical fibers from high germanium content (15–30 atm. %) glass systems of Ge–S, Ge–Se, Ge–As– S, Ge–As–Se, Ge–As–Se–Te are potentials for operation at 200–300 °C. Ga–La–S, Ge–Ga–La–S ChG glasses with rare-earth elements doping are most suitable for amplifiers and fiber lasers, but the purification process for the glasses is complex and difficult to realize. In order to use ChGs in fiber optics, these following physico-chemical and optical properties should be known firstly: transparency range, minimum optical losses, linear and nonlinear refractive index, spectral dependence, crystallization tendency, characteristic temperatures (usually glass transition temperature T g , temperature for the onset of crystallization T c , temperature of the maximum of crystallization peak T p , liquidus temperature T l ), density, thermal expansion coefficient, heat conductivity, heat capacity, viscosity, Young’s modulus and microhardness. The next section will consider the key characteristics of glass fiber including thermal properties, viscosity and spectral properties related to fiber fabrication.

4.1 ChG Glasses Thermal Properties ChG glasses are prepared by rapid cooling from a high melting temperature. For glass preparation, the required composition is prepared by weighing the constituent elements in their desired atomic percentages and which are then sealed in quartz ampoules under high vacuum degree. The sealed ampoules are kept in a furnace at the temperature which melts the components. During the melting, ampoules are frequently rocked for 12 h to homogenize the melt. Quenching is done by air-cooling where an air blower blows air on the heated ampoules or dropping the quartz ampoules into ice-cooled water or even liquid nitrogen. Not all the materials can be transformed into glassy state even at the fastest cooling rates. Systematic research is needed to determine the glass phase diagram for a specific series of glassy alloys by checking the glassy nature by X-ray diffraction or differential scanning calorimetry (DSC). One of the key properties of fiber glasses are their thermal properties. By understanding the thermal properties of ChG fiber glasses, it is possible to select glass compositions with low crystallize tendency, optimum time–temperature modes for glass samples and effective glass purification technology, optical fiber production process, an operating temperatures range, and an acceptable threshold value for laser

4.1 ChG Glasses Thermal Properties

71

Table 4.1 Thermal characteristics and CCR value of some ChGs [18, 19] Glass

Tg (°C)

Tc (°C)

Tl (°C)

Tc –Tg (°C)

CCR (K/s)

Se

30.2

–

217

–

≥ 0.33

As2 S3

185

–

310

–

2.4 × 10–6

As2 Se3

170

298

370

128

9 × 10–3

GeS2

495

550

820

55

17

GeSe2

380

580

740

200

≈1

As2 S1.5 Se1.5

175

–

–

–

–

As2 Se1.5 Te1.5

150

231

300

81

@ 5 × 10–3

(GeTe4 )85 (AgI)15

147

–

–

–

–

radiation transmission power. Glasses with low crystallize tendency are sutiable for fiber fabrication. On the other hand, glasses with increased crystallization ability can also find applications in certain areas, such as storage devices and optical switches. The crystallization tendency of glass is determined by many factors, including the difference between the crystallization temperatures T c and glass transition T g . Another well known rule is the critical cooling rate (CCR) of the glass-forming melt. The higher the CCR, the more possible of crystallization could happen during melt solidification. Table 4.1 shows the thermal characteristics and CCR values of some ChGs. The glass transition temperature of two-dimensional network glass systems (As2 S3 , As2 Se3 ) is 170–185 °C, and that of three-dimensional network (GeS2 , GeSe2 ) is above 300 °C [18]. In the following, eight kinds of ChG glass systems are considered as typical ChG fiber materials.

4.1.1 As–S As–S ChG glasses have a good glass forming stability and high stability against crystallization in a binary glass system. The transparency condition in the visible wavelength range can be solved by using sulfur as chalcogenide element instead of selenium or tellurium. In Terutoshi [12] reported that appropriate compositions of As–S glass for low-loss fibers drawing are limited to a narrow range in the glassforming region. Glasses in the arsenic-rich region have a crystallize tendency during the drawing process. In the glass with a S-rich region, sublimation of S would lead to surface irregularities, which is a problem as these non-homogeneities can increase the fiber loss by two orders of magnitude higher than that of a homogeneous fiber does. Figure 4.3 shows the range of glass composition As–S which are suitable for different fiber drawing methods (fiber-drawing region). The appropriate composition of glass for fiber drawing by the preform method is limited to the region of 58–80 at.% sulfur

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4 Chalcogenide Glass Composition, Processing and Structure Characterization

Fig. 4.3 Fiber-drawing regions of As–S system

and 20–42 at.% arsenic. For the case of fiber drawing by the crucible method, the composition is in the range of 61–80 at.% S and 20–39 at.% arsenic. The difference in the composition regions shows that the crucible drawing method requires a higher glass stability than other method, due to its suffering from longer time for higher temperature. By contrast, the glass extrusion benefits much from the low temperature for preform reforming, so it keeps near the same glass region as bulk glass.

4.1.2 As–Se As–Se glasses possess good thermal stability, higher non-linearity and can be used for manufacturing hypotoxic IR fibers. The difference value between the temperatures for crystallization and glass transition for As2 Se3 glass is 128 °C. On the basis of the experimental data [20], its CCR value is 0.009 K/s, which is in agreement with the calculated value of 0.0034 K/s [20]. Using differential thermal analysis (DTA), the critical heating rate (CHR) for As–Se glass system is determined as the minimum heating rate in which the proportion of crystalline phase in the sample is less than 5% of total sample volume. CHR of As2 Se3 composition is 0.04 K/s. For As2 Se3 glass, the experimental dependencies of nucleation and crystal growth rate were determined in Ref. [20]. The maximum value crystal growth rate is 7 × 10–5 cm/s at 344 °C. The maximum nucleation rate W max at 210 °C is about 130 cm–3 /s. The linear rate of crystal growth, the nucleation rate of the crystalline phase, and the temperature dependence of these parameters are closely related with the critical information for the ‘glass-crystal’ transition. Figure 4.4 shows the calculated dependence of the ratio of crystalline phase volume [21] in the glass matrix α(b) as a function of the potential change of the applied critical cooling rate for As2 Se3 glass. It can be seen that the change of cooling rate by one order of magnitude in the given interval can produce a same level of volume change for crystalline phase in the glass matrix.

4.1 ChG Glasses Thermal Properties

73

Fig. 4.4 Dependence of crystalline phase volume ratio in As2 Se3 glass matrix as a function of CCR

4.1.3 Te–As–Se It is well known that, Se is a very good glass former, even for itself [22]. As– Se ChG glass systems with numbers of different compositions can be produced with high stability against crystallization, and they can be shaped into sophisticated optical devices such as optical fibers, planar guides and lenses. Nevertheless, the transparency of As–Se glasses is limited to about 12 μm (depending on the thickness or length of optical systems) due to its relatively small mass of Se element. The search for glasses with longer transmission cutoff wavelength in the infrared region is motivated by the development of IR fibers with the widest optical window. Since the IR light must propagate through certain length of fibers in some typical applications, it is critical to decrease the fiber loss in the low phonon region as much as possible. The best way is to increase the average relative atomic mass of the ChG components using a Se/Te combination compatible with glass stability, or even completely replace the Se with Te. It is well-known that partly substitution of Se by Te is beneficial in getting low vibrational modes, but it is also risky because of the strong tendency of Te to produce electron delocalization and easy formation of microcrystals [23], that will increase the scattering loss violently. It is challenge to develop a glass composition that maintains the Se character of the mixed chalcogens chains. In another words, the Te atoms should be sufficiently separated to avoid electron exchange by π bonding. This has been solved in the Se/Te/As ternary compositions located at the glass forming domain as shown in Fig. 4.5. The most stable glass composition free of any crystallization is Te20 As30 Se50 . In this so-called selected TeAsSe (TAS) glass, the proportion of Se is twice that of Te which allows for Te dilution in the melt avoiding the formation of any Te–Te bond and ion clasters. In this glass system, the mixed—Se–As–Se–Te–Se– As–Te–Se–As–Se—chains are cross-linked by the As atoms and form a ramified 2D structure. The glass transition temperature T g of TAS glass is low to 137 °C, which is

74

4 Chalcogenide Glass Composition, Processing and Structure Characterization

Fig. 4.5 Te–As–Se system including the Te20 As30 Se50 composition (called TAS glass). The TAS can transmit IR light from 2 to 18 μm with no crystallization peak on DSC curve. This glass can be easily drawn into fiber that is a good candidate for remote IR spectroscopy (Adapted from Ref. [22], with permission from the MDPI)

significantly lower than traditional chalcogenide glass. As shown in Fig. 4.5, a TAS glass can transmit IR light up to the multiphonon region near 18 μm at thickness of near 2 mm. Due to its high anti-crystallization stability, TAS is also suitable for drawing optical fibers from a glass rod perform. Additionally, it is possible to taper the optical fiber to design suitable shapes for the application of optical sensors based on detecting the IR signatures of molecules with IR fibers.

4.1.3.1

Ge–As–Se/Te

As observed in many glassy systems, the strategy to develop stable glasses resistant to crystallization is to intentionally introduce several specialized potential crystalline species, with the objective that the competition between the species can delay the devitrification process. To meet this demanding goal, complex compositions are always preferred. For instance, one of the industrial glass compositions selected for molding infrared complex optics is the so-called GAS (GeAsSe) IR glass from the company UMICORE-IR. Figure 4.6 represents the glass forming area in the GAS system where the selected Ge22 As20 Se58 composition is marked as a red dot. In this Ge22 As20 Se58 , the Se network is highly reticulated by the trivalent As and tetravalent Ge leading to a glass with a high T g (292 °C). When heated above T g in the thermal regime where the materials become viscous, this glass does not show any tendency toward crystallization and is therefore suitable for molding optics under moderate pressure, although the fiber drawing demand is far from this criterion. Figure 4.7 shows some sophisticated infrared optics including aspheric diffractive lenses which can be directly inserted into a thermal imaging system such as an infrared camera based on chalcogenide S/Se/Te glasses system. The company UMICORE-IR produces such IR glass lenses for the automobile market and car manufacturers such as BMW or TOYOTA. The lenses are specially designed to equip systems for night vision driving assistance [24], life decting and other mid-infrared optical systems.

4.1 ChG Glasses Thermal Properties

75

Fig. 4.6 The glass-forming region in Ge–As–Se system. Their thermomechanical properties make them easy to be molded into sophisticated diffractive (above on the right) and aspheric (below on the right) lenses (Adapted from Ref. [22], with permission from the MDPI)

Fig. 4.7 The glass formation domain in Ge–As–Ch (S/Se/Te) system

Ge–As–Se glasses have a long cutoff wavelength larger than 10 μm. As the Gecontained glasses have relatively high softening temperatures, Suchet [9] supposed that the insertion of arsenic element is realized by the simultaneous substitution of two neighboring selenium atoms with the formation of a bridge between two double chains:

76

4 Chalcogenide Glass Composition, Processing and Structure Characterization

Fig. 4.8 Fiber-drawing region in Ge–As–Se system (Adapted from Ref. [12], with permission from the IEEE)

Ge 〈

Se–As–Se | 〉 Ge Se–As–Se

In Ge–As–Te system, there are two narrow glass formation domains (Fig. 4.7), a feature maybe related to the two ternary eutectics existence that decreases the crystallization rate of the alloys drastically. Typically, the GeTe6 alloy with a small amount of Te substituted by As is the easiest way to vitrify. Such alloys will crystallize easily by heating or intense illumination, resulting in the formation of two separated phases of tellurium or GeTe crystals. Figure 4.8 shows the compositional region where the Ge–As–Se glasses are fiber drawable from preform. The glasses with compositions outside this region exhibit a tendency toward crystallization during fiber-drawing and could not be drawn into homogeneous low-loss fiber. The appropriate glass compositions for drawing monoindext fibers are limited to the region of 0–20 at.% Ge, 0–45 at.% As, and 55–95 at.% Se, which is significantly narrower than the glass-forming region [12]. In [25] presented a glass forming region with the variation of glass transition temperatures in Ge–As–Se system. Here, the glass transition temperatures for this ternary glass system are increasing directly with the Ge content increasing. Others, a broad overview of the viscosity parameters that allow relevant comparison for the ternary systems was also reported [25]. Figure 4.9 presents a similar plot with Nemilov’s data and several other points published by different authors [26–28]. It is evident that the addition of specialized metal into the glassy selenium matrix causes greater interconnection between different structural units and the glass transition temperatures thus increase rapidly. This phenomenon is more apparent in the case of germanium addition. The small local maximum for As–Se glass system can be

4.1 ChG Glasses Thermal Properties

77

Fig. 4.9 Glass forming region of the Ge–As–S system; the full circles represent the Gex Asy Sz glasses under study (Adapted from Ref. [30], with permission from the Elsevier), the insert is images of typical Ge–As–S bulk glasses for fiber fabrication

S

20 Mol.% AS

30

As2S3

observed in the region between the AsSe and As2 Se3 stoichiometric compounds, indicating that the glass structure from this compositional region is more rigid than others. Furthermore, breaks can be observed on the isoviscosity curves in the vicinity of the straight line that connects the As2 Se3 and GeSe2 compounds and prone to more chemical stablility. The reason for these breaks is the increase of structure rigidity in the case when the matrix is composed solely of the AsSe3/2 and GeSe4/2 units. Structures without any selenium chains seem to be more stable.

4.1.4 Ge–As–S In 1982, M. Pisarcik studied the Raman spectra of Gex Asy Sz glasses in the ternary composition region of GeS2 –As2 S3 –S. Here, the structural units forming the disordered Gex Asy Sz structural network were identified as below segments: AsS4 pyramids, GeS4 tetrahedra, As–S–S–As bent chains and Sn (n = 2–4) chains. When the concentration of sulfur is high, part of S is dissolved in the glass network in S8 molecules form and increases the viscosity. The Ge–As–S system has a large glass forming region. So, these glasses can be obtained easily by air cooling. Gex Asy Sz glasses can be prepared up to the content of ~ 50 at.% of As and ~ 40 at.% of Ge, although it may prone to phase separation. The glass forming region of the Ge–As–S glasses was denoted by dash lines in Fig. 4.9. All the prepared samples were homogeneous and their color varied from deep-red to bright-red and yellow–brown depending on the differrent atom ratio of As/Ge/S. At higher sulfur concentrations, the structrual units of Gex Asy Sz glasses are containing AsS3 , GeS4 , As–S–S–As and Sn chains. When the sulfur concentration exceeds 75 at. %, part of sulfur grows in S8 molecules form dissolved in the glass network. The glass formation ability of Gex Asy Sz containing a large amount of S

78

4 Chalcogenide Glass Composition, Processing and Structure Characterization

molecules may be associated with a layer-type structure of As2 S3 network. Similar layer-type structure is also found in GeS2 -based glasses [29]. For the region of S-low concentration, the structure of Gex Asy Sz is more complicated and still a subject of study at present, especially for developing high power laser transmitting fiber based on this glass host. In Kobelke [31] reported a metal Ge improved As–S glass, and found that the effect of Ge and excess S on viscosity during the crucible drawing method could be estimated by T g and fiber elongation/tension force measurement [10] using Eqs. (4.1) and (4.2):  log ηGe = (0.41 ± 0.02) × Ge

(4.1)

 log η Sy = (0.13 ± 0.01) × Sy

(4.2)

where log ηGe is the difference of viscosity with germanium adding (log units), comparing relatively to that of As40 S60 caused log ηSy is the difference in viscosity caused by excess S (log units).

4.1.5 Ge–As–Se–Te Adding Ge atoms into As–Se–Te glass can improve its mechanical strength and expand its operating temperature range [32]. For laser power delivery, glasses with T g > 150 °C and expansion coefficient less than 17 × 10–6 K–1 are necessary. Unfortunately, the glass transition temperature of As–Se–Te is not high enough. This limits the applications of As–Se–Te fibers. The addition of Ge element to As–Se–Te glass can improve its thermal–mechanical properties by decreasing the expansion coefficient and increasing the glass transition temperature, as well as the chemical stability. Because As40 Se40 Te20 is the most stable glass in terms of crystallization in the As40 Se60-x Tex series, it was used as the starting compositions and subsequently a portion of As was substituted by Ge [33]. Glasses with a Gex As40-x Se40 Te20 (x = 0–40) composition have T g values in a range of 140–320 °C, depending on the Ge content. Whatever the heating rate is, glasses with x ≤ 35 exhibit no obvious exothermal peaks of crystallization„ indicating a high glass-forming ability. In Ge– As–Se–Te system with high Te content, some of the glasses have low tendency to crystallization [34]. Results showing that the glasses containing 16–24 at.% Ge and 42–48 at.% Te have glass transition temperatures above 200 °C and do not crystallize during annealing over 20 h at temperatures in the range 50–100 °C above T g . In Inagawa [32] investigated an IR fiber of Ge–As–Se–Te glass host, and reported their glass forming region, thermal and IR transmission properties. The glasses were developed with an expectation to deliver CO2 laser in the region of 3–13 μm. The appropriate Te content for low-loss fiber at 10.6 μm wavelength was optimized about

4.1 ChG Glasses Thermal Properties

79

Fig. 4.10 Glass forming region in Ge–As–Se–Te system (Adapted from Ref. [32], with permission from the Elsevier)

50 at.%. The glass forming region with typical 5 classes of (a), (b), (c), (d), and (e) was shown in Fig. 4.10.

4.1.6 Ge–Te–AgI While Se is helpful in improving Te-based ChG glass formation, it also introduces a large level of absorption near the wavelength of 10 μm. Therefore, research efforts were focused on decreasing Se content or completely removing Se in glasses such as Ge–Te–X (X = Cl, Br, I), but it failed due to anti-deliquescence and stability for fiber development until 2012, when Wang [35] proposed a new Te-based ChG glass with addition of halide AgI (up to 30 mol%). It is transparent up to nearly 25 μm with strong anti-hydrability, and thus becomes an optimized material for far-IR application. In this, AgI was incorporated into the glasses acting as a glass modifier. With the help of AgI, a higher glass transition temperature T g (151 z C) can be obtained. The detailed thermal properties of the glasses were analyzed by DTA. The infrared optical transmission spectra were studied by FTIR. A purifying process was adopted to eliminate the impact of impurities in the tellurium glass. This series of glasses can meet nearly all the the requirements for applications in far-IR optical imaging and sensing. Table 4.2 shows the prepared glass compositions. The glass forming region of Ge– Te–AgI system is presented in Fig. 4.11. This region is narrow but clearly different from that of Ge–Te–Ag [36] and Ge–Te–I [23] glass systems. The difference is from that the status of the raw material, the silver ions were supposed to be in the I ions tetrahedral environment, forming α-AgI-like tetrahedral clusters in the network. Based on the previous work on a similar glass system, the properties and structure

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4 Chalcogenide Glass Composition, Processing and Structure Characterization

Table 4.2 Calorimetric properties of (GeTe4 )100-x (AgI)x glasses and Te-based glasses previously published [17, 19]

Composition

Tg (°C)

Tx (°C)

T(°C)

GeTe4

140

217

77

(GeTe4 )95 (AgI)5

150

247

97

(GeTe4 )90 (AgI)10

145

/

/

(GeTe4 )85 (AgI)15

147

/

/

(GeTe4 )80 (AgI)20

141

/

/

(GeTe4 )75 (AgI)25

128

231

103

Ga10 Ge15 Te75

172

285

113

Ge20 Te73 I7

150

274

124

Ge21 Te76 Se3

160

283

123

Fig. 4.11 Glass-forming region of the Ge–Te–AgI glass system, the inset shows the appearance of the typical glass sample (Adapted from Ref. [35], with permission from the Elsevier)

of Ge–Te–AgI are related to the ratio of Ge/Te. Two glasses series with different ratio of Ge/Te have been investigated. The results presented in Fig. 4.11 show that the region lies along the Ge–Te4.3 side in the Ag-rich glass. This is rather surprising that up to 30 mol% AgI can be dissolved into Ge–Te–AgI glass system. It was found that the addition of AgI could decrease the number of Te homopolar bonds and lead to an increase in the glass stability of the Ge–Te–AgI system. Iodine atoms can form covalent bonds with Te by trapping the metallic electrons and decreasing the formation probability of Te microcrystal. Besides that, silver is essentially bonded by an I ion. Then, in 2012, Conseil [19] revisited the data on Ge–Te–AgI with higher purity raw material. The thermal analysis results are presented in Table 4.2. Some Te-based glasses did not shown any exothermic crystallization peak. For example, even the Se-contained tellurium glasses would show some crystallization peaks. Three of the studied compositions (x = 10, 15 and 20) possess such strong thermal stability. But a previous study reported opposite results: crystallization peaks were observed in

4.1 ChG Glasses Thermal Properties

81

Fig. 4.12 IR transmission window of (GeTe4 )90 (AgI)10 compared to different Te-rich based glasses reported in literature. The inset shows a (GeTe4 )90 (AgI)10 bulk glass (Adapted from Ref. [19], with permission from the Optical Society)

the similar composition Te glasses. This may be caused by the differences in the synthesis process (especially for the raw materials of less purity) [35]. It can be seen from Fig. 4.12 that (GeTe4 )90 (AgI)10 glass exhibits a large IR window from near-IR 2 μm up to far-IR 25 μm. The onset cut-off wavelength beyond 20 μm is associated with phonon vibration. This long-wavelength cut-off edge was attributed to the lightest element of Ge in the glass. Thus, as the covalent bond energy of Ge–Te (456 kJ/mol) is about twice of that of Ge–I (212 kJ/mol), we can conclud that the cut-off multiphonon is attributed to Ge–Te bonds. This conclusion is supported by the following observations. First, the cut-off edges remain unchanged with a change in AgI percentage (from 5 to 20%). So, neither I nor Ag play a key to the glass transmission. Second, as shown in Fig. 4.12, Te–Ge–Ga, Te–Ge–I and TeGeAgI glasses show the similar mid-IR cut-off wavelength, confirming that this limitation is attributed to the common elements Ge and Te in these three tellurium glass families. The Raman peak of Fig. 4.13 also associated with the low phonon energy of Ge–Te–AgI glasses. An optical fiber with a core glass composition of (GeTe4 )90 (AgI)10 has been drawn and demonstrated for CO2 gas detection. So, glasses from (GeTe4 )100-x (AgI)x system are interesting candidate for space application in the ESA’s Darwin mission [37], as the CO2 gas absorption band (~15 μm) is clearly visible in its entirety.

4.1.7 Ga Contained Chalcogenide/Chalcohalide Glasses Ga is mostly three coordinated, which is similar to that of trivalent rare earth ions. Therefore, Ga-contained chalcogenide glasses can usually act as hosts for rare earth ion doping to achieve fluorescence and laser emission [38], but the fiber with low loss is difficult to prepare using this kind of glass composition. The first example is Ga-La-S glass system. The crystallization kinetic of Ga–La–S glass has been investigated by Dianov [39]. In this system, the (Ga2 S3 )0.7 (La2 S3 )0.3 composition shows the highest glass forming ability among Ga, La and S. Figure 4.14

82

4 Chalcogenide Glass Composition, Processing and Structure Characterization

Fig. 4.13 Raman Spectra of modified GeTe4.3 glasses with different AgI content. The inset shows a peak situated at 275 cm−1 under a large abscissa scale (Adapted from Ref. [35], with permission from the Elsevier)

Fig. 4.14 Temperature dependences of nucleation rate 1 of W and rate 2 of crystal growth of V for Pr-doped glass; 3, nucleation rate for undoped glass at 870 K; 4, linear rate of crystal growth for undoped glass at 870 K

4.1 ChG Glasses Thermal Properties

83

shows the temperature dependence of the nucleation rate W and linear rate of crystal growth V for Pr-doped (Ga2 S3 )0.7 (La2 S3 )0.3 glass in comparison with undoped glass. The second is ChG glasses containing halide, i.e. chalcohalide (ChH) glasses. The glasses which combine chalcogen with halogen or metal halide offer a route to tuning of the optical properties of the glass [40]. Optical band-gap, transparent window, refractive index, etc. could be scaled down or up by appropriate halogen/metal halide doping. ChH glasses have been studied in past several decades for potential applications in the infrared [41–46], such as multispectral imaging from visible to longwave infrared [47], or large-scale rare-earth doping for fiber lasers and amplifiers [48]. One such ChH glasses is GeSe2 –Ga2 Se3 –CsI [49], which has good thermal stability and excellent transparency from the visible up to the 14 μm atmospheric window. However, this type of glass possesses poor resistance of water and chemical surroundings. In Zhao [50] reported a series of ChH glasses of 36(GeSe2 )– 24(Ga2 Se3 )–40(CsI) and 55(GeSe2 )–25(Ga2 Se3 )-20(CsI)), doped with the largest amount of CsI up to 40 at.%. Its optical absorption edge of short wavelength blue shifts speedily, and the material dispersion is scaled down to near 2 μm but high nonlinearity is still retained in the ChH glasses. These Se-based ChH glasses show a high laser damage threshold, as well as its superior environment friendliness (“As” free). The ChH glasses considered here have a material ZDW at 3.5 μm for 55(GeSe2 )– 25(Ga2 Se3 )–20(CsI) and 2.2 μm for 36(GeSe2 )–24(Ga2 Se3 )-40(CsI), smaller than As2 S3 at 4.33 μm or As2 Se3 at 7.2 μm, as shown in Table 4.3. With increasing CsI content, the optical absorption edge wavelength increases while the ZDW decreases [52]. Furthermore, ChH glass containing 40% CsI has a large optical absorption edge of 2.25 eV, which is promising for excellent transmission at shorter wavelength Table 4.3 Comparison of ChH and ChG glasses Glass

n2 (× 10–18 m2 /W)

n0 (@2 μm) ZDW Tg (°C) Laser (μm) damage threshold (kW/cm2 )

55(GeSe2 )–25(Ga2 Se3 )–20(CsI) 6.67

2.214

3.5

320

884 (single pulse) 114 (continuous)

36(GeSe2 )–24(Ga2 Se3 )–40(CsI) 6.47

2.05

2.2

290

–

As2 S3

2.3 [51]

2.432

4.3

185

822 (single pulse) 150 (continuous)

As2 Se3

10.5 [51]

2.798

7.2

178

682 (single pulse) 147 (continuous)

(n2 is nonlinear refractive index at 1.55 μm, and n0 is linear refractive index at 2 μm)

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4 Chalcogenide Glass Composition, Processing and Structure Characterization

as well as a high laser damage threshold. Besides, in Carcreff [53] reported a ChH fiber based on Ge–Ga–S glass system. This demonstrated the possibility to be used as a rare-earth doped glass host and a permanent photo-written material using a femtosecond laser.

4.2 Transparent Windows and Phonon Energy As is well known, ChG can be used as host materials for rare earth ion doping and thus achieve light emission in the mid-infrared region for active optical applications because of their low phonon energy and wide transparent window, as shown in Table 4.4. The glasses with low phonon energy are favored as a means to obtain high quantum efficiency for wavelength conversion and mid-IR laser applications. The spectral characteristics of several glasses are reported concerning S-, Se-, Te-based ChG glasses [54]. The absorption coefficient corresponding to the multi-phonon absorption of the host glass is represented by the four curves, from the top to down, values of 10, 5, 2, 1, 0.5, and 0.1 cm–1 were shown as Fig. 4.15 for increases in the low phonon Te content. With changes in the glass component, the multi-phonon energy of the glasses is changed, and the infrared cut-off edge of the transparency window is blue or red shifted, as shown in Fig. 4.16, which shows the transparent windows of typical mid-IR glass fibers. It is indisputable that ChG fibers usually have a relatively narrow transmission windows compared with their bulk counterparts, such as S-, Se-, and Te-based fibers, which can be sourced from the absorption of multi-phonon and multi-photon. These fibers can transmit light within the wavelength region of about Table 4.4 Approximate transmission windows and phonon energy of typical glasses

Glass-forming system

Transparent window (μm)

Phonon energy (cm−1 )

References

Sillicate

0.4–2.4

1100

[55]

Germanium

0.5–4

850

[56]

Tellurite glass

0.4–4

780

[57]

ZBLAN

0.25–4.5

550

[58]

InF3

0.3–5

507

[59]

As–S

1–7

340

[60]

As–Se

1.5–10

230

[61]

Ge–As–Se

0.84–16

230–300

[62]

Te–As–Se

2.0–16

238

[63, 64]

As–S–Se

0.75–12.5

230–340

[30]

Ge–Sb–S

0.64–11

314

[65, 66]

Ga–La–S

0.57–9.5

425

[67, 68]

Ge–As–Se–Te

1.7–20

200–230

[64]

Ge–Te–AgI

2.0–25

159

[35]

4.2 Transparent Windows and Phonon Energy

85

Fig. 4.15 Effect of increasing Te concentration on multi-phonon absorption edge in Ge–Se–Te glasses

Fig. 4.16 Typical mid-IR fiber loss and transparent window

0.8–7 μm, 1–10 μm and 2–12 μm, respectively, depending on their compositions [7].

4.3 Viscosity and Glass Reformation Viscosity at a given temperature is one of the most fundamental issues in glass formation and fiber drawing, perhaps even more critical than the glass transition temperature (T g ), because many critical parameters can be estimated from the viscosity

86

4 Chalcogenide Glass Composition, Processing and Structure Characterization

curve of the glass. The viscoelastic behavior is critical in all temperature-related process, as it dictates the time and temperature scale needed to produce optical elements precisely, from slow, low-temperature extrusion to rapid, high-temperature fiber drawing. Aside from its technological importance, the viscosity–temperature relationship has shown significant value, for the aim of building the connections between the atomic structure of amorphous solids and the glass properties of physics, chemics, mechanics and optics.

4.3.1 Fundamental Theory Resonable knowledge of the temperature dependent viscosity for various chalcogenide glasses is essential for the development of all the viscosity related glass heat treatment technology. Shear viscosity is a basic physical discription for a given material, it is dependent on temperature, pressure as well as shear stress. Most works dealing with viscosity of ChG focus on the temperature dependence of this property because of temperature affecting viscosity significantly. General viscosity theories are summarized in the text below, which can be used for ChG. Other summaries and important observations can be found in the good works of Ojovan [69] or Mauro et al. [70]. The temperature dependence of viscosity for typical non-crystalline materials is usually plotted as the dependence of the log of the viscosity on the reciprocal temperature (commonly 1000/T [K − 1]), just as shown in Fig. 4.17. Such dependences in

Fig. 4.17 Viscosity comparison of typical oxide glasses and ChG glasses

4.3 Viscosity and Glass Reformation

87

the measurable regions of undercooled melt and glass can be described by a simple Arrhenius-type equation: log η = log η0 +

Eη ln 10·RT

#

(4.3)

Here, η is the viscosity, T is the thermodynamic temperature, η0 is a preexponential factor, R is the universal gas constant, and E η is the apparent activation energy of viscous flow. The above equation is empirical, and the parameter logη0 has no clear physical meaning. The Arrhenius-type equation is not only suitable for the description of viscosity data in an undercooled region but can also be used for the data description in the region of melt (alone). On the other hand, if the viscosity data needs to be defined over a broad temperature interval (including the melt, undercooled melt and glass regions), the Arrhenius-type equation is not suitable for the description of a viscosity curve. It is apparent from Fig. 4.17 that two empirical parameters of the Arrhenius equation are not enough for the description of whole viscosity curve, and it is necessary to add other parameters. Three-parameter viscosity equations are often used for the description of viscosity data in a broad temperature interval. In this regard, the most well-known and widespread equation in the fields of non-crystalline science and glass industry is the Vogel–Fulcher–Tammann (VFT) equation: log η = A +

B # T −T0

(4.4)

where A (logη0 ), B, and T 0 are temperature independent constants for a given system. The frequently quoted work [71] by Vogel was the first publication that introduced this equation but in a form of somewhat different. With only formal differences (constant A has a negative sign and constant B is divided by 103 ), the classic form of the VFT equation was usually used for the viscosity behaviors description of oxide glasses by Fulcher [72] and of organic melts by Tammann and Hesse [73, 74] (thus Eq. (4.4) is sometimes called Vogel–Fulcher–Tammann–Hesse equation). Several attempts to create a comprehensive viscosity theory were published over the last few decades, suuch as the theories introduced by Eyring [75] and Weymann [76], free volume theories introduced by Doolittle [77], Cohen and Turnbull [78], or theories combining some of the previously reports (such as the Macedo and Litovitz theory [79] being only partially successful). These theories are often formally similar to either the Arrhenius-type or the VFT equations, and their typical application scenarios are limited to similar groups of materials as those can be readily described by the Arrhenius and VFT equations. But the intrisic physical meaning of these parameters in these equations is still not clear, and the applicability of these equations is not general. Another well-known viscosity theory was reported by Avramov and Milchev [80]. The model basis here is the assumption that activation energies for molecular transport are statistically randomly distributed. Along with corresponding measurement techniques, Koštál [6] extracted datasets for 36 pure ChGs, 262 binary, 265 ternary and 33 multi-component ChGs systems. The result show that none of the three tested equations can describe the experimental

88

4 Chalcogenide Glass Composition, Processing and Structure Characterization

data completely, and prove that the behavior of viscosity is complex. However, the compositional dependences show a certain variability, which is again related to the complexity of the viscosity behavior and complicates any related predictions. All the observed behaviors originate from the simple fact that the theoretical principles of viscous flow VS temperature are still not fully discovered. The correlation is still lacked between heat capacity and viscosity, with classifications of fragile and strong liquids in the over constrained region. Based on Adam-Gibbs equation and mode decoupling method, U. Senapati [81] put forward an explanation. They measured the viscosity data between 109 –106 Pas as a function of the average coordination number (r) in Ge–Sb–Se and Ge–Se glasses, and adopted the viscosity data and previously published heat capacity data to verify the ‘fragile’ and ‘strong’ liquids. Viscosity value was calculated from the deformation rate of dh/dt by the equation of Gent [82]: η=

2π Mgh 5 3V (dh/dt)(2π h 3 +V )

(4.5)

where η is viscosity, M is the placed mass (500 g) on load pan, g is the acceleration of gravity, h is the sample thickness at time t, and V is the sample volume. The following modified Gibbs-DiMarzio equation of (4.6) shows a good leastsquare fit to the experimental data T g and the T 0 in Se-rich region but fails in the Se-poor region. The applicability of this Gibbs-DiMarzio equation in describing T g has already been discussed [83], as follows: Tg =

T0 [1−β(r −2)]

(4.6)

Here, β varies between 0.55 and 0.75 depends on the glass system in use, < r > = 2.4 displays optimized bonding. It has an implication on the Angell’s concept of fragile and strong liquids. It indicates that both the glass transition temperatures T g and the softening temperatures T 0 can be determined as a function of ≺r using the above modified Gibbs-DiMarzio equation.

4.3.2 Techniques In addition to the important role of viscosity in heat-treatment and glass shaping applications that will be discussed in the next Section, a scientific report of the viscoelastic behavior of glasses provides a powerful insight into the atomic-level structure of amorphous networks. For example, the dimensionality, or connectedness, of an amorphous network has a direct impact on the viscous flow behavior usually exhibited by bulk glass. A network composed of intertwined chains of atoms will flow differently compared with a network composed of sheets or clusters of atoms. Understanding how the viscosity behavior of a glass family evolves with variations in composition can shed light on the underlying network structures and dynamically

4.3 Viscosity and Glass Reformation

89

Fig. 4.18 An overview of the viscosity regions for a ChG glass, showing the ranges for measurements (left) and hot-forming applications (right)

display that of giving rise to the behavior. The viscosity of the supercooled liquids in a typical ChG glass is shown in Fig. 4.18. Figure 4.18 shows the typical temperature dependence of viscosity for the model of ChG glass-forming system [84]. The viscosity curve is plotted in a form of the dependence of the log of the viscosity value on the reciprocal temperature, which is usually used for non-crystalline materials. Two important temperature lines divide this curve into three regions. The first important point is the melting temperature T m . The second is the glass transition temperature T g determined from the viscosity data. This point was established by definition as the temperature where the viscosity is 1012 or (formerly) 1012.3 Pa s. Nowadays, the value 1012 Pa s is popularly used, and the temperature corresponding to this viscosity value is denoted as the viscosity glass transition temperature T g . Below this temperature, it can be assumed that the viscous liquid is frozen into a macroscopic solid material, referred to as a glass. Also, the ChG glass-forming systems can be also classified with strong and fragile liquids. The correlation between the two different classifications is based on the data for viscosity and heat capacity [81], since, there might be an obvious devitrification region between the temperatures for fiber drawing and glass extrusion, as shown Fig. 4.17. Various viscometrical techniques are used to collect the thermal data in practice. The viscosity of ChG glasses usually changes over 17 orders of magnitude at a range from the room temperature to a temperature of 500 K, meaning that the viscosity ranges is appropriate for various hot-forming methods, as that may only be accessible in small (< 50 K) ranging of temperature windows.

90

4 Chalcogenide Glass Composition, Processing and Structure Characterization

Figures 4.19 and 4.20 shows some typical methods for the viscosity measurements of ChG materials. Firstly, the regions of applicability and frequency of adopted for the measuring techniques are shown in Fig. 4.19, they are the Falling Ball [85], Free fiber elongation or Rod elongation—RE, Penetration method—P [86] and Torsion

Fig. 4.19 Typical measuring techniques of a Falling ball, b free fiber elongation or rod elongation— RE, c penetration—P, d torsion oscillating cup—TOC

Fig. 4.20 Overview of the experimental methods for the determination of chalcogenide materials’ viscosities (Adapted from Ref. [6], with permission from the Maney Publishing)

4.3 Viscosity and Glass Reformation

91

oscillating cup method—TOC [87, 88]. All the experimental methods used for the determination of chalcogenide materials’ viscosities are summarized and shown in Fig. 4.20. The approximate regions of applicability and the frequency of use for chalcogenide glass systems (in %) are depicted for the most common methods. The group ‘Others’ includes the so-called methods of magnetic bearing torsional creep, Eisenberg–Tobolsky, low-temperature torsion and falling sphere. The TOC method is an abbreviation for a standard torsional oscillating cup method [6]. Each method was counted once for single composition and single literature source, regardless of the viscosity region in which the composition was measured detailedly. It is apparent that the penetration and TOC methods are most frequently used and approved. The penetration method, which is suitable for the region of undercooled melt and glass, was widely used by Nemilov [89] and his co-workers and followers. The dominant part of ChG melt viscosities can also be measured by this method. Assuming that fiber fabrication is the objective, the focus is commonly on the fiber elongation method. The viscometrical technique can be used to measure the intermediate viscosities between those probes by the beam bending and parallel plate methods described above. For drawing optical fibers, a viscosity of 104 Pa s is usually kept. As the glass composition varies, the temperature required to reach this viscosity is decided by the molecule activation energy. In particular, the fiber elongation technique can be used to precisely calculate the Littleton softening temperature of the glass, where the viscosity is defined as h = 106.6 Pa s. Knowledge of the Littleton softening temperature is critical in the analysis of fiber drawing parameters for chalcogenide glasses, as it describes the temperature at which a glass fiber will deform under its own weight drawing. This viscosity–temperature value is measured by recording the deflection rate of the fiber sample contained in a furnace. The viscosity can be calculated from this deflection rate with this equation: 

η=

 2 L p+ p2 ρg− γrp 3 ddtp

(4.7)

where η is the viscosity of the glass in unit of Pa s, L is the length of fiber outside of the furnace, p is the length of fiber inside the furnace, ρ is the glass density, g is the acceleration due to gravity, γ is the surface tension of the glass, and dp/dt is the elongation rate of the fiber.

4.3.3 Applications Figure 4.18 shows the viscosity ranges for several hot-forming methods commonly adopted to shape ChG glasses: extrusion, precision glass molding, and fiber drawing from bulk chalcogenide glass preforms. Extrusion is the most versatile of these techniques in terms of total possible viscosity range covered by simple or complicated technique.

92

4 Chalcogenide Glass Composition, Processing and Structure Characterization

However, in order to achieve optimal device performance, different applications (i.e., different geometries) require different extrusion viscosities; extrusion can be used to form glass or preform shpes with a viscosity similar to that of a soft solid being forced through a die (109 Pa s), or like that of a viscous liquid being forced into a channel (102 Pa s). Precise molding covers the next widest viscosity range of the three techniques, and is a process for precisive molding optical lenses with chalcogenide glass in which the glass has the viscosity of a slightly softened solid (107 –109 Pa s). Drawing optical fiber from ChG requires a viscosity below that of the Littleton softening point (106.6 Pa s) where a glass fiber would deform under its own weight, and is typically done in the viscosity range around 105 Pa s. Each technique is used to obtain precise geometries, dimensions, and surface characteristics by taking advantage of the wide range of viscosities ChG glasses and exhibit comparatively short temperature ranges comparing to oxides glasses. a.

Extrusion

Extrusion is a widely spread manufacturing process for fabricating glass and polymer preforms that can be used to make optical fibers. The extrusion of chalcogenide glasses has no significant difference from that of polymers in many ways: a viscoelastic material is forced into a channel or through a die to form a desired geometry under an appropriate temperature, then the material is cooled down below its glass transition to ‘lock in’ this geometry in a solid structure. Depending on the desired viscosity (extrusion can be applied to form glasses with viscosities from 102 to 109 Pa s), there may or may not be danger of volatilization of the ChG during processing of relative low temperature; in the high viscosity region, glass can be extruded only up to 25–50 K above their glass transition temperature. Nowadays, the extrusion method is widely adopted to produce preform pieces for specialized or micro-structured optical fibers, as complex geometries cannot be accessible through drilling, stacking, or other structure-forming methods [90, 91]. In addition to the production of more complex geometries, extrusion can also be used to produce pieces or preforms from complicated material combinations such as ChG glasses paired with oxide glasses, polymers, or metals, as well as fine surface and interface. An example of such a combination is shown in the next chapter [92]. b.

Fiber drawing

To date, the determination of the working regions (for both precisive molding and fiber drawing) for a new ChG glass type has been done using empirical rather than deterministic methods. Recently, researchers have made large advances in the effective prediction of these thermal regimes using numerical modeling methods [93]. Drawing down a ChG preform to form an IR optical fiber requires raising the preform’s temperature to as much as 100 K or more above its glass transition temperature in order to make glass flow sufficiently easily (h = 105 Pa s), allowing it to be pulled thin to a diameter up to 1000 times smaller than that of starting bulk. For the volatilization of As, S and Se, including As–S, As–Se and Ge–As–Se, the volatilization temperature can be approximately 100 K higher than T g , so care must

4.3 Viscosity and Glass Reformation

93

be taken to prevent out-gassing in the glass surface, which would be detrimental to the composition, and also harmful to humans. The viscosity curves of ChG glasses are much steeper than those of oxide glasses, the temperature window at which the glass with a viscosity of 105 Pa s is comparatively smaller, thus precise temperature controlling and narrow heating region are critical to ChG fiber drawing. The crystallization stability window, T, discussed in Sect. 4.1, of a given ChG glass can also limit the available composition options; a typical Ge20 Te80 composition may be quenched into an amorphous bulk [94], but when it is heated to the draw temperature, it will crystallize completely. Despite this limitation, ChGs have achieved great success in fiber optics applications. Photonic crystal fibers have been demonstrated in ChG glasses [95], which show potentials for novel optical properties such as dispersion tailoring [96], supercontinuum generation [11], bio-sensing and single-mode infrared guidance [97]. Beside viscosity control, there are also other reports on the special observations and controls of viscosity. In Brazhkin [98] presented an in-situ high-temperature VS high-pressure study of liquid AsS by x-ray diffraction, resistivity measurements, and quenching speed. The data provide direct evidence for the existences of two transformation in the melt: one is from a moderate viscosity molecular liquid to a high-viscosity nonmetallic polymerized liquid at pressure of 1.6–2.2 GPa; the other is from the latter to a low-viscosity metallic liquid at pressure of 4.6–4.8 GPa. By rapid cooling, molecular and metallic liquids may crystallize to normal and highpressure phases, respectively, while a polymerized liquid is easily quenched to form a new kind of AsS glass. In Fig. 4.21, the open circles correspond to the experimental results of AsS crystal melting temperature points, while the red line and dash lines are approximations of the melting temperature line and boundaries between liquid states, respectively. Green triangles correspond to phase transitions between these crystalline phases, determined at near isobaric heating. Blue triangles corresponds to the metallization of liquid AsS. Dashed lines are the approximation of experimental transition temperature lines (kinetically dependent) from AsS type I (right line) and AsS type II (left line) crystalline phases, while dashed-dotted line is an approximation line of I-II equilibrium. Thus, the AsS melt under pressure shows typical three liquid states with different structures and properties: a molecularbased liquid at a pressure of < 1.6 GPa, a polymerized viscous liquid at pressures in the range 2.2 to 4.5 GPa, and metallic liquid at a pressure of > 4.8 GPa. The existence of several phase transformations is observed in simple isotropic liquids and the corresponding presence of more than two critical points are retained in a substance; all the results remain a challenge for various theoretical models of simple melts. In addition, the compositional dependences show a certain variability of viscocity, which is connected with the complexity of the viscosity behavior and complicates with some related predictions. All the observed behaviors originate from the simple fact that the current theoretical principles of viscous flow are still under-developed as a means to fully describe the observed experimental results.

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4 Chalcogenide Glass Composition, Processing and Structure Characterization

Fig. 4.21 Pressure–temperature phase diagram of AsS in crystalline and liquid states (Adapted from Ref. [98], with permission from the American Physical Society)

4.4 Conclusion In this chapter, typical ChG glass systems including S-/Se-/Te-based ChG glasses and its typical properties of fiber drawing are presented and analyzed. The glass forming region as well as fiber drawing region are presented and discussed detailedly. In particular the viscosities of the ChG glasses along the temperature and pressure are discussed in detail. To date, it remains a challenge to fabricate a low-loss ChG fiber due to its micro-crystal, impurities and other defects in the glasses. However, it can be improved by glass purification process and preform preparation technology, which will be discussed in the chapters which follow.

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Chapter 5

Chalcogenide Glass Preparation, Purification and Fiber Fabrication Xiange Wang, Kai Jiao, Gerald Farrell, and Xunsi Wang

Chalcogenide glasses are non-oxygen glasses that are usually prepared under vacuum. Since the glasses are mostly used as optical devices working in mid and farinfrared region, where impurities such as oxygen, carbon, transition metal dust and others could seriously compromise the performance of the glasses, it is essential to purify the raw material before glass preparation, as well as remove the microdefects induced during fiber fabrication. In this chapter, we will review the major factors that can affect the optical properties of chalcogenide glass fiber with a coverage that ranges from fundamental theory to glass preparation methods and on to fiber fabrication technologies.

5.1 Loss Mechanisms in Fiber Optics The optical transparency of a medium is quantitatively characterized as attenuation of the light flux passing through the sample and is described by the well-known Buger-Lambert–Beer equation [1]: J = Jo exp(−βl)

(5.1)

where Jo and J are the light fluxes at the sample input and output; l is the optical path length, β is the absorption coefficient (attenuation or optical losses) with a reciprocal length dimension. The transmission loss in an optical fiber consists of intrinsic and extrinsic losses. Each of these can be divided into absorption loss and scattering loss. For intrinsic losses, there are four types: electronic absorption, Rayleigh scattering, multi-phonon absorption, and free carrier absorption. For extrinsic losses, the main sources of loss are due to impurity absorption and scattering loss due to structural imperfection such as pores and scattering centers, macro-and micro-bending of the fiber, and © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Wang et al., Mid-Infrared Fluoride and Chalcogenide Glasses and Fibers, Progress in Optical Science and Photonics 18, https://doi.org/10.1007/978-981-16-7941-4_5

99

100

5 Chalcogenide Glass Preparation, Purification …

imperfections at the boundary between the core and cladding [2]. The total loss is the sum of the intrinsic loss and the extrinsic loss and thus is equal to the absorption loss plus the scattering loss:  f G H4 H2 a a b β = Ae λ + D0 e λ B T + B0 e λ + 4 + F0 e λ + εi xi + + +J λ  λ λ    Intrinsic Loss

(5.2)

Extrinsic Loss

where λ is the wavelength; T is the temperature; x i is the content of the i-th impurity; εi is the extinction coefficient of the i-th impurity; A0 , B0 , G, F 0 , D0 , B, H, J, a, b, f , d, are the constants. The components of the right-hand part of equation are characterized in turn by the electron absorption and the flow loss due to the so-called ‘tail of weak absorption’, multi-phonon absorption; the Rayleigh scattering on density fluctuations; the absorption on free charge carriers. The last four components represent the extrinsic losses due to impurity absorption and scattering due to impurities and structure defects. Optical attenuation in a dielectric or semiconductor material is usually given as absorbance (cm−1 ) whereas fiber attenuation is conventionally expressed as loss (dB/m or dB/km). The conversion is 2.3 × 10–3 cm−1 per dB/m. The loss (A) of a fiber is expressed in dB as α=

  Po 10 log10 L Pi

(5.3)

where L is the fiber length (m), and Po , and Pi are the output and input power, respectively.

5.1.1 Intrinsic Losses 5.1.1.1

Electronic Absorption

The short wavelength cut-off (λc ) point in the transmittance of solids is due to electronic absorption. This excitation occurs when bound valence electrons absorb sufficient photon energy to jump across the optical gap into the conduction band. The short wavelength cut-off determined by the optical gap energy E o [2] is λc =

hc Eo

(5.4)

where h is Planck’s constant and c is the velocity of light. In the short wavelength cutoff region (λ < λc), the electronic absorption (Aelectronic ) is exponentially dependent on the wavelength [2],

5.1 Loss Mechanisms in Fiber Optics

101 a

αelectr onic = Ae λ

(5.5)

where A and a are material-and temperature-dependent constants. In crystalline materials such as Cu, Au, and Fe metals, at an atomic-scale the structure possesses periodic arrays of atoms with a long-range order repeating in three dimensions of space. In non-crystalline materials, long range order is absent. The structure is viewed as three dimensional networks or arrays, lacking symmetry and periodicity, in which no unit of the structure is repeated at the regular intervals. At a microscopic level, the structure of the disordered solids consists of tetrahedral and pyramidal units, connects together to form a random network in which no unit of the structure is repeated at regular intervals in three dimensions. Even with such local atomic arrangements, the structure has short range order. In disordered solids, the electronic edge is not perfectly sharp. It possesses a weak tail often known as the Urbach edge [3]. Since disordered solids possess a distribution of short-range order networks, each network has its own electronic energy levels. An effective optical gap is determined by the overall energy levels. However, absorption can occur below the optical gap due to the existence of other configurations with lower energy levels [4]. Thus, the Urbach edge is caused by the structural disorder induced broadening of the electronic edge. In covalent materials, the structural disorderinduced defect is known as dangling bonds. The Urbach absorption also follows the simple exponential form [3] of b

αUr bach = Be λ

(5.6)

where B and b are material-and temperature-dependent constants.

5.1.1.2

Rayleigh Scattering

Rayleigh scattering occurs due to compositional and density fluctuations in the material on a microscopic level. These lead to localized changes in the dielectric constant. This fluctuation in the dielectric constant and the resultant variation in the refractive index yields a λ−4 wavelength dependence for Rayleigh scattering. The scattering absorption coefficient (ARS ) is derived theoretically [5] as αRS =

83 8 2 CRS n p βT k B Tg = 4 4 3λ λ

(5.7)

where n is the index of refraction, p is the photoelastic Pockels constant βT is the isothermal compressibility, k B is the Boltzmann constant, Tg is the glass transition temperature, and C RS is the Rayleigh scattering coefficient. Because of the wavelength dependence, the loss due to Rayleigh scattering decreases rapidly with increasing wavelength. In lower refractive index glasses and multicomponent glasses with decreased glass transition temperatures, Rayleigh scattering losses are also reduced.

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5 Chalcogenide Glass Preparation, Purification …

Specifically, to determine the acceptable size of nanoparticles resulting in Rayleigh scattering for a given application, one can use the following formula to estimate Rayleigh loss [6, 7]:  α Rayleigh

dB m

 = 4.34 × C Rayleigh × N × 

(5.8)

where N is the particle density (m−3 ), Γ is the overlapping factor between the guided mode and the region of the fiber containing the nanoparticles and C Rayleigh (m2 ) is the Rayleigh scattering coefficient such as [6, 7]: C Rayleigh =

d6 (2π )5 × 4 × n 4m × 48 λ



n 2n − n 2m n 2n + 2n 2m

2 (5.9)

where d is the diameter of the nanoparticle, nn and nm are the refractive indices of the matrix and the nanoparticle at the wavelength under consideration, respectively.

5.1.1.3

Multi-phonon Absorption

The long wavelength cut-off is due to multi-phonon absorptions resulting from infrared-active vibrational modes of the atoms in the solid. For a simple free linear diatomic molecule with atomic masses m1 and m2 , the fundamental absorption frequency due to vibration is 1 vo = 2π



k M

 21 (5.10)

where k is the force constant (related to the bond strength), and M is the reduced mass (1/M = 1/m1 + 1/m2 ). In solids, multi-phonon absorption of light is directly related to nonlinearities in the electric dipole moment [8], and indirectly related to anharmonic interactions between IR-active phonons and photons [9]. In IR-active solids, anharmonic absorption occurs as a result of photon excitation of many phonons according to the Planck distribution function < n > = (ehw/kT − 1)−1 . The wavelengthdependent multi-phonon absorption (Amp ) is defined as αmp = De− λ d

(5.11)

where D and d are material dependent constants and λ is the phonon excitation wavelength in the infrared, respectively.

5.1 Loss Mechanisms in Fiber Optics

5.1.1.4

103

Free Carrier Absorption

Free carrier absorption is an indirect process which involves both a photon and phonon. This occurs when electrons in the lower conduction band that absorb the incident electromagnetic radiation are excited to the higher lying conduction bands (intraband absorption) or states in different bands (intraband absorption). As temperature increases, these electrons are scattered inelastically by lattice vibrations, transferring energy to the lattice of the material. The intraband absorption, Afc , follows the empirical relationship [10] α f c = Gλm

(5.12)

where G is the constant for optical phonon scattering and m is an empirical constant with 1.5 ≤ m ≤ 3.5. Materials with a higher carrier mobility have a larger m value. The absorption depends upon the free carrier concentration which obeys Fermi statistics. Of all the intrinsic losses, at room temperature free carrier absorption loss is small compared to electronic and multi-phonon absorption, and Rayleigh scattering losses. Therefore, it is reasonable to neglect free carrier absorption and assume intrinsic losses depend only on the electronic absorption, Rayleigh scattering loss, and multi-phonon absorption. The theoretical minimum attenuation of an optical fiber is determined by the intersection of these intrinsic losses. Calculated V-shape loss curves comparing the ultimate losses of fluoride, oxide, and chalcogenide glasses are illustrated in Fig. 5.1a, and the theoretical loss of fluoride and chalcogenide are even lower than oxide [11]. For example, the minimum intrinsic optical losses in As2 S3 and As2 Se3 glasses are theoretically estimated to be ~ 0.08 dB/km [12–14]. However, because of the presence of higher Rayleigh scattering, electronic absorption and multi-phonon absorption [11], shown as Fig. 5.1b, the practical loss can reach several dB/km.

Fig. 5.1 a Projected minimum loss of IR glasses; b Intrinsic and extrinsic losses in chalcogenide fibers

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5 Chalcogenide Glass Preparation, Purification …

5.1.2 Extrinsic Losses 5.1.2.1

Impurity Absorption

Foreign compositions such as transition metal ions, rare earth ions, and hydroxyl radical ions (OH− ) are the main cause of impurity absorption. Each impurity absorbs at a specific wavelength corresponding to the electronic and vibrational resonances associated with the impurity, as listed in Table 5.1. In IR-transmitting materials (As– S–Se and Ge–As–Se–Te glass systems), typical impurity absorption peaks include H–S, H–Se, H–Ge, OH, C–O, C, and H2 O. These impurity-related vibrations of H– S, H–Se, H–Ge, OH, C–O, C, and H2 O are responsible for selective absorptions. The selective absorptions can create energy levels within the optical gap so that Table 5.1 Impurity molecule or functional group position of absorption bands [15] Glass System

Wave Number (cm−1 )

As–S

1825; 925

5.48; 10.8

AsO–H

6950; 5210; 4370; 3440

1.44; 1.92; 2.29; 2.91

SO–H

Wavelength (μm)

Bond

3610; 1580

2.77; 6.32

H2 O

4880; 3940; 3215; 2710; 2480

2.05; 2.54;3.11; 3.69; 4.03;

S–H

3570

2.80

SO–H

2500

4.00

S–H

2030

4.92

Ge–H

3420

2.92

SeO–H

3600; 3520

2.78; 2.84

OH

1585

6.30

H2 O

1992

5.02

As–H

2830; 2430; 2190

3.53; 4.12; 4.57

Se–H

904; 936

10.68; 11.06

Se–O

Ge–Se

1280; 800; 500

7.8; 12.5; 20.0

Ge–O

Ge–As–Se

3420

2.92

OH

2190

4.57

Se–H

Ge–S

As–Se

1270

7.90

Oxide

Ge–Se–Te

765; 1230

8.13; 13.07

Ge–O

Others

1204

8.3

P–O

2298

4.35

P–H

2309; 2320; 667

4.33; 4.31; 15.0

CO2

2020

4.95

COS

1282

7.8

CSe2

2150; 1497

4.65; 6.68

CS2

1333;1265;1123;1020;943;787;649

7.5;7.9;8.9;9.5;10.6;12.7;15.4;

As–O

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lower energy absorption can take place for excitation from the imperfection level to the conduction band. The absorption coefficient due to vibrational impurities is wavelength dependent and can be expressed as [2] αimpurit y = H (λi )

(5.13)

where i corresponds to the individual impurity-related bond vibration. The presence of trace levels (parts per billion) of metals such as copper, iron, vanadium, chromium, nickel, and manganese in the chalcogenide and chalcohalide precursor chemicals cause electronic absorption in or near the visible part of the spectrum. Due to their low-lying electronic configurations, these impurity atoms can create electrically active energy states in the optical gap. These impurity-related energy levels cause a weak absorption tail. The weak absorption tail follows the empirical relation f

αT ail = Fe λ

(5.14)

where F and f are material depended constants.

5.1.2.2

Extrinsic Scattering

Extrinsic scattering arises from impurities, in-homogeneities or imperfections in the material. The wavelength dependence of scattering is dictated by the size of the imperfection. When the size of the imperfections is greater than or equal to the wavelength of the scattered light, wavelength independent (λ0 ) and Mie (λ−2 ) scattering occurs and can contribute significantly to absorption in optical material. The absorption coefficient due to imperfection scattering is given by [2, 16]. αimper f ection =

E λ0−2

(5.15)

where E is a material dependent constant.

5.1.2.3

Bending Losses

Bending losses occur when the fiber is bent. There are two types of bending: macrobending and micro-bending. Macro-bending occurs when the fiber is bent as a result of particular operational configuration of the cable containing the fiber. Macrobending can extend through a large curvature or even a full loop. For macro-bending, the power transmission of a fiber with a bend of radius R (R  2a) is approximated by Gloge [16] as

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5 Chalcogenide Glass Preparation, Purification …

Po =1− Pi



α+2 2α



  23

3λ 2a + R 4π n 2 R

(5.16)

where Po and Pi are the power output and input in mW, respectively, a is the fiber radius, Δ is the index difference, A is the profile grading factor, n2 is the refractive index of the cladding, and λ is the optical wavelength. Equation 5.16 indicates that as the wavelength and the bend radius R increases, macro-bending loss increases. Micro-bending is caused by fiber twisting during the cabling process, via compressive stresses exerted on the fiber during the fiberization process, or via stains induced by a plastic jacket surrounding the fiber. Micro-bending loss is a form of radiation loss arising from mode coupling generated by random bends of the optical fiber [17]. The micro-bending loss for a step-index fiber is approximated by Gambling [17] as 6 V4  1 0.65 + 1.62V −1.5 + 2.88V −6 αmicr o =  2 2 Ro 32

(5.17)

a

where Ro is the curvature of bending, a is the core radius, and the parameters Δ.

5.1.2.4

Geometrical Effects

Irregularities in the overall fiber size as well as fluctuations in the core diameter contribute to the total loss of the fiber. Non-concentricity of the core relative to the cladding also gives rise to additional loss. In addition, imperfections such as pores at the boundary between the core and cladding cause the loss to increase. Furthermore, the cladding should have the right thickness to prevent scattering of light into the cladding. In chalcogenide glasses, the main limiting impurities are oxygen, hydrogen, carbon, coming from the starting materials or from silica glass reactor walls, as well as heterophase inclusions (dispersed carbon and SiO2 ) [18]. The concentration and physicochemical nature of the limiting impurities depend on the macro-composition of the glass, the degree of purity of the starting materials and the silica ampoules, the synthesis temperature, and other factors [19]. Chemical reactions of impurities with each other and with glass macro-components cause a change in the chemical form of impurities. The most effective way to reduce the negative consequences associated with an uncontrolled change in the chemical form of impurities is the synthesis of glass forming compounds from high purity starting materials or the deep purification of glass-forming compounds. The high adhesion of chalcogenide glasses to silica glass can lead to the contamination of surface layers of chalcogenide glass samples by nano-sized and micro-sized silica particles [20, 21]. During the subsequent operation of the glass, for example, for fiber drawing, the particles contaminate the melt causing the additional optical losses and a decrease in mechanical strength. To achieve an optical loss in chalcogenide fibers that is at the level of intrinsic losses, the content

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of the main limiting gas-forming impurities should not exceed 0.1–10 ppb, and the content of impurity micro-inclusions should not exceed 103 cm−3 [14, 15, 18, 22].

5.1.3 Distillation Purification with Subsequent Vacuum Distillation To remove some impurities (carbon, oxygen, hydrogen), the preliminary special chemical treatment of arsenic and germanium chalcogenides can be used. For example, to remove the oxygen-containing impurities it is possible to use aluminum or magnesium [23, 24], while for removing the hydrogen-containing ones it is possible to use chlorine and chlorides of glass-forming elements [25, 26]. In melting process with micro amounts of Al or Mg, the oxygen is converted to low-volatile oxides Al2 O3 or MgO. In the case of micro amounts of AlCl3 , SeCl4 or TeCl4 , the hydrogen is converted to high volatile HCl. After melting at a temperature of 750–850 °C, the glass melt is vacuum distilled [14]. In 2002, Nguyen et al. [27, 28] reported a purification process based on the addition of 0.1 wt% tellurium tetrachloride (TeCl4 ) to the glass. During melting, the chlorine from TeCl4 reacts with the hydrogen impurities to produce volatile products (e.g., HCl) that can be removed by subsequent dynamic distillation. The processing conditions have been modified accordingly to give very low H–Se impurity content. Consequently, the loss associated with the H–Se absorption band centered at 4.57 μm has been reduced from tens of dB/m to 0.2 dB/m. The preparation of the glass samples with the additions of TeCl4 based on this technique was described as a four-step process. The first two steps are exactly the same as that of the traditional melt quenching method. The third step involves the addition of 0.1 wt% of TeCl4 to the glass cullet. The samples were heated at 750 °C for 24 h to allow melting and reaction to occur between the TeCl4 and hydrogen impurities. Next, the ampules containing the melts were rapidly quenched in water from 650 °C to preserve the integrity of the byproducts of the reaction with TeCl4 . In step three, the glass cullet containing TeCl4 was subsequently distilled under dynamic vacuum at 550 °C for 15 h to leave behind particulate matter impurities and eliminate any gaseous byproducts from the reaction with TeCl4 . Finally, in step four, the distillate was melted at 750 °C for 24 h followed by quenching from 400 °C by immersion of the ampules in water for about 2 s and then annealed at 180 °C. In 2004, Shiryaev et al. [28] prepared unclad optical fibers based on high-purity Te–As–Se glasses using both chemical and physical methods of purification. The minimum optical fiber losses were 0.07 dB/m at 7.3 μm for Te25 As40 Se35 glass fiber and 0.04 dB/m at 6.7 μm for Te20 As30 Se50 glass fiber, as shown in Fig. 5.2b. The arsenic, selenium and tellurium components were purified by double vacuum distillation with low rate of evaporation. Additionally, 700 ppm wt aluminum was used as an oxygen getter. The appropriate content of initial high purity substances and aluminum were loaded into synthesis reactor and evacuated in an oil-free high

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5 Chalcogenide Glass Preparation, Purification …

Fig. 5.2 a Set-up for preparing high purity Te–As–Se glasses: 1-ampoule with Te–As–Se glass melt after interact with Al getter; 2-intermediate ampoule; 3-synthesis reactor; 4-trap for light volatile impurities; 5-glass partition; 6-magnetic hammer-breaker; 7-glass tube; 8-wire heater; 9-furnace; b Spectrum of the total optical loss of Te25 As40 Se35 glass and fiber (Vs = 240 cm−1 (As–Se); Vb = 340 cm−1 (As–Se–As)). Adapted from Ref. [28], with permission from the Elsevier

vacuum (10–7 Torr). Then, the sealed ampoule was placed into a muffle rocking furnace and was heated at 850 °C for 7 h. After being cooled to 400 °C, this ampoule was sealed to a silica system (Fig. 5.2a) for double distillation of chalcogenide glass. After double distillation with low rate, the melt was homogenized at 700 °C for 7 h (Fig. 5.3). In 2007, Churbanov et al. [29] prepared melting extra-pure-grade elements with the addition of 0.05 wt % Al and 0.07 wt % TeCl4, followed by double distillation and glass homogenization. In 2013, Danto et al. [30] explored four purification methods on As2 Se3 glasses to improve their optical and mechanical properties. These methods involve oxides removal by thermal treatment of the reagents, and addition of AlCl3 impurity-getters in the melt followed by distillation. These techniques are very effective on the removal of hydroxyls, water, and oxide impurities. A recurrent increase in the Se-H vibrational Fig. 5.3 Total optical loss spectra of As–S–Se glass fibers: (1) As35.2 S35.2 Se29.6 /As35.8 S41.9 Se22.3 [AsSSe(II)] glass fiber prepared from extra-pure-grade elements (first-drawn portion), (2, 3) As37.6 S28.3 Se34.1 /As36.5 S36.4 Se27.1 [AsSSe(III)] glass fiber prepared using chemical distillation with an Al + TeCl4 getter (first-drawn portion and back end, respectively). Adapted from Ref. [29], with permission from the Pleiades

5.1 Loss Mechanisms in Fiber Optics

109

band is observed with the concentration of the Se-H species escalating to several tens-of-ppm, depending on the purification method. To achieve lower loss levels in ChGs, they designed and constructed a new dynamic-vacuum purification system. The experimental design consists of a three-tube manifold quartz system in which the different glass-former elements are introduced into separate tubes where they are subjected to element-specific presynthesis purification under dynamic vacuum. (In this specific work, only two elements are introduced in the setup: arsenic and selenium). Elements are individually heated while under vacuum at the appropriate thermal processing temperature, whereby the metal–impurity bond can be broken, and the impurity removed by vacuum. Subsequent to this treatment, the now purified elemental materials (As and Se) are collected in the final reaction tube and sealed under vacuum. The mixture is then heated up to the melting (liquids) temperature, rocked, and quenched in the standard manner as the same as that described previously for the unpurified ampoule. All four investigated purification procedures are, as compared with the reference material, extremely effective in reducing the intensity and hence concentration of the hydroxyl species, molecular water, and oxide constituents. The impurity content of Se-H was reduced to 24 ppm. In 2015, Shiryaev et al. [31] prepared arsenic selenide glass with the low content of residual impurities (hydrogen 60.02 ppm wt, oxygen < 0.1 ppm wt, carbon < 0.5 ppm wt, silicon 60.1 ppm wt, and metals 0.01–0.1 ppm wt). The minimum optical loss in an unclad As35 Se65 glass fiber of diameter 200 μm is 70 dB/km at wavelengths of 2.7 and 3.8 μm. The minimum optical loss in core/cladding As40 Se60 /As38 Se62 glass fiber is 67 dB/km in the 6–6.5 μm spectral range. The starting substances (with purity 6 N) in the form of small pieces with an addition of chemical getters of impurities (Al and/or TeCl4 ) were batched in an inert atmosphere box into a silica glass ampoule, which then was evacuated and sealed off. The mixture in the evacuated silica ampoule was melted in a rocking furnace at a temperature 800 °C for 8–12 h. During melting, the impurity oxygen reacting with Al formed a non-volatile Al2 O3 , and chlorine from TeCl4 reacted with impurity hydrogen to form volatile HCl. Then, the purification of the As–Se melt from these limited impurities by multiple vacuum distillation was carried out (Fig. 5.4). In 2015, Tang et al. [24] fabricated high-purity glasses by a combination of chemical and vacuum distillation methods. The fiber baseline loss is 83–90 dB/km across 5.6–6.8 μm, a Se–H impurity absorption band of 1.4 dB/m at 4.5 μm is superposed and other impurity bands (e.g. O–H, As–O, Ge–O) are ≤ 20 dB/km (Fig. 5.5). In the fiber preform preparation, Ge (5 N purity), As (7N5 purity, prior heat treated at 310 °C under vacuum (10−3 Pa)), Se (prior purified by distillation at ~ 450 °C under vacuum (10−3 Pa), from a source of 5 N purity), 1000 ppmw TeCl4 (3 N purity) and 750 ppmw Al (6 N purity) were batched (nominal composition of Ge10 As21 Se69 ). Then they were melted (12 h at 850 °C) in a silica glass rig and experienced double distillation at ~ 750 °C under vacuum (10−3 Pa). In 2015, Zhang et al. [32] prepared high-purity Ge–As–Se and Ge–As–S chalcogenide glasses by modified physical and chemical purification techniques with subsequent vacuum distillation shown as Fig. 5.6. The fiber exhibited a background loss of < 1 dB/m, and a peak loss of ~ 5 dB/m at 4.6 μm.

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5 Chalcogenide Glass Preparation, Purification …

Fig. 5.4 Spectrum of total optical losses of core–clad As40 Se60 /As38 Se62 glass fiber of 250/400 μm diameter. Adapted from Ref. [31], with permission from the Elsevier

Fig. 5.5 Optical loss spectrum of the unclad. Ge10 As21 Se69 No. I fiber. Inset shows more clearly the sub-100 dB/km loss characteristic [24]. Adapted from Ref. [24], with permission from the Optical Society

The tube containing the raw materials was sealed after being evacuated down to < 10–5 mbar. The mixture was melted at 850 °C for 12 h in a rocking furnace. Subsequently, the tube containing the melt was quenched in water. The glass soobtained was then loaded into section A of a specialized distillation quartz tube (see Fig. 5.6) which has two sections connected with a capillary. The tube was evacuated

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Fig. 5.6 Schematic of the distillation tube

and sealed at position 1 (the other side was stopped using a rubber stopper during evacuation). Thereafter it was put into a tube furnace with section A in the hightemperature zone and section B in the low-temperature zone (the rubber stopper was removed and a vacuum line connected to that side). The glass was distilled into section B after section A was held at 800 °C for 2 h. The oxide impurities (e.g. Al2 O3 ) remained in section A because of their very low vapor pressures. The hydrogen- and carbon-related impurities (e.g., HCl, CCl4 ) were pumped away through the vacuum line because of their very-high vapor pressures. In 2019, Meneghetti et al. [33] prepared Ge10 As22 Se68 samples using a double distillation method, using different combinations of chlorides and metals as getters for the physico-chemical elimination of carbon, oxygen and hydrogen impurities. Comparing the attenuation spectra of the different samples, the choice of the getters seems to be a very significant factor in the quality of the glass. Different glasses by melt quenching technique were prepared in sealed tube with a double distillation to remove impurities: the charge and getters, placed in a sealed silica tube under high vacuum, were heated up to 800 °C and subsequently quenched in water. During the melting of the charge, the reaction of the getters with the impurities occurs: the halides react with hydrogen and carbon, following reactions like the ones presented as an example below [27, 34]: 4Se − H + TeCl4  4Se + Te + 4HCl

(5.18)

C + TeCl4  Te + CCl4

(5.19)

while the metals react with the oxygen in a way similar to the one shown below for the case of magnesium. As − O + Mg  As + MgO

(5.20)

The resulting glass underwent a two-step distillation. The first one happens in an open setup, in connection with a pump actively removing gases (dynamic distillation). The second one is performed in a sealed setup, once again in high vacuum atmosphere (static distillation). A schematic of these processes can be found in Fig. 5.7.

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5 Chalcogenide Glass Preparation, Purification …

Fig. 5.7 Schematics of the dynamic (a) and static (b) distillation processes

During dynamic distillation the glass migrates from the hot zone, at a temperature of 600 °C, to the cold zone. Metal oxides and residuals of silica from the crucible, being refractory, do not migrate, while water and hydrophilic acids, being volatile, are sucked into the liquid nitrogen cooled trap by the vacuum pump, and condense there. The second step removes traces of metal oxides that could have remained inside the glass. After the distillations, the glass was melted, quenched and annealed again with the same procedure used for the first synthesis. This last step is fundamentally required in order to homogenize the material, which during the distillations was separated into compounds with different melting temperatures (Fig. 5.8).

Fig. 5.8 Comparison between the attenuation spectra of Ge−As−Se fibers prepared using magnesium in combination with different hydrogen getters (a) and TeCl4 in combination with different oxygen getters [33] (b). In both graphs, an example of non-purified glass is shown. Adapted from Ref. [33], with permission from the Elsevier

5.1 Loss Mechanisms in Fiber Optics

113

5.1.4 Distillation Purification with Subsequent Static Distillation In order to synthesize a chalcogenide glass with low impurity level, it is necessary to remove impurities from the raw materials. Vacuum distillation technique has attracted great attentions for its low cost and less pollution to our environment, as it drives high purity vapor flowing into target room from a raw material surface under vacuum. Besides, the boiling point of metal will decrease a lot under vacuum condition and that helps to speed up the distilling process. Meanwhile, the efficiency of distillation is dependent on the maximum admissible temperature of container and the vapor pressure of raw material. Generally, the vapor pressure varies between different metals or metal-oxides (such as impurity), therefore certain metals can be separated and purified according to the difference of vapor pressures between metal and impurities. The relationship between the vapor pressure of the metal and the temperature can be expressed by the Clausius–Clapeyron equation [35, 36]: dp L = dT T (V1 − V2 )

(5.21)

where p is the vapor pressure of the metal, T is the temperature of the metal, L is the vaporizing latent heat of the metal. V 1 and V 2 represent the molar volume of gaseous metal and liquid metal, respectively. The value of (V 1 –V 2 ) is approximately of the molar volume of gaseous metal (V 1 ) because V 1 is far larger than V 2 . Under low vapor pressure, a gas obeys ideal gas law: V 1 = RT /p, where R is a gaseous constant and equal to 4.575. Therefore, this equation can be rewritten as: dp L dT = · 2 p R T

(5.22)

The vaporizing latent heat (L) relates to the temperature of the metal: L = L 0 + aT + bT 2 + cT 3 + ···, so Eq. (5.22) can be approximately expressed as the forms of Eqs. (5.23)–(5.24): lg p = −

b a L 0 −1 T + lgT + T+D 4.575 1.987 4.575

lg p = AT −1 + BlgT + C T + D

(5.23) (5.24)

where A, B, C and D are four constants varying with different metals [37]. The vapor pressures of Ge, Ga, Te, Si and Fe at different temperatures are calculated based on Eq. (5.24) and are shown in Fig. 5.9. Here, the critical P means the value of vapor pressure when distillation starts. The critical T means the lowest requirements of the temperatures to reach the critical P, decided by the thermodynamic data of the metals or metal-oxides. As shown in Fig. 5.9, it is clear that a high temperature of

114

5 Chalcogenide Glass Preparation, Purification …

Fig. 5.9 The vapor pressures of Ge, Ga, Te, Si and Fe at different temperatures. Adapted from Ref. [38], with permission from the Elsevier

1400 °C (1673 K) is needed to vaporize and distill Ge and Ga at a vacuum of 10–3 Pa in theory. In fact, a component containing Ge and Te will appear and the vapor pressure of Ge-containing compound will increase enormously so that Ge can be easily purified by distillation at a temperature of 1050 °C (1323 K) [39], which is not the case for the Ga-containing glasses. As is known to all, quartz glass container can be used stably under 1050 °C (1323 K), and the short-term maximum temperature can be up to 1450 °C (1723 K). Thus, it is necessary to find a new way to fast vaporize metals and keep the stability of the quartz container at an ultimate temperature, while leaving the oxides as residues (metallic oxides are usually more stable than the metals, such as Al2 O3 with boiling temperature of 3250 K, and Ga2 O3 with melting point of 2173 °C). In 1995, King et al. [40] prepared arsenic selenide glass samples using a combination of heat treatment and distillation techniques to remove impurities. The As and Se were weighed out inside the glovebox to make 25 g batches of As2 Se3 with an accuracy of + 0.01 g. Distilled samples were prepared using the looped tube, shown in Fig. 5.10a, b bi-level furnace. The glass batch was placed into the loop portion of the tube, along with a 75–100 mg strip of Mg metal, which acted as an oxygen getter, and the tube was sealed. Glass batches before distillation were placed in a straight tube and sealed off. Tubes were sealed by alternately evacuating and purging the tube with nitrogen or argon before finally sealing under vacuum (~ 15 μm Hg) with an O2 /gas flame. Distillation was accomplished by heating the upper portion of the tube to 600–700 °C and the lower portion to 300–350 °C in the bilevel furnace, as shown in Fig. 5.10b. At these temperatures, the As and Se volatilized in the upper portion and condensed in the lower portion over a period between 16 and 24 h. In this way, oxygen impurities were gettered by the Mg metal, and nonvolatile impurities were left in the upper portion of the tube. After the distillation

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Fig. 5.10 a Looped tube used for distillation of As and Se components; the Mg strip acts as an oxygen getter; b Distillation occurs in a bi-level furnace, with top and bottom sections held at different temperatures

was complete, the upper portion of the tube was pulled off with the O2 /gas flame and discarded, leaving the purified batch still under vacuum and ready for melting. The vacuum integrity was checked for all samples by heating a small area near the top of the tube and observing whether that area was significantly pulled in. In 2009, Troles et al. [23] prepared several GeSe4 glasses with different chemical and physical purification steps and thorough distillation of selenium under dynamic vacuum. The lowest the optical losses of GeSe4 fiber are lower than 0.5 dB/m between the wavelengths 1.7 and 7.5 μm except at 4.5 μm where losses are equal to 2.8 dB/m because of Se-H absorption (Figs. 5.11 and 5.12). In 2014, Xu et al. [38] adopted a rapid heating furnace and the fast distillation method based on vapor evaporation plus deposition under vacuum condition to decrease the content of impurities and micro-crystal particles in prepared GeGaTe glass samples. Dependence of optical loss on the types of oxygenic getters and their contents and glass quenching temperature was also studied. Minimum optical losses

Fig. 5.11 Experimental set-up for the purification and the distillation of the Se. a Step 1: purification and distillation of Se, b step 2: first synthesis (800–850 °C) with or without oxygen getter (Mg), step 3: distillation of the glass and step 4 s synthesis and homogenization in final silica tube in a rocking furnace (750 °C)

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5 Chalcogenide Glass Preparation, Purification …

Fig. 5.12 a Attenuation spectrum of an optical fiber prepared with glasses 2 and 3; b Attenuation spectrum of an optical fiber prepared with glasses 4 and 5 (inset: expanded view of optical losses between 0 and 3 dB/m.). Adapted from Ref. [23], with permission from the Elsevier

of 0.042 dB/mm at 9 μm and 0.037 dB/mm at 12 μm were obtained after an appropriate purification process, which represents the lowest loss for an GGT chalcogenide glass currently (Figs. 5.13 and 5.14). In 2016, Zhao et al. [41] purified a low-loss Te-based chalcogenide (ChG) glasses for the first time, and fabricated a step-index fiber by an isolated extrusion method. The fiber has an optical loss of 2–3 dB/m at 6.2–10.3 μm and 3.2 dB/m at 10.6 μm. In 2019, Jiao et al. [42] prepared a well-structured tellurium chalcogenide (ChG) fiber with a specialized double cladding structure by an improved extrusion method. The step-index fiber had an optical loss of 0.69 dB/m at 7.87 μm (Figs. 5.15 and 5.16). Subsequently, Wang et al. [43] fabricated a single-mode Te-rich chalcogenide (ChG) fiber for mid-infrared via an isolated extrusion method. The optical loss of the fiber is 3–4 dB/m in a range of 6.5–10.5 μm.

Fig. 5.13 a Glass distillation and preparation flow chart. (I) Vacuum evacuating process, (II) vacuum distilling process: the mixtures in section A are heated to 1400 °C and hold for ten minutes to guarantee that all the raw materials are distilled into section B, then the tube is sealed at position 3. Here, 1: sealing point after evacuated, 2: high temperature region, 3: sealing point after distillation, A: raw materials containing tube, B: glass synthesizing tube; b Optical losses of glasses with different quenching temperatures (all glass samples were distilled with 500 ppm Al, and the quenching temperature of G2 is 600 °C, G1 is 750 °C, G3 is 900 °C) [38]. Adapted from Ref. [38], with permission from the Elsevier

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Fig. 5.14 a Optical transmission of the bulk glass with a thickness of ∼2 mm thick [41]; b measured optical loss in the fiber. Adapted from Ref. [41], with permission from The Optical Society

Fig. 5.15 a Transmission spectra of the glasses (~2 mm); b Fiber Optical loss spectrum. Adapted from Ref. [42], with permission from The Optical Society

Fig. 5.16 a Measured optical loss of the single-mode fiber; b Cross-section image of the fiber. Adapted from Ref. [43], with permission from The Optical Society

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5 Chalcogenide Glass Preparation, Purification …

In 2019, Jiao et al. [44] prepared a novel low-loss selenium-based chalcohalide fiber with a low zero-dispersion wavelength by an innovative preparation process. The composition optimized fiber has a wide transmission range of up to 11.5 μm, a lowest fundamental mode zero-dispersion wavelength of 4.03 μm, and a minimum optical loss of 1.12 dB/m at 6.4 μm (Figs. 5.17). In 2020, Feng [45] et al. fabricated a low-loss and Arsenic-free sulfur-based chalcogenide fiber (Ge–Sb–S) by chemical purification and static distillation. The fiber has an optical loss of < 2 dB/m in a range from 5 to 6 μm with a minimum loss of 0.87 dB/m at 5.1 μm (Fig. 5.18). In 2020, Liang et al. [46] prepared a chemically stoichiometric chalcogenide (ChG) glass fiber with ultra-high germanium-containing for the first time. The fiber Fig. 5.17 Measured fiber loss (insert shows the output light spot diagram). Adapted from Ref. [44], with permission from The Optical Society

Fig. 5.18 Measured fiber loss (insert shows the output light spot diagram at1.55 μm). Adapted from Ref. [45], with permission from the Elsevier

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119

Fig. 5.19 a Transmission spectrum of the glass (~2 mm); b Measured fiber loss (inset shows the output light spot diagram). Adapted from Ref. [46], with permission from the Elsevier

exhibits an optical loss < 3 dB/m in a range of 5.1–7.9 μm, and the minimum loss of 2.11 dB/m was obtained at 6.61 μm (Fig. 5.19).

5.1.5 Glass Preparation Using Volatile Compounds Arsenic monosulfide, As4 S4 , can be used as the arsenic-containing component for production of glasses with As/S ratio equal to 1/1 or less. This compound is more suitable for ultra-purification, especially for decreasing the number of submicron particles, due to low viscosity of the melts. Two- and three-component glasses can be produced by melting purified arsenic monosulfide with the required amount of chalcogens, e.g.: As4 S4 + S2 → 2As2 S3

(5.25)

As4 S4 + Se2 → 2As2 S2 Se

(5.26)

To prepare high purity chalcogenide glasses, the initial high-purity elements such as As, Se, arsenic monosulfide can be loaded into a synthesis reactor by evaporation from intermediate ampoules in the evacuated all-sealed system [14, 47]. Sufficiently low content of hydrogen impurity (5.27–5.29 × 10−5 % mole) in glasses in the form of SeH-groups, calculated with the use of extinction coefficient of this impurity for As2 Se3 glass [48], is probably due to interaction of germanium tetraiodide, antimony triiodide and molecular iodine with hydrogen selenide and SeH-groups of glass network, leading to formation of gaseous hydrogen iodide: GeI4 + 2H2 Se = GeSe2 + 4HI

(5.27)

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5 Chalcogenide Glass Preparation, Purification …

2SbI3 + 3H2 Se = Sb2 Se3 + 6HI

(5.28)

I2 + H2 Se = Se + 2HI

(5.29)

which is removed from glass-forming melt [49]. In 2001, Churbanov et al. [50, 51] prepared As–S–Se glasses using arsenic monosulfide as an arsenic-containing component, and the fiber was drawn by the doublecrucible method. The minimum loss measured by the two-point method was equal to 0.7 dB/m at 5.5 μm. As4 S4 was purified by vacuum distillation at low evaporation rate. Then As–S–Se glasses for core and cladding were obtained from the purified arsenic monosulfide via melting it with the required amounts of selenium and As2 Se3 glass. In 2011, Velmuzhov et al. [52] reported a description of a preparation method of Ge–Sb-S-I glass by interaction of germanium iodide (IV) and antimony iodide (III) with sulfur melt. The iodides of germanium (IV) and antimony (III) (99.98%), subjected to double sublimation, and sulfur (99.999%), purified additionally by distillation in vacuum, were used for preparation of glasses. The quartz ampoules used were sequentially rinsed with mixture of hydrochloric and nitric acids, hydrofluoric acid and distilled water, and then were annealed in the tube muffle furnace at 800 °C for 12 h. A schematic of the set-up for the preparation of the glass forming melt, made from quartz glass, is shown in Fig. 5.20. Chemical interaction of charge components was carried out in reactor 7 under heating. The escaping iodine vapors together with vapors of volatile iodides ascended through mass-transfer Section 9 where separation took place. Iodine, as a low-boiling component, concentrated in the upper part of mass-transfer section and was collected in receiver 10. Vapors of high-boiling iodides condensed in the bottom part of separating section and drained down into the melt. It provided the selective removal of iodine from the glass-forming melt with simultaneous retention of other macro-components in the melt. Iodine was

Fig. 5.20 Set-up for glass production: 1, 2-loading ampoules; 3, 4-magnetic hammer-breaker; 5, 6, 8-resistance heating furnaces; 7-reactor; 9-mass-transfer section; 10-iodine receiver

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Fig. 5.21 Absorption spectra of glasses: sample No. 1 is produced in environment of high purity argon; samples No. 3 and No. 5 are produced in a vacuum. Adapted from Ref. [49], with permission from the Elsevier

periodically extracted in the temperature range 300–650 °C. Rings from quartz glass with height and diameter of 6 mm were used as filling. The as-prepared samples had a silicon impurity of 1–2 ppm wt. and metal impurities of less than 0.5 ppm wt. The glasses are characterized by high transparency in 2–10 μm spectral range. In 2014, Velmuzhov et al. [49] prepared Ge–Sb–Se–I system by reacting germanium tetraiodide and antimony triiodide with selenium in a set-up which provides selective removal of iodine from the melt. In the purest examples of these glasses, the content of residual impurities is: hydrogen—0.1 ppm mol, silicon—0.02 ppm wt., and transition metals-less than 0.5 ppm wt (Fig. 5.21). To prepare the Ge–Sb–Se–I glasses, the commercial germanium iodide (IV) of purity 99.98%, antimony iodide (III) of purity 99.999%, and selenium of purity 99.999%, additionally purified by double sublimation or distillation in vacuum, were used. The germanium iodide (IV), synthesized by reaction of iodine (99.999%) vapors and metallic germanium (99.9999%) and purified by vacuum sublimation, were also used. The silica-glass ampoules, used for the preparation of Ge–Sb–Se–I glasses, were routinely cleaned: they were washed out in the mixture of nitric and hydrochloric acids, hydrofluoric acid, distilled water, and then heated at a temperature of 800 °C within 12 h in an effort to remove the hydrogen impurity. Experimental implementation of the proposed method in production of chalcogenide glasses via volatile iodides is described in detail in paper [52]. Syntheses were carried out in environment of high-purity argon and in vacuum. The maximum temperature of syntheses was 550 °C; the glass-forming melt was homogenized at 600–650 °C.

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5 Chalcogenide Glass Preparation, Purification …

5.1.6 Chemical Vapor Transport Reactions Technique The decrease in the temperature and the duration of synthesis of chalcogenide glasses is possible due to the use of volatile and low melting iodides instead of simple substances. The formation of glass-forming compounds during such a process can be described by chemical reactions: GeI4 + 2S(Se) ↔ GeS(Se)2 + 2I2 ↑

(5.30)

2Ge2 S(Se)3 I2 ↔ 3GeS(Se)2 + GeI4 ↑

(5.31)

GeI4 + S(Se) ↔ GeS(Se)I2 + I2 ↑

(5.32)

2SbI3 + 3S(Se) ↔ Sb2 S(Se)3 + 3I2 ↑

(5.33)

To shift the equilibrium of the reactions toward the formation of chalcogenides of germanium and antimony, it is expedient to remove iodine from the reaction melt. This is achieved by carrying out the process in a reactor with a “cold” receiver, which provides condensation of the evolved iodine vapors. The integrated intensity of the bands of selective impurity absorption due to Se–H and S–H bonds in glasses produced via volatile iodides of germanium and antimony is by 10–60 times lower than that in glasses obtained by the direct melting method. It is caused by performing reactions that transfer the hydrogen impurity into high volatile HI [14]: H2 S(Se) + I2 ↔ S(Se) + 2HI ↑

(5.34)

2H2 S(Se) + GeI4 ↔ GeS(Se)2 + 4HI ↑

(5.35)

GeI4 + 2H2 O ↔ GeO2 + 4HI ↑

(5.36)

Thus, the use of volatile and low-melting iodides instead of the simple substances makes it possible to reduce the content of optically active impurities in glasses [14]. In 2014, Shiryaev et al. [53] prepared the high-purity glass samples using chemical distillation for purification of the Ge–As–Se base-glass. Next, a new vapor phase transport approach of metallic Ga transfer in a GaI3 flow is developed to purify and add the batch of metallic gallium into the silica glass reactor for the Ge–As– Se-Ga glass synthesis. A thermodynamic equilibrium-based vapor phase transport model is presented and discussed. In the best examples of these glasses, the content of residual impurities is: hydrogen—0.15 ppm, oxygen—< 1 ppm, and transition metals-less than 0.1 ppm.

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Fig. 5.22 Silica glass set-up for making Ga–Ge–As–Se glass including the delivery of gallium, purified by the chemical vapor transport route. 1-Evaporator containing gallium (III) iodide. 2Metallic gallium (Ga[0])-containing ampoule and receiver for the vapor transported Ga[III]I3 which react to form Ga[I]I. 3-Reactor with three processes: a receipt of Ga[I]I vapor which, as it deposited the purified Ga[0] metal here, was converted back to Ga[III]I. 3-this left the reactor to end up in the receiver at 4; b receipt of chemically distilled Ge–As–Se to join the vapor-deposit and purified Ga[0] metal (via smashed baffle) and c the final glass melting of Ga–Ge–As–Se, executed in order (a–c). 4-Receiver for the gallium (III) iodide by-product. A, B-waists for flame fusing

The reactor (3rd section, Fig. 5.22) temperature was lowered to 180–200 °C; the temperature of the 2nd section (Fig. 5.22), containing the gallium, was adjusted to 550 °C, while the 1st section (Fig. 5.22) containing the gallium iodide (III) was heated to only 220–230 °C. It is then that gallium (III) iodide vapor moved from the 1st to the 2nd section (Fig. 5.22) and reacted with, and hence oxidized, the metallic gallium (Ga0 → GaI ), already present in the 2nd section, in accordance with the reaction: Ga[III]I3 + 2Ga[0] = 3Ga[I]I

(5.37)

Next, the product of the forward reaction the gallium (I) iodide vapor, on forming, entered the reactor (3rd section, Fig. 5.22) and disproportionated back again (reverse direction of Eq. 5.37) due to the temperature reduction encountered. In other words, the temperature of the reactor (3rd section) was lower than that of either the 1st or the 2nd sections. During this disproportionation, which was the reversal of Eq. 5.37, metallic gallium was found to deposit as droplets in the reactor essentially “falling out” of the Ga[I] vapor, which itself concomitantly reverted back to Ga[III]I3 . Next this gallium [III] iodide filled the receiver, i.e., the 4th section of the rig shown in Fig. 5.22 and, theoretically could be redirected back to the first section of the rig (Fig. 5.22) from whence it had originated. The rate of transport transfer of Ga to the synthesis reactor was measured to be about 0.008 g/min. After the required amount of vapor-transported and hence purified gallium had been received, the heating of both ampoules comprising the 1st and 2nd sections of the rig (Fig. 5.22) was switched off. Then the synthesis reactor (3rd section of the rig, Fig. 5.22) was heated up to 250 °C with the aim of simply removing the excess Ga[III]I3 from the 3rd section into the 4th section of the rig (Fig. 5.22). Thereafter, the synthesis reactor (3rd section, Fig. 5.22) was sealed-off at points A and B. The remaining part of the glass composition, i.e. the Ge–As–Se alloy, was loaded into the synthesis reactor (3rd section, Fig. 5.22) by vacuum evaporation via the deliberately smashed silica-glass baffle (smashed with a magnetic metal lug, sealed inside a silica glass sheath, situated inside the system by

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5 Chalcogenide Glass Preparation, Purification …

movement of a magnet outside the system). Distillation of the preliminary produced high purity Ge–As–Se alloy was carried out in the closed vacuum system. Glass melting of the complete required composition: Ga–Ge–As–Se was then executed in the rocking muffle furnace at a temperature of 800 °C with subsequent air-quenching and glass annealing at 220 °C. In 2016, Karaksina et al. [54] developed the multi-stage technique for synthesis glasses. It is based on chemical distillation purification of glass components and the transport reaction for purification of gallium. The content of limiting impurities of oxygen, carbon and hydrogen in the glass samples was ≤ 0.2 ppm wt, just shown as Fig. 5.22. The 1300–3000 ppm wt Pr3+ -doped Ga–Ge–Sb–Se bulk glasses exhibit an intensive photoluminescence in the spectral range of 3.5–5.5 μm. The method for preparation of multi-component Ga–Ge–Sb–Se glasses is based on the technique for purification and synthesis of high-purity chalcogenide glasses previously [53] (Fig. 5.23). In 2015, Velmuzhov et al. [55] prepared Ge–S-I glasses having an iodine content of 4.0–22.1 at. % and the x(S)/x(Ge) ratio of 1.7–2.1 by melting and subsequent cooling of the products of thermal decomposition of Ge2 S3 I2 . The compositions and properties of the resulting glasses and the impurity content in samples obtained under different conditions of Ge2 S3 I2 decomposition were investigated. The purest samples of prepared glasses were characterized by high optical transparency in the

Fig. 5.23 Absorption spectra of samples: 1—Ga3 Ge24 Sb11 Se62 glass prepared using the developed technique [54]; 2—Ga3 Ge24 Sb11 Se62 glass prepared by direct melting of specially pure elements; 3—Ga3 Ge16 As17 Se64 glass reported in paper. Adapted from Ref. [53, 54] with permission from the Elsevier

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125

Fig. 5.24 a A scheme of reactor and set-up for synthesis and thermal decomposition of Ge2 S3 I2 : 1, the section with the initial Ge2 S3 I2 ; 2, the connecting (and rod-forming) tube; 3, the waist for sealing off; 4, the receiver for volatile decomposition products; 5, the resistance heating furnaces; b Absorption spectra of Ge–S–I glass samples obtained by the traditional method (Ge31 S57 I12 ) and by melting the products of thermal decomposition of Ge2 S3 I2 (GSI-7 and GSI-9) [55]. Adapted from Ref. [55], with permission from the Elsevier

spectral range of 2–10 μm and the low content of impurities: silicon (0.1 ppm wt), metals ( 2.405, more than two modes can propagate (so called multi-mode propagation). The numerical aperture (NA) can be calculated by the equation [9]: 1/2  N A = n 21 − n 22

(6.20)

The Sellmeier coefficients can be calculated out with the Sellmeier equation based on measured refractive indices [10]: n(λ) = 1 +

 A i λ2 λ2 − λi2 i

(6.21)

where λ is the wavelength in free space, where Re(n e f f ) is the real part of the effective index of fundamental mode and c is the light velocity in free space. Then the chromatic dispersion D(λ) can be calculated from the equation [11] with linear refractive index n(λ) and neff :   

λ d 2 Re n e f f D(λ) = − (λ) ps/nm/km + D m C dλ2

(6.22)

Dispersion is one of the important characteristics of optical fiber. For chalcogenide fibers, the total dispersion is usually described as a combination of waveguide dispersion and material dispersion. The zero dispersion wavelength (λd ) is 1.31 μm in a typical silica fiber. Figure 6.3 shows the dispersion parameters of single-mode

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6 Chalcogenide Fiber Structures: Design and Performance Analysis

Fig. 6.3 Dispersion profile of optical fiber

Dispersion [ps/(nm.km)]

20

Material Total Waveguide

10

0

-10 d

-20 1200

1400

1600

Wavelength ( m)

silica fiber [5]. It can be seen that waveguide dispersion makes the zero-dispersion wavelength (ZDW) of material dispersion red shift to a longer wavelength. When the wavelength meets λ < λd , the total dispersion D < 0, the fiber has normal dispersion, otherwise it is called abnormal dispersion. In addition to the refractive index and dispersion distributions of fiber, nonlinearity is another key parameter. The third order refractive index n 2 and nonlinear coefficient (γ ) play a significant role in areas of optical frequency comb and supercontinuum generation and so on. The value of n 2 is proportional to the third-order nonlinear   susceptibility χ(3) e and linear refractive index n 0 . In As2 Se3 glass, the n 2 [12] can be estimated by the following formula: n 2 = 4.27 × 10−16

(n 20−1 )4 2 −1

m /W n 20

(6.23)

The effective mode area Ae f f [10] represents the effective area of propagating light mode that can be defined as: 

Ae f f

2

2 ) (S Z2 d→

r  = = π w 2 μm2 2 S Z2 d→

(6.24)

r

The nonlinear coefficient γ can be evaluated by the equation: γ =

2π λ

 n 2 −1 −1

W km · Ae f f

(6.25)

Numerical analysis of an optical pulse propagating in the chalcogenide glass fiber is very important since optical nonlinearity in chalcogenide glasses is hundreds or even thousands of times higher than that in silica glasses, making them attractive

6.1 Overview

179

in various applications. Here, the nonlinear Schrodinger equation describing the ultrashort pulse state in optical fiber and the split step Fourier algorithm for numerical analysis of the equation are briefly described. Generally, in order to simplify the calculation, the sharp change component of the electric field is ignored and a slowly changed envelope is used as an approximation. In time domain, with the movement of group velocity of the pulse, the envelope function A = (z, T ) of the optical pulse, and the propagation equation are all in the following form [14]: N  i k βk ∂ k A ∂A α + A−i ∂z 2 k! ∂ T k K ≥2



∞     i ∂ = iγ 1 + (A(z, T ) R t  |A z, t − t  |2 dt  w0 ∂ T

(6.26)

0

Here, γ is a nonlinear coefficient, which can be obtained from the above formula (6.25). In this formula, the second term on the left represents the fiber loss, and the third term represents the effect of wavelength dispersion including higher-order dispersion. On the right side, there are self-phase modulation, four-wave mixing, stimulated Raman scattering and other nonlinear effects. R(t) is the Raman response function, which includes the contributions of electronic transition and vibrational transition. By means of this formula, the spectrum broadening of a short pulse in different nonlinear optical fibers can be calculated, mainly the supercontinuum generation in highly nonlinear chalcogenide optical fibers. The frequency domain equation is shown in the formula (6.27). i sgn(β2 ) ∂ 2 U ∂U + ∂ξ 2 L D ∂τ 2 sgn(β3 ) ∂ 3 U e(−αξ ) = +i 3 6L D ∂τ LNL   2 ∂(|U | U ) ∂|U |2 |U |2 U + is · − τRU ∂τ ∂τ

(6.27)

For the simulation of the propagation equations, many methods have been reported. Here we discuss just two typical methods, the first is the Finite Difference Time Domain (FDTD), and the second is the Finite Element Method (FEM). During the past decades, the FDTD method has become the most widely used simulation tool, which is characterized by the solution of Maxwell’s curl equations in the time domain after replacement of the derivatives by finite differences. It has been applied to solve many problems in propagation, radiation and scattering of electromagnetic waves. The classical FDTD method employs a second order finite centered approximation to the space and time derivatives in Maxwell’s curl equations, giving rise

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6 Chalcogenide Fiber Structures: Design and Performance Analysis

to a new discrete electromagnetism with its own inherent peculiarities. The FDTD defines an orthogonal cubic spatial grid; each field component is sampled and evaluated at a particular space position, and the magnetic and electric fields are obtained at time instants delayed by half the sampling time step. The materials are modeled by specifying their characteristic constants at every grid point, usually in homogeneous regions, where interfaces proper continuity conditions are needed. The time advancing algorithm is explicit, that is, the fields at each time instant are obtained as a function of previous values in time. The simulation of open problems is carried out by placing Absorbing Boundary Conditions (ABCs) in the terminating planes of the grid. Assume that all the currents are ohmic, and we also assume that as all the media are linear, isotropic and non–dispersive dielectrics, therefore we can omit the displacement and magnetic flux density vector fields. In the FDTD method, the space can be discretized into a set of points uniformly spaced in each direction, as integer and semi–integer multiples ofx,  y, z, and similarly time parameter can be divided in time instants, that is integer and semi–integer multiples of a given intervalt. It is possible to define the fields at each spatial–temporal position of this grid with the following notation. The central difference method and the numerical curl method are used to calculate out the correlated solutions via the following equals (but not limited to). ψi,n j,k ≡ (x = ix, y = jy, z = kz, t = nt)

(6.28)

In the electric field, the polarizations and the flux density of the nonlinear material have the following relationship [15]: ε0 ε∞ E y = D y − PL − PR − PK

(6.29)

where ε0 and ε∞ are the permittivity of free space and the saturated value of the relative constant in the limit for that frequency is infinity. The polarizations PL , PR , and PK are linear polarization, Raman scattering, and Kerr effect, respectively. The linear dispersion is given by the following convolution between the electric field and the linear susceptibility χ (1) (t) in time domain:  PL (t) = ε0

t

χ (1) (t − τ ) · E (τ ) dτ

(6.30)

0

 where χ (1) (t)  = γ L exp(α L t)sin(β L t), α L = ω L δ L , β L = ω L 1 − δ L 2 and γ L = ω L (εs − ε∞ )/ 1 − δ L 2 . The FDTD is easy to use, but it is less efficient when dealing with complex fiber designs, such as graded-index fibers, multi-cladding fibers, as well as photonic crystal fibers (PCFs). Those structures are usually dealt with numerically using finite element methods (FEM) [16]. The FEM obtains the eigenvalue equation of the optical fiber from Maxwell equations, builds the model and discrete gridding according for a

6.1 Overview

181

calculation region, then synthesizes the solutions in all small regions. One can set the parameters for the fiber, such as wavelength, material refractive index and boundary conditions in the model. The FEM approach has also been applied to a diverse range of singlemode waveguides by approximating the index profile and enforcing boundary conditions across multiple interfaces. The result is a matrix eigenvalue equation in a generalized form. First, Maxwell’s equations are transformed to ordinary differential equations in eigenvalue form. In one case, gradient index terms are neglected, and in the second, they are considered to be first order so that their effect can be monitored. Next, finite element analysis, using the Galerkin technique reduces these differential equations to matrix equations in standard eigenvalue form. The propagation modes and effective mode index of an optical fiber can be then calculated by FEM [17]. A full-vectorial FEM can be used efficiently in determining the quasi-TE and quasi-TM fundamental and higher order modes. A real value eigenvalue problem determined by the H field formulation can be solved with higher computing efficiency, compared to other methods. The important optical properties of a fiber that define its use in a particular application include attenuation, mode field diameter, effective area, cut off wavelength, dispersion, and bending losses. In the modal solution approach based on the FEM, the intricate cross section of a fiber can be accurately represented by using nearly a million triangles of different shapes and sizes. The flexibility of the irregular mesh makes the FEM more preferable than the FDM, which not only avoids inefficient regular spaced meshing, but also can be used to represent curved dielectric interfaces. The optical modes in a fiber with two-dimensional confinement and high index contrast core/clad interfaces are also hybrid in nature, with all six components of the E and H fields [18]. Hence, only a vectorial formulation can be used to accurately calculate the fiber modal solutions. The H field formulation with the augmented penalty function technique is given below [19]:  ω = 2

∇×H

∗

   · ε−1 (∇ × H )d + (η/ε0 )(∇ · H )d  H ∗ · μH d

(6.31)

where H is the full-vectorial magnetic field, ε and μ are the permittivity and permeability, respectively, of the waveguide, ε0 is the permittivity of the free space, and ω2 is the eigenvalue, where ω is the angular frequency of the wave. The dimensionless parameter η is used to impose the divergence-free condition of the magnetic field in a least squares sense to eliminate spurious solutions. A highly efficient sparse solver with the subspace iteration technique is used to solve the resulting large eigenvalue equations with orders often larger than 100,000. The coordinate transformation allows a bent optical waveguide in the x plane to be represented by an equivalent straight waveguide with modified refractive index distribution,n eq (x, y): n eq (x, y) = n(x, y)(1 +

x ) R

(6.32)

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6 Chalcogenide Fiber Structures: Design and Performance Analysis

where n(x, y) is the original refractive index profile of the bent waveguide,n eq (x, y) is the equivalent index profile of a straight guide, R is the radius of the curvature, and x is the distance from the center of the waveguide. This equation is valid for the range 2x R, which is well within the ranges considered. Subsequently, a straight waveguide with a transformed index profile can be analyzed by a number of modal solution techniques, such as the eigenmode expansion [20], the lines method [21], the FDM [22, 23], the variational method [23], the matrix approach [24], the Wentzel-Kramers–Brillouin (WKB) analysis [25], and the FEM approach [26–28]. With the development of chalcogenide glass fiber, there are many fiber structures designed for different applications, such as traditional step-index fiber and various novel structured chalcogenide fibers. The key fiber structure designs and their performance are developed in the sections which follow.

6.2 Traditional Step-Index Fiber A traditional step-index fiber is the simplest fiber type, as it just possesses one core and one cladding, with no variations in refractive index within the core of cladding. Usually, the most important fiber parameters are the light mode number, propagation constant and mode field parameters. The fundamental state of LP01 light radial mode for the step-index fiber is near-Gaussian [5], and the number of modes can be determined with the aid of Fig. 6.2. The fiber mode field diameter (MFD) and effective area Aeff are defined by 2 ∫∞ 0 |E(r )| r dr and    dE 2 ∫∞ 0 dr r r dr

2 ∞ ∫0 |E(r )|2 r dr = 2π ∞ ∫0 |E(r )|4 r dr

M DF2 = 2

Ae f f

(6.33)

respectively. The theoretical cutoff wavelength for a step-index single-mode fiber,λth c is defined as λth c =

2π n 1 a √ 2· 2.405

(6.34)

where n 1 is the refractive index of the core,  is the core radius, and  is the relative index difference between the core and cladding. At wavelengths greater than λth c , the transverse propagation constant βt2 of the first higher order LP11 mode in the cladding region becomes a real number. This changes the solution for the electric field in the cladding from a decaying, evanescent field to an oscillatory, propagating field, thus resulting in radial energy flow (i.e., one that carries energy away from the fiber axis). The bound mode becomes a leaky mode.

6.2 Traditional Step-Index Fiber

183

Considering the behavior of the LP11 at wavelengths shorter than λth c , then at , the LP mode is tightly confined within the core wavelengths much lower than λth 11 c region and losses will generally be comparable to those of the fundamental mode. As the wavelength increases, the LP11 mode becomes less tightly confined to the core. When the fiber has axial imperfections, such as micro-bends or macro-bends, the decreasing mode confinement gives rise to excess LP11 mode loss that can be expressed as [29]. α=−

20π Im(β) ln10

(6.35)

6.2.1 Standard Chalcogenide Single-Mode Fiber Design In 1983, Okamura et al. [30] described the design for single-mode chalcogenide fiber whose operation wavelengths are in the 2.5–6 μm range leading to ultralow loss < 0.01 dB/km and evaluated the excess loss caused by uniform bending, microbending, and splicing with the formula used for silica-glass fiber. In that paper also, the possibility of preparing an ultralow loss fiber was discussed. In 1998, Mossadegh et al. [31] fabricated a long length (>150 m) single-mode chalcogenide optical fiber by the double crucible method. Single-mode transmission through a 10 m long fiber was demonstrated using an F-center laser at 2.7 μm. Figure 6.4 shows the image of fiber cross-section, the core is indeed quite distinct, circular, centered in the fiber, and easily distinguished from the cladding. In 1999, Kobelke et al. [32] described a praseodymium doped arsenic sulphidebased single mode fiber with optical loss of 3.3 dB m−1 at 1.3 μm. The composition of the core glass was As37.8 Ge1.3 Ga0.5 S60.4 doped with 760 ppm wt% praseodymium. Fig. 6.4 Core image near the end of the single-mode fiber (core is 12 μm in diameter). Adapted from Ref. [31], with permission from the Institute of Electrical and Electronics Engineers Inc.

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6 Chalcogenide Fiber Structures: Design and Performance Analysis

Fig. 6.5 Cross section of the optical fiber. Adapted from Ref. [33], with permission from the Elsevier

The fiber of small core and large numerical aperture was produced by a combination of the double crucible method and rod-in tube processing. In 2007, Patrick et al. [33] achieved a single mode optical fiber at 1.55 μm, based on GeSe4 glass. The single mode fiber is performed by an improved method of drawing named “fiber-in-tube”. A photo of the fiber cross section observed with an optical microscope is shown in Fig. 6.5. This method allows for the realization of a single mode fiber in only one drawing step with the core/clad “fiber-in-tube” preform. The optical loss of this fiber is 4 dB/m at 1.55 μm. When light propagates in a transparent material with birefringence, the state of polarized light will be changed. Polarization maintaining fiber (PMF), also known as birefringent fiber, is designed to prevent the random fluctuation of birefringence from changing the polarization state of the light. For instance, it will induce birefringence in optical fiber via adding asymmetric stress in the core, provided by two stress rods embedded in the fiber. PMF has large birefringence, so the group velocity of light along the two orthogonal axes is different. When the effective refractive index is small in particular axial direction, the group velocity of the propagating light will be larger, so that axis is called the fast axis. Conversely, a direction with a large effective refractive index is called the slow axis. In 2019, Ren et al. [34] fabricated a step-index chalcogenide fiber with a rectangular core and a circular cladding through an approach that combines an extrusion technique and a multiple-stage rod-in-tube method (Ge12 As24 Se64 /Ge10 As24 S66 , core/cladding). Figure 6.6 shows that the GVD of the fiber becomes anomalous starting from ~ 3.2–3.8 μm when the rectangular core size is 3–5 μm × 6–10 μm. However, when the core size is < 2.5 μm × 5 μm, the fiber shows a behavior of all-normal dispersion. The polarization fiber also has high nonlinearity.

6.2 Traditional Step-Index Fiber

185

Fig. 6.6 Calculated group velocity dispersion (GVD) of the Ge–As–Se/Ge–As–S fibers with different core size. Adapted from Ref. [34], with permission from the Elsevier

6.2.2 Multi-mode Fiber In 2015, Zhang et al. [35] fabricated a step-index chalcogenide fiber with a large numerical aperture (~1.3) and small core (~5.5 μm). The measured refractive index of the cladding glasses Ge–As–Se and core glass Ge–As–S was larger than 2.6 or lower than 2.2 separately. In 2017, Luo et al. [36] fabricated an As2 Se3 –As2 S3 multi-material chalcogenide glass fiber with ultra–high numerical apertures (NA ~ 1.4). With a 10-μm-diameter As2 Se3 core and a 246–μm–diameter As2 S3 cladding, the fiber exhibited a zerodispersion wavelength (ZDW) near 4.5 μm. The black and blue lines of Fig. 6.7a show the refractive indices of the As2 Se3 glass and As2 S3 glass, respectively. The red

Fig. 6.7 a Measured refractive indices of bulk As2 Se3 (black) and As2 S3 (blue), and calculated dispersion (red) of the small-core fiber. b The calculated NA of the fiber. Adapted from Ref. [36], with permission from the Institute of Electrical and Electronics Engineers Inc.

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line shows the dispersion curve of the chalcogenide glass fiber. Figure 6.7b shows the calculated NA of the fiber. In 2017, Wang et al. [37] reported a chalcogenide fiber taper with an ultra-high numerical aperture. A chalcogenide step-index fiber consisting of As2 Se3 core and Axs2 S3 cladding was fabricated by using an isolated stacked extrusion method. Measured refractive indices of the core and cladding glasses, and the calculated NA are shown in Fig. 6.8. In 2020, Zhong et al. [38] prepared an step-index chalcogenide (ChG) fiber (SIF) with ultra-large core, high numerical aperture (NA) and ultra-large mode area that exceeded 43,600 μm2 at a wavelength of 5.5 μm by an improved extrusion method. The refractive index and NA from 1.5 to 15 μm for the As2 Se3 and As2 S3 glasses are shown in Fig. 6.9. The fiber has a core diameter of 339.0 μm and ultra-thin cladding that is only 30 μm thick (with outer diameter of 399.0 μm). The beam spot diagram for the fiber is shown in Fig. 6.10. This ultra-large core fiber possesses the potential for high-power infrared laser transmission, as the power density in the core is kept low. Fig. 6.8 Measured refractive indices of the core and cladding glasses, and the calculated NA. Adapted from Ref. [37], with permission from the Elsevier

Fig. 6.9 Refractive index from 1.5 to 15 μm for the As2 Se3 and As2 S3 glasses and calculated NA. Adapted from Ref. [38], with permission from the Wiley-Blackwell

6.2 Traditional Step-Index Fiber

187

Fig. 6.10 Beam spot diagram. Adapted from Ref. [38], with permission from the Wiley-Blackwell

Fig. 6.11 Refractive indices and calculated NA. Adapted from Ref. [39], with permission from the Institute of Electrical and Electronics Engineers Inc.

In 2020, Liu et al. [39] prepared a novel fiber consisting of three different glasses with W-type double-cladding structure and a constant core-to-cladding ratio, using the extrusion method. The fiber had an ultra-high numerical aperture (NA ≥ 2.17) which indicates that this fiber is capable of collecting and confining light over a wide wavelength range. Refractive indices of the glasses and the NA are shown in Fig. 6.11.

6.2.3 Damage Threshold for Chalcogenide Fibers Usually, the glass components for chalcogenide fibers are selected from a limited range of possibilities: As–S, Ge–As–Se, Ge–Se–Te and Ge–As–Se–Te. Because these fiber glasses have a relatively wide transmission window of 1–12 μm, they

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6 Chalcogenide Fiber Structures: Design and Performance Analysis

are expected to be used in high power IR laser transmission, for example for laser types that include: 2.94 μm Er: YAG laser, 5.3 μm CO laser and 10.6 μm CO2 laser. The energy transmission characteristics and applications of chalcogenide fibers were firstly reported for As–S fiber. In 1988, Francois et al. [40] reported that an As2 S3 fiber with core diameter of 400 μm and fluorinated ethylene propylene (FEP) cladding can transmit a CW (continuous wave) laser power of 16.9 W at a wavelength of 5 μm, which equates to a power density about 13.5 kW/cm2 ). In 1992, Nishii et al. [41] reported an output power of 10.7 W for a 19.4 W launched laser power (an efficiency of 55.2%) at 10.6 μm using Te-based fibers. The next year, Sato et al. [42] made a further improvement with transmission for a 62 W CW laser transmission with core diameter of 700 μm [43], to a new record of 226 W for CO laser power transmission (equivalent to a CW power density of about 28.8 kW/cm2 ) using a larger core diameter of 1000 μm in the fiber. One of the factors which allows for such an improvement is that Sato et al. used room temperature nitrogen to continuously cool the fiber end surface. In addition, As-S component is stoichiometric and has a simple component makeup, so the refractive index is relatively insensitive to temperature change, at 9.3 × 10–6 °C−1 . This is nearly an order of magnitude lower than that of Ge–As–Se or Ge–Se–Te glasses, which greatly reduces the fiber end surface damage caused by self-focusing [44]. Later in 1996, L. Busse [45] reported that As2 S3 fiber can withstand more than 108 MW/cm2 of pulsed power density (about 100–200 MW/cm2 ), with energy density of 0.5–1.0 kJ/cm2 at a 3.48 μm wavelength (5 ns pulse width, 10 Hz) without any observable surface damage. Then, in 2007, Papagiakoumou [46] demonstrated a record of handling power intensity of 38 MW/cm2 for a long pulse (80 μs). In theory, maximum power density of 500 MW/cm2 for short pulsed (5 ns) laser at 3.48 μm can be permitted at the sulfide fiber end facet [47]. Small core diameter (n2 Ѳ1

Ѳ2

Ѳc

kr

Reflected light

Ѳc

Ѳ1>Ѳc

n1

Fig. 6.33 Schematic diagram of evanescent wave

An evanescent wave is a form of electromagnetic wave generated close to the interface of two different media due to total reflection, as shown in Fig. 6.33. Because its amplitude decreases exponentially with the increase of the depth perpendicular to the interface, and its phase changes with the tangential direction, it is also a form of surface wave. Generally, total reflection occurs when the light wave is incident from the dense to sparse medium, and the incident angle is greater than the critical angle. At this point, although the light wave cannot pass through the critical interface of the two media, the light wave will be generated along the parallel direction of the critical interface. The complex amplitude of the electric field and magnetic field will decrease exponentially with an increase in the distance from the critical interface. Evanescent waves are widely used in various fields, such as optical fiber communication and fiber sensing. In order to obtain the optical fiber with high sensitivity and wide application range, reprocessing of optical fiber has become more and more attractive. When the fiber is drawn into tapered fiber, the taper waist is very thin. At this point, the original fiber core becomes very small, and the transmission mode is that of an air-cladding fiber. Because the cone waist is very thin, which is of the same order of magnitude as the wavelength of light, most of the energy of evanescent wave field penetrates into the cladding. The tapering of optical fiber can effectively improve the sensitivity of optical fiber. This is an important reason why the evanescent wave in a tapered fiber shows great application potential. In 2000, Hocde et al. [87] used a cone shaped TeAsSe optical fiber as a sensing device for measuring the different concentration of acetone in methylene chloride liquid. In 2001, Hode et al. [88] explored the chemical sensing and the feasibility of the detection in tumor and cancer formation process, where a tapered chalcogenide fiber was adopted as a probe. In 2003, Michel et al. [89] used a Te2 As3 Se5 (TAS) tapered fiber to measure the concentration of C2 Cl4 pollutants in the water source with only 1 ppm concentration. The diameter of the fiber was 400 μm, and the sensing area of taper was only 100 μm. The experiment verified the feasibility of detecting pollutants with the choridium-based fiber sensor. In the same year, LeCoq et al. [90] also adopted the TAS taper fiber with core-cladding structure to effectively detect trace gases such as ethanol and chloroform with low concentration in the air. In 2009, Anne et al. [91] prepared a Ga5 Ge25 Sb10 Se65 tapered fiber with a fiber

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6 Chalcogenide Fiber Structures: Design and Performance Analysis

diameter of 400 μm and a tapered region of 100 μm, which was successfully used in characterizing a resin polymerization process by recording the C-O key function of the absorption spectrum and the curing time of the diagram. This demonstrates that the chalcogenide tapered fiber can be used to investigate chemical reaction processes.

6.6 PCF Photonic crystal fibers (PCFs), also called as micro structured or holey fibers; were proposed for the first time by Knight [92] in 1996. Their unique characteristics and innovative properties were attractive. PCFs is usually prepared by undoped silica and the low-index region in PCFs is typically provided by air-holes running along their entire length. There are two main categories of PCFs: high-index guiding fibers and photonic bandgap ones. PCFs belonging to the first category are similar to conventional optical fibers, because light is confined in a solid core by exploiting the modified total internal reflection mechanism [93]. In fact, there is a positive refractive index difference between the core region and the photonic crystal cladding, where the air-hole presence causes a lower average refractive index. The guiding mechanism is defined as “modified” because the cladding refractive index is not a constant value, as in standard optical fibers, but rather it changes significantly with the wavelength. When the PCF core region has a lower refractive index than the surrounding photonic crystal cladding, light is guided by a mechanism different from total internal reflection, that is, by exploiting the presence of the photonic bandgap (PBG). In fact, the air-hole microstructure which constitutes the PCF cladding is a two-dimensional photonic crystal that is a material with periodic dielectric properties characterized by a photonic bandgap, where light in certain wavelength ranges cannot propagate. The PBG effect can be also found in nature, for example, the beautiful and bright colors seen in butterfly wings. PCFs with a low index core are created by introducing a defect in the photonic crystal structure, for example, an extra air-hole or an enlarged one, and light is confined because the PBG makes propagation in the microstructured cladding region impossible. This guiding mechanism cannot be obtained in conventional optical fibers and it opens a new set of interesting possibilities and applications. Analogies with classical theory of conventional fibers, two different formulations of the V parameter are considered. The first one is V1 =

2π  2  n e f f − n 2F S M λ

(6.41)

which has been recently proposed for triangular PCFs [94, 95]. neff and nFSM are the effective indices of the fundamental guided mode and the fundamental space-filling mode in the air-hole cladding, respectively, which have been evaluated using a freely available software package [96]. The choice of  as the effective core radius can be

6.6 PCF

205

adopted also for the PCFs studied here, since it is the natural length scale of both the triangular and the square lattices [94, 95]. The second V parameter definition considered, more similar to the one used for conventional fibers, is V1 =

 2π ρ n 2co − n 2F S M λ

(6.42)

where nco is the refractive index of the core at the operation wavelength and ρ is the effective core radius. Then, using the effective index of the guided mode neff , the normalized propagation constant is determined as:     βn = n 2e f f − n 2F S M / n 2co − n 2F S M

(6.43)

Substituting the β n value into the characteristic equation for the step-index fibers with 1/2  N A = n 2co − n 2F S M

(6.44)

a new normalized frequency V is obtained. Finally, the effective core radius is given by the ratio ρ = V/V t . By plotting ρ versus the normalized air-hole diameter d/ , it can be shown that, in the limit of short wavelengths compared to the air-hole size, that is d/λ ≥ 2, and for low air-filling fractions, that is d/  ≤ 0.5, the effective core radius tends to a constant value regardless of d/ . An FEM analysis can be used to evaluate the guided-mode field distribution in PCFs, as well as the effective area and the nonlinear coefficient. First, the magnetic field H = (H x , H y , H z ) on the fiber cross-section is calculated and then, from the expression of H, the electric field E = (E x , E y , E z ) is obtained through the Maxwell equation. Hence, from the definition of the Poynting vector, the normalized intensity is given by   ∗ E×H 1 ·z i(x, y) = Re p 2 

(6.45)

where P is the integral of the intensity over the section of the PCF, that is,  ∗ E×H · zˆ dxdy P= Re 2 s   ¨ E x × Hy∗ · zˆ dxdy = Re 2 ¨



(6.46)

s

Then, the effective area of the PCF fundamental guided mode can be calculated according to

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6 Chalcogenide Fiber Structures: Design and Performance Analysis

1

Ae f f = ˜ s

i 2 (x,

y)dxdy

(6.47)

where i(x, y) is the guided-mode normalized intensity distribution, the nonlinear coefficient can be evaluated as ¨ γ = (2π/λ) · n 2 (x, y)i 2 (x, y)dxdy (6.48) S

where n2 (x, y) is 3×10−20 m2 /W in the silica bulk and 0 in the air-holes, and i(x, y) is the normalized intensity. In a PCF with an infinite number of air-holes in the photonic crystal cladding, the propagation is theoretically lossless. However, in the fabricated fibers the number of air-holes must be finite, so the guided modes are leaky and loss occurs. The confinement loss CL of the mode is deduced by C L = 20αlog10 e = 8.686α(dB/m)

(6.49)

Until 2000, the host materials of PCF were based on oxide glass, then Monro et al. [97] fabricated the first chalcogenide glass micro structured optical fiber, in which the chalcogenide glass gallium lanthanum sulphide was used. In 2003, Burak et al. [98] reported a hollow optical fiber lined with an interior omnidirectional dielectric mirror. The hollow core is surrounded by AsSe glass (n = 2.8) and a thermoplastic polymer, polyether sulphone (PES) (n = 1.55), respectively. The result shows that the fundamental and high-order transmission windows are determined by the layer dimensions, and can be tuned from a wavelength of 0.75–10.6 μm. In 2005, Shaw et al. [99] reported on broadband IR light source generation in As-Se based PCF. In 2005, the first Holey Fibers (HF) in GaGeSbS chalcogenide glass were manufactured by Brilland [100]. They demonstrated the possibility of fabrication of complex structures up to three rings using the “Stack & Draw” technique. In 2006, Smektala et al. [101] fabricated several holey fibers based on Ge20 Ga5 Sb10 S65 chalcogenide glass, and demonstrated the possibility to achieve complex structures using up to 3 rings of holes, again using the stack and draw process. In 2006, Ju et al. [102] investigated single-polarization single-mode operation in a highly birefringent PCF using a full-vector finite-element method with anisotropic perfectly matched layers, and found that, the cutoff wavelengths of the two linearly polarized principal states could be designed by varying the structure parameters of the PCF. In 2007, Smektala et al. [103] reviewed the recent achievements in fabrication of chalcogenide glasses PCFs by the stack and draw technique. Figure 6.34 shows the cross-section images of the chalcogenide PCFs obtained with by varying the processing parameters. It was demonstrated that the effective areas can be changed typically from 1000 to 10 μm2 by changing the structure as shown in Fig. 6.35. The working wavelength range of these fibers is from 1 to 6 μm. The losses at 1.55 μm can reach up to 6 dB/m.

6.6 PCF

207

Fig. 6.34 Chalcogenide PCFs obtained with variable process parameters. Adapted from Ref. [103], with permission from the SPIE

Fig. 6.35 Chalcogenide PCFs with various effective area (Aeff ). Adapted from Ref. [103], with permission from the SPIE

In 2008, Désévédavy et al. [104] prepared small-core Ge15 Sb20 S65 chalcogenide microstrutured fiber with regular profiles by improved “Stack & Draw” technique. Figure 6.36 shows the cross-section of the optical fiber. In 2009, Désévédavy et al. [105] presented a chalcogenide microstructured optical fiber based on Te–As–Se glass. Figure 6.37 shows the cross-section of the optical fiber. In 2010, El-Amraoui et al. [106] fabricated As2 S3 MOFs with high purity arsenic sulfide glasses. Mechanical drilling was used for the preparation of the preforms and then MOFs were drawn out including suspended core fibers. A cross-section of the optical fiber is shown in Fig. 6.38. Low losses MOFs are obtained in this way, with a background loss less than 0.5 dB/m.

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6 Chalcogenide Fiber Structures: Design and Performance Analysis

Fig. 6.36 Cross-section of the fiber. Adapted from Ref. [104], with permission from the Optical Society

Fig. 6.37 Cross-section of the optical fiber. Adapted from Ref. [105], with permission from the Optical Society

Fig. 6.38 Cross-section of the optical fiber. Adapted from Ref. [106], with permission from the Optical Society

6.6 PCF

209

Fig. 6.39 Cross-section of the optical fiber. Adapted from Ref. [107], with permission from the Academic Press Inc.

In 2015, Zhang et al. [107] proposed a mechanical drilling method for the preparation of PCFs based on Ge20 Sb15 S65 chalcogenide glasses. Figure 6.39 shows the cross-section of the PCFs. In 2019, Han et al. [108] designed a novel all-normal flat near-zero dispersion chalcogenide PCF. The fiber cores were made of As2 Se3 glass, the uniform air holes in the cladding were selectively filled with As2 S5 glass. Figure 6.40 shows the cross-section image of the PCF, the plots of dispersion versus wavelength (a) for three different d values and (b) for three different  values, respectively. The results demonstrate that an all-normal flat near-zero dispersion and a high nonlinearity is possible for this fiber.

Fig. 6.40 Cross-section of the optical fiber and plots of D versus wavelength. Adapted from Ref. [108], with permission from the IOP Publishing Ltd. a for three different d values and b for three different  values, respectively. Adapted from Ref. [108], with permission from the IOP Publishing Ltd.

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6 Chalcogenide Fiber Structures: Design and Performance Analysis

6.7 Conclusion Different optical fiber structures can be applied in different situations. The total chromatic dispersion of chalcogenide multi-cladding fiber can be tuned if a high degree of flexibility is allowed in the fiber structure design. For example, blue-shifting of the ZDW can be realized, or/and all positive dispersion can also be obtained. In the M-typed double-cladding fiber, the mode dispersion in the core can be blue shifted to less than 3 μm by limiting the mode number in the core and controlling the mode type in the inner cladding. The ZDW can also be blue-shifted by optimized the structure of suspended core fiber. A high nonlinearity and small ZDW can be obtained for tapered fiber. The emergence of photonic crystal fiber makes the fiber structure even more innovative. Dispersion tuned PCFs and polarized PCFs can be obtained with the special PCF structures. These reports form the foundation for developing new optical fiber structures in the future, such as mid- and far-infrared high-transparency optical fiber, multi-core optical fiber. To some degree, they can also improve the laser damage threshold of chalcogenide fiber and achieve other performance enhancements, for example, negative curvature fibers can realize ultralow loss transmission using a special structure. These fibers can be expected to have great potential for applications in mid- and far-infrared integrated laser technology in the future.

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Chapter 7

Mid-Infrared Spectral Properties of Rare Earth Ion Doped Chalcogenide Glasses and Fibers Haitao Guo, Jian Cui, Chenyu Xu, Yantao Xu, and Gerald Farrell

Rare earth (RE) ion doped transparent optical materials such as laser glasses and laser fibers are increasingly being deployed in a variety of applications. At present, most of the substrates for these photoelectric functional materials are oxides or fluorides. The mid-infrared (MIR) luminous efficiency of lanthanide ions in the matrix is related to the phonon energy of the matrix. Compared to oxide and fluoride glasses, chalcogenide glasses have a low phonon energy (200–350 cm−1 ), low multi-phonon relaxation probability and high radiation transition rate for the RE ions. As a result of their exceptionally wide infrared transmission window, the MIR luminescence of RE ions doped chalcogenide glasses has drawn great attention in past few years. In addition, chalcogenide glass have a larger refractive index (n > 2.3) than silica. According to Judd-Ofelt (J-O) theory, it has a large dipole oscillation intensity, which causes a strong local electric field around RE ions in the chalcogenide glass and an induced larger stimulated emission cross section. For applications in the MIR wavelength region, a coherent light source based on fiber lasers and amplifiers has greater potential, in many areas such as the remote sensing, range finding, environmental detection, bioengineering, medical and military, etc. [1]. For now, fiber lasers with wavelengths greater than 2.5 μm are mainly made of fluoride-based RE-doped fibers, and the longest wavelength of the reported fiber laser is 3.95 μm [2] with a cooled working environment. The working wavelength of fiber lasers continues to move toward longer wavelength. To enable new low-phonon energy gain fiber, firstly dielectric materials must be developed to reduce the probability of non-radiative transition. Secondly, we must improve the coordination environment of the matrix material and increase the doping concentration and quantum efficiency of RE ions.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Wang et al., Mid-Infrared Fluoride and Chalcogenide Glasses and Fibers, Progress in Optical Science and Photonics 18, https://doi.org/10.1007/978-981-16-7941-4_7

217

218

7 Mid-Infrared Spectral Properties of Rare Earth …

7.1 RE Ion Species and MIR Energy Level Transition Mechanism Because of their special electronic layer structures, RE ions have some special spectral properties. RE luminescence covers almost the entire category of solid luminescence. That is why the presence of RE ions are almost essential when it comes to luminescence. RE atoms have an underfilled 4f electronic configuration shielded by an outer layer, so they have abundant electronic energy levels and long-lived excited states. There are more than 200 thousand energy level transition channels, which can produce a variety of radiation absorption and emission phenomena. Their spectrum has about 30 thousand observable lines, and they can emit electromagnetic radiation of various wavelengths from ultraviolet light, visible light to infrared light region [3]. The rich energy levels of RE ions and the transition characteristics of 4f electrons make it a very fertile environment from which more new luminescent materials can be discovered, constituting many luminous and laser materials. The average lifetime of some excited states of RE ions is as long as 10–2 – −6 10 s (metastable state) [4]. Because the spontaneous transition between 4f-4f is a forbidden transition, and the transition probability is very small, so the excitation state life is very long. Due to the special spectral characteristics of RE ions, they can be used as fluorescent materials and laser materials. At present, more than 90% of laser materials are RE ions doped materials. Three divalent RE ions and eleven trivalent RE ions have been discovered, which can be used as laser materials, and the laser wavelength ranges from the ultraviolet to MIR.

7.1.1 The Electronic Layer Configuration of RE Elements RE elements are the general term for scandium, yttrium and lanthanides in group IIIB of the periodic table, as shown in Fig. 7.1. The characteristic of the electronic layer structure of the lanthanide RE element is that the electrons are filled on the 4f orbital of the outer third layer, the angular quantum number of the 4f orbital (marked as l) is equal to 3, and the magnetic quantum number m can be 0, ± 1, ± 2, ± 3 for the 4f sublayer with 7 tracks. According to Pauli’s principle of incompatibility, there are no two electrons with the same four quantum numbers in the same atom, that 21 Sc 3d14s2 44.96 39 Y 4d15s2 88.91 58 Ce 59 Pr 60 Nd 61 Pm 62 Sm 63 Eu 64 Gd 65 Tb 66 Dy 67 Ho 68 Er 69 Tm 70 Yb 71 Lu La 57 1 La 2 1 1 2 3 2 4 2 5 2 6 2 7 2 8 2 9 2 10 2 11 2 12 2 13 2 14 2 14 1 2 - 5d 6s 4f 5d 6s 4f 6s 4f 6s 4f 6s 4f 6s 4f 6s 4f 6s 4f 6s 4f 6s 4f 6s 4f 6s 4f 6s 4f 6s 4f 5d 6s 138.9 Lu 138.9 140.1 140.9 144.2 145.0 150.4 152.0 157.3 158.9 162.5 164.9 167.3 168.9 173.0

Fig. 7.1 Types of RE ions

7.1 RE Ion Species and MIR Energy Level Transition Mechanism

219

is, an atomic orbit can only accommodate two electrons with opposite spins. The 4f sublayer can only accommodate 14 electrons. From La to Lu, 4f electrons increase from 0 to 14. The electron shell configurations of scandium and yttrium are: Sc 1s2 2s2 2p6 3s2 3p6 3d1 4s2 ; Y 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s2 ; The electron shell configuration of lanthanide atoms is: [Xe] 4fn 5dn’ 6s2 , n = 0–14, n’ = 0 或 1; where [Xe] is the electronic layer structure of xenon atoms, 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 . When forming trivalent RE ions, the 6 s and 5d electrons are lost first, so that the trivalent RE ions have a 4f n electronic structure that increases sequentially, n = 0, 1,…, 14, corresponding to La3+ , Ce3+ , …, Lu3+ ions, respectively. Y3+ and La3+ without 4f electrons and Lu3+ (4f14 ) full of 4f electrons all have a closed shell, so they are colorless and are optically inert, which means they are very suitable as matrix elements for luminescent materials.

7.1.2 RE Ions and Energy Level Transitions that Produce MIR Transitions Lanthanides have the same outer electronic structure 4fn 5s2 5p6 6s2 , and usually lose two electrons from 6 s and one electron from 4f to form a trivalent ion, and 5 s and 5p electrons do not participate in bonding. The remaining 4f electrons are shielded by the outer electron layers 5 s and 5p, and the coordination field has little effect on them. The transitions caused by optical absorption and emission all occur in the 4f layer. Since there are 1639 transition energy levels within the f configuration, the number of possible transitions between energy level pairs is as high as 199, and the observable spectral lines are as high as 30,000. Coupled with the transition between f-d configurations, the number is even more. But in order to achieve a 3–5 μm transition, the energy interval between the upper and lower transition levels needs to be between 2000 and 3300 cm−1 . According to classical spectroscopy theory, the total transition probability (W ) of RE ions in an excited state is equal to the sum of the radiation transition probability (Arad ) and the non-radiation transition probability (W nr ). Therefore, the probability of MIR radiation transition between RE ion energy levels is often affected by the matrix material, that is, the interaction between the RE ion and the matrix. This non-radiation transition process is generally regarded as a multi-phonon relaxation process. The non-radiative transition probability W nr caused by multi-phonon relaxation can be described by the Miyakawa-Dexter equation [5]: Wnr = W0 exp(−α

E ) hν

(7.1)

220

7 Mid-Infrared Spectral Properties of Rare Earth …

where α = ln(P/g) − 1 and p = E . W 0 is the transfer rate when the band gap hν is zero and there is no phonon emission, which is a constant. g is the electron– , where E is the energy phonon coupling strength, P is the phonon order, P = E hν interval between energy levels, and hν is the phonon energy. It can be seen that the non-radiative transition probability of a multi-phonon process is first determined by the phonon order, that is, the energy interval between energy levels and the phonon energy. The former is determined by the RE ion’s energy level structure and the latter is determined by the matrix itself. From Eq. 7.1, when the energy interval E between the two energy levels is fixed, the multi-phonon relaxation rate is mainly determined by the high-energy phonons in the material lattice vibration. The higher the phonon frequency, the greater the probability of multi-phonon non-radiation relaxation. Sudo et al. [6] shows the relationship between the multi-phonon relaxation rate W nr of RE ions in different glass matrix materials and the two-level energy interval E of the transition. According to the phonon energy of the glass matrix, oxide glasses (including the borates, phosphates and silicates) have high phonon energy, and the multi-phonon relaxation rate is large, whereas the multi-phonon relaxation rate of chalcogenide glass is small due to its low phonon energy (200–350 cm−1 ). In common oxide glasses, due to the generally high phonon energy of the matrix material, the RE ion has a high probability of multi-phonon relaxation, resulting in a very small probability of the upper and lower energy level radiation transition corresponding to the MIR band transition, which makes it difficult to obtain fluorescence in the infrared range. To achieve fluorescence above 3 μm in RE ions doped glass materials, two conditions must be met. One is that the phonon energy of the material must be small thus reducing the multi-phonon relaxation rate and improving the quantum efficiency of the MIR transition; The second is that the material itself needs to have good transmission characteristics both for the pump light and the MIR emission band to ensure that the RE ions doped therein can be excited to a higher energy level efficiently. Both chalcogenide and fluoride glasses meet these two conditions. In addition, the larger refractive index in combination with lower phonon energy of chalcogenide glasses make the oscillator strength and quantum transition efficiency of RE ions doped therein higher than those in fluoride glasses. So far, at room temperature, MIR fluorescence greater than 3 μm can be obtained in RE ions doped chalcogenide glasses. Six kinds of RE ions involved in the MIR fluorescence are Er3+ , Ho3+ , Dy3+ , Pr3+ , Tm3+ and Tb3+ . Figure 7.2 shows the simplified energy level diagrams of these RE ions, and the transitions corresponding to the 3–5 μm fluorescence are marked in the diagram. The branch ratio (β) is an important parameter to evaluate the efficiency of a certain transition. It equals to the transition probability ratio between a required spontaneous transition and all possible transitions from a certain high energy state to low energy levels. Based on the absorption spectra of RE ions in chalcogenide glasses, the researchers calculated the transition branch ratios of the six RE ion species above to produce 3–5 μm transitions through the J-O theory. These data are summarized in Table 7.1 [5, 7–11].

7.1 RE Ion Species and MIR Energy Level Transition Mechanism

221

Fig. 7.2 The energy level structure of RE ions emitting 3–5 μm fluorescence and the comparison of emission spectra in different matrix glasses Table 7.1 Mid-infrared fluorescence transition characteristics of different RE ions [5, 7–11] Rare-earth Transition ions Pr3+

Er3+ Ho3+ Tb3+ Tm3+ Dy3+

3H → 3H 4 5 3H → 3H 6 5 3F → 3H 2 5 4I 4 9/2 → I11/2 4F 4 9/2 → I9/2 5I → 5I 6 7 5I → 5I 6 5 7F → 7F 4 5 7F → 7F 6 5 3H → 3F 4 5 6H 6H → 11/2 13/2 6H 6 13/2 → H15/2

λex /μm λem /μm ASE/S−1 β/% σ e /(× τ /ms 10–20 cm2 ) 2.02

σ e τ /(× 10–23 cm2 s)

4.815

54.5

100

8.56

18.34 156.99

4.619

66.8

45.5

8.89

14.97 133.08

3.537

139.5

87.3

6.39

7.17

45.82

4.53

8

0.8

0.88

125

110

3.452

–

–

–

–

–

2.815

104

17.4

1.93

9.62

18.57

3.867

43

10.5

2.84

23.25 66.03

7.5

169.5

8.76

7.59

5.9

44.78

5.01

84.7

–

10.80

11.8

127.44

0.8

3.8

32

0.04

1.97

31.25 61.56

0.81

4.38

29.8

9.3

3.24

33.5

108.54

2.86

113.8

100

2.25

8.79

19.78

0.804 0.9 2.950

222

7 Mid-Infrared Spectral Properties of Rare Earth …

The purpose of the study on RE doped chalcogenide glasses lies in the MIR fiber lasers and amplifiers, thus it is necessary to investigate their gain characteristics. The excited transition gain of three-level and four-level RE ions in the fiber can be expressed by the formula 7.2 [12]. From the formula 7.2, it can be seen that the gain g(L) is proportional to the product of the stimulated emission cross section and the fluorescence lifetime; therefore, one can use this product as the parameter of fiber laser performance-laser quality factor (σe τ ). Table 7.1 shows the transition characteristics of RE ions with emission wavelengths around and above 3 μm in chalcogenide glasses. The laser quality factors (σe τ ) corresponding to different wavelengths of each RE ion are calculated. The quality factor of the Yb3+ ion laser in the quartz fiber is only 0.6 × 10–23 cm2 s, which is dozens or hundreds times smaller than that in the chalcogenide glass. However, the laser output from chalcogenide RE doped fiber is mainly limited by the large fiber loss. Theoretical simulation n shows that when the fiber loss is less than 1 dB/m, Dy3+ doped chalcogenide fiber can provide a laser output of 4.5 μm with a slope efficiency of 0.16 [13]. g(L) = −σa N0 ηs L + (1 + γs )

σe τ Pabs F ξ hν p A η p

(7.2)

By comparing the statistical and calculated parameters in Table 7.1, it is easy to find that the Pr3+ and Dy3+ ions have the largest laser quality factors. Therefore, according to formula 7.2, it can be seen that they have the largest gain coefficients and are most likely to achieve MIR laser output. The Pr3+ : 3 H5 → 3 H4 transition has the largest fluorescence branch ratio of 100%. In addition, the lifetime of the upper and lower energy levels of the laser is also an important factor affecting the laser generation; the long lifetime of the upper energy level will result in a larger σe τ , but the radiation probability of the upper energy level will be small. A shorter upper energy lifetime means the material cannot store a large amount of energy or even form an effective population inversion. The lifetime of lower energy level should be as low as possible, in order to clear the number of particles at the lower energy level in time and maintain a large number of inverted particles (n). For the Dy3+ ion 6 H11/2 → 6 H13/2 transition, the lower-level fluorescence lifetime is as long as 8.79 ms, and it cannot be emptied in time to affect the operation of the laser. It is necessary to adopt the form of a cascade laser to reduce the particles on the lower-level 6 H13/2 . For the Pr3+ ion 3 H5 → 3 H4 transition, the lower energy level of the laser is close to the ground state to form a quasi-four-level system without the bottleneck of the accumulation of lower energy levels.

7.2 Local Field Characteristics of RE Ions …

223

7.2 Local Field Characteristics of RE Ions in the Chalcogenide Glass Structure 7.2.1 Multi-phonon Relaxation Multi-phonon relaxation is an important factor affecting fluorescence lifetime and quantum efficiency. The multi-phonon relaxation rate Wmp can be obtained from the measured fluorescence lifetime τm and the calculated theoretical fluorescence lifetime τ R . The calculation formula is shown in formula 7.3 [14]: Wmp =

1 1 − τm τR

(7.3)

Yong et al. [15] studied the multi-phonon relaxation changes of RE ions before and after the introduction of halide into Ge–Ga–S glass (Ge0.25 Ga0.1 S0.65 in composition). They found that the introduction of CsBr will greatly reduce the multi-phonon relaxation rate. Taking Dy3+ for example, the 6 F11/2 /6 H9/2 energy level is the upper energy level of 1.31 μm fluorescence. After the introduction of CsBr, the multiphonon relaxation rate is reduced by 4 orders of magnitude. In order to understand the quantitative changes of local phonon modes, the temperature dependence of the Dy3+ excited state’s multi-phonon relaxation rate was studied [16]. With the temperature increasing from 50 to 150 K, the multi-phonon relaxation rate increases slowly, and the temperature continues to increase, the multi-phonon relaxation rate increases rapidly. Formula 7.4 provides the relationship between multi-phonon relaxation and temperature T. For glass without CsBr, it is assumed that the phonon energy of a single vibration mode is 375 cm−1 , and five phonons participate in the transition process. Wmp (T ) = Wmp (0)



(n i + 1) pi

(7.4)

i

In formula 7.4, W mp (0) and ni represent the multi-phonon relaxation rate and the Bose–Einstein occupancy number at temperature of 0 K, respectively. pi is the number of phonons required to reach the energy level difference ΔE, and the vibration of 375 cm−1 is caused by the asymmetric stretching vibration of the GeS4 tetrahedron [17]. However, these parameters are not applicable to glass containing CsBr. It indicates that the glass containing CsBr has different phonon energies, and the stretching vibration of the Ga-Br bond is the main phonon mode participating in the relaxation process.

224

7 Mid-Infrared Spectral Properties of Rare Earth …

7.2.2 Extended X-Ray Absorption Fine Structure Spectrum

Fourier transformation/a.u.

Extended X-ray absorption fine structure spectroscopy (EXAFS) can be used to study the local structure of atoms in the glass. For example, researchers have used EXAFS to investigate the local structure of Ge, Ga and Tm3+ in (1 − x) (Ge0.25 Ga0.10 S0.65 )xCsBr (x = 0.00, 0.05, 0.10 and 0.12) glasses [18]. To prevent the distortion of EXAFS spectrum, Tm2 S3 and TmBr3 are tested in transmission mode. The glass containing Tm3+ ions is tested by a fluorescence method using a Laiter detector equipped with a Co filter. There are standard methods and detailed procedures can be found in the literature for analyzing EXAFS data [18]. Figure 7.3 shows the Fourier transform spectra of the Ga ion test data in (1x)(Ge0.25 Ga0.10 S0.65 )-xCsBr glasses. This figure shows that doping of CsBr will cause the change of the radial distribution function of Ga ion in the glass. The main peak indicating the distance between atoms in the figure is derived from the Ga–S bond in the glass (after phase correction) [19]. After adding CsBr, the intensity of this main peak increases. At the same time, due to the formation of Ga-Br bonds, a new peak appears at 2.36 ± 0.03 Å (after phase correction) [20]. The curve fitting results show that Ga ions are surrounded by four S atoms in the Ge0.25 Ga0.10 S0.65 glass. On the other hand, as the concentration of CsBr increases, that is, as the coordination number of Br increases, the average coordinated S around Ga decreases accordingly. In particular, in the 0.90(Ge0.25 Ga0.10 S0.65 )-0.10CsBr glass, the number of S in the first coordination layer immediately adjacent to Ga is reduced to three, and the fourth

Distance/Å Fig. 7.3 Radial distribution function (RDF) curves of Ga ions in (1 − x) (Ge0.25 Ga0.10 S0.65 )-xCsBr glasses without phase correction, a x = 0.00, b x = 0.05, c x = 0.10, d x = 0.12 [18]

7.2 Local Field Characteristics of RE Ions …

225

Table 7.2 Coordination number (N), bond length (R), Debye–Waller factor and bond length R fitting deviation of Tm-S and Tm-Br bonds in glasses and crystals [18] Chemical bond

Composition

R(Å)

N

σ2 (Å2 )

R factor

Tm–S

Tm2 S3 crystal

2.74 (0.01)

6.50

0.0115 (0.0015)

0.013

Ge0.25 Ga0.1 S0.65 glass

2.77 (0.01)

6.77 (0.85)

0.0107 (0.0014)

0.015

TmBr3 crystal

2.79 (0.01)

6.00

0.0066 (0.0013)

0.001

0.9(Ge0.25 Ga0.1 S0.65 )-0.1CsBr glass

2.79 (0.01)

5.86 (1.58)

0.0083 (0.0018)

0.020

Tm–Br

coordination ion becomes Br. After adding of CsBr, the main peak in the RDF curve of Ga changed and became similar to the RDF curve of TmBr3 . These results indicate that in the 0.90(Ge0.25 Ga0.10 S0.65 )-0.1CsBr glass, Tm3+ ions are mainly surrounded by Br− ions. The fitting results in Table 7.2 show that in the Ge0.25 Ga0.10 S0.65 glass, Tm3+ ions are surrounded by 6.77 (±0.85) S2− ions. After adding of CsBr, the first coordination shell surrounding the Tm3+ ion is composed of 5.86 (±1.58) Br− ions. This result supports the theory that Tm3+ ions in the 0.90(Ge0.25 Ga0.10 S0.65 )-0.1CsBr glass are mainly surrounded by Br− ions [21].

7.3 MIR Luminescence Characteristics of RE Doped Chalcogenide Glasses The MIR 3–5 μm wavelength range, which is corresponds to the characteristic molecular absorption or vibration energy bands of most gas, liquid and solid phases is of great importance; and it is located within the atmospheric transmission window, which further emphasizes the importance of the MIR spectral range. The characteristic absorption of the target molecule enables one to detect specific gases, which is very useful for detecting harmful gases in the air or chemical reaction gases in the manufacturing process. Biological tissues also have typical fingerprint spectra in the MIR spectrum, and MIR light sources can be used for biological detection and modification. In addition the atmospheric transmission window ensures that major requirements can be met for military purposes, such as secure communications, missile guidance, object detection, etc. [1]. However, there is still no reliable solution for high-power MIR light sources such as lasers, which hinders large-scale development and the expansion of applications. III-V diode lasers based on antimonides (such as AlGaAsSb, InGaAsSb, and InAsSbP) or IV-VI diode lasers based on lead salts (such as PbSSe or PbSnTe) have shown promise for MIR lasers, but their output power (less than 1 mW) and the quantum efficiency is very low. The quantum cascade (QC) structure can also generate

226

7 Mid-Infrared Spectral Properties of Rare Earth …

laser in the MIR range, but requires a low temperature environment and the output power is again low at only a few mW [22, 23]. The near-infrared frequency conversion using optical parametric oscillator (OPO) and difference frequency generation (DFG) can produce tunable MIR laser, but this requires complex optical devices and results in low output power [23]. ZBLAN glass doped with Ho3+ , Er3+ and Dy3+ ions can obtain lasers greater than 1 W below 3 μm [24–26]. Although lasers operating above 3 μm can be implemented by increasing the pump source power [2, 27, 28], this is difficult to achieve. Therefore, it is very difficult to manufacture compact, high-power and economical all-fiber lasers with traditional fluoride glass fibers. Compared with fluoride glass, chalcogenide glass has lower phonon energy (< 350 cm−1 ) and a wider infrared transmission window of more than 10 μm. The quantum efficiency and radiation lifetime of the MIR transition of RE ions largely depend on the energy gap width and the phonon energy of the glass matrix [29]. So far, MIR radiation has been reported in chalcogenide glasses [10, 30–34]. In recent years, the increasing demand and interest in environmental protection, biomedical and military applications have further promoted the research of chalcogenide glass as a MIR gain medium. The United States, Russia, the United Kingdom, Germany, France, Japan, South Korea, and China have all carried out research in this area. In 1995, Shin et al. [35] of Pohang University of Science and Technology in South Korea first reported 2.9 μm fluorescence, which was obtained in Ho3+ : Ge–As–S glass. Subsequently, Heo et al. [5] also observed a fluorescence output of 2.9 μm in Dy3+ : Ge-As(Ga)–S glass. In 1997, Schweizer et al. [36] of Southampton University in the United Kingdom obtained 3.4, 3.6 and 4.3 μm fluorescence in Pr3+ and Er3+ ion-doped Ga–La–S glass. Then in 1999, Shin et al. [37] observed 4.4 μm fluorescence related with Dy3+ :6 H11/2 → 6 H13/2 transition in Ge–Ga–S chalcogenide glass. Subsequently, many RE-doped chalcogenide glasses have achieved 3–5 μm fluorescence output. The optical fiber amplification output of 4.0, 4.5 and 5.0 μm signals were realized experimentally for the first time in the Pr3+ doped selenium-based chalcogenide glass fiber, and the optical conversion efficiency of the pump light signal at 4.5 μm was 45% using 2 μm reverberation pumping method [38]. Table 7.3 summarizes the reported data of 3–5 μm MIR fluorescence in RE ions doped chalcogenide glasses. The research progress highlights for chalcogenide glasses doped with different RE ions were listed in Table 7.3.

7.3.1 MIR Luminescence of Dy3+ Doped Chalcogenide Glass The energy level structure of a dysprosium ion (Dy3+ ) and the corresponding energy level transitions are shown in Fig. 7.4. Dy3+ can produce near-infrared and MIR emissions at 1.34, 1.76, 2.86 and 4.36 μm. The 1.76 μm emission produced by Dy3+ ions are exactly in the L-band of optical communication systems. On the other hand, the transition from the ground state 6 H15/2 energy level to the excited state 6 H5/2 energy level of Dy3+ ions is just around 800 nm, which is very suitable for pumping

0.1–0.3 mol.% 0.1–0.3 mol.%

2000 ppm

500–1000 ppm

500–5000 ppm

0.1 mol.%

1000 ppm

500 ppm

500 ppm

500 ppm

500–1000 ppm

1000 ppm

500 ppm

10,000 ppm

500 ppm

500 ppm

500 ppm 1000 ppm

Er3+

Pr3+

Tb3+

Dy3+

Pr3+

Dy3+

Tb3+

Dy3+

Er3+

Tb3+

Pr3+

Pr3+

Tm3+

Pr3+

Pr3+

Pr3+

Ge–As–Ga–Se

Ge–As–Se–Ga(In,I)

Ga5 Ge20 Sb10 Se65

Ga0.8 As39.2 S60

Ge10 As24 Ga4 Se62

Ge–Sb–S–I

GeAsGaSe

Ga8 Sb28 As4 S60

Ga–La–S

GeAsGaSe

GeAsGaSe

Ge16.5 Ga3 –As16 Se64.5

Ga0.8 As39.2 –S60

GeAsInSe

GeGaAsSe

GeAsInSe

–

–

–

193

226

–

–

–

236

–

286

–

194

272

–

–

–

2 × 1019 cm−3

Tb3+

Ge36 Ga5 Se59

Pr3+

T g /°C

Concentration/ppm

Doped ions

Composition

4.7

1.70 4.50–5.0

3.5–6.0

1.2 1.4 1.8

3.5–6.0

4.0/4.5/5.0

4.8

3.6

4.1

4.7

4.3

4.80

4.2

8

4.8

2.7

4.9–5.5

Output wavelength/μm

Table 7.3 Summary of mid-infrared output of RE doped chalcogenide glasses

10.1 9.0

0.27 11.5

–

0.68 0.12 –

7.80

–

12.8

0.11

1.38

–

5.4

10.74

1.6

–

–

–

–

Lifetime/ms

Fluorescence

Fluorescence

Fiber Loss

Fluorescence

Fluorescence

Amplifier

Fluorescence

Modeling

Loss

Modeling

Fluorescence

Loss

Fluorescence

Fluorescence

Fluorescence

Modeling

Laser oscillation

Remarks

2014

2014

2014

2015

2015

2015

2015

2015

2015

2017

2017

2017

2018

2018

2018

2020

2020

Year

(continued)

[53]

[52]

[51]

[50]

[49]

[38]

[48]

[47]

[43]

[46]

[45]

[44]

[43]

[42]

[41]

[40]

[39]

References

7.3 MIR Luminescence Characteristics of RE Doped … 227

– –

1000–2000 ppm 500–1500 ppm 500–1500 ppm

(3 × 1019 ion cm−3 )

10,000 ppm

(7 × 1019 ion cm−3 )

1000 ppm

200 ppm 200 ppm

–

750 ppm

(1.57 mol%)

Dy3+ Pr3+ Tb3+

Dy3+

Er3+

Dy3+

Er3+

Pr3+ Dy3+

Dy3+

Pr3+

Er3+

Ge16.5 Ga3 –As16 Se64.5

GeGaAsSe

Ga5 Ge20 –Sb10 S65

GeGaAsSe

Ga5 Ge20 –Sb10 S65

GeGaAsSe

‘selenide’

‘selenide’

70Ga2 S3 :30La2 S3 (O3 )

–

–

–

–

300

–

–

–

500–10,000 ppm 500–10,000 ppm

Dy3+ Pr3+

Ge20 Ga5 –Sb10 S65

T g /°C

Concentration/ppm

Doped ions

Composition

Table 7.3 (continued)

3.62 4.53

3.50–5.5

~ 4.50

– –

4.50

4.20–4.7

4.5–4.65

4.60

4.60 4.89 7.50

4.20–4.5 3.50–5.5

Output wavelength/μm

0.10 0.59

–

–

– –

0.72

–

–

–

2.20 2.70 5.90

2.30 –

Lifetime/ms

Fluorescence

Fluorescence

Fiber Loss

Fiber Loss

Fluorescence

Modeling

Modeling

Modeling

Fluorescence

Fluorescence

Remarks

1997

1999

2000

2000

2008

2008

2009

2010

2012

2013

Year

[13]

[34]

[33]

[58]

[8]

[57]

[56]

[55]

[11]

[54]

References

228 7 Mid-Infrared Spectral Properties of Rare Earth …

7.3 MIR Luminescence Characteristics of RE Doped …

229

Fig. 7.4 Dy3+ ion energy level and related absorption spectrum

with common commercial solid-state lasers. It can be seen from the figure that the Dy3+ ion contains a variety of MIR fluorescences, among which the 6 H11/2 → 6 H15/2 , 6 H13/2 → 6 H15/2 transitions correspond to the 2.86 and 4.36 μm fluorescence; when the particles are pumped to 6 H11/2 energy level, the 2.86 μm transition belongs to the laser three-level system; when pumped to higher energy levels such as 6 H9/2 , 6 F11/2 , the 4.36 μm fluorescence radiation satisfies the laser four-level system. However, because the lifetime of 6 H13/2 level is much longer than the upper 6 H11/2 one, leading to the laser self-termination effect, the cascade laser output mode must be used to solve the particle accumulation at the laser lower energy level to achieve efficient 4.36 μm laser output. The reasonable distribution of Dy3+ ion energy levels form a threelevel or four-level system, which is more conducive to reversing the accumulation of population and laser oscillation output. In fact, research on Dy3+ ion-doped chalcogenide glasses can be traced back to the research on the base materials of fiber amplifiers for the 1.3 μm band in the 1990s. The RE ions which can achieve 1.3 μm optical amplification mainly include Nd3+ ion (4 F3/2 → 4 I13/2 with fluorescence center wavelength at 1300 nm), Pr3+ ion (1 G4 → 3 H5 transition with fluorescence center wavelength at around 1300 nm) and Dy3+ ion (6 F11/2 , 6 H9/2 → 6 H13/2 transition with fluorescence center wavelength at around 1320 nm). In 1994, K. Wei of Rutgers University in New Jersey and Tanabe of Kyoto University in Japan proposed the application of Dy3+ ion doped chalcogenide glass for a 1.3 μm fiber amplifier [59, 60]. Their research showed that the stimulated emission cross sections of Dy3+ : 6 F11/2 , 6 H9/2 → 6 H15/2 are four times larger than that of Pr3+ : 1 G4 → 3 H5 , and the transition fluorescence branch ratio exceeds 90%. However, when sulfide is used as the matrix glass, the lifetime of Dy3+ ions at 1.31 μm is only 35 μs, which is only one tenth of Pr3+ , and the quantum efficiency is as low as ~ 17% [60]. Since 2000, Shin et al. [16] have reported on Dy3+ doped Ge–Ga–S chalcogenide glasses with alkaline halide addition. The glass system has significantly increased luminescence lifetime and quantum efficiency at 1.3 μm. Therefore, research on the luminescence properties of Dy3+ doped chalcogenide glasses has attracted more and more attention.

230

7 Mid-Infrared Spectral Properties of Rare Earth …

The near-infrared and MIR fluorescence obtained in the Dy3+ : Ge25 Ga5 S70 chalcogenide glass under 810 nm laser pumping are shown in Fig. 7.5 [9]. The three fluorescence transition characteristics of Dy3+ : (6 H9/2 , 6F11/2 ) → 6 H15/2 , 6 H11/2 → 6 H15/2 and 6 H13/2 → 6 H15/2 are shown in Table 7.4. Table 7.5 presents the relationship between the fluorescence lifetime and the doping concentration. As the Dy3+ ion

6

Dy3+:Ge25Ga5S70

Dy3+:Ge25Ga5S70

H11/2→6H15/2

H9/2, 6F11/2 6

H13/2→6H15/2

H15/2

Pumping 810nm 6

H11/2→6H13/2

Intensity/a.u.

6 6

→

Intensity/a.u.

Pumping 810nm

Wavelength/nm

Wavelength/nm

Fig. 7.5 Fluorescence spectra of 0.4wt% Dy3+ : Ge25 Ga5 S70 chalcogenide glass pumped by 810 nm laser [9]

Table 7.4 Dy3+ : Radiation characteristics of the three main fluorescence peaks in Ge–Ga–S glass [9] Radiation characteristics

(6 H9/2 ,6 F11/2 ) → 6 H15/2

6H 11/2

Central peak/μm

1.34

1.76

→ 6 H15/2

6H 13/2

→ 6 H15/2

2.86

FWHM/nm

85

130

264

Radiation lifetime/μs

227

2945

7339

Fluorescence lifetime/μs

38

1130

6000

Quantum efficiency/%

16.8

38.4

81.8

Peak gain cross-section/pm2

4.35

0.64

0.99

Table 7.5 Dy3+ : 6 F11/2 , 6 H11/2 and 6 H13/2 energy level lifetimes in Ge–Ga–S glass with changes in concentration [9] Energy lifetime 6F 11/2

6H 11/2

6H 13/2

0.1wt%Dy3+

0.4wt%Dy3+

1.0 wt%Dy3+

2.0 wt%Dy3+

τm1 /μs

38

35

26

24

τm2 /μs

41

38

30

27

τm3 /μs

46

46

40

35

τm1 /μs

1.13

0.987

0.593

0.41

τm2 /μs

1.33

1.16

0.745

0.557

τm3 /μs

1.56

1.29

0.895

0.742

τm1 /μs

6.00

3.75

1.48

0.69

τm2 /μs

6.23

3.71

1.58

0.799

τm3 /μs

6.13

3.63

1.81

1.01

7.3 MIR Luminescence Characteristics of RE Doped …

231

Table 7.6 The multi-phonon relaxation rates of Dy3+ in GAS and GGS glasses calculated by two different methods [5] Transition 6F 6 6 11/2 , H9/2 → H11/2 6H 6 11/2 → H13/2 6H 6 13/2 → H15/2

Wmp =

1 τmeas

−

1 τrad

hυ

Wmp = B(1 − e− kT )− p e−αE

GAS

GGS

E

GAS

GGS

–

21.9 × 103

1925

13.7 × 103

13.7 × 103

0.92 × 103

0.58 × 103

2288

4.71 × 103

4.71 × 103

3497

2.26 ×

2.26 × 103

30.7

15.8

103

concentration increases from 0.4 to 2.0 wt%, the lifetimes of three energy level obviously show a decreasing trend. Among them, the lifetimes for the 6 H11/2 and 6 H13/2 level decrease remarkably, which is mainly due to the increase of the doping ion concentration. Due to the reduction of the distance between ions, the energy transfer effect is enhanced, which leads to an increase in the non-radiation transition and a decrease in lifetime. The author’s opinion is that the doping concentration of Dy3+ in Ge–Ga–S glass should be controlled below 0.1 wt%. Heo and Yong [5] studied the MIR luminescence characteristics of 0.2 wt% Dy3+ : Ge30 As10 S60 (GAS) and 0.5wt% Dy3+ : Ge25 Ga5 S70 (GGS) glasses. The 1.75 and 2.9 μm fluorescence were detected under the 808 nm LD pumping. The multi-phonon relaxation rates of 6 F11/2 , 6 H9/2 → 6 H11/2 (5.27 μm), 6 H11/2 → 6 H13/2 (4.36 μm), 6 H13/2 → 6 H15/2 (2.86 μm) transitions were evaluated based on two methods. The first method is calculated by formula 7.1, and the second is calculated from the measured fluorescence lifetime τ m and radiation transition lifetime τ R by using formula 7.3 [5]. The calculated results of the two methods are shown in Table 7.6. The calculated multi-phonon relaxation data of the two are quite different, but overall, the multiphonon relaxation rates of Dy3+ ions in Ge–Ga–S or Ge–As–S glasses are 1/3–1/10 of those calculated for ZBLAN glasses. Schweizer et al. [61] studied the fluorescence emission of 1.8, 3.0 and 4.3 μm in Dy3+ doped Ga–La–S glass. According to the J-O theory, the Fuchtbauer-Ladenburg equation and McCumber theory, the stimulated emission cross-section and fluorescence lifetime of each wavelength band are calculated, as shown in Table 7.7. The theoretical calculated σ em· τ m at 4.3 μm of Dy3+ in the Ga–La–S glass is ~ 4000 times larger than that in the YLF crystal, which means that by using the Dy3+ : Ga–La–S glass as active materials, the 4.3 μm laser threshold will be reduced to 1/4000 of YLF crystal. At the same time, it shows that the fluorescence of Dy3+ at 4.3 μm perfectly covers the absorption peak of carbon dioxide, which indicates that Dy3+ doped chalcogenide glass or fiber is a good candidate material for preparing carbon dioxide detectors. Yong et al. [62] used the EXAFS method to study the local field characteristics of Dy3+ ions in Ge–As–S glass. As shown in Table 7.8, for the Dy3+ doped Ge–As–S glass, the nearest neighbor atoms of Dy are 6.7 ± 0.5 S ones, which are less than those in the Dy2 S3 crystal (7.5 S atoms). Moreover, the Dy–S bond length in Ge– As–S glass is 2.78 ± 0.01 Å, which is smaller than that in the Dy2 S3 crystal (2.82 ±

6H 6 11/2 → H13/2 6 → H15/2 6H 6 13/2 → H15/2

Transition

42

338

129

1.76

2.83

Aed (s−1 )

4.27

λ (μm)

30

–

15

Amd (s−1 )

100

86

14

β (%)

Table 7.7 Radiative properties of Dy3+ doped Ga–La–S glass [61]

6289

2532

2532

τr (μs)

3600

1300

1300

τm (μs)

57

44

7.4

η (%)

0.92

0.57

1.17

σem, FL (10−20 cm2 )

1.16

0.64

–

σem, MC (10−20 cm2 )

232 7 Mid-Infrared Spectral Properties of Rare Earth …

7.3 MIR Luminescence Characteristics of RE Doped …

233

Table 7.8 Structural parameters optimized from single-shell fits to Dy L3 -edge EXAFS spectra [62] Sample

EXAFS Dy–S distance (Å)

XRD Coordination number

Debye–Waller factor (Å2 )

R-factor

Dy–S distance (Å)

Coordination number

2.83

7.5

Dy2 S3

2.82 ± 0.01

7.5 (fixed)

0.010 ± 0.001

0.0016

GAS

2.78 ± 0.01

6.7 ± 0.5

0.009 ± 0.001

0.0034

Table 7.9 Calculated oscillator strength of Dy3+ ions in different glass substrates (×10–8 ) [63] 6H 15/2

→

6H 13/2 6H 11/2 6F 6 11/2 , H9/2 6 F ,6 H7 9/2 /2 6F 7/2 6F 5/2

Wavelength/nm

Ge30 Ga5 Se65

Ge25 Ga5 S70

ZBLAN

30PbO-70PbF2

2829

395

128

–

–

1704

236

195

98

39

1294

2203

1741

365

382

1108

496

454

218

207

912

350

312

181

154

812

143

81

108

91

0.01 Å). Fewer coordination numbers and shorter bond length indicate that the Dy–S bond in Ge–As–S glass has greater propensity to form covalent bonds, which is not conducive to the incorporation of Dy ions. In order to improve the solubility of RE ions, it is necessary to control the electronegativity and local coordination structure to reduce the propensity to form covalent bonds for the Dy–S bond. Nemec et al. [63] studied the MIR luminescence characteristics of Dy3+ ions in selenium-based glasses with lower phonon energy. The specific composition of the glass is (100–x)(Ge30 Ga5 Se65 )–x(Dy2 Se3 ), where x = 0, 0.01, 0.05, 0.1, 0.2, 0.3 and 0.5. The oscillator strength of Dy3+ ions in selenium-based glass was calculated and compared with those in other host glasses (see Table 7.9). The results show that the oscillator strength of the Dy3+ ion transition in selenium-based glass is quite large. The electronegativity of Se (2.4) is slightly smaller than that of S (2.5), which makes the 2 value of selenium-based glass larger than that of sulfur-based glass, indicating the covalence of selenium-based glass is stronger. Five fluorescence peaks at 1150, 1340, 1760, 2470 and 2950 nm have been obtained when pumped by a 905 nm laser, and three fluorescence peaks at 1760, 2470 and 2950 nm were obtained when pumped by a 1300 nm laser. Bong et al. [64] studied the characteristic MIR transition of Dy3+ ions in selenide glass (Ge30 Ga2 Sb8 Se60 ) under 1.8 μm laser excitation (as shown in Fig. 7.6). The strong broadband emission at 2.9 and 4.3 μm is attributed to the transitions of Dy3+ : H13/2 → 6 H15/2 and Dy3+ : 6 H11/2 → 6 H13/2 . The dip at 4.26 μm in the spectrum is caused by the characteristic absorption of CO2 in the air in the direction of light travel. On the other hand, it can be seen that the intensity ratio between the peak at 2.9 μm and the total spectrum gradually decreases with the increase of the Dy3+

234

7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.6 Fluorescence spectra of different Dy3+ ion concentrations in Ge–Ga–Sb–Se glass under 1.8 μm laser pumping: (a) 0.01, (b) 0.02, (c) 0.05, (d) 0.1 and (e) 0.2 mol%; the small inset figure shows the ratio of the peak intensity at 2.9 μm to the total intensity of the spectrum as a function of the Dy3+ ion concentration [64]

ion concentration. This is because with the increase of Dy3+ ion concentration, the decrease of distance between Dy3+ ions promotes cross relaxation between 6 H13/2 → 6 H15/2 and 6 H11/2 → (6 H7/2 , 6 F9/2 ) both relaxation and excited state absorption (ESA,6 H13/2 → (6 H7/2 , 6 F9/2 )) will reduce the number of particles in the 6 H13/2 state. Therefore, when determining the optimal concentration of Dy3+ ions, it is necessary to comprehensively consider the influence of the interaction between ions. In order to increase the emission intensity of the MIR fluorescence of Dy3+ ions, a co-doping sensitization process can be used to increase the pump energy utilization rate through energy transfer and increase the number of Dy3+ particles at the 6 H13/2 energy level. The RE ions that meet the energy level matching requirements are Pr3+ , Tb3+ , Ho3+ and Tm3+ . The corresponding energy level distribution is shown in Fig. 7.2. In the case of co-doping, the fluorescence of Dy3+ 2.9 μm is generally enhanced with a 2.05 μm laser pumping, and the fluorescence peak of 2.9 μm has the largest intensity when Dy3+ -Pr3+ is co-doped (as shown in Fig. 7.7). However, there will be reverse energy transfer from Dy3+ :6 H13/2 to Pr3+ :3 H5 . When the concentration of Pr3+ increases, the number of particles at the Dy3+ :6 H13/2 energy level will decrease significantly. In the case of Dy3+ single doping, since there is almost no absorption of 2.05 μm laser power, no fluorescence is observed, while Pr3+ , Tb3+ , and Ho3+ ions have relatively strong characteristic absorption at 2.05 μm, which promotes Dy3+ in the energy transfer and enhances the fluorescence at 2.9 μm. On the other hand, both Tm3+ ions and Dy3+ ions have absorption at 1.8 μm, and Tm3+ can transfer energy to Dy3+ . As shown in Fig. 7.7, after adding Tm3+ ions, the radiation intensity at 2.9 μm will increase, which is mainly due to the effective energy transfer from Tm3+ :3 F4 to Dy3+ :6 H13/2 . At the same time, energy transfer from Dy3+ :6 H11/2 to Tm3+ :3 F4 can also occur after the synchronous excitation of the pump source, which is proven by the decrease of the radiation intensity at 4.3 μm. Since 2010, in order to draw the prepared bulk glass into an optical fiber with good optical properties, Tang et al. [65] have systematically studied the crystallization of chalcogenide glass caused by Dy3+ doping. First, they studied the internal and

Intensity/a.u.

Intensity/a.u.

7.3 MIR Luminescence Characteristics of RE Doped …

Wavelength/nm

235

Pumping

Pumping

Wavelength/nm

Fig. 7.7 Fluorescence spectrum of Ge–Ga–Sb–Se glass co-doped (left picture) under 2.05 μm pump, (right picture) Dy3+ single doped and Tm3+ -Dy3+ co-doped under 1.8 μm pump [64]

surface crystallization on 0–2000 ppm Dy3+ (DyCl3 ) doped GeAsGaSe bulk glass, and found that the internal and surface crystallization of the bulk glass have different mechanisms, but they are all related to the level of Dy3+ doping. With the increase in Dy3+ doping, the baseline of the infrared absorption spectrum rises significantly. XRD characterization shows that the α-Ga2 Se3 crystal phase is generated inside the glass, and it is believed that the scattering caused by the generation of this crystal phase makes the chalcogenide glass produce additional absorption. Although Dy3+ did not precipitate internally, the author believes that it plays the role of a heterogeneous nucleation point. On the surface of chalcogenide glass, as the amount of Dy doping increases, spots are generated on the glass surface, as shown in Fig. 7.8. These spots are not only rich in Dy elements, but also Si elements and O elements. This means that glass with higher Dy content corrodes the quartz ampoule. The possible reactions are: 2[≡Ge–Se–H] + 2Cl– → [≡Ge–Se–Se–Ge≡] + 2HCl↑ and 2DyCl3 + 3H2 O

Fig. 7.8 Glass melting results of a 2000 ppm (left, exhibits spots at surface) and b 1000 ppm (right, more shiny fewer spots) Dy3+ -doped GeAsGaSe glasses [65]

236

7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.9 SEM BSE cross-sectional images for the remelted 6000 ppm Dy-doped sample after the extended heat treatmet: 20–390 °C at 2 °C/min, then 390 °C hold for 4 h indicating that the clustered crystals grew up in the vicinity of submicrometer very bright spots. The submicrometer, very bright spots (Spectrum 3) are proposed to by Dy-associated [66]

→ Dy2 O3 + 6HCl↑. It can be deduced that using DyCl3 as a Dy source will cause erosion of the quartz ampoule. Because the introduction of Dy in the form of chloride will corrode the quartz ampoule, when exploring the influence of Dy3+ on the crystallization of GeAsGaSe glass at the near-optical fiber drawing temperature in 2012, the Dy source was replaced with Dy foil [66]. The internal crystalline state of GeAsGaSe glasses with doping amounts of 0, 1000, 3000 and 6000 ppm was observed after heat treatment at 390 °C for 4 h. The results show that α-Ga2 Se3 crystals are produced inside all the glasses, but the number of tiny crystal phases produced inside the GeAsGaSe glass with Dy added decreases, which means that the addition of Dy strengthens the glass structure. However, in the backscattered electron image of the 6000 ppm Dydoped glass section, as shown in Fig. 7.9, it is found that there are bright small spots around the α-Ga2 Se3 crystal, and its Dy content is up to 32,000 ppm. This shows that Dy is indeed involved in the crystallization process of α-Ga2 Se3 as a heterogeneous nucleation point. But its form of existence is doubtful. Dy may exist in the form of crystalline or semi-crystalline compound. Dy may be present in Dy2 S3 , but it is more likely to be Dy2 O3 or Dy hydrate, Dy hydroxide or Dy Si/O compound. During the fiber drawing process, α-Ga2 Se3 appears near the Dy-enriched area on the glass surface, resulting in a decrease in the surface quality of the fiber. In 2014, Tang et al. [67] studied the local field characteristics of Dy in GeAsGaSe glass doped with DyCl3 and Dy foil as the Dy source, and proved that when the doping concentration of DyCl3 is above 1000 ppm, Dy3+ stays in the highly complex DyCl3 crystal grid without melting into the glass grid, and Dy foil will not. This once again proves that Dy foil is a relatively good source of Dy. However, the heterogeneous nucleation with the participation of Cl element proposed in 2010 cannot explain the erosion of the quartz ampoule by the glass doped with Dy foil. In 2016, Tang et al.

7.3 MIR Luminescence Characteristics of RE Doped …

237

[68] used the Dy foil-doped GeAsGaSe glass selected area EDS to conclude that the Dy element itself will corrode the quartz ampoule. In particular, when melting above 900 °C, quartz will decompose to produce free O, which further promotes the reaction between Dy and the quartz ampoule. Therefore, when preparing chalcogenide glasses containing Dy elements, three principles should be followed: the first is to select a proper Dy source such as Dy foil; the second is to reduce the melting temperature; the third is to reduce the melting time. A feasible way is to melt the matrix components in advance and then add the Dy source to melt for a short time, which can greatly improve the quality of the glass. Cui et al. [69] studied the Dy3+ doped Ga–As–S chalcogenide glass. This matrix not only has good RE solubility, but also maintains the good thermal stability of As2 S3 and has a similar fiber-forming ability. The Ga0.8 As39.2 S60 glass shows a desirable large Dy3+ ion solubility (3000 ppm in fiber form), which has been increased by an order of magnitude compared to As2 S3 glass. These Dy doped Ga–As–S glass have obvious characteristic absorption in near-infrared band. As the concentration of Dy ions increases from 0 to 5000 ppm, the absorption coefficients first increase then decrease, and the coefficients reach the highest when the doped Dy ions concentration equals to 3000 ppm, as shown in Fig. 7.10a. There is strong fluorescence emission at 2.9 microns and 4.3 microns, as shown in Fig. 7.10b. The change of fluorescence intensity with rare earth ions is similar to the change of absorption coefficient. This phenomenon can be investigated and explained by the use of highresolution transmission microscope images. From the Fig. 7.11, it can be seen that when the Dy3+ concentration is less than 3000 ppm, the glasses are homogeneous and show the obvious amorphous states. However, when the Dy3+ concentration is higher than 3000 ppm, the homogeneous state is broken, and crystallization phase appears in local region. From the EDS result for the GAS 0.5% sample, it can be seen that the Dy3+ ions gather in a certain local region, indicating the formation of Dy-rich nano-crystallines in matrix. The similar phenomenon shows in both selenide

Fig. 7.10 a Variations of absorption coefficient at each peak wavelength with Dy3+ concentration in GAS samples. b Mid-infrared fluorescence spectra of Dy3+ ions doped GAS samples pumped at 1707 nm [69]

238

7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.11 a TEM image of GAS 0.3%. b–d HR-TEM images of GAS 0.1%, GAS 0.3% and GAS 0.5%. e The gathering of Dy3+ ions in GAS 0.5% (EDS). f Local zoom of HR-image of GAS 0.5% [69]

glasses and sulfide glass, indicating that the principles of melting of rare-earth doped chalcogenide glass proposed by A. B. Seddon are universal. In order to improve the solubility of rare earth ions and enhance their fluorescence efficiency in chalcogenide glasses, researchers have adopted the method of adding heavy metal halides to the glass matrix or rare earth co-doping. Shin et al. [37] studied the variation of multi-phonon relaxation rate and MIR fluorescence characteristics by introducing Br− ions into Dy3+ : Ge–Ga–S chalcogenide glasses. The lifetime changes of Dy3+ : 6 F11/2 , 6 H11/2 and 6 H13/2 energy levels with introducing different content of Br− ions present that the lifetimes of three energy levels gradually increase with the addition of Br ions (see Table 7.10). This is mainly due to the fact that Br− ions reduce the multi-phonon relaxation rate, which is accompanied by an increase Table 7.10 The lifetimes of the 6 F11/2 , 6 H11/2 and 6 H13/2 of samples with different content of Br ions [37]

6F 6 11/2 , H9/2 6H 11/2 6H 13/2

Ge25 Ga10 S65

0.95(Ge25 Ga10 S65 ) + 0.05Br

0.95(Ge25 Ga10 S65 ) + 0.15Br

τ c /ms

τ m /ms

τ c /ms

τ c /ms

0.205

0.034

0.332

0.045

0.405

0.051

2.94

1.09

3.59

1.53

4.3

1.39

6.71

3.92

6.12

11.2

6.31

10.0

τ m /ms

τ m /ms

7.3 MIR Luminescence Characteristics of RE Doped …

239

in the degenerate states’ width of 6 F11/2 and 6 H9/2 levels, which leads to a decrease in the thermal effect of the lowest Stoke level, making the phonon energy of Br involved glass decreases. In addition, because the MIR multi-phonon relaxation of Dy3+ ion is dominated by five asymmetric [GeS4 ] groups vibrating phonons (375 cm−1 ) in the Dy3+ : Ge–Ga–S glass, therefore Shin et al. [70] also studied the influence of sulfur content on the MIR fluorescence characteristics in Dy3+ : Ge–Ga–S glass. The results show that the content of sulfur has a great influence on the MIR fluorescence characteristics of Dy3+ . The deficiency of sulfur content will lead to the decrease of non-radiative transition rate. As a result, the absorption cross- sections, stimulated emission cross-sections and lifetimes of Dy3+ : 6 H11/2 and 6 H13/2 corresponding to 2.9 and 4.4 μm are all increased accordingly. It appears + that compared with the stoichiometric composition (GeS2 -GaS2 ) glass, a composition with proper shortage of sulfur is more suitable as a doping host while considering the emission properties. In 2009, Tao et al. [71] introduced Cd element to improve the solubility of Dy3+ in chalcogenide glass and prepared GeGaCdS glass. The glass transitions temperature (Tg ) of the matrix glass is above 420 °C, which means that its working temperature range is wider; the difference value between Tg and Tx is greater than 120 °C, indicating a good fiber-forming ability. At the same time, its Vickers hardness is above 221 kgf/mm2 , indicating that it has good mechanical properties. Dy-doped GeGaCdS chalcogenide glasses of 1000–50,000 ppm were prepared, and the absorption coefficient of 1.3 μm was measured, as shown in Fig. 7.12. It can be seen that the Dy3+ ion absorption coefficient has a linear relationship with its mass fraction, which proves that Dy3+ has sufficient RE solubility in chalcogenide glasses. Although fluorescence quenching is observed in glasses with a doping content of more than 10,000 ppm,

Fig. 7.12 The relationship between Dy3+ ions concentration and the absorption coefficient at the peak wavelength of 1.3 μm in Dy3+ ions doped in GeGaCdS glasses [71]

240

7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.13 Energy transition processes in Dy3+ , Tm3+ -codoped glasses [72]

GeGaCdS glass is still a good matrix material for MIR lasers and fiber amplifiers due to its excellent thermal properties, good mechanical properties, and sufficient doping concentration. In 2009, Guo et al. [72] introduced CdI2 in the form of halide, and hoped to increase the fluorescence efficiency of Dy3+ ions by co-doping with Tm3+ , and prepared Dy3+ doping and Dy3+ /Tm3+ Co-doped Ge-Ga-S-CdI2 glasses. The addition of CdI2 reduces the phonon energy of the glass, thereby reducing the probability of nonradiative transition of the Dy3+ : 6 H11/2 energy level, resulting in an enhancement of 1.7 μm fluorescence emission. Since the 4.3 μm fluorescence and the 1.7 μm fluorescence transition share the same upper energy level, the 4.3 μm fluorescence emission enhancement can be expected. The co-doping of Tm3+ is to use the energy transfer process (ET) between Tm3+ and Dy3+ , which use the energy transfer from Tm3+ : 3 F4 energy level to Dy3+ 6 H11/2 energy level, as shown in Fig. 7.13, to enhance 2.9 and 4.3 μm fluorescence emission efficiency of Dy3+ ion. When CdI2 and Tm3+ are introduced at the same time, the introduction of CdI2 can reduce the ethane-like structure [S3 (Ga)Ge–Ge(Ga)S3 ] in Ge–Ga–S glass, making more likely that Tm3+ will agglomerate, which is beneficial to the Tm3+ : 3 F4 energy transfer process from Dy3+ :6 H11/2 energy level. In 2015, Dai et al. [73] studied the MIR luminescence characteristics of Tm3+ /Dy3+ : Ge–Ga–S–CsI glass and observed an increase in fluorescence intensity at 2.9 μm. The Dy3+ ion has a weak absorption band near the commercial nearinfrared laser 808 nm, and the output MIR fluorescence intensity is also very weak. By introducing other sensitizing ions and relying on the energy transfer mechanism, the luminous efficiency in the 2–5 μm band can be improved. When a certain amount of Tm3+ ions are introduced, the fluorescence intensity of Dy3+ at 2.9 μm can be increased by more than 5 times. The fluorescence spectra of 1.48 and 1.8 μm when Tm3+ /Dy3+ co-doped under 800 nm laser pumping are produced by Tm3+ : 3 H4 → 3 F4 and 3 F4 → 3 H6 transitions, respectively. The 3 H4 and 3 F4 energy level lifetimes

7.3 MIR Luminescence Characteristics of RE Doped … Table 7.11

3H , 3F 4 4

241

energy level lifetime measured by Tm3+ ion in the sample [73]

Dy3+ concentration

0 wt%

0.5 wt%

0.7 wt%

1 wt%

e-fold time (3 H4 )/μs

354

348

335

320

(3 F4 )/μs

980

450

325

210

e-fold time

of the Tm3+ ions in the measured samples obtained by fitting the fluorescence attenuation at 1.48 and 1.8 μm are shown in Table 7.11. It can be seen that the energy level lifetime of Tm3+ ion 3 F4 decreases rapidly and monotonously with the increase of Dy3+ ion, indicating that there is an energy transfer of Tm3+ : 3 F4 → Dy3+ : 6 H11/2 under Tm3+ /Dy3+ co-doping. Wang et al. [74] investigated 64GeS2 ·16Ga2 S3 ·20CdI2 chalcohalide glasses doped with 0.2%-3% weight of Dy3+ ions, and their spectroscopic properties were investigated. It was shown that the host glass could dissolve as much as 3% Dy3+ ions without deteriorating the glass transparency. However, the concentrations of impurities, especially H2 O, O–H and S–H, are significantly enhanced because of the hygroscopic property of halide. Three additional 0.4% weight of Dy3+ -doped samples were prepared using different purity grades of raw materials and additional dehydration processes in order to study the effect of impurities on fluorescence. They were labeled as GGC0.4%1 , GGC0.4%2 and GGC0.4%3 , respectively, as shown in Table 7.12. It can be seen that the concentrations of O–H and S–H impurities both have an obvious decrease after the dehydration process. Heating raw materials at 100 °C for 1 h under vacuum mainly removed the surface moisture from the mixtures, inducing the decrease of O–H and S–H concentrations in GGC0.4%1 . Repeatedly distilling S at 180 °C is to remove the hydrate impurities, and the TeCl4 can further reduce the S–H and O–H impurities in the glasses through the following chemical reactions: TeCl4 + 4S − H(in glass) → Te(in glass) + 4HCl ↑ (gas) + 4S(in glass) (7.5) TeCl4 + 4O − H(in glass) → Te(in glass) + 4HCl ↑ (gas) + 4O(in glass) (7.6) These processes are responsible for the decrease of O–H and S–H concentrations in GGC0.4%2 . Besides, TeCl4 also reacts with carbon impurities, resulting in much weaker absorption bands at 6.63 mm, as shown in Fig. 7.14a. TeCl4 + C − S(in glass) → Te(in glass) + CCl4 ↑ (gas) + S(in glass)

(7.7)

It is worth mentioning that the GGC0.4%3 glass contains more S–H impurities (224 ppm) than the GGC0.4%2 glass. This may be due to that the reaction (7.5) mainly happens at melting temperature, but the TeCl4 has partly decomposed during the dehydration processes at 100 °C under vacuum for a long time. The decomposition of TeCl4 reduced its effect in reaction (7.5). The fluorescence decay curves at 1.75 and 2.95 μm emissions of Dy3+ in all the GGC glasses were measured and the lifetimes

242

7 Mid-Infrared Spectral Properties of Rare Earth …

Table 7.12 The different methods to eliminate impurities in the GGC glasses [74] Sample

Methods

Impurities O–H

S–H

CO2

H2 O

GGC0.4%

No purification

80

706

26

41

GGC0.4%1

First step: high purity raw materials Second step: heated under vacuum at 100 °C for 1 h to remove the surface moisture

70

301

7

23

GGC0.4%2

First step: high purity 62 raw materials Second step: S was firstly distilled repeatedly at 180 °C for 8 h in the quartz tube to remove the hydrate impurities Third step: the raw materials were mixed with 0.1 wt.% TeCl4 , and heated at 180 °C for 1 h to reduce S–H and O–H impurities

167

–

–

GGC0.4%3

The same as method 2, except for the third step: heated at 180 °C for 8 h

224

–

–

59

Fig. 7.14 a Infrared absorption spectra of GGC0.4%, GGC0.4%1 , GGC0.4%2 and GGC0.4%3 glasses [74]

7.3 MIR Luminescence Characteristics of RE Doped …

243

obtained are shown in Fig. 7.14b as a function of Dy3+ concentration. The lifetimes are supposed to decrease with increasing doping concentrations. The lifetimes in GGC0.4%2 and GGC0.4%3 are much higher than those in the other GGC glasses, which implies that the impurities in the glasses have significant negative effects on the MIR fluorescence decays. In 2017, Guo et al. [45] studied the influence of the introduction of halogen I2 on the structural, thermal and optical properties of Ge–Sb–S glass. A series of (100 − x) Ge25 Sb10 S65 -xI (x = 0, 5, 10, 15, 20, 25 and 30 wt%) chalcohalide glasses and ones doped with 0.1 wt% Dy3+ ions were synthesized and their structural, thermal and optical properties were systematically studied. The results show that the addition of I2 into Ge25 Sb10 S65 glass significantly decreases the connectivity of the glass network and average bond energy, and modifies the associated properties. The glass transition temperature and refractive index decrease monotonically, and the density increases almost linearly with the increasing of iodine content in glass. The thermal stability is improved, which has a maximum of T max = 174 °C, while the optical bandgap has a peak value at x = 10 wt%. The absorption spectra and MIR fluorescence spectra of Dy3+ ions doped glasses were investigated together with the J-O analysis. GeS2 nano-crystals were found in the sample with x ≥ 15 wt%, which is supposed to induce enhancements of MIR fluorescence at 2.95 and 4.32 μm. At present, the main focus of research on rare earth-doped chalcogenide bulk glass is as shown above. The solubility of rare earth is improved by adding Ga or doping halide to the glass system. More rare earth ions can be introduced by adding halide, but, the concentrations of impurities especially H2 O, O–H and S–H are prominently enhanced because of the hygroscopic property of halide, and the glasses have high crystallization tendency during fiber drawing because of large amount ionic bonds in glass. In addition, to obtain a useful active fiber for applications, all chalcogenide glasses need to consider the laser quality factor. Table 7.13 shows the laser quality factor of different Dy3+ doped chalcogenide glasses. It can be seen that the Ge-Ga-Sb-S has the highest σemi (emission cross section), and the Ge-As-Ga-Se has the longest Table 7.13 The laser quality factor of different Dy3+ doped chalcogenide glasses Sample

λp (nm)

σemi (10−20 cm2 )

τmea (ms)

σemi × τmea (10−23 cm2 ·s)

References

Ge–La–S

4270

1.17

1.30

1.52

[61]

Ge–As–Ga–Se

4500

0.82

2.00

1.64

[30]

Ge–Ga–S

4400

0.36

1.31

0.47

[75]

Ga–Sb–S

4400

0.38

1.42

0.54

[75]

Ga–Sb–As–S

4400

0.36

1.38

0.50

[43]

Ge–Ga–S–CdI2

4290

1.27

0.72

0.91

[74]

Ga–As–S

4186

1.06

1.60

1.70

[69]

Ge–Ga–Sb–S

4210

1.86

1.41

2.62

[76]

244

7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.15 Emission spectrum of 0.02 mol% Dy3+ doped Ge10 As25 S65 glass fiber. The insert is the IR transmission image of core-cladding structural fiber [77]

τmea (measured lifetime). But the product σemi × τmea reaches 2.62 × 10−23 cm2 ·s in Ge–Ga–Sb–S glass, which is the highest among these glasses. Park et al. [77] drew Dy3+ -doped Ge–As–S glass into an optical fiber and explored the possibility of its application in a 2.96 μm fiber laser. A Ge10 As25 S65 and Ge9 As23 S68 composition were selected as the core and cladding of the fiber, respectively, and optical fiber was prepared by using the double crucible method. They found that the crystallization readily occurred when Dy3+ was doped into Ge10 As25 S65 glass. In order to improve the RE solubility, 0.1 mol% of Ga was introduced to obtain Dy3+ a glass mandrel successfully doped with 0.02 mol% Dy3+ . The fiber’s fluorescence spectrum was tested under 1.7 μm laser pumping, as shown in Fig. 7.15. The authors pointed out that due to the O–H and S–H vibration absorption in the 3 and 4 μm bands, in order to obtain the laser output, it is necessary to purify the raw materials to reduce the loss of these impurities. In 2015, Yang et al. [43] studied Dy3+ doped Ga–Sb–S chalcogenide glass. This matrix has excellent infrared transmittance and RE doping ability. It has a relatively high glass transition temperature and strong MIR emission at 2.95 and 4.40 μm. The low phonon energy of Ga–Sb–S glass makes it exhibit a longer infrared cut-off edge (~14 μm), and also helps reduce the multiphonon relaxation rate of the excited state of the Dy3+ energy level. In Ga7.8 Sb32 S60 Dy0.2 glass, the quantum efficiencies of 2.95 and 4.4 μm fluorescence are 88.1 and 75.9%, respectively, and the emission crosssections are 1.1 × 10–20 and 0.38 × 10–20 cm2 , respectively. However, devitrification occurred during the fiber drawing process. In order to improve the thermal stability of Ga–Sb–S glass, 4 wt.% As was used instead of Sb, and a 0.05 wt.% Dy3+ doped Ga–Sb–As–S chalcogenide fiber was successfully prepared, and its background loss was found to be less than 2 dB/m over the range 2–6 μm. Based on the optical properties shown in Fig. 7.14, Wang et al. [74] chose the GGC0.4%2 for the fiber fabrication. A carefully purified preform (11 mm in diameter) with good surface quality was prepared. The preform was then drawn into core-only fibers with a diameter of 210 mm. In order to avoid possible crystallization, the fiber

7.3 MIR Luminescence Characteristics of RE Doped …

245

Fig. 7.16 a GGC0.4%2 preform. b End-face image of GGC 0.4%2 fiber [74]

drawing temperature was set to be as low as possible. In this case, the glass showed relatively high viscosity, and therefore an object with a weight of about 200 g was attached on the bottom of the preform to help the fiber drawing at the start and high tension was subsequently applied for the following continuous drawing of the fibers. Figure 7.16 shows the preform. Optical inspections with a microscope indicated that there was no surface crystallization or microcrystals inside the fibers. Cui et al. [69] chose 0.3 wt% Dy3+ doped Ga0.8 As39.2 S60 glass for fiber drawing. This glass shows good MIR spectral properties, as shown in Fig. 7.10 and Table 7.13. The large value of the Criterion Temperature (T = 182 °C) indicates that 0.3 wt% Dy3+ doped Ga0.8 As39.2 S60 glass maintains a good thermal stability, which is large enough for a fiber drawing process. An experimental fiber drawing operation successfully fabricated a core-only fiber with a diameter of 300 μm. To ensure no crystallization exists during the fiber drawing process, the fiber’s powders are observed under TEM, as shown in Fig. 7.17. The morphology of this powder and the HR-TEM image proves that no crystalline phases or clusters exists in the fiber. Figure 7.18 shows the MIR fluorescence spectra of the 0.3 wt% Dy3+ doped Ga0.8 As39.2 S60 bulk glass and fiber, for which a 20 cm long fiber was used for measurement. A red-shift about 25 nm appears in the fiber’s MIR fluorescence, which makes for the two emission bands center at around 2910 and 4210 nm, respectively. Xiao et al. [76] fabricated a series of Dy3+ -doped Ga5 Ge20 Sb15 S60 bulk glasses and evaluated their potential for developing mid-infrared fiber laser beyond 4 μm. As shown in Fig. 7.19, the fluorescence intensity reached its highest value when the Dy3+ concentration reached 7000 ppm. However, the measured lifetimes showed that fluorescence quenching is occurring when the concentration of Dy3+ was above 3000 ppm. Because the 3000 ppm Dy3+ doped GGSS glass had the highest laser

246

7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.17 The TEM and HR-TEM images of 0.3 wt% Dy3+ doped Ga0.8 As39.2 S60 glass [69] Fig. 7.18 The mid-infrared fluorescence spectra of 0.3 wt% Dy3+ doped Ga0.8 As39.2 S60 bulk glass and fiber, the insert image is the section view of the fiber. [69]

Fig. 7.19 a Mid-infrared fluorescence spectra of Dy3+ -doped GGSS glass pumped at 1707 nm. b Variation in the measured fluorescence of two excited states with different Dy3+ concentration in GGSS Samples [76]

7.3 MIR Luminescence Characteristics of RE Doped …

247

Fig. 7.20 Measured mid-infrared transmission spectra for GGSS glass (before and after Cl2 purification) [76]

quality factor (shown in Table 7.13), it was chosen for impurities removal and to be drawn into an optical fiber. To remove the S–H and O–H impurities of glass, chemical purification using chlorine gas was carried out. The Ga5 Ge20 Sb15 S60 bulk glass samples were firstly fabricated by the conventional melt-quenching technique mentioned previously. The glass samples were placed in a new ampoule and the argon gas was injected to form a protective atmosphere. Then, the glass samples were heated to 650°C and the chlorine gas was introduced, where the flow rate and time were controlled to be approximately 4 mL/min and 100 min, respectively. Subsequently, the purified glass clinkers were subjected to dynamic distillation to remove the HCl gas so that finally high-purity chalcogenide glasses were obtained after the remelting process. After the purification process, the values of the absorption coefficients α O–H and α S–H were drastically reduced from 0.24 and 2.29 cm−1 to 0.0069 and 0.0048 cm−1 , from which it can be deduced that there is a decrease in O–H and S–H impurity concentration from 10.42 and 397.82 ppm to 0.3 and 0.85 ppm, respectively, as shown in Fig. 7.20. As the same time, the lifetimes of both 6 H11/2 and 6 H13/2 energy increased, as shown in Fig. 7.21. Their values were markedly improved from 3.88 and 1.41 ms to 4.61 and 1.82 ms, respectively. This is a result of the reduction in the impurity content decreasing the multiple-phonon relaxation rate which in turn results in a reduction in the nonradiative energy transfer rate between ions and impurities. The bulk glass samples with Cl2 purification were used for fiber drawing. For the first time, Dy3+ -doped, single-mode and double-cladding type chalcogenide fibers with core/cladding ratios of 125:60:11 and 125:66:11.5 respectively were fabricated by a multistage rod-in-tube-fiber drawing process and extrusion method, respectively. Both the methods have their own specific advantages, the former has the advantage of a more flexible core/cladding structure fabrication and the latter has superiority given its simpler operation and higher success rate. However, in terms of the control of fiber concentricity, the extrusion method has a distinct advantage due to is molding

248

7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.21 Fluorescence decay curves of a 6 H11/2 , b 6 H13/2 before and after the Cl2 purification [76]

Fig. 7.22 End-face view (transmitted light): a the fiber fabricated by multistage rod-in-tube method, b the fiber fabricated by extrusion method [76]

technology (the deviation distance were 5.62 and 1.98 μm for rod-in-tube method and extrusion method, respectively, as shown in Fig. 7.22), which is beneficial for fabricating fiber with a lower loss. With a FTIR spectrometer using the cut-back method, the fiber loss was measured and the corresponding spectrum was presented in Fig. 7.23. With the benefit of the chemical purification and an appropriate fiber drawing method, the loss values of the two impurity absorption peaks at around 2.9, 4.1 μm were observed to be as low as 3.0 and 2.4 dB/m, which represent one of the best results for GeS2 -based fiber. Quimby et al. [13] simulated the possibility of outputting lasers in the Dy-doped GeAsGaSe fiber at a wavelength around 4.5 μm. The simulation adopts the 6 H11/2 , 6 H13/2 and 6 H15/2 levels which correspond to the three-level structure shown in Fig. 7.24. The 6 H13/2 level is depopulated by cascade lasing at 3.3 μm through to the 6 H13/2 → 6 H15/2 transition, and the population inversion between the 6 H11/2 and 6 H15/2 level is realized, resulting an efficient ~ 4.5 μm laser output. The simulation results show that the cascade laser method can produce a higher efficiency

7.3 MIR Luminescence Characteristics of RE Doped …

249

Fig. 7.23 Measured optical loss spectrum in the undoped fiber, the insert shows the end-face view of the fiber used [76]

Fig. 7.24 a Cascade lasing scheme with simultaneous lasing at λ1 ~ 4.5 and λ2 ~ 3.3 μm. Fiber Bragg gratings FBG1 and FBG 2 are tuned to λ1 and λ2 . b Transition rates included in model, with thin lines spontaneous transitions and thick lines stimulated transitions [13]

laser output at 4.6 μm compared to the traditional single-laser wavelength mode, as shown in Fig. 7.25. In addition, it is also important to reduce the fiber loss to the range of 1–3 dB/m. Xiao et al. [74] theoretically investigated a Dy3+ doped chalcogenide fiber laser operating at 4.3 μm based on the rate equations and propagation equations. Two main pump bands for 1319 and 1707 nm corresponding to the 6 H15/2 → 6 H9/2 and 6 H15/2 → 6 H13/2 transitions are discussed. The 1707 nm pumping was determined to be the most efficient, achieving a predicted slope efficiency of ∼ 15.1% which is approximately twice that associated with 1319 nm pumping. It is also found that acceptable laser operation is to be expected for fiber loss below 6 dB/m, as shown in Fig. 7.26. Furthermore, among the three pumping structures, each of them has their own unique advantage and some unavoidable shortcomings, in which the backward

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7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.25 Calculated output power versus incident pump power. Idler (light solid curve) reaches threshold at 0.2 W, above which the slope efficiency of 4600-nm output (dark solid curve) abruptly increases. With no idler (dotted curve), the output saturates at ≈ 0.028 W [13]

Fig. 7.26 The dependences of the predicted slope efficiency on the fiber loss for the signal and idler lights [74]

and bidirectional pumping show relatively better laser performances (as shown in Fig. 7.27), but, the forward pumping allows for a simpler and easier to implement optical system.

7.3.2 MIR Luminescence of Pr3+ Doped Chalcogenide Glasses The energy level structure of the praseodymium (Pr3+ ) ion and the corresponding infrared absorption transition are shown in Fig. 7.28. The Pr3+ ion is pumped to 3 F4

7.3 MIR Luminescence Characteristics of RE Doped …

251

Energy level/cm-1*1000

Fig. 7.27 The variation in the calculated slope efficiencies and threshold pump power for three different pump configurations when the fiber length is varied [74]

Pr3+

Wavelength/μm

Fig. 7.28 Energy levels and related transitions of Pr3+ , mid infrared absorption spectra of 1000 ppm Pr3+ : GAGS glass

(3 F3 ) energy level by a 1.5 μm or 2.0 μm laser, and the radiation transition to the lower energy level can produce 3–5 μm MIR broadband fluorescence. Based on the energy level diagram, when Pr3+ ions are excited to the (3 F2 , 3 H6 ) energy state by a 2.05 μm pump source, the MIR emission centered at 3.57 and 4.74 μm has the potential to generate a MIR laser output. The strong broadband luminescence is formed by the superposition of the ~ 3.7 μm fluorescence peak produced by the (3 F2 , 3 H6 ) → 3 H5 transition and the ~ 4.7 μm fluorescence peak produced by the 3 H5 → 3 H4 transition. The absorption depression at 4.24 μm is derived from the absorption of carbon dioxide in the air of the optical path inside the spectrometer. The complete Pr3+ emission spectrum can be obtained

7 Mid-Infrared Spectral Properties of Rare Earth …

(b)

Emission/a.u.

Energy level/(×1000 cm-1)

(a)

Absorption/a.u.

252

Pr3+

/nm

Fig. 7.29 a schematic diagram of energy transfer between Pr3+ ions in selenium-based glass; b overlapping phenomenon of emission spectrum of Pr3+ : (3 F2 , 3 H6 ) → 3 H5 and absorption spectrum of Pr3+ : 3 H4 → 3 H5

by compensating and correcting for the absorption of CO2 [78]. Pr3+ ion’s ultrawideband luminescence range is mainly caused by many Stark split energy levels of the 3 H5 → 3 H4 transition energy level and the difference in different lattice positions in the matrix. On the other hand, the bandwidth of the emission spectrum increases with the increase of the Pr3+ ion concentration, and is accompanied by a decrease in the 3.7 μm radiation peak; this is due to the enhancement of the interaction between the excited state energy levels of the Pr3+ ion. With the increase of Pr3+ ion concentration and the decrease of distance between ions, the probability of a cross relaxation (CR1) transition between Pr3+ ions increase (as shown in Fig. 7.29a). This process can be expressed as (3 F2 , 3 H6 ) → 3 H5 and 3 H4 → 3 H5 . Figure 7.29b shows that the corresponding spectra of the two transitions have a large overlap interval; therefore, the fluorescence peaks of A and B can be effectively reabsorbed, and the number of inverted particles at the 3 H5 energy level is increased to promote the corresponding fluorescence emission from D to J. In addition, as shown in Fig. 7.29a, there is another cross-relaxation (CR2) energy transfer between (3 F2 , 3 H6 ) → 3 H5 and (3 F2 , 3 H6 ) → (3 F3 , 3 F4 ). This process also can consume the fluorescence band intensity of A, B and C. The excited 3 F3 and 3 F4 states transition downward to radiate high-energy photons, achieving the effect of up-conversion luminescence. In addition, the excited state absorption of the 3 H5 energy level can also contribute to the emission bands H, I and J. The related transition process is shown by the dotted line in Fig. 7.29a. Taking into account the interaction between the excited states of Pr3+ ions, it is believed that when the gain medium such as optical fiber has a long interaction length, 3.7 μm MIR radiation is difficult to use for laser output, while for the 3 H5 energy level is easier to achieve particle number inversion and turn obtain a laser output. The measurements showed that the fluorescence lifetime of the Pr3+ :3 H5 state in the selenium-based glass is longer than the actual 4 ms. The initial increase in fluorescence lifetime with doping concentration is due to the cross relaxation (CR1) process increasing the number of inverted particles at the 3 H5 energy level. The subsequent

Fig. 7.30 Luminescence spectra of Pr3+ -doped se based glass: a 0.05 mol% Pr3+ single doped, b 0.05 mol% Pr3+ and 0.2 mol% Tm3+ Co doped, c 0.05 mol% Pr3+ and 0.2 mol% Ho3+ Co doped, with pump wavelength of 2.05 μm

253

Intensity/a.u.

7.3 MIR Luminescence Characteristics of RE Doped …

Wavelength/nm

decrease in fluorescence lifetime with increased concentration can be explained by the typical energy transfer between Pr3+ ions; this is because as the concentration increases, the distance between internal ions decreases without increasing radiation relaxation. Considering the actual use of Pr3+ ions as the active ions of the MIR light source, a compromise needs to be reached between lifetime and bandwidth. For example, in Ge–Ga–Sb–Se glass, the Pr3+ doping concentration is 0.02–0.05 mol%, which can avoid the interaction between Pr3+ as mentioned above. As mentioned in the previous discussion of the role of Dy3+ , reasonable sensitizer can further increase the MIR fluorescence intensity for activated ions. For Pr3+ ions, Tm3+ and Ho3+ ions are a more suitable means to increase sensitivity. They have a similar absorption band near 2 μm and there is no energy level lower than the Pr3+ : 3 H5 energy level and can eliminate the possible reverse energy transfer from the Pr3+ ion to improve the quantum efficiency of the Pr3+ : (3 F2 , 3 H6 ) and 3 H5 energy levels. The comparison spectrum of co-doping and single-doping is shown in Fig. 7.30. It can be seen that Tm3+ -Pr3+ co-doping can effectively increase the MIR luminescence intensity of Pr3+ ions. The high energy transfer efficiency is due to the strong thermal coupling between theTm3+ : 3 F4 and Pr3+ : (3 F2 , 3 H6 ) energy levels, and the efficient absorption of the pump light with the large absorption cross-section of the Tm3+ : 3 F4 energy level. However, Ho3+ -Pr3+ co-doping weakens the MIR luminescence intensity of Pr3+ ions under the pump of 2.05 μm. Although the position of the Ho3+ : 5 I7 energy level is very close to the Pr3+ : (3 F2 , 3 H6 ) energy level, the reverse energy transfer from Pr3+ : (3 F2 , 3 H6 ) to Ho3+ : 5 I7 energy level suppresses the intensity of the radiation transition of the Pr3+ ion. At the same time, the fluorescence radiation lifetime at 4.7 μm became shorter, but the fluorescence lifetime of the Tm3+ -Pr3+ co-doped samples did not change significantly. In 2014, Guo et al. [79] of the Xi’an Institute of Optics and Mechanics studied the influence of the (100 − x)(0.8 GeS2 ·0.2 Ga2 S3 ) × CdI2 (x = 5, 10, 15 and 20) glass system composition on Pr3+ ion MIR fluorescence. Under the pumping of 2.01 μm Tm3+ : YAG laser, 4.6 μm fluorescence emission of 3 H6 → 3 H5 transition was observed, and the effective linewidth of fluorescence reached 106–227 nm. It can be seen from Fig. 7.31 that the intensity of MIR fluorescence has a strong effect on the components. Because CdI2 has a lower phonon energy, it can effectively reduce

254

7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.31 Mid infrared fluorescence of Pr3+ -doped (100 − x) (0.8GeS2 ·0.2Ga2 S3 ) xCdI2 (x = 5, 10, 15, 20) glass samples. The response curve of InSb detector is illustrated [79]

the non-radiative transition of the 3 H6 energy level, so the fluorescence is increased with the increase of CdI2 concentration. Liu et al. [44] studied the MIR luminescence characteristics of Pr3+ in Ge–As–Ga– Se glass. As shown in Fig. 7.32, Pr3+ ions have a very wide (2.8–5.5 μm) emission range under the excitation of a 2 μm pump source, and the optimal doping concentration is 0.1 mol%. In the case of higher concentration, as the distance between ions decreases, the energy migration rate becomes larger, causing serious non-radiation loss, leading to fluorescence quenching. In addition, the phenomenon of excited state absorption occurred under the excitation of the 2 μm laser pump source, as shown by the dotted line in the energy level distribution of Fig. 7.32; this also explains the 2.9 and 3.3 μm peak in the fluorescence spectrum. In order to verify the existence of

Fig. 7.32 Mid infrared fluorescence and transition of Pr3+ doped Ge–As–Ga–Se glass sample

7.3 MIR Luminescence Characteristics of RE Doped …

255

ESA transitions, near-infrared spectroscopy test also found fluorescence at 1.3 and 1.6 μm. In 2005, Chung et al. [80] studied the feasibility of 0.02 mol% Pr3+ doped selenium-based glass fiber and studied the advantages and disadvantages of combined pumping by a 1.48 μm laser diode and 2.05 μm fiber laser. A fluorescence spectrum similar to that from bulk glass was obtained from the optical fiber, which proved its potential as a MIR gain medium when pumped at 1.48 and 2.05 μm. The 5.25 μm emission peak under 1.48 μm laser pump is caused by the second-order diffraction from (3 F3 , 3 F4 ) → 3 H5 , transition. Generally speaking, the 2.05 μm pumping scheme can generate a strong 4.7 μm fluorescence, which is more conducive to the generation of fiber laser. The Pr3+ doped fiber is excited to the (3 F3 , 3 F4 ) energy level by a 1.48 μm laser pump. Under continuous pumping, the excited state absorption of the Pr3+ doped fiber will appear and the electronics jump to a higher energy level, such as 1 G4 energy level, will lead to the decrease in the number of particles of 3 H6 and 3 H5 energy levels and prevention the gain of MIR. In addition, for the 1.48 μm pump source, the two-photon absorption of selenide glass and the thermally coupled interaction of the lowest unoccupied molecular orbital limit its maximum usable power. In 2014, Sojka et al. [54] prepared a Pr3+ doped Ge–As–Ga–Se chalcogenide fiber by the extrusion method, and tested the loss of the fiber at 6.65 μm with a result of 2.8 dB/m. For a 1550 nm laser pump, in the 120 mm long Pr3+ : Ge–As–Ga–Se core/clad chalcogenide fiber, 3.5–5.5 μm MIR fluorescence output was successfully obtained. In addition, the author tested the radiation lifetime of MIR 4.5–5 μm fluorescence. The fitting test result (11.52 ms) is close to the theoretical calculation result (10 ms). In order to reduce the influence of fluorescence re-absorption phenomenon on the life test results, a 34 mm length optical fiber was selected. In 2017, Liu et al. [44] studied the preparation and MIR emission spectra of Pr3+ doped selenide chalcogenide glass and fiber. They successfully prepared 0.1 mol% Pr3+ -doped step refractive index fiber and characterized its spectral characteristics. According to the calculated J-O intensity parameters t and oscillator strengths, they found that compared with other glass, Pr3+ : GAGS glass has the largest oscillator strengths 2 and 4 /6 , as shown in Table 7.14. When the concentration of Pr3+ ions increase to 0.3 mol%, t jumps suddenly. This indicates that as the concentration of Table 7.14 The values of J–O intensity parameters t (t = 2, 4, 6), calculated from absorption spectra of Ge10 As24 Ga4 Se62 Prx (x = 0.05, 0.1, 0.2, 0.3, 0.4) samples [44] Glass sample

2 (×10−20 cm2 )

4 (×10−20 cm2 )

6 (×10−20 cm2 )

4/ 6 (× 10−20 cm2 )

GAGS-0.05

2.01

17.74

8.65

2.05

GAGS-0.1

1.99

11.92

8.07

1.48

GAGS-0.2

1.99

10.34

7.47

1.38

GAGS-0.3

3.08

4.92

5.24

0.94

GAGS-0.4

3.24

4.56

5.01

0.91

256

7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.33 Optical loss spectrum of the 0.1 mol% Pr3+ : GeAsGaSe core/GeAsGaSe clad. Step-index glass fibre drawn from the co-extruded core/clad preform. The inset is the optical photomicrograph of the fibre end face [71]

Pr3+ ions increase, the ratio of clustered Pr3+ ions to non-clustered Pr3+ ions become larger and the emission intensity becomes weaker. In GAGS-0.1 glass, due to the dispersive effect of Ga, the highest emission intensity can be obtained by forming a Pr3+ -Se-Ga bond, but for higher Pr3+ concentration, the intensity will be greatly reduced. When using an excitation wavelength from a 2.0 μm laser, ESA occurs in the 3 F2 → 1 H4 transition, which helps to achieve nearinfrared emission of 1.3, 1.6, 2.9 and 3.3 μm. In addition, the optical loss spectrum of the 250/125 μm cladding diameter/core diameter, 0.1 mol% Pr3+ step-index glass fiber exhibited a lowest loss of 3.5 dB/m at 6.9 μm, as shown in Fig. 7.33. Karaksina et al. [41] studied the luminous intensity and laser pumping of the Prdoped Ge–As–Se–Ga (In, I) glass fiber in the 2.4–6 μm spectral range. The ability of selenium-based glass fiber to withstand pump powers up to 1600 mW was demonstrated for the first time. The luminescence spectrum of a fiber sample was tested for the first time. In addition its length corresponds to the value required for the numerical model of the fiber laser characteristics calculated for the chalcogenide fiber material. For the first time, an experimental study on the luminescence characteristics of coreclad Pr chalcogenide glass fibers using two 50–1600 mW pump light sources was demonstrated. The results in Fig. 7.34 show that, with the use of Ga-In alloy cover for fiber input and output, compared with previous data, the pump power that the fiber sample can withstand is several times higher. For the first time, the emission of a fiber sample with a length of less than 100 cm was detected in the wavelength range of 2–6 μm. As the pump power increases, the dependence of the luminous intensity on the pump power becomes nonlinear. Figure 7.35 shows the luminescence band with maximum at 4.7 μm recorded for different fiber lengths. Fibers have been pumped

7.3 MIR Luminescence Characteristics of RE Doped …

257

Fig. 7.34 Dependences of the luminescence intensity at 4.7 μm on the pump power at 1.56 μm for fibers: 1–90 μm core, 2000 ppmw Pr-doped Ge15 As16 Se63 In3 I3 fiber, 30 cm length; 2–40 μm core, 2000 ppmw Pr-doped Ge15 As16 Se63 In3 I3 fiber, 30 cm length; 3–16 μm core, 1300 ppmw Pr-doped Ga3 Ge17 As18 Se62 fiber, 20 cm length [41]

Fig. 7.35 Luminescence spectra for 90 μm core fibers of different lengths: 1–5 cm; 2–20 cm; 3–30 cm; 4–40 cm; 5–80 cm. The pump power at 1.97 μm is 300 mW [41]

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7 Mid-Infrared Spectral Properties of Rare Earth …

with the use of the laser at 1.56 μm, which is most likely a result of the lower level of optical fiber losses in these spectral ranges as compared with data shown before. But it is important to note as well that these new results are promising and demonstrate the possibility of preparation of fiber samples in accordance with previously developed lasing models. Shaw et al. [81] studied the application of Pr3+ doped fiber as an infrared search system (Infra-Red Search System, IRSS) laser source. The matrix material is a Sebased chalcogenide glass, this type of matrix material not only has superb fiberforming ability, but also has the advantages of low phonon energy, wide infrared transmission window, and low optical loss. Fluorescence emission in the infrared wavelength range of 3.5–5.5 μm can be observed in Se-based chalcogenide fiber with 750 ppm Pr3+ ion doping, under the pump of 1.5 μm laser. The loss at 4.26 and 4.5 μm is mainly caused by the absorption of CO2 and H-Se impurities. Figure 7.36 shows a 300 μm diameter Pr3+ doped Se-based chalcogenide fiber pump scheme and IRSS system structure diagram. This fiber has the characteristics of high temperature resistance and good flexibility. The 5 × 5 array of this fiber is used as the IRSS laser source. As shown in Fig. 7.36, when an infrared imaging simulation is performed on it, it is found that the imaging effect is basically uniform and clear. However, since the array fibers are bare, there is a crosstalk error of about 2%, which can be improved by using appropriate cladding/core structure fibers.

Silicon fiber

Pr3+ doped selenide glass Mid-infrared fluroscence

Semiconductor laser

Computer

Infrared fiber pixel array

Pump light Laser scanning

Fig. 7.36 Pumping scheme of Pr3+ doped se based chalcogenide fiber and array structure of IRSS system

7.3 MIR Luminescence Characteristics of RE Doped …

259

In 2015, Hu et al. [38] reported the amplification characteristics of Pr3+ doped GeAsGaSe fiber, and experimentally observed the fiber amplifier output signals at 4.0, 4.5, and 5.0 μm for the first time. The core diameter is 5 μm, the numerical aperture is 0.3, and the doping concentration is 0.055 mol%. Figure 7.37a is a schematic diagram of the fiber amplifier testing, the pumping method used is backward pumping. Figure 7.37b is the gain coefficient curve of signal light at different wavelengths, and the entire line type is consistent with the fluorescence spectrum line type. Figure 7.37c shows the signal output power of the fiber amplifiers at 4.0, 4.5, and 5.0 μm when the signal input power is 100 mW and the laser pump power is 10 W. Figure 7.38 shows the output signal intensity of 4.5 μm signal light under different pump power conditions. When the signal light power is 10 mW, the light conversion efficiency reaches 45% and nearly half of the pump energy is converted into signal light energy. When the input signal power is 10 and 100 mW, amplification gain coefficients of 25 and 16 dB, respectively, can be achieved within a bandwidth from Pr3+doped fiber

Coupler output

power/W

Pump laser

Slope efficiency

input

wavelength/μm

wavelength/μm

Fig. 7.37 Pr3+ doped chalcogenide fiber (left) schematic diagram of amplification system; (right) slope efficiency of output wavelength between 4 and 5 μm [38]

Signal output power/W

Fig. 7.38 Relationship between output signal power and pump power [38]

Input pump power/W

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7 Mid-Infrared Spectral Properties of Rare Earth …

4.25 to 4.55 μm. Therefore, high gain and low noise amplification of mid-infrared light are achieved at the same time. In 2020, Sojka and others of the Wroclaw University of Technology in Poland studied a MIR spontaneous emission source based on chalcogenide fibers doped with Pr3+ [82]. For the first time, they demonstrated a spontaneous emission fiber source operating in the MIR wavelength range of 3.5–8 μm, with an output power of at least 1 mW. The experimental results demonstrated the feasibility of a spontaneous emission source fabricated from Pr3+ -doped chalcogenide fibers pumped by a commercially available laser diode operating at 1.470, 1.511, and 3.5–8 μm. Figure 7.39 shows the measured dependence of MIR output power on the 1.47 μm pump power. A maximum output power of at least 1mW can be provided in the spectral range. In addition, the emission of Pr3+ chalcogenide selenide fiber in the 6.2–8.0 μm region was measured for the first time, as shown in Fig. 7.40, and it was confirmed that the Fig. 7.39 Measured dependence of MIR output power on pump power for a Pr3+ -doped Ge–As–Ga–Se spontaneous emission fiber source of length 70 mm. The pump wavelength was 1.47 μm. Output power was measured after passage through a long-pass filter with cut-on at 3 μm. BD is black diamond lens [82]

Fig. 7.40 Normalized MIR PL spectrum from the 70-mm-long Pr3+ -doped Ge–As–Ga–Se fiber across the spectral region: 6–8 μm, pumped with a 300 mW diode laser operating at 1.47 μm, and the PL was measured after passage through a long-pass filter with cut-on at 6.15 μm [82]

7.3 MIR Luminescence Characteristics of RE Doped …

261

Fig. 7.41 Measured photoluminescence decay at: a 6.5 μm; b 7 μm; c 7.4 μm and d 1.6 μm in 1000 ppmw Pr3+ -doped GeAsGaSe chalcogenide glass fiber. The laser excitation was at 1.47 μm [82]

lifetime emission measured at 7.4 and 1.6 μm came from the 3 F3 energy level as shown in Fig. 7.41. This observation is an important step towards the first MIR fiber laser operating at wavelengths above 4 μm. In 2020, Gan et al. of Ningbo University studied the near and MIR luminescence characteristics of Er3+ /Pr3+ co-doped Ge–As–Ga–Se glass [40]. The results show that the introduction of Pr3+ can effectively shorten the decay time of Er3+ : 4 I13/2 from 2.34 to 0.31 ms under 980 nm excitation, as shown in Fig. 7.42. Therefore, the decay time of Er3+ : 4 I13/2 is shorter than that of Er3+ : 4 I11/2 , and a number of particles reversal can be realized. In addition, they also studied the laser characteristics of the fiber by establishing a cascade laser model for numerical simulation and the test device is shown in the Fig. 7.43. Figure 7.44 shows the variation characteristics of output power of signal on fiber length under different ion doping concentrations. When the ion concentration of Er3+ and Pr3+ is 0.3 mol%, under the excitation of 980 nm, 5 W pumping, the output power of the 2.7 μm laser can reach 0.43 W, and the slope efficiency is 14.21%. These results proved that this system has great potential to be used for MIR fiber lasers.

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7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.42 a Fluorescence decay curves of 0.1 mol% Er3+ : 4 I11/2 and 4 I13/2 states in GAGS glass. B–d The measured fluorescence lifetimes of two energy levels for Er3+ -, Er3+ /Pr3+ -doped GAGS samples with different concentrations [40]

Fig. 7.43 Schematic diagram of Er3+ /Pr3+ -doped chalcogenide fiber cascade laser [40]

7.3.3 MIR Luminescence of Tm3+ Doped Chalcogenide Glass The energy level structure of thulium ion (Tm3+ ) and the corresponding energy level transitions are shown in Fig. 7.45. Tm3+ can produce near-infrared and MIR transitions of 1.2, 1.46, 1.8, 2.3, and 4.0 μm. Among them, 1.8, 2.3 and 4.0 μm lasers can be used in chemical sensing, medical and environmental detection. The 1.48 μm transition produced by Tm3+ ions overlaps exactly in the S band of communication.

7.3 MIR Luminescence Characteristics of RE Doped …

263

Fig. 7.44 a Output signal power of the Er3+ /Pr3+ co-doped fiber laser of different concentrations versus the fiber length. b Output signal power of the Er3+ /Pr3+ co-doped fiber laser of different concentrations versus the pump power [40]

Fig. 7.45 Tm3+ ion energy level and related transition 800 nm laser pumping 0.4 wt% Tm3+ : Ge25 Ga5 S70 chalcogenide glass fluorescence spectrum [9]

On the other hand, the transition from the ground state 3 H6 energy level to the excited state 3 H4 energy level of Tm3+ ions is just around 800 nm, which is very suitable for pumping with common commercial solid-state lasers. However, there are few reports in the literature on the MIR luminescence of Tm3+ doped chalcogenide glasses. Wei et al. [9] reported the near-infrared and MIR fluorescence spectra of 0.4 wt% Tm3+ -doped Ge25 Ga5 S70 chalcogenide glass for the first time in his doctoral dissertation. The obtained near-infrared and MIR fluorescence spectrum of the chalcogenide glass pumped by 800 nm laser is shown in Fig. 7.46, and the relevant spectral characteristic parameters are shown in Table 7.15. The fluorescence branching ratios of Tm3+ : 3 H5 → 3 F4 (3.98 μm) and 3 H4 → 3 H5 (2.325 μm) transitions in Ge25 Ga5 S70 glass are only 2.7 and 1.9%, respectively. However, due to the lower phonon energy of Ge–Ga–S glass, infrared fluorescent radiation becomes a reality. Although the transitions corresponding to 3.98 μm (3 H5 → 3 F4 ) and 1.46 μm (3 H4 → 3 F4 ) in Tm3+ ions are four-level transitions, they are suitable for small-signal MIR optical amplifiers due to their relatively small fluorescence branch ratio.

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7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.46 Fluorescence spectrum under 2.0 μm laser pumping, a Tm (0.2%): GLS sample; b Tm (1.5%): GLS sample; c Tm (1.5%)/Tb (0.2%): GLS Co-doped sample [20]; Tm3+ and Tb3+ ion energy level structure and Tm3+ → Tb3+ energy transfer mechanism [83]

Table 7.15 Spectral characteristic parameters of 3 F4 , 3 H5 and 3 H4 energy levels of Tm3+ ions in Ge25 Ga5 S70 chalcogenide glass [9] Transition

λ/nm

λ/nm

β/%

σse /pm2

τrad /s

τmeas s

3F → 3H 4 6 3H → 3F 4 5 3H → 3H 6 5 3H → 3H 4 5 3H → 3F 4 4

1826

188

100

1.48

1143

2535

3984

198

2.7

1.21

802

1218

50

97.3

1.53

2325

200

1.9

0.49

1461

82

7.5

0.76

160

267

Schweizer et al. [83] studied the MIR luminescence characteristics of Tm3+ singledoped and Tm3+ /Tb3+ co-doped Ga–La–S (GLS) glass, and observed fluorescence at 3.8 μm when a 2.0 μm laser pump is used and a 4.8 μm fluorescence output (as shown in Figure 7.46), where 3.8 μm corresponds to the Tm3+ :3 H5 →3 F4 transition, and 4.8 μm corresponds to the Tb3+ :7 F5 →7 F6 transition. The principle of Tm3+ /Tb3+ energy transfer is shown in Figure 7.46. Dai et al. [84] studied Tm3+ doped GeGaS–CsI chalcogenide glass. Due to the addition of CsI, there is no concentration quenching phenomenon when Tm3+ is doped above 10,000 ppm. Although the fluorescence branch ratios of Tm3+ 3 H5 → 3 F4 and 3 H4 → 3 H5 are low, a large amount of Tm3+ doping still makes the glass produce obvious fluorescence of 2.3 and 3.8 μm. Based on J-O theoretical analysis and fluorescence lifetime test, the quantum efficiencies of 2.3 and 3.8 μm fluorescence are only 1% and 2%, which also means that the use of Tm3+ doped chalcogenide glass as the working medium of the fiber laser will result in a larger lasing thresholdfor a working MIR laser. They also prepared and characterized 0.5T m2 S3 : 80GeS2 ·20Ga2 S3 doped chalcogenide glass ceramics, and discussed the mechanism

7.3 MIR Luminescence Characteristics of RE Doped …

265

of MIR luminescence enhancement [85]. After heat treatment at 458°C for 25 h, Ga2 S3 crystals with a size of ~ 50 nm were obtained. Spectral studies found that the 3.8 μm luminescence intensity of Tm3+ gradually increased with an extension of the crystallization treatment time; the MIR luminescence intensity of the sample after 25 h was increased by about five times compared with the untreated sample (as shown in Fig. 7.47). The fluorescence lifetime of 3.8 μm also has a consistent change rule, and the sample after treatment for 25 h is extended by about 76 μs compared with the untreated sample. The element distribution electron energy loss spectra of Ge, S, Ga and Tm (shown in Fig. 7.48) indicate that the luminescence enhancement effect is related to the formation of germanium-rich regions in the glass ceramic sample. This

Fig. 7.47 The left picture is the XRD curve of the sample before and after the GGT-20 glass sample is processed, the small picture is the STEM image of the sample after crystallization; the right picture is the MIR fluorescence spectrum of the sample at different processing times, and the table shows the corresponding fluorescence lifetime [85]

Fig. 7.48 High-angle annular dark field (HAADF) image of GGT-20 glass sample treated at 458 °C for 25 h, and the electron energy loss spectrum (EELS) diagram of Ge, S, Ga and Tm element distribution [85]

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7 Mid-Infrared Spectral Properties of Rare Earth …

conclusion is also verified by studying the luminescence spectra of glass samples with different GeS2 content and different β-GeS2 crystallinity.

7.3.4 MIR Luminescence of Er3+ Doped Chalcogenide Glass The energy level structure of erbium ion (Er3+ ) and the corresponding energy level transition are shown in Fig. 7.49. Ground state particles are excited to the high energy level 4 I11/2 through ground state absorption (GSA), and the radiation transition between 4 I11/2 and 4 I13/2 will produce a 2.77 μm MIR transition; energy level transitions between 4 I9/2 and 4 I11/2 will produce a 4.5 μm MIR transition, and the quantum efficiency of the 4 I9/2 radiation transition can reach 64%. The 1.5 μm transition produced by Er3+ ions is exactly in the C-band of optical communications systems. On the other hand, the transition from the ground state 4 I15/2 energy level to the excited state 4 I9/2 energy level of Er3+ ions are just around 800 nm, which is very suitable for pumping with common commercial solid-state lasers. Wei [9] reported the preparation and properties of Er3+ ion-doped Ge25 Ga5 S70 glass in his doctoral thesis. The absorption spectrum and energy level lifetime of Er3+ -doped glass were measured, and the fluorescence of erbium ions at the MIR band of 2.7 μm (as shown in Fig. 7.49) was observed, corresponding to the Er3+ :4 I11/2 → 4 I13/2 transition. Ye et al. [86] studied the spectral properties of Er3+ :0.7Ga2 S3 :0.3La2 O3 chalcogenide glass. The absorption spectrum and energy level lifetime were measured, and the fluorescence of Er3+ :4 I11/2 → 4 I13/2 transition at 2.7 μm was observed. The results show that the peak absorption and emission cross-sections of erbium iondoped chalcogenide glass are 2.5 times larger than that of erbium-doped silicate

Fig. 7.49 Schematic diagram of 4f energy level distribution of Er3+ ion and the fluorescence spectrum of 0.4wt% Er3+ : Ge25 Ga5 S70 chalcogenide glass pumped by 800 nm laser [9]

7.3 MIR Luminescence Characteristics of RE Doped …

267

glass. Due to the higher refractive index, the radiation transition rate is 5 times that of silicate glass. The sub-energy is low, and the multi-phonon relaxation rate is very low. In addition, RE ions have high RE solubility in the Ga–La–S glass matrix. In 1997, Schweizer et al. [36] used a gas laser (670 nm) to pump Er3+ ion-doped Ga–La–S glass bare fiber, the Er3+ ion doping concentration was 24,400 ppm, and the fiber length was 8.6 cm, the diameter is 270, and 3.6 μm MIR fluorescence is obtained. The fluorescence peak corresponds to the Er3+ :4 F9/2 → 4 I9/2 transition. In 1997, Schweizer et al. [13] studied the infrared emission spectra of erbium-doped 70Ga2 S3 :30La2 S3 (GLS) glass and optical fiber, detected MIR transitions of 2.0, 2.75, 3.6 and 4.5 μm and studied their characteristics. The Er3+ ions were excited by a DCM dye laser at 660 nm (4 F9/2 ), and the fluorescence was collected at one end of a fiber doped with 1.57 mol% Er3+ or on one side of a bulk sample. A black body emission source was used to correct the corresponding atmospheric absorption spectrum of the system. The results show four infrared Er3+ emission bands, at 2.0, 2.75, 3.6 and 4.5 μm. This non-uniform broadened emission peak shape provides the glass matrix with tunability over a wide range of wavelengths. The Er3+ radiation lifetime was measured, and the results showed that although the energy level width of the 4 I9/2 energy level is smaller than that of the 4 F9/2 energy level, its lifetime is longer than the latter. This shows that the attenuation rate of Er3+ ions in GLS glass is mainly radiation attenuation, and the proportion of non-radiation attenuation rate is very small. In 2000, Choi et al. [87] studied the luminescence of erbium ions in Ge–Ga–As–S glass doped with Er3+ and Er3+ /Tm3+ co-doped at 2.7 μm. A differential scanning calorimetry analysis of Ge–Ga–As–S glasses with different proportions found that Ge30 Ga2 As2 S62 glass is the best choice as a glass matrix due to its good thermal stability, greater solubility of RE ions, and smaller cut-off wavelength in the visible wavelength range. After co-doping with thulium ions, the energy transfer of Er3+ :4 I13/2 → Tm3+ :3 F4 occurs, which reduces the energy level lifetime of Er3+ :4 I13/2 . The analysis by the authors also took the view that the glass co-doped with erbium and thulium may achieve the population inversion of Er3+ :4 I13/2 and 4 I11/2 . Moizan et al. [8] studied the spectral parameters and MIR infrared spectra of Ge20 Ga5 Sb10 S65 glass and fiber with different Er3+ ion doping concentrations (500, 1000 and 10,000 ppm), and measured its absorption spectrum and fluorescence spectrum and energy level lifetime etc. At a wavelength of 5.2 μm, the loss of the Er3+ ion-doped Ge20 Ga5 Sb10 S65 (2S2G) fiber can reach 1.5 dB/m, and through experimental tests, two fluorescences of Er3+ ions in the MIR band at 1.75 and 4.6 μm are observed in the fiber (Fig. 7.50), with the 4.6 μm fluorescence being the longest wavelength fluorescence reported by Er3+ ion so far. Table 7.16 lists the theoretical calculation results and experimental results of the transition characteristics of the 2S2G glass with an Er3+ doping concentration of 500 ppm in the infrared range and in addition to 4 I9/2 to 4 I15/2 , the JO theoretical calculations and experimental results of other energy levels. The results are consistent and the deviation between the theoretical values of 4 I9/2 to 4 I15/2 and the experimental results is mainly due to the impurity absorption of SH. The research results also show that the Ge–Ga–Sb–S quaternary system has good glass-forming properties and is suitable for Er3+ ion-doped glass substrates.

268

7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.50 804 nm laser pumped 1000 ppm Er3+ : the fluorescence spectrum of Ge20 Ga5 Sb10 S65 bulk glass and optical fiber, a fluorescence at 1.5 and 1.7 μm; b fluorescence at 4.5 μm in bulk glass [8]

Table 7.16 Theoretical calculation results and experimental results of the transition characteristics of Er3+ doped 2S2G glass in the infrared range [8] Transition 4I 4 13/2 → I15/2 4I 4 11/2 → I15/2 →4I 13/2 4I 4 9/2 → I15/2 4 → I13/2 → 4 I11/2

E/cm−1

λ/μm

Aed /S−1

Amd /S−1

β/%

τrad /ms

τexp /ms

6528

1.531

10,138

0.986

431

115

100

1.8

1.9 ± 0.1

630

25

86.2

1.4

3609

2.771

75

1.4 ± 0.1

12,346

0.810

745

5817

1.719

174

18.8

1.1

10.7 ± 0.1

2208

4.529

4

0.8

13.8 4

80.4

Han et al. [47] designed an on-chip MIR laser source based on Er3+ doped GaLaS glass with multimode micro-resonators. The cavity is carefully designed to have a high Q factor at both the pump wavelength (0.66 μm) and the lasing wavelength (3.6 μm), and it is noteworthy that the separation between them exceeds two octaves. This is achieved by using a novel idea that operates the cavity as a miniature disk at the pump wavelength and as a miniature ring at the laser wavelength. Detailed numerical analysis shows that the high-Q characteristics of the cavity greatly reduces the laser threshold to 7.6 μW, and the laser slope efficiency is 10%, as shown in Fig. 7.51.

7.3 MIR Luminescence Characteristics of RE Doped …

269

Fig. 7.51 Lasing performance for gap = 100 nm and LC = 0.6 μm, and accordingly the power enhancement factor η = 50.64 at the pump wavelength and cavity Q = 2.2 × 105 at the signal wavelength. a The influence of the power enhancement on the lasing threshold, b the influence of the cavity Q-factor is shown [47]

7.3.5 Mid-Infrared Luminescence of Ho3+ Doped Chalcogenide Glass The energy level structure of holmium ion (Ho3+ ) and the corresponding energy level transitions are shown in Fig. 7.52. Ho3+ can produce near-infrared and MIR transitions at 1.2, 1.66, 2.03, and 2.9 μm. In 1994, Wei [9] reported the spectral characteristics of Ge25 Ga5 S70 glass doped with Ho3+ ions in his doctoral dissertation. Under the excitation of 900 nm laser, the MIR bands of 2.0 and 2.9 μm fluorescence were observed (as shown in Fig. 7.52), corresponding to Ho3+ : 5 I7 → 5 I8 and 5 I6 → 5 I7 transitions. Shin et al. [88] studied the MIR luminescence characteristics of Ho3+ ion-doped Ge30 As10 S60 chalcogenide glass and 56PbO–27Bi2 O3 –17Ga2 O3 heavy metal oxide

Fig. 7.52 Ho3+ ion energy level and related transitions [12]; the fluorescence spectrum of 0.4wt%Ho3+ : Ge25 Ga5 S70 chalcogenide glass pumped by 900 nm laser

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7 Mid-Infrared Spectral Properties of Rare Earth …

Table 7.17 Spectral parameters of Ho3+ ions in 70GaS–30La2 S3 glass [7] Transition

λ/μm

Aed /S−1

Amd /S−1

β/%/

τr /ms

τm /ms

η/%

σem /10–20 cm2

→

2.00

228

95

100

3.10

3.0

97

1.08

1.19

489

81

1.65

2.86

73

44

19

0.90

212

1.7

36

→ 5 I7

1.67

239

→ 5 I6

3.90

28

0.76

24

8

→ 5 I7

1.25

114

41

6

→ 5 I6

2.19

114

41

6

→ 5 I5

4.92

19

10

1

5I 7 5I 6

→

5I 8 5I 8

→ 5 I7 5I 5

5I 4

→ 5 I8

→ 5 I8

42 20

8

2.22 2.00

48

41

0.71

10

9

2.29

3.58

0.5

1

1.27

glass, and calculated the relevant spectral parameters. Samples pumped with a 905 nm laser achieved fluorescence at 2.0 and 2.9 μm. In 1996, Kim et al. [69] studied the MIR luminescence characteristics of Tm3+ /Ho3+ ion co-doped Ge25 Ga5 S70 glass 2.02 μm under 798 nm pumping. The results showed that when Ho3+ ions are introduced with a concentration of 0.7wt%, the Ho3+ :5 I7 → 5 I6 transition corresponding to the fluorescence intensity at 2.02 μm gradually increased with the increase of Tm3+ ion doping concentration from 0.1 to 1.0 wt%, mainly due to the energy transfer effect of Tm3+ :3 F4 → Ho3+ :5 I7 . Schweizer et al. [7] studied the MIR luminescence characteristics of Ho3+ :Ga– La–S glass, using 760 nm laser to pump 1.5 wt% Ho3+ :70GaS–30La2 S3 glass. Eight fluorescence bands at 1.2, 1.25, 1.67, 2.0, 2.2, 2.9, 3.9 and 4.9 μm were observed, among which the fluorescence at 4.9 μm is the longest wavelength fluorescence reported by Ho3+ ions so far. Table 7.17 lists the spectral parameters of 1.5 wt% Ho3+ :70GaS–30La2 S3 glass. Lee et al. [89] studied the influence of CsBr composition on the 2.0 μm fluorescence of the 5 I7 → 5 I8 transition in Ho3+ : Ge–Ga–S glass, and found that when the amount of CsBr introduced increased, Ge–Ga–S glass had a narrower absorption linewidth at 2.0 μm (as shown in Fig. 7.53), and its fluorescence spectrum around 2.0 μm is also narrower, when pumped by a 897 nm laser. This is mainly due to the increase in the CsBr content, especially when the level of CsBr content is close to the Ga content, so that a new [GaS3/2 Br]-group is formed in the glass system (as shown in Fig. 7.53), which will directly affect the Ho3+ ion coordination field structure. Dai et al. [90] studied the MIR fluorescence properties of Ho3+ : GGS sulfur halide glass, and calculated the vibrator intensity parameters and the spectral parameters etc. of Ho3+ ion in Ge–Ga–S–CsI glass by using J–O theory. The vibrator intensity parameters i of GGS glass samples with a doping concentration of 1.0 wt% are 2 = 8.38 × 10−20 cm2 , 4 = 1.91 × 10–20 cm2 , 6 = 1.29 × 10–20 cm 2 . This shows that Ge–Ga–S–CsI glass has lower covalent properties, compared with traditional oxide glass. Table 7.18 shows the comparison of strength parameters and theoretical

7.3 MIR Luminescence Characteristics of RE Doped …

271

Fig. 7.53 (1−x) Ge0.25 Ga0.10 S0.65 –xCsBr glass near 2.0 μm absorption spectrum, and possible structural units in the glass, a GaS4/2 : single tetrahedron, b Ga2 S6/2 : Ethane-like structure group, c Ga2 S2 S4/2 : colateral double tetrahedron, d [GaS3/2 Br]–: mixed tetrahedral unit [89]

Table 7.18 Comparison of Judd–Ofelt intensity parameters and theoretical oscillator intensity between GGS glass samples and oxide glass [90] Judd–Ofelt parameters i /(×10–20 , cm2 ) fcal

/(×10–6 )

References

GGS

Germanate

Fluoride

Phosphate

Silicate

Tellurite

2

8.38

3.30

2.28

3.33

3.60

6.92

4

1.91

1.80

2.08

3.01

3.15

2.81

6

1.29

0.17

1.73

0.61

1.31

1.42

5I → 5I 8 7 5I 6 5I 5 5F 5

1.898

0.90

1.44

–

1.54

1.95

1.045

0.26

0.72

0.63

0.93

1.00

0.134

–

0.12

–

0.25

0.24

5.693

1.24

2.67

2.65

3.69

4.56

[24]

[2]

[27]

[28]

[2]

[27]

oscillator strength between GGS glass samples with a doping concentration of 1.0 wt% and different oxide glasses. Like most chalcogenide glasses, Ge–Ga–S–CsI glass also has lower phonon energy. This advantage greatly reduces its multiphonon relaxation rate, making Ho3+ : possible to generate MIR fluorescent radiation at 2.81 and 3.86 μm. In the same year [91], they synthesized a series of chalcogenide glasses based on Ge25 Ga10 Se65 system co-doped with Ho3+ / Pr3+ ions of different ratios by melt-quenching technique. The absorption spectra, mid-infrared fluorescence and lifetime of glass samples under 908 nm laser excitation were measured. The emission cross section of Ho3+ :5 I7 → 5 I8 and absorption cross section of Pr3+ :3 H4 → 3 F2 were calculated. The energy transfer efficiency between Ho3+ and Pr3+ ions with different Pr3+ ion concentrations have been discussed. The results prove that Pr3+ is an efficient

272

7 Mid-Infrared Spectral Properties of Rare Earth …

Table 7.19 Value for the theoretical radiative lifetimes (τrad ), experimentally measured lifetimes (τexp ), quantum efficiencies (η), branching ratios (β), effective linewidths (ν), and emission crosssections (σemi ) for the MWIR and LWIR emitting levels of Tb3+ in GAGSe glass [30] Transition Initial state

Final state

7F 5 7F 4

7F 6 7F 6 7F 5 7F 5 7F 4

7F 3

λ (μm)

τrad (ms)

τexp (ms)

η (%)

β

ν (cm−1 )

σem (× 10–20 cm2 )

4.8

15.0

11

73

1

305

1.05

3.1

8.0

0.012

0.15

0.88

195

1.37

7.5

8.0

0.012

0.15

0.12

248

0.83

4.7

4.1

–

–

0.17

–

–

10.5

4.1

–

–

0.12

–

–

means to improve sensitivity which enhances the Ho3+ :2.9 μm fluorescence intensity significantly.

7.3.6 MIR Luminescence of Tb3+ Doped Chalcogenide Glass The energy level structure of terbium (Tb) ion and the corresponding infrared absorption transition are shown in Fig. 7.54a. Tb3+ ions have a large number of transition fluorescence emissions in the mid-to-far infrared band (3–12 μm), and can be easily excited by 1.5–2 μm commercial diode lasers. Figure 7.54b shows the characteristic absorption bands in the Tb3+ doped selenide glass composed of Ge30 Ga2 Sb8 Se60 , which can be measured by UV/VIS/NIR and FTIR spectrometers. Based on the energy level diagram, when Tb3+ ions are excited to the 7 F4 energy state by a 1.5 μm pump source, the MIR emission centered at 3.2, 4.8 and 7.5 μm has the potential to generate an MIR lasing output. Tb3+ has a longer-wavelength fluorescence emission in the MIR band, with 7 F5 → 7 F6 energy level fluorescence emission (central wavelength 4.8 μm) and 7 F4 → 7 F5 energy level fluorescent emission (central wavelength 7.5 μm) dominating, as its doped matrix glass is generally selenide or telluride. Table 7.17 show the parameters of different transitions. Among them, the 7 F5 → 7 F6 energy level transition has a large emission cross-section, a long fluorescence lifetime and a high quantum efficiency. Shaw et al. [30] fabricated Tb3+ doped GeAsGaSe glass and obtained a fluorescence emission cross section of 1.5 × 10–20 cm2 at 4.8 μm, which is compared with the emission cross sections of BIGGSe [92], GAT [92] and GLS [93]. The transition of 7 F4 → 7 F5 belongs to excited state energy levels, but its fluorescence branch ratio is large enough, and its emission cross-section is high, which is beneficial for producing MIR fluorescence with a center wavelength of 7.5 μm. However, the quantum efficiency of the 7 F4 energy level in Se-based chalcogenide glass is extremely low, and the fluorescence quenching effect is significant, which would result in high threshold operation in a lasing application.

Fig. 7.54 a Rare-earth energy diagram of the lower lying levels of Tb3+ with energies < 12 000 cm−1 . Potential MWIR and LWIR laser transitions are marked. Levels expected to be thermally coupled at room temperature are shaded; b Absorption spectrum of 1000-ppm Tb3+ -doped GAGSe glass [30]

7.3 MIR Luminescence Characteristics of RE Doped … 273

274

7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.55 a Comparison of absorption and emission cross sections of 4.8 μm fluorescence; b Comparison of normalized Tb3+ doped GeAsGaSe glass and optical fiber fluorescence spectra (2013 nm excitation) [48]

For the above reasons, the early research on Tb3+ mainly focused on the fluorescence emissions at the 7 F5 → 7 F6 energy level transition (center wavelength at 4.8 μm). Churbanov et al. [94] investigated Tb3+ -doped chalcohalide glasses with different components. Through fine-tuning the components, the Se–H and S– H components in the glass were reduced, in order to increase the lifetime of the 7 F5 energy level. Then optical fibers were drawn by using the high-performance glass. Studies have shown that increasing the content of I element or Ge element can increase the lifetime of 7 F5 energy level of Tb3+ ion. Ge5 As32 Ga0.5 Se57.5 I5 glass has the longest energy level lifetime of 16.1 ms. At the same time, this component glass contains the least Se–H and S–H groups, which is manifested in the optical fiber loss spectrum with a lowest loss of 1.5 dB/m over the range 6–9 μm. In 2015, Sojka et al. [48] conducted a detailed study on the spectroscopic properties of the 4.7 μm fluorescence in Tb3+ doped GeAsGaSe system glass and optical fiber. In this system, Tb3+ has a large emission cross-section and an absorption crosssection, as shown in Fig. 7.55a, which could produce lasing in the range 4.3–6.0 μm. The experiments determined that the fluorescence lifetime of the 7 F5 energy level of Tb3+ ion is 12.9 ms. In addition, the results indicated that compared with the self-absorption effect of bulk glass in the fluorescence emission of Tb3+ -doped fiber, the center wavelength of the 7 F5 → 7 F6 transition shifted to the long-wave direction by 200 nm, as shown in Fig. 7.55b. It is confirmed again that the 7 F4 energy level of Tb3+ ions has a strong quenching effect in chalcogenide glasses, the electrons of the 7 F4 energy level will relax to the 7 F5 energy level quickly, causing the population inversion, so the 7 F4 , 7 F5 and 7 F6 form a quasi-three-level system with the potential for 4.7 μm lasing without the need for a cascade laser approach. Based on this, Sojka et al. [46] simulated the use of 2.013 and 2.95 μm lasers to pump 500 ppm Tb3+ doped GeAsGaSe fibers in 2017. The simulation shows that when the pump power density is greater than 20 MW/m2 @2.013 μm or 10.7 MW/m2 at 2.95 μm, a laser with a center wavelength of 4.7 μm can produce a positive gain, as shown in Fig. 7.56a. The damage threshold of GeAsGaSe glass is 250 MW/m2 , which meets

7.3 MIR Luminescence Characteristics of RE Doped …

275

Fig. 7.56 a Calculated material gain N = σem N2 − σabs N1 as a function of the pump intensity for different pumping wavelengths and for laser emission at 4.7 μm in Tb3+ -doped Ge–As–Ga–Se; b Calculated output power as a function of input pump power for different fiber background losses. Results were calculated for the signal wavelength and the pump wavelength set to 4.7 and 2.95 μm, respectively [46]

the requirements for laser pumping. Fiber loss has a great influence on laser slope efficiency and pump power threshold. As shown in Fig. 7.56b, under the pumping of the 2.95 μm laser, as the fiber loss decreases from 9 to 1 dB/m, the slope efficiency of the fiber increases from 8 to 42%, and the pump light threshold was also reduced from 0.08 to 0.031 W. This shows that the fiber loss will seriously influence the slope efficiency of output laser. In 2020, Churbanov et al. [39] prepared Tb3+ doped Ge36 Ga5 Se59 glass by using a new vapor transport method for impurity removal, gallium-RE intermetallic compound method, etc.. The overall optical loss is reduced to 1.5 × 10–2 cm−1 and with the polished bulk glass in the optical system shown in Fig. 7.57a, a laser oscillation of 4.9–5.5 μm was obtained by using a 2.93 μm laser pump based on a Er:YAG laser, as shown in Fig. 7.57b. This is the first time that MIR laser action produced by REs was observed in chalcogenide matrix glass. The author believes that if the fiber can be configured properly, continuous laser output becomes possible, and it is expected that the RE-doped chalcogenide fiber will produce a viable MIR laser. With the extension of the working range of optical fiber sensors and detectors to the mid- and far-infrared bands, the special spectroscopic performance of Tb3+ which can generate a 7.5 μm wavelength fluorescence has aroused the interest of researchers. Sojka et al. [95] simulated the laser action of 7.5 μm wavelength in Tb3+ doped GeAsGaSe glass. They used the J-O theory to simulate and calculate the lifetime of each energy level of Ge16.5 As16 Ga3 Se64.5 glass doped with 500–1500 ppm Tb3+ . Among them, the calculated energy level lifetime of the 7 F4 energy level τ3-cal = 5.9 ms, and the calculated energy level lifetime of the 7 F5 energy level is τ2-cal = 11.8 ms, so the 7 F4 → 7 F5 transition has a self-terminating effect. Corresponding to the energy level diagram of Tb3+ ions, it can be found that in order to generate lasing at 7.5 μm, the lower energy level 7 F5 needs to be emptied to produce an idle light output of 5.1 μm to eliminate the self-termination effect. By using a 2.95 μm

276

7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.57 a Experimental laser setup; b Typical oscillograms of the 2.94 μm pump pulse (top) and of ~ 5 μm Tb3+ 7 F5 → 7 F6 laser oscillations (bottom) [39]

laser as the pump source, lasing action at 7.5 μm in a Tb3+ doped fiber with a loss of 1 dB/m at different powers was simulated. In addition, the Tb3+ doped fibers with different losses pumped by a constant power of 5 W were also simulated. The result is shown in Fig. 7.58. The left figure shows that with sufficient laser pump power, the slope efficiency of the 7.5 μm laser output can reach 9%; while the right figure shows that even at a higher fiber loss of 3 dB/m, the 7.5 μm laser can have an obvious output. However, as mentioned above, the 7 F4 energy level of Tb3+ ions is affected by the multi-phonon coupling effect in Se-based chalcogenide glasses, and the quenching phenomenon is serious. Most electrons relax to the 7 F5 energy level through non-radiative transitions. This suggests great difficulties in the realization of efficient lasing action at 7.5 μm. In 2018, Starecki et al. [42] reported for the first time that a Tb3+ doped Ga5 Ge20 Sb10 Se65 fiber produced a fluorescence output of 8 μm and improved the signal-to-noise ratio of the 8 μm fluorescence output signal through time-resolved spectroscopy experiments. In the experiment, two kinds of fibers with Tb3+ doping

7.3 MIR Luminescence Characteristics of RE Doped …

277

Fig. 7.58 a Calculated dependence of the output signal power on the fiber length with different pump power for Tb3+ ; b Calculated dependence of the output signal power on the fiber length with different levels of fiber optical losses for Tb3+ [95]

of 500 and 1000 ppm were prepared, with some impurity removal treatment carried out during the preparation process to reduce the optical loss at 8 μm to several dB/m (for 500 ppm Tb3+ the doped fiber loss is 3.9 dB/m, and the 1000 ppm Tb3+ doped fiber loss it is slightly higher). By using a 2.05 μm pump laser, a fluorescence output at 8 μm is obtained, and the signal-to-noise ratio is 60, as shown in Fig. 7.59. Taking into account the different test lengths of chalcogenide fibers with different doping concentrations, it is necessary to consider the influence of absorbed power. In fact, the fluorescence efficiency of 500 and 1000 ppm Tb3+ doped fibers are similar. However, since the 7 F4 energy level is severely quenched by the multiphonon coupling effect in Se-based glass, the fluorescence signal is extremely weak. In the same year, Abdellaoui et al. [96] and others tried to reduce the phonon energy of the glass, increase the refractive index, and increase the lifetime of the Tb3+ : 7 F4 energy level by replacing part of Se with Te in the glass system Ga5 Ge20 Sb10 Se65 to obtain more stronger 8 μm fluorescence. In this article, two glasses, Ga5 Ge20 Sb10 Se65 and Ga5 Ge20 Sb10 Se45 Te20 , doped with 500 ppm Tb3+ were selected and drawn into Fig. 7.59 8 μm fluorescence output in a 26 cm long 500 ppm Tb3+ doped Ga5 Ge20 Sb10 Se65 fiber and a 17 cm long 1000 ppm Tb3+ doped Ga5 Ge20 Sb10 Se65 fiber. Insert figure: Energy level structure of Tb3+ transition [42]

278

7 Mid-Infrared Spectral Properties of Rare Earth …

Fig. 7.60 Emission spectra of 500 ppm Tb3+ doped Ga5 Ge20 Sb10 Se65 and Ga5 Ge20 Sb10 Se45 Te20 fibers under excitation at λ = 2.05 μm: a 7 F4 → 7 F6 (3.1 μm) transition; b 7 F5 → 7 F6 (4.7 μm) transition; c 7 F4 → 7 F5 (8.0 μm); d Tb3+ low-lying manifolds energy diagram corresponding to the measured a–c spectrums [96]

optical fibers. However, the results of the fluorescence test did not match expectations. The fluorescence of Se-Te fiber in all wavelength bands is weaker than that of Se-based fiber, as shown in Fig. 7.60 (it can be seen that the fluorescence intensity of the upper energy level transition of 7 F4 is far weaker than that of 7 F5 → 7 F6 transition, the fluorescence signal intensity of 3.1 and 8.0 μm is only one tenth and one ten thousandth of the signal intensity of 4.8 μm respectively. This is mainly due to: the dispersion of RE ions in Te-containing glass is not as good as Se-based glass; the loss of Se-Te fiber is high, and the introduction of Te does not effectively reduce the overall phonon energy of the glass. Therefore, although the introduction of Te can in theory be beneficial to the generation of 8 μm fluorescence, a new impurity removal process is needed to further reduce the loss of the fiber. At present, pure Se glass is still a better choice for Tb3+ doped host materials.

7.4 Problems and Prospects Because chalcogenide glass has low phonon energy and high transparency in the MIR band, chalcogenide glass doped with RE ions have become one of the best choices as

7.4 Problems and Prospects

279

a gain medium for MIR fiber amplifiers and lasers. However, chalcogenide glasses have a relatively low solubility of REs compared to oxide glasses. By appropriately adjusting the glass composition, the RE doping characteristics and luminescence characteristics can be effectively improved. At present, the solubility of RE ions is mainly improved by adding elements such as Ga and In. The amplifier and laser applications of doped chalcogenide glass in the MIR needs to be realized in the form of optical fiber. At present, the loss of passive optical fibers can be reduced to a very low level, and passive optical fibers of As2 S3 and As2 Se3 have been commercialized. However, due to the significant absorption of impurities in the optical communication window and MIR range, the rare-earth-doped fiber needs to be further optimized and purified. In addition, the fusion splicing between chalcogenide gain fiber and conventional silica fiber also needs to solve the insertion loss caused by the huge difference in refractive index and glass thermal properties. At the same time, in order to increase the output power of MIR light, the laser damage threshold of the fiber needs to be increased. For 1.8 and 2.0 μm laser pumping, damage tolerance is particularly important. Furthermore, mirrors are essential in fiber lasers, so coating mirrors or fiber gratings also need to be resolved. In addition, for the gain media doped with Dy3+ , Pr3+ and Tb3+ chalcogenide glasses, the study of solid and sheet lasers is also a future research direction.

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Chapter 8

Supercontinuum Generation in Mid-Infrared Glass Fibers Shixun Dai, Yingying Wang, Gerald Farrell, and Peiqing Zhang

When intense optical pulses propagate through nonlinear media, their temporal as well as spectral evolution will be affected by not only the dispersion properties but also the optical nonlinearities, leading to a wide spectral broadening of the pulses and so-called supercontinuum (SC) generation. The emergence of photonic crystal fiber (PCF) in 1996 allowed for great progress in SC generation based on silica fibers. However, SC sources generated in silica fibers are limited to operation below 2.5 μm due to the strong material absorption of silica. Mid-infrared (MIR) sources have attracted much attention because of their great potential in various applications such as spectroscopy, sensing, and optical coherence tomography. Therefore, soft-glass fibers, made of fluoride, tellurite, and chalcogenide (ChG) glasses which are transparent in MIR region, have been investigated and adapted extensively nonlinear media for producing MIR SC sources. In this Chapter, we will briefly introduce the development process and generation mechanism of SC sources in the first section. Sections 8.2 and 8.3 then focus on the MIR SC generation in fluoride and ChG fibers, respectively. Section 8.4 is devoted to the soft-glass fiber cascading scheme for MIR SC generation. Finally, we will illustrate the typical applications of MIR SC sources in the last section.

8.1 Overview of SC Sources 8.1.1 A Brief History SC generation in nonlinear media originates from the interaction of the dispersion and nonlinear, such as self-phase modulation (SPM), four wave mixing, stimulated Raman scattering (SRS), soliton self-frequency shift, etc [1]. It is a process in which narrow band pump pulses with high peak power are broadened continuously in the © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Wang et al., Mid-Infrared Fluoride and Chalcogenide Glasses and Fibers, Progress in Optical Science and Photonics 18, https://doi.org/10.1007/978-981-16-7941-4_8

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8 Supercontinuum Generation in Mid-Infrared Glass Fibers

Fig. 8.1 SC generation in nonlinear media. Reproduced from Shen et al. 2018 [4]

spectral domain in nonlinear media (shown in Fig. 8.1). SC Sources have been studied extensively in various nonlinear media since 1970, when it was first reported by Alfano et al. in BK7 glass pumped by a mode-locked Nd: glass laser [2, 3]. Early research on SC generation was mainly based on bulk solids, liquids and gases. The required pump power was very high and the generated SC sources exhibited narrow bandwidth and low output power due to the short interaction length between the pump light and nonlinear media and the lack of good light confinement. As manufacturing technology improved and transmission loss reduced, silica fibers were considered as ideal media for SC generation given their larger nonlinearity compared to bulk glasses. However, the bandwidth and coherence properties were still limited by the dispersion of silica fibers. Along with performance improvements in pulse lasers and the emergence of the photonic crystal fiber (PCF) in 1996 [5], SC generation in optical fibers was extensively developed. PCFs were immediately used for SC generation experiments thanks to their flexible structural design, high nonlinear coefficients and controllable dispersion. The first SC generation in PCF was reported in 2000 by Ranka et al., with SC generation covering a range from 390 to 1600 nm, obtained by injecting 100 fs pulses at 790 nm into a 75-cm-long PCF, as shown in Fig. 8.2 [6]. Since then, SC generation has become a strong research focus in the field of nonlinear optics given their significant potential for use in diverse applications, such as spectroscopy, hyperspectral microscopy, and optical coherence tomography. As a result of huge efforts by a large number of researchers, remarkable progress and development have been achieved in SC generation based on silica fibers, and SC sources with an output power in Watts spanning the entire transmission window of silica glass are now commercially available [7]. During the past two decades, many researchers have been devoted to extending the SC spectral range from visible and near-infrared (NIR) to ultraviolet (UV) and MIR even far-infrared (FIR). In particular, the MIR spectral region is regarded as an important target for SC sources because most molecules exhibit fundamental vibrational absorption bands in this region, leaving distinctive spectral fingerprints

8.1 Overview of SC Sources

287

Fig. 8.2 a Micrograph image of the silica PCF, b SC generated in a 75-cm-long silica PCF pumped by 790 nm pulses. The dashed curve represents the pump pulses. Adapted from Ranka et al. 2000 [6] with permission from the Optical Society

which are critical for applications, such as biomedical science, sensing, and defense and security [8–10]. However, silica fibers are not suitable for generating MIR light sources (> 2.5 μm) because of their very high material loss in the MIR spectral region. Various non-silica soft glass fibers made of fluoride, tellurite, and ChG glasses have been used to generate MIR SC sources, and numerous advances have been accomplished. Fluoride and tellurite glasses are both transparent in the wavelength range below 5.5 μm, while ChG glasses have a comparatively wider transparency window and higher optical nonlinearity, making them excellent candidates for MIR SC generation. To date, an ultrabroad SC generation spanning the range from 2 to 16 μm has been successfully demonstrated in a Te-based ChG fiber [11], and a SC source with high output power reaching 30 W has also been realized in a fluoride fiber [12]. Meanwhile, a commercial MIR SC source with a spectral range of 1–5 μm and an output power of 1 W is available [13].

8.1.2 SC Generation Mechanism The evolution of an optical pulse, having a slowly varying electric field amplitude represented by an envelope function A(z, t), propagating along a nonlinear fiber can be described by the solution to the generalized nonlinear Schrödinger equation as follows [1]:    i k+1 ∂ k A ∂A α i ∂ + A− βk k = iγ 1 + ∂z 2 k! ∂t ω0 ∂t k≥2

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8 Supercontinuum Generation in Mid-Infrared Glass Fibers

∞ × (A(z, t)

R(t )|A(z, t − t )| dt ) 2

(8.1)

−∞

where α is the fiber transmission loss in unit of 1/m, β k is the kth derivate of the propagation constant β, and γ is the nonlinear coefficient. ω0 is the pulse central frequency. R(t) is the Raman response function, which can be defined as: R(t) = (1 − f R )δ(t) + f R

    t τ21 + τ22 t sin exp − 2 τ2 τ1 τ1 τ2

(8.2)

where f R is the fractional contribution of the Raman response, τ 1 and τ 2 are the Raman period and lifetime, respectively. It has been proved that Eq. 8.1 is quite successful as a means to simulate SC generation in nonlinear fibers and that simulation results are consistent with experimental results. It is necessary to consider what nonlinear processes pump pulses undergo in a nonlinear fiber. It is known that the fiber dispersion will affect the group velocity of the pulses and further the intensity and phase of the pulses. This will cause the pump pulses with different dispersion characteristics to experience a range of diverse nonlinear effects. In turn, a variety of SC sources can be implemented which will possess great differences in temporal and spectral performance.

8.1.2.1

Pumping in the Anomalous Dispersion Regime

In general, efficient and broadband SC generation can be obtained by pumping in the anomalous dispersion regime close to the zero dispersion wavelength (ZDW) of the fiber. In this case, the SC generation in the fiber pumped with a high peak power is dominated by soliton dynamics, such as soliton fission and soliton selffrequency, shifting toward the long wavelength region, and dispersion waves in the short wavelength region [14]. Firstly, the pump pulse transforms into a higher-order soliton, which shows periodic spectral and temporal evolution over a soliton period. Higher-order dispersion and SRS are the two most important effects that can perturb the ideal periodic evolution of the higher-order soliton and induce pulse breakup into multiple fundamental solitons through soliton fission. Then each soliton experiences a continuous shift to a longer wavelength due to the Raman soliton self-frequency shift. Solitons that are generated earlier by the input pulse have relatively shorter temporal durations and faster propagation group velocities, leading to a greater frequency red-shift. Under higher-order dispersion perturbations, solitons emit dispersion waves in the normal dispersion regime via the Cherenkov radiation at the wavelengths that are phase-matched with solitons. Finally, solitons trap the dispersion waves by group velocity matching, and cause them to shift to short wavelengths.

8.1 Overview of SC Sources

289

Additional frequency components can also be generated by the cross-phase modulation and four wave mixing, and these effects can further broaden the spectral bandwidth of the SC source.

8.1.2.2

Pumping in the Normal Dispersion Regime

During the process of spectral extension in the normal dispersion regime, the nonlinear effects of SPM, optical wave breaking (OWB), and the SRS play important roles [14]. The SPM effect during the initial propagation in the fiber ensures the broadened pump pulse spectra always keeping a nearly symmetrical shape with typically an oscillatory structure, which is induced by spectral interference at different temporal locations within the pulse where the same spectral components exist. Then, OWB occurs on the trailing and leading edges of the pulse, which is responsible for the additional broadening on both sides of the spectrum. Moreover, the slightly asymmetric extension of the generated spectrum and the slightly increased energy of the relatively longer wavelengths are attributed to the SRS. However, these nonlinear effects are limited by the pump peak power and dispersion slopes, which inevitably impairs the achievable spectral broadening. In addition, SC sources generated from fibers pumped with periodic and ultrashort pulse trains can exhibit strong correlations between different pulses when the spectral broadening mechanism is highly reproducible, even though these pulses may still have complex spectral intensity and phase profiles. A commonly used method to obtain coherent SC sources is to pump fibers with optimized all-normal dispersion profiles. The SC spectral broadening in this case is mainly caused by SPM and OWB, which can maintain a deterministic phase relation as well as high coherence between the newly generated wavelengths and the pump pulse, and the noise-sensitive soliton dynamics are eliminated.

8.2 MIR SC Generation in Fluoride Fibers 8.2.1 Fluoride Glass Fiber Properties Fluoride fiber is multi-composite glass optical fiber composed of several heavy metal fluorides, and many different types of fluoride fibers exist depending on the material composition. Because of the difficulty in fabricating fluoride fibers, step-index structures are always employed in SC generation and other fields. ZrF4 -based ZBLAN (ZrF4 –BaF2 –LaF3 –AlF3 –NaF) and InF3 fibers have been used frequently to generate SC sources. Compared with silica fibers, fluoride fibers have higher transmission loss in the range of > 2 μm, but much lower transmission loss of < 1 dB/m in the range of

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8 Supercontinuum Generation in Mid-Infrared Glass Fibers

Fig. 8.3 Transmission losses of silica fiber and several single-mode fluoride fibers

Table 8.1 Optical properties of silica, ZBLAN and InF3 glasses [9] Glass

n

n2 a /n2 (silica)

Optical damage threshold at 2 μm (MW)

Silica

1.45

1

15.1

ZBLAN

1.49

1.2

12.0

InF3

1.50

1.2

12.0

a

n2 is the nonlinear refractive index coefficient of the glass

1–4 μm. As displayed in Fig. 8.3, < 0.5 dB/m loss can be achieved from visible to 4 μm for ZBLAN fiber and from 2 μm to longer than 4.5 μm for InF3 fiber. Meanwhile, fluoride fibers have the slightly larger nonlinear coefficient than silica fibers, as shown in Table 8.1. Thus it is expected to obtain SC sources spanning from visible to wavelengths beyond 4 μm, which far exceeds the long wavelength cutoff edge caused by the material loss in silica fibers. As described in Sect. 8.1.2, the fiber dispersion characteristic has an important effect on the SC broadening. The ZDWs of single-mode fluoride fibers are usually located around 1.5 μm. Figure 8.4 shows the dispersion properties of ZBLAN material and two commercial ZBLAN fibers with core diameters of 7 and 9 μm. Therefore, the SC sources generated in fluoride fibers can be greatly extended by the soliton dynamics when pumped by 1.55 or 2 μm laser pulses.

8.2.2 SC Generation The first SC source generated from fluoride fibers spanned a wavelength range from 1.8 to 3.4 μm with an output power of 5 mW, which was reported in 2006 by pumping a 91-cm-long ZBLAN fiber with a 900 fs 1550 nm mode-locked fiber laser [15]. In

8.2 MIR SC Generation in Fluoride Fibers

291

Fig. 8.4 Dispersion properties of two single-mode ZBLAN fibers

this case, the output power is only on the milliwatt level and its spectral width is also limited. Subsequent ot this demonstration, a large amount of effort has been devoted to increasing the output power and extending the spectral width of the SC sources from fluoride fibers.

8.2.2.1

SC Output Power Improvement

In order to improve the output power of SC sources, the simplest and the most straightforward method is to increase the pump power launched into fibers. The light transmitted in fluoride fibers at wavelengths shorter than 4.5 μm would not be absorbed significantly thanks to the relatively low fiber loss. The output power of SC sources can increase linearly with the pump power as long as the pump power is below the optical damage threshold. As is well known an SC spectrum is broadened in a fiber through nonlinear effects, the generation process is related to the pump peak power instead of the average power. Hence, the average power of the generated SC can be scaled up by increasing the pulse repetition rate while maintaining a similar pulse peak power. Figure 8.5a shows a setup for SC generation in ZBLAN fibers [16]. The pump laser is a MOPA system with a DFB seed operating at 1542 nm. Three amplification stages are included. The system outputs the amplified signal with a peak power of 6.7 kW for 1 ns pulse. As seen in Fig. 8.5b, when the pulse repetition rate is set to < 5 kHz, an SC source is generated with an output power of only 1.6 W. In contrast, the average power of SC source can be scaled up to 10.5 W by a factor of 6.5 by increasing the pulse repetition rate to 3.3 MHz. The SC spectra exhibit nearly identical spectral shape. Therefore, the SC generation system can improve its average power linearly by increasing the repetition rate of pumping pulse while keeping the same peak power. Increasing the pump power through multiple amplification stages can improve the output power of an SC source. However, the above output power beyond 2.5 μm of

292

8 Supercontinuum Generation in Mid-Infrared Glass Fibers

Fig. 8.5 a Experimental setup for SC generation in a ZBLAN fiber pumped with 1542 nm laser pulses. b SC output power scaling up by varying pulse repetition rate and pump power. c SC spectrum in the ZBLAN fiber pumped by ns pulses at a repetition rate of 3.33 MHz. Reproduced from Xia et al. 2009 [16] with permission from IEEE

the generated SC source is only 3 W since it is pumped by a 1.55 μm fiber amplifier. The MIR SC generation efficiency can be improved by pumping a ZBLAN fiber at a longer wavelength. With the development of TDFLs and TDFAs in recent years, SC generation in ZBLAN fiber pumped by 2 μm fiber lasers has become an interesting topic for researchers. The experimental setup for SC generation is illustrated in Fig. 8.6a [17]. Similar to that in the 1550 nm band, the pump is a 2 μm MOPA laser with multistage amplification. A mode field adapter (MFA) is used to achieve the mode conversion from the TDF-25/250 to ZBLAN fiber. By optimizing each 790 nm pump power, the power before the MFA can reach 31.5 W. By injecting such a high pump power into the ZBLAN fiber, the output power of SC source can be scale up to 13 W with a power of 6.85 W for wavelengths longer than 2.5 μm (Fig. 8.6b). Meanwhile, the SC spectrum is extended to cover a range of 1.9–4.3 μm, as shown in Fig. 8.6c. However in the above results, the ZBLAN fibers used in these experiments were mechanically spliced or butt-coupled to silica fibers, resulting in system instability and extra power loss and limiting the output power of SC sources. The development of low-loss fusion splicing between ZBLAN and silica fibers makes the low-loss coupling and robust system possible. Benefiting from this technology, even higher SC source output powers can be expected. A pump laser shown in Fig. 8.7a [12] included four parts: 1) a pulsed seed laser operating at 1550 nm; 2) a two-stage Erbium ytterbium co-doped fiber amplifier (EYDFA), to amplify the seed; 3) a section of SMF to frequency-shift the laser towards 2.4 μm; and 4) a TDFA to, amplify the wavelengths lying in the gain-band

8.2 MIR SC Generation in Fluoride Fibers

293

Fig. 8.6 a Experimental setup for SC generation in a ZBLAN fiber pumped with 2 μm laser pulses. b Output power of each stage in the setup versus the pump power. c SC spectra in the ZBLAN fiber with different output powers Reproduced from Yang et al. 2014 [17] with permission from the Optical Society

Fig. 8.7 a Experimental setup for the 30 W SC sources. b Output spectra from the TDFA under different pulse width and repetition rate. c SC spectra from the ZBLAN fiber under different pulse width and repetition rate. Reproduced from Yang et al. 2019 [12] with permission from the Optical Society

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8 Supercontinuum Generation in Mid-Infrared Glass Fibers

of Tm3+ ions. As a result, the pigtail of the TDFA emits 3 ns laser pulses under 3 MHz repetition rate with a spectrum covering 1.9–2.6 μm (Fig. 8.7b). The pigtail of the TDFA is fusion spliced with the ZBLAN fiber showing a splicing loss of 0.25 dB at 2 μm. A SC generation from 1.9 to 3.35 μm with a record output power of 30 W in Fig. 8.7c was obtained under TDFA output power of 41 W. It should be noted that the ZBLAN fiber used has a large core of 10 μm, making a traditional MFA unnecessary and thus further enhance the SC source output power. However, a drawback of the typical method to generate MIR SC sources in fiberbased system in undoped nonlinear fibers is a low power ratio beyond 3 μm because a large portion of pump energy cannot be converted to longer wavelength region due to the relatively short pump wavelength. Gauthier et al. have proposed an approach called in-amplifier MIR SC generation to overcome this problem [18]. In this concept, the rare earth gain and nonlinear effects combining into one doped fiber can contribute simultaneously to the broadening of the SC spectrum inside the amplifier. By such a scheme, Deng et al. presented a MIR SC generation setup [19], as shown in Fig. 8.8a. The structure of the broadband pump laser is similar to that in Fig. 8.7a. Output SC generation from TDFA spans from 2 to 2.5 μm with a repetition rate of 500 kHz. To cover the gain-band of Er3+ at 2.75–2.85 μm, these pulses are injected into a 10-mlong undoped ZBLAN double-clad fiber (DCF) for further SC spectral broadening to 2.0–3.5 μm, which is necessary for efficiently exciting the Er3+ -doped ZBLAN fiber amplifier. Then the broadened SC source was injected into a 7 mol % Er3+ -doped ZBLAN DCF (core diameter of 15 μm). Each connection between every two fibers involves fusion splicing, enabling highly efficient coupling and greatly enhancing

Fig. 8.8 a Experimental setup for SC generation in an Er3+ -doped ZBLAN fiber amplifier. b SC generation in the Er3+ -doped ZBLAN fiber amplifier pumped with different powers. c Power percentage of the SC source beyond 3 μm with increases in the pump power. Reproduced from Deng et al. 2020 [19] with permission from the Optical Society

8.2 MIR SC Generation in Fluoride Fibers

295

the system stability. Figures 8.8b, c show that the SC source output power reached 4.96 W with a spectral width of 2.7–4.2 μm when a maximum pump power of 23 W was launched into the Er3+ -doped ZBLAN DCF. As the pump power increases, the power ratio above 3 μm rises rapidly and achieves a maximum of 71.9% at a pump power of 20.8 W. Great progress has been made in SC source with high output power based on ZBLAN fibers. However, the fiber endcap is prone to damage during the long-term operation of SC sources because of the tendency of fluoride glasses to absorb water. The whole system must be cooled for efficient heat dissipation especially when operating with a high output power. In addition, the SC spectrum toward the longer wavelength is only extended to 4.75 μm at most which is mainly restricted by the ZBLAN fiber absorption loss.

8.2.2.2

SC Spectral Coverage Extension

To overcome the limitation of SC spectrum broadening in ZBLAN fibers and extend the spectrum beyond 5 μm while maintain a high output power, InF3 glass has emerged as a promising candidate. InF3 fiber has similar dispersion and nonlinearity properties with ZBLAN fiber, but a much lower transmission loss that reaches 1 dB/m at 5 μm. Much effort has been expended to increase the output power as well as the spectral coverage of MIR SC sources in InF3 fibers since the first demonstration in 2013 of 2.6–4.8 μm SC generation with an output power of 0.1 mW in InF3 fiber, which was pumped by an OPA at 3.4 μm [20]. With the development of fiber manufacturing and processing technology, the performance of the generated SC sources in InF3 fibers has been improved in recent years. In terms of SC spectral coverage in InF3 fiber, the spectrum has been extended to over 5 μm. By pumping a 15-m-long InF3 fiber with a pump system consisting of an OPG at 2.8 μm followed by a section of Er3+ -doped ZrF4 fiber, as shown in Fig. 8.9a, a MIR SC spectrum extending to 5.4 μm can be generated and shown in Fig. 8.9b. The spectral coverage is up to the long wavelength edge of the InF3 glass transparency window. This result shows the potential of InF3 fibers for wider MIR SC generation [21]. As discussed above, it is difficult to achieve a high power ratio for a wavelength range longer than 3 μm for SC sources in undoped ZBLAN fibers because of the sharply increased fiber loss above 4 μm. In contrast, the relatively low loss of InF3 fibers allows the pump energy to transfer to long wavelength region. Pumped with a MOPA laser similar with that in Fig. 8.7a, a high output power SC source of 11.8 W with spectral coverage of 1.9–4.9 μm was obtained in a InF3 fiber. The power in the wavelength beyond 3.8 μm is up to 2.18 W, corresponding to a power ratio of 18.5%, which can be seen in Fig. 8.10 [22]. In applications such as UV and MIR spectroscopy and metrology, a broadband SC spanning from UV to MIR is necessary. The material absorption in MIR and UV-induced optical damage limit silica fibers to broaden the SC spectra into MIR and reduce the lifetime of fibers. Among the non-silica glasses, ZBLAN glass can

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8 Supercontinuum Generation in Mid-Infrared Glass Fibers

Fig. 8.9 a Experimental setup for SC generation in an InF3 fiber. b SC generation in the InF3 fiber pumped with different powers. Reproduced from Gauthier et al. 2016 [19] with permission from the Optical Society

Fig. 8.10 a Experimental setup for SC generation in an InF3 fiber. b SC generation in the InF3 fiber pumped with different powers. Reproduced from Yang et al. 2020 [22] with permission from the Optical Society

transmit from UV to MIR. Therefore, a SC spectrum covers these wavelengths can be expected in ZBLAN fibers as long as the pump wavelength and the fiber ZDW can be well matched. Jiang et al. fabricated a solid-core ZBLAN PCF instead of a step-index type to blue-shift the fiber ZDW to around 1 μm [23]. Then a 1042 nm laser with a pulse duration of 140 fs and a repetition rate of 75 MHz was used as the pump source to excite the ZBLAN PCF with a fiber length of only 4 cm. A broadband and long-term stable SC source was generated spanning more than three octaves from deep-UV (200 nm) to MIR (2500 nm) using a pump power of ~ 60 mW (a peak power of only ~ 6 kW), as shown in Fig. 8.11. Although the SC source shows a low output power of 20 mW, it can be scaled up by optimizing the pump conditions such as increasing the single pulse energy. In addition, the SC spectral width from UV to MIR can be further expanded by improving the peak power of pump pulse to effectively enhance the intensity of nonlinear effects in fibers, and shortening the ZBLAN fiber length to dramatically reduce the power consumption caused by fiber loss. Assuming the pump average

8.2 MIR SC Generation in Fluoride Fibers

297

Fig. 8.11 SC spanning more than three octaves from deep-UV to MIR generated in a 4-cm-long solid-core ZBLAN PCF Reproduced from Jiang et al. 2015 [23] with permission from Springer Nature

power and pulse duration are fixed, the peak power can be increased by decreasing the pulse repetition rate. In a pump system with an operating wavelength of 1450 nm and a pulse duration of 180 fs, the pump peak power can be as high as 50 MW when the pulse repetition rate is set to 1 kHz even though the average pump power is only 20 mW. By using such a pump laser, an ultra-broadband SC generation from 350 nm to 6.28 μm (Fig. 8.12) was generated in a 2-cm-long ZBLAN step-index fiber [24]. Fig. 8.12 SC generation spanning from UV to 6.28 μm in a 2-cm-long ZBLAN fiber. Reproduced from Qin et al. 2009 [24] with permission from AIP Publishing

298

8 Supercontinuum Generation in Mid-Infrared Glass Fibers

8.2.3 Fluorotellurite Fiber for SC Generation Fluorotellurite glass, a form of glass in which fluoride is introduced into tellurite glass, has been widely investigated for high power MIR fiber lasers in the last several years given their broad transmission window, stable chemical and physical properties compared to fluoride glasses [25, 26]. Recently, 70TeO2 –20BaF2 –10Y2 O3 (TBY) glass has been developed for MIR SC generation. TBY glass exhibits a broadband transmission window of 0.35–6 μm (Fig. 8.13a) [27]. The addition of Y2 O3 can increase the glass transition temperature to 425 °C, higher than that for fluoride glasses is (250–300 °C), improving the glass stability of chemical and physical properties. Moreover, TBY glass has better water resistance than fluoride glasses [27]. As can be seen in Fig. 8.13b, after placing the 2-mmthick ZBLAN glass in deionized water for one hour, its transmittance is reduced dramatically. Two absorption bands appear at 2.9 and 6.1 μm, originating from vibrations resulting from OH− and H2 O absorption. In contrast, there is no obvious change in transmittance for TBY glass even though it is submerged in water for 12 days. Choosing TBY glass as core, and 33AlF3 –9BaF2 –17CaF2 –12YF3 –8SrF2 – 11MgF2 –10TeO2 as cladding, a fluorotellurite fiber with a core diameter of 6.8 and a ZDW at 1810 nm was fabricated by Yao et al. [27]. The setup for SC generation is shown in Fig. 8.14a. The pump source is a 2 μm fiber laser with a pulse duration of 200 fs and a repetition rate of 50 MHz. With the increase of the pump power, the SC spectrum from the fluorotellurite fiber is gradually extended, and its output power also increases. When the pump power is 15.9 W, an SC generation spanning from 947 to 3934 nm is obtained with an output power of 10.4 W (Fig. 8.14b, c). The output power of SC source can be scaled up further by increasing the pump power and enlarging the fiber core diameter to improve its power handling capbability. As shown in Fig. 8.15a [28], the pump source is a high power laser output from a TDFA with a repetition rate of 50 MHz. When the launched average pump power injected into a fluorotellurite fiber with a core diameter of 11 μm is increased to

Fig. 8.13 Transmittance of 2-mm-thick a TBY glass after putting the in in water for 0 and 12 days, and b ZBLAN glass after putting the glass in water for 0 and 1 h. Reproduced from Yao et al. 2018 [27] with permission from the Optical Society

8.2 MIR SC Generation in Fluoride Fibers

299

Fig. 8.14 a Setup for SC generation in a fluorotellurite fiber. b SC spectra generated from the fluorotellurite fiber pumped with different powers. c Dependence of the SC output power on pump power. Reproduced from Yao et al. 2018 [27] with permission from the Optical Society

Fig. 8.15 a Setup for SC generation in a fluorotellurite fiber. b SC spectra generated from the fluorotellurite fiber pumped with different powers. Reproduced from Li et al. 2020 [28] with permission from the Optical Society

39.7 W, broad SC generation spanning from 0.93 to 3.95 μm is possible. The output power of the SC source can reach 22.7 W (Fig. 8.15b), corresponding to a power conversion efficiency of 57.2%. These results indicates that fluorotellurite fibers could be used as the nonlinear media for constructing over 20 W or even higher power level MIR SC sources with long-term stability.

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8 Supercontinuum Generation in Mid-Infrared Glass Fibers

8.3 MIR SC Generation in Chalcogenide Fibers 8.3.1 ChG Glass Fiber Characteristics ChG glass has been known for more than half a century as infrared (IR) optical materials. A highly important property of ChG glass is its broad IR transparency range, which extends the long wavelength cut-off edge from 12 to 20 μm depending on the mass of anionic elements present in the ChG glasses (viz. S, Se, and Te, as shown in Fig. 8.16 [29]), having significant advantages over silica and fluoride glasses. The IR coverage of ChG glass means that they it is the unique medium which can extend the SC spectrum to the MIR or even the FIR region (longer than 10 μm). ChG glass possesses a high third-order optical nonlinearity up to a thousand times greater than that of silica glasses, attributed to their high refractive indices (n, for example, ~ 2.9 for As2 Se3 and ~ 2.5 for As2 S3 ), as listed in Table 8.2. Furthermore, low phonon energy and long multiphoton absorption edge also make ChG glass excellent candidates for MIR SC generation.

Fig. 8.16 Typical IR transmission spectra of S-, Se-, and Te-based ChG bulk glass samples. Reproduced from Tao et al. 2015 [29] with permission from the Optical Society

Table 8.2 Optical properties of silica and ChG glasses

Glass

n

n2 a /n2 (silica)

Transmission range (μm)

Silica

1.44

1.0

0.2–2.5

As2 S3

2.45

200

0.6–12

As2 Se3

2.81

600

1.0–16

Te-based

~ 3.2

~ 1000

1.5–20

a

n2 is the nonlinear refractive index coefficient of the glass

8.3 MIR SC Generation in Chalcogenide Fibers

301

Fig. 8.17 Geometrical profiles of ChG MOFs. a Photonic crystal fiber with regular hexagonal geometry and three air hole rings; b Suspended-core fiber with three air holes; c Suspended-core fiber with six air holes; and d Hybrid MOF. Reproduced from [30] and Cheng et al. 2014 [31] with permission from the Optical Society

8.3.1.1

Step-Index Fibers

The zero material dispersion wavelength (ZMDW) of ChG glass is always located at long wavelengths. For example, the ZMDW of As2 Se3 glass is ~ 8 μm, and that of As2 S3 is 5 μm. Depending partly on material dispersion, the ZDWs of ChG step-index fibers also lie in long wavelengths, resulting in a large normal and steep dispersion at the relatively shorter wavelengths, which will reduce the effective optical nonlinearity and distort the ultrashort optical pulses propagating in fibers. Therefore, fiber dispersion must be carefully engineered with waveguide dispersion for specific applications such as the cases of wavelength conversion and SC generation, etc.

8.3.1.2

Microstructured Optical Fibers

Microstructured optical fibers (MOFs) have become popular topics in recent years because of their unique optical properties such as strong light confinement, adjustable dispersion, and enhanced optical nonlinearity as the core diameter is decreased by air holes arranging along the transverse section of the fiber. As optical media which can provide great flexibility in the fiber structure design for the MIR, MOFs based on ChGs have already been reported frequently in the literature. Several typical geometrical profiles of ChG MOFs are shown in Fig. 8.17. Despite the long ZMDWs of ChG fibers, large values of anomalous waveguide dispersion can be used to engineer the total dispersion into a zero dispersion or even anomalous regime. MOFs have great advantages in terms of dispersion control because of their flexible design. ChG MOFs with ZDWs shorter than 2 μm have been reported experimentally [32, 33].

8.3.1.3

Tapered Fibers

As a well-known fiber post-processing technology, tapered fibers (Fig. 8.18) can also dramatically increase the optical nonlinearity and engineer the total dispersion

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8 Supercontinuum Generation in Mid-Infrared Glass Fibers

Fig. 8.18 Schematic of a typical tapered fiber

of fibers. During the tapering process, the fiber diameter is decreased continuously, reducing the effective mode area (Aeff ) of the tapered fiber, and leading to a large nonlinear coefficient. The material dispersion is compensated with waveguide dispersion, resulting in the blue-shifting of ZDW. These results are of great significance for SC spectral broadening in ChG fibers.

8.3.2 SC Generation MIR SC generation in ChG fibers have attracted wide attention since the first reported SC source from an As–Se fiber in 2005 [34]. In the following years, a large number of reports on SC generation have been demonstrated in ChG fibers. Similar to Sect. 8.3.1, the MIR SC generation in ChG fibers is introduced in this section from the perspective of three types of fiber structure: SC generation in step-index fibers, MOFs, and tapered fibers.

8.3.2.1

Step-Index Fibers

The study of SC generation in ChG step-index fibers started in 2005. Wei et al. [35] demonstrated an SC generation using a 1.5-m-long As2 S3 single-mode step-index fiber pumped in the normal dispersion regime. A passively 1.5 μm mode-locked Er3+ -doped fiber laser with a pulse duration of 100 fs at a repetition rate of 20 MHz was used as the pump source. With an input power of 16.4 mW, the output spectrum (Fig. 8.19) was broadened with a bandwidth of 310 nm. Although the spectral width of the SC generation was not wide, the observed spectral broadening was very attractive and it confirmed the possibility of using ChG fibers for MIR SC generation. In the following years, researchers reported SC generation in ChG step-index fibers which were excited by fiber lasers at short wavelengths. However, broadband MIR SC generation was hardly achieved in ChG step-index fibers pumped in the normal dispersion regime with a short pump wavelength from a fiber laser. The ZDWs of ChG step-index fibers are usually located at long wavelengths (>5 μm). It is difficult to match the ZDWs of ChG fibers with the operating wavelengths of commercial fiber lasers. Moreover, the use of ChG step-index fibers in SC generation was limited by the lack of availability of high peak power pump sources in the MIR

8.3 MIR SC Generation in Chalcogenide Fibers

303

Fig. 8.19 First SC generation in an As-S step-index fiber. Reproduced from Wei et al. 2005 [35] with permission from the Optical Society

until optical parametric amplifiers (OPAs) and oscillators (OPOs) were successful employed as MIR pump sources to excite fibers in the anomalous dispersion regime. An ultrabroad MIR SC generation spanning the range from 1.4 to 13.3 μm was generated experimentally [36] when ~ 100 fs pulses with a central wavelength of 6.3 μm and a repetition rate of 1 kHz were launched into an 85-mm-long step-index fiber made of As2 Se3 core and Ge10 As23.4 Se66.6 cladding. The pump wavelength was located in the anomalous dispersion regime, just above the fiber ZDW of 5.83 μm. Figure 8.20 shows the setup used for SC generation and the measured result. This was

Fig. 8.20 a Setup for SC generation in the ChG fiber. BD-2, black-diamond-2 aspheric lenses; CM, concave mirror; LPF, long-pass filter; NDF, neutral density filter; NDFG, noncollinear difference frequency generation. b Measured SC generation in the ChG fiber pumped at 6.3 μm Reproduced from Petersen et al. 2014 [36] with permission from Springer Nature

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8 Supercontinuum Generation in Mid-Infrared Glass Fibers

the first time that an SC source with a spectral width of > 10 μm was achieved in ChG fibers, thus representing a breakthrough in broadband MIR light source research. Researchers subsequently focused on the enhancement of SC spectrum broadening. ChG fibers with short length, small core diameter, high nonlinearity, and optimized pump conditions were designed and used to generate SC spectra with good performance. For example, an As-Se fiber with a length of 3 cm and a ZDW of 5.5 μm was pumped by pulses of 9.8 μm with 170 fs pulse duration and 1 kHz repetition rate produced from a difference frequency generation. When the peak power was 2.89 MW, SC generation spanning from 2 to 15.1 μm was obtained [37]. When the pump conditions are fixed and cannot be changed, ChG fiber with a large-core, which can relax the focusing of the OPA beam, can be alternatively adopted to increase the coupling efficiency and improve the output power. The two other ways to achieve high output power are by reducing the fiber loss resulting from the impurity band and shortening fiber length used for SC generation.

8.3.2.2

Mictrostructured Optical Fibers

The ability to tailor the fiber dispersion profile is an underlying theme across all glass and fiber designs. The use of MOF technology provides primary flexibility in dispersion control to match pump source conditions. By changing the structural parameters, the waveguide dispersion of MOFs can be controlled, so that the total dispersion can be engineered precisely. To ensure efficient SC generation, researchers always choose a suitable pump wavelength to excite MOFs in the anomalous dispersion regime. Møller et al. [38] demonstrated SC generation in a 18-cm-long As38 Se62 fiber with a core diameter of 4.5 μm and ZDW of 3.5 μm pumped at different wavelengths with 320 fs pulses from an OPA. When the pump wavelength was shifted from 3.5 to 4.7 μm, the solitonic long wavelength edge moved to longer wavelengths in the simulated spectra, and accordingly the dispersive waves were shifted to shorter wavelengths. Consequently, the broadest numerical spectrum spanning from 1.65 to 7.7 μm and the largest experimental output power of 15.6 mW were obtained in this work. To engineer the dispersion profile more precisely, hybrid MOFs fabricated by two materials for the core and cladding glasses were proposed and studied theoretically and experimentally, and the adjustability of which can greatly facilitate the effective of SC generation. By using a hybrid four-hole AsSe2 –As2 S5 MOF (Fig. 8.17d) with a ZDW of 3380 nm and γ of 4.9 × 104 W−1 km−1 , a 1.25–5.4 μm SC source was obtained when pumped at 3389 nm, as shown in Fig. 8.21. The introduced As2 S5 cladding exhibits improved transmission properties compared with that of As2 S3 glass in the visible and NIR range [31]. From the above discussions, ChG MOFs have the advantages of wide dispersion adjustable range and large γ, but the generated SC spectra are not as wide as that from step-index fibers due to the low power coupling efficiency. In addition, the design of MOFs with air holes implies that the fiber core can be in contact with the ambient

8.3 MIR SC Generation in Chalcogenide Fibers

305

Fig. 8.21 SC spectrum in a hybrid four-hole AsSe2 –As2 S5 MOF. Reproduced from Cheng et al. 2014 [31] with permission from the Optical Society

atmosphere. Extra losses induced by this exposure can reduce the transmission efficiency in the MOF with time, and a certain beam power is inevitably consumed during the propagation [39, 40]. Therefore, the impact of optical aging in MOFs on SC generation necessarily needs to be considered for long-term applications in non-controlled atmospheric conditions.

8.3.2.3

Tapered Fibers

Another important method to tailor the dispersion property is to taper fibers, which can engineer the overall dispersion from normal to zero or even anomalous region and enable to increase the nonlinearity dramatically by reducing the effective mode area Aeff . Owing to the improvement of fiber fabrication and tapering techniques, ChG tapered fibers have been used in many applications, such as frequency conversion, chemical sensing, and SC generation. In particular, by using tapered fibers, the pump power required for SC generation can be decreased and the pump wavelength can be shortened. In 2011, Hudson et al. increased γ to 12,400 W−1 km−1 in an As2 S3 tapered fiber with a taper waist diameter of 1.3 μm and a mode area of 0.8 μm2 , exhibiting a ZDW around 1.4 μm. The pump pulses had a duration of 250 fs at a repetition frequency of 38.6 MHz and were generated from an Er-fiber-based mode-locked laser. An SC spectrum spanning from 970 to 1990 nm was achieved from the fiber taper by using only 77 pJ pump pulse energy (3 mW average power) [41]. Apart from increasing pump power, enhancing nonlinearity and reducing power consumption for higher-order modes of fibers are both effective methods to broaden SC spectral bandwidth and improve the output power in ChG tapered fibers. Tapered MOFs with large Aeff can also be used for SC generation, thus a high coupling efficiency and damage threshold are obtained simultaneously for the pump power injected to the fiber end facet, while the tapered waist offers strong nonlinearity and anomalous dispersion at the pump wavelength. Moreover, the MOF structure

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can ensure single-mode propagation, which improves the beam quality and reduces losses in the taper due to higher-order mode stripping. Petersen et al. tapered a 15.1 μm core Ge10 As22 Se68 PCF into a core diameter of 5.9 μm at the taper waist. The higher-order modes excited in the large-core fiber were stripped during the propagation in the transition region. By using an OPA source with 250 fs pulse duration and 21 MHz repetition rate at 4 μm as the pump source, a 35.4 mW SC source with the broadest bandwidth from 1 to 11.5 μm, or the highest output power of 57.3 mW SC source with a spectral bandwidth spanning from 1 to 8 μm were generated in the tapered PCFs, as shown in Fig. 8.22 [42]. Several improvements were also considered to guide the future optimization of tapered fibers. Relatively shorter taper waists, higher pitch-to-hole ratio, and shorter fiber length before the taper are all advantageous as a means to extend SC spectra to longer wavelengths.

Fig. 8.22 Taper characterization. a Illustration of the longitudinal sections of the tapered fiber: Length before the taper (LBT ), down-taper length ( LDT ), waist length (LW ), up-taper length (LUT ), and length after the taper (LAT ). b, c Same magnification SEM images of the 12.7 μm fiber crosssection in LBT and LW , respectively. d Normalized FTIR transmission through the three tapers before cut-back. e Typical measured outer diameter profile in the taper transition region. f SC generation spanning from 1 to 8 μm with an output power of 57.3 mW. g SC generation spanning from 1 to 11.5 μm with an output power of 35.4 mW. Reproduced from Petersen et al. 2017 [42] with permission from the Optical Society

8.3 MIR SC Generation in Chalcogenide Fibers

307

Fig. 8.23 Measured SC spectra generated from 12-cm-long As-S ChG tapered fibers with transition region length varying from 6.7 to 10.5 mm pumped at 3.25 μm.

In addition, several structural parameters of tapered fibers, such as taper waist diameter and transition region length, can affect the performance of SC sources. Some researchers have demonstrated that a wider SC generation can be achieved in a tapered fiber with a smaller diameter. A small fiber diameter produces a small Aeff and a large γ in ChG tapered fibers, which will increase the laser power intensity at the taper waist. Nonlinear power intensity dependent effects, such as SPM and SRS, are significant, and can lead to a wider SC generation. The impact of the length of the transition region of an As-S tapered fiber on the SC spectral behavior was investigated by Wang et al. As shown by the experimental results in Fig. 8.23, when the diameter of the transition region changes slowly with a relatively longer transition length, it is always accompanied by a less number of higher-order modes and higher transmission efficiency of light, which results in a higher output power and a larger spectral broadening from the tapered fiber. A 1.4– 7.2 μm SC generated [43] with an average power of 1.06 mW was achieved from a 12-cm-long As-S tapered fiber with a transition region length of 10 mm pumped by an OPA with 150 fs pulse duration and 1 kHz repetition rate at 3.25 μm.

8.3.3 Novel ChG Fibers for MIR SC Generation In addition to the commonly used ChG fibers including As2 S3 , As2 Se3 , and Ge-AsSe fibers, novel ChG fibers have also been investigated and employed as nonlinear media for MIR SC generation to further extend the spectral coverage and increase the output power.

308

8.3.3.1

8 Supercontinuum Generation in Mid-Infrared Glass Fibers

As-Free Fibers

Most ChG fibers for SC generation, such as As2 S3 and As2 Se3 , contain the toxic element arsenic. Thus, health and safety issues can arise when these toxic materials are exposed to a laser power that is beyond the damage threshold of the materials, as evaporation of As product can take place. Obviously, these evaporated gases are extremely harmful to health. The replacement of highly toxic arsenic with antimony (Sb) makes the Ge–Sb–Se glasses more environmentally favorable. Widely used As2 S3 and As2 Se3 glass have relatively low glass transition temperature (Tg) at 185 and 178 °C, respectively. By contrast, Ge–Sb–Se glasses exhibit higher thermal and mechanical durability (as listed in Table 8.3) and have been demonstrated to be suitable for molded infrared-transmitting lenses. Furthermore, the nonlinear refractive index of the Ge15 Sb25 Se60 glass (19 × 10–18 m2 /W) is greater than that of As2 Se3 glass, and also greater than that of Ge–As–Se because of the replacement of As by the more metallic Sb. The most important thing is that visible damage arises at a beam intensity of 3674 GW/cm2 for Ge15 Sb25 Se60 glass at 3.0 μm, a value which is more than twice than that of As2 Se3 glass, which is 1524 GW/cm2 [44]. The microscope images of laser-induced damage are shown in Fig. 8.24. This indicates that Ge15 Sb25 Se60 fibers can endure relatively higher peak power. Such a large nonlinear refractive index and a high laser-induced damage threshold of the glass, demonstrates that Ge–Sb–Se based fibers have the potential to generate an ultra-broad MIR SC source with high output power. Ge–Sb–Se fiber fabrication and SC generation have been demonstrated experimentally. In a fiber with the core and cladding made of Ge15 Sb25 Se60 and Table 8.3 Refractive index and transition temperature of As2 S3 , As2 Se3 , and Ge–Sb–Se glasses Glass

As2 S3

As2 Se3

Ge15 Sb20 Se65

Ge15 Sb25 Se60

Ge20 Sb15 Se65

Ge28 Sb12 Se60

n@3 μm

2.42

2.79

2.73

2.92

2.61

2.63

Tg (°C)

185

178

227

236

285

300

Fig. 8.24 Optical microscope images of laser-induced damage sites after irration with a 3 μm fs laser at 30 mW and 20 s duration. a As2 Se3 glass; b Ge15 Sb25 Se60 glass. Reproduced from Ou et al. 2016 [44] with permission from the Optical Society

8.3 MIR SC Generation in Chalcogenide Fibers

309

Fig. 8.25 Broadband MIR SC generation in a Ge–Sb–Se fiber. Reproduced from Zhang et al. 2016 [45] with permission from John Wiley & Sons

Ge15 Sb20 Se65 glasses, respectively, SC generation spanning from 1.8 to 14 μm was achieved in a 20-cm-long fiber with a core diameter of 23 μm by pumping with 150 fs pulses at 6.0 μm in the anomalous dispersion regime [44]. Similarly, a Ge15 Sb15 Se70 /Ge20 Sb80 fiber with a core diameter of 6 μm and length of 11 cm was pumped at 4.485 μm with 330 fs pulses, and SC generation covering the 2–12 μm spectral range with an average output power of ~ 17 mW was achieved [45], as shown in Fig. 8.25. These results indicate the significant potential of the Ge–Sb–Se fibers as nonlinear media for a practical and bright MIR SC source.

8.3.3.2

Te-Based Fibers

As high optical nonlinearity is beneficial to SC generation, it is well known that the replacement of a lighter chalcogen element by a heavier one can enhance the optical nonlinearity of the materials and extend the transparency to much longer wavelengths. Te-based glasses have low phonon energies and high linear refractive indices, which result in wide optical windows and high nonlinear refractive indices (see Table 8.2). Thus, wider SC generation extending to FIR can be expected from Te-based fibers. However, the introduction of Te always shift the ZDW of ChG fibers to wavelengths longer than 9 μm. Although the waveguide dispersion can be engineered by fiber structure design such as microstructured and tapered technologies, the degree of the ZDW blue shift is very limited. As can be seen in Fig. 8.26a, b, when a fiber core diameter is less than 20 μm, it tends to exhibit no ZDW property [46, 47]. Figure 8.26c is a SC source generated from a Te-based fiber with all-normal dispersion spanning from 2 to 14 μm, showing a flatness in 2.9–13.1 μm within −10 dB dynamic range [48].

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8 Supercontinuum Generation in Mid-Infrared Glass Fibers

Fig. 8.26 a Calculated material dispersion of As2 S3 and Ge20 As20 Se20 Te40 glasses and fiber dispersion curves with various core diameters Reproduced from Yuan et al. 2020 [46] with permission from the Optical Society. b Calculated fundamental mode dispersion characteristic curves of Te-based ChG fibers with various core diameters. Reproduced from Zhang et al. 2019 [47] with permission from the Optical Society. c SC generation from a Te-based fiber pumped at different input powers at 5 μm. d Optical loss of a Te-based fiber. Reproduced from Jiao et al. 2019 [48] with permission from the Optical Society

In addition, the metallic characteristic of Te can lead to a greater tendency for crystallite formation, which may prevent the production of low-loss optical fibers due to the scattering effects of the microscopic crystals. A second consequence of the metallic characteristic of Te is a low bandgap which results in significant background carrier absorption caused by thermally excited charge carriers at room temperature, leading to high transmission loss for Te-based fibers (see in Fig. 8.26d). Though these drawbacks can be alleviated by substituting a small amount of Te by Se to lower the conductivity and dramatically increases the resistance to crystallization while retaining a wide optical window in the long wavelengths, the optical fiber loss still needs to be decreased further to suit the requirements of SC generation and other applications.

8.3 MIR SC Generation in Chalcogenide Fibers

8.3.3.3

311

Chalcohalide Fibers

Chalcohalide (ChH) glass combining chalcogen with halogen or metal halide offers a path to tuning of optical properties, such as optical band-gap, transmission window, refractive index, etc. Taking GeSe2 –Ga2 Se3 –CsI as an example [49], the optical bandgap increases gradually with the content of CsI, as shown in Fig. 8.27a, resulting in the increase of absorption edge and excellent transmission at shorter wavelength. Simultaneously, the GeSe2 –Ga2 Se3 –CsI glass exhibits better transparency in IR than traditional ChG glass. Moreover, the higher the content of CsI in ChH glass, the shorter the ZDW of the fiber. As can be seen in Fig. 8.27b, 55GeSe2 –25Ga2 Se3 – 20CsI fiber shows a ZDW of 3.5 μm, and 36GeSe2 –24Ga2 Se3 –40CsI fiber even has a shorter ZDW of 2.2 μm. When pumping a GeSe2 –Ga2 Se3 –CsI fiber with 150 fs pulses at a repetition rate of l kHz and a wavelength of 6.3 μm, which is located at the water absorption peak, a broadband SC spectrum spanning from 1.05 to 13.0 μm was achieved (Fig. 8.27d). Such a spectral broadening is dominated by soliton dynamics in the anomalous dispersion regime. Due to the increasing band-gap resulting from metal halide doping, a blueshift of SC spectrum down to 1 μm is obtained, which is unreachable in As– S fibers. However, because of the volatility and deliquescence of halide, the raw material cannot be purified via chemical deposition or vacuum distillation. In addition, high temperature during the fiber fabrication would boost the volatilization of halide and contaminate the glass preform. Therefore, further spectral broadening was hindered by the high fiber loss with a minimum loss of 4.15 dB/m at 7 μm (Fig. 8.27c). By peeling off the defective surface layer of ChH glasses to reduce the volatilization of halide [50], a low-loss Ge10 As22 Se68 –I ChH fiber with a minimum fiber loss of 1.12 dB/m @ 6.4 μm (Fig. 8.28a) was fabricated [51]. Meanwhile, the fiber fundamental mode ZDW is adjusted to 4.03 μm from the MZDW of 7 μm. Pumped with 150 fs pulses at a repetition rate of l kHz at several different wavelengths, experimental SC spectra were generated and shown in Fig. 8.28c. As can be seen, the SC spectrum pumped at 8 μm exhibits a wideset spectral coverage of 1.2–15.2 μm. It can be concluded that ChH fibers have great potential for various applications in the whole IR region.

8.4 Cascading SC Sources A cascading approach has been demonstrated in which a fiber-based SC generation is used to pump another fiber to further extend the spectral coverage. This cascading approach is mainly based on the SC spectrum that contains many solitons with fs pulses and high peak powers in the long wavelength range from the first fiber with anomalous dispersion. As long as the second fiber has a ZDW below the SC long wavelength edge, the solitons can continue to redshift in the second fiber when two fibers are cascading, which can then lead to a wider SC generation. Therefore, the

Fig. 8.27 a optical absorption edge of the Ge–Ga–Se–CsI glasses (the inset shows the absorption edge values). b Calculated material dispersion of ChH glasses. c optical loss of the ChH fiber. d SC generation in the ChH fiber pumped at 2.9 μm. Reproduced from Zhao et al. 2019 [49] with permission from John Wiley & Sons

312 8 Supercontinuum Generation in Mid-Infrared Glass Fibers

8.4 Cascading SC Sources

313

Fig. 8.28 a Fiber loss of the Ge10 As22 Se68 -I ChH fiber. b calculated the material dispersion and the fundamental mode dispersion curves of the Ge10 As22 Se68 –I ChH fiber. c SC spectra from the Ge10 As22 Se68 –I ChH fiber pumped with different wavelengths. Reproduced from Jiao et al. 2019 [48] with permission from the Optical Society

choice of these fibers is particularly important because it can affect the efficiency and dynamics of SC generation.

8.4.1 Two-Stage Cascading The cascading scheme for wide MIR SC generation was numerically proposed by Kubat et al. in 2014 [52]. In this scheme shown in Fig. 8.29, a two-stage cascading scheme was adopted where the SC spectrum was extended to 9 μm in a 10-cm-long As2 Se3 MOF with a core diameter of 5 μm by pumping with a 0.9–4.1 μm SC source generated from a ZBLAN fiber. Subsequently, this concept has been proved to be feasible by many experimental results. As shown in Fig. 8.30, Petersen et al. experimentally demonstrated an SC spectrum which was extended to 7 μm with 6.5 mW output power in a 4-μm-core As38 Se62 SCF, pumped by an SC source with long wavelength edge of 4.4 μm and power of 51.4 mW from a 5-m-long ZBLAN fiber. Here the pump source was a

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8 Supercontinuum Generation in Mid-Infrared Glass Fibers

Fig. 8.29 Setup of the cascading MIR SC source.

Fig. 8.30 Broad experimental spectrum (solid line) from cascaded SCG with a total output power of 6.5 and 1.5 mW above 4.5 μm, and the corresponding simulated spectrum (dashed line). The inset shows the near-field image of the output beam superimposed on a scanning electron microscope image of the suspended-core region from Perfos. Reproduced from Petersen et al. 2016 [53] with permission from the Optical Society

1.25 W TDFA seeded by a 1.55 μm pulse laser with a pulse duration of 3 ns and a repetition rate of 40 kHz [53]. The SC output power of the above case is relatively low, resulting not only from the limited pump power from TDFA, but also the extra power consumption by the multiple sets of lens coupling in the cascading process. Yan et al. used a MOPA pump laser shown in Fig. 8.31a to increase the average power to several tens of watts from the TDFA with a repetition rate of 600 kHz. They also constructed an all-fiber structure by fusion splicing silica and ZBLAN fibers, and butt-coupling ZBLAN and As2 S3 fibers to eliminate the power loss caused by the free-space system. Eventually, the maximum output power of the SC source from the 7-m-long ZBLAN was increased to 5 W with a spectrum broadening to 4.2 μm (Fig. 8.31b). The SC spectrum generated from the 4-m-long As2 S3 fiber was extended from 2 to 6.5 μm, with a high output power reaching 1.13 W (Fig. 8.31c) [54]. In these cases, fluoride fibers are always chosen as the first fiber in the cascading scheme because of the following excellent properties: (1) high damage threshold; (2) a capability to generate SC sources with > 3.5 μm long wavelength edge extending above the ZDW of typical step-index fluoride fibers around 1.5 μm; (3) high output powers of up to 30 W. As for the second fiber, As2 Se3 fibers are usually adopted rather than As2 S3 fibers thanks to their stronger nonlinearity and especially longer MIR transmission edge. Even so, the long wavelength edge of the generated SC

8.4 Cascading SC Sources

315

Fig. 8.31 a Setup for all-fiber cascading SC generation. b SC spectra from ZBLAN with different output powers. c SC spectra from As2 S3 fiber with different output power versus the input power. Reproduced from Yan et al. 2021 [54] with permission from the Optical Society

source using this scheme is still limited to 8 μm due to the longer ZDW of As2 Se3 fibers which hinders the soliton dynamics.

8.4.2 Three-Stage Cascading To increase the SC generation efficiency and further broaden the SC spectrum range, it is necessary to add another suitable fiber between fluoride and As2 Se3 fibers to mitigate the effect of the longer ZDW of As2 Se3 fibers. As2 S3 fibers are the most ideal intermediary fibers because they have not only the shorter ZDW, but also the high nonlinearity and handling power. In this way, the cascading scheme is extended to three-stage. By the three-stage cascading scheme, Martinez et al. [55] realized a broadband SC generation through utilizing a MOPA to pump the cascading ZBLAN, As2 S3 , and As2 Se3 fibers. Figure 8.32a shows the corresponding setup for SC generation. The As2 S3 fiber was excited by the SC source that was extended to 4.5 μm from the ZBLAN fiber, generating a SC source with a long wavelength edge of 7 μm, which is beyond the ZDW of the As2 Se3 fiber (6 μm). Finally, the cascading SC generated in the As2 Se3 fiber is shown in Fig. 8.32b, spanning a broadband spectrum from 1.6 to > 11 μm with a high output power of 417 mW. Such an SC generation exhibits the

316

8 Supercontinuum Generation in Mid-Infrared Glass Fibers

Fig. 8.32 a Setup of MOPA and cascading scheme for all-fiber SC generation. b Measured SC spectra from ZBLAN (black), AS2 S3 (red), and As2 Se3 (blue) fibers. Reproduced from Martinez et al. 2018 [55] with permission from the Optical Society

properties of wide spectral coverage and high average power required for practical applications. Cascading scheme makes MIR SC sources more compact, robust, and low-cost thanks to its all-fiber structure and stable SC output, which are necessary for applications in practical fields. In addition, the length of each fiber should be optimized carefully to be short enough to prevent excess long wavelength loss but simultaneously long enough to maximize spectral expansion in cascading scheme. Coupling between different kinds of fibers is also a great challenge for cascading scheme. Lowloss splice fusion has been achieved between silica and ZBLAN, silica and As2 S3 , As2 S3 and As2 Se3 fibers to date. The technical problem to be solved in the future is mainly finding a way to fuse fluoride and ChG fibers.

8.5 Applications of MIR SC Sources The recent development of MIR light sources, such as OPAs/OPOs, Raman lasers, and quantum cascade lasers, opens up attractive prospects for molecule detection, especially beyond 3 μm, where most molecules exhibit distinctive spectral fingerprints. These lasers allow high sensitivity and accurate concentration measurements in many fields including industrial manufacture, emission control, and pollution monitoring. However, they show single frequency characteristics or limited tuning

8.5 Applications of MIR SC Sources

317

range, and they may not be able to cover certain wavelengths. Hence these lasers always target a single absorption line of specific molecules, which will limit their practicality. In contrast, MIR SC sources possess more suitable properties: wide spectral range, high brightness, and excellent spatial coherence. Hence, SC sources are of great significance for applications where wide spectral range is required to detect multicomponent molecules and/or where high brightness and excellent spatial coherence are needed for remote sensing. Besides, MIR SC sources based on soft glass fibers can provide good mechanical robustness and durability which are important in practice. Several applications of MIR SC sources will be considered below.

8.5.1 Gas Sensing Several criteria are required for an efficient spectroscopic gas sensing system, which are proposed by Thorpe et al. and shown as follows [56]: (i) wide spectral coverage for multiple species measurements; (ii) sufficient spectral resolution for species determination; (iii) high sensitivity for trace gas detection; (iv) fast spectral acquisition response for real-time monitoring. However, there is always a competition between these requirements in many gas sensors. For instance, a high spectral resolution can be achieved but at the expense of long spectral acquisition time or narrow spectral bandwidth. Therefore, one often needs to make a trade-off between these sensing performances in accordance with the demand in gas detection. Compared to the visible and NIR region, many gases exhibit stronger (by an order of magnitude or more) absorptions in MIR. Especially in 2–5 μm which covers the first and second atmospheric transmission windows, numerous organics and pollutants can be detected in this wavelength range, in particular the molecules involving a C-H bond, as shown in Fig. 8.33 [10]. Single-path SC laser absorption spectroscopy is the most straightforward method to detect the multi-component gas, with a scheme as shown in Fig. 8.34 [10]. The MIR SC source generated from soft glass fibers is collimated and launched into a gas cell, then it is absorbed at specific wavelengths by the gas. After that, the absorbed SC source is collected into an optical spectrum analyzer. By comparing the SC spectra before and after absorption by the molecules, one can determine the species of the detected gas. Although a scheme based on single-path SC laser absorption spectroscopy has the advantages of low cost and system simplicity, it also creates problems of low sensing sensitivity and large accuracy error due to its short sample absorption path, which means it is not suitable for trace gas sensing. Cavity enhanced absorption spectroscopy is one simple and robust method to increase the optical path and thus the interaction length between SC sources and gas to be detected, resulting in enhanced sensitivity. As shown in Fig. 8.35a [57], the SC source with a spectral coverage of 0.9–3.7 μm and an output power of 160 mW (at the top of Fig. 8.35a) is generated from a 7-m-long ZBLAN step-index

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8 Supercontinuum Generation in Mid-Infrared Glass Fibers

Fig. 8.33 Atmospheric transmission and absorption spectra of several gases. Reproduced from Nicolas et al. [10]

Fig. 8.34 Setup for single-path SC laser absorption spectroscopy. Reproduced from Nicolas et al. [10]

fiber. Then it is firstly spectrally filtered to match the spectral bandwidth of the 1-mlong optical confocal cavity formed by two mirrors with high reflection coefficient > 99.95%, resulting in an effective optical path length about 300 m. After the SC source is coupled into the cavity and absorbed constantly by the gas, it is then coupled into a detection device. Note that the SC power intensity at the cavity output end is relatively low because of the high reflectivity of the mirrors, thus the detection device is required to be highly sensitive, which means a lock-in amplifier must always be used. Using the cavity enhanced absorption spectroscopy based on MIR SC sources, trace gases of CH4 and C2 H2 are detected simultaneously with a sub-ppm accuracy, as shown in Fig. 8.35b.

8.5.2 Solid Detection Explosives, dangerous chemicals, toxic industrial compounds, etc., are required to be detected by a contactless method. Diffuse reflection spectroscopy is a widely used

8.5 Applications of MIR SC Sources

319

Fig. 8.35 a Experimental setup for gas sensing based on cavity enhanced absorption spectroscopy. b comparison between the measured and modeled absorption spectra of C2 H2 and CH4 . Reproduced from Amiot et al. 2017 [57] with permission from AIP Publishing

technology to analyze IR active samples qualitatively and quantitatively. Therefore the high coherence and brightness of MIR SC sources are particularly important and advantageous to realize safe remote chemical detection with a distance between the testing device and the object and simultaneously distinguishing the components in different samples. The scheme of a setup for stand-off detection with diffuse reflection spectroscopy is shown in Fig. 8.36a [8]. First, the SC source shown in Fig. 8.36b from the ZBLAN fiber is collimated achromatically to achieve a divergence. The sample is kept at a distance of a few or dozens of meters from the collimating mirror. Then the light is diffusely reflected when it strikes the sample surface. A concave mirror with a large

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8 Supercontinuum Generation in Mid-Infrared Glass Fibers

Fig. 8.36 a Setup for stand-off detection based on MIR SC source. b SC spectrum from the ZBLAN fiber. c absorbed spectra by ammonium nitrate powder based on SC source (orange) and FTIR (blue). Reproduced from Kilgus et al. 2018 [8]

diameter is put several centimeters to the side of the collimating mirror to collect a fraction of the reflected light and finally focus the light into a detector. Then the reflection spectrum with the sample information can be obtained. Ammonium nitrate (NH4 NO3 ) powder was selected as a detection demonstration in this demonstration, given its explosive properties under high temperature or pressure. The absorbed spectra of the sample by the SC sources is shown in Fig. 8.31c together with a reference absorbed spectrum recorded by the attenuated total reflection with the FTIR spectrometer. Both curves exhibit the same absorption peak at 3.27 μm of NH4 + bond, verifying the validity of the stand-off detection by SC sources.

8.5.3 Spectral Imaging Spectral imaging technology is a new technique for diagnosing healthy and diseased tissues in the field of biomedical imaging in recent years. Because many biochemical tissues have distinguished absorption spectra in MIR, the absorption characteristics of these fingerprint spectra can be used to identify the types of trace chemicals, so as to provide data support for the discrimination of various diseases.

8.5 Applications of MIR SC Sources

321

The demonstration of multispectral imaging based on MIR SC sources at wavelengths beyond 5 μm was firstly reported by Petersen et al. The SC source used for spectral imaging is generated from a tapered ChG PCF pumped at 4.3 μm, spanning 2–7.5 μm with an output power of 25 mW. Then the SC source was long-pass filtered at 4.5 μm, allowing the power to be focused onto the sample of 9–10 mW. By raster-scanning the sample at a beam focus in 5 μm step, the imaging was realized over a 600 μm × 600 μm region. The sample was a section of a tumor free colon tissue with 7 μm thickness. A grating spectrometer for wavelength selection was used before a lock-in MCT detector. A reference image of the sample stained by gold-standard haematoxylin and eosin (H&E) is shown in Fig. 8.37a where the darkly stained regions represent the outer nuclear while the lightly stained ones represent the inner cytoplasmic, connective tissue, thin muscle layer, and mucin secretions, and a visible light transmission mosaic image is also shown in Fig. 8.37b. MIR SC source point-scan images at 6.03, 6.45, and 7.30 μm are shown in Fig. 8.37c–e, respectively. These wavelengths were chosen for their relation to cancer in terms of spectral shape and intensity exhibited by some key molecular absorptions. As can be seen in Fig. 8.37f which is a composite image of the three discrete frequency images by spectral-spatial mapping, one can distinguish

Fig. 8.37 Comparison between a the confocal image of the H&E stained tissue section with identification of the various histological regions. A, muscle layer; B/C, cytoplasmic/nuclear regions of the colonic crypts; D, connective tissue; E, mucin secretions. b transmission image of the unstained sample at visible light. c, d Mid-IR absorbance images of the protein rich amide regions highlighting the nuclear regions of the colonic crypts, and e the mucin secretions and surface epithelial walls. f Composite image showing the spectral-spatial mapping of c–e. Reproduced from Petersen et al. 2018 [58] with permission from the Optical Society

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8 Supercontinuum Generation in Mid-Infrared Glass Fibers

the amide rich nuclear regions of the colonic crypts from the surrounding connective tissue. Compared with the commonly used FTIR imaging technology, this method can reduce the noise intensity of each image along the scanning length as long as the SC power spectral intensity is sufficient. It can be concluded then that MIR SC sources can be effectively used for multispectral tissue imaging, making it a promising approach early diagnosis of many diseases, which is often the key to successful treatment.

8.6 Summary Fluoride and ChG fibers provide very wide IR transmission window, excellent nonlinear intensity as well as dispersion engineering ability for SC generation. Significant and remarkable progress has been made with the realization of broadband and high output power MIR SC generation. To date, SC sources based on fluoride and ChG fibers can reach above 30 and 1 W in average power, whilst also spanning the entire transmission window in spectral coverage, respectively. The advantages of wide spectrum, high brightness and coherence for SC sources have also been successfully verified and applied in several fields. In the near future with improvements in the technology for pumping and fusion splicing between soft glass fibers, and the maturity of related fiber-optic components and devices, it is believed that MIR SC sources will be used in an increasingly wide range of applications.

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Chapter 9

Industrial, Medical and Military Applications of Fluoride and Chalcogenide Glass Fibers Haitao Guo, Hao Zhang, Lutao Liu, Xusheng Xiao, and Gerald Farrell

Owing to their low phonon energies, fluoride and chalcogenide glasses have wide optical transparent windows extending from the visible to mid- or even far-infrared region. The initial application of the glasses was infrared light transmission, including IR signal transmission and image transmission. Some chalcogenide glasses (ChG) such as As2 Se3 and Ge28 Sb12 Se60 have been widely used in the infrared lens manufacturing field because of their relatively low cost and low refractive index temperature sensitivity. For the fibers, some fluoride and ChG transmission fibers have also been mature now and become the commercial products, such as ZBLAN and As2 S3 fibers. However, compared to the silica glasses, the infrared transmission fibers are still developing. Fluoride and ChG photonic crystal fibers with large optical mode area, infrared optical fiber imaging bundles with higher resolution are still being studied. In addition, the low phonon energy of the glass matrix also makes many infrared transitions possible where Rare Earth (RE) elements are doped in the glass. This indicates their potential application in mid-infrared lasers or amplifiers, although the loss of the fibers, especially after doping with RE elements, is still relatively high and has hindered the development of infrared lasers. On the other hand, ChG fibers have high non-linear index and high Raman gain index. These makes them suitable as the basis for lasers for nonlinear frequency transformation and as a stimulated Raman laser. The fabrication of infrared laser using such fibers also needs other fiber devices, such as an infrared fiber coupler for the coupling of pumping light and a fiber Brag grating (FBG) for the formation of laser cavity. Due to the weak thermal and mechanical properties of these glasses, the development of such devices is still in laboratories. It is worth pointing out that development of infrared FBGs would not only be of benefit in the development of high-efficiency mid-infrared lasers, they could also be applied in infrared sensing, which have indispensable applications in many fields. This chapter will discuss the above devices or applications.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Wang et al., Mid-Infrared Fluoride and Chalcogenide Glasses and Fibers, Progress in Optical Science and Photonics 18, https://doi.org/10.1007/978-981-16-7941-4_9

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9.1 Laser Power Delivery As the performance of IR sources continues to improve, there will be an increasing demand for applications in materials processing, laser surgery, military and other fields. For many applications, fiber delivery is beneficial and necessary.

9.1.1 Step-Index Fiber The advantage of ChG fibers is that laser power from 2 to 12 µm can be delivered by changing their composition. This section will introduce the ChG step-index fiber as a means to realize the laser transmission from short to long wavelengths. In the medical field, a considerable amount of laser surgery is performed at 2.94 µm using Er: YAG lasers. The Er: YAG laser radiation absorption is very strong because laser wavelength is practically in the center of the maximum absorption band of cellular water. Since biological tissues contain up to 70–90% of water, Er: YAG laser is extremely efficient for their high precision cutting and vaporization. In 2007 Papagiakoumou et al. demonstrated an As–Se–Te fiber transmitting at a wavelength of 2.94 µm with a loss of 0.7 dB/m and an As40 S60 fiber which had a loss of 1.5 dB/m [1]. In order to get the best possible approximation to a Gaussian input beam profile into the fiber, the experiment used a 150 mm CaF2 focusing lens and a variable diameter pinhole to focus the beam. The energy limit for As–Se–Te fibers, before fiber end damage was observed, was 1.8 mJ independent of core diameter (power density of 3.30–30.6 MW/cm2 , depending on the pinhole diameter), while for As40 S60 fibers the threshold for damage was 2.3 mJ (power density of 1.90–38.2 MW/cm2 ). In 1998, NRL reported the delivery of energy from a medical-free electron laser (MFEL) operating between 2 and 10 µm through a ChG fiber [2]. The MFEL can emit more than 10 MW of peak power in a femtosecond pulse which is equivalent to an average power greater than 10 Watts. Laser energy from the free-electron laser operating at a wavelength of 2.94 µm has also been launched into an As40 S60 fiber for potential ophthalmic surgery with an energy requirement of 1–2 mJ. In addition, laser surgery based on protein bond breakage at 6.1 µm or 6.45 µm should be mentioned, because they are more efficient and have less tissue degeneration and scarring. These results showed that the laser surgery should be possible using a ChG fiber laser. For 2–5 µm mid-infrared lasers, another important application is in the military field such as infrared countermeasures (IRCM) or laser tactical systems. Transmitting high power IR lasers by ChG fibers can deflect or dazzle the infrared target seeking system. This application puts a higher demand on the power handling capability of the fiber (typically tens of watts). Figure 9.1 shows the relation between the output and input powers through a As40 S60 fiber using a pulsed laser [3]. The peak power is 26.9 kW which corresponds to a peak power destiny 1.07 GW/cm2 without fiber end damage for 1.5 × 107 pulses. This damage threshold is close to the predicted

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Fig. 9.1 Pulsed high energy laser transmission in the 2–5 µm region [3]

value of 3.0 GW/cm2 . This result requires very good polishing of the fiber end face in order to avoid end face damage at a relatively lower power. In 2018, Sincore et al. [4] at the University of Central Florida examined the potential of ChG fibers to handle three high power sources, i.e. a 2.053 µm Tm: silica fiber laser, a 2.520 µm Cr: ZnSe MOPA, and a 4.102 µm Fe: ZnSe oscillator. Figure 9.2 shows the optical images of the As-S fibers with polished facets. The large polymer jacket enables excellent mechanical robustness. CHG-A fiber has a 12 µm core and CHG-B has a 25 µm core diameter. These fibers were AR-coated by depositing a layer of Al2 O3 on the facets. As shown in Fig. 9.3, the uncoated fibers delivered ~8 W and the AR-coated fiber enabled the delivery of 10.3 W at 2.053 µm laser through ChG-A fiber. As shown in Fig. 9.4, facet damage occurred after coupling ~ 1.3 W 2.520 µm laser into a 20 cm long AR-coated ChG-A fiber. But the damaged fiber facet sustained multi-Watt transmission without total failure (no transmission through the fiber). It also demonstrated a Gaussian profile while the transmitted beam profile is at ~ 1 W. For the 4.102 µm laser, as shown in Fig. 9.9.5, 500 mW could be delivered through a 20 cm length uncoated ChG-B fiber but transmission failure occurred when coupling ~ 1.1 W through the 40 cm length fiber. However, the researchers also stated that the poor coupling was likely the cause of fiber failure (see the inset of Fig. 9.5). With an improved compact telescope design and better alignment, strong core confinement should be attainable similar to the 2.053 and 2.520 µm source delivery. Above results indicate that by improving the anti-reflection coatings and using a high beam quality mid-infrared source, ChG fibers can reliably deliver > 10 W in a

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Fig. 9.2 Optical image of the polished ChG fiber facets, along with their dimensions and calculated V-numbers [4]

Fig. 9.3 Measured transmission of the 2053 nm source through 20 cm length of uncoated and AR-coated ChG-A fibers [4]

single mode, in the 2–5 µm region. These results are very encouraging and further improvements are expected. At wavelengths > 5 µm, ChG fibers can be used for laser surgery, industrial cutting and welding applications by transmitting the output from high power CO (5.4 µm) and CO2 (10.6 µm) lasers. In addition, the transmission of laser power through optical fiber enables remote operation. Early in 1964, a CO laser with its high power and high efficiency potential was developed at Bell Labs and initially studied as a laser weapon candidate, but its emission spectrum overlapped with the strong absorption

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Fig. 9.4 Measured transmission of the 2520 nm source through 20 cm length of AR-coated ChGA fiber. Inset: Beam profile when transmitting 1 W demonstrates strong core confinement with Gaussian profile [4]

Fig. 9.5 Measured transmission of the 4102 nm source through a 20 and 40 cm length of uncoated ChG-B fiber. Inset: Beam profile diverging from the ChG output facet. A Gaussian profile is located in the center surrounded by substantial cladding light [4]

band in the atmosphere and this leads to strong atmospheric propagation attenuation. However, CO lasers show great advantages in laser surgery and materials processing. In these applications, complex systems of mirrors are commonly used to transmit the laser radiation to the target. But IR fibers allow the use of optical fibers for the delivery and manipulation of the laser beam, in which it is mechanically much simpler and inherently easier to change if required. Transmitting the laser power through fibers also improves the potential for automation of applications using robotics or other complex systems. Overall good progress has been made in ChG fibers for transmission in the 5–6 µm region.

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Fig. 9.6 Experimental arrangement for the power transmission measurements [6]

In 1985, Shusuke et al. [5] at Keio University showed that an As-S ChG glass system has a minimum optical transmission loss in the wavelength region around 5 µm and is therefore the best optical waveguide for transmitting CO laser. By using a 1 m long As-S fiber with core diameter of ~ 1000 µm, a maximum incident power of 59 W of CO laser was achieved without damaging to the fiber. In this case, the laser power intensity at the fiber output end was 7.5 kW/cm2 . In 1993, Shunichi et al. [6] at the Institute of Research and Innovation prepared As40 S60 and Ge10 As30 S60 fibers with Teflon claddings, which have the transmission losses of 0.45 dB/m and 0.30–0.45 dB/m at 5.4 µm, respectively. The maximum delivery power for the 1 m-long and 1000 µm-core-diameter fibers reached as high as 226 and 180 W, respectively. The experimental arrangement for the power transmission measurements is shown in Fig. 9.6. A CO laser beam was focused with a ZnSe lens and then injected into a polished fiber input end which was placed near the focal point of the lens. The laser oscillator used was a 1 kW, fast axial flow, radio frequency discharge excited continuous wave (CW) CO laser which could produce a stable and low order multimode output beam. Both the fiber ends and the fiber transmission path were protected by Nitrogen gas in room temperature. The corresponding output power intensities were 29 and 23 kW/cm2 for the As40 S60 and Ge10 As30 S60 fibers, respectively. Figure 9.7 shows the relationship between the incident and transmitted laser power. The theoretical maximum transmission efficiency is 70% because of the Fresnel loss. To reduce the Fresnel losses at the fiber ends, PbF2 antireflection (AR) coatings were added to a 1 m-long fiber with 700 µm core diameter. No damage was observed on the coatings both for the As40 S60 and Ge10 As30 S60 fibers in a transmitted power range of up to ~ 100 W. The output power could be increased by cooling the fiber and AR coatings. Although larger core diameters can transmit higher power lasers, laser transmission quality and modes must also be considered, which requires the development of small core diameter fibers. In the 1990s, the U.S. Naval Research Laboratory began

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Fig. 9.7 Relation between the incident and transmitted laser power under forced air-cooling conditions [6]

to report their efforts on the development of small-core ChG fibers. Small core diameter (~200 µm) As40 S60 fibers showed a tolerance of about 126 kW/cm2 at a power density of 5.4 µm [7] (see Fig. 9.8). CO2 lasers are similar to CO lasers in many ways. For technical, medical and some specific areas of application, CO2 lasers are still the most desired type of laser. In the medical field, ablative surgery using CO2 lasers can cause less damage to surrounding tissue than those using shorter wavelength lasers. The IR cutoff wavelength for Sbased glasses is around 10 µm (the fiber cutoff band is around 6 µm) and for Sebased glasses is around 14 µm (the fiber cutoff band is around 11 µm), respectively. Te-based ChGes glasses have a wider IR cutoff wavelength (greater than 25 µm) compared to S- and Se- based ones, which can better meet the requirements of CO2 laser transmission. In 1991, Inagawa et al. [8] investigated the CO2 laser power delivery in the Se25 Te30 I45 fiber, which has the lowest transmission loss of 0.9 dB/m at 10.6 µm. The length and diameter of fiber were 100 cm and 400 µm, respectively. The relation of input power and output power through the fiber is shown in Fig. 9.9. CO2 laser power of 0.82 W was delivered through the un-clad fiber while the input power was 2.85 W. In order to increase the damage threshold of Te-based ChG fibers, an appropriate amount of Ge can be introduced. In 1992, Nishii et al. [9] investigated the properties of Ge–Se–Te glass fiber for CO2 laser power transmission. Core and cladding glasses are Ge–Se–Te and Ge–As–Se–Te, respectively, and the diameters of the core/cladding

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Fig. 9.8 CO laser transmission of small-core-diameter As40 S60 fiber [7]

Fig. 9.9 Relation between input power and output power through the fiber (Se25 Te30 I45 ) on CO2 laser delivery; Solid line A and dashed line B indicate the experimental and estimated lines [8]

were 450/550 µm. The CW beam emitted from the CO2 laser was focused by a ZnSe lens (f = 30 mm) into 250 µm spot and launched into the 100 cm long fiber. Nitrogen gas at a rate of 500 cm3 /min was blown onto the polished fiber ends to inhibit their thermal rupture during power transmission. The maximum input and output power attained were 19.4 and 10.7 W, respectively, and the efficiency was therefore as well as 55.2%. The maximum input power density was 12 kW/cm2 . The fibers possessed a PbF2 AR coating and were cooled with water to prevent thermal lensing. In 1996, Busse et al. [10] at NRL reported using a core-cladding Ge30 As10 Se30 Te30 fiber with the diameter of 270 µm and length of 100 cm to deliver CO2 laser power. The core-

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335

Fig. 9.10 Experimental setup for CO2 laser power transmission [11]

and cladding- diameters for this fiber were 162 and 270 µm, respectively, and the fiber ends were cleaved. The maximum input and output power attained were 1.73 and 0.6 W without cooling, and the efficiency was therefore was as good as 34.7%. The maximum input power density was 27 kW/cm2 with no resultant damage to the fiber. With the development of purification technology and drawing processes in recent years, lower loss optical fibers of Ge15 As25 Se40 Te20 glass purified by combined chemical and physical methods were successfully drawn. The transmission loss of this fiber is 5 dB/m at 10.6 µm [11]. Figure 9.10 shows the experimental setup which comprise a tunable 10.6 µm CO2 laser with a maximum output power of 75 W, a ZnSe beam splitter, a focusing ZnSe lens (f = 40 mm), 1 m long Ge15 As25 Se40 Te20 fiber with diameter of 400 µm, and two power meters. In this experiment, the duration time of the laser irradiation was limited to 60 s to avoid excessive laser heating. Free air convection was adopted to cool the fiber. The dependence on the input and output power of CO2 laser through the fiber is shown in Fig. 9.11. The output power increases almost linearly with the input power. When a laser power of 6.16 W was launched into the fiber, a maximum transmitted power of 1.37 W was achieved without any damage in the fiber, and the corresponding laser power density at the fiber input and output ends was 4.9 and 1.09 kW/cm2 , respectively. Both the CO and CO2 laser transmission results show promise for ChG fiber applications in high average power delivery.

9.1.2 Microstructured Fiber Although great progress has been made in ChG step index fibers, with the increase in transmission power, traditional step-index fibers are incapable due to material limitations. Hollow-core microstructure optical fibers (HC-MOFs) and large-mode-area

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Fig. 9.11 Dependence on the input and output CO2 laser power through the coated fiber [11]

photonic crystal fibers (LMA PCFs) have become effective approaches to enhance the capability of power handling of ChG fibers. In order to achieve high power laser delivery, increasing the mode field area is the most direct and effective solution, and at the same time, to ensure the beam quality, transmission in the fiber is required to be single-mode. It is difficult to meet both the need for a large mode field area and the need for single mode transmission in a traditional step-index fiber, as even if large mode field/single mode transmission is achieved, it will inevitably cause an increase in the fiber bending loss, which is very undesirable. The emergence of photonic crystal fiber provided a new means to solve this problem. ChGs have excellent IR transmission properties, high refractive index, low phonon energy, and tunable components, making them ideal materials for the design and preparation of infrared micro structured optical fibers. A large amount of research has been undertaken in recent years on this topic, but ChGs have a low damage threshold and the use of the effective mode field area of the fiber to regulate nonlinearity and transmission performance has become a hot research topic. In 2006, Le Person et al. [12] from the University of Rennes I prepared a LMA PCF with a mode field area of 108 µm2 using the Ge20 Ga5 Sb10 S65 glass, and investigated single-mode transmission by near-field spectroscopic characterization. With the increased maturity of the preparation process for ChG, Troles et al. [13] prepared a LMA PCF with Ge20 Ga5 Sb10 S65 as the substrate material in 2007. Figure 9.12 shows that the mode field area was 150 µm2 when the fiber outer diameter was 240 µm, the ratio of hole diameter to hole spacing d/ was 0.31 and the hole spacing  was 14 µm. When the fiber outer diameter changed to 400 µm and the hole spacing  was 28 µm, the mode field area was 1000 µm2 .

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Fig. 9.12 ChG PCFs with different geometries [13]

The all-solid design was proposed in the development of LMA PCFs, which is because all-solid compared to porous micro structured cladding can avoid defects such as air-hole deformation generated during the thermal processing of fiber drawing. In turn this eliminates the influence of environmental factors on fiber transmission as well as preventing water vapour contamination from the air. In 2019, Ren et al. [14] from Jiangsu Normal University prepared all-solid LMA PCF (see Fig. 9.13) with a mode field area of 5200 µm2 at 4 µm and a fiber loss of 5.2 dB/m. In 2020, Feng et al. [15] demonstrated a novel few-mode ultra-large mode area ChG photonic crystal fiber for mid-infrared high-power applications. Figure 9.14 shows the preform prepared by the stacking technique. As is shown in Fig. 9.15, the output power increases almost linearly with the increase of the incident power, until the fiber facet was damaged under an incident power of 11.8 W. The maximum permissible incident laser power density for 2 µm CW laser irradiation is estimated to be 150 kW/cm2 . The forced water cooling is beneficial for power handling. With the development of this fiber technology, researchers extended the transmission band of fiber by using hollow core micro structured fiber, which reduces the

Fig. 9.13 a Preform obtained by stacking method. b SEM images of all-solid LMA PCF [15]

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Fig. 9.14 a–d Photographs of various parts of the preform. e SEMphoto of ChG ULMA PCF [15]

Fig. 9.15 Photographs of a PCF sandwiched between two aluminum plates, b incident end of PCF, c input facet of PCF was damaged, d relation between incident and output power through a length 47 cm fiber [15]

transmission loss of fiber. This is because the energy is mainly transmitted in the air core, and this can also ensure the low-loss transmission band is located in the mid-infrared region by an appropriate design of the fiber structure to provide a so called “negative curvature” fiber, which uses the interference effect resulting from the design of the structure to guide light in the hollow core and minimize the overlap with the glass of fiber. In 2002, Temelkuran et al. [16] firstly reported a hollow-core Bragg fiber composed of layers of As2 Se3 and thermoplastic polymer poly (ether sulfone, PES) with high glass-transition temperature (see Fig. 9.16). The fundamental and highorder transmission windows of the fiber are determined by the layer dimensions and can be altered to lie wavelength range from 0.75 to 10.6 µm. The transmission losses are found to be less than 1.0 dB/m and the maximum laser power density coupled into these fibers was approximately 300 W/cm2 . The drawing of such multilayer fibers is relatively difficult and requires consideration of the thermal matching of the ChG and polymer layers.

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Fig. 9.16 Cross-sectional SEM micrographs at various magnifications of hollow cylindrical multilayer fiber mounted in epoxy [16]

The development of negative curvature fibers is an encouraging advance in fiber technology that combines lower theoretical loss over wide bandwidths with higher tolerance to fabrication imperfections. A tolerance to fabrication imperfections is particularly important for ChG fibers. In 2011, the Fiber Optics Research Center of the Russian Academy of Sciences prepared a negative curvature fiber from Te20 As30 Se50 glass [17]. The fiber preforms were made from a substrate tube and eight capillaries by the “stack and draw”technique, and the fibers were drawn at 240 °C. Figure 9.17 shows the fiber cross-section with the help of electron microscope. When the fibers are drawn, a certain amount of overpressure is maintained in the capillaries to prevent their collapse. The transmission loss of a negative curvature fiber is measured by the tangent method, in which the light propagates not only through the hollow core but also through the matrix tube and the cladding capillary. Therefore, the measured loss is the average of the losses in the hollow and glass. The loss is measured as 13.5 dB/m at a wavelength of 10.6 µm. The distribution of CO2 laser intensity in Fig. 9.17 Fiber cross-section photograph [17]

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Fig. 9.18 Measured optical loss spectra of the ChG micro structured hollow-core fiber in the region of the CO2 -laser radiation (left) and the CO2 -laser radiation intensity (in a.u.) distribution over the fiber core (right) [17]

the cross-section of the fiber has also been investigated, in which the ZnSe lens was made to transmit the laser into the fiber, while the intensity distribution on the output end face of the fiber was observed with the help of a thermal imaging camera. It has been found that the high power laser can be transmitted through the core, as shown in Fig. 9.18 (right), where the CO2 laser is well confined in the core, and it has also been determined that the propagation mechanism in the core is multimode. As2 S3 negative curvature fibers were also prepared at the Fiber Optics Research Center of the Russian Academy of Sciences in 2014 [18], with substrate tubes obtained by centrifugal casting, capillaries by the double crucible method, and a preform by the stacking method. The fabricated negative curvature fibers exhibited higher losses compared to step-index fibers due to different light conduction mechanisms and fiber defects, mainly because of the associated difficulties in the preparation of ChG negative curvature fibers: (1) The high temperature gradient required for stretching due to viscosity (2) The high refractive index of ChG increases the requirement for capillary thickness (3) The capillaries made by the double crucible method are not yet comparable to the quality of commercial quartz capillaries. To obtain glass tubes with high fabrication tolerances, prefabricated rods based on ChG were fabricated and drawn into optical fibers using an extrusion method at the U.S. Naval Laboratory in 2016 [19], and the fibers showed record low losses in the range of 9.75–10.5 µm, i.e. 2.1 dB/m at 10.0 µm. Figure 9.19 illustrates the use of pneumatic pressure to control the internal structure size of the fiber.

Fig. 9.19 Cross-sectional view of the fiber at different inner tube pressure [19]

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Due to the unique structure of a negative curvature fiber, the manufacturing process comes with a range of strict requirements. In order to achieve high power laser delivery, it is necessary to reduce the loss of fibers. The compatibility and coupling of the negative curvature fibers with conventional fibers and devices are also problems that needs to be solved in the future.

9.2 Infrared Optical Fiber Imaging Bundles The roles of IR fiber bundles is typically to collect and transfer the IR optical images with the help of a large array or bundle of coherent IR fibers (sometimes referred to as a fiber filament in this context). Here, the optoelectronic signal transformation process can be omitted, unlike in a traditional optical fiber communication system. The key is that the fiber bundle provides optical image transmission from an object to the viewer with each fiber in the bundle acting as an independent pixel. For fibers used in bundles operating in the visible or near IR light wavelength region, there are many material options such as plastic or quartz optical fiber. However, very few fiber types exist which can extend the wavelength range to greater than 2 µm. In the development of IR optical fiber bundles, using an IR optical fiber bundle based on chalcogenide glass is the option with the greatest potential for success and which could be widely adopted in the fields of medical, industrial, scientific research, military, aerospace, and many others for its excellence of IR transmission window and the stability of the fiber or glass forming processes. Because an IR optical fiber image bundle is a form of passive optical device that can accurately deliver images by assembling numerous single fiber filaments with a certain length into bundles according to certain rules, the most important requirement is that the arrangement of the individual optical fiber at both end faces of the optical fiber bundle should be strictly aligned to each other. In addition, each optical fiber should have good optical isolation (or low crosstalk), which means the optical signals in each fiber cannot be affected by light from other adjacent fibers’ [20]. In this way, the surface of each fiber can be regarded as an image sampling point, carrying a pixel in the independent light transmission process. The size of the pixel is the size of the sampling hole, and the number of pixels is equal to the number of optical fibers at the end faces. In other words, the greater the number of fiber filaments evenly arranged in the image bundle, the larger the number of pixels in the image bundle, and the better the image resolution and thus quality. In addition to transmitting images in their original shape and size, it is also possible to realize simple image processing such as zoom in, zoom out, or recombination, such as a split screen display of the image, scaling of image display size, and deliberate alteration of the image pixel arrangement order for specific applications [21], where the pixels are rearranged from a square pattern to a line pattern. The exploration of the design of fiber bundles has greatly expanded the range of possible applications in the future.

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The idea of using optical fiber to transmit images originated in the 1920s, but it was not until the mid-1950s that H. Hopkins and N. S. Kapany first developed a practical optical fiber image bundle, and published articles on image transmission by optical fiber image bundle [22, 23]. Since the 1960s, the United States, Japan, Germany, the former Soviet Union and other countries have carried out substantial research work on visible band optical fiber image bundles. In the early 1980s, the Olympus Company of Japan introduced the idea of making optical fiber image bundles by an acid-leaching technology. At present, the diameter of an image bundle has reached 8 mm, the resolution has reached 50 line-pairs per mm (lp/mm), with the number of pixels reaching 320,000, with very high image quality. The medical endoscope, a device which utilizes a fiber bundle, plays an important role in the field of cancer diagnosis, prevention and treatment, was first developed by Olympus in 1950. With regard to fiber imaging bundles for the IR region, the most representative company is the American AMI, whose As-S optical fiber image bundle 10-M-2 shows good performance (the pixel number is 2898), and has entered the preliminary application stage [24]. In 1999, Beijing Glass Research Institute reported As-S infrared optical fiber image bundle with 70 cm long, had 10,000 pixels and 6.4 × 6.3 mm end-face size [25]. In 2001, the institute reported As-Se-Te IR glass optical fiber image bundle with 2 m length and 10,000 pixels (arranged as an array of 100 × 100). An IR image of a blackbody furnace was delivered using this bundle and a clear thermal image was obtained by the thermal imager, demonstrating conclusively that the IR image bundle is one of the best design schemes for thermal image transmission in special environments [26]. In 2015, Zhan et al. [27] of the Xi’an Institute of Optics and Precision Mechanics of the Chinese Academy of Sciences developed an IR fiber array for line to planar shape conversion, which is used together with area array CCD to realize linear push-broom imaging. On the basis of the traditional lamina-stacking method, by introducing a high-precision grating ruler and accurately controlling the position of each optical fiber, a special-shaped As2 S3 fiber image bundle with square pixel arrangement was prepared. The area array end specification is 128 × 64, and the line array end specification is 4096 × 2. The IR push-broom imaging experiment was carried out by using the image bundle, and a good quality IR image was obtained, as shown in Fig. 9.20. Zhang et al. [28] in 2015 developed another technique for fabricating chalcogenide optical fiber image bundles, a so called stack-and-draw approach. A chalcogenide fiber bundle which consisted of about 810,000 single fibers each with an As2 S3 glass core of 9 µm in diameter and a polyetherimide (PEI) polymer cladding of 10 µm in diameter was fabricated. The resolution of this bundle reaches 45 lp/mm. However, this bundle has poor application potential because the optical properties of the cladding material are poor and the thickness of the fiber cladding is too thin to constrain the optical signal to the core, which means the optical loss of this bundle is high. This bundle fabricating technique also has a major disadvantage, that is, it cannot be used to make special-shaped optical fiber bundles. Since 2014, a new round of scientific and technological demands have emerged, especially for a 8– 12 µm high-performance thermal image transmission optical fiber which can work in fiber bundles in difficult electromagnetic environments. A good example is the

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IR detector Camera (f=240mm) Fiber bundle fixture

Taken

Line-plane-switching infrared bundle

Taken by IR detector directly

by

push-broom

imaging system

Fig. 9.20 The line-plane-switching chalcogenide fiber array, IR push-broom experimental system and obtained IR images of target [27]

recent goal of the US Navy is to develop 512 × 512 thermal image optical fiber bundle. These requirements are encouraging scientists to develop high-performance mid- and far- infrared transmission fibers for this purpose [29]. As a convenient passive image transmission optical device, imaging bundles have been widely used in recent years and given the growing demand for IR image bundles has prompted more and more research. So far, significant progress has been made in the preparation and application of chalcogenide fiber image bundles. Even though many image bundles have been developed as practical applications, there are still some problems to be further studied and improved. At present, the development challenges are mainly the following: 1.

2.

At present, the preparation technology for chalcogenide glass fiber has matured and an optical fiber image bundle only needs a short transmission distance comparing to the much longer distances common to communication optical fiber. Nevertheless transmission loss is still too large for the transmission of high quality IR images and negatively affects the contrast of images. Thus there is a need to be further reduced the transmission loss in order in practical applications. The appropriate design of fiber structural parameters such as cladding thickness, core diameter is also the key factor in improving the image transmission quality of the image bundle, that is, increasing the efficient area of imaging bundle can improve its transmittance and resolution. But this method is limited by the physics of optical fiber transmission theory. A thin cladding will increase the signal crosstalk between fibers and reduce the image clarity; increasing the fiber

344

3.

4.

5.

6.

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diameter will lead to low resolution, which also degrades image quality. Therefore, there is an urgent problem to improve the transmittance and resolution of IR image bundle to avoid adverse impacts on the image quality. At present, the method used is to reduce the fiber diameter and increase the number of pixels in unit cross section, which can improve the diffraction efficiency and resolution of the optical fiber image plane. The applications for some special fields require that the image bundle must meet corresponding strict technical specifications. For example, the line-planeconversion image bundle used in a hyperspectral imager has strict requirements on the positional accuracy of optical fiber, the straightness of optical fiber array, minimal error in the specified string length and the alignment error when the output end array is superimposed. However, it is difficult to meet the above requirements by the conventional lamina-stacking method and acid-leaching method used to fabricate bundles, so it is necessary for researchers to develop a new preparation process to meet the above needs. At present, the 47th Research Institute of China Electronic Science and Technology Group has put forward a method of using a Si-V groove to make an image bundle [30], and some samples have been developed, but the technology is not mature and needs further exploration. Due to the discrete structure and the existence of an adhesive layer, the image transmission effect of providing for mechanical flexibility in an optical fiber bundle has an intrinsic defect, grid phenomenon. Moreover, the mechanical strength of the chalcogenide glass fiber is poor, thus the rate of broken fibers in IR fiber bundles is difficult to reduce in the process of manufacturing and in use, which will inevitably produce dark areas in the field of view, and may even lose important image information during transmission, resulting in low image quality. However, to solve this problem, researchers have proposed the use of dynamic scanning technology and wavelength division multiplexing technology to reduce the problem and have achieved good results. The preparation process for chalcogenide fiber image bundles is extremely difficult, due to the poor mechanical strength of chalcogenide glass fiber. So, the number of pixels of the IR image bundles is generally low so far. The crosssectional area of IR image bundles is always large, thus the traditional acidleaching technology cannot be carried out easily. The performance tests for IR image bundles are also not mature, which limits their application potential. Finally there is also an urgent need to establish a standard image quality evaluation system. The traditional method to evaluate the quality of optical imaging system is used the optical transfer function (OTF), which means the object it applies to must be a linear space invariant system. The fiber image bundle is a discrete sampling imaging device and it does not meet the above conditions, so the traditional OTF theory is not applicable to it. New image quality evaluation systems have been reported. For example, in 2005, Changchun Institute of optics, precision mechanics and physics, Chinese Academy of Sciences proposed the “evaluation method of modulation transfer function of linear optical fiber image bundles” [31].

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9.3 Fiber Lasers Mid-IR lasers operating in the range 3–5 µm overlap the important atmospheric transmission window with the lowest attenuation, and cover many molecular absorption peaks, having a wide range of application prospects and high market demand in the fields of national defense security, remote sensing telemetry, space communication, atmospheric monitoring, bio-chemical sensing and spectral analysis. At present, the typical technical method of realizing a mid-IR laser can be mainly divided into two types of directly generated excited radiation and nonlinear frequency transformation [1, 1]. Among them, quantum cascade lasers (QCLs), solid lasers based on the transition metal doped sulfur compounds (e.g. Fe2+ –ZnSe), solid laser pumping optical parametric oscillator (OPO) and fiber lasers have become the focus of current research and development. QCLs are the most commonly used mid-infrared lasers in commercial applications, with a wide spectral tuning range and a compact physical size, but most of the pumping power is converted into thermal noise, therefore it is difficult to achieve a high-power single-mode IR laser output with good beam quality. Transition metal doped sulfur compound solid lasers have small size, high electro-optical efficiency and their tunable spectral range is also wide. Their disadvantage is that the output efficiency is seriously reduced with increasing temperature. Nonlinear frequency conversion achieves mid-infrared laser output mainly by means of optical parametric conversion. It has wide tunable wavelength, high beam quality, which is an effective way to achieve high power IR laser output. But the structure of such lasers is complex, which causes low electro-optical efficiency and difficult thermal management. An additional disadvantage is that the IR OPO crystal in the system is hard to be fabricate. Compared with the above methods, fiber-based mid-infrared laser technology has advantages in terms of large spectral coverage, high efficiency and beam quality, good performance stability and ease of use. At present, 1–2 µm near-infrared band fiber laser has achieved a great success, fiber laser technology is also probably likely to become one of the mainstream technologies of mid-IR laser light sources into the future. In mid-infrared fiber lasers, there are mainly two approaches: Direct laser generation based on RE ions doped IR fibers, or, shifted laser output based on a nonlinear conversion, such as stimulated Raman scattering.

9.3.1 Direct Mid-Infrared Laser Generation Based on RE Doped Fluoride Fibers For direct laser generation, present developments were mainly focus on RE doped fluoride glass fibers, because the level of RE ions doping in chalcogenide glass is limited and the fiber’s loss is still high at present.

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Research about RE ions doped fluoride fiber laser started from ZBLAN fiber in 1987. By using the multimode ZBLAN fiber doped by Nd3+ as a gain medium, researchers obtained a laser output at 1 µm under pumping from a 514 nm argon ion laser. In 1992, researchers from Brunswick University of Technology obtained 8.5mW laser output at 3.48 and 3.54 µm based on a 1 mol% Er3+ doped ZBLAN fiber of 12 cm length, where a 655 nm dye laser was used as pump source [33]. This is the first time a fiber laser produce an output beyond 3 µm at room temperature. By 2018, with the continuous development of gain fiber and laser technology, a laser output of 41.6 W at 2.82 µm had been achieved in a 7 mol% Er3+ doped ZBLAN fiber, which is the maximum power of fluoride fiber laser until now [34]. Based on Er3+ doped ZBLAN fiber, ~ 2.8 and ~ 3.5 µm lasers are the two main midinfrared output wavelengths based on the 4 I11/2 → 4 I13/2 and 4 F9/2 → 4 I9/2 transitions. The ~ 3.5 µm continuous fiber laser was not thoroughly investigated after the 8.5 mW ~ 3.5 µm fiber laser was realized in 1992. By 2014, Ori Henderson-Sapir et al. [35] from University of Adelaide had improved the ~ 3.5 µm Er3+ doped ZBLAN fiber laser’s power to 260 mW, by adopting dual-wavelength pumping. In 2016, Vincent Fortin and others [36] at Laval University in Canada fabricated the fiber Bragg gratings (FBGs) into 1 mol% Er3+ doped ZBLAN fibers as feedback and output reflectors, and also used dual-wavelength pumping at 974 and 1976 nm to obtain a 3.44 µm W laser output (power = 1.5 W). In the same year, Ori HendersonSapir et al. [37] used diffraction gratings as tuning elements to obtain adjustable laser outputs with a wavelength coverage of 450 nm, with the laser output at a wavelength of 3.47 µm having an output power up to 1.45 W. In 2017, Maes et al. [38] from the University of Laval wrote FBGs with a reflectivity of 90 and 30% respectively at both ends of the ZBLAN fiber, built a single integrated fiber laser cavity, and increased the power of the 3.55 µm laser to 5.6 W, which is currently highest power for a 3–5 µm laser on record. In 2018, Xie et al. [39] of Shanghai Jiaotong University used black phosphorus as a saturated absorbent, and achieved a 3.5 µm Q switched pulse laser output in 1 mol% Er3+ doped ZBLAN fiber. The energy is 1.83 µJ, the pulse width is 2.05 µs, the repetition frequency is 66.33 kHz, and the average power of the laser is 120 mW. Based on Ho3+ doped ZBLAN fiber, ~3.9 and ~3.0 µm are the main two midinfrared output wavelengths based on the 5 I5 → 5 I6 and 5 I6 → 5 I7 transition. In 1995, Schneider [40] from Brunswick University of Technology used a 640 nm laser as the pumping source to obtain a laser output of 3.92 µm of 1 mW in 2000 ppm Ho3+ doped ZBLAN fiber cooled by liquid nitrogen. In 1997, by using 640 and 890 nm lasers as pump sources, they obtained cascading laser outputs of 3.98, 1.38 and 1.2 µm in a 5000 ppm Ho3+ doped ZBLAN fiber, in which the 3.9 µm laser output power was 11 mW [41]. In 1998, they used a 532 nm Nd3+ : YAG frequencydoubling laser as the pumping source, and obtained an 11 mW laser output with a wavelength of 3.22 µm in 2000 ppm Ho3+ doped ZBLAN fiber at room temperature [42]. In 2011, researchers from the University of Sydney used a 1150 nm laser as a pumping source and a diffraction grating as a wavelength modulation device, in turn obtaining a laser output at 3.002 µm in a 1.2 mol% Ho3+ doped ZBLAN fiber with the power of 770 mW and slope efficiency of 12.4% [43]. In 2012, Li

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347

et al. [44], again from the University of Sydney, used a sound-optical modulator as a modulator to obtain a cascading pulse laser output with a pulse width of 350 ns and a wavelength of 3.002 µm in 1.2 mol% Ho3+ doped ZBLAN fiber. In the same year, they obtained a cascading pulse laser output of 3.005 µm with a pulse width of 380 ns and pulse energy of 29 µJ. In 2015, Li [45] of the University of Electronic Science and Technology used Fe2+ : ZnSe crystal saturation absorber to obtain stable wavelength outputs at 2919.1–3004.2 nm in 1.5 mol% Ho3+ doped ZBLAN fiber. The passive Q-switch pulse laser output of 3004.2 nm has a pulse width of 1.23–2.35 µs and a repeat frequency of 43.56–96.1 kHz. Based on Dy3+ doped ZBLAN fiber, a wavelength at 2.8–3.4 µm is the main midinfrared output based on the 6 H13/2 → 6 H15/2 transition. In 2016, researchers from the University of Adelaide used the tandem pumping method to obtain a 3.26 µm laser output in the 2000 ppm Dy3+ doped ZBLAN fiber with a power of 80 mW and a slope efficiency of 51% [46]. In the same year, they used the same pumping method to obtain a tunable laser output covering wavelengths from 2.95 to 3.35 µm [47]. In 2018, they achieved 3.15 µm laser outputs with power of 1.06 W and slope efficiency of up to 73% [48]. In 2019, Majewski et al. [46] of Macquarie University used a 2.83 µm home-made Er3+ doped ZBLAN fiber laser as pumping source, and obtained a 10 W laser output of 3.24 µm in a 2000 ppm Dy3+ doped ZBLAN fiber by tandem pumping (Table 9.1). It is not difficult to conclude, from the above research achievements for 3–4 µm mid-infrared lasers using RE doped ZBLAN fiber, that relatively high-power 3– 3.5 µm laser powers can be obtained in RE doped ZBLAN fiber, but for wavelength above 3.6 µm, the laser power and slope efficiency are very low. For example, the wavelength of laser based on Ho3+ doped ZBLAN fiber can reach 3.95 µm, but the output power and slope efficiency are only 11 mW and 3.7% respectively, and this wavelength has reached the theoretical limit of laser output. S.D. Jackson of the University of Sydney pointed out that the maximum phonon energy of ZBLAN glass is ~ 565 cm−1 , and only the radiative transition in rare earth ions of greater than 2825 cm−1 (that is, the wavelength is less than 3.5 µm) is allowed at room temperature in ZBLAN glass. Therefore, in order to achieve higher power laser output above 3.6 µm, the gain fiber material with a lower phonon energy must be used [63]. The glass materials with a lower phonon energy than ZBLAN glass are largely ChG and InF3 glasses. InF3 glass is a form of fluoride fiber with the lowest phonon energy and the widest transmission window (as shown in Table 9.2). It has obvious advantages in the development of 3.6–4.0 µm mid infrared fiber lasers. In 2018, researchers from the University of Adelaide, using a 1.7 µm Raman fiber laser as the pump source, obtained a laser output with wavelength of 2.95 µm, a power of 80 mW and slope efficiency of 14% in a 2000 ppm Dy3+ doped InF3 fiber. Moreover, they also observed 4.1–4.5 µm luminescence in the fiber’s spectrum, which is the longest wavelength fluorescence signal detected in fluoride fiber so far [65]. In the same year, researchers from Laval University, using 888 nm semiconductor laser as a pumping source and 10 mol% Ho3+ doped double clad InF3 fiber

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Table 9.1 Development status of typical rare-earth doped CW mid-infrared fiber lasers Laser wavelength (µm)

Host

Structure

Power (W)

Institution

Time

references

2.7

Er-ZBLAN

Spatial

0.19

BT Labs

1988

[49]

2.8

Er-ZBLAN

Spatial

24

Kyoto University

2009

[50]

2.8

Er-ZBLAN

All-fiber

41.6

Laval University

2018

[51]

2.86

Ho-AlF3

Spatial

0.057

Jinlin University

2018

[52]

2.9

Ho/Pr-AlF3

Spatial

0.16

Harbin Engineering University

2020

[53]

3.04

Dy-ZBLAN

Spatial

0.08

Adelaide University

2016

[54]

3.15

Dy-ZBLAN

Spatial

1.06

Macquarie university

2018

[55]

3.24

Dy-ZBLAN

All-fiber

10

laval University 2019

[56]

3.44

Er-ZBLAN

All-fiber

1.5

laval University 2016

[57]

3.48

Er-ZBLAN

Spatial

0.08

TU Braunschweig

1992

[58]

3.5

Er-ZBLAN

Spatial

0.26

Laval University

2014

[59]

3.55

Er-ZBLAN

All-fiber

5.6

Laval University

2017

[60]

3.92

Ho-InF3

Spatial

0.2

Laval University

2018

[61]

3.95

Ho-ZBLAN

Spatial

0.011

TU Braunschweig

1997

[62]

Table 9.2 Optical properties of several fluoride glasses/fibers [64] Glass composite

ZBLAN

ZBYA

InF3 -based

(cm−1 )

~ 576

~ 580

~ 507

Transparent window (1 mm thick) (µm)

0.2–10

0.2–10

0.25–12

Transparent window in fiber (µm)

0.3–4.5

0.3–4

0.3–5.5

Phonon energy

as a gain medium, obtained a 200 mW laser output at 3.92 µm at room temperature, which is the highest fiber laser power of ~ 3.9 µm reported so far [66]. To conclude, direct laser generation in RE doped fluoride fiber can provide for laser outputs with wavelength as far as ~ 3.9 µm. But the laser power and slope efficiency drop with longer laser wavelength output, because of the high phonon energy of fluoride glass means it is no longer able to allow the necessary transitions

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349

in RE ions in the glass matrix. Lasers with longer wavelength output requires medium materials with lower phonon energy.

9.3.2 Mid-Infrared Laser Generation by Stimulated Raman Scattering in Chalcogenide Fiber Chalcogenide glass has very high Raman gain coefficient gR (10–11 m/W, two orders of magnitude higher than quartz and fluoride) [67]. Therefore, ChG is very suitable as a fiber material to develop stimulated Raman fiber lasers in mid-infrared region. Theoretically, it is the only active medium capable of generating Raman laser with a wavelength above 4 µm. A Raman fiber laser is based on the stimulated Raman scattering (SRS) effect. When a strong pumped incident light that exceeds the Raman threshold is directed into the fiber, the level of the lower frequency light known as Stokes light increases dramatically due to nonlinear action, and most of the energy of the pumped light is converted to Stokes light. It was first observed by Stolen et al. [68] from Bell Laboratory, who used a multiplier Nd3+ : YAG laser to pump single-mode quartz fiber. Based on the effect, Raman fiber lasers and amplifiers using quartz fiber have been developed and widely used, and the 1–2 µm band has achieved > 100 W laser output at any wavelength in that spectral band [69]. A Raman laser with longer wavelength requires lower phonon energy and higher Raman gain coefficient, both of which are limited in silica fiber. Therefore, ChG fiber got more and more attention as an attractive active medium for mid-infrared Raman lasers. In 2003, Thielen et al. [70] from the U.S. Navy Laboratory numerically simulated the Raman fiber laser based on the ChG fiber for the first time, and studied the performance of non-cascading Raman fiber lasers for different ChG fiber losses. In 2006, Jackson et al. [71] from the University of Sydney experimentally realized a Raman fiber laser output based on a ChG fiber for the first time, which led to research on a mid-infrared sulfur fiber-optic Raman laser. First, they used an 805 nm LD to pump a Tm3+ doped silica fiber, producing a 2.051 µm laser with power of 10 W. Secondly, they used the 2.051 µm laser to pump a single-mode As2 Se3 ChG fiber with core diameter of 6 µm and length of 4 m to produce first-stage Stokes light output at 2.063 µm. Two-color mirror and gold-plated mirror at both ends of the fiber composed the resonant cavity. The slope efficiency was 27%. The optical pumping threshold power was 0.76 W, and when the input pump laser power was increased to 2.5 W, a secondary Stokes light was generated at 2.074 µm with a slope efficiency of 82%. In 2013, Bernier et al. [72] from the Laval University firstly inscribed three FBGs in ChG fiber to form a low-loss F-P cavity for a Raman fiber laser, as shown in Fig. 9.21. They used a quasi-continuous Er3+ -fluoride fiber laser operating at 3.005 µm as a pumping source to pump As2 S3 single-mode fiber with core diameter of 4 µm and length of 3 m. FBG1 working at 3.34 µm with a reflectivity greater than

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Fig. 9.21 3.34 µm cascade Raman fiber laser based on As2 S3 fiber [72]

99% and a bandwidth of 2.2 nm was inscribed at the input end of the fiber, acting as an input cavity mirror. FBG2 with a working wavelength of 3.34 µm, reflectivity of 63% and bandwidth of 0.6 nm, and FBG3 with working wavelength of 3.005 m, reflectivity of 95% and bandwidth of 4 nm were inscribed at the output end of the fiber, acting as an output cavity mirror for Stokes light and a reflector for the pumping light, respectively. The results showed that when the average power of the pumped laser was 245 mW, the maximum average power of the 3.34 m laser was 47 mW, the corresponding peak power was 0.6 W, and the slope efficiency reached 39%. In 2014, Bernier et al. [73] further implemented a 3.766 µm laser output in As2 S3 ChG fiber using a cascaded F-P cavity structure. This is the longest working wavelength to date obtained in Raman fiber lasers at present. They used a 980 nm LD to pump a 5.2 m Er3+ doped ZBLAN fiber to produce a 3.01 µm laser output, and then coupled the laser into the As2 S3 fiber (core diameter of 4 µm and length of 2.8 m) with a total coupling efficiency of 38% through a pair of aspheric lenses and a long-wavelength passband filter. Two pairs of FBGs with working wavelengths of 3.34 and 3.766 µm are inscribed at both ends of the fiber as input and output couplers for the first and second-stage Stokes light, respectively. When the pumping power reaches a maximum of 371 mW, the average laser output at 3.766 µm was 9 mW and the slope efficiency was 8.3%. To conclude, the laser power and slope efficiency of both direct mid-infrared fiber laser generation based on RE doped fluoride fibers and Raman fiber laser based on ChG fibers have developed greatly over recent years. The laser working wavelengths possible have been expanding and the laser power or slope efficiency has been increasing. All these improvements not only rely on the availability of fluoride and ChG fibers with better performance, but also rely on the IR optical devices and laser technology. For example, FBGs inscribed in the fibers can form cavities directly, without introducing unwanted loss that would result from a splice between the IR fibers and silica fiber devices. On the other hand, optical coupling technology is also essential in the mid-infrared fiber laser generation. Because the pumping

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351

efficiency also affect the power limit and the slope efficiency, the improvements in mid-infrared fiber laser is closely related to the improvement of fluoride and ChG glass/fiber devices, such as fluoride and chalcogenide FBGs and fiber couplers. This latter device will be considered in the next section.

9.4 Optical Fiber Couplers With the growing needing for fiber lasers with higher power, many new fibers including double-cladding fiber, large mode area fiber, chirally coupled fiber and etc. have been reported and fabricated. In addition, many new pumping and coupling techniques including cascading end face pumping, side pumping, fiber bundling and etc. were successfully developed. Both the new fibers and new fiber technologies have resulted in very large improvments in the power of fiber lasers [17]. A key element in achieving high output power is a fiber coupler, as a pumping or signal fiber coupler. The coupler plays an important role to guide one or more pump sources into a doublecladding fiber, and its laser power damage threshold directly defines the maximum power of the fiber laser. The fiber optic coupler made by side pumping technology can coupling the pumping light signal from the side of the double-envelope fiber to the inner core of the fiber, and it has the advantage that it does not occupy either ends of the fiber or affect the output and transmission of the signal laser. Without cleaving or tapering the signal fiber, side pumping technology can reduce the loss of signal light, improve the coupling efficiency, guarantee good beam quality and realize a laser cascade, all of which cannot be realized by end face pumping. Side pump couplers have been a great success in fiber lasers made of silica fiber. However, the development of mid-infrared fiber pump couplers has been slower due to the huge differences between IR soft-glasses and silica. Side pumping couplers are usually achieved by means of side polished mechanical docking and melting tapers, as shown in Fig. 9.22 [74]. The mechanical docking method is more flexible to operate, which can achieve effective control of the divided light ratio, but it requires very high accuracy in polishing and machining. Moreover, the mechanical properties of fluoride and ChG fiber are poor, and the laser damage threshold of a mechanical coupling point is limited, which implies that it cannot work in a mid-infrared fiber laser of high power for a long time. On the other hand, a fiber taper prepared by controlled drawing with heat is more structurally stable and can theoretically withstand higher laser power. But owning to the poor mechanical properties and low glass transition temperature of fluoride and ChG glasses, the tapering process requires highly accurate control. Related research about the mid-infrared fiber couplers started from 1989, when British Telecom declared a patent for a multi-mode ZBLAN fiber coupler [75]. In 1993, NTT realized Er3+ doped broadband fiber amplifier output, covering 2.72– 2.81 µm, by adopting low loss fluoride fiber couplers. Experiment results show that the fluoride fiber coupler can not only achieve efficient coupling, but also induce a lower insertion loss compared with that of previous two-color mirror coupling. The

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Fig. 9.22 Schematic diagram of melting taper coupler (a) and mechanical docking of side polishing (b) [74]

fiber coupler has the advantages of easy coupling, easy integration, lower insertion loss (less than 2.8 dB), and can also improve the gain by optimizing the structure, increase the isolation, suppress spontaneous radiation backlighting, and increase the amplification gain [76]. In 2016, British Telecom developed a molten tapering platform that can process mid-infrared soft-glass fibers, and has successfully developed a coupler based on single-mode ZBLAN fibers [77]. In 2018, Samsung Diamond Industries Co., Ltd., in collaboration with Osaka University, developed a side grinding docking system plus carbon dioxide laser fusion process to fabricate high-power laser based on fluoride fiber side-pump couplers with coupling efficiency of up to 83.3% [20]. In 2019, Osaka University, on the basis of the above coupler, demonstrated first-stage MOPA amplification, which increased the laser power at 2.8 µm to 33 W, which is currently the coupler with the high power handling capability in the world for all-fiber coupling amplification [78]. To conclude, there are great challenges in the fabrication technology of midinfrared glass fiber couplers. Because the IR soft-glasses have low mechanical strength and thermal stability, it is very difficult to couple by polishing and mechanical docking, and the power tolerance of the mechanically coupled nodes is very limited, which means it cannot be applied to the high-power mid-infrared laser system. Tapering provides more stable optical properties than mechanical docking, but due to the low softening temperature of the mid-infrared glass (glass softening operating temperature range is small), and the heating process can easily cause the oxidation and crystallization of optical fibers, it is necessary to accurately control the temperature, speed and atmosphere protection of the tapering and other process

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parameters. In addition, because of the great difference between the softening temperature and materialization characteristics of IR soft-glasses fiber and silica fiber, the current well understood and mature silica fiber processing platforms cannot be used to process IR soft-glasses fiber.

9.5 Optical Fiber Gratings An Optical fiber grating is a passive filter device, which is formed by introducing an axialy periodic modulation of the refractive index into the fiber core. Because the grating fiber has the advantages of small size, small fusion loss, resistance to interference and is full compatibility with interconnected optical fiber, it has been widely used in the field of fiber laser, optical fiber communication and sensing.

9.5.1 Fiber Grating in Rare-Earth Doped Fluoride Fibers An RE ion doped fluoride fiber with a fiber Brag grating (FBG) inscribed at both ends can directly form a viable fiber laser cavity and which is also beneficial in reducing losses and increasing the laser power and slope efficiency. The study of FBGs in fluoride fibers began with ZBLAN fibers. In 1994, Taunay et al. [79] fabricated an FBG in a Cr3+ doped ZBLAN fiber with reflectivity of about 10% by using ultraviolet lithography. In 2013, Saad et al. [80] fabricated an FBG in a Cr3+ /Tm3+ co-doped ZBLAN fiber with reflectivity of 96% by using a 248 nm excimer laser. Although fluoride fiber gratings can be fabricated by a continuous UV laser, because of its linear absorption, it needs to utilize the photosensitivity of the material. The refractive index modulation of the gratings is small, and the thermal stability and laser damage resistance are poor. In contrast, femtosecond laser writing of a fiber grating is based on the nonlinear absorption effect of materials, and the prepared fiber grating has high refractive index modulation depth and good thermal stability. Moreover, the femtosecond laser is not dependent on increasing the photosensitivity of the material, and can be applied directly to fluoride fibers. As a result, femtosecond laser etching of fluoride fiber gratings has become the most effective method to obtain high spectral quality and high stability fiber gratings. The actual detail of the teachnique involves one of two methods: the phase mask method (see Fig. 9.23) and laser direct writing method (see Fig. 9.24). Considering the phase mask method, in 2007, Bernier et al. [83] from Laval University were the first to use femtosecond laser pulses with a wavelength of 800 nm, a repetition rate of 1 kHz and a pulse width of 115 fs to write ~ 5 mm long FBGs in Tm3+ doped and undoped ZBLAN fibers respectively, and achieved refractive index modulation depth in the order of 10–3 . However, the maximum refractive index modulation depth of the fabricated gratings decreased by 50% after heat treatment

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Fig. 9.23 Sketch of the phase mask method used to write FBGs [81]

Fig. 9.24 Schematic of femtosecond laser direct writing method to make FBG [82]

at 125 °C for 30 min. Subsequently the researchers fabricated an FBG with central wavelength of 2.8 µm and reflectivity of 95% in a highly Er3+ doped ZBLAN fiber, and finally obtained 5 W laser output with 32% laser conversion efficiency [84]. In 2015, Fortin et al. [85] of Laval University prepared high reflectivity gratings with a central wavelength of 2.938 µm and reflectivity of more than 99% in ZBLAN fiber using 800 nm femtosecond laser. In 2018, Aydin er al [86], again from Laval

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355

University wrote a pair of FBGs with high and low reflectivity of 99 and 8% respectively at a center wavelength of 2.824 µm in a Er3+ doped ZBLAN fiber with 6.5 m length. A laser output power of 41.6 W was then obtained by bidirectional pumping. In 2019, Maes et al. [87] of Laval University wrote a pair of FBGs with high and low reflectivity of 96% and 55% respectively at the center wavelength of 3.42 µm in a highly Er3+ doped ZBLAN fiber with 2.5 m length. Using 1976 and 976 nm lasers to pump the core and cladding of doped fiber at the same time, a laser output with an efficiency of 38.6% at laser wavelength of 3.42 µm was obtained in the fiber laser cavity. The phase mask method has high optical quality and good repeatability. However, the phase mask method can only write fiber gratings with specific wavelengths each time, and it is difficult to prepare fiber gratings in more complex fiber structures such as double-cladding fiber and photonic crystal fiber. In addition, due to the close distance between the phase mask and the fiber, the low damage threshold of the phase mask limits the pulse energy of femtosecond laser. All the drawbacks are avoided in femtosecond laser direct writing method. In 2013, Hudson et al. [88] from University of Sydney wrote a FBG in fluoride fiber using the femtosecond laser direct writing method for the first time. By using 800 nm femtosecond laser, an FBG was written with a length of 20 mm and center wavelength at 2.9 µm in the Ho3+ /Pr3+ co-doped ZBLAN fiber. In 2017, Bharathan et al. [89] from Macquarie University wrote a 15 mm long second-order FBG in the uncoated double cladding Ho3+ /Pr3+ : ZBLAN fiber with central wavelength of 2.88 µm and grating reflectivity of about 50%. In 2018, Goya et al. [90] of Osaka University wrote a 2.5 mm long first-order FBG with central wavelength of 2.8 µm in double cladding Er3+ : ZBLAN fiber using a 513 nm femtosecond laser. The refractive index modulation was 1.1 × 10–3 , and the reflectivity reached 97%. A laser output efficiency of up to 29.1% was obtained when the FBG was used in fiber lasers. The femtosecond laser direct writing method is flexible, and can directly write high-quality grating structure in fibers with complex structure, such as double cladding or microstructured fiber. However, the type-I modulation is the main modulation in the fluoride fiber. The refractive index modulation depth is insufficent, the mode overlap coefficient is small, the writing time is long, and the accuracy of the translation stage needs to be high enough to create reliable displacement at a nanometer scale.

9.5.2 Fiber Gratings in Chalcogenide Fibers The fabrication method for ChG fiber gratings is similar to those of silica and fluoride fibers. The main methods are double beam holographic interferometry, the mask method and the point-by-point direct writing method. The inscription laser source types include continuous and femtosecond lasers. Because the photosensitivity of ChG glass lies in the visible and ultraviolet region, a CW laser can be used to write chalcogenide fiber gratings.

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In 1995, Tanaka et al. [91] of Hokkaido University firstly reported a FBG with a reflection wavelength of 1.55 µm fabricated in As2 S3 fiber by using He–Ne laser holographic interferometry with a power of 5 mW and wavelength of 632.8 nm. However, the thermal stability of this grating is poor, and it can be erased by low temperature annealing. In 2008, Eggleton et al. [92] from University of Sydney fabricated a FBG in single-mode As2 Se3 fiber by 785 nm continuous laser interference. The refractive index modulation depth order of magnitude is 10–3 . However, due to the photosensitivity of As2 Se3 glass in the visible light band and low laser damage threshold, there is no ideal effect in laser nonlinear measurement. In 2011, Ahmad et al. [93] of McGill University fabricated FBGs in As2 Se3 fiber using 632.8 nm and 1.55 µm laser, respectively, and studied the variation of their photorefractive index. It was found that the refractive index of As2 Se3 fiber was changed differently under different two wavelengths of laser irradiation. The refractive index’s change (n) of As2 Se3 fiber for 632.8 nm laser irradiation is negative, the refractive index is reduced, whereas for 1.55 µ m laser irradiation, n is positive, that is the refractive index increases. In 2017, Zou et al. [94] of Nanchang University used 532 nm laser combined with phase mask to write an FBG in As2 S3 fiber. The average order of magnitude of the refractive index change was 10–3 . During the experiment, it was found that the influence of exposure time on the FBG is very obvious. With the increase of exposure time, the reflection wavelength of the FBG first moves rapidly to a shorter wavelength, and then slowly recovers to a longer wavelength. Although a CW laser has a high beam quality and the light path is simple for the purposed of writing chalcogenide FBGs, due to its linear absorption, it needs to use the photosensitivity of the material, which has a wavelength dependence on the irradiation laser, and the grating has small refractive index modulation, poor thermal stability and laser damage resistance, which were observed in many experiments. The optical band gap is obvious in the CW laser inscribed ChG fiber gratings, but there have been no reports regarding the application of fiber gratings in the fields of fiber laser and sensors. In contrast, femtosecond laser writing technology is the most powerful way to fabricate high quality ChG fiber gratings. In 1998, Meneghini et al. [95] of Laval University successfully fabricated a grating structure in ChG using a 800 nm femtosecond laser for the first time, as shown in Fig. 9.25, where the two-photon absorption effect is the basic energy absorption mechanism. Its refractive index modulation depth is large, and the grating structure can be seen clearly under the optical microscope. In 2008, Florea et al. [96] of the U.S. Navy Laboratory obtained a fiber grating with period of 4 µm in As2 S3 fiber by using 800 nm femtosecond laser direct writing technology. The maximum refractive index change for the grating is up to 0.06. It was also found that the pulse width and repetition rate of femtosecond laser can affect the refractive index modulation of the grating. However because of the relatively large grating period, the reflection band gap was not successfully measured. In 2012, Bernier et al. [97] of Laval University used a 800 nm femtosecond laser to write a fiber Bragg grating into As2 S3 fiber without a polymer layer script. Then, they used the FBG to build an all- fiber Raman laser, and obtained 3.34 µm laser output. In 2014, they further used single-mode As2 S3 fiber and femtosecond inscribed

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Fig. 9.25 Optical microscope image of a grating written during 11 min onto a 2-mm As2S3 thin film at a 780-nm wavelength. The average input powers in the two interfering beams are 640 and 740 mW, respectively, whereas the pulse duration is 2 ps. The encircled regions (a, b) indicate different levels of refractive index change [95]

fiber grating to build a 3.77 µm Raman cascade laser. Using a 3.9 W pump laser, the 3.77 µm laser achieved an output power of 100mW, which is the longest output wavelength currently obtained in Raman fiber laser [97]. Although ChG FBGs have been successfully inscribed by a femtosecond laser, the spectral quality of the fiber gratings still needs to be improved. In the femtosecond writing process, the precise control of refractive index modulation is the key to achieve high quality fiber gratings. Because ChG is quite different from traditional quartz glass in its physicochemical and optical properties, research on this problem remains challenging. Masselin et al. [98] of the Centre National de la Recherche Scientifique (CNRS) found that there is almost a linear relationship between the refractive index change in glass and the laser power distribution under the action of a low power laser, but when the laser power exceeds a certain threshold, the n changes in a more complex manner. The refractive index change is positive in the central region with high laser energy density, while it is negative in the edge region with lower laser energy density. The complex refractive index change of ChG induced by a femtosecond laser means that the accurate control the refractive index change of fiber grating is a challenge. The mechanism and accurate control method of refractive index change of chalcogenide glass induced by femtosecond laser still need to be further explored, and the controllable writing process of chalcogenide glass fiber grating still need further improvement.

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9.6 Optical Fiber Sensor Fluoride and chalcogenide glass fiber and above fiber devices are not only used in supercontinuum generation and mid-infrared fiber laser, but also in sensor field. The mid-infrared and even far-infrared transparent window of these fibers covers the socalled molecule fingerprint spectral region, making them promising candidates for biological, liquid and gas sensing. The low glass transition temperature of soft-glass fibers means greater dimensional changes and higher strain when the temperature changes, altering the transmission of guided light. The high refractive index and nonlinear index of the fibers are also beneficial in sensing field. the sub-section which follow summarize existing research results for a biological sensor, a temperature sensor, a solution concentration sensor and a gas sensor.

9.6.1 Biological Sensor IR fiber biosensors can be used for on-line in situ monitoring of various cell metabolic abnormalities. They have the advantages of small size, are simple and fast, offering high sensitivity and good reproducibility. The tissue and blood of living organisms are mainly composed of proteins, carbohydrates, fats, sugars, etc., and the IR characteristics of different tissues and cells, or the same type of cells in different states, are different, so one can obtain useful pathological information by detecting the changes of their IR characteristic spectrum. This technology can be applied to the early and rapid diagnosis of tumor and cancer formation in medical treatment. In 2003, Keirsse et al. [99] from University of Rennes I built an optical fiber sensing device by using the tapered Te2 As3 Se5 (TAS) ChG fiber, which was used as shown in Fig. 9.26 in a system using a Fourier transform infrared spectrometer (FT-IR), a Hg-Cd-Te detector and fiber evanescent wave spectroscopy (FEWS) methods. Using the evanescent wave, the IR spectrum of rat liver tissue under different metabolic conditions (starvation and normal feeding) was measured, and the liver cancer was detected effectively. In 2004, Bruno et al. [100] of Arizona Materials Laboratory

Fig. 9.26 Schematic representation of the experimental set-up used for FEWS [99]

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359

Fig. 9.27 Human lung cell infrared spectra recorded with the TAS glass fiber. The asymmetric and symmetric CH2 bands decrease in intensity regularly whereas the methyl CH3 group around 2870 cm−1 increases and slightly change in position

successfully detected the IR characteristic spectrum of healthy human lung cells with a device composed of TAS ChG fiber, and tracked the spectral characteristic changes of the effect of poison Triton X-100 on healthy lung cells, as shown in Fig. 9.27 . In 2005, by using TAS fiber, Pierre et al. [101] from University of Arizona successfully detected the IR spectrum changes of lung epithelial cells exposed to poison Triton X-100 in the range of 2800–3000 cm−1 where the vibration of methyl and methylene hydrocarbons is located. The detection verified the feasibility of using a ChG fiber in a cell-based bio-optical fiber sensor. In 2014, Bruno et al. [102] from University of Rennes I studied the application of Se chalcogenide fiber in the diagnosis of human metabolic diseases and detection of liver cirrhosis in human serum based on the principle of FEWS. ChG fiber is not sensitive to biomaterials, so it can be used in various pathological diagnosis.

9.6.2 Temperature Sensor The high thermal expansion coefficient and high thermos-optical coefficient of fluoride and chalcogenide glass fibers make them suitable for use in the field of temperature sensing. Depending on the operating principle of the sensors, the fiber temperature sensor can be classified as one of three types: fiber grating type, interference type and new materials type. Fiber grating temperature measurement is a mature temperature measurement technology, which is mainly based on the change of the FBG grating period caused by temperature change, which affects the position of characteristic wavelength of

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FBG. Traditional silica fiber grating temperature measurement has been widely used, but the research of soft glass fiber grating is still in its infancy. In 2018, Qian et al. [103] from Ningbo University reported a simulation calculation for different ChG fiber grating temperature sensors. The temperature sensors with different core and cladding materials are considered. It can be seen from the simulation results in Table 9.3 that the temperature sensitivity of ChG fiber grating is higher than that of traditional quartz fiber grating. In addition, the temperature sensitivities of two kinds of ChG fiber gratings at different working wavelengths are calculated. The Bragg wavelengths of 1550, 2000 and 3390 nm are calculated respectively. The results in Table 9.4 show that the temperature sensitivity clearly increases with an increase in the working wavelength. Because of the wide transmission range of chalcogenide fiber, the working wavelength of the grating can be set in the mid-infrared band, thus greatly improving the sensitivity of the temperature sensor [103]. A long period ChG fiber grating can also be used for the temperature measurement. The change of refractive index with external environment will not affect the result of temperature measurement, which gives the sensor immunity from unwanted environmental influence. In addition to the soft glass fiber grating temperature measurement, there are also some temperature sensors based on other theories. In 2017, She et al. [104] from Harbin Engineering University proposed a ChG fiber temperature sensor based on the principle of multimode interference. The sensor structure is shown in Fig. 9.28. A section of multi-mode ChG fiber is spliced between two Table 9.3 Simulation results of Ge–Sb–Se MM-FBG based on different cladding materials [103] Core

Cladding

Temperature sensitivity (nm/°C)

Ge28 Sb12 Se60 (2.6601)

Ge25 Sb15 Se60 (2.6500)

0.07567

Ge28 Sb12 Se60 (2.6601)

Ge20 Sb15 Se65 (2.4621)

0.07666

Ge28 Sb12 Se60 (2.6601)

Ge15 Sb25 Se60 (2.4500)

0.07666

Ge25 Sb15 Se60 (2.6500)

Ge20 Sb15 Se65 (2.4621)

0.065333

Ge25 Sb15 Se60 (2.6500)

Ge15 Sb25 Se60 (2.4500)

0.065333

SiO2 (1.4628)

SiO2 (1.4681)

0.011

Table 9.4 Simulation results of Ge-Sb–Se MM-FBG based on different Bragg wavelengths [103]

Core materials

Cladding materials

Bragg wavelength (nm)

Temperature sensitivity (nm/°C)

Ge28 Sb12 Se60

Ge25 Sb15 Se60

1550

0.07567

2000

0.098

3390

0.166

1550

0.065333

2000

0.084667

3390

0.145

Ge25 Sb15 Se60

Ge20 Sb15 Se65

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Fig. 9.28 Schematic of an SMS fiber structure based on two single-mode silica fiber and a multimode ChG fiber [104]

sections of traditional silicon single-mode optical fiber to form a single-mode/multimode/single-mode (SMS) interference sensor structure. The length and refractive index of the ChG fiber centre section is dependent on the temperature and thus the SMS structure can be used for temperature sensing. The sensor operates near the 2 µm wavelength, which belongs to the near-infrared band, resulting in improved sensitivity compared to the short wavelength band. But there are also drawbacks at the same time, such as the lower softening temperature of soft glass fiber limiting the temperature measurement range. This kind of interference structure is a kind of common optical fiber sensor structure. The theoretical and experimental methods are relatively mature, and it is suitable for a variety of soft glass fiber components, and can also be applied to different measurands, such as humidity, pH, gas concentration and several other parameter. Fluorescent material is also a convenient and practical temperature measuring material. Temperature will affect the lifetime or intensity of fluorescence in the material. The thermal conductivity of soft glass fiber is good, so the combination of soft glass fiber and fluorescent material can realize a temperature sensing function. In 2017, Haouari et al. [105] from Université de Monastir proposed to use the green light to excite Er3+ doped fluorine-tellurium glass fiber to realize temperature detection, as shown in Table 9.5. From the comparison results, it can be seen that the sensitivity for temperature measurement of soft glass doped with RE elements has been improved. Finally, the high non-linear refractive index of ChG glass has also been applied in temperature sensing. In 2013, Trung et al. [106] from University of Sydney realized the distributed temperature sensing by using the nonlinear effect within a ChG fiber. The Stokes and anti-Stokes phonons produced by spontaneous Raman scattering were analyzed in the time domain to monitor the temperature change. Compared with quartz fiber, ChG fiber has better temperature sensing performance because of its high refractive index and Raman coefficient.

9.6.3 Solution Concentration Sensor Soft glass fiber concentration sensors can be classified as either a refractive index sensor or a liquid molecular concentration sensor, depending on the different working principle of the sensor, that is the fluorescence effect or evanescent field.

1060

800

Chalcogenide: Er3+ /Yb3+

Fluorine-tellurium: Er3+

379

379

Tellurite: Er3+

Germanate:

978

Silica: Er3+

Er3+

Stimulate wavelength (nm)

Glass matrix and RE element

300–500

293–498

453–713

313–713

296–673

Temperature range (K)

10.25

8.85

18.8

13.6

1.87

Pre-exponential factor

Table 9.5 Spectroscopic and sensing parameters for various Er3+ hosting matrix [105]

765

645

827

781

512

Energy Band (cm−1)

0.0054

0.0052

0.0066

0.0085

0.0023

Max sensitivity (K−1 )

547

493

473

596

Max temperature (K)

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The photoluminescence intensity of fluorescent materials will be affected by the change of external contact environment, for example, when the concentration of the solution to be measured changes. In 2016, Chahal et al. [107] from Université de Rennes I proposed a chloroform detection device based on this principle, which used dissolved praseodymium ions in ChG fiber. The fiber was excited by a light source to generate fluorescence and the sensor realized the detection of chloroform concentration based on the change in the absorption spectrum intensity. When a fiber is tapered, then bent or wound as coil, the evanescent field extends further into the cladding. The evanescent field can interact more readily with the surrounding environment or material, thus allow sensing or measurement to take place. In 2014, Houizot et al. [108] from Université de Rennes I proposed a new type of wound sensor probe. Its structure is shown in Fig. 9.29. The ChG fiber is heated and softened before winding. The ChG glass has good ductility and can readily realize fiber bending. Compared with the taper sensor and the bending sensor, this kind of winding sensor has the highest detection sensitivity, small size and as a point sensor is convenient to use in many applications and can be used in a variety of detection environments. In 2015, Markos et al. [109] from University of Patras realized the detection of biomolecular concentration by using a ChG fiber with tapered structure to form evanescent field in the fiber tapered region. In 2018, Romanova et al. [110] from Saratov State University directly bent a multi-mode chalcogenide fiber into a tube containing acetone. Due to the evanescent field generated by the bending, the acetone concentration was measured by measuring the change of acetone characteristic absorption peak. The structure and fabrication process of the evanescent field sensor are simple, and there are many applications, but the mechanical strength of the fiber will be affected by the pretreatment. Fig. 9.29 Chalcogenide winding sensor probe [108]

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9.6.4 Gas Sensor A gas sensor based on soft glass fiber mainly uses the fluorescence effect, the infrared characteristic absorption spectrum of the gas and infrared imaging. In 2015, Starecki et al. [111] from Université de Rennes I proposed a CO2 detection device with simple structure, as shown in Fig. 9.30. Dy3+ ions were doped in Ga5 Ge20 Sb10 S65 glass fiber to realize the detection of gas concentration based to the influence of different CO2 concentrations on fluorescence intensity. In the same year, Pele et al. [112] from Université de Caen used a similar method, using erbium-doped ChG fiber and 810 nm pump light to generate a 4.4 µm signal to realize the detection of gas concentration. The results laid a foundation for the development of an all-optical gas sensor, which can be combined with quartz fiber to realize remote detection of various gases. In 2015, Dai et al. [113] from Ningbo University realized the detection of combustion gas using infrared imaging method, and the sensor demonstrated a high sensitivity. Because each kind of gas has its unique characteristic spectrum, the existence of gas in the combustion environment can be selectively and quantitatively detected by analyzing the mid-infrared spectrum, therefore realizing the real-time detection on site. This method can not only be applied to the detection of combustion gas, but also detect other gases, which makes this approach a very useful high-performance detection method. Although most of the current research on soft glass fiber gas sensors focus on the detection of CO2 gas, this detection method can also be applied to the detection of other gases. The wide transmission range of soft glass fiber allows it to operate across a large portion of the infrared band. Most of the molecules’ characteristic absorption peaks are concentrated in the infrared band, which can be used to realize the online detection of gas concentration, even multiple gases simultaneously. Therefore soft glass fiber used in the mid infrared band for gas sensing has wide range of potential applications in the future.

Fig. 9.30 CO2 concentration detection device [111]

9.6 Optical Fiber Sensor

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Soft glass fiber has a variety of components, which can fully adjust the optical properties of the fiber to achieve the best structural design for sensing. In addition, due to its high non-linear coefficient, high refractive index and wide transmission window, soft glass fiber has a wide range of applications in temperature detection, liquid concentration detection, gas concentration detection and biosensor fields. With the ongoing development of soft glass fiber research and processing technology, it is expected to be applied in the fields of infrared radiation measurement, infrared imaging and other biological detection areas.

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Chapter 10

Conclusion Pengfei Wang, Jibo Yu, and Gerald Farrell

Optical fiber communications became a reality with the first demonstration of optical fiber transmission in 1977. Since then the entire field of information communication has undergone unprecedented change and evolution. The growth of the Internet has seen a near-exponential rise in the demand for transmission bandwidth and with it the global deployment of optical fiber. As evidence for this, currently in excess of 2.5 Quintillion bytes (2.5 Million TB) of data are produced globally every day. Optical fiber systems now span the globe and literally provide the essential backbone for all global communications. To put this in perspective, the 2018 “Marea” undersea cable, which is just one of many fiber links between the USA with Europe, has a capacity of 208 terabits per second. Optical fiber communication technology uses light as the carrier and optical fiber as the waveguide for information transmission. The key advantages of optical fiber include its ability to deliver high-speed and large-capacity communication, very low transmission loss, small size, light weight and immunity to electromagnetic interference. The bandwidth and thus the information transmission capacity of fiber is immense compared to traditional copper and radio frequency media. The limit on the information capacity is set by a number of factors, which include the modulation technology and multiplexing technique in use but fundamentally one of the key limiting factors is the fiber spectral bandwidth available for transmission. Silica based fiber is currently the material of choice for almost all the fiber deployed globally. The spectral bandwidth available over silica fiber is approximately 400 nm, between 1240 nm and 1640 nm. The lower limit is set by the condition for singlemode transmission. The upper limit circa of 1640 nm arises because the loss in the mid-infrared and infrared wavelength band for silica fiber is very high due to the large increase in fiber absorption loss which occurs beyond 1640 nm in silica. While the 400 nm window available for silica fiber can accommodate terabit transmission capacities, in the longer term as the demand for bandwidth continues to grow (the current growth rate of global data is 25% per year), wider transmission windows in the mid-infrared and infrared wavelength ranges will be needed © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Wang et al., Mid-Infrared Fluoride and Chalcogenide Glasses and Fibers, Progress in Optical Science and Photonics 18, https://doi.org/10.1007/978-981-16-7941-4_10

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to allow for much larger tranmission capacities per fiber. For now improvements in modulation and multiplexing technology can provide some increased capacity over silica fiber, for example using advanced quadrature modulation, a group at University College London in 2020 managed to transmit data at a rate of 178 Tbits/s. However in the long term, exploiting the wider bandwidth available through using non-silica fiber working at longer wavelengths will become a necessity to deliver the world’s inexhaustible need for data transmission capacity. In the past few decades, a large amount of research effort has been dedicated to finding and optimising optical glass materials which can successfully operate at longer wavelengths with low optical transmission loss. Fluoride and chalcogenide glass materials are considered to be promising glasses with strong potential to be useful in a range of applications. These glasses are attractive as candidates for intensive development as they possess a range of useful advantages: lower phonon energy, a very wide transmission window along with good thermal and chemical stability. These advantages make fluoride and chalcogenide glasses suitable for the production of optical devices for practical applications such as: optical fiber, planar waveguides, optical lenses, optical switches, sensors, fiber lasers, amplifiers, infrared imaging arrays and a range of other all-fiber devices. As a result fluoride and chalcogenide glass materials show excellent potential to provide innovative solutions in fields such as remote sensing, range finding, environmental detection, bioengineering and medical surgery. Research on these two materials has seen significantly increased investment and funding and this is reflected in virtually every chapter of this book with reports of novel landmark research results and advances. Over the last few decades in-depth research on all aspects of glass structure along with improvements in optical fiber preparation technology has enabled the realization and production of the low-loss fluoride and sulfide glass optical fibers. These successes in turn have led to a range of new developments, for example when used as active fibers, lasing is possible in these fibers. As an example successes have included a mid-infrared laser with an output of several tens of watts in a fluoride fiber laser doped with rare earth ions, and a continuous wave laser with an output wavelength at 3.92 µm fabricated using a double-clad fiber. Chalcogenide glass fiber has been widely recognized and extensively used as a means to broaden the spectrum of a supercontinuum laser due to its high nonlinear coefficient, low phonon energy and wide wavelength transmission range. In addition the need for higher pump efficiency of fiber lasers has further stimulated the investigation of fiber gratings and fiber couplers and other devices fabricated in fluoride and chalcogenide glass fibers. The performance of such devices has confirmed the significant advantages and development prospects that exist for fluoride and chalcogenide glass materials for the mid-infrared band. Although great progress has been made in many ways, limitations remain in the preparation of fluoride and chalcogenide glass, mainly because the stability of these infrared glass materials is lower than that of silica. Also there is currently no glass material available that satisfies all of necessary conditions such as low phonon energy, a very wide transmission window and good stability. The result is that researchers

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working on glass materials in the mid-infrared and infrared range face many challenges on the journey from theoretical research to viable technical applications. In order to improve performance and expand the range of application areas, a number of key areas for further research and study of fluoride and chalcogenide glass materials need to be pursued, such as: maximizing the available transmission window; improving transmission efficiency and lowering loss, improved anti-crystallization properties, better mechanical properties, providing a wider temperature operating range, etc. For now the main focus of research on glass materials is divided into two distinct areas. Firstly there is a need to undertake focused research on fluoride and chalcogenide glass materials to improve performance to meet the requirements mentioned above. Secondly, there is a need to improve the glass material preparation process and strictly control the level of water molecules and other impurities introduced into the material, using technologies such as distillation purification, reaction impurity removal, physical adsorption and other methods. The main objective is to reduce to the greatest extent possible the difference between the theoretical loss and the experimental loss achieved. However it must be recognized that even if the necessary improvements in glass materials are achieved, there remains the challenge of manufacturing large volumes of fiber based on these materials, with very high quality. whilst also repeatably meeting stringent specifications. For the manufacture of optical fibers made from fluoride and chalcogenide glass materials there remain a number of technical difficulties in material preparation for fiber drawing and an optical fiber that can realize ultra-long-distance optical transmission has not yet been developed. Furthermore beyond material preparation, the optical fiber fabrication process for flouride and chalcogenide still needs to be further developed and improved to solve problems associated with impurity mixing, lowering structural defects (such as cracks, bubbles and poor low mechanical strength). Compared with the mature technology of silica optical fiber, significant research and development effort is required to improve the production technology of fluoride and chalcogenide optical fibers. The authors of this book have reviewed the basic properties of the fluoride and chalcogenide glass fibers and the significant advances and progress which has been achieved in the mid-infrared field as of 2021. We believe that through the ongoing and sustained efforts of many international scholars and research teams, fluoride and chalcogenide glass optical fibers will see further breakthrough research results in the future and in time such fibers will occupy a larger share in the optical fiber market, driven as always by the insatiable demand for communications bandwidth and to meet global challenges in sensing and medicine.