The ABCs of Fiber Optic Communication [1 ed.] 9781630814168, 9781630814144

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Photonics / Communications

WARIER

This book presents the use of standard tools like laser source and power meter (LPM) to overcome common issues related to optical patching and fiber plants and also discusses the use of specialized tools including the optical time domain reflectometer (OTDR) for issues with fiber plants and locating fiber breaks. Readers gain an understanding of the architecture of core TDM, IP, and optical access networks including PON. Specific methodologies are explored for assessing OTN, DWDM, IP/MPLS, optical access networks– PON/GPON or FTTx networks. Key parameters that influence the choice of fiber based on the network and application type are discussed. This book also provides an overview of the current and future developments in optical fibers, interfaces, transceivers, and backbone networks.

“The contents of the book truly reflect the ABCs of optical fiber technology. The book, an immense help to beginners and experts, provides an insight to the latest technology in its entirety while educating in the most simplistic manner what the next decade would unfold in terms of technology and its horizons.” – Vinod Hingorani, Regional Director, South Asia, Telecom Italia Sparkle

Sudhir Warier is a Principal Mentor at Cognitio. He is a Chartered Engineer from IETE and received his M.S. in philosophy from Alagappa University, his master of financial management (MFM) from Pondicherry University, and his B.E. in electronics and telecommunications engineering from Karnatak University.

Include bar code ISBN 13: 978-1-63081-414-4 ISBN: 1-63081-414-8

The ABCs of Fiber Optic Communication

This unique, practical handbook is the only one of its kind to provide the conceptual framework and troubleshooting tactics related to the manufacturing, selection, and installation of optical fiber plants for designing and deploying modern photonic networks. It discusses optical transceivers, test and measurement equipment, and network architecture of SDH, OTN, IP/MPLS, FTTx networks, and passive optical network (PON). This resource includes the latest technological advancements and industry applications while covering the entire fiber ecosystem from installation to troubleshooting.

The ABCs of Fiber Optic Communication SUDHIR WARIER

ARTECH HOUSE BOSTON I LONDON

www.artechhouse.com

Black

PMS 7488

Photonics / Communications

WARIER

This book presents the use of standard tools like laser source and power meter (LPM) to overcome common issues related to optical patching and fiber plants and also discusses the use of specialized tools including the optical time domain reflectometer (OTDR) for issues with fiber plants and locating fiber breaks. Readers gain an understanding of the architecture of core TDM, IP, and optical access networks including PON. Specific methodologies are explored for assessing OTN, DWDM, IP/MPLS, optical access networks– PON/GPON or FTTx networks. Key parameters that influence the choice of fiber based on the network and application type are discussed. This book also provides an overview of the current and future developments in optical fibers, interfaces, transceivers, and backbone networks.

“The contents of the book truly reflect the ABCs of optical fiber technology. The book, an immense help to beginners and experts, provides an insight to the latest technology in its entirety while educating in the most simplistic manner what the next decade would unfold in terms of technology and its horizons.” – Vinod Hingorani, Regional Director, South Asia, Telecom Italia Sparkle

Sudhir Warier is a Principal Mentor at Cognitio. He is a Chartered Engineer from IETE and received his M.S. in philosophy from Alagappa University, his master of financial management (MFM) from Pondicherry University, and his B.E. in electronics and telecommunications engineering from Karnatak University.

Include bar code ISBN 13: 978-1-63081-414-4 ISBN: 1-63081-414-8

The ABCs of Fiber Optic Communication

This unique, practical handbook is the only one of its kind to provide the conceptual framework and troubleshooting tactics related to the manufacturing, selection, and installation of optical fiber plants for designing and deploying modern photonic networks. It discusses optical transceivers, test and measurement equipment, and network architecture of SDH, OTN, IP/MPLS, FTTx networks, and passive optical network (PON). This resource includes the latest technological advancements and industry applications while covering the entire fiber ecosystem from installation to troubleshooting.

The ABCs of Fiber Optic Communication SUDHIR WARIER

ARTECH HOUSE BOSTON I LONDON

www.artechhouse.com

Black

PMS 7488

The ABCs of Fiber Optic Communication

For a complete listing of titles in the Artech House Applied Photonics Series, turn to the back of this book.

The ABCs of Fiber Optic Communication Sudhir Warier

Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the U.S. Library of Congress. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Cover design by John Gomes

ISBN 13: 978-1-63081-414-4

© 2017 ARTECH HOUSE 685 Canton Street Norwood, MA 02062

All rights reserved. Printed and bound in the United States of America. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing from the publisher. All terms mentioned in this book that are known to be trademarks or service marks have been appropriately capitalized. Artech House cannot attest to the accuracy of this information. Use of a term in this book should not be regarded as affecting the validity of any trademark or service mark.

10 9 8 7 6 5 4 3 2 1

To my family

Contents Preface

xix

Acknowledgments

xxi

Part 1 The Conceptual Framework

1

1

Fundamentals of Optical Communication

3

1.1

Chapter Objectives

3

1.2

Electromagnetic Spectrum

4

1.3

Light—Key Concepts

5

1.4

Fundamentals of Light Transmission

7

1.5 1.5.1 1.5.2

Estimating Channel Capacity of a Communication Link 10 Noiseless Channel 10 Noisy Channel 11

1.6 1.6.1

Scales Logarithmic Scales

12 13

1.7

Optical Power Measurements

15

vii

viii

The ABCs of Fiber Optic Communication

1.7.1 1.7.2

Absolute Power Measurements Logarithm Rules

16 17

1.8 1.8.1 1.8.2

Modes of Light Propagation in Optical Fiber Acceptance Angle and Numerical Aperture Modal Propagation

17 18 19

1.9

Dispersion

20

1.10

Effects of Fiber Nonlinearities

22

1.11

Summary

23

1.12

Referred Standards

24

1.13 1.13.1 1.13.2 1.13.3

Review Review Questions Exercises Research Activities

25 25 26 27

1.14 1.14.1 1.14.2

Selected Bibliography Books URLs References

27 27 28 28

2

Essentials of Fiber Optic Communication

29

2.1

Chapter Objectives

29

2.2

Introduction

30

2.3

Optical Fiber Design Specifications

31

2.4

Optical Fiber Classification

37

2.5

Standard Optical Fiber Designs

40

2.6

Safety Standards

42

2.7

Optical Fiber Composition

43

2.8

Fiber Geometry

44

2.9

Fiber Selection Criteria

45

Contents

ix

2.10

Common Fiber Plant Deployment

47

2.11

Summary

49

2.12 2.12.1 2.12.2 2.12.3

Review Review Questions Exercises Research Activities

50 50 52 52

2.13

Referred Standards

53

2.14 2.14.1 2.14.2

Selected Bibliography Books URLs References

53 53 54 54

3

Optical Fiber Splicing and Interfaces

55

3.1

Chapter Objectives

55

3.2

Introduction

56

3.3

Splices and Connectors

56

3.4 3.4.1 3.4.2 3.4.3

Optical Transmitters Optical Sources Modular Optical Interfaces Key Parameters (Transmitter)

65 66 69 72

3.5

Optical Receivers

76

3.6

Optical Modulation Techniques

77

3.7 3.7.1 3.7.2 3.7.3 3.7.4

Link Loss Budgeting Transmitter Launch Power Receiver Sensitivity and Dynamic Range Power Budget and Margin Computations Span Analysis

78 79 79 80 82

3.8

Summary

85

3.9 3.9.1 3.9.2 3.9.3

Review Review Questions Exercises Research Activities

86 86 88 88

x

The ABCs of Fiber Optic Communication

3.10

Referred Standards

89

3.11 3.11.1 3.11.2

Selected Bibliography Books URLs References

89 89 89 90

4

Fiber Plant Manufacturing, Installation, Maintenance, and Diagnostic Techniques

91

4.1

Chapter Objectives

91

4.2

Introduction

92

4.3

Manufacturing of Optical Fibers

93

4.4

Fiber Laying Techniques

94

4.5 4.5.1 4.5.2 4.5.3

Cable Preparation, Splicing, and Termination Cable Preparation Splicing Fiber Termination

97 98 98 99

4.6 4.6.1 4.6.2 4.6.3 4.6.4

Safety Guidelines Causes of Injury Maximum Permissible Exposure Accessible Emission Limits Fiber Handling Techniques

100 101 101 104 104

4.7 4.7.1 4.7.2 4.7.3 4.7.4 4.7.5 4.7.6

Network Diagnostic Techniques Fiber Route Locator Visual Connector Inspection Optical Power Measurements Power Measurements Optical Time Domain Reflectometer—Working Fiber and Connector Cleaning

106 106 107 107 108 108 109

4.8

Summary

110

4.9 4.9.1 4.9.2 4.9.3

Review Review Questions Exercises Research Activities

111 111 112 112

4.10

Referred Standards

113

Contents 4.11 4.11.1 4.11.2

xi

Recommended Reading Books URLs References

113 113 114 114

Part 2 Optical Network Architectures

115

5

Photonic Transport Networks

117

5.1

Chapter Objectives

117

5.2

Introduction

118

5.3

Transport Network—An Overview

118

5.4

Transport Network—Needs, Benefits, and Function

120

5.5

Synchronous Optical Networks—Evolution

121

5.6

Transport Network—Architecture

123

5.7 5.7.1 5.7.2 5.7.3

Transport Network—Components Network Equipment Media Network Topologies

126 127 135 137

5.8

Summary

137

5.9 5.9.1 5.9.2 5.9.3

Review Review Questions Exercises Research Activities

138 138 140 140

5.10

Referred Standards

140

5.11 5.11.1 5.11.2

Recommended Reading Books URLs References

141 141 141 141

6

Dense Wavelength Division Multiplexing

143

6.1

Chapter Objectives

143

xii

The ABCs of Fiber Optic Communication

6.2

Introduction

144

6.3

What Is Wavelength Division Multiplexing?

146

6.4

Standardization

147

6.5

WDM Fundamentals

148

6.6

Bandwidth Explosion

155

6.7 6.7.1 6.7.2

Optical Transmission Challenges Linear Characteristics Nonlinearities

158 158 160

6.8 6.8.1 6.8.2 6.8.3 6.8.4 6.8.5 6.8.6 6.8.7 6.8.8 6.8.9

WDM Network Components Optical Transmitters Optical Receivers Couplers Optical Amplifiers Regenerators Passive Routers Wavelength Selective Switch Wavelength Converters Wavelength Add/Drop Multiplexers

162 162 162 163 163 164 165 165 165 166

6.9

DWDM Links

167

6.10

Summary

169

6.11 6.11.1 6.11.2 6.11.3

Review Review Questions Exercises Research Activities

170 170 171 171

6.12

Referred Standards

171

6.13 6.13.1 6.13.2 6.13.3

Recommended Reading Books URLs Journals References

172 172 172 173 173

7

Next Generation Optical Networks

175

7.1

Chapter Objectives

175

Contents

xiii

7.2

Introduction

176

7.3 7.3.1 7.3.2 7.3.3 7.3.4

Optical Transport Networks OTN Hierarchy OTN Architecture OTN Interfaces Key Features of an OTN

177 178 178 180 181

7.4 7.4.1 7.4.2 7.4.3

IP over DWDM Architecture Enhanced Router Interfaces Advanced ROADM Technology Transparent Optical Transport with Traffic Protection

183 184 185 185

7.5

IP/MPLS Optical Core Networks

186

7.6 7.6.1 7.6.2

187 187

7.6.3

Next-Generation Packet Optical Transport Network Generalized Multiprotocol Label Switching Packet Optical Evolution or the Packet Optical Transport Service Space Division Multiplexing

189 189

7.7

Summary

190

7.8 7.8.1 7.8.2 7.8.3

Review Review Questions Exercises Research Activities

191 191 193 193

7.9

Referred Standards

193

7.10 7.10.1 7.10.2 7.10.3

Recommended Reading Books URLs Journals References

194 194 194 194 195

8

Optical Access Networks

197

8.1

Chapter Objectives

197

8.2

Introduction

198

8.3 8.3.1

Broadband Access Networks (Cable) DOCSIS

199 200

xiv

The ABCs of Fiber Optic Communication

8.4

Converged Cable Access Platform

201

8.5 8.5.1

202

8.5.2 8.5.3

Optical Fiber Access Networks Passive Optical Network—Architecture and Functioning WDM Technologies Next Generation PONs

205 207 208

8.6

Summary

208

8.7 8.7.1 8.7.2 8.7.3

Review Review Questions Exercises Research Activities

209 209 210 211

8.8

Referred Standards

211

8.9 8.9.1 8.9.2 8.9.3

Selected Bibliography Books URLs Journals References

212 212 213 213 213

Part 3 Operation, Maintainance, and Troubleshooting of Optical Networks

215

9

Troubleshooting Fiber Plants

217

9.1

Chapter Objectives

217

9.2

Introduction

218

9.3 9.3.1 9.3.2 9.3.3

Visual Inspection Techniques Tracing Visual Fault Location Using Fiberscopes

219 219 219 220

9.4 9.4.1 9.4.2

Optical Power Measurements Absolute Versus Relative Power Measurements Procedure for Measuring Optical Power

221 221 222

9.5

Loss Measurements

222

Contents

xv

9.6 9.6.1 9.6.2 9.6.3

Cleaning Connectors Important Considerations Standard Precautions Cleaning Procedure

223 224 225 225

9.7

Splicing Techniques

227

9.8 9.8.1 9.8.2 9.8.3 9.8.4 9.8.5 9.8.6 9.8.7 9.8.8

Using an Optical Time Domain Reflectometer Working Operational Parameters OTDR Testing Prerequisites Precautions Testing Procedure Analyzing OTDR Traces Distance Measurement Estimating the Attenuation Coefficient

228 228 229 231 232 232 232 232 233

9.9

Summary

234

9.10 9.10.1 9.10.2 9.10.3

Review Review Questions Exercises Research Activities

236 236 238 238

9.11

Referred Standards

238

9.12 9.12.1 9.12.2 9.12.3

Selected Bibliography Books URLs Journals References

238 238 239 239 239

10

Optical Network Testing and Troubleshooting Procedures

241

10.1

Chapter Objectives

241

10.2

Introduction

242

10.3 10.3.1 10.3.2

Testing and Troubleshooting Long Haul Networks General Guidelines Preinstallation Checks

242 243 243

xvi

The ABCs of Fiber Optic Communication

10.3.3 10.3.4

Postinstallation Tests Test Description

243 243

10.4 10.4.1 10.4.2 10.4.3 10.4.4

Troubleshooting Optical Termination Points/ Short Haul Segments/Networks Continuity Testing Power Measurement Connector Inspection End-to-End (E2E) Loss (Single/Double Ended)

248 249 250 250 250

10.5 10.5.1 10.5.2 10.5.3 10.5.4 10.5.5 10.5.6 10.5.7

Troubleshooting Cabling and Connecterization Issues Optical Cabling—General Issues Damaged Optical Cables Stretched Cables Connecterization Issues Optical Cabling—General Guidelines Testing Optical Patch Cords Troubleshooting Optical Patchcord Issues

251 252 252 252 253 253 254 254

10.6 10.6.1

Troubleshooting FTTh Networks Troubleshooting FTTx Networks Based on PON Architecture

255 255

10.7

Troubleshooting DWDM Networks

256

10.8

Standardization Bodies

257

10.9

Summary

259

10.10 Review 10.10.1 Exercises 10.10.2 Research Activities

259 259 259

10.11

Referred Standards

260

10.12 10.12.1 10.12.2 10.12.3

Selected Bibliography Books URLs Journals References

264 264 264 264 265

Contents

xvii

Appendix

267

Acronyms

269

About the Author

281

Index

283

Preface

This book is the result of my desire to share, with fellow engineers and technicians, my experience in maintaining and troubleshooting optical fiber networks. I have had unique and varied exposure, for more than 22 years, in the fields of computer hardware, data networking, and telecommunications, spanning the entire value chain from network planning, installation and commissioning, operations and maintenance, troubleshooting, optimization, learning and development, to talent and competency management. The economic development and the liberalization of the telecom sector facilitated the entry of new players in the market with the resultant investments creating a huge employment basket. This is especially true with the world’s largest telecom markets—China and India. However, there is a major knowledge gap, vis-à-vis the formal education imparted in engineering colleges and the needs of the industry. These rapid technological developments in the fields of semiconductor engineering, fiber-optic technology, as well as computer networks, broadband, and telecommunication engineering has widened the gap between the needs of the industry and the academic offerings. Thus, most of the telecom technicians/engineers learned the tricks of the trade on the job itself. As a result, many of these technicians/engineers as well as managers knew “how” to tackle a problem, but not necessarily “why.” Most of the books available in the market provide a mathematical treatise on the subject thus alienating it from the masses. The lack of suitable reference books also hindered the inclusion the practical aspects of installing, maintaining, and troubleshooting optical fiber networks from the syllabi of most technology schools. xix

xx

The ABCs of Fiber Optic Communication

The architecture of the network, the interfaces, transmitters, and receivers have undergone a sea of change over the past few years. Even as I write this book, new innovative technologies and products are fast replacing existing ones. In addition, there is no single book that provides the conceptual framework as well as the latest technology offerings in optical fibers, network architecture, interfaces, lasers, and receivers networks. This book attempts to provide a systematic progression from the core concepts to the practical aspects covering the entire gamut of activities associated with the setup, maintenance, and troubleshooting of modern photonic networks. This book is unique in its presentation. The theoretical concepts of light transmission through a fiber, types of fiber, key parameters of a fiber, optical interfaces, testing tools and techniques, practical guide for maintaining and troubleshooting fiber networks and systems, along with the architecture and components of synchronous digital hierarchy, optical transport networks, Internet protocol (IP)/multiprotocol lable switching (MPLS) core and dense wavelength division multiplexing are presented in this concise and well-illustrated text. The book also presents the optical access networks including fiber to the x (FTTx), passive optical networks and its various flavors. This book would be useful to all individuals associated with data networking, cable television, and telecom industries, irrespective of their functional role—planning, engineering, network management and operations, installation, and commissioning, as well as operations and maintenance. This book will be especially relevant, as a practical handbook, to the massive technical workforce tasked with installation and maintenance of our vast optical networks and as a reference text for graduates, postgraduates, and diploma students of allied engineering disciplines. Happy reading!

Acknowledgments

Authoring a book, especially technical one, involves a lot of reading, a thorough understanding of the fundamental concepts, introspection, data structuring, visualization, and, last but not least, the writing itself! After the hard work is done, the realization dawns that the end result is the cumulative effort of all the researchers, fellow authors, industry, family, and the social ecosystem. I wish to thank all direct and indirect contributors to this work. A special mention for my family—my mother and especially my wife and little sons—for having given me the space to complete this work and being patient as the estimated few days of work stretched to days, weeks, and months. I also wish to thank Ms. Aileen Storry and her team for bringing out this book in a relatively short span of time, nudging me along the way to stick to deadlines, and getting the book reviewed at the speed of light! I also wish to thank the reviewer(s) for their spot-on reviews that have brought tremendous additional value and coherence to this book.

xxi

Part 1 The Conceptual Framework

1 Fundamentals of Optical Communication 1.1

Chapter Objectives

Telecommunication refers to the transmission of voice signals over a distance to facilitate communication. In olden days our ancestors relied on ingenious methods for communication, including the usage of smoke signals, drums, or beacons. In modern times this process involves transmitting rays of light through a fiber optic cable. In the current technology-driven era, telecommunication has become all pervasive and permeates into the realms of data networks, radio, and television as well. Telecommunication involves interconnection of a vast array of networks connecting a myriad of devices and providing multiple services including voice, data, and video—triple play applications. This has led to the development of a multitude of applications from the simple electronic mails to video chats to video-on-demand services (VOD) and more complex medical imaging applications. This chapter introduces the basic concepts related to the propagation of light through a medium. It provides the conceptual basis for understanding complex issues surrounding the design, deployment, and troubleshooting of modern day next generation optical networks that are presented in the later chapters of this book. This book focuses on the practical application of the key concepts presented, and hence the mathematical content has been kept to a bare minimum. There are myriad books available in the market that focus exclusively on the mathematics of designing optical networks. This chapter would equip the reader, irrespective of their background, with the necessary skills to understand the more sophisticated concepts related to designing, deploying, and troubleshooting optical networks. 3

4

The ABCs of Fiber Optic Communication

Key Topics

• Understanding the electromagnetic spectrum; • Describing the basic principles related to transmission of light; • Estimating channel capacity of a communication link scales; • Describing optical power measurements; • Understanding the concept of modal propagation within an optical fiber; • Understanding the concept of dispersion and other nonlinearities and describe their effects on an optical link.

1.2

Electromagnetic Spectrum

Electromagnetic (EM) radiation refers to the energy radiated by matter subjected to electromagnetic processes. The radiation involves a stream of particles, which do not have any mass, referred to as photons. The photons travel at the speed of light in a wave-like pattern. The waves are transverse with the electrical and magnetic perpendicular to each other in the direction of propagation. The term electromagnetic refers to the presence of interrelated/interdependent alternating magnetic and electrical fields. An electromagnetic process is one that involves transfer or change in energy levels (e.g., excitation of atoms using an external energy source). Electromagnetic radiation can travel in space (vacuum) without any medium, like air or water. This radiation spans a range of frequencies collectively referred to as the EM spectrum. The spectrum includes visible light, microwaves, infrared light, ultraviolet rays, X-rays, and gamma rays. The measurement reference and the corresponding units for EM radiation include: 1. Energy—electron volts or Joules; 2. Wavelength—meters and it subunits; 3. Frequency—cycle per second or Hertz (Hz). The measurement unit chosen depends upon the application context. The amount of energy is directly proportional to the frequency, which is inversely proportional to the wavelength. Wavelength refers to the distance between two similar points on successive waves. As highlighted in the previous section, electromagnetic spectrum includes [1]:

Fundamentals of Optical Communication

1. 2. 3. 4. 5. 6. 7.

5

Gamma rays; X-rays; Ultraviolet rays; Visible spectrum; Infrared rays; Microwaves; Radio waves.

Light, generally referred to as visible light, is the radiation that can be deciphered by the human eyes and is responsible for sight. There are numerous theories that describe the characteristics/behavior of light. These include: 1. Corpuscular theory: Light as composed of tiny particles that travel in a straight line; 2. Wave theory: Light as a waveform; 3. Quantum theory: Light as a stream of photons; 4. Electromagnetic theory: Light exhibiting electromagnetic properties. The corpuscular theory, also referred to as particle theory, proposed by Newton is based on the premise that light emits a number of tiny particles referred to as corpuscles that travel in a straight line. In contrast, wave theory assumes that earth is surrounded by a medium referred to ether, and light travels in the form of waves through this ether. Quantum theory states that light contains of tiny particles, referred to as photons, that exhibit properties similar to a wave. This concept is similar to “matter” that is assumed to be composed of protons, neutrons, and electrons. The electromagnetic theory propagated by Maxwell states that light propagated as electric and magnetic waves that are transverse in nature. This transverse nature of light can help in explaining the phenomenon of polarization.

1.3

Light—Key Concepts

Light is an electromagnetic radiation that forms the part of the electromagnetic spectrum. The term light is synonymous with visible light, has a wavelength of around 400 to 700 nm, and falls between the infrared and the ultraviolet rays within the electromagnetic spectrum (described in the previous section). This corresponds to a frequency range of 430 to 770 THz [1]. The key properties of light include:

6

The ABCs of Fiber Optic Communication

1. 2. 3. 4. 5.

Intensity; Direction of propagation; Frequency/wavelength; Speed; Polarization.

Light contains a stream of photons that exhibits the properties of particles as well as waves. This property is referred to as wave-particle duality. The speed of light, in vacuum, is defined to be 299,792,458 meters per second (mps) or 186,282.397 miles per second or 2, 99,792.458 kilometres per second or 1,079,252,848.8 kilometres per hour (kph). In contrast, the speed of sound is 343.2 meters per second (in dry air) [2]. This is the reason that lightning is seen prior to hearing the sound of thunder. Optics refers to the study of the interaction of light with matter. There are two alternative sets of mechanism to measure the intensity of light: 1. Radiometry: Measurement of light power at specific wavelengths; 2. Photometry: Based on standardized model related to perception of light by humans, it provides a measure to quantify illumination based on specific requirements. The concepts of optical power are covered in detail later in this chapter. Wavelength refers to the distance over which the shape of a wave repeats. This concept is illustrated in Figure 1.1. As observed in the Figure 1.1, wavelength is a measure of the distance between the repetitions of a wave (peaks or troughs). The measurement units

Figure 1.1 Wavelength of a waveform.

Fundamentals of Optical Communication

7

are meters and its sub units—millimetres, micrometres, and nanometres, dependent upon the frequency of the waves under consideration. The wavelength of a waveform (traveling at a constant speed) is provided by the following equation: λ = v/f Where λ refers to the wavelength and v is referred to as the phase speed of the waveform under consideration having a frequency of f. The speed of light in vacuum is denoted by c and is taken to be 3 * 108 meters/sec. Example 1.1

Calculate the wavelength of an electromagnetic wave of 112 MHz. In case of electromagnetic radiation (light) traveling through space the phase speed V = 3 * 108 meters/sec. f = 112 MHz Therefore wavelength can be calculated as: λ = 3 * 108 /1.12 * 108 = 2.68 meters

1.4

Fundamentals of Light Transmission

The effect on a ray of light that passes through different media is described by Snell’s Law. As per the law: sinΘ1/sinΘ2 = v1/v2 = λ1/λ2 = n2/n1 Where Θ is the angle measured from the normal of the boundaries of the two medium, V is the velocity of light in mps within the respective medium, and n refers to the refractive indices of the respective medium. sinΘ1 and sinΘ2 refers to the angles of incidence and refraction respectively [3]. The refractive index (RI) of a material describes the manner in which light propagates through a medium other than free space. Since the RI is a ratio of two variables, there are no dimensions attached to it. RI can be defined by the following formula: (RI) = n = c/v Where n = RI of a medium, c = speed of light in vacuum, and v represents the phase velocity of light in a specific medium.

8

The ABCs of Fiber Optic Communication

This section explores the fundamental principles governing the transmission of light through a medium. The understanding of the possible effects of a ray of light incident on a medium is a precursor to understanding the transmission of light. A ray of light incident on a medium can undergo any of the following effects (Figure 1.2): 1. Reflection: change in the direction of the ray of light at the junction or boundary of two different media; 2. Refraction: the bending of a ray of light as it passes from one transparent medium (medium of varying refractive indices) to another; 3. Absorption: the complete attenuation of the incoming ray of light (converted to heat); 4. Scattering: the diffusion of the incoming ray of light into multiple directions. This behavior of the ray of light incident on a medium is dependent on several factors including: 1. The RI of the medium is a measure for how much the speed of light is reduced as it passes through the medium. For example, if a medium has an RI of 2, then a ray of light passing through it would travel at 1/2 = 0.5 times the speed in air or vacuum.

Figure 1.2 Propagation of light through different media.

Fundamentals of Optical Communication

9

2. The angle of incidence (Øi) is the angle at which the ray of light strikes the medium with reference to the normal (line perpendicular to the surface of the medium). 3. Transmission of light over a distance necessitates the use of a wave guide or a channel over which the rays of light can propagate. A fiber optic cable, also referred to as optical fiber, is the most commonly used medium for light transmission. The details of the different types of optical fibers are included in Chapter 2. 4. Optical fibers are made of glass, plastic (generally used for multimode fibres). or both and contain an inner conductor, referred to as core, surrounded by an outer conductor referred to as cladding. The RI of the core is greater than that of the cladding. 5. The transmission of a ray of light through an optical fiber or fiber optic cable (FOC) is due to the principle of total internal reflection (TIR). This is based on Snell’s law, which determines the effect on light incident on a medium. The law states that the angle at which light is reflected is dependent on the RI of the two media under consideration. In the case of an optical fiber, these are the core and the cladding. The lower RI of the cladding (with respect to the core) causes the light to be angled back into the core. The propagation of light through an optical fiber is graphically illustrated in Figure 1.3. As highlighted in the figure, the incident ray of light cannot travel through the cladding and is confined to the core through which it propagates due to the successive internal reflections. Another important parameter that aids in the transmission of light through an FOC is the critical angle Øc. The following list highlights the relationship between the angle of incidence and the critical angle: 1. If the angle of incidence is less than the critical angle, the ray of light incident on the core of a fiber will get refracted through the cladding. 2. If the angle of incidence is equal to the critical angle, the ray of light incident on the core of a fiber will travel along the boundary of the core and cladding. The ray would be weakly guided and likely to be refracted from the fiber at some point in time. 3. If the angle of incidence is greater than the critical angle, then the ray of light incident on the core of a fiber will undergo total internal reflection and propagate through the fiber.

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The ABCs of Fiber Optic Communication

Figure 1.3 Transmission of light through an optical fiber.

The principle of TIR can also have other applications including: 1. 2. 3. 4. 5.

1.5

Medicinal (endoscopes); Fiber diagnostic techniques (fiberscope); Sensors; Optical instruments; Fingerprinting devices.

Estimating Channel Capacity of a Communication Link

There are two distinct approaches to estimating the theoretic capacity of a communication link. The first one is important from a theoretical perspective, while the second mirrors the ground realities. 1.5.1 Noiseless Channel

Assuming a noiseless channel, the channel capacity of a communication link can be estimated using the Nyquist theorem: C = 2B log2 Ln Where: C = Channel capacity in bps B = Bandwidth (Hertz) log2 = Logarithmic scale to the base 2 L = Number of signal levels n = Number of bits per symbol

Fundamentals of Optical Communication

11

1.5.2 Noisy Channel

Assuming a noisy channel, the channel capacity of a communication link can be estimated using Shannon’s theorem: C = 2 B log2 (1 + SNR) Where: C = Channel capacity in bps B = Bandwidth (Hertz) log2 = Logarithmic scale to the base 2 SNR = Signal-to-noise ratio The data rate of a communication channel is dependent on three factors: 1. Available bandwidth; 2. Signal levels; 3. Quality of the communication channel (noise levels).

Note 1

The number of signal levels employed has a direct co-relation with an increase in the probability of occurrence of errors (due to the decrease in spacing between the signals). Example 1.2

Compute the maximum bit rate of a communication channel with a bandwidth of 2500 Hz transmitting a signal with three levels. Assume that the channel is noiseless. The maximum bit rate (MBR) of a noiseless channel can be computed by using the Nyquist theorem. C = 2B log 2 Ln B = 2500 Hz L=3 MBR = C = 2 * 2500 * log 2 3 = 2 * 2500 *1.584 = 7920 bps

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The ABCs of Fiber Optic Communication

Note 2

Use online logarithmic scales for calculations (e.g., http://logbase2.blogspot. in). Example 1.3

A signal of 372 Kbps needs to be transmitted over a communication channel with a bandwidth of 30 kHz. How many signal levels would be required to transmit the signal, assuming that the channel is noiseless? Since the channel is assumed to be noiseless, Nyquist theorem would be applicable. Accordingly: C = 2B log 2 Ln C = 372 kbps = 372,000 bps B = 30 kHz = 30000 Hz 372,000 = 2 * 30000 * log 2 L log 2L = 372,000 60,000 = 6.2 L = 26.2 = 73.51 The resultant level is not a power of 2. In order to ensure conformance, the following options can be considered: 1. Reduce the bit rate (e.g., 360 kbps, which will result in a signal level that is a power of 2). 2. Increase the number of levels. The next highest power of 2 would correspond to 128 levels. This would increase the bit rate to: MBR = 2 * 30000 * 7 = 420 kbps

1.6

Scales

Scale is a technique used for arranging, measuring, or quantifying events or objects or figures in a specific sequence. Linear scales are used commonly to represent data, due to its inherently simplistic usage. However, such scales are useful only when the data to be represented is within a small range. The scales represent the information in uniform intervals that may result in improper conclusions, especially when working with large data sets. Exponential changes in a data set cannot be represented by linear scales.

Fundamentals of Optical Communication

13

Example 1.4

A $5 increase in the price of a commodity whose original price was $10 represents a 50 percent increase (from $10 to $15), while an increase of $5 in the price of a commodity whose original price was $20 represents a 25 percent increase only (from $20 to $25). This change may not be apparent in a chart using a linear scale, since the prices are depicted as equidistant points on the scale. Linear scales can be broadly classified as: 1. Category scale: The category scale is also referred to as nominal or qualitative scale and is used for sequencing numbers or words without any quantitative significance. 2. Interval scale: Interval scale or quantitative scale is used for sequencing numbers in a specific order (e.g., representing distance and temperatures). 3. Sequence scale: This scale is also referred to as ordinal scale. It consists of uniformly spaced rank entities that have no quantitative significance (e.g., Richter scale, pH scale).

1.6.1 Logarithmic Scales

Logarithm refers to the number of instances a number has to be multiplied with itself to obtain another number. Example 1.5

How many multiples of 2 will be equal to 16? 2 * 2 * 2 * 2 = 16 This implies that multiplying 2 four times will result in the number 16. In other words: 24 = 16 Where 2 represents the base and 4 represents the exponent. This equation can be represented on a logarithmic scale as: log2 (16) = 4 Where 2 represents the base and 4 the logarithmic exponent.

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The ABCs of Fiber Optic Communication

Note 3

Exponent calculations can be performed on a physical or electronic calculator using the keys éxp’ of ‘xy. Common logarithm refers to a base of 10’. Natural logarithm uses a base of é’ also referred to as the Euler’s number. É’ refers to the number of times a number has to be multiplied in order to obtain a specified number. This is represented by “ln” on a scientific calculator. E or Euler’s number = 2.71828 Example 1.6

ln (6) = 1.792 Or 2.718281.792 = 6 Or loge (6) = 1.792 Or 2.71828 with an exponent of 1.792 equals 6. Logarithm computation is similar to a multiplication operation. A logarithm of a negative number corresponds to a “divide” operation. An example is provided in Table 1.1: Example 1.7 highlights the advantages of a logarithmic scale in comparison with a linear scale.

Number 1000 100 10 1 0.1

0.01 0.001

Table 1.1 Logarithmic Table Example No. of 10’s Log (Base 10) Result 3 1 * 10 * 10 * 10 log10(1000) 2 1* 10 * 10 log10(100) 1 1 * 10 log10(10) 0 0 * 10 log10(1) –1 –1 * 10 log10(0.1) Or 1 /10 –2 1/(10 * 10) log10(0.01) –3 1/(10 * 10 * 10) log10(0.001)

Fundamentals of Optical Communication

15

Example 1.7

In a controlled temperature environment bacteria, within a lab culture, multiply at a constant rate of 20 percent per hour. As the temperature is increased, there is an exponential growth of bacteria with 50 percent increase in every 20 minutes. The number of cells, at room temperature, can reach over a million in just over eight hours. A linear chart representing this growth is as illustrated in Figure 1.4. It may be noted that the data for the first six hours cannot be interpreted due to the inherent nature of the linear scales. The same data is plotted using a logarithmic scale as illustrated in Figure 1.5. It may be observed in a better manner.

1.7

Optical Power Measurements

The following are the measurement units for optical power (measured in linear units): 1. Milliwatts; 2. Microwatts; 3. Nanowatts. Absolute power measurements are generally difficult, hence changes in power or power referenced to another unit (or signal), referred to as relative

Figure 1.4 Example of linear chart.

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The ABCs of Fiber Optic Communication

Figure 1.5 Example of logarithmic chart.

power is generally used. The relative difference between the strength of the two signals is expressed as decibels (dB) (e.g., optical power losses in a fiber). For power measurements in linear units (mW, μW, nW), dB is calculated as follows: Power (dB) = 10 log (P1/P2) Where P1 refers to the measured power and P2 is the reference power. 1.7.1 Absolute Power Measurements

Absolute power measurements are generally referenced to mW and is expressed as: Power (dBm) = 10 log (P/1mW) dB is expressed as the logarithm to the base 10 of the ratio of power of two signals. Note 4

1. Unit of loss measurement = dB; 2. Unit for power measurement = dBm.

Fundamentals of Optical Communication

17

1.7.2 Logarithm Rules

A basic knowledge of the common logarithmic calculations is desirable in order to decipher design documents as well as perform basic theoretical computations like optical link budget design. A list of the common functions are presented here: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

log10(X * Y) = log10(X) + log10(Y) log10(X/Y) = log10(X) – log10(Y) log10(1/A) = –log10(A) log10(0.01) = –log10(100) = –2 log10(0.1) = –log10(10) = –1 log10(1) = 0 log10(2) = 0.3 log10(4) = 0.6 log10(10) = 1 log10(20) = 1.3 log10(100) = 2 log10(1000) = 3 log10(10000) = 4

Example 1.8

log10 (2 *10) = log10 (2 ) + log10 (10) = 0.3 + 1 = 1.3 Decibels relative to a mW = dBm = 10 log10 (P/1mW) Table 1.2 provides a co-relation between power expressed in mW and dBm.

1.8

Modes of Light Propagation in Optical Fiber

It may be noted that the properties of light can be explained using wave as well as particle theory. The earlier discussion related to the propagation of light through an optical fiber assumed a ray of light guided through an optical fiber by a series of reflections. The discussion may not be complete since a beam of light has a finite radius (beam radius) and divergence (beam divergence)

18

The ABCs of Fiber Optic Communication Table 1.2 Power (mW) Versus dBm Power (mW) Ratio dBm 1 0 1/1 = 1 2

2/1 = 2

3

5

5/1 = 5

7

10

10/1 = 10

10

0.1

100/1 = 100

20

1W

1000/1 = 1000

30

5W

5000/1 = 5000

37

as explained by the wave theory. It may be confusing to readers that different properties of characteristics of light are explained with the alternate use of the wave or ray theories. 1.8.1 Acceptance Angle and Numerical Aperture

Acceptance angle, denoted by Θa , refers to the maximum angle (from normal) at which a ray of light can be injected onto an optical fiber or a waveguide. The numerical aperture (NA) defines the range of angles over which light can be transmitted over an optical fiber. A ray of light that is incident on a fiber at an angle greater than Θa will have its angle of incidence Θi lesser than the critical angle Θc and will get transmitted through the core-cladding interface. Acceptance cone (cone of acceptance) refers to the cone (three dimensional) in which optical power can be coupled onto a fiber and propagated down its length using the principle of total internal reflection. The cone (Figure 1.2) is formed by rotating the acceptance angle around the fiber axis. The concepts described in this section can be cemented by the following simple examples. Example 1.9

The refractive indices of the core and the cladding, of an optical fiber, are 1.28 and 1.24, respectively. Compute the critical angle for transmission of light through optical fiber. For light to be transmitted through an optical fiber, the following condition must be satisfied: Θi > Θc Θc = sin–1(n2/n1) n1= 1.28, n2 = 1.24 Θc = sin–1(1.28/1.24) Θc = 75.63o

Fundamentals of Optical Communication

19

Example 1.10

With reference to Example 1.9 compute the acceptance angle Θa. With reference to Example 1.9: Θc = 75.63o This implies that: αc = 90° – Θc o = 90° – 75.63 = `14.37 Sin Θa = n1 sin αc (assuming refractive index of Air na = 1) Θa = sin-1(n1 sin αc) Θa = sin-1(1.28 sin 14.37°) Acceptance angle = Θa = 18.52o Example 1.11

With reference to the Example 1.9 compute the numerical aperture. Numerical aperture = NA = SinΘa sinΘa can also be computed as (n12– n22)1/2 = (1.282−1.242)1/2 0.317 Note 5

Optical fibers used on a long haul network typically have a NA of 0.1 to 0.3. The NA defines the amount of power coupled onto a fiber. 1.8.2 Modal Propagation

The ray of light traveling through the core of an optical fiber exhibits variations in its intensity as it travels down the fiber. These variations are referred to as modes. Modes can be thought of as rays of light. The number of modes (constituents) depends upon the dimensions of the core and the variation in the refractive indices of core and cladding. The modes are numbered in the ascending order. As the name implies, a single-mode fiber (SMF), because of its small core diameter, allows only a single ray of light to travel along its length. In contrast, a multimode fiber has a large core diameter, which permits multiple modes to travel down its length. Optical fibers can be classified based on their modes of propagation and refractive index profiles as follows: 1. Single-mode step index: Propagation of a single lower order mode of light through a narrow cylinder over the axis of the optical fiber. 2. Multimode step index: The RI of the core is greater than the cladding. The larger core diameter of the core results in multiple mode trans-

20

The ABCs of Fiber Optic Communication

mission (can have more than thousand modes based on the fiber diameter) with the higher modes leaking onto the cladding as well as conversion of optical energy to heat due to absorption. The higher levels of attenuation limit the transmission distance to a few meters to a kilometer. 3. Multimode graded index: The RI decreases gradually from the center of the core toward the cladding. This results in reduction in dispersion through differential mode delay1. The core diameter of a SMF is typically in the range of 8–12 μm, while that of a MMF-GI is in the range of 50–100 μm and that of MMF-SI in the range of 50–200 μm. The cladding size is usually standardized at 125 μm.

1.9

Dispersion

Dispersion refers to the spreading of light pulses as it travels down the length of an optical fiber. Dispersion is one of the primary factors that limits the data transmission rates on an optical network. Dispersion can occur due to a variety of reasons. One of them is due to the difference in RI for different wavelengths leading to changes in velocity and angle of refraction, since the RI is a function of the wavelength passing through a medium. The ratio of the speed of light in a medium to the speed of light in vacuum is referred to as the refractive index. Dispersion causes broadening of pulses and is dependent upon the wavelength of the signal under consideration. As the bit rate of an optical link increases, the width of each bit decreases and renders the bit stream susceptible to effects of dispersion. This will cause bits to overlap onto adjacent time slots leading to transmission errors. The received data stream with overlapping pulses cannot be decoded by the receivers leading to transmission losses due to intersymbol interference. Dispersion is expressed in seconds. For optical transmission over a fiber, the units can be expressed in ps. The normalized dispersion (dispersion per unit length of a fiber) can be expressed2 as ps/ (nm*km). Dispersion can be primarily classified into two types: 1. Intermodal (modal) dispersion: Modal dispersion is caused by the incident ray (ray incident on a core of the fiber) splitting into its different modes (MMF only), each of which takes different path (shorter or longer modal path lengths) through the fiber. The number of paths or 1. Differences in group velocity among propagating modes due to variations (imperfections) in the RI profile of the fiber. 2. Applicable only for chromatic dispersion and not for the other forms of dispersion.

Fundamentals of Optical Communication

21

modes can range from 1 to 1,000,000 depending upon the type of fiber and supported wavelengths. This causes the modes to arrive at the other end of the fiber at differing times causing light to “spread” (in the time domain). This spreading of light is referred to as modal dispersion. The core diameter of the multimode fiber is large and hence susceptible to modal dispersion. The effects of modal dispersion can be negated by using a SMF or reduced by using a fiber with smaller core diameter or the use of a GI fiber. 2. Intramodal dispersion or chromatic dispersion is caused by the materials used in the manufacture of optical fibers. The two types of intramodal dispersion are: a. Material dispersion: As governed by the equation n=c/v, the different wavelengths (within a SMF) travel through an optical fiber at differing velocities, due to inherent variations in the material properties (hence the name) of an optical fiber. b. Waveguide dispersion: The wavelength of a ray of light incident on a SMF is not significantly greater than its core diameter. This causes some portion (modes) of the ray to travel along the cladding. The RI of the core is greater than the cladding, resulting in the portion of light traveling along the cladding to be faster than the portion traveling along the core.3 This results in waveguide dispersion. The area (diameter) over which the light travels through the fiber is referred to as mode field diameter (MFD). MFD is wavelength dependent, hence higher wavelengths will have higher MFD. These concepts are summarized in Table 1.3. Chromatic Dispersion

Chromatic dispersion (CD) is a combination of material and waveguide dispersion and primarily occurs due the fact that rays of light are propagated at differTable 1.3 Types of Dispersion Dispersion Type Cause Intermodal Modal Propagation Intramodal — Material Variation in RI Waveguide Fiber type 3. The mode travels at a speed determined by the proportion of that mode in the core and cladding. This phenomenon accounts for the difference in speeds.

22

The ABCs of Fiber Optic Communication

ent speeds at different wavelengths through the fiber. The effects of CD can be minimized by the use of light sources (lasers) with low spectral width and use of optical fibers optimized for specific wavelengths. CD is expressed in ps per nm per km and represents the pulse (time) spreading in ps for a source with a spectral width of 1 nm traveling over 1 km of a SMF. CD affects all transmissions over SMF at all bandwidths. CD can cancel out at certain wavelengths. Polarization Mode Dispersion (PMD)

A SMF supports two polarizations of the signal to be transmitted. The two polarizations are perpendicular to each other and will propagate at the same speed over a fiber with circular core. However due to the nature of the manufacturing process and the consequent stresses on the fiber, the core is slightly elliptical in shape. The elliptical core causes the two polarizations of a signal, transmitted over a fiber, to travel at different speeds and arrive at the endpoint at different times. This difference in time between the two polarizations is referred to as differential group delay (DGD). PMD is the representation of this time difference over a normalized length of the fiber. PMD is a major contributory factor for losses in optical links at high transmission speeds (40 Gbps and above). The units of PMD are ps/km1/2. The effects of PMD may be highly pronounced in links with transmission speeds of 40 Gbps and above.

1.10

Effects of Fiber Nonlinearities

Numerous types of losses typically account for more than 40% of the optical energy coupled onto a fiber. These include inherent losses, due to materials used, and the manufacturing techniques employed. Some of the major contributors include [4]: 1. Fresnel loss: A portion of the ray of light incident on the core of a fiber gets reflected due the difference in refractive index of the air and that of the fiber core. Fresnel loss typically accounts for 4% losses at both ends of an optical link. 2. Cladding losses: The RI of the cladding is less than that of the core, and this prevents the ray of light from leaving the fiber (high attenuation). 3. Stimulated Brillouin scattering: Scattering of light from its path is a consequence of the irregularities (nonlinear effect) in the propagation medium. Brillouin scattering is a result of interaction between the electromagnetic wave (light) and vibrations in the medium (in the opposite direction). As a direct consequence of SBS, the maximum amount of optical power that can be coupled onto the fiber is limited to a certain value. This value is referred to as SBS threshold.

Fundamentals of Optical Communication

23

4. Stimulated Raman scattering: Stimulated Raman scattering (SRS) refers to the inelastic (unyielding) scattering of photons on interaction with matter. This is in contrast to the elastic scattering of (Rayleigh scattering) wherein scattered photons exhibit the same frequency and wavelength as that of the incident photons. In SRS the scattered photon may have lower energy than the incident photon (Stokes Raman scattering) or the reverse referred to as anti-Stokes Raman scattering. The effects of SRS are insignificant in single channel systems but cause transfer of power from lower wavelengths to higher wavelengths in case of dense wavelength division (DWDM) systems. 5. Four wave mixing: Four wave mixing is a phenomenon affecting systems that use multiple wavelengths for communication dense wavelength division multiplexing (DWDM) systems. Interaction of multiple wavelengths results in the generation of single or multiple wavelengths that are not supported by the system. 6. Self-phase modulation: When a pulse of very short duration is launched over an optical fiber, it causes changes in the RI of the core, which in turn introduces a phase shift in the pulse and consequentially a change in its frequency spectrum. This phenomenon is referred to as selfphase modulation (SPM) [5]. 7. Cross-phase modulation: The concept of cross-phase modulation (XPM) is similar to SPM. However, XPM is caused by the change in RI of the fiber when two rays of light (pulses) travel along the length of the fiber. In case the pulses combine they may affect other pulses by similar interaction. XPM can also be used to facilitate DWDM transmission.

1.11

Summary

The chapter provided the conceptual basis for understanding the causes and effects of propagation of light through a medium. The basic concepts conform to the fundamental laws of physics and can be expressed mathematically. This book has been designed with the view of providing a comprehensive reference to field engineers and hence the mathematical treatise has been kept to a bare minimum. There are myriad books on the subject that a reader could refer to for additional mathematical content. The chapter introduces theory related to the propagation of light through a medium and provides the background for understanding complex issues surrounding the design, deployment, and troubleshooting of modern day next generation optical networks

24

The ABCs of Fiber Optic Communication

The chapter also provides an understanding of linear and logarithmic scales and their application in designing and maintaining telecom networks. Scales is a technique used for arranging, measuring, or quantifying events or objects or figures in a specific sequence. Linear scales are used commonly to represent data, due to their inherently simplistic usage. A basic knowledge of the common logarithmic calculations is desirable in order to decipher design documents as well as perform basic theoretical computations like optical link budget design. The chapter describes modal propagation and its effects on light transmission through a fiber. A ray of light traveling through the core of an optical fiber exhibits variations in its intensity as it travels down the fiber. These variations are referred to as modes. The number of modes (constituents) depend upon the dimensions of the core and the variation in the refractive indices of core and cladding. The modes are numbered in the ascending order. The data carrying capacity of a fiber is theoretically high (defined by Shannon’s theorem). However there are practical limits imposed due to various causes. The primary ones are related to fiber nonlinearities. These losses can typically account for more than 40% of the optical energy coupled onto a fiber. Notable among them is dispersion, which causes broadening of pulses and is dependent upon the wavelength of the signal under consideration. The chapter provides the groundwork for understanding complex concepts related to the design, deployment, and maintenance of optical networks.

1.12

Referred Standards

G.652—Characteristics of a single-mode optical fiber cable. G.653—Characteristics of a dispersion shifted single-mode optical fiber cable. G.654—Characteristics of a cut-off shifted single-mode optical fiber cable. G.655—Characteristics of a nonzero dispersion shifted single-mode optical fiber cable. G.656—Wideband nonzero dispersion-shifted fiber. G.657—Characteristics of a bending-loss insensitive single-mode optical fiber and cable for the access network . GR-761-CORE— Generic criteria for chromatic dispersion test sets. GR-761-CORE—Generic criteria for chromatic dispersion test sets. IEC 60793-1-42—Measurement methods and test procedures for chromatic dispersion . ITU-T G.650.1—Definitions and test methods for linear, deterministic attributes of single-mode fiber and cable.

Fundamentals of Optical Communication

25

TIA FOTP-175-B—Chromatic dispersion measurement of single-mode optical fibers . TIA FOTP-175-B—Chromatic dispersion measurement of SMF.

1.13

Review

1.13.1 Review Questions

1. Electromagnetic radiation can be expressed using the following units: a. Energy—electron volts or joules b. Wavelength—meters and it subunits c. Frequency—cycle per second or Hertz (Hz) d. All the mentioned units 2. Visible light corresponds to a frequency range of 430 to 770 THz within the electromagnetic spectrum. a. True b. False 3. he wavelength of a waveform (travelling at a constant speed) is provided by the following equation: λ = v/f a. True b. False 4. _____________ describes the manner in which light propagates through a medium other than free space. a. Reflection b. Refraction c. Scattering d. Refractive index 5. The transmission of a ray of light through an optical fiber is due to the principle of _______________. a. Total internal reflection b. Reflection c. Refraction d. Absorption 6. If the angle of incidence is less than the critical angle, the ray of light incident on the core of a fiber will get ______________ by the cladding.

26

The ABCs of Fiber Optic Communication

a. Reflected b. Refracted c. Absorbed d. Scattered 7. The channel capacity of a communication link, assuming a noisy channel, can be estimated using: a. Nyquist theorem b. Shannon’s theorem c. Rayleigh theorem d. Raman effect 8. Logarithmic scales are used only when the data to be represented is within a small range. a. True b. False 9. Sequence scale is a commonly used type of logarithmic scale. a. True b. False 10. The effects of __________may be highly pronounced in links with transmission speeds of 40 Gbps and above. a. PMD b. CD c. SBS d. SRS 1.13.2 Exercises

1. List the important design considerations for an optical network. 2. List and describe nonlinearities in fiber. 3. Describe the commonly used light sources in an optical fiber network. List their key parameters. 4. Which type of fiber is suited for commercial deployment? Provided a detailed reasoning for your stand. 5. Explain the concept of scales and their relation to network engineering.

Fundamentals of Optical Communication

27

1.13.3 Research Activities

1. Describe in detail the types of dispersion and illustrate with examples their effect on network design and performance. 2. Write a note on e-dispersion compensation. 3. Describe space division multiplexing. Detail its relation with optical fiber communication and modal propagation.

1.14

Selected Bibliography

1.14.1 Books Carson, Mary Kay, Alexander Graham Bell: Giving Voice to the World, New York: Sterling Publishing, 2007, pp. 76–78. Bell, Alexander Graham, “On the Production and Reproduction of Sound by Light,” American Journal of Science, Third Series. XX (118): 305–324. Huber, John C., Industrial Fiber Optics Networks, Instrument Society of America, 1995. Betti, Silvello, Coherent Optical Communications Systems, New York: John Wiley & Sons Inc., 1995. Senior, John, Optical Fiber Communication: Principles & Practice, Harlow, England: Prentice Hall, 2008. Keiser, G., Optical Fiber Communication, Tata McGraw Hill, 2008. Idachaba, F., D. U. Ike, and O. Hope, “Future Trends in Fiber Optics Communication,” Proceedings of the World Congress on Engineering, Vol. 1, 2014. “Guide To Fiber Optics & Premises Cabling.” The Fiber Optics Association. Retrieved December 22, 2015, http://www.thefoa.org/tech/ref/contents.html. Buzzelli, S., et al. “Optical Fibre Field Experiments in Italy: COS1, COS2 and COS3/FOSTER.” International Conference on Communications, Seattle, 1980. Alwayn, V., “Propogation of Light,” November 20, 2016. Retrieved from Cisco Press: http:// www.ciscopress.com/articles/article.asp?p=170740&seqNum=5. “Bell Labs Breaks Optical Transmission Record, 100 Petabit Per Second Kilometer Barrier,” Phys. org, September 29, 2009. Rigby, P., “Three Decades of Innovation,” Lightwave, Vol. 31, No. 1, 2014, pp. 6–10. Stone, Jack A., and Jay H. Zimmerman, “Index of Refraction of Air,” Engineering Metrology Toolbox, National Institute of Standards and Technology, December 28, 2011.

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1.14.2 URLs http://rapidtables.com/calc/math/Exponent_Calculator.htm. http://rapidtables.com/calc/math/Sin_Calculator.htm. http://waset.org/publications/4157/modal-propagation-properties-of-elliptical-core-opticalfibers-considering-stress-optic-effects. http://www.convertunits.com/from/megahertz/to/hertz. http://www.rapidtables.com/convert/frequency/hz-to-mhz.htm. https://en.wikipedia.org/wiki/Isotropy. https://en.wikipedia.org/wiki/Light. https://en.wikipedia.org/wiki/Numerical_aperture. https://www.rp-photonics.com/acceptance_angle_in_fiber_optics.html. http://waset.org/publications/4157/modal-propagation-properties-of-elliptical-core-opticalfibers-considering-stress-optic-effects.

References [1]

Elert, Glenn, The Electromagnetic Spectrum, The Physics Hypertextbook, Hypertextbook. com. Retrieved October 16, 2010.

[2]

Bannon, M., and Frank Kaputa, “The Newton-Laplace Equation and Speed of Sound,” Thermal Jackets, Retrieved May 3, 2015.

[3]

Joannopoulos, John D.; S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd ed., Princeton NJ: Princeton University Press, 2008.

[4]

Fiber Technology Incorporated, “Transmission Loss,” November 20, 2016. Retrieved from fiberopticstech.com, http://www.fiberopticstech .com/technical/transmission_loss. php.

[5]

Stolen, R., and C. Lin, “Self-Phase-Modulation in Silica Optical Fibers.” Phys. Rev. A, Vol. 17, No. 4), April 1978, pp. 1448–1453.

2 Essentials of Fiber Optic Communication 2.1

Chapter Objectives

The developments in the field of optical fiber technology have spanned more than 40 years and are still continuing. The challenge lies in designing and developing fibers that can meet the challenges of the future, namely in terms of the magnitude of bandwidth carried as well as the number of wavelengths supported. As a result of continuous research and development efforts, fibers with a high level of glass purity and performance approaching the theoretical limits have been manufactured and put into operational use. The high purity levels in consonance with the allied developments in semiconductor technology have enabled fibers to transmit optical signals over large distances without amplification. This chapter introduces the basic concepts of fiber optic transmission along with a detailed treatise of the different types of optical fibers and interfaces. The last few years have witnessed revolutionary growth in the areas of fiber manufacturing and semiconductor technology resulting in the development of a new generation of optical sources, receivers, and modulation techniques, thereby shattering a few myths and breaking hitherto unbreakable barriers of speed. These developments are covered in Chapter 7 of this book. Key Topics

• Optical fiber design specifications; • Optical fiber classification;

29

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The ABCs of Fiber Optic Communication

• Optical fiber design; • Safety standards; • Optical fiber composition; • Commonly deployed fibers; • Fiber selection criteria; • Common fiber plant deployments.

2.2

Introduction

The media, an important component of a telecommunication network, directly impacts the network speed, accuracy, and transmission distance. This implies that the choice of media has a significant impact on the capital expenditure (CAPEX) and operational expenditure (OPEX) of a telecommunication service provider along with the quality of service (QoS). As the number of people having access to telecommunication services (teledensity) and the number of services provided increase, the bandwidth requirements spirals exponentially. In fact the increase in the number of users and services may often be co-related. The legacy networks were based on copper wires. The media type and the synchronization method were the two major impediments in the development of high-speed/ high-capacity networks. The electrical properties of copper cables hamper signal transmissions due to the effects of resistance and interference. An optical fiber consists of a strand of glass or plastic that allows the passage of light in a manner similar to the conduction of electricity through copper cables. Light travels through the fiber due to reflection from its inner surfaces. The possibility of using a ray of light to transmit data originated from visual observations and experiments originating in the late nineteenth century. The year 1880 witnessed the initial patent on a method to transmit light using glass pipes. This led to subsequent developments in this field followed by the first demonstration of a system based on optical transmission. Significant progress in the growth of fiber optic technology was achieved in the early twentieth century. The initial success was attributed to the development of a device referred to as a fiberscope. The fiberscope (from which the name fiber optics was derived) employed a glass fiber and functioned as an image transmitting device. The major limitation of this method was the excessive optical losses of the fiber, which severely limited the transmission distance. To counter the excessive loses a glass fiber with two layers, the inner core and the outer cladding, was developed. The core facilitated the transmission of light while the cladding prevented the light from leaking out of the core by reflecting the light within the boundaries of the core.

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The initial challenge was to offset the drastic losses during transmission of light through the optical fiber. These losses were primarily due to purity of glass used in the fiber construction and the effects of dispersion. The losses, which were very high, were of the magnitude of 1000 dB/km. The technological developments, primarily the ability to manufacture high purity glass fibers, reduced the losses to less than 20 dB/km. Corning has been a world leader in developing cutting edge technologies relating to the manufacture of glass and is responsible for pioneering efforts in the development of the optical fiber with some early patents on the fabrication process. These technology advances brought about a continuous decrease in the attenuation characteristics of the glass fiber and in the process facilitated guided light communication. The development of the plastic optical fiber as well as plastic clad silica (PCS) cable can be directly attributed to these developments. Figure 2.1 illustrates the block diagram of a basic fiber optic transmission/ receiving system. The electrical signal, either analog or digital, representing the user signal is fed to the input of a light source—light emitting diode (LED) or light amplification by simulated emission of radiation (laser)—which converts the electrical energy into a ray of light. This light is focused on to the core of the optical fiber which carries the signal to the receiver consisting of a diode—an avalanche photo diode (APD), which converts the incoming light rays to electrical pulses again.

2.3

Optical Fiber Design Specifications

An optical fiber consists of two layers of different types—varying refractive indices (RI)—of highly pure glass or plastic arranged to form the core and cladding. The core refers to the inner layer, while the cladding forms the outer layer. The cladding is usually covered by multiple layers of protective coating to help it withstand different types of stresses during deployment. The protective coating generally comprises of a soft inner layer, designed to cushion the fiber, followed by a hard outer layer designed to withstand stresses during installation

Figure 2.1 Basic block diagram of an optical system.

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The ABCs of Fiber Optic Communication

or termination. This section introduces the key design parameters of a fiber optic cable (FOC). These include the attenuation characteristics, modes of light propagation, types of optical fibers, and safety standards, among others. The cross section of an optical fiber is illustrated in Figure 2.2. 1. Transmission losses: There are different types of losses encountered during the propagation of light through a medium. The following section lists down the primary causes of attenuation of light traveling through an optical fiber. Figure 2.3 illustrates the attenuation characteristics of a standard (single mode) optical fiber: a. Absorption: Light consists of a stream of photons. Absorption refers to the loss of signal energy due to the absorption of the propagating photons through a media and its subsequent conversion to heat. b. Scattering: Scattering results in the redirection of light through the core into the cladding.

Figure 2.2 Sectional view of an optical fiber.

Figure 2.3 Attenuation characteristics of an optical fiber.

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c. Dispersion: Dispersion refers to the elongation of the light waves as it travels along the fiber, since the phase velocity of a wave is dependent on its frequency. Dispersion (Figure 2.4) causes the propagating signal through the fiber to get severally degraded over distance. There are generally two sources of dispersion: intramodal dispersion and intermodal dispersion. Intramodal dispersion can occur in all types of fiber and can be further classified as material dispersion and waveguide dispersion. Material dispersion refers to the frequency dependent response of a media to the propagating light. Waveguide dispersion is due to the relationship between the physical dimensions of the waveguide (core diameter) and the propagating light signal. Material dispersion refers to the pulse spreading caused by the specific composition of the glass while waveguide dispersion results from the light traveling in both the core and the inner cladding glasses at the same time, but at slightly different speeds. The two types can be balanced to produce a wavelength of zero dispersion anywhere within the 1310-nm to 1650-nm operating window. 2. Transmission wavelengths or windows: In the early days of fiber optic communication the LED was employed as a light source. The LEDs mostly operated at 780-nm or 850-nm wavelengths. This region is referred to as the first transmission window. LEDs are not suitable for high bandwidth transmissions over a long distance. Laser is the normal choice as a light source in equipment deployed in telecommunication networks. Lasers operate in two wavelength regions namely 1310 nm and 1550 nm that are commonly referred to as the second and the third optical transmission windows. The effects of dispersion are zero, or near zero, in the 1310-nm window, whereas the losses are the least in the 1550-nm window. The modern long distance fiber optic networks operate around 1550 nm. This wavelength band is

Figure 2.4 Dispersion types.

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The ABCs of Fiber Optic Communication

also particularly important to the WDM systems that are increasingly being deployed in networks worldwide. These networks use amplifiers to counter the effects of attenuation. The commonly deployed amplifiers are the erbium-doped fiber amplifiers (EDFAs) that provides signal amplification across a range of wavelengths from 1530 to 1565 nm. This window is commonly referred to as the EDFA window. The addition of dopants like aluminum can further extend the EDFA window by around 30 nm. The optical transmission windows are illustrated in Figure 2.5. 3. Propagation modes: Light is propagated through the core of an optical fiber by the principle of total internal reflection, discussed in Chapter 1. The optical fiber thus acts as a waveguide. Light traveling through a FOC exhibits certain modes or variations in the intensity of the light as it travels over the fiber. The number of modes that can exist is dependent on the fiber dimensions as well as the refractive indexes (RIs) of the core and cladding. Fibers that support only a single mode are called single-mode fibers (SMF), while those providing multiple propagation paths referred to as multimode fibers (MMF). The commercial fibers available in the early days of optical communication had a large core diameter facilitating multiple modes of light to simultaneously propagate through the fiber. The larger core diameter of these multimode fibers also facilitated the use of lower-cost optical transmitters. A single-mode fiber has a small core diameter that allows only one mode of light to propagate through it. Hence the optical

Figure 2.5 Optical transmission windows.

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signal can travel over longer distances in the absence of modal dispersion. It is interesting to note that there, as commonly perceived, the multimode fiber does not provide a higher transmission bandwidth as compared to a single mode fiber. The SMF are the preferred choice for deployment in telecommunication networks as well as other environments requiring long distance, high bandwidth applications. MMF is used principally for systems that need to communicate over short distances (typically less than 2000m)1. This could include communication between equipment within an industry or communication inside optical multiplexing equipment or between communication equipment within an exchange. 4. Dimensions: The dimensioning of an optical fiber is crucial and governs the various losses that affect the propagation of light waves through it. A large core diameter would ensure higher coupling of light energy from the source onto the fiber. This can however cause saturation problems at the receiver. Typically, the fiber dimensions are expressed in the form: x/y Where x indicates the diameter of the core and y indicates the diameter of the cladding. Example 2.1

8/120 indicates a core diameter of 8 microns and a cladding diameter of 120 microns. The common dimension of a SMF is 8/125 while that of a multimode fiber is either 50/125 or 62.5/125. These dimensions are standardized internationally so as to maintain compatibility between connectors and allied accessories. The standardization of the communication media and its associated interfaces are an important prerequisite for universal communication. Accordingly, the standard for outer cladding diameter of single-mode glass optical fibers is 125 microns (μm) and 250 μm for the coating. SMF have a miniature core size, approximately 6 to 10 μm in diameter, while common MMFs have core sizes of 50 to 62.5 μm in diameter, as illustrated in Figure 2.6.

1. It may be noted that the developments in the fiber manufacturing technologies has resulted in the development of MMF capable of supporting high transfer rates. The details are included in Chapter 7 of this book.

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The ABCs of Fiber Optic Communication

Figure 2.6 Single and multimode fibers—cross-sectional view.

5. Strength: The glass used for normal household use is highly brittle. However, the optical fiber is extremely durable with a high tensile strength in excess of 600-plus pounds per square inch. However, flaws or microscopic cracks are developed during the fiber manufacturing process. The manufactured fibers are subjected to load tests to ensure their tensile strength is within permissible limits. 6. Longevity: Optical fibers are designed to provide service for a lifetime as long as the recommended installation and splicing guidelines are adhered to. A number of tests can be used to gauge the service life of fiber installations. Telcordia provides a comprehensive range of testing services that can be availed of by a service provider. In practice the useful lifespan of a fiber depends upon its type, deployment, and operating environment. The biggest factor for obsolescence is the technological advancements. 7. Bending diameter: One of the biggest advantages of an optical fiber is the ease of installation due its lightweight structure, small size, and flexibility. However, extreme care should be taken to ensure that tight bends (macro bending) are avoided. Bends cause high losses of the rays of light traveling through a fiber. Also a bend along an inherent flaw (as discussed earlier) can lead to breakage. Standard tests as well as experience highlights that most fibers can sustain a bend diameter of two or three inches. However, it is advisable to use extra caution to avoid bends while employing fiber handling devices like splice trays, fiber risers, or racks to minimize bending losses.

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2.4

37

Optical Fiber Classification

As discussed in the previous sections FOCs are used extensively in building modern day convergent telecommunication networks. Thus, these fibers are deployed to link geographically distant sites (intercity networks) as well as within a campus or building. Based on the deployment we can classify the cables as: 1. Outside plant (OSP): As the name suggests these cables are deployed extensively by telecom/Internet service providers for intracity as well as intercity connections. These may be blown into ducts that are laid in trenches (the depth may be regulated by state/central/federal authorities or municipal corporations in order to accommodate multiple service providers as well as the need to keep rodents at bay). Depending upon the terrain these cables may also be strung up over poles or may also be laid underwater (submarine cables) for international connectivity. Outside plant installations are nearly always single mode fiber (detailed in the subsequent section), and cables often have very high fiber counts. Cable designs are optimized for resisting moisture and rodent damage. 2. Inside plant (ISP): As opposed to OSP cables ISP cables involves short lengths and are installed typically within a building or campus and may be of few hundred feet with 2 to 48 fibers per cable. The fiber may be single mode, multimode, or hybrid cable with both multimode and single mode fibers, and may be of glass or plastic. There are two primary types of FOCs that are employed in communication networks. They are single-mode fibers and multimode fibers. The primary difference between the cables is with respect to bandwidth carrying capacities due to the differences in their attenuation characteristics, material composition, and manufacturing techniques. The attenuation is principally induced due to the double effect of absorption and scattering. The attenuation of light within the fiber is dependent on the transmitted wavelength and can be minimized by choosing an optimal window within the optical spectrum. The wavelengths that are used by communication networks include 780 nm, 850 nm, 1310 nm, 1550 nm, and 1625 nm. The 780- and 850-nm wavelengths correspond to the high loss region but were used earlier due to the low equipment cost. The 1310nm (zero dispersion) and the 1550-nm wavelengths fall in the low loss region and are used in telecom networks. Table 2.1 highlights the profiles of some of the commonly employed FOCs. 1. Single-mode fibers consist of a strand of fiber made of glass with a core diameter of around 8.3 micrometers with a single transmission mode.

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The ABCs of Fiber Optic Communication

Mode Multimode Multimode Multimode Multimode Multimode Multimode Multimode Multimode Multimode Multimode Multimode Multimode Multimode Multimode Multimode Multimode Single mode Single mode Single mode Single mode

Material Glass Glass Glass Glass Glass Glass Glass Glass Glass Glass Glass Glass Plastic Plastic Plastic Plastic Glass Glass Glass Glass

Table 2.1 MMF/SMF RI Profiles [3] Refraction Size Profile (micrometers) Step 62.5/125 Step 62.5/125 Graded 62.5/125 Graded 50/125 Graded 62.5/125 Graded 50/125 Graded 85/125 Graded 85/125 Graded 85/125 Graded 100/140 Graded 100/140 Graded 100/140 Step 485/500 Step 735/750 Step 980/1000 Step 200/350 Step 3.7/80 Step 5/80 or 5/125 Step 9.3/125 Step 8.1/125

Wavelength (nm) 800 850 850 850 1300 1300 850 1300 1550 850 1300 1550 650 650 650 790 650 850 1300 1550

Attenuation (dB/km) 5.0 4.0 3.3 2.7 0.9 0.7 2.8 0.7 0.4 3.5 1.5 0.9 240 230 220 10 10 2.3 0.5 0.2

The relatively narrow diameter of the core facilitates the propagation of only a single mode of light typically at 1310 or 1550 nm. These fibers can carry significantly higher bandwidths than a multimode fiber over very large distances. However, the small core size necessitates the use of a small (concentrated or intense) light source having constricted spectral width. Single-mode fibers are extensively used for most telecom/data applications2 including wavelength division multiplexing (WDM) systems. The 1310-nm wavelength is preferred because of the zero-dispersion characteristics; however, the losses are the least in the 1550-nm region. Single-mode fibers can have modified RI profiles to suit different applications. The RI can include fibers with abrupt changes in the RI or a gradually reducing RI (from the outer surface of the core toward its central axis). There are three basic classes of singlemode fiber used in modern telecommunications systems [1].

2. Refer to Footnote 1.

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a. Standard SMF has been in existence since the beginning of optical communication and is still used extensively. These fibers are intended for use with the 1310- and 1550-nm wavelengths, which corresponds to the low loss band. b. Dispersion shifted fibers (DSF) was introduced to offset the transmission losses over standard single-mode fiber at 1550nm (as explained earlier). In a DSF the zero-dispersion point is moved to the 1550-nm region by the use of suitable dopants. These fibers, however, exhibit severe nonlinearities when multiple wavelengths in the 1550-nm band are transmitted as in the case of WDM systems. c. Nonzero-dispersion shifted fibers (NZ-DSF) were developed to address the issue of nonlinearities associated with DSF. NZ-DSF is available with positive as well as negative dispersion characteristics and is deployed extensively in modern day telecommunication networks. 2. MMFs [2] allow multiple modes of light to propagate through it. The diameter of a MMF is greater than a SMF and is normally of the order of 50, 62.5, or 100 micrometers. The amount of light that can be coupled on the MMF (light gathering capacity) is higher than the SMF, due to the higher core diameter. However, it has a lower bandwidthdistance limit, as compared to SMF, due to propagation of multiple modes (modal propagation). In most short-range applications a twofiber (2F) MMF is deployed. As per ISO/IEC 11801 standards the following classes of MMF have been defined: a. OM1: Multimode fiber type 62.5 μm core; minimum modal bandwidth of 200 MHz·km at 850 nm; b. OM2: Multimode fiber type 50 μm core; minimum modal bandwidth of 500 MHz·km at 850 nm; c. OM3: Multimode fiber type 50 μm core; minimum modal bandwidth of 2000 MHz·km at 850 nm; d. OM4: Multimode fiber type 50 μm core; minimum modal bandwidth of 4700 MHz·km at 850 nm. 3. MMF cables can be classified into the following types: a. Step-index multimode fiber contains a core with a uniform RI and a sharp decrease in RI at the core-cladding interface. Thus, the cladding has a lower RI. This type of fiber is named step-index due to the sharp change in the RI.

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The ABCs of Fiber Optic Communication

b. In graded-index multimode fiber, the RI of the core gradually reduces outward from the central axis toward the cladding. This causes the rays of light to take helical or sinusoidal paths within the core. In a MMF there can be multiple modes of propagation. Accordingly, certain modes would take the helical path, which is longer but faster due the varying RI, while other modes would travel along the shorter path along the central axis at a slower rate. This characteristic ensures that all the modes arrive almost simultaneously at the receiving end. GI multimode fibers are more expensive than typical single-mode fibers. SMF has lower cost, attenuation, and different dispersion characteristics to MMF since MMF tends to be dominated by modal dispersion. MMF-SI are available in plastic, while MMF-GI are available in glass only. The usage of MMF glass is more common. Table 2.2 highlights the application context of the different types of fibers mentioned in the preceding section.

2.5

Standard Optical Fiber Designs

The two basic cable designs commonly available are: 1. Loose-tube cable: The loose-tube cable (Figure 2.7) has a modular design that typically holds up to 12 fibers per buffer tube with a maximum per cable fiber count of more than 200 fibers. These optical fibers are housed and protected in color-coded plastic buffer tubes. A gel filling compound effectively blocks out water infiltration. Routine bending and installation stresses are prevented by employing excess fiber length (relative to buffer tube length). A central steel member or a dielectric (glass reinforced plastic) provides antibuckling support. The primary tensile strength member for the cable core is generally steel Table 2.2 Operational Wavelength for Common Fibers [4] Wavelength Attenuation Fiber Type (nm) (dB/km) Plastic optical fiber 650 1 dB/m (POF) 850 MMF-GI 850 1 1300 3 SMF 1310 0.2–0.4 1490–1625

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Figure 2.7 Loose tube design.

wire or aramid yarn (for specialized cables due to higher cost) with an outer polyethylene jacket is extruded over the core. In case of armored cables a corrugated steel tape (CST) or steel wire armoring (SWA) is wound over a jacketed cable and an additional jacket extruded over the armor. The two types of armoring are for different situations as they have different advantages and drawbacks. The modular buffertube facilitates easy drop-off of fibers without any interference to the protected buffer tubes being routed to other locations. The loose-tube design also helps in the identification and administration of fibers in the system and hence is used for OSP installation applications both overhead and underground. 2. Tightly buffered cable: This type of rugged cable design has the buffering material in direct contact with the fiber. It provides a cable structure that protects individual fibers during handling, routing, and during dropping connections. In order to prevent installation related tensile stress on the cable yam strength members are deployed. This unique design makes these cable types highly suitable for jumpering or connecting OSP cables to terminal equipment and also for linking various communication devices within premises. Figure 2.8 illustrates the tightly buffered cable. In addition to the cable types listed earlier, multifiber cables are also available. These cables are constructed with strength members that can better resist crushing and bending forces generally associated with cable installation. The outer cable jackets are OFNR (riser rated), OFNP (plenum rated), or LSZH (low-smoke, zero-halogen rated). These specifications are explained in the next section. The primary advantage of this multi-fiber tightly-buffered cables is

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The ABCs of Fiber Optic Communication

Figure 2.8 Tightly buffered cable.

their routing and handling flexibility. In addition to the mentioned cable types ribbon cables are also available. These are generally used extensively for Internet service provider (ISP) and datacenter applications. Individual cables have a flat ribbon-like structure facilitating increased cable concentration and savings in space.

2.6

Safety Standards

The National Fire Protection Association (NFPA) is responsible for creating and maintaining minimum standards and requirements for fire prevention and suppression activities. This includes everything from building codes to the personal protective equipment utilized for firefighting operations. Underwriters Laboratories Inc. (UL) is an independent product safety certification organization that has been testing products and writing standards for safety for more than a century. UL tests raceways and fittings for installation of nonconductive optical fiber cables in accordance with the various established procedures and standards. The basic standard used to classify FOC raceways is UL 2024, “Optical Fiber Cable Raceway.” The salient features are as listed [5]: 1. Optical fibers that are intended to be used in ducts or conduits used for air circulation/exhaust needs to be enclosed within raceways with “Plenum” marking. These cables need to be of OFNP type. 2. For vertical installation, between floors of a building, a raceway marked “Riser” needs to be used along with optical cables marked OFNP and OFNR.

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3. Raceways with neither the marking “Plenum” nor “Riser” are suitable for general purpose use. Optical cables marked OFC can be used for general purposes. In the European Union (EU) the Construction Products Regulation (CPR) provides regulatory standards for power and communication (copper and fiber) cables used in construction works. The standards are formulated by members of the European standardization organizations: European Committee for Standardization (CEN), the European Committee for Electrotechnical Standardization (CENELEC), and the European Telecommunications Standards Institute (ETSI). CEN is the association of the National Standardization Bodies of 33 countries. The standard EN 13501-6 provides fire classification of construction products and building elements—Part 6: Classification using data from reaction to fire tests on electric cables. The term electric cable includes all power, control, and communication cables including optical fiber cables. All products conformant to the above standard will have obligatory CE marking from July 01, 2017.

2.7

Optical Fiber Composition

In earlier days, glass or silica was used in the construction of an optical fiber. However, technological developments have facilitated the development of an optical fiber made of plastic. Three different types of materials can be used to manufacture optical fibers. The basic types of fiber based on composition are as listed: 1. Glass fiber: This type of fiber has the core as well as the cladding made of high purity silica. The attenuation characteristics of the glass fiber are the lowest and hence these are the preferred choice for the high bandwidth long distance telecommunication networks. A pure-glass fiber-optic cable has a glass core and a glass cladding and hence provides the lowest attenuation. Silicon dioxide or fused quartz is used to manufacture the glass FOC. Appropriate impurities like titanium, germanium, fluorine, and boron are added to vary the RI of the cables as per the required profile. The former two will cause an increase in the RI, while the latter two will cause a decrease in RI. 2. Plastic fiber: As the name suggests, the plastic fiber uses a plastic compound for the core as well as the cladding. It is fairly obvious that the conduction of light through plastic would not be as effective as through glass. The plastic FOC has the highest attenuation as compared to the glass and plastic clad silica (PCS) cables. The plastic FOC

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The ABCs of Fiber Optic Communication

is thicker than the other cable types since it is predominantly used for short-distance communication within industrial environments. The dimensions of the plastic FOC would be 480/500 or 735/750 or 980/1000 μm. A major advantage of these cables is their low system cost, ruggedness, as well as the ability to withstand tight bends. However, a major disadvantage, other than the attenuation characteristics, is that the cables are not fire retardant. 3. Plastic-clad silica fiber: The PCS fiber is a hybrid variety of the glass and plastic fibers. It consists of core made of glass (silica) core and a plastic cladding. The plastic cladding has a lower RI as compared to the glass core. The major disadvantage of this cable type is the inability to mate with connectors owing to the cable plasticity. Due to the same reason, adhesive bonding is also not possible with these cables, thereby severely limiting its usage. A variation this cable referred to as hard PCS has found extensive usage in medical applications in the visible, near infrared, and the near ultra violet (UV) regions.

2.8

Fiber Geometry

Fiber optic cables are available in standard reel sizes and hence need to be joined or spliced together for long-distance installations. The quality of the joint has a profound impact on the quality of system performance and the cost of installation. The dimensions of an optical fiber, glass geometry are one of the primary contributors to splicing losses. Tightly controlled fiber geometry helps reduce system costs and highly reduces splicing losses, besides providing for better splicing quality and faster splicing times. This holds true for single fibers as well as ribbon fibers that are mechanically as well as fusion spliced. The three fiber geometry parameters that have the greatest impact on splicing performance include the following: 1. Cladding diameter: The cladding diameter refers to the outer diameter of the fiber. Splicing losses can be reduced by ensuring that the fibers being spliced are almost exactly of the same size, thereby facilitating better alignment during splicing. 2. Core/clad concentricity: This parameter ensures that the core is correctly centered with respect to the cladding. This parameter is determined/controlled during the initial stages of the fiber manufacturing process. A tight core/clad concentricity ensures better axial alignment of the optical fibers being spliced resulting in better quality fibers. As discussed earlier, the core diameter of a MMF is much higher than

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that of a SMF. The MMF thus has a higher light collection capacity, as compared to SMF, facilitating the use of low-cost light sources like VCSEL at 850/1300 nm wavelengths. However, the larger diameter of the core supports multiple propagation modes resulting in modal dispersion and lower bandwidth-distance product (as compared to SMF). The bandwidth (BW) of multimode fiber is a function of the type of MMF and the wavelength of light source used to transmit the signals. For example, the BW of a 50/125 μm MMF would be 600–1000 MHZ-km at 1300 nm and 500 MHz-km at 850 nm. The BW of the fiber is inversely proportional to distance. 3. Fiber curl: The stresses, especially thermal, introduced during the fiber manufacturing process introduces an inherent curvature along the length of the optical fiber. Tighter fiber-curl tolerances reduce the possibility that fiber cores will be misaligned during splicing, thereby impacting splice loss.

2.9

Fiber Selection Criteria

The single mode fiber is extensively deployed for communication purposes. Accordingly, the key optical performance parameters for single-mode fibers are attenuation, dispersion, and mode-field diameter. Optical fiber performance parameters can vary significantly among fibers from different manufacturers and can be a significant contributing factor (positive or negative) affecting the overall system performance. The primary parameters are as listed: 1. Attenuation: Attenuation refers to the reduction of signal strength or the power of light over the length of the fiber and is measured in decibels per kilometer (dB/km). The attenuation of light through fiber is a varying function of wavelength with normal values of 0.35 dB/km at 1300 nm (single-mode fibers) and 0.25 dB/km at 1550nm. This allows propagation of optical signals through an optical fiber possible in the range of kilometers (typically around 100 km depending upon the fiber type, splicing losses, and the transmitted wavelength and power) without regeneration or amplification. Attenuation is caused by several different factors, primary factors being scattering and absorption. Scattering of light occurs due to molecular level irregularities in the glass structure. Residual materials like metals or water ions within the fiber core and cladding results in absorption losses. The water ions have a broadening effect on the propagating ray of light through a

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The ABCs of Fiber Optic Communication

fiber and contribute to attenuation losses for nearby wavelengths as well. 2. Dispersion: Modal dispersion results in the elongation or broadening of a pulse and occurs due to the time difference in the propagation of the different components (modes) of an optical signal traveling through the fiber. In digital transmission dispersion limits the maximum data rate, the maximum distance, or the information-carrying capacity of multimode and single-mode fiber links. Figure 2.9 highlights the effects of dispersion on a signal propagating through the fiber. In contrast CD varies with wavelength and can be controlled by fiber design. The wavelength at which CD equals zero is called the zero-dispersion wavelength (λ0). The maximum bandwidth carrying capacity of the fiber can be exploited at this wavelength. In case of a standard SMF λ0 is at 1310 nm. 3. Mode-field diameter: Mode-field diameter (MFD) is a functional parameter that defines the performance of an optical fiber. MFD refers to the size of the light carrying portion of the fiber. For single-mode fibers, this includes the fiber core as well as a small portion of the surrounding cladding glass. MFD is an important parameter for determining a fibers resistance to bend induced losses as well as splicing losses. MFD is a function of wavelength, core diameter, and the RI difference between the core and the cladding. 4. Cutoff wavelength: Cutoff wavelength is the wavelength above which an optical fiber propagates only a single mode of light. An optical fiber may allow single mode propagation at a particular wavelength but may allow two or more modes at wavelengths lower than the cutoff wavelength. The cutoff wavelength of a fiber is dependent on its length. A longer fiber would have a lower effective cutoff wavelength. The bend radius of a fiber has a direct effect on the cutoff wavelength as a bend reduces the cutoff wavelength.

Figure 2.9 Dispersion characteristics of an optical fiber.

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5. Environmental performance: A key factor that affects the performance of the optical system is the losses due to micro bending. Microbends [6] are small-scale flaws or distortions (of the order of microns) along the fiber axis that are inherent in the fiber manufacturing process. Microbends can also result from pressures exerted due to cabling and connecterization. These distortions cause seepage of light from the fiber and can also be caused as a result of the difference in the coefficient of thermal expansion (between the core/cladding and coating material) at low temperatures. This may result in increased rigidity in the coating and cable due to contractions leading to further microbends. The coating is an important component of a FOC that can impact reliability and performance. Table 2.3 summarizes the concept:

2.10

Common Fiber Plant Deployment

The most commonly deployed fiber type is G.652 for SDH and DWDM network applications including IP. It is also referred to as standard single-mode fiber. SMF G.652 is a single-mode optical fiber and cable that has zero-dispersion wavelength around 1310 nm. The fiber was originally optimized for use in the 1310-nm wavelength region, but can also be used in the 1550-nm region. G.652 can also be used for WDM applications. However, the losses due to the high chromatic dispersion at the 1500- to 1625-nm wavelength window, generally used for long haul and DWDM (C band and L band) transmission, would need to be countered. This can be done by the use of dispersion compensation modules (DCM). The cost of DCM modules is decreasing by the day but complexities may arise due to the need for additional amplifiers. DCM provides bulk (composite wavelength) and per-wavelength compensation. There are several methods of manufacturing a DCM. A popular method is dispersion compensation fiber (DCF), which consists of coiled spools of fiber with negative values of CD to cancel out the CD (bulk compensation) of the span. Another technique that is popular is e-DCM (electronic DCM), which provides per wavelength dispersion compensation. Nortel (now Ciena) was one of the companies that provided e-DCM facilities nearly a decade ago. The low cost of the G.652 fiber makes it the most widely used fiber for access, collector, and metro networks. G.653 is a single-mode optical dispersion shifted fiber (DSF) with a coreclad index profile tailored to shift the zero-dispersion wavelength from the natural 1300 nm in silica-glass fibers to the minimum-loss window at 1550 nm. G.653 fibers have the lowest dispersion profile in the C band and support signal rates up to 40 Gbps. However, these fibers are not suitable for DWDM

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The ABCs of Fiber Optic Communication

S.N 1.

2.

3.

4.

5.

Table 2.3 FOC Coating Features Glass (FOC Characteristics) Coating Optical Attenuation Adhesion to glass Macro/micro bending Defects Bandwidth/dispersion Cutoff wavelength MFD OMD Geometric Diameter Diameter Circularity Strip force Concentricity (core-cladding) concentricity Mechanical Strength Clarity/translucence/yellowing Fatigue Environmental performance Ageing resistance (hydrogen) Temperature stability Ageing (temperature) Humidity stability Water stability Application Transmission capacity Fiber protection Splicing loss Cabling ability Connecterization loss Microbending resistance Environmental stability Handling Stripping and termination

networks due to its vulnerability to four wave mixing effects and not used any longer. The ITU-T G.654 fiber is optimized for operation in the 1500-nm to 1600-nm region with the low loss region corresponding to 1550-nm band. The G.654 fibers have a larger core area and hence can handle higher power levels. However, they suffer from the effects of high chromatic dispersion at 1550 nm. These fibers have been designed for extended long-haul undersea applications. The effects due to nonlinear characteristics of a fiber can be reduced by shifting the zero-dispersion wavelength outside the 1550-nm operating window. The fibers are referred to as nonzero dispersion-shifted fiber (NZDSF). These fibers have a small amount of chromatic dispersion at 1550 nm, which minimizes nonlinear effects, such as FWM, SPM, and XPM. This is especially suitable for DWDM networks and eliminates or reduces the need for dispersion compensation, which can add to the system complexity. There are two

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types of fiber families referred to as NZD+ and NZD–, in which the zerodispersion value falls before and after the 1550-nm wavelength, respectively. G.655 is a NZDSF fiber that is optimized for DWDM transmission in the C- and L bands. Note 1

TPG Telecom, a service provider in Australia, holds the record for fiber density by deploying Prysmian FlexTube FOC with 2112 fibers contained in a single sheath of 24-mm diameter [7]. Table 2.4 lists some of the commonly used fibers in the industry:

2.11

Summary

The chapter provided the essential concepts required to understand, design, deploy, and maintain the modern day fiber optic backbone networks—both data as well as telecommunications. It presented a sequential introduction to the various components that are required to build optical networks along with the background theory and technology. The treatise is kept nonmathematical,

S.N Fiber Type 1 ITU-T G652B

Table 2.4 Commonly Used Fiber Plants Common Description Applications Standard SMF Telcom networks, with 9/125 cable TV and micron core/ other utilities cladding dimension Low water peak MAN, WDM fiber Networks

Features Low 1383 nm (water-peak attenuation, long term reliability (coating), excellent bending resistance, PMD of 0.06 ps/ /√km

2

ITU-T G652D

3

ITU-T G655E/656

Nonzero dispersion shifted fiber (NZDSF)

4

ITU-T G657A

5

ITU-T G657B

6

ITU-T G651.1

Bend insensitive Access networks SMF Bend insensitive Short distances SMF at end of access networks (buildings) Optical Cost effective, supports 1 Gbps 50/125 μm broadband access transmission over links up to 550m MMF-GI (buildings) using 850-nm transceivers

Core/backbone networks (Long Haul) (SDH, OTN, IP, DWDM)

Low optical loss over 1260 nm to 1625 nm (O, E, S, C and L band), PMD 0.15 ps/ /√km, optimized dual layer coating Supports 10/40G speeds high bit-rate, multiwavelength transmission, CD of 8ps/nm.km, eliminates need for dispersion compensation for up to 200km Supports bending radius of 10mm, compatible with G.652D fibers Supports bending radius of 5 mm, compatible with G.652D fibers

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The ABCs of Fiber Optic Communication

deliberately, with the objective of presenting the key concepts and technology in a simple fashion. Following are key points covered in this chapter. 1. The media is an important component of a telecommunication network and has a direct impact on the network speed, accuracy and transmission distance. 2. An optical fiber consists of a strand of glass or plastic that allows the passage of light in a manner similar to the conduction of electricity through copper cables. 3. An optical fiber consists of two layers of varying refractive indices of high purity glass or plastic arranged to form the core and cladding. 4. Dispersion is one of the major contributors to the losses in light transmission over an optical fiber and results in the elongation of the light waves as it travels along the fiber. 5. Dispersion causes severe degradation in the signal being transmitted through a fiber optic cable. 6. Lasers operate in two wavelength regions, namely 1310 nm and 1550 nm, that are commonly referred to as the second and the third optical transmission windows. 7. The effects of dispersion is zero at the 1310-nm window whereas the losses are the least at 1550-nm window. 8. Fibers that support only a single-mode of transmission are called single mode fibers (SMFs), while those providing multiple propagation paths referred to as multimode fibers (MMFs). 9. The key criteria for selecting a fiber optic cable include attenuation, dispersion, mode-field diameter, cutoff wavelength, and environmental performance.

2.12

Review

2.12.1 Review Questions

1. Dispersion is one of the major contributors to the losses in light transmission over an optical fiber and results in the elongation of the light waves as it travels along the fiber. a. True b. False 2. The primary cause of attenuation in an optical link is ___________.

Essentials of Fiber Optic Communication

3.

4.

5.

6.

7.

8.

51

a. Scattering b. Dispersion c. Absorption d. Operational wavelength In fiber optic transmission zero dispersion in standard SMF occurs at _________ wavelength. a. 1310 nm b. 1550 nm c. 850 nm d. 1625 nm The bandwidth carrying capacity of a MMF is higher than that SMF owing to the multiples modes of transmission. a. True b. False The channel bandwidth of a PCM system is __________. a. 48 Kbps b. 64 Kbps c. 36 Kbps d. 128 Kbps The following statement is true with respect to DSF: a. The low loss band is shifted from 1550 nm to 1310 nm to coincide with the property of zero dispersion. b. The low loss band is shifted from 1310 nm to 1550 nm to coincide with the property of zero dispersion. c. The property of zero dispersion is shifted from 1550 nm to 1310 nm to coincide with the low loss band. d. The property of zero dispersion is shifted from 1310 nm to 1550 nm to coincide with the low loss band. Material dispersion refers to the frequency dependent response of a media to the propagating light. a. True b. False The refractive index of the core is greater than that of the cladding. a. True b. False

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9. MFD refers to the size of the light carrying portion of the fiber. a. True b. False 10. The most commonly employed light source in telecommunication systems (telecom) is _________. a. LED b. Laser c. VCSEL d. CFL 2.12.2 Exercises

1. List the key parameters for choosing a fiber optic cable. 2. What is an e/o unit? Describe its function in detail. 3. Describe the meaning of the term extinction ratio. What bearing does it have on the design of a fiber optic link? 4. List the commonly used fiber types for intracity communication along with their key characteristics. 5. List the commonly used fiber types for intercity communication along with their salient features.

2.12.3 Research Activities

1. Prepare a table that lists the standards mentioned in this chapter along with a brief description. 2. List the latest developments or the likely developments in the future in the areas of fiber optic cables. 3. Track down and subscribe to one online newsletter in the area of fiber optic engineering.

2.13

Referred Standards

EIA-440-A. EIA-455-A.

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EN 13501-6. EN 86 275-801. EN 86 275-802. G.651—Characteristics of a 50/125 μm multimode graded index optical fiber cable. G.652—Characteristics of a single-mode optical fiber cable. G.653—Characteristics of a dispersion shifted single-mode optical fiber cable. G.654—Characteristics of a cut-off shifted single-mode optical fiber cable. G.655—Characteristics of a nonzero dispersion shifted single-mode optical fiber cable. G.656—Characteristics of a fiber and cable with non-zero dispersion for wideband optical transport. G.657—Characteristics of a bending-loss insensitive single-mode optical fiber and cable for the access network. GR-1081. GR-1435. GR-326. IEC 61754-15. IEC TR 62221 Test methods for microbending. ISO/IEC 11801 Information technology—generic cabling for customer premises. TIA/EIA 604-16. TSB-62.

2.14

Selected Bibliography

2.14.1 Books Chomycz, B., Fiber Optic Installer’s Field Manual, New York: McGraw Hill, 2000. Huber, J. C., Industrial Fiber Optics Networks, New York: Instrument Society of America, 1995. Betti, S., Coherent Optical Communications Systems, New Jersey: John Wiley & Sons, 1995. Senior, J., Optical Fiber Communication: Principles & Practice, Harlow, UK: Prentice Hall, 2008. Keiser, G., Optical Fiber Communication, New Delhi, India: Tata McGraw Hill, 2008 Hayes J., Connector Identifier, Fiber Optic Association, http://www.thefoa.org/tech/connID.htm, retrieved May 24, 2008.

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2.14.2 URLs http://ecmweb.com/training/electrical_basics/electric_basics_fiber_optics_7/ http://ecmweb.com/mag/electric_basics_fiber_optics_4/ http://www.arcelect.com/ http://www.nfpa.org http://www.ul.com/telecom http://www.ciscopress.com http://www.hubersuhner.com/ http://www.avap.ch/ http://en.wikipedia.org/ http://www.telebyteusa.com/ http://search.techrepublic.com.com/search/fiber-optics.html http://www.fiberopticproducts.com http://www.ask.com/questions-about/Fiber-Optics http://www.optiwave.com/ http://www.fibersolutionsonline.com/

References [1]

Underwriters Laboratories, Optical Fiber Raceway, http://www.ul.com, retrieved April 22, 2008.

[2]

Multimode Fibers, http://www.fiberoptics4sale.com/Merchant2/multimode-fiber.php, retrieved April 22, 2008.

[3]

Schneider, Dr. K. S., Primer on Fiber Optic Data Communications for the Premises Environment, http://www.telebyteusa.com/foprimer/fofull.htm, retrieved June 10, 2008.

[4]

Hayes, J., Understanding Wavelengths in Fiber Optics, Fiber Optic Asssociation, http:// www.thefoa.org/tech/wavelength.htm.

[5]

The Basics of Fiber Optic Cable, http://www.arcelect.com/fibercable.htm, retrieved April 22, 2008.

[6]

Gardner, W. B., and D. Gloge, “Microbending Loss in Coated and Uncoated Optical Fibers,” Topical Meeting on Optical Fiber Transmission, Williamsburg, VA, 1975.

[7]

http://www.lightwaveonline.com/articles/2016/04/australia-s-tpg-deploys-2112-fibersvia-prysmians-flextube-fiber-optic-cable.html.

3 Optical Fiber Splicing and Interfaces 3.1

Chapter Objectives

Optical fiber is the preferred medium for deployment in modern day metro as well as long distance networks. However the length of a fiber is finite, and hence they need to be joined together or spliced as per the network requirements. The ends of the fiber also need to be connectorized at the termination points. This is achieved through the use of splicing and connectorization techniques. These techniques are highly critical and have a bearing on the network costs as well as the system performance. There has been major development in the field of fiber optic interfaces or connectors. Technology developments have resulted in the transition from the old ferrule core connectors to the recent small form factor—plus interfaces. The size, cost, and the attenuation characteristics of the interfaces have reduced drastically, while the density of optical interface ports have significantly increased. This chapter introduces the basic splicing techniques along with a detailed treatise of the different types of optical fiber interfaces. The chapter concludes with an in-depth discussion on optical link budgeting and span analysis that would be particularly useful for field engineers and network planners. Additional details on the developments in the area of connectors is included in the appendix. This will provide an opportunity to the readers to develop their core concepts and understand the rationale behind the development of new age connectors.

55

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Key Topics

• Splices and connectors; • Optical transmitters; • Optical receivers; • Optical modulation techniques; • Link loss budgeting.

3.2

Introduction

Fiber comes in fixed lengths. Hence they need to invariably be joined together or spliced as per the network requirements. The ends of the fiber also need to be connectorized (fixing of suitable connectors) prior to termination. Splicing and connectorization are highly critical in determining the installation costs as well as the system performance.

3.3

Splices and Connectors

Splicing refers to the specific alignment of the cores of two fiber segment resulting in a smooth junction through which light signals can be transmitted without significant attenuation. The two techniques (please note several more are available) that are widely used for joining fibers are as detailed here: 1. Splicing: Splicing provides permanent connections between fibers segments in a network. They can be of different types, with variations in terms of cost and performance. The two commonly used types are as outlined: a. Fusion splicing: Fusion splicing provides a fast, reliable, low-loss, fiber-to-fiber connection by creating a homogenous joint between the two fiber ends. The fibers are melted or fused together by heating the fiber ends using an electric arc or a similar arrangement. Fusion splices provide a high-quality joint with lowest loss (in the range of 0.01 dB to 0.15 dB for SMF) while eliminating the possibility of reflections. Fusion splicing cannot be used for MMF (SI/ GI) since the fusion would cause changes in the RI profile and lead to higher losses b. Mechanical splicing: Mechanical splicing is an alternative method of making a permanent connection between fibers. In the days gone by this method of splicing suffered from the disadvantages of

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high losses, poor performance, and low reliability. Technological advancements however have contributed to the significantly increased performance of these types of splices—second generation splices. In comparison with fusion splicing, which requires a costly fusion splicer, the overall cost of mechanical splicing is lower. The per splice cost of a mechanical splice is significantly higher than a fusion splice. Mechanical splices may be used as a temporary fix to a fiber cut before being fusion spliced at a later stage. 2. Connectors: Connectors [1] provide remateable connections and hence are used generally at termination points where flexibility is required in routing an optical signal (e.g., from equipment pigtails to the receivers or wherever re-configuration is necessary or for normal termination purposes). These remateable connections simplify system reconfigurations to meet changing customer requirements. The list of connectors that are included in this section are the ones that were used in earlier days. Chapter 6 provides the list of connectors that are used currently by the telecom/datacom industry. a. Epoxy and polish connectors: Epoxy and polish style connectors were the original connectors used for termination purposes and are still deployed extensively. These connectors provide a wide range of options including ST, SC, FC, LC, D4, SMA, MU, and MTRJ. Their key advantages include the following: i. Robustness: Ability to withstand higher levels of mechanical/ environmental stress; ii. Cable diameter: These connectors can be employed for cables of varying diameters from small to big; iii. Multiway connectors: These connectors support single as well as multiple fibers (up to 24 fibers) in a single connector. b. Preloaded epoxy or no-epoxy and polish: The primary advantage of these connectors is the simplicity of installation and the consequent reduction in skills for handling these connectors. These connectors can be classified into two types: connectors with preloaded epoxy and connectors without epoxy. These connectors employ an internal crimp mechanism to stabilize the fiber and are available in ST, SC, and FC connector styles. c. No-epoxy and no-polish: These connectors offer the advantages of simple design and highly reduced installation and training costs while facilitating fast restorations. They are available in ST, SC, FC, LC, and MTRJ connector styles.

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There are numerous connectors, both proprietary as well as standardized, that are employed in the field of telecom, data, as well as cable/television and other industrial uses. The ones included in this text include those that were used earlier as well as the ones that are in use currently. These include [2]: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Biconic connector; Ferrule core connector (FC); Subscriber connector (SC); SMA connector; ST connector; Plastic fiber optic cable connectors; Lucent connector (LC); Fiber distributed data interface connector (FDDI); Enterprise systems connection connector (ESCON); LX-5 connector; Optijack connector; MT-RJ connector; Volition connector; MT connector; MU connector; E2000 connector.

A brief description of the connectors is as follows: 1. Biconic (obsolete): Biconic connector, now obsolete, was one of the earliest connectors employed over FOC links. This connector is characterized by the presence of a tapered sleeve that fixes on to the FOC. The tapered end facilitates the location of the proper position on the FOC. The caps along with the guided rings provided fits over the ferrule and screws onto the threaded sleeve securing the connection. A newer design referred to as RACE connector is used in some fiber networks. Figure 3.1 illustrates the bionic connector. 2. Ferrule connector (FC): The ferrule connector (Figure 3.2) was used extensively in fiber optic networks until very recently. Its use, however, is now dwindling. The FC connector employs a position locatable notch and a threaded receptacle to offer precise positioning of the SMF in relation to the optical source and the receiver. Once installed, the position is maintained with absolute accuracy.

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Figure 3.1 Biconic connector. Source: The Fiber Optic Association, 2009, http://www.thefoa. org. Reprinted with permission.

Figure 3.2 Ferrule connectors. Source: The Fiber Optic Association, 2009, http://www.thefoa. org. Reprinted with permission.

3. Subscriber connector (SC): SC connectors (Figure 3.3) are low-cost, rugged, and simple connectors that employ a ceramic ferrule for providing accurate alignment of SMF. The SC is a push on-pull off connector with a clipping mechanism. Its low manufacturing cost makes it suitable for FTTH/FTTP subscriber end terminations. 4. SMA connector: The SMA connector, now obsolete for datacom/telecom applications, was the predecessor to the ST connector. This connector has been replaced by the ST and subsequently by the SC con-

Figure 3.3 Standard connector. Source: The Fiber Optic Association, 2009, http://www.thefoa.org. Reprinted with permission.

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The ABCs of Fiber Optic Communication

nectors. The connector is however still used for specialist applications. Figure 3.4 illustrates the SMA connector. 5. ST connector: The ST connector (Figure 3.5) is similar in design (keyed bayonet) to a Bayonet Neill-Concelman or Bayonet nut connector (BNC) connector. The connector finds wide application for MMF. Its performance was hampered by stability issues and hence did not find wide application for SMF. The connector, characterized by the ease of use, also finds application in the cable TV industry. This connector is available in two versions referred to as ST and ST-II. Both the connectors are spring loaded, keyed, and employ push-in and twist design. 6. Plastic fiber optic cable connectors: There are fewer connector options for a plastic connector when compared to the glass fiber. These connectors are designed for easy application and are relatively cheaper. There are generally no epoxy and polished options available. Connectors for plastic FOC include both proprietary designs and standard designs. Connectors like ST or SMA that are primarily designed for use with glass FOC can also be used with plastic FOC.

Figure 3.4 SMA connector. Source: The Fiber Optic Association, 2009, http://www.thefoa. org. Reprinted with permission.

Figure 3.5 ST connector. Source: The Fiber Optic Association, 2009, http://www.thefoa.org. Reprinted with permission.

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7. Lucent connector (LC): Lucent connector or little connector (LC) is a small form-factor fiber optic connector that employs a 1.25 mm ferrule (Figure 3.6). These connectors have widespread use in the telecommunication industry. There can be different polishing specifications, as in case of other connectors, that can be applied. These are described in the next section. 8. FDDI connectors: Fiber distributed data interface (FDDI) is a dual ring token network that provides data transmission at 100 Mbps in a LAN within the range of less than100 kilometers (Figure 3.7). The FDDI connector is used to connect network equipment (NE) to a wall outlet. The connector houses two 2.5-mm ferrules that mate onto SC/ST connectors through the use of adapters. 9. ESCON connectors: Enterprise systems connection or ESCON is a connector (Figure 3.8) developed by IBM for interfacing their mainframes to peripheral storage devices like tape drives. ESCON is a serial half-duplex interface that employs FOC.

Figure 3.6 Lucent connector. Source: The Fiber Optic Association, 2009, http://www.thefoa. org. Reprinted with permission.

Figure 3.7 Fiber distributed data interface connector. Source: The Fiber Optic Association, 2009, http://www.thefoa.org. Reprinted with permission.

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The ABCs of Fiber Optic Communication

Figure 3.8 Enterprise systems connection connector. Source: The Fiber Optic Association, 2009, http://www.thefoa.org. Reprinted with permission.

10. LX-5 connector: The LX-5 connector [3] (Figure 3.9) is a standardized small form factor connector that employs a 1.25-mm ferrule with an automatic metal shutter technology providing high-performance, density, and reliable connections. LX-5 connectors are designed for use in the modern day high-performance telecommunication networks as well as CATV applications. The LX-5 connector offers high packing density due to its small form factor while delivering high performance and enhanced safety due to its automatic metal shutter. The insertion loss of this connector (SMF) would be between 0.2 to 0.4 dB. 11. Optijack: The optijack (Figure 3.10) is a rugged plug-and-jack (male/ female) duplex connector with two ST type ferrules in a package resembling the ubiquitous RJ-45 connector. 12. MT-RJ: MT-RJ (Figure 3.11) is single polymer ferrule duplex connector with alignment. It is available in a plug-and-jack format or male/ female connectors. 13. Volition: Volition (Figure 3.12) is a plug-and-jack duplex connector that is unique in the fact that it does not employ any ferrule. The fibers are aligned in a V-shaped groove.

Figure 3.9 LX-5 connector. Source: The Fiber Optic Association, 2009, http://www.thefoa.org. Reprinted with permission.

Optical Fiber Splicing and Interfaces

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Figure 3.10 Optijack connector. Source: The Fiber Optic Association, 2009, http://www.thefoa.org. Reprinted with permission.

Figure 3.11 MT-RJ connector. Source: The Fiber Optic Association, 2009, http://www.thefoa. org. Reprinted with permission.

Figure 3.12 Volition connector. Source: The Fiber Optic Association, 2009, http://www.thefoa.org. Reprinted with permission.

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The ABCs of Fiber Optic Communication

14. MT connector: MT (Figure 3.13) is a ribbon cable with 12 fiber connectors that are used for cabling systems and factory terminated cable assemblies. 15. MU connector: MU connectors [3] are small (reduced foot print) new generation connectors used for dense applications. The connector is (Figure 3.14) of square shape and features a push-pull mating mechanism. There are different variants of this connector available and are as listed here: a. Single-mode MU UPC/APC fiber optic connector; b. Multimode MU UPC fiber optic connector. The MU connector is used for SDH, SONET, WDM, LAN, ATM, as well as CATV applications. 16. E2000 connector: The E2000 connector [3] (Figure 3.15) is being increasingly used in the modern day telecommunication networks. The unique feature of this connector is the inclusion of an integrated

Figure 3.13 MT connector. Source: The Fiber Optic Association, 2009, http://www.thefoa.org. Reprinted with permission.

Figure 3.14 MU connector. Source: The Fiber Optic Association, 2009, http://www.thefoa. org. Reprinted with permission.

Optical Fiber Splicing and Interfaces

65

Figure 3.15 E2000 connector. Source: The Fiber Optic Association, 2009, http://www.thefoa. org. Reprinted with permission.

spring loaded shutter that protects the ferrule from dust, dirt, and scratches. The connector is of latched push-pull locking type. E2000 is a trademark of Diamond SA based in Losone, Switzerland. This connector is available in the following variants: a. Single-mode E2000 UPC/APC connector; b. Multimode E2000 UPC connector. The primary advantages of this connector are the high performance and enhanced safety due the monobloc ferrule and the shutter mechanism. The application of this connector includes telecommunication networks, broadband applications, fiber CATV, LAN (fiber-inthe-loop [FITL], fiber-to-the-home [FTTH], and fiber-to-the-desk [FTTD]), and data networks. Figure 3.16 summarizes the various types of connectors described in this section.

3.4

Optical Transmitters

The term optical transmitters refer to the source of light in an optical system. It is fairly obvious that the light sources are an integral and a crucial component of a fiber optic network. The span length, the number, as well as the placement of regenerators and amplifiers are all dependent directly or otherwise on the transmitter types. In all commercial applications as well as equipment, the conversion of signals from electrical to optical and vice versa is done by an integrated electro-optical (e/o) unit that functions as a transmitter and receiver. The issues of thermal dissipation, efficacy of heat sinks, size, and method of coupling the optical power onto a fiber are taken care of by the manufacturer, and in most cases the users would not have much say in the same. The modern day architecture consists of racks that are equipped with cards also referred to as plug-

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The ABCs of Fiber Optic Communication

Figure 3.16 Commonly employed fiber optic connectors.

in-units (PIUs) or plug-in-cards (PICs). The e/o unit is integrated with other functional components and would be in the form of a PIC—the user can only decide among the various types of cards offered by the manufacturers, based on certain parameters discussed later in this chapter. This section introduces the transmitter which serves two major functions: 1. Provide a source of light for transmission over the fiber; 2. Modulation of the source in accordance with the transmitted data. The former involves emission and the subsequent coupling of light onto a waveguide, which is nothing but the fiber. The second function (modulation) involves the variation of the intensity of light being coupled onto the fiber. 3.4.1 Optical Sources

The olden day transmitters were fabricated using discrete electrical as well as electro-optical components. These were subsequently replaced by integrated circuits (ICs) of varying designs starting from small scale integration (SSI) to the current multilayer very large scale integrated (VLSI) chipsets. The commonly employed optical sources include light emitting diodes (LEDs), laser diodes (LDs), as well as vertical cavity surface emitting lasers (VCSELs). Following are some of the important design considerations for a transmitter:

Optical Fiber Splicing and Interfaces

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1. The physical dimensions must be compatible with the size of the FOC being used. 2. This implies that the emitted light must be coupled into a cone with a cross sectional diameter 8–100 mm. 3. The optical source must be able to generate enough optical power so that the desired bit error rate (BER) can be met. 4. In order to ensure the optimal usage of the high bandwidth offered by the FOC, the optical source must be capable of being modulated by a high frequency electrical signal. 5. The coupling efficiency (the amount of power that is actually transmitted along the fiber) should be high. 6. The source should have a high coupling efficiency. 7. The linearity of the optical source should facilitate the suppression of harmonics and intermodulation distortion. 8. The source must be made available in a compact low footprint, low weight package with high reliability. The listed requirements are satisfied by the light emitting junction diode matches and hence find wide use in transmitters. There are two types of light emitting junction diodes that can be used as optical source in transmitters. As mentioned earlier they are the LEDs and the LDs. LEDs are simpler but generate a low power incoherent light, while the LDs are more complex but provide a coherent, higher power light. Figure 3.17 plots the optical power output of an LED and a LD as a function of current and wavelength. The LED has a relatively linear P-I characteristic while the LD has a strong nonlinearity or threshold effect.

Figure 3.17 Spectral characteristics of LED and laser.

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Table 3.1 presents a comparison between an LED and a laser. The coupling efficiency of an LED is very low (around 2 percent) owing to its divergent output. The output of a laser has low divergence (collimated) resulting in higher coupling efficiencies. However, the output of an LD is elliptical while the fiber cross-section is circular. This limits the amount of power coupled by the source onto the fiber (typically 50 percent). This disadvantage is overcome by another type of light source—VCSEL. The beam output of a VCSEL is circular resulting in higher coupling efficiency. Note 1

The key difference between an LED and laser is their light emission characteristics. The output of an LED is highly diverged while that of the laser is collimated (low divergence). Further the spectral width, 3 dB optical power width in nm, of the LD is smaller than that of the LED. The values for an LED are around 40 nm for operation at 850 nm and 80 nm at 1310 nm while that of the LD are 1 nm for operation at 850 nm and 3 nm at 1310 nm. The basic types of lasers include: 1. Fabry-Perot (FP) lasers: Fabry-Perot lasers have a high signal to noise (S/N) ratio and are slower than DFB, but are highly economical. FP lasers emit light at a number of discrete wavelengths. There are two commonly used FP laser types—buried hetero (BH) and multiquantum well (MQW) types. BH lasers were extensively used earlier. MQW are used in present day applications. There are several advantages of MQW lasers over BH lasers. These include: • Low threshold current; • High efficiency; • Low noise; • Enhanced linearity; • Stability over a wide range of temperatures; • Lower operating and manufacturing costs.

Table 3.1 LEDs Versus Lasers LED Laser High reliability Higher optical power Enhanced linearity High coupling efficiency Lower cost Capable of being modulated at high frequencies High MTBF Lower MTBF

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2. Distributed feedback (DFB): DFB lasers are monochromatic (emit a single color of light) and they are quieter and faster than the MQW lasers. They are high-performance lasers that are used extensively in high-speed applications (analog as well as digital). They offer enhanced linearity and have narrow spectral widths. 3. Vertical cavity surface emitting laser (VCSEL): VCSEL are lasers with a large output aperture that emits circular low-divergence beams. This facilitates a high coupling efficiency with optical fibers (highest in comparison to LED and LD). The VCSEL has a low threshold current that results in low power consumption and high intrinsic modulation bandwidths. The major disadvantages of a VCSEL include complex manufacturing process and support for 850-nm and 1310-nm applications only. However, the manufacturing costs of VCSEL are comparatively lower than the other types of lasers. The commonly used types of VCSEL are the pigtailed 850 nm VCSEL and the 10 Gbps MM VCSEL. It may be observed that the commercial versions of VCSELs were only available for the 850-/1310-nm wavelengths, effectively limiting their use in telecom networks. VCSELs are also used for sensing applications (proximity sensing), high-power applications including line-of-sight (LOS) systems, atomic clocks, and magnetometers. Other applications include spectroscopy, broadband transmission (analog), laser printers, and optical mouse. Table 3.2 provides a comparison between the VCSEL and laser.

3.4.2 Modular Optical Interfaces

In the days prior to the development of the SFP, the failure of an optical port warranted the replacement of an entire plug-in-unit or a functional card. The case was similar in case of equipment upgradation. The development of the SFP has facilitated an easy or faster method of recovering from connector failures and a cost-effective method for card or capacity upgradation.

Table 3.2 Lasers Versus VCSEL VCSEL Conventional Lasers High coupling efficiency Coupling efficiency less than VCSEL Available at 850 nm/1310 nm only Available for 1550-nm and WDM wavelengths Used for shorter spans Longer span distances

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1. SFP modules: Small form-factor pluggable (SFP) modules are compact, hot swappable optical modular transceivers that are capable of operating at speeds up to several Gbps. The development of the SFP reflects the significant strides made in the field of semiconductor and electronic engineering. The benefits of an SFP cannot be understated. In most of the conventional multiplexers deployed in modern day telecom networks, a capacity quadrupling calls for the replacement of only an SFP. In fact an electrical SFP (operating at E4/STN-1e) with mini BNC connectors are also available. The development of the SFP has resulted in lower capital expenditures (CAPEX) as well as operational expenditures (OPEX) for deploying and maintaining fiber optic networks. The inherent benefits of an SFP have brought together several leading telecom as well as data organizations with the common objective of standardizing the SFPs. The common goals were to have an interface with a small footprint facilitating easy installation as well as maintenance. SFPs are available at STM-1/4/16 as well as STM-1e bandwidths. However complete portability has not yet been achieved [4]. For example, an STM-1 SFP from vendor A, used in a telecom equipment, manufactured by company X may not work in a similar equipment from company Y. Figure 3.18 illustrates the front view of an SFP. 2. XFP modules: The 10-Gb small form factor pluggable (X representing 10) [XFP] [5] is a protocol-independent, hot-swappable optical transceiver operating at 850 nm, 1310 nm, or 1550 nm. It finds use in a wide range of applications including: • SDH/SONET networks;

Figure 3.18 Short form factor pluggable module—front view.

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• Fiber channel (FC); • Gigabit Ethernet (Gibe); • Dense wavelength division multiplexing (DWDM). The XFP (as illustrated in Figure 3.19) supports varying interfaces ranging from very short reach (VSR) to long reach (LR). The standard offerings are short reach/intraoffice, intermediate office/short haul, and long reach/long haul. The XFP module, as in case of the SFP, includes the transmission as well as well as receiving functions integrated onto a compact, flexible, and low-cost format supporting up to16 XFP modules on a typical card (European standard). As in case of the SFP, the XFP specifications was drawn from a multisource agreement. The family also includes an XFI or “ziffy” high-speed serial electrical interface with a nominal baud rate of 9.95–11.1 Gbps. 3. SFP+ modules: SFP (Figure 3.20) represents the next generation transceiver module with a miniature form factor as specified by the ANSI T11 group. The module is designed for operation at 8.5 Gbps and 10 Gbps. These bandwidths correspond to that of fiber channel and

Figure 3.19 XFP module [5]. Source: The Fiber Optic Association, 2009, http://www.thefoa. org. Reprinted with permission.

Figure 3.20 SFP module [6]. Source: The Fiber Optic Association, 2009, http://www.thefoa. org. Reprinted with permission.

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Ethernet applications. The SFP+ form factor is 30 percent less in comparison to an XFP. In fact, form factors of tune of 1/6 of an SFP (this holds good for the power dissipation also) have also been successfully designed. To achieve this reduced size, certain traditional functions including the serial/parallel conversion, clock recovery, and signal conditioning functions have been relocated to the associated plug-in-card or module. The primary benefits of an SFP+ module are as listed next: • Reduced footprint and hence higher port density (up to 24 and even 48 ports on a standard equipment card); • Lower costs; • Reduced power since many functions are relocated to the associated plug-in-unit. Table 3.3 lists the various industry standards governing the usage of fiber optic connectors, while Table 3.4 lists the equivalent Bellcore standards. Note 2

The SFP standards are laid down by American National Standards Institute (ANSI) Technical Committee (T11) along with inputs from more than thirty fiber-optic system vendors. 3.4.3 Key Parameters (Transmitter)

The following are some of the key (often overlooked) parameters that significantly impact the choice of an optical source and its performance: 1. Backreflection: Backreflection refers to the optical energy that is reflected back onto the waveguide especially at the spliced/terminating end. Backreflections cause distortions in the light propagating through a FOC besides increasing the effective noise. Strong back reflections can cause some lasers to become unstable and render them useless in certain applications. It can also generate nonlinearities, referred to as kinks, in the laser response. This is unsuitable for most analog applications and some digital applications. The importance of controlling backreflection depends on the type of information being sent and the particular laser. The laser design may render some lasers susceptible to backreflection. A crucial factor that has significant impact on the amount of backreflections is the coupling of the fiber and the connector. A low-power laser generally has weak coupling to the fiber with only 5–10% of the laser power being directed onto its core. This implies that only 5–10 percent of the

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TSB-62

EIA-440-A EIA-455-A EIA-455-1A EIA-455-9 EIA/TIA-455-13 EIA-455-17A EIA-455-21A EIA-455-26A EIA-455-34A TIA/EIA-455-158 EIA-455-172 EIA/TIA-455-187 EIA/TIA-4750000-B EIA/TIA-475C000 TIA/EIA-475EA TIA/EIA-475EB TIA/EIA-475EC00 TIA/EIA-604

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Table 3.3 Standards for Fiber Optic Connectors Informative test methods for fiber-optic fibers, cable, opto-electronic sources and detectors, sensors, connecting and terminating devices, and other fiber optic components Fiber-optic connector terminology Standard test procedure for fiber-optic fibers, cables, transducers, sensors, connecting and terminating devices, and other components Cable flexing for fiber optic interconnecting devices Fiber optic test procedure for bundle connector Visual and mechanical inspection of fibers, cables, connectors, and/or other fiber-optic devices Maintenance aging of fiber-optic connectors and terminated cable assemblies Mating durability for fiber-optic interconnecting devices Crush resistance of fiber-optic interconnecting devices Interconnection device insertion loss test Measurement of breakaway frictional force in fiber-optic connector alignment sleeves Flame resistance of firewall connector Engagement and separation for fiber-optic connector sets Generic specification for fiber-optic connectors Sectional specification for type FSMA connectors Blank detail specification for connector set for optical fiber and cables, type BFOC/2.5, environmental category I Blank detail specification for connector set for optical fiber and cables, type BFOC/2.5, environmental category II Blank detail specification for connector set for optical fiber and cables, type BFOC/2.5, environmental category III Fiber optic connector intermateability standards

Table 3.4 Connector Reflectance Connector Reflectance Reflectance Type (dB) (Percent) Flat –20 1 PC –30 to –40 0.01 SPC –40 0.001 UPC –40 to –50 0.001 APC –60 or higher 0.0001

backreflection would also be coupled into the laser cavity. This makes the laser relatively immune to backreflections. On the other hand, a high-power laser may have 50–70% of the laser output coupled to the fiber. This would therefore imply that 50–70% of the backreflection

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would also be coupled back into the laser cavity, making those lasers more susceptible to backreflections. The losses induced when coupling light onto a fiber, due to misalignment and/or air gap between components like connectors or due to absorption, are termed as insertion losses. With advancements in manufacturing technologies, the insertion loss is less than 0.2 dB. Insertion loss is expressed in dB as: IL = 10 log10 (Pout/Pin) Where Pin refers to the incident power and Po the output power. The losses induced due to reflectance or backscattered light is referred to as return loss. A higher value of return loss indicates a better quality connection. Return loss is expressed in dB as: RL = 10 log10 (Pi /Pr) Where Pi refers to the incident power and Pr the reflected power. Normal (flat) fiber connectors leave a small air gap between the ferrules due to the surface imperfections. In order to increase the coupling efficiency and reduce return losses, fiber-optic connectors are polished. The common polishing specifications include: a. Physical contact (PC): The connector ends are polished to form a spherical cone (reducing the overall size of the end face). This lowers the air gap between the connectors resulting in lower return loss. b. Super physical contact (SPC): The connector end face has a higher degree of polish, as compared to the PC connectors. c. Ultraphysical contact (UPC): The connector end face is subjected to an extended polishing technique that produces a finer surface finish. This results in lower backreflection than standard PC connectors besides lower insertion loss. d. Angled physical contact (APC): PC/UPC connectors provide low insertion losses; however, the quality of the connector starts to deteriorate with repeated usage. Also applications like FTTx that provides triple play services require connectors with very low backreflection due to the nature of the signals being transported (constantly changing, as opposed to digital data being carried through normal telecom networks). The APC connector, as the name suggests, provides an 8° angle to the end face that reduces the surface

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area and provides for tighter connections. Due to the angle provided, the backreflections would actually be reflected out of the fiber through the cladding. 2. Extinction ratio: Normally optical systems employ two levels of optical power, where the higher power level represents a binary 1 and the lower power level represents a binary 0. These two power levels can be represented as P1 and P0, where P1 > P0, the unit of representation being watts. The extinction ratio re expresses the relationship of the power used in transmitting a binary 1 to the power of transmitting a binary 0. The extinction ratio is used to describe the efficiency with which the transmitted optical power is modulated over the fiber-optic channel. It can be defined as a linear ratio—P1/P0—or as a power measurement (dB)—10 * l log (P1/P0)—or as a percentage—(P0/P1) * 100. Small changes in extinction ratio can make a relatively large difference in the power required to maintain a constant bit error rate (BER). Thus, variations in the extinction ratio affect the performance of an optical link. For an ideal transmitter, P0 would be zero and hence ER would be infinite. Most optical transmitters have a finite amount of optical power at the low level and hence P0 > 0. (Lasers must be biased so that P0 is in the vicinity of the laser threshold.) At the receiving end, two important decisions must be taken: a. The instant or time to sample the received data; b. Decision as to whether the sampled value represents a binary 1 or 0. The receiver decision circuit simply compares the sampled voltage to a reference value known as the decision threshold. The associated circuitry is generally a part of a clock and data recovery (CDR) block. P0 is ideally equal to zero, making the optimum extinction ratio infinite. When the extinction ratio is not optimum, however, the transmitted power must be increased in order to maintain the same BER at the receiver. This increase in transmitted power due to nonideal values of extinction ratio is called the power penalty. Note 3

Small changes in the extinction ratio make a relatively large difference in the power required to maintain a constant BER. This effect is especially pronounced when the values of the extinction ratio are less than seven. The change in the value of the extinction ratio by one necessitates a corresponding change of 10 percent in the average power of the laser. This additional required power is re-

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ferred to as power penalty. It is important to note that increasing the power of a laser will reduce its MTBF.

3.5

Optical Receivers

The detection of the optical signal, at the receiving end, is performed by a photodiode and the demodulation process is carried out on the resulting electrical signal. One or more stages of amplifiers, filtration, and equalization circuits are employed to boost the weak incoming signals as well as to reshape (amplitude distortion correction) them. The receiver component of an e/o unit primarily serves two functions: 1. The receiver must sense or detect the light from the FOC and subsequently convert the light into an electrical signal. 2. It must then demodulate this light to determine the identity of the binary data represented by the incoming signal In addition to the primary functions, a receiver also performs multiple secondary functions including: 1. Clock recovery: This is true for NEs that are part of a synchronous network like SDH or SONET and employ line timing; 2. Line decoding: 4B/5B decoding; 3. Error detection/recovery: The receiver must be capable of high level of detection (sensitivity), support high data rates, and have low noise characteristics. A high sensitivity receiver would be capable of detecting highly attenuated signals. A high bandwidth handling capacity would characterize swift response to state transitions ensure demodulation of the high-speed incoming data. In order to ensure the link BER, the receiver must have low noise characteristics. There are two types of photodiode that are commonly employed in optical equipment. They are positive intrinsic negative (PIN) diode and the avalanche photo diode (APD). The PIN is the preferred choice in short distance applications due to its standard power supply requirements (can be operated on a standard 5V or 15V supply) and lower costs. APD devices have much better sensitivity (5 to 10 dB more sensitivity) and can accommodate twice the bandwidth than that possible by a PIN. The major disadvantage, however, is that APDs require a stable power supply and are also priced considerably higher. The demodulation performance of the receiver is characterized by the BER. This in turn is determined by the modulation scheme, the received optical

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signal power, the receiver noise, and the processing bandwidth. A typical sensitivity curve is illustrated in Figure 3.21. Note 4

The receiver performance is characterized by a parameter referred to as receiver sensitivity. The receiver sensitivity is indicated by a curve on a graph representing the minimum optical power that a receiver can detect versus the data rate required in order to achieve a particular BER.

3.6

Optical Modulation Techniques

The modulation technique is a key element in realizing large high-throughput telecommunication networks. This is especially crucial for systems operating at 10/40 Gbps, as there is severe performance degradation due to the interaction between dispersion and fiber nonlinearities. Optical modulation refers to the controlled variation of the amplitude of the optical signal prior to its propagation through the network. This concept is illustrated in Figure 3.21. There are several different schemes for carrying out the modulation function. These include nonreturn-to-zero (NRZ), return-to-zero (RZ), alternate mark inversion (AMI), sinusoidal same phase modulation (SaPM), and intensity modulation among others. NRZ modulation is used on links up to 10 Gbps, including DWDM systems. The performance of systems based on RZ have empirically been proven to provide better quality of signals for longer distances. Intensity modulation also referred to as amplitude shift keying (ASK) or on-off keying (OOK) is an incoherent modulation scheme wherein the receiving end looks for the presence or absence of energy during the bit interval. This modulation scheme is generally preferred for shorter distances and can be employed for LED as well

Figure 3.21 Optical signal modulation.

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as LD sources. The IM scheme is particularly suitable with LEDs because of its incoherent nature. The output of an optical source using the IM modulation follows the input binary sequence, as represented in Figure 3.21. Modern day telecommunication systems are fairly complex and are capable of very high throughputs ranging from 10 Gbps to 400 Gbps. These systems have characteristics that are vastly different from their predecessors and are expected to be highly reliable over wide range of operating conditions. This has led to advanced research into optical modulation formats. Most of the modulation formats presented in the preceding section have been rendered obsolete by the changed network requirements. Fiber-optic transmission requirements are vastly different in comparison to microwave transmission or CATV systems. No modulation format can accommodate all sources of performance degradation. However, a judicious selection of advanced modulation formats improves system performance by minimizing the effects of some of the performance degradations.

3.7

Link Loss Budgeting

The link loss is relative to the transmit power of a light source and represents the range of optical loss for an operational fiber optic link while meeting its operating specifications. The calculation and the subsequent verification of a fiber optic system’s operating characteristics are done with the help of span analysis that encompasses the following multiple functions (depicted in Figure 3.22): 1. Type of fiber. 2. Fiber link parameters—length and routing. 3. Operational wavelengths.

Figure 3.22 Link loss budget—key parameters.

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4. Characteristics of the active as well as passive components involved in the link. (Active components: system gain, transmitter power, receiver sensitivity, dynamic range, and so on; passive loss: fiber loss, splice loss, couplers/splitters loss.) 5. Dispersion: Pronounced at higher bit rates and minimized by employing dispersion compensation modules (DCM). Links with speeds of 40G and above may need additional compensation for effects of polarization mode dispersion (PMD). The preceding analysis would result in a link loss budget that includes a safety margin to offset losses due to temperature changes (1 dB) as well as component aging and any maintenance work in the future (3 dB). The actual or the true loss is measured using an optical meter and can be compared to the computed loss. Following are some of the key parameters to be considered when preparing a link budget. 3.7.1 Transmitter Launch Power

The transmitter launch power measured in dBm refers to the transmitter output at a specific wavelength. A higher power generally yields better results (higher extinction ratio and consequentially lower BER). However, care should be taken that the power transmitted does not increase beyond the receiver’s sensitivity or saturation level (dynamic range). A higher transmit power offsets the effects of link attenuation but can induce nonlinear effects degrading system performance at higher bit rates. 3.7.2 Receiver Sensitivity and Dynamic Range

Receiver sensitivity and dynamic range refers to the minimum acceptable value of received power that is required to obtain an acceptable BER or performance. This takes into account the power penalties caused by use of a transmitter with worst-case values of jitter, receiver connector induced distortions, extinction ratio, pulse rise and fall times, measurement tolerances, as well as optical return loss. However the receiver sensitivity does not include dispersion or backreflection power penalties. These effects are compiled separately under the heading of maximum optical path penalty. It, however, takes into account the worst-case operating and end-of-life (EOL) conditions. For example, a receiver with a high optical input of –3 dBm and a worst case value of –29 dBm requires an optical dynamic range of 26 dB.

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3.7.3 Power Budget and Margin Computations

A section (span) power budget that reflects the maximum transmit power is required to be computed to ensure that an optical link has adequate power for normal operation. This budget is computed on the assumption of worst-case scenarios—minimum transmitter power and minimum receiver sensitivity in order to simulate practical conditions. This ensures a margin for compensation for variations of transmitter power and receiver sensitivity levels. Accordingly,

PBS (Span Power Budget) = PTmin (Min. Transmtter Power) −PRmin (Min. Receiver Sensitivity) As noted earlier span losses are the summation of the various linear and nonlinear losses and include factors like fiber attenuation, splice attenuation, connector attenuation, chromatic dispersion, and other linear and nonlinear losses. The typical attenuation characteristics of various kinds of fiber-optic cables are provided in Table 3.1. The reader is advised to refer to vendor provided information for actual values. Single-mode connectors that are factory made and fusion spliced onto the optical fiber will have insertion loss values ranging from 0.3 dB to 0.5 dB, while field terminations can have a higher value of up to 0.75 dB. Multimode connectors have an insertion loss values ranging from 0.25 dB and 0.5 dB. The newer connectors would typically have minimal insertion loss at 0.2 dB. In case of PON, the losses due to the use of optical couplers would also need to be included. A 1:N coupler would introduce an additional loss of 10logN dB. In case there is no coupler, the value of N=1 and the additional loss would be equal to zero. The following formula can be used to compute the span loss: Ps (span loss) = [(fiber attenuation*km) + (splice attenuation * no. of splices) + (connector attenuation * no. of connections) + (inline device loss) + (coupler losses) + (nonlinear losses) + (safety margin)] Power margin (PM) represents the amount of power existing in the system after accounting for linear and nonlinear span losses with a positive value indicating sufficient receiver power levels [6]. The PM is computed as follows: Pm (power margin) = PBS (power budget) – PS (span loss) The input power should not exceed the receiver sensitivity PRmax after adjusting for all span losses. In case the received power exceeds the receiver sensitivity receiver saturation would occur. To compensate for the additional power at the input ports attenuators would have to be used. The input signal

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level is denoted as PIN and the maximum launch power or transmitter power is denoted as PTmax. The span loss PS would, however, remain constant. The receiver power is computed as follows: PREC (received power) = PTmax (max. transmitter power) – PS (span loss) To prevent receiver saturation and ensure efficient system operation the following condition must be satisfied: PREC (received power) = PRmax (max. receiver sensitivity) If PREC is greater than the maximum receiver sensitivity PRmax, passive attenuation must be considered to reduce signal level and bring it within the dynamic range of the receiver. The following section presents a few examples of preparing optical link loss budgets. Example 3.1

Figure 3.23 depicts an SDH link operating at STM-1 or 155 Mbps spanning a distance of 5 km with MMF fibers. The link would contain two patch panels that would employ mechanical splicing and standard values for attenuation as well as component and nonlinear losses are assumed. Following are the key link parameters: 1. 2. 3. 4. 5. 6.

PTmin = –13 dBm; PTmax = –4 dBm; PRmax = –5 dBm; PRmin = –31 dBm; λ = 1310 nm; Link length = 5 km;

Figure 3.23 A sample STM-1 link.

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7. Link capacity = 155 Mbps; 8. Fiber type = 50/125 μm graded index (GI).

3.7.4 Span Analysis

For a link operating at a lower throughput of STM-1 the following effects can be discounted: • Self-phase modulation (SPM); • Polarization mode dispersion (PMD); • Stimulated Raman scattering (SRS)/stimulated Brillouin scattering; • Cross phase modulation (XPM); • Four wave mixing (FWM). The effects of chromatic dispersion need to be accounted irrespective of the link speed. The span analysis is as depicted in Table 3.5. PBS (span power budget ) = PTmin (min. transmitter power) −PR min (min receiver sensitivity) = −13dBm + 31 dBm = 18 dBm

The power margin is computed as follows: Pm (power margin) = PBS (power budget) − PS (span loss) Pm = 18 dB − 10.4 dB Pm = 7.6 dB

S.N 1 2 3 4 5 Total

Table 3.5 Example 3.1—Span Analysis Factor Computation MMF GI 50/125 μm fiber (1310 nm) – 5km @ 0.7 dB/km SC connectors 2 @ 0.5 dB/km Mechanical splice 2 @ 0.7 dB/splice Patch panel 2 @ 0.75 dB/panel Excess power margin — (safety factor)

Loss (dB) 3.5 1 1.4 1.5 3 10.4

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The total span loss is within the power budget of 18 dB (maximum allowable loss over the span). Further, as explained in the previous section, the power at the receiver received by the receiver should not exceed the maximum receiver sensitivity (PR max) after adjusting for span losses. This is to prevent saturation at the receiver. This input signal level is denoted as (PIN) and the maximum transmitter power is represented by PT max. With the span loss (PS) remaining constant, PT max represents the launch power and accordingly the input power (PIN) is computed as follows: Pin (input power) = PT max (max. transmitter power) − PS (span loss) PIN = −4 − 10.4 PIN = −14.4 dBm −14.4 dBM (PIN ) < −5 dBM (PR max ) The value of PIN satisfies the receiver sensitivity constraint while ensuring the viability of the optical system operating at the rate of STM-1 rate over a span of 5 km without any amplification stages or usage of attenuators. Example 3.2

For this example let us consider a bidirectional link with a higher throughput (STM-64 or 10 Gbps) operating over a larger distance of 60 km (Figure 3.23(b)). The minimum optical transmitter launch power is assumed to be –9 dBm and the maximum optical transmitter launch power is + 2 dBm at 1550 nm. The minimum receiver sensitivity is assumed to be –32 dBm and the maximum receiver sensitivity is –5 dBm at 1550 nm. There would be a total of two patch panels involved and fusion splicing would be employed with a maximum of six splices spanning the 60-km distance. The fiber used is a step index 8.1/125 μm SMF cable and standard assumptions for attenuation, component, and nonlinear losses are taken. Accordingly, following are the key parameters for the link under consideration: 1. 2. 3. 4. 5. 6.

PTmin = –9 dBm; PTmax = +2 dBm; PRmax = –5 dBm; PRmin = –32 dBm; λ = 1550 nm; Link length = 30 km;

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7. Link capacity = 10 Gbps; 8. Fiber type = 8.1/125 μm SMF step index (SI). The system is operating at 10 Gbps or approximately 10 GHz. At such high bit rates, SPM, PMD, and SRS/SBS margin requirements must be taken into consideration while allowing for a degree of chromatic dispersion [7]. XPM or FWM margins are not taken into account for a single-wavelength system. The link loss and viability calculations are as follows: PBS (span power budget) = PTmin (min. transmitter power) −PR min (min. receiver sensitivity) = −9 dBM + 32 dBm = 23 dB

Table 3.6 presents the span analysis: Pm (power margin) = PBS (power budget) − PS (span loss) Pm = 23 dB − 20.62 dB Pm = 2.38 dB ( > 0 dB) Pm (power margin) = PBS (power budget) − PS (span loss) Thus, it is evident that the 20.6-dB span loss is well within the 23-dB power budget (maximum allowable loss over the span). The input power received by the receiver (PIN), after accommodating the span losses must not

Table 3.6 Example 3.2 Span Analysis [7] S.N 1 2 3 4 5 6 7 8 9 Total

Factor SMF step index (SI) 8.1/125 μm LC connectors Fusion splices Patch panels SPM margin PMD margin SRS/SBS margin Dispersion margin Optical safety and repair margin

Loss Computation (dB) 60 km@ 0.2 dB/km 12 2 @ 0.5 dB/connector 1 6 @ 0.1 dB/splice 0.6 2 @ 0.75 dB/panel 1.5 — 0.5 — 0.5 — 0.5 — 1 — 3 20.6

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exceed the maximum receiver sensitivity specification (PR max) so as to prevent receiver saturation. Pin (input power) = PT max (max. transmitter power) − PS (span loss) Pin = −0 − 20.6 Pin = −20.6 dBm −20.6 dBm (PIN ) 40 dB) to the point of reflection, where the loss is 1.5 dB lower than peak. It represents the minimum distance after a Fresnel reflection for detecting another event or the minimum length of fiber needed between two reflective events. 4. Accuracy: • Accuracy represents the correctness of the value measured with respect to the true value in reference to the loss and distance measurements. • In case of OTDR where the measurement quantity is not an absolute value, linearity is used to judge the loss accuracy. • The distance accuracy depends on the setting of fiber refractive index, OTDR calibration, and clock stability.

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• An error will occur in the distance measurement unless an accurate group refraction index is specified. 5. Attenuation dead zone: Attenuation dead zone indicates the distance between the leading point of the reflected trace where the return loss is 40 dB and the point where the back scattered level is within ±0.5 dB of the normal level. Attenuation dead zone refers to the minimum distance after which the OTDR can measure accurately the loss of consecutive events after Fresnel reflection. 6. Averaging: • Averaging is used to remove noise from the data of OTDR trace and produces less noisy trace facilitating events to be measured more accurately. • The values are to be set based on the applications. • It is set to lesser values for quick fault measurement and higher values for measuring the events over a longer link. 7. Pulse width: • This parameter defines the duration (time and distance) of the pulse, launched onto the fiber, from the OTDR laser source. • A higher pulse carries more energy and hence has to be set for longer length measurement. The drawback is that multiple events that are close together may appear as a single event. • A shorter pulse width is used for short distance range, and it measures the closer events more accurately.

Example 9.1

Determine the maximum distance range for SMF G652 single mode fiber using an OTDR with 40-dB dynamic range. The fiber loss is specified as 0.20 dB/ km at 1550 nm. Distance range (max) = dynamic range (max)/fiber loss per km (min) Distance range (max) = 40dB/0.20dB/km = 200 km

9.8.3 OTDR Testing Prerequisites

1. OTDR (capable of working at the wavelength and the fiber type being tested);

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2. Launch and receive (not required for testing for length of span or link) reference cables (compatible—type, connectors, and size with the cable plant under test); 3. Adapters (for cable mating, if required); 4. Cleaning kit. 9.8.4 Precautions

1. Ensure adequate length of the reference cables to ensure that the initial test pulse settles back to the baseline. A long cable helps in countering the problems caused by dead zone. 2. Watch for uncontaminated connectors or cleaning of connectors. 3. Compatible adapters are available. 9.8.5 Testing Procedure

1. 2. 3. 4. 5. 6.

Switch on the OTDR with sufficient time to warm up; Ensure that connectors and adapters are cleaned as per established OP; Attach cables (launch and receive, as applicable); Program the test parameters; Acquire trace (Figure 9.6); Analyze trace (Figure 9.6).

9.8.6 Analyzing OTDR Traces

1. The slope of the fiber trace depicts the attenuation coefficient of the fiber expressed in dB/km. 2. A fairly long fiber is required to prevent overloading due to large reflections. 3. Connectors and splices, referred to as events, reflect loss with the height of the peak indicative of the amount of reflection. 9.8.7 Distance Measurement

1. Place one of the markers (marker A) before the reflectance peak on the trace reflecting the connection between the launch cable and the cable under test. 2. Place the second marker (usually called marker B) just before the reflectance peak from the connection between the cable under test and

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Figure 9.6 OTDR trace and analysis. Source: The Fiber Optic Association, 2009, http://www. thefoa.org. Reprinted with permission.

the receive cable or the reflectance peak from the final connector on the cable under test, in case there was no receive cable used. 3. The segment length between the markers will determine the distance.

9.8.8 Estimating the Attenuation Coefficient

1. Place one of the markers (marker A) away from any splice or connection in the cable under test. 2. The second marker (marker B) is placed further away on the same segment. 3. The loss of the segment between the markers is computed along with the distance and expressed in dB/km.

Example 9.2

The sample test settings for distances between 10 and 20 km is presented in Table 9.1. Example 9.3

The sample test setting for distances less than 100 km is presented in Table 9.2. Example 9.4

The sample test setting for distances greater than 100 km is presented in Table 9.3.

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S.N 1 2 3 4 5 6 7 8 9 10 11 12 13

Table 9.1 Sample Test Settings (Distances Between 10 and 20 km) Parameter Setting Mode Manual mode Wavelength 1550nm/1310nm Refractive index Appropriate for fiber type Cabling factor 0 percent Backscatter coefficient –81 dB Power on setting Last used Pulse width 200ns (20m) Range 50 km Test time1 (number of pulses being 150,000 averages) Event loss threshold 0.01 dB Reflectance threshold –40 dB End of fiber 3 dB Event loss limit 0.1 dB

1. Test time setting can be set to auto. In manual mode, test times can be set to 15 seconds, 30 seconds, 1 minute, or 3 minutes per wavelength.

Table 9.2 Sample OTDR Test Settings for Distances Less Than 100 kms S.N Parameter Setting 1 Mode Manual mode 2 Wavelength 1310nm/1550nm 3 Refractive index Appropriate for fiber type 4 Cabling factor 0 percent 5 Backscatter coefficient –81 dB 6 Power on setting Last Used 7 Pulse width 1000ns (100m) 8 Range 100 km 9 Test (number of pulses 150,000 being averages) 10 Event loss threshold 0.01 dB 11 Reflectance threshold –40 dB 12 End of fiber 3 dB 13 Event loss limit 0.1 dB

9.9

Summary

The common tests on an optical network includes tests for continuity and losses (insertion, splicing). In addition, it involves cleaning and/or replacement of connectors in case of damage or contamination. Contaminated connectors

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Table 9.3 Sample OTDR Test Settings for Distances Greater Than 100 kms S.N Parameter Setting 1 Mode Manual mode 2 Wavelength 1550 nm 3 Refractive index Appropriate for fiber type 4 Cabling factor 0 percent 5 Backscatter coefficient –81 dB 6 Power on setting Last used 7 Pulse width 5000 ns (500m)/10000 ns (1000m) 8 Range 200 km 9 Test time (number of 100,000 pulses being averages) 10 Event Loss threshold 0.01 dB 11 Reflectance threshold –40 dB 12 End of fiber 3 dB 13 Event loss limit 0.1 dB

must be cleaned using suitable kits and appropriate techniques. Testing and certifying of long-haul links, which involve a number of splices, calls for measurement of splicing losses and its location. This necessitates that use of more complex tools like an OTDR. Visual inspection techniques involve physical inspection of the optical termination points, patch panels, connectors, and physical interfaces on network line terminals or segments of the optical network directly using the naked eye and/or visual tracers. Tracing is used to ascertain damage to cables including reels prior to installation. Visual inspection uses tools like visual fault locator (VFL), which uses a high-power visible laser source to detect fiber faults. Checking for the contamination of the fiber connector end faces necessitates the use of magnifying device, since the diameter of a fiber core is smaller than a strand of human hair. An optical fiberscope provides magnification ranging from 100X (older models) to 400X (commonly used) and beyond. They are used for inspection of fiber optic to confirm proper polishing and find faults like scratches, polishing defects, and dirt. The use of video scopes provides another method of viewing connectors that are in areas that are difficult to reach and checking for damage or contamination of the fibers using handheld device Measurements of optical power and losses are important parameters that define the health of the fiber system. Optical poser is expressed in units of dBm where m refers to the reference power of 1 milliwatt (mW). The measurement of the optical power of a transmitter or the input to a receiver are absolute optical power measurements, since the actual value of the power is being measured. In contrast, loss is a relative power measurement equal to the difference between

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the power coupled into a cable, splice, or a connector and the power that is transmitted through it. Cable loss is the difference between the power coupled into the cable at the transmitter end and the available power at the receiver end. Insertion loss testing involves measuring the optical power loss in a cable due to attenuation, connectors, and splices. The quality of connectors in the optical network significantly impacts network uptime. To ensure trouble-free network operations, connectors needs to be free from contamination. It is empirically established that dirty connectors are one of the leading contributors that impact network availability. Fusion splicing is the preferred method of splicing optical fibers. It involves joining the two fibers end to end using heat in such a manner that the light passing through the fibers is not scattered or reflected back by the splice. OTDR is a potent test instrument for testing fiber-optic cable plants. The use of an OTDR, however, requires knowledge of some key concepts, terminologies, and skill in performing the test and interpreting the results. This chapter provided an understanding of the primary tests that are required for verifying the optical network installation quality, as well as identifying and troubleshooting common issues that results in network downtime or cause serious performance degradation in the network.

9.10

Review

9.10.1 Review Questions

1. Optical fiber is one of the most reliable and cost effective (bandwidth carrying capacity) transmission medium and will continue to remain in the near foreseeable future. a. True b. False 2. VFL can also be used to locate/identify splices in splice trays while locating breaks in fiber cables and/or high loss connectors. a. True b. False 3. Optical power is measured in _______ whereas optical loss is measured in _______. a. dBm and dB b. dB and dBm c. dB and mW d. mW and dB

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4. dB = 10 Log (measured power/reference power). This equation is ______. a. True b. False 5. The measurement of the optical power of a transmitter is an example of _________ optical power measurements. a. Relative b. Absolute 6. The _________ is the initial activity performed during splicing of optical fibers. a. Cleaving b. Stripping c. Precleaning d. Alignment of fiber ends 7. The OTDR sends a high-power laser beam pulse into the fiber and tracks the return signals from backscattered light in the fiber. a. True b. False 8. ____________________ indicates the distance between the leading point of the reflected trace where the return loss is 40dB and the point where the back scattered level is within ±0.5dB of the normal level a. Dynamic range b. Dead zone c. Attenuation dead zone d. Accuracy 9. When using an OTDR, setting a higher pulse width results on higher energy being coupled on the links under measurement, and hence is useful for ____________ measurement. a. Longer length b. Short length 10. A setting of 5000ns on a OTDR indicates that the link under test is __________. a. Long haul link b. Metro link c. Short haul link

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9.10.2 Exercises

1. List and compare the testing procedures for short haul and long haul optical cable networks. 2. Write a short note on the procedure for cleaning DWDM cables? 3. Outline the key safety measures when using an optical fiberscope for checking the fiber and connector end-faces. 4. Write a short note on visual inspection techniques with respect to optical networks? 5. List the various methods of splicing optical cables and discuss the merits and demerits of each method?

9.10.3 Research Activities

1. Write a detailed note on the commercially available fiberscopes (optical and video) with a comparison of the primary features. 2. List and describe the advantages of different types of optical cleaning products. 3. Compile a list of OTDR models and their key features. 4. Draw a list of standard tests for ascertaining the quality of installation of fiber plants. 5. Write a short note on the testing procedures for long haul optical links.

9.11

Referred Standards 1. 2. 3. 4.

9.12

Fiber optic connector intermatibility standard (FOCIS) ISO 14763-3 TIA 568C.0 TIA 568C.0

Selected Bibliography

9.12.1 Books Gower, J., Optical Communications Systems, Englewood Cliffs, NJ: Prentice Hall, 1984.

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Liao, D. D., and Forber, A. E., “Method for Finding and Measuring Optical Features Using an Optical Time-Domain Reflectometer,” U.S. Patent 5,442,434. Marcuse, D., Principles of Optical Fiber Measurement, New York: Academic Press, 1981. Parks, T. W., and A. L. Vandervort, “Method for Testing Optical Waveguide Fibers to Detect Reflection-Type Discontinuities,” U.S. Patent 5,450,191.

9.12.2 URLs http://ebooks.cawok.pro/Academic.Press.Troubleshooting.Optical.Fiber.Networks.Understanding.and.Using.Optical.Time-Domain.Reflectometers.May.2004. eBook-DDU.pdf http://ebooks.cawok.pro/Academic.Press.Troubleshooting.Optical.Fiber.Networks.Understanding.andUsing.Optical.Time-Domain.Reflectometers.May.2004. eBook-DDU.pdf http://foa.org/ http://www.thefoa.org/tech/ref/testing/test/OFSTP-14.html https://www.google.co.in/search?q=troubleshooting+fiber+optic+networks&ie=&oe=

9.12.3 Journals Garrett, I., and C. J. Todd, “Review: Components and Systems for Long-Wavelength Monomodefiber Transmission,” Optical Quantum Electronics, Vol. 14, 1982, pp. 95–143. Heffner, B. L., and P. R. Hernday, “Measurement of Polarization-Mode Dispersion,” HewlettPackard Journal, February 1995. IEC TC 86/WG4/SWG2, Calibration of Optical Time-Domain Reflectometers, 1994. Jeunhomme, L., “Single-Mode Fiber Design for Long Haul Transmission,” IEEE Trans. MTT, Vol. 30, 1982, pp. 573–578, and IEEE Journal of Quantum Electronics, Vol. 18, 1982, pp. 727–732. Moeller, W., K. Hube, and D. Huenerhoff, “Uncertainty of OTDR Loss Scale Calibration Using a fiber Standard,” Journal of Optical Communications, Vol. 15, No. 1, Jan. 1994, pp. 20–28. Neumann, E. G., “Theory of the Backscattering Method for Testing Optical fiber Cables,” Electronic Communications, Vol. 34, 1980, pp.157–160.

References [1]

Jones, W. B., Introduction to Optical Fiber Communications Systems, New York: Holt, Rinehart and Winston, 1988.

[2]

Marcuse, D., Principles of Optical Fiber Measurement, New York: Academic Press, 1981.

[3]

Parks, T. W., and A. L. Vandervort, “Method for Testing Optical Waveguide Fibers to Detect Reflection-Type Discontinuities,” U.S. Patent 5,450,191.

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[4]

Jauvtis, H. I., “Locating Fiber Faults in FITL Systems,” Fiberoptic Product News, July, 1994.

[5]

Liao, D. D., and A. E. Forber, “Method for Finding and Measuring Optical Features Using an Optical Time-Domain Reflectometer,” U.S. Patent 5,442,434.

[6]

Bellcore, Generic Requirements for Optical Time Domain Reflectometer (OTDR) Type Equipment, GR-196-CORE, 1995.

10 Optical Network Testing and Troubleshooting Procedures 10.1

Chapter Objectives

The growth of converged network infrastructure has been exponential over the past decade. The developments in mobile broadband technologies, increasing teledensity (especially in developing nations), and development of new services have led to a massive increase in traffic in the access network and necessitated the shift to optical broadband access solutions. Consequently, FTTx deployments have witnessed a manifold increase in the past decade. The bandwidth requirements at the access network have spurred the use of DWDM technologies in the metro and backbone networks and development of spatial multiplexing techniques. This trend is expected to continue in the foreseeable future. The demand for trained manpower for installation and commissioning O&M of optical networks is at its peak and will continue to remain so over the next decade. Service providers are expected to provide reliable network services on a 24/7 basis necessitating the use of in-service performance monitoring and optimization tools and shorter maintenance windows. A thorough understanding of the installation and troubleshooting best practices is essential in order to equip planners, field engineers, and technicians with the necessary skills to build, operate, and ensure trouble-free network operations.

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Chapter 9 provided detailed inputs related to industry best practices for test and troubleshooting optical cabling along with the commonly used test tools. This chapter is designed to equip field engineers and technicians with the necessary skills required to acquire more specialized competencies for maintaining optical networks of varying complexities. This chapter, based on standard test procedures and current industry best practices, is designed to equip field engineers and technicians with the end-to-end attributes required to maintain and troubleshoot optical backbone, metro, WDM, and access networks. Key Topics

• Methodologies for testing, certifying, and troubleshooting: • Backbone networks; • DWDM networks; • Access networks (FTTx); • Cabling and connecterization tests; • Testing of optical patchcords.

10.2

Introduction

Ensuring network performance and reliability calls for specialized skills related to optical diagnosis, characterization, and performance monitoring. Network reliability is highly dependent on test and measurement. Testing is the only way to ensure that network specifications have been met prior to rollout. Testing and diagnosis ensures network operation in accordance with the established client SLAs. Proactive maintenance of the network elements, fiber plants, and allied infrastructure is crucial for ensuring high levels of quality and availability of the services with a direct impact on the OPEX of a service provider.

10.3

Testing and Troubleshooting Long Haul Networks

A network is designed for trouble-free operation for a number of years. While network planning is a specialized activity, the benefits of proper planning can be realized only by ensuring installation quality of network equipment (NE), fiber plants, and interconnection mechanisms like fiber-management systems (FMS), optical distribution frames (ODF), cable connecterization, and patch cords. The extra time and effort spent in ensuring installation as per standard industry practices or specified organizational design can go a long way in ensuring trouble-free network operation. This section outlines the methodology of testing and certifying long haul optical systems [1].

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10.3.1 General Guidelines

Table 10.1 provides a generic list of prerequisites prior for testing/certification of network segments. 10.3.2 Preinstallation Checks

It is recommended that the cables (reels) are tested for continuity prior to installation. Cable continuity can be established using visual tracers (see Section 9.2 in Chapter 9). Cables that exhibit signs of damage would need to be inspected using an OTDR. 10.3.3 Postinstallation Tests

Postinstallation tests ensure: 1. Cable plant installation is as per industry standards or specific organizational specifications. 2. Operationalization of the link (ensuring terminal equipment parts are able to communicate). 3. Design specification/objectives are met. 4. A baseline document is created for future reference. The series of tests performed on an optical fiber span to test its integrity, installation quality, and performance is referred to as fiber characterization. The five tests are outlined in Table 10.2. 10.3.4 Test Description 10.3.4.1 E2E Loss Measurement

Loss measurement is performed by using laser source and power meter (LSPM) test set (loss test set) or in other words using commonly available light sources Table 10.1 Prerequisites for Testing Fiber Plant Installations S.N Prerequisites Description 1 Understand the network Understanding the overall network architecture and points of being tested interconnection 2 Appropriate tool kit and test Ensure availability of tools as per the nature of activity equipment intended to be performed 3 Standard testing process and Knowledge of standard test procedures and ability to correct usage of tools/test correctly use the test equipment for various testing scenarios equipment

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Table 10.2 Fiber Characterization [2] S.N Test Objective Tools 1 E2E loss Measure end-to-end Power meter, test measurement (E2E) loss on an optical source, reference (using LTS) span cable(s) 2 Fiber integrity Check for fiber OTDR inconsistencies, Check quality of splices 3 Optical return loss Ensure losses are as Power meter, test (ORL) per link budget source, reference cable(s) 4 Chromatic Measurement of CD CD dispersion test set dispersion (CD) 5

Polarization mode dispersion (PMD)

PMD measurement

PMD dispersion test set/OSA

Standard FOTP-171 / EIA-455171 FOTP-59 / TIA/EIA455FOTP-107 / TIA/EIA455-107A FOTP-175 / TIA-455175-B/ IEC 60793-142/GR-761 TIA-455-124 – FOTP124

and power meters. The total optical losses along a fiber link can be quickly and reliably established by using a light source with known characteristics and power meter. Refer to Section 9.3 for details of optical power measurements and Figure 10.1 for the test setup. The steps involved are as listed: 1. Establish the reference value for the optical power by measuring the optical power of the source—POUT. 2. Identify the wavelength of the optical signal being used on the link. 3. Connect one end of the link to the optical source being used. 4. Connect the power meter at the other end of the link with proper wavelength selection. 5. Record the value of the power at the receiving end—PIN. 6. Compare the received power value with the established reference value.

Figure 10.1 Optical power measurements.

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7. Loss (dB) = POUT – PIN. 8. If the loss is within the link budget, the link can be certified. Note 1

dBm = 10 log POUT/1mW—this isn’t a step in the test process. The LSPM test will, however, be able to point to neither the causes of the losses on the optical link nor their location. The test can also not provide any pointers to events that may occur in the near future. 10.3.4.2 Fiber Integrity

Refer to Section 9.7 for detailed OTDR functionalities and usage. It may be noted that fiber characterization is usually performed on a single-mode fiber (SMF). The wavelengths used for longer fibers are 1550 nm and /or 1625 nm, which correspond to the C-band and L-band transmission windows. 10.3.4.3 Optical Return Loss

Optical return loss (ORL) [3] is defined as the ratio of reflected power to the incident power at the input of a network device. It refers to the ratio of cumulative back reflectance to the optical power at the input of a NE. The primary factors that causes ORL is Rayleigh backscattering and Fresnel back reflection. A portion of the light ray incident on the core of a fiber gets reflected, back to the transmitter, due the difference in refractive index of the air and that of the fiber core. ORL is a measure of the light reflected back to the transmitter by optical connectors (due to air gaps, contaminants, geometric misalignments, and manufacturing defects). The total reflected or returned power (open ends of fiber, cracks in fiber, and mechanical splices) is denoted as TORL. ORL = 10 log10 (PR/PIN) Where PR is the reflected power and PIN is the input power and ORL is represented as a negative number in dB (positive value indicates reflectance). IEC 61300-3-6 standard outlines four tools for measuring ORL: a. b. c. d.

Optical continuous-wave reflectometer (OCWR); Optical frequency-domain reflectometer (OFDR); Optical low-coherence reflectometer (OLCR); Optical time-domain reflectometer (OTDR).

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Figure 10.2 illustrates the ORL measurement test setup using an OTDR:1 The steps involved in the measurement are: 1. The device under test (DUT) is connected to an OTDR using a standard patch cord with suitable connectors. Without the use of a standard cable, the actual reflection and attenuation of the DUT will be masked by the high reflection from the internal connector of the OTDR. 2. The test parameters are set on the OTDR. These include: a. Index of refraction: Index of refraction (IOR) measures the speed of light over the optical fiber: n = c/v Where n = IOR, c = velocity of light in vacuum (3 * 108 m/sec), and v = speed of light in the optical fiber. A large value of IOR indicates that light travels slowly in the medium. If the IOR value is set low, OTDR will compute the network distance to be lower than actual. Conversely if the IOR value is set high, OTDR will measure a longer distance. b. Backscatter coefficient: Backscatter coefficient refers to the amount of light scattered back onto the OTDR by the test pulse. OTDR

Figure 10.2 ORL measurement using OTDR.

1. Refer to Section 9.7 for detailed OTDR functionalities and usage.

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requires the backscatter information about the relative backscatter level to establish reflection when working with new fibers and sections with different cables. The value of backscatter coefficient is specified by fiber manufacturers (in product information sheets). It may be noted that the backscatter coefficient varies along the length of the fiber. Further, the backscatter coefficient cannot be adjusted in tune with changes in fiber characteristics of the network. This may the case in case the network is a combination of different fiber standards. c. Helix factor: The length of the fiber is different from that of the cable, as the fiber spiral around the central strength member (loose tube design). This difference in lengths is referred to as the helix factor. OTDR measures the fiber distance as opposed to the cable distance. The value of helix factor can be obtained from the cable manufacturer. d. Pulse width: A short pulse width provides higher measurement resolution over a shorter distance as compared to a higher pulse width that provide lower measurement resolution over a larger distance. e. Measurement distance: The measurement distance should mirror the distance of the network under test. The far end of the network will not be tested if the distance value is set low. A higher value will result in low resolution of the network under test. f. Acquisition time: A test with a small value of acquisition time would result in higher noise. However, a large value of acquisition time will result in a longer completion time for the test. Most versions of the newer OTDRs support automatic settings detection that can be used in case some the values of some of the test parameters are not available. 10.3.4.4 Chromatic Dispersion

Chromatic dispersion (CD) is a result of different wavelengths traveling at varying speeds, resulting in differences in arrival time of each wavelength. The differences in arrival time of each wavelength cause pulse spreading or CD. CD is measured in ps/nm. The two methods employed for CD measurement (Figure 10.3(a)) are: a. Modulation phase-shift method (MPS); b. Differential phase-shift method (DPS).

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Figure 10.3 Measuring dispersion.

DPS method is generally used for CD measurement since the process is faster as compared to MPS2, which provides superior accuracy. The test systems available tests for both CD and PMD and provide automated testing options and a user-friendly GUI. 10.3.4.5 Polarization Mode Dispersion (PMD)

The elliptical core causes the two polarizations of a signal, transmitted over a fiber, to travel at different speeds and arrive at the endpoint at different times. This difference in time between the two polarizations is referred to as differential group delay (DGD). This differential delay is PMD and is measured in picoseconds (ps). A test setup for PMD is illustrated in Figure 10.3(b).

10.4 Troubleshooting Optical Termination Points, Short Haul Segments, and Networks A systematic approach is essential for understanding and minimizing the recurrence of common issues that cause failure and/or performance degradation of the network at the points of termination and/or interconnection of network segments [4]. Table 10.3 lists the prerequisites for initiating a troubleshooting protocol. The contamination of connector end-face and improper connector-

2. The differential phase shift technique is not really quicker than the phase shift technique, but generally it uses a smaller number of measurement points to determine the dispersion curve. By contrast the phase-shift technique (MPS in this chapter) is able to provide significantly more test points on the dispersion curve, and that tends to lead to greater accuracy.

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Table 10.3 Troubleshooting Prerequisites S.N 1

2

3

Prerequisites Understand the details of termination points

Description Understanding the overall network architecture and points of interconnection including the interface type and description Appropriate tool kit and Ensure availability of tools and accessories test equipment related to visual inspection of connectors and cleaning kit Standard testing process Knowledge of standard test procedures and ability and correct usage of to correctly use the test equipment for various tools/test equipment testing scenarios

ization techniques are the major causes of network failures and/or performance degradation. Service providers must include periodic visual inspection and cleaning of connectors as a standard operating procedure to ensure reliable and trouble-free network operation. Table 10.4 summarizes the common tests and their purpose along with the list of tools. The list of tools required for performing the tests outlined in Table 10.4 is included in Table 10.5. 10.4.1 Continuity Testing

Cable continuity can be established using visual tracers that contains a source (commonly LED) that can be mated to a fiber connector. Cable continuity is established either by visual confirmation or using a power meter. See Section 9.2 for additional details.

Table 10.4 Testing of Short Haul Networks/Segments/Endfaces S.N Test Objective Tools 1 Continuity Check for breaks Fiber optic tracer/ in fiber visual fault locator 2 Power measurement Measure transmit Power meter, ANSI/TIA/EIA- 455-95or receive power reference cable A-2000 3 Connector inspection Check for Fiberscope FOTP-13/EIA-455-13 damaged connectors 4 End-to-end (E2E) loss Ensure losses are Power meter, test TIA/EIA-455-171 (single/double ended) as per link budget source, reference cable(s)

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The ABCs of Fiber Optic Communication Table 10.5 Tool List S.N Tools 1 Adapters (depending on the cable type being tested) 2 Reference (test) cables 3 Optical power source 4 Optical loss test set (OLTS) 5 Optical power meter 6 Fiber tracer or visual fault locator 7 Cleaning materials—dry cleaning kits or lint-free cleaning wipes and isopropyl alcohol (IPA)

10.4.2 Power Measurement

The measurement of the optical power of a transmitter or the input to a receiver are absolute optical power measurements, since the actual value of the power is being measured. See Section 9.3 for additional details. 10.4.3 Connector Inspection

The diameter of the fiber core is extremely small, in comparison with a strand of human hair (90 μm), which can be up to nine times larger, and it needs high levels of magnification to inspect the end face. The inspection is carried out by using a device referred to as a fiberscope. See Section 9.2 for additional details. 10.4.4 End-to-End (E2E) Loss (Single and Double Ended)

Loss measurements can be classified into the following two types: 1. Single-ended test: a. The cable under test is connected to a reference launch cable at one end (near end) and the power (POUT ) at the other end (far end) is measured using an OPM. b. The specifications of the reference and connectors will match those of the cable and connectors under test. c. This test provides the measurement of the loss in the connector, coupled to the launch cable, along with losses in the fiber and splicing losses. d. To check for the connector at the far end, the test setup will have to be reversed. 2. Double-ended test:

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a. In case of a double-ended test, the cable under test is connected to a reference launch cable at the near end and an OPM is connected at the far end using another reference cable. b. The specifications of the reference and connectors will match those of the cable and connectors under test. c. This test provides the measurement of the loss in the connectors at both the ends as well as fiber and splicing losses. E2E loss testing is performed by using LSPM test set or, in other words, using commonly available light sources and power meters. The test can be used to quickly eliminate the fiber patch cables and its end points as potential causes of network issues, including performance degradation. The total optical losses along a fiber link can be quickly and reliably measured by using a light source with known characteristics and power meters. The key steps in the testing include the following: 1. Establish the reference value for the optical power by measuring the optical power of the source—PREF. 2. Identify the wavelength of the optical signal being used on the link. 3. Connect one end of the link to the optical source being used. 4. Connect the power meter at the other end of the link with proper wavelength selection. 5. Record the value of the power at the receiving end—POUT. 6. Compare the received power value with the established reference value. 7. Loss (dB) = POUT – PREF. 8. If the loss is within the link budget, the link can be certified The LSPM test will, however, be able to point to neither the causes of the losses on the optical link nor their location. The test can also not provide any pointers to events that may occur in the near future. Links that are certified may have connectors that are not conformant to industry standards.

10.5

Troubleshooting Cabling and Connecterization Issues

SDH, OTN, IP, and WDM core network elements have a significant number of termination ports connected to them using a fiber management system (FMS) or optical distribution frames (ODF). It is essential that industry cabling best practices are followed during cable installation to ensure proper cable routing,

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identification, and troubleshooting. This section presents the common causes of problems related to cabling and connecterization. The prerequisites for successful testing and/or troubleshooting exercises is presented in Table 10.6. 10.5.1

Optical Cabling—General Issues

Macrobending, Stretching, Twisting, and Winding

1. It is essential that the lengths of the fiber be determined prior to installation. The use of long cables increases the possibility of bending (due to looping), winding, and/or twisting. 2. The use of short cable lengths can increase the chances of macrobending with the associated losses. 3. The layout of equipment and the cable routing should be planned with eye toward optimizing the lengths and ensuring proper routing of cables. 10.5.2 Damaged Optical Cables

1. Damaged optical cables can be detected by using visual inspection techniques outlined in Chapter 9. 2. It is usually difficult to detect cable damage by inspection with the naked eye. 3. Continuity testing with the help of visual traces is recommended. 4 In case of isolation, the cable has to be replaced/reinstalled. 10.5.3 Stretched Cables

1. As discussed in Chapters 1 and 2, fiber optic cables are extremely fragile and susceptible to damage due to poor handling and/or installation techniques.

S.N 1 2 3

Table 10.6 Cable/Connector Troubleshooting Prerequisites Prerequisites Description Gather basic information Understanding the details and cabling layout along with connector details Appropriate tool kit and test Ensure availability of LTS, fiberscope for visual equipment inspection of connectors and cleaning kit Standard testing process and Knowledge of standard test procedures and ability correct usage of tools/test to correctly use the test equipment for various equipment testing scenarios

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2. As discussed earlier, cables may get stretched, especially of if their length is shorter than the actual requirements. 3. The problem can be rectified by cable rerouting, changes in equipment layout, or cable replacement. 4. Cable stretching can also occur if pressure is exerted on the fiber jacket/connectors during cable routing. 10.5.4 Connecterization Issues

1. It is obvious that an optical link can function optimally only if the cable and connectors are properly installed and are free of damage. 2. In case of connectivity issues, after the cable problems are eliminated, the connectors must be removed and reseated (this is an option that yields results in a large number of cases). 3. If the issue persists, the connector end faces must be checked for contaminants using the process outlined in Chapter 9 and using a suitable type of fiberscope. 4. In case of contamination, the connector end faces must be cleaned using standard procedures (outlined in Chapter 9). The troubleshooting summary for cables and connectors is outlined in Table 10.7. 10.5.5 Optical Cabling—General Guidelines

1. Identify the source and destination ports of the optical cable being investigated. 2. Check the LED status on the source and destination equipment (if applicable) to understand the status of the link (active/inactive). 3. Check the cable routing between the source and destination. Ensure that the cables are not pulled tightly; if so, ensure that appropriate slack is provided. 4. In case of bundled cables, ensure enough slack. 5. Cables that have sharp bends need to be rectified. 6. Ensure hanging cables are reinforced. 7. Ensure that no bare cables are routed through the floor. There is possibility of the cables getting damaged by footfalls or movement of heavy objects.

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Table 10.7 Potential Issues in Cabling and Connecterization Potential S.N Problem Cause(s) Tests Solution 1 Improper Macrobending, Visual inspection Reinstallation or rerouting installation stretching, of cables twisting, and winding 2 Damaged optical Improper Visual inspection Continuity testing with the cables installation, help of visual tracers is damage in transit mandated. 3 Stretched cables Poor handling, Connector cleaning Cable rerouting, changes in installation, cables equipment layout. or cable of shorter lengths replacement. 4 Connecterization Connector damage Visual check/use of Connector replacement/ issues or end-face fiberscope cleaning contamination

10.5.6 Testing Optical Patch Cords

Optical patchcords are used to interface network elements (multiplexers, line terminals, routers) to the installed cables using FMS and/or ODF. Optical patchcords may also be used as reference cables for testing insertion losses described in the earlier sections. It is, however, preferred to use test reference cords terminated with reference grade connectors compliant with ANSI/TIA-52614-C and IEC 61280-4-2. It is obvious that the patchcord type must (usually) match or be compatible with the fiber type of the cable plant. Further, the connector type of the patch cords must match that of the patch panel in use. The test and troubleshooting procedure for the optical patchcords is defined by the TIA FOTP-171/OFSTP-7/ISO/IEC 14763 standards [5]. The most common problem with optical patchcords is damage to connectors due to contaminants or improper matings. Patchcords used as reference cables may have their end faces damaged due to frequent use. 10.5.7 Troubleshooting Optical Patchcord Issues

Optical patchcords must be necessarily tested for insertion loss prior to usage. This is especially true for patchcords used as reference cables. 1. A LTS is used to test patchcords using single-ended FOTP-171 test procedure with one reference cable used as a launch cable. 2. This will test the connector mated to the reference cable and the fiber in the patchcord. 3. Establish the reference value for the optical power by measuring the optical power of the source—POUT.

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4. 5. 6. 7.

Connect one end of the link to the optical source being used. Connect the power meter at the other end of the link. Record the value of the power at the receiving end—PIN. Compare the received power value with the established reference value. 8. After testing in one direction, reverse the patchcord and test the other end. 9. In case of high losses, the connector end faces must be examined for damage and/or contamination using a fiberscope.

10.6

Troubleshooting FTTH Networks

An FTTH network [6] consists of number of optical network units (ONU) as CPEs connected to splitters that form part of the optical distribution network (ODN) connected to an optical line terminal (OLT) at the central office through a feeder network. The common issues with PON architectures involve faulty drop cables, cable macro bending, or contaminated connectors. The prerequisites to successful troubleshooting is outlined in Table 10.8. 10.6.1 Troubleshooting FTTx Networks Based on PON Architecture

As discussed in Chapter 8, PON-based network architecture is gaining traction globally. PONs involve passive components, optical splitters, in the distribution network. The splitters usually provide 2 to 32 end-user connections. The first step involves identification or localization of the failure. There are three distinct segments that the fault could be localized to, as outlined in Table 10.9. 1. If all customers are affected, the problem is likely to be with the OLT and/or feeder cables.

Table 10.8 Troubleshooting Prerequisites S.N Prerequisites Description 1 Understand the details of the Understanding the PON network architecture including distribution network the interconnection of splitters 2 Appropriate tool kit and test Ensure availability of LTS and fiberscope for visual equipment inspection of connectors and cleaning kit 3 Standard testing process and correct Knowledge of standard test procedures and ability to usage of tools/test equipment correctly use the test equipment for various testing scenarios

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2. If individual customers are affected, the problem is most likely with the CPE and/or distribution network, with drop cables being the suspect. Table 10.10 outlines the network troubleshooting methodology, and the list of recommended tools are included in Table 10.11.

10.7

Troubleshooting DWDM Networks

DWDM technology has emerged as a potent technology to meet the bandwidth requirements of the present as well as the future. DWDM technology provides capacity expansions of optical links by supporting multiple independent transmission channels, of 40/100 Gbps, (80 λ in the C band with a channel spacing of 50 GHz) over a single fiber link. DWDM networks provide a converged platform for integrating OTN- and IP-based services. With transmission capacity in the L band largely untapped and the potential for using channel spacing

S.N 1 2 3

Table 10.9 Localizing Network Faults in PONs Equipment Network OLT Feeder FDH Distribution ONU/ONT Home/customer premise

Table 10.10 Troubleshooting PON-Based FTTH Networks [7] S.N Problem Potential Cause(s) Tests Solution 1 All Failure of OLT or Check OLT Replace failed line cards. customers status are affected Break/damage of OTDR Splicing. feeder cable 2 Problem Check optical power at Power If power is low, use OTDR at with customer premise measurement specified test wavelengths to isolate individual fiber fault; repair/replace drop cable connections in case of damage. In case drop cable is ok, check distribution cable for damage. Damaged or Connector Check connectors for damage or contaminated cleaning contaminants using fiberscope. connector end faces Device failure CPE check If connectors are ok, check connectivity using spare port on splitter. If issue persists replace ONT.

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Table 10.11 Tool List S.N Tools 1 Adapters (depending on the cable type being tested) 2 Reference (test) cables 3 Optical power source (CWDM compatible may be required in some cases) 4 Optical loss test set (OLTS) 5 PON power meter 6 Fiberscope 7 OTDR (Portable PON OTDR) 8 Cleaning materials—dry cleaning kits or lint-free cleaning wipes and isopropyl alcohol (IPA)

of 25 GHz and 12.5 GHz, wavelength switching capabilities DWDM offers a scalable solution to meet the bandwidth requirements of the future and is the cornerstone for all optical networks of the future. The evolution of DWDM networks is captured in Table 10.12. In comparison with TDM/IP networks, DWDM networks are difficult to troubleshoot. Table 10.13 lists the various stages in the deployment of DWDM networks along with the recommended tests and required tools (Table 10.14). In addition there could be physical layer issues like amplifier saturation, fiber type mismatch (in large networks with a combination of legacy and new fibers), and link BER.

10.8

Standardization Bodies

International Telecommunication Union–Telecommunication Standardization Sector (ITU-T) addresses interoperability issues at the system level in the form Table 10.12 Evolution of DWDM Networks S.N Feature(s) Description 1. Enhanced bandwidth support Support for 40 channels, in C band, at STM16/64 or OC/48/192 rates 2. Optical add/drop capabilities OADM rings 3. Consolidation of multiple network services Multiservice provisionable platform/ multiservice transport platform 4. Simplified network, traffic switching Managed ROADM capabilities at wavelength level Switch IP traffic directly in the optical Switched OTN domain, use of GMPLS Meet additional bandwidth requirements Support for 40 Gbps/100 Gbps rates

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Table 10.13 DWDM Network Deployment Stages, Tests, and Tools S.N Stages Purpose Tests 1 Preinstallation: Establish 1. Loss/ORL establish compatibility of WDM 2. E2E fiber characterization network equipment and the 3. Multipath interference compatibility fiber plant 4. CD measurement 5. PMD measurement 2 Installation Establish system 1. Optical power/OSNR commissioning functionality and 2. Insertion loss transmission integrity 3. Jitter and transmission delay 4. Channel wavelength 5. Channel isolation/drift 6. Gain 7. Amplifier noise 3 Postinstallation: Ensure network Spectral analysis operations and availability and 1. Central wavelength maintenance reliability 2. Distance between adjacent center wavelengths 3. Channel spacing 4. Insertion loss 5. OSNR 6. Peak power

Tools OTDR, CD/PMD analyzer, LTS

Optical spectrum analyzer (OSA)

OSA

Table 10.14 Tool List for Troubleshooting DWDM Networks S.N Tools Tests 1 CWDM analyzer CWDM channel power 2 Optical spectrum analyzer (OSA)/optical Spectral analysis channel analyzer 3 Optical power source (CWDM/DWDM Power measurements compatible may be required in some cases) 4 Optical loss test set (OLTS) Loss measurement 5 CD/PMD analyzer Measurement of CD/PMD 6 Fiberscope/video scope Fiber/connector end-face examination 7 OTDR (portable PON OTDR) Compute splicing losses, localize network faults 1 Cleaning of connector end faces 8 Cleaning materials—dry cleaning kits 1. Wet cleaning of connectors is not recommended on DWDM networks.

of advisory documents referred to as recommendations. ITU-T has a number of study groups (SG) that addresses specialized issues. In addition, there are national/regional standardization bodies like American National Standards Institute (ANSI), European Telecommunications Standards Institute (ETSI), British Standardization Institution (BSI), and German Institute for Standardization (DIN), among others. ANSI has accredited the Telecommunications

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Industry Association (TIA) for developing standards in the area of ICT. TIA is an offshoot of the Electronic Industries Alliance (EIA), which is a standards and trade organization. The International Electrotechnical Commission is the international standards and conformity assessment body for all fields of electrotechnology. This includes standards for testing and troubleshooting optical fiber plants, the focus of this chapter.

10.9

Summary

A network is designed for trouble-free operation for a number of years. While network planning is a specialized activity, the benefits of proper planning can be realized only by ensuring installation quality of network equipment, fiber plants and interconnection mechanisms like fiber management systems, optical distribution frames, cable connecterization, and patch cords. The extra time and effort spent in ensuring installation as per standard industry practices or specified organizational design can go a long way in ensuring trouble-free network operation. Proactive maintenance of the network elements, fiber plants, and allied infrastructure is crucial ensuring high levels of quality and availability of the services with a direct impact on the OPEX.

10.10

Review

10.10.1 Exercises

1. Explain the significance of optical return loss (ORL). Describe the test setup for measuring ORL in a long-haul network. 2. What is the need for using reference cables for loss measurements in an optical fiber? 3. How can a technician quickly localize and troubleshoot problems in a PON? 4. Describe the test setup and important test parameters for a DWDM network. 5. Write brief note on single- and double-ended tests for loss measurement in an optical cable.

10.10.2 Research Activities

1. Describe the functions of an optical spectrum analyzer (OSA). 2. What is meant by the term spectral analysis?

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3. Prepare a detailed note on the developments in test and measuring equipment for optical networks. 4. Prepare a test setup and troubleshooting methodology for GPON. 5. Prepare a test setup and troubleshooting methodology for OTN.

10.11

Referred Standards

EIA-472—General specification for fiber optic cable EIA-472B—Sectional specification for fiber optic communication cables for underground and buried use EIA-472C—Sectional specification for fiber optic communication cables for indoor use EIA-4750000-B—Generic specification for fiber optic connectors FOTP- 179—Inspection of cleaved fiber end faces by interferometry FOTP-1—Cable flexing for fiber optic interconnecting devices (ANSI/TIA/ EIA-455- 1-B-98) FOTP-111—IEC 60793-1-34 Optical fibers—Part 1-34: Measurement methods and test procedures—fiber curl FOTP-122—Polarization mode dispersion measurement for single mode optical fibers by Stokes parameter evaluation FOTP-124—Polarization-mode dispersion measurement for single-mode optical fibers by interferometry method FOTP-164—Measurement of mode field diameter by far-field scanning (single-mode) FOTP-178—IEC 60793-1-32—Optical fibers—Part 1-32: Measurement methods and test procedures—coating stripability FOTP-194—Measurement of fiber pushback in optical connectors (ANSI/ TIA/EIA-455- 194-99) FOTP-195 IEC 60793-1-21 Optical fibers—Part 1-21: Measurement methods and test procedures—coating geometry FOTP-196—Guideline for polarization-mode measurement in single-mode fiber optic components and devices (ANSI/TIAEIA-455- 196-99) FOTP-197—Differential group delay measurement of single-mode components and devices by the differential phase shift method (ANSI/TIA/ EIA-455-197-2000)

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FOTP-2—Impact test measurements for fiber optic devices (ANSI/TIA/ EIA-455-2-C- 98) FOTP-20—IEC 60793-1-46 optical fibers—Part 1-46: Measurement methods and test procedures—monitoring of changes in optical transmittance FOTP-206—IEC 61290-1-1 Optical fiber amplifiers—basic specification— Part 1-1: Test methods for gain parameters—optical spectrum analyzer (ANSI/ TIA/EIA-455-206- 2000) FOTP-208—IEC 61290-1-3 Optical fiber amplifiers—basic specification— Part 1-3: Test methods for gain parameters—optical power meter (ANSI/TIA/ EIA-455-208-2000) FOTP-209—IEC 61290-2-1 Optical fiber amplifiers—basic specification— Part 2-1: Test methods for optical power parameters—optical spectrum analyzer (ANSI/TIA/EIA- 455-209-2000) FOTP-21—Mating durability for fiber-optic interconnecting devices FOTP-210—IEC 61290-2-2 Optical fiber amplifiers—basic specification— Part 2-2: Test methods for optical power parameters—electrical spectrum analyzer (ANSI/TIA/EIA-455-210-2000) FOTP-211—IEC 61290-2-3 Optical fiber amplifiers—basic specification— Part 2-3: Test methods for optical power parameters—optical power meter (ANSI/TIA/EIA-455- 211-2000) FOTP-212—IEC 61290-6-1 Optical fiber amplifiers—basic specification— Part 6-1: Test methods for pump leakage parameters—optical demultiplexer (ANSI/TIA/EIA-455- 212-2000) FOTP-213—IEC 61290-7-1: Optical fiber amplifiers—basic specification— Part 7-1: Test methods for out-of-band insertion losses—filtered optical power meter (ANSI/TIA/EIA-455-213-2000) FOTP-214—IEC 61290-1 Optical fiber amplifiers—Part 1: Generic specification (ANSI TIA/EIA-455-214-2000) FOTP-218—Measurement of end-face geometry of optical connectors FOTP-219—Multifiber ferrule end-face geometry measurement FOTP-221 IEC 61290-5-1—Optical fiber amplifiers—basic specification— Part 5-1: Test method for reflectance parameters—optical spectrum analyzer FOTP-226 -IEC 61746—Calibration of OTDR FOTP-227 IEC 61300-3-24—Fiber optic interconnecting devices and passive components—basic test and measurement procedures—Part 3-24: Examination and measurements—keying accuracy of optical connectors for polarization maintaining fiber

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FOTP-228—Relative group delay and chromatic dispersion measurement of single-mode components and devices by the phase shift method FOTP-229—Optical power characterization FOTP-231 IEC 61315—Calibration of fiber optic power meters FOTP-239—Fiber optic splice loss measurement methods FOTP-29—Refractive index profile (transverse interference method) FOTP-45—Microscopic method for measuring fiber geometry of optical waveguide fibers FOTP-46—Spectral attenuation measurement (long length graded index optical fibers) FOTP-59—Measurement of fiber point defects using an OTDR FOTP-61—Method for measuring the effects of nuclear thermal blast on optical waveguide fiber FOTP-61—Measurement of fiber or cable attenuation using an OTDR FOTP-62 IEC 60793-1-43—Measurement methods and test procedures—numerical aperture FOTP-78 IEC 60793-1-40—Optical fibers—Part 1-40: Measurement methods and test procedures—attenuation FOTP-8—Measurement of splice or connector loss and reflectance using an OTDR GR-2947-CORE—Generic requirements for portable polarization mode dispersion (PMD) test sets IEC 60793—Optical fibers IEC 60793-1-42—Measurement methods and test procedures—chromatic dispersion IEC 60793-1-48—Measurement methods and test procedures—polarization mode dispersion IEC 60794—Optical fiber cables IEC 60869—Fiber-optic attenuators IEC 60874—Connectors IEC 60875—Fiber-optic branching devices IEC 60876—Fiber-optic spatial switches IEC 61073—Splices for optical fibers and cables IEC 61202—Fiber-optic isolators IEC 61274—Fiber-optic adaptors IEC 61280—Fiber-optic communication subsystem basic test procedures

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IEC 61281—Fiber-optic communication subsystems IEC 61282—Fiber-optic communication system design guides IEC 61290—Optical amplifier test methods IEC 61291—Optical amplifiers IEC 61292 TRs—Optical amplifiers technical reports IEC 61300—Test and measurement IEC 61313—Fiber-optic passive components IEC 61314—Fiber-optic fan-outs IEC 61751—Laser modules used for telecommunication IEC 61753—Fiber optic interconnecting devices and passive components performance standard IEC 61754—Fiber-optic connector interfaces IEC 61755—Fiber-optic connector optical interface IEC 62074—Fiber-optic WDM devices IEC 62077—Fiber-optic circulators IEC 62099—Fiber-optic wavelength switches IEC/TS 61941—Technical specifications for polarization mode dispersion measurement techniques for single-mode optical fibers ITU-T G.650.1—Definitions and test methods for linear, deterministic attributes of single-mode fiber and cable ITU-T G.650.2—Definition and test methods for statistical and nonlinear attributes of single-mode fiber and cable ITU-T G.652—Characteristics of a single-mode optical fiber and cable ITU-T G.653—Characteristics of a dispersion-shifted single-mode optical fiber and cable ITU-T G.654—Characteristics of a cut-off shifted single-mode optical fiber and cable ITU-T G.655—Characteristics of a nonzero dispersion-shifted single-mode optical fiber and cable ITU-T G.656—Characteristics of a fiber and cable with nonzero dispersion for wideband transport TIA FOTP-175-B—Chromatic dispersion measurement of single-mode optical fibers TIA TIA-4920000-B—Generic specification for optical fibers TIA TIA-492A000-A—sectional specification for class Ia graded-index multimode optical fibers

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TIA TIA-492CAAA—Detail specification for class IVa dispersion-unshifted single-mode optical fibers TIA/EIA -455- FOTP-122A—Polarization mode dispersion measurement for single-mode optical fibers by Stokes parameter evaluation TIA/EIA -455- FOTP-124A—Polarization mode dispersion measurement for single-mode optical fibers by interferometry TIA/EIA TSB 107—Guideline for the statistical specification of polarization mode dispersion on optical fiber cables TIA/EIA-455 FOTP-113—Polarization mode dispersion measurement for single-mode optical fibers by the fixed analyzer method

10.12

Selected Bibliography

10.12.1 Books Liao, D. D., and A. E. Forber, Method for Finding and Measuring Optical Features Using an Optical Time-Domain Reflectometer, U.S. Patent 5,442,434. Marcuse, D., Principles of Optical Fiber Measurement, New York: Academic Press, 1981. Agrawal, G. P., Nonlinear Fiber Optics, San Diego, CA: Academic Press, 1995. Derickson, Dennis, Fiber Optic Test and Measurement, Upper Saddle River, NJ: Prentice Hall, 1998. Anderson, Dwayne R., L. M. Johnson, F. G. Bell, Troubleshooting Optical Fiber Networks: Understanding and Using Optical Time-Domain Reflectometers, MA: Academic Press, 2004.

10.12.2 URLs http://www.thefoa.org/tech/ref/1pstandards/ http://www.fiber-optic-transceiver-module.com/pon-passive-optical-network-troubleshootingoverview.html http://www.senko.com/technical/pdf/Optical%20Return%20Loss%20Measurement.pdf https://aresu.dsi.cnrs.fr/IMG/pdf/evolution_DWDM.pdf

10.12.3 Journals Jeunhomme, L., “Single-Mode fiber Design for Long Haul Transmission,” IEEE Trans. MTT, Vol. 30, 1982, pp. 573–578, and IEEE Journal of Quantum Electronics, Vol. 18, 1982, pp. 727–732. Moeller, W., K. Hube, and D. Huenerhoff, “Uncertainty of OTDR Loss Scale Calibration Using a fiber Standard,” Journal of Optical Communications, Vol. 15, No. 1, Jan. 1994, pp. 20–28.

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Neumann, E. G., “Theory of the Backscattering Method for Testing Optical fiber Cables,” Electronic Communications, Vol. 34, 1980, pp.157–160

References [1]

Keiser, G., Optical Fiber Communication, New York: McGraw Hill, 2000.

[2]

EXFO, Multifunction Loss Tester User Guide, FOT-930, Quebec City, Canada: EXFO, 2006.

[3]

Lee, B., Optical Return Loss Measurement, http://www.senko.com/technical/pdf/Optical%20Return%20Loss%20Measurement.pdf.

[4]

Agrawal, G. P., Nonlinear Fiber Optics, San Diego, CA: Academic Press, 1995.

[5]

Derickson, Dennis, Fiber Optic Test and Measurement, Upper Saddle River, NJ: Prentice Hall, 1998.

[6]

Varghese, S., and K. R. S. Nair, FTTH Compliant Truly Fused 1x4 Couplers for Passive Optical Network (PON) Applications, 2009, http://www.optername.com/images/ ftthcoup.pdf, Opterna M.E., Dubai, retrieved May 15, 2015.

[7]

Anderson, D. R., L. Johnson, and F. G. Bell, “Analyzing Passive Networks Containing Splitters and Couplers,” in Troubleshooting Optical Fiber Networks, 2004.

Appendix Table A.1 Optical Transceivers S.N Transceivers Description 1 AOC Active optical cable (AOC) is employed for interconnection and shortrange multilane data communication applications. AOC consists of MMF, optical transceivers, and control circuitry, and performs electrical-to-optical conversion at the cable ends to improve speed and distance performance of the cable while ensuring compatibility with standard electrical interfaces. 2 C-BIDI SFF Compact bidirectional small form factor* transceiver module supports bidirectional communication over a single fiber at rates up to 2.5Gbps at 1310-/1490-nm wavelengths. It’s used in access networks including fiber-inthe-loop and PON. 3 CFP C-form factor pluggable transceiver interface with 100-Gbps support (10*10Gbps streams or 4*25-Gbps streams). 4

CFP2

5

CFP4

6

CFP8

7

QSFP+

Functionally similar to CFP with half the form factor (41.5 mm as compared with 82 mm CFP). Supports 10*10-Gbps streams, 4*25 Gbps. Functionally similar to CFP with one-fourth the form factor (21.5 mm as compared with 82 mm CFP). CFP8, with physical dimensions similar to CFP2 module, is a next generation optical transceiver designed for 400G Ethernet applications. It can support 16x25G and 8x50G electrical I/O along with 4x100GE. Quad SFP+ is a hot pluggable transceiver supporting optical as well as electrical TDM and IP interfaces. The interface was developed as a part of the multisource agreement (MSA) by the Small Form Factor Committee. The original specification supported 4*4-Gbps channels (GigE or FC or DDR Infiniband). The variations of the interface supports 4*14-Gbps FDR Infiniband or SAS-3 channels (QSFP14) and 4* 28-Gbps 100 GigE or EDR Infiniband.

267

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Table A.1A (continued) S.N Transceivers Description 8 Tunable XFP Tunable XFP 10-Gbps multiprotocol optical transceiver providing a highspeed serial link from 9.95 Gbps to 11.1 Gbps in conformance with 10-gigabit small form factor pluggable (XFP) multisource agreement (MSA). 9 XGPON 10 Gbps PON specification, wavelength—downstream: 1578 nm ± 3 nm; upstream: 1270 nm ± 10 nm. Data rates: 9.953 Gbps downstream, 2.488 Gbps upstream as per ITU-T G.987 standards. * Does not support hot swapping (SFP supports hot swapping)

1

2

3

S.N S.N

Table A.2 New Generation Fibers Description Hollow core Microstructured cladding with light guided through a h ollow (air) core (which can be filled with gas or particles depending upon the application context). Photonic-crystal fiber Similar to Hollow Core fibers. PCF fibers represent a new class of micro structured optical fibers that guides light rays through structural modifications as opposed to changes in refractive index. The different types includes photonic-bandgap fiber, Holey fiber, and Bragg fiber. Few mode fibers MMF with fiber properties modified to support 2, 4, or up to 10 modes on a single fiber. Few mode fibers are used for spatial division multiplexing. Fiber Type Fiber Type

Acronyms µm µW 3r 4G 5G ADM AEL ADSS Agg ALR ALS AM AMP ANSI AON APC APD APON APR APS ASK ATM

micrometer microwatts reshape, retime, retransmit fourth generation wireless technology fifth generation wireless system add/drop multiplexer accessible emission limit all dielectric self-supporting aggregate automatic laser restart automatic laser shutdown amplitude modulation asynchronous mapping procedure American National Standards Institute active optical networks angled physical contact avalanche photo diode ATM PON automatic power reduction automatic protection switching amplitude shift keying asynchronous transfer mode 269

270

BER BGP BPL BPON BS BSI BTS CAPEX CBR CCAP CCAT CCITT

The ABCs of Fiber Optic Communication

bit error rate Border Gateway Protocol broadband over power lines broadband PON basic switching center British Standardization Institution base transceiver station capital expenditure constant bit rate converged cable access platform contiguous concatenation Consultative Committee on International Telephone and Telegraph CD chromatic dispersion CD/C ROADM colorless directionless contentionless ROADM CDMA code division multiple access CEN European Committee for Standardization CENELEC European Committee for Electrotechnical Standardization CESAR converged edge services access router CMAP converged multiservice access platform CMTS cable modem termination system CO central office CPON composite PON CPRI common public radio interface CST corrugated steel tape CWDM coarse wavelength division multiplexing dB decibel dBm decibel per milliwatt DCM dispersion compensation module DGD differential group delay DIN German Institute for Standardization DOCSIS data over cable service interface specification DRA distributed Raman amplifier DSCM dispersion slope compensation module

Acronyms

DSF DSL DSP DTH DUT DVB-C DWDM E/O E2E e-DCM eDCO EDFA EIA EIGRP EPoC EPON EPON eROADM ESCON ETSI EU FBT FC FC FC/PC FCA FDD FDDI FDM FEC FICON FM FMS FOAS

271

dispersion shifted fiber digital subscriber line digital signal processing direct-to-home device under test digital video broadcasting–cable dense wavelength division multiplexing electro/optical end-to-end electronic dispersion compensation electronic dispersion compensation erbium-doped fiber amplifier Electronic Industries Alliance Enhanced Interior Gateway Routing Protocol EPON over coaxial Ethernet over passive optical network Ethernet PON electronically reconfigurable optical add/drop multiplexer Enterprise Systems Connection European Telecommunications Standards Institute European Union fused or fused biconical taper ferrule core fiber channel ferrule core/physical contact fiber connector angled frequency division duplexing fiber distributed data interface frequency division multiplexing forward error correction Fibre Connection frequency modulation fiber management system future optical access system

272

FOCIS FOTP FPA FR FRR FSAN FSC FSK FTTB FTTD FTTDp FTTF FTTH FTTK FTTN FTTO FTTP FTTx FWM GbE Gbps Gbs GeCl4 GFP GHz GI GI GigE GMP GMPLS GPON GPON GSM GUI

The ABCs of Fiber Optic Communication

fiber optic connector intermateability standard fiber optic test procedure Fabry-Perot amplifiers frame relay fast reroute full service access network fiber-switch capable frequency shift keying fiber-to-the-building fiber-to-the-desktop fiber-to-the-distribution point fiber-to-the-factory fiber-to-the-home fiber-to-the-kerb fiber-to-the-node/neighborhood fiber-to-the-office fiber-to-the-premises fiber to the x four wave mixing gigabit Ethernet gigabit per second gigabits per second germanium tetrachloride generic framing procedure gigahertz galvanized iron graded index gigabit networks generic mapping procedure generalized MPLS gigabit passive optical network gigabit PON Global System for Mobile Communication graphical user interface

Acronyms

HD HDD HDPE HDX HFC Hi-Cap IaDI IC IEC IETF ILA IM IOR IoT IP IP/DOCSIS IPA IR IrDI IS-IS ISO ISP ITU-R ITU-T km kph L2SC LAN-PHY laser LC LD LDP LDPC

high definition horizontal directional drilling high density polyethylene high density digital cross connect hybrid fiber coaxial high capacity intradomain interface integrated circuit International Electrotechnical Commission Internet Engineering Task Force inline amplifier intensity modulation index of refraction Internet of Things Internet Protocol IP over DOCSIS isopropyl alcohol infrared interdomain interface intermediate system to intermediate system International Standards Organization inside plant International Telecommunication Union–Radio Communication Sector International Telecommunication Union– Telecommunication Standardization Sector kilometer kilometres per hour Layer-2 switch capable local area network physical interface light amplification by stimulated emission of radiation lucent/little connector laser diode Label Distribution Protocol low density parity check

273

274

LED LIB LOS low-cap LR LSC LSPM LSR LTE LTS MAC Mbps MBR MCVD MEMS MFD MIC MMF MMF MMF–GI MMF-SI MPE MPLS-TP mps MSC MSPP mW NA NE NE NFPA NFV NG-PON NG-PON2

The ABCs of Fiber Optic Communication

light emitting diode label information base line-of-sight low capacity long reach lambda switch capable laser source and power meter label switched router Long-Term Evolution loss test set media access control megabits per second maximum bit rate modified chemical vapor deposition microelectromechanical systems mode-field diameter media interface connector multimode multimode fibers multimode fiber-graded index multimode fiber–step index maximum permissible exposure multiprotocol label switching–transport profile meters per second mobile switching center multiservice provisionable platform milliwatts numerical aperture (OTN) network element Network equipment National Fire Protection Association network function virtualization next generation passive optical network 40-gigabit-PON

Acronyms

NG-PON2+ NIST nm ns nW NWDM NZ-DSF OADM OAM OC Och OCWR ODF ODFA ODN ODU OEO OEO OFC OFC OFCG OFCP OFCR OFDM OFDR OFN OFNG OFNP OIF OLCR OLT OLTS OMA&P OMS

275

next generation n*gigabit PON National Institute of Standards and Technology nanometer nanosecond nanowatt narrowband wavelength division multiplexing nonzero dispersion shifted fiber optical add/drop multiplexer operations administration and maintenance optical carrier optical channel optical continuous-wave reflectometer optical distribution frame optical doped fiber amplifier optical distribution network optical data unit optical-electrical-optical optical-to-electrical-to-optical optical fiber cable optical fiber, conductive optical fiber, conductive, general use optical fiber, conductive, plenum optical fiber, conductive, riser orthogonal frequency-division multiplexing optical frequency-domain reflectometer optical fiber, nonconductive optical fiber, nonconductive, general use optical fiber, nonconductive, plenum Optical Interworking Forum optical low-coherence reflectometer optical line terminal optical loss test set operations, administration, maintenance and provisioning optical multiplex section

276

ONE ONT ONU O-O OPEX OPM OPU ORL OSP OSPF OTDR OTN OTS OUT OVD P2MP P2P PAL PBB PBB-TE PBB-TE Pbps PC PCS PDH PHY PIC PIC PIN PIU PLC PM PMD POH

The ABCs of Fiber Optic Communication

optical network element optical network terminal optical network unit optical-to-optical operational expenditure optical power meter optical payload unit optical return loss outside plant open shortest path first optical time-domain reflectometer optical transport network optical transmission section optical transport unit outside vapor deposition point-to-multipoint point-to-point phase alternating line provider backbone bridge provider backbone bride–traffic engineering provider backbone bridge traffic engineering peta bits per second physical contact plastic-clad silica plesiochronous digital hierarchy physical layer photonic integrated circuit plug-in-card positive intrinsic negative plug-in-unit planar splitter or planar lightwave circuit phase modulation polarization mode dispersion path overhead

Acronyms

PoS POTS P-OTS PSC PSK PSTN QAM QoS RI ROADM ROW RSVP S/N SAN SBS SC SC SDH SDN SFEC SFF SFP SG-15 SI SiCl4 SLA SMA SMC SMF SNR SOA SOH SOLA

packet over SDH/SONET plain old telephone service packet optical transport service packet switch capable phase-shift keying public switched telephone network quadrature amplitude modulation quality-of-service refractive index reconfigurable optical add-drop multiplexer right-of-way resource reservation protocol signal-to-noise storage area network stimulated Brillouin scattering square connector/stick and click subscriber/standard Siemon standard connector Synchronous Digital Hierarchy software-defined networking super concatenated forward error correction small form factor small form-factor pluggable Study Group 15 step index silicon tetrachloride service-level agreement subminiature A subminiature C single-mode fiber signal-to-noise ratio semiconductor optical amplifier section overhead semiconductor optical laser amplifier

277

278

SONET SPC SPM SRS ST STM SWA TA TCM TDD TDM TDP TE TIA TIR Tribs TriFEC TSA TSB TSI TWA UL UPC UPC UV VAS VCAT VCSEL VFL VLSI VoIP VSR WAN-PHY WDM

The ABCs of Fiber Optic Communication

Synchronous Optical Network super APC self-phase modulation stimulated Raman scattering stab and twist synchronous transport module steel wire armoring terminal amplifier tandem connection monitoring time division duplexing time division multiplexing Tag Distribution Protocol terminal equipment Telecommunication Industry Association total internal reflection tributaries triple forward error correction time slot assigner Telecommunication Standardization Bureau time slot interchange traveling wave amplifier Underwriters Laboratories Inc. ultra polished connector ultraphysical contact ultraviolet value-added service virtual concatenation vertical cavity surface emitting lasers visual fault locator very large scale integration voice-over-IP very short reach wide area network physical interface wavelength division

Acronyms

WDM WDM-PHY WPON WRS WSS WSXC WWDM XGXG(S)-PON+ XGS-PON XGS-PON XPM

wavelength division multiplexing wavelength division multiplexing physical interface WDM PON wavelength-routing switch wavelength selective switches wavelength selective cross connect wideband wavelength division multiplexing 10-gigabit-PON 40-gigabit-PON 10-gigabit symmetrical–PON 40-gigabit symmetrical PON cross phase modulation

279

About the Author Sudhir Warier, entrepreneur, human capability management coach, consultant, author, freelance trainer, and speaker, has more than 22 years of work experience with more than 18 years leading the L&D and human capital management functions for the telecommunication and data networking industries. He has hands-on experience designing, deploying, managing, and troubleshooting state-of-the-art convergent triple play next generation telecommunication networks. His specialization is in the area of engineering next generation photonic backbone networks, and he is a visiting faculty and guest lecturer for some of the leading technology institutions in India. He also serves as member of the board of studies and advisor to leading colleges and polytechnics in India and serves on the Mumbai University external examination panel for post-graduate engineering programs. He has authored 10 books and published more than 30 high-quality research papers in international conferences and peer-reviewed journals. His book Knowledge Management is a best seller and a reference text for university degree and post-graduate courses in management, engineering, and science.

281

Index Attenuation coefficient, estimating, 233 Automatic laser restart (ALR), 105 Automatic laser shutdown (ALS), 105 Automatic power reduction (APR), 105 Avalanche photo diode (APD), 31, 76 Averaging, OTDR, 231

Absolute power measurements, 15, 16 Absorption, 8, 32 Absorption losses, 159 Acceptance angle, 18 Accessible emission limits (AEL), 104 Access layer, 124–25 Accuracy, OTDR, 230–31 Acquisition time, 247 Acronyms, this book, 269–79 Active optical networks (AONs), 202 Add/drop multiplexers (ADMs), 128–30 Aerial cables, 96 American National Standards Institute (ANSI), 258 Amplifiers, 134–35 Angled physical contact (APC), 74–75 Angle of incidence, 9 Architecture DOCSIS/CCAP, 201 IP over DWDM, 183–86 multiplexer/demultiplexer, 131 optical transport networks (OTN), 177, 178–80 passive optical network (PON), 205–7 transport network, 123–26 Attenuation dead zone, OTDR, 231 as fiber selection criteria, 45–46 optical fiber characteristics, 149 splice, 99 as transmission challenge, 158–59

Backreflection, 72–75 Backscatter coefficient, 246–47 Base transceiver stations/basic switching centers (BTS/BSC), 126 Bending diameter, 36 Biconic connector, 58, 59 British Standardization Institution (BSI), 258 Broadband PON (BPON), 204 Cable loss, 236 Cable modem termination system (CMTS), 201 Capital expenditure (CAPEX), 30 Category scale, 13 Channel capacity estimating, 10–12 noiseless channel, 10 noisy channel, 11–12 Chromatic dispersion defined, 21–22, 247 measurement methods, 247 measuring, 248 Cladding diameter, 44 Cladding losses, 22 283

284

The ABCs of Fiber Optic Communication

Cleaning connectors, 109, 223–26 important considerations, 224 overview, 223–24 procedures for, 225–26 standard precautions, 225 steps, 226 techniques, 226 Clock recovery, 76 Collection/aggregation layer, 125–27 Common logarithm, 14 Connectors biconic, 58, 59 cleaning, 109, 223–26 contaminated, 224 defects, 220 E2000, 64–65 epoxy and polish style, 57 ESCON, 61–62 FDDI, 61 ferrule, 58, 59 inspecting, 250 lucent, 61 LX-5, 62 MT, 64 MT-RJ, 62, 63 MU, 64 no-epoxy and no-polish, 57 optijack, 62, 63 overview, 57 plastic fiber optic cable, 60 preloaded epoxy, 57 reflectance, 73 SMA, 59–60 ST, 60 standards, 73 subscriber, 59 types of, 58, 66 visual inspection of, 107 volition, 62, 63 Consultative Committee for International Telephone and Telegraph (CCITT), 119, 122–23 Continuity testing, 249 Converged cable access platform (CCAP), 201–2 Converged edge services access router (CESAR), 201 Converged multiservice access platform (CMAP), 201 Core, 126

Core/clad concentricity, 44–45 Corpuscular theory, 5 Corrugated steel tape (CST), 41 Critical angle, 9 Cross connects defined, 132 functional blocks, 133, 134 key parameters, 133 types of, 132 Cross-phase modulation (XPM), 23, 160 Cutoff wavelength, 46 Data over cable service interface specification (DOCSIS), 198, 200, 201 Dead zone, OTDR, 230 Demultiplexers, 127–32 Dense wavelength division multiplexing (DWDM), 23, 135–36, 143–69 branched network, 169 C band, 157 central frequencies, 150–51 grid wavelengths, 152, 153–55 integrated systems, 144 introduction to, 144–46 IP over, 183–86 L band, 157 links, 167–69 network deployment, tests, and tools, 258 network evolution, 257 network troubleshooting, 256–57 transmission grids, 156, 157 See also Wavelength division multiplexing (WDM) Destination end, 129 Digital mapping, 185 Dimensioning, 34 Directly buried cables, 96–97 Dispersion chromatic, 21–22, 247, 248 defined, 20, 33 as fiber selection criteria, 46 intermodal (modal), 20–21 intramodal, 21 material, 21, 33 polarization mode (PMD), 22, 248 as transmission challenge, 159–60 types of, 33 waveguide, 21 Dispersion compensation fiber (DCF), 47

Index Dispersion compensation modules (DCM), 47, 85, 168 Dispersion shifted fibers (DSF), 39, 47, 136 Distance measurement, OTDR, 232–33 Distance range of operation, OTDR, 229 Distributed feedback (DFB) lasers, 69 Distributed Raman amplifier (DRA), 164 DOCSIS/CCAP architecture, 201 Doped fiber amplifiers (DFA), 163–64 Dynamic range, OTDR, 230 E2000 connector, 64–65 Electrical safety, 101 Electromagnetic spectrum (EM) defined, 4 elements of, 5 measurement reference and units, 4 Electromagnetic theory, 5 Electronics Industries Alliance (EIA), 259 End-to-end (E2E) loss, 250–51 Environmental performance, 47 Epoxy and polish style connectors, 57 Erbium-doped fiber amplifiers (EDFAs), 34, 149, 152, 164 Error detection/recovery, 76 ESCON connector, 61–62 Ethernet passive optical network (EPON), 201 European Telecommunications Standards Institute (ETSI), 258 Extinction ratio, 75 Fabry-Perot (FP) lasers, 68 Fabry-Perot amplifiers (FPA), 163 FDDI connector, 61 Ferrule connector, 58, 59 Fiber and connector cleaning, 109–10 Fiber characterization, 244 Fiber cleaving, 227–28 Fiber curl, 45 Fiber drawing, 93–94 Fiber handling techniques, 104–6 Fiber integrity, 245 Fiber laying techniques aerial cables, 96 directly buried cables, 96–97 horizontal directional drilling (HDD), 97 micro trenching, 96

285 mini trenching, 95–96 overview, 94–95 trenching and ducting, 95 Fiber management systems (FMSs), 99–100 Fiber nonlinearities, 22–23 Fiber optic cable (FOC) coating features, 48 key design parameters, 32 light transmission through, 9 profiles, 37–40 types of, 37 Fiber optics essentials of, 29–50 introduction to, 30–31 Fiber plants cleaning connectors, 223–26 commonly used, 49 deployment, 47–49 installation of, 218 loss measurements, 222–23 optical power measurements, 221–22 OTDR use and, 228–34 splicing techniques, 227–28 troubleshooting, 217–36 visual inspection techniques, 219–21 Fiber route locator, 106–7 Fiberscopes, 220–21 Fiber stripping, 227 Fiber-to-the-home (FTTH) networks, troubleshooting, 255–56 Fire safety, 100–101 Four wave mixing (FWM), 23, 160 Frequency division multiplexing (FDM), 120 Fresnel loss, 22 FTTx networks architecture, 203 defined, 202 troubleshooting, 255–56 variants, 203 Full service access network (FSAM), 202 Fusion splicing, 56, 98–99 Generalized MPLS (GMPLS), 187–88 German Institute for Standardization (DIN), 258 Gigabit-capable PON (GPON), 204–5 Glass fiber, 43 GMP asynchronous mapping procedure (AMP), 181–82

286

The ABCs of Fiber Optic Communication

Graded-index multimode fiber, 40 Helix factor, 247 Hollow fibers, 190 Horizontal directional drilling (HDD), 97 Hybrid fiber coaxial (HFC), 198 Index of refraction (IOR), 246 Injury, causes of, 101 Inside plant (ISP), 37 Intermodal (modal) dispersion, 20–21 International Telecommunication Union - Telecommunication (ITU-T), 257–58 Interval scale, 13 Intramodal dispersion, 21 IP/MPLS optical core networks, 186–87 IP over DWDM architecture advanced ROADM technology, 185 enhanced router interfaces, 184–85 illustrated, 184 overview, 183–84 transparent optical transport with traffic protection, 185–86 Lasers automatic laser restart (ALR), 105 automatic laser shutdown (ALS), 105 automatic power reduction (APR), 105 classes, 102–3, 104 distributed feedback (DFB), 68 Fabry-Perot (FP), 68 LEDs versus, 68 VCSEL versus, 69 vertical cavity surface emitting (VCSELs), 66, 68, 69 Last-mile networks, 199 Light characteristics/behavior of, 5 defined, 5 effects, 8 intensity of, 6 key concepts, 5–7 propagation through different media, 8 properties of, 5–6 speed of, 6, 7 transmission, 7–10 Light emitting diodes (LEDs), 31, 33, 66 lasers versus, 68

long distance communication and, 149 spectral characteristics of, 67 Line decoding, 76 Link loss, 78 Link loss budget key parameters, 78 power budget and margin computations, 80–82 receiver sensitivity and dynamic range, 79 span analysis, 82–85 transmitter launch power, 79 Logarithmic scales, 13–15 Logarithm rules, 17 Long haul network certification chromatic dispersion, 247–48 E2E loss measurement, 243–45 fiber integrity, 245 general guidelines, 243 optical return loss (ORL), 245–47 polarization mode dispersion (PMD), 248 postinstallation tests, 243 preinstallation checks, 243 test description, 243–48 testing and, 242–48 Loose-tube cable, 40–41 Loss measurements, 222–23 Lucent connector, 61 LX-5 connector, 62 Manufacturing, optical fiber, 93–94 Material dispersion, 21, 33 Materials safety, 100 Maximum bit rate (MBR), 11–12 Maximum permissible exposure (MPE), 101–3 Mechanical splicing, 56–57 Media, transport networks, 135–37 Micro trenching, 96 Microwave links, 136–37 Mini trenching, 95–96 Mobile switching centers (MSCs), 126 Modal propagation, 19–20 Mode-field diameter, 46 Modified chemical vapor deposition (MCVD), 93 Modular optical interfaces, 69–72 MT connector, 64 MT-RJ connector, 62, 63

Index MU connector, 64 Multimode fibers (MMF) cable classifications, 39–40 classes of, 39 core diameter, 190 cross-sectional view, 36 defined, 34, 39 graded-index, 40 profiles, 38 step-index, 39 Multimode graded index, 20 Multimode step index, 19–20 Multiplexers bidirectional connection, 132 classifications, 128 connection types, 129, 131 functional architecture, 131 linear configuration, 129 point-to-point configuration, 128 ring configuration, 129, 130 types of, 127 unidirectional connection, 131 Multiplexing multistage, 181–82 as primary transport network function, 127 Multiprotocol label switching (MPLS) components, 186 defined, 186 generalized (GMPLS), 187–88 integration of technologies, 187 transport profile (MPLS-TP), 187, 189 Multiservice provisionable platform (MSPP), 184 Multistage multiplexing, 181–82 National Fire Protection Association (NFPA), 42–43 Natural logarithm, 14 Network components amplifiers, 134–35 cross connects, 132–34 multiplexers and demultiplexers, 127–32 regenerators, 135 See also Transport networks Network diagnostic techniques fiber and connector cleaning, 109–10 fiber route locator, 106–7 optical power measurements, 107–8

287 OTDR - working, 108–9 overview, 106 power measurements, 108 visual connector inspection, 107 Network topologies, 137 New generation fibers, 268 Next Generation Passive Optical Network (NG-PON), 208 No-epoxy and no-polish connectors, 57 Noiseless channel, 10 Noisy channel, 11–12 Nonlinearities, 22–23, 160–61 Nonzero-dispersion shifted fibers (NZ-DSF), 39, 48–49 Numerical aperture (NA), 18 Nyquist theorem, 10 OM1, 39 OM2, 39 OM3, 39 OM4, 39 Operational expenditure (OPEX), 30 Operations, administration, maintenance, and provisioning (OAM&P) costs, 118 input and equipment, 119 Optical access networks, 202–8 broadband access, 199–201 converged cable access platform (CCAP), 201–2 DOCSIS, 200 introduction to, 198–99 summary, 208–9 Optical add/drop multiplexers (OADMs), 166–67 Optical amplifiers types of, 164 wavelength division multiplexing (WDM), 163–64 channel capacity estimation, 10–12 dispersion and, 20–22 electromagnetic spectrum and, 4–5 fiber nonlinearities and, 22–23 fundamentals of, 3–24 light concepts and, 5–7 light propagation modes and, 17–20 light transmission and, 7–10 optical power measurements, 15–17 scales and, 12–15

288

The ABCs of Fiber Optic Communication

Optical communication (continued) summary, 23–24 techniques, 92 Optical distribution frames (ODFs), 99, 251 Optical fiber attenuation characteristics, 32, 149 bending diameter, 36 classification, 37–40 cleaning, 109–10 composition, 43–44 construction of, 30 cross section, 32 designs, 40–42 design specifications, 31–36 dimensioning, 35 fiber drawing, 93–94 geometry, 44–45 handling techniques, 104–6 laying techniques, 94–97 light propagation modes in, 17–20 longevity, 36 loose-tube cable, 40–41 manufacturing of, 93–94 as preferred medium, 92–93 propagation modes, 34–35 selection criteria, 45–47 splices and connectors, 56–65 strength, 36 termination, 99–100 testing, 93, 94 tightly buffered cable, 41, 42 transmission losses, 32–33 transmission of light through, 9 transmission wavelengths (windows), 33–34 Optical fiber cables (OFCs) connecterization issues, 253 damaged, 252 fiber termination, 99–100 general guidelines, 253–54 general issues, 252 preparation, 98 splicing, 98 stretched, 252–53 troubleshooting, 251–55 use of, 92 Optical fiber loss (OFL) defined, 245 measurement steps, 246–47 with OTDR, 246

tools for measuring, 245 Optical modulation techniques, 77–78 Optical patch cords, 254–55 Optical power measurements, 15–17, 107–8 absolute, 221 fiber plants, 221–22 illustrated, 222, 244 procedure for, 222 summary, 235–36 Optical power meter, 108 Optical receivers defined, 76 functions of, 76 photodiodes, 76–77 sensitivity and dynamic range, 79 wavelength division multiplexing (WDM), 162–63 Optical safety, 101 Optical sources, 66–69 Optical system, block diagram, 31 Optical termination points, troubleshooting, 248–51 Optical time domain reflectometer (OTDR) accuracy, 230–31 attenuation coefficient, estimating, 233 attenuation dead zone, 231 averaging, 231 in checking splicing losses, 218 dead zone, 230 defined, 108 distance measurement, 232–33 distance range, 229 dynamic range, 230 link access, 109 operational parameters, 229–31 operation of, 108–9, 228–29 optical fiber loss (OFL) measurement, 246 precautions, 232 problem detection, 229 pulse width, 231 sample test settings, 234 testing prerequisites, 231–32 testing procedure, 232 trace analysis, 232, 233 using, 93, 228–34 Optical transceivers, 267–68 Optical transmission attenuation, 158–59 challenges, 158–61

Index dispersion, 159–60 linear characteristics, 158–60 nonlinearities, 160–61 Optical transmission bands channel bandwidth, 151 illustrated, 150 lambda capacity, 156 Optical transmission windows, 150 Optical transmitters backreflection, 72–75 defined, 65 extinction ratio, 75 key parameters, 72–76 launch power, 79 modular optical interfaces, 69–72 optical sources, 66–69 overview, 65–66 wavelength division multiplexing (WDM), 162 Optical transport networks (OTN), 152, 177–83 advantages of, 182–83 architecture, 177, 178–80 bit rates, 178 control plane, 179 defined, 177 hierarchy, 177–78 integration, 177 interfaces, 180–81 internal switching support, 182 key features, 181–83 line rates, 179 next generation architecture illustration, 188 optical entities, 180 packet, 187–90 standards, 178 Optics, 6 Optijack connector, 62, 63 Outside plant (OSP), 37 Outside vapor deposition (OVD), 93 Packet optical evolution, 189 Packet optical networking platforms (PONP), 189 Packet optical transport service (P-OTS), 187, 189 Packet transport network (PTN), 137 Passive optical network (PON) architecture, 205–7

289 architecture illustration, 207 broadband (BPON), 204 FTTH networks, troubleshooting, 256 functioning, 205–7 gigabit-capable (GPON), 204–5 localizing network faults in, 256 next generation, 208 passive elements, 205–7 technologies, 197 types, 206 Passive routers, 165 Payload interchange, 133 Photometry, 6 Photonic transport networks. See Transport networks Physical contact (PC), 74 Plastic-clad silica fiber, 44 Plastic fiber, 43–44 Plastic fiber optic cable connector, 60 Plesiochronous digital hierarchy (PDH) networks, 118, 136 Point-to-multipoint (P2MP) architecture, 204 Point-to-point (P2P) architecture, 204 Polarization mode dispersion (PMD), 22, 248 Polishing incorrect, defects due to, 220 proper, confirming, 235 specifications, 61, 74–75 Power budget computations, 80–82 Power measurements absolute, 15, 16, 221 optical, 15–17 with optical power meter, 108 relative, 221 short haul segments/networks/endfaces, 250 Preform manufacturing, 93 Preloaded epoxy connectors, 57 Propagation modes, 34 Provider backbone bridge traffic engineering (PBB-TE), 189 Pulse width, 231, 247 Quality of service (QoS), 30 Quantum theory, 5 Radiometry, 6

290

The ABCs of Fiber Optic Communication

Reconfigurable optical add/drop multiplexers (ROADMs), 166–67 Reflection, 8 Refraction, 8 Refractive index (RI) defined, 7 of medium, 8 Regenerators types of, 165 use of, 135 wavelength division multiplexing (WDM), 164–65 Return loss, 74 Safety accessible emission limits (AEL), 104 electrical, 101 fiber handling techniques and, 104–6 fire, 100–101 guidelines, 100–106 injury causes, 101 materials, 100 maximum permissible exposure (MPE) and, 101–3 optical, 101 standards, 42–43 Scales category, 13 defined, 10 interval, 13 logarithmic, 13–15 sequence, 13 Scattering, 8, 32 Scattering losses, 159 SDH/SONET networks, 143, 144 OTN comparison, 182–83 OTN integration, 177 standards, 145 use of, 176 Self-phase modulation (SPM), 23, 160 Semiconductor optical amplifier (SOA), 163 Sequence scale, 13 Service level agreements (SLAs), 118 SFP+ modules, 71–72 SFP modules, 70, 71 Short haul segments, troubleshooting, 248–51 Single-mode fiber (SMF), 19, 34 classes of, 38–39 cross-sectional view, 36

Single-mode fibers (SMF) defined, 37 profiles, 38 Single-mode step index, 19 SMA connector, 59–60 Snell’s Law, 7 Source end, 129 Space division multiplexing (SDM), 189–90 Span analysis, 82–85 Speed of light, 6, 7 Splicing attenuation, 99 fiber cleaving and, 227–28 fiber stripping and, 227 fusion, 56, 98–99 mechanical, 56–57 techniques, 227–28 Standardization bodies, 257–59 Standard SMF, 39 ST connector, 60 Steel wire armouring (SWA), 41 Step-index multimode fiber, 39 Stimulated Brillouin scattering (SBS), 22, 161 Stimulated Raman scattering (SRS), 23, 160–61 Storage area networks (SANs), 143 Subscriber connector, 59 Super physical contact (SPC), 74 Synchronous digital hierarchy (SDH), 117, 136 Synchronous optical network (SONET) defined, 117 evolution of, 121–23 network element models for, 119 optical base carrier rate, 123 standards, 122–23 See also SDH/SONET networks Tandem connection monitoring (TCM), 183 Telecommunications Industry Association (TIA), 258–59 Terminal multiplexers (TMs), 128, 129 Testing continuity, 249 long haul networks, 242–48 in manufacturing optical fiber, 94 optical patch cords, 254 OTDR, 231–32

Index short haul segments/networks/endfaces, 249 Tightly buffered cable, 41, 42 Time division multiplexing (TDM), 120, 136, 144, 176 Time slot interchange (TSI), 133 Total internal reflection (TIR), 9, 10 Trace analysis, OTDR, 232, 233 Tracing, 219 Transmission losses, 32 Transmission wavelengths, 33–34 Transport networks abstract view, 124 access layer, 124–25 analogy, 125 architecture, 123–26 collection/aggregation layer, 125–27 components, 126–37 core, 126 introduction to, 118 layer functions, 126 media, 135–37 multiplexing, 127 needs, benefits, and function, 120–21 network equipment, 127–35 overview of, 118–20 SONET, 117, 119, 121–23 summary, 137–38 topologies, 137 Traveling wave amplifiers (TWA), 163 Trenching and ducting, 95 Troubleshooting cabling and connecterization issues, 251–55 DWDM networks, 256–57 fiber plants, 217–36 FTTH networks, 255–56 long haul networks, 242–48 optical patch cords, 254–55 optical termination points, 248–51 short haul segments, 248–51 Ultraphysical contact (UPC), 74 Vertical cavity surface emitting lasers (VCSELs), 66, 68, 69 Video fiberscopes, 220 Visual connector inspection, 107 Visual fault locator (VFL), 219

291 Visual inspection fault location, 219 fiberscopes, 220–21 summary, 235 techniques, 219–21 tracing, 219 See also Fiber plants Volition connector, 62, 63 Waveguide dispersion, 21 Waveguide losses, 159 Wavelength defined, 6 illustrated, 6 transmission, 33–34 of waveform, 6–7 Wavelength counters, 165–66 Wavelength division multiplexing (WDM), 38, 143–44 classification, 147 couplers, 163 defined, 145, 146 evolution of, 148 fundamentals, 148–55 guardband in, 155 illustrated, 146 network components, 162–67 optical amplifiers, 163–64 optical receivers, 162–63 optical transmitters, 162 passive routers, 165 proprietary grids, 157 regenerators, 164–65 standardization, 147–48 technologies, 207–8 wavelength add/drop multiplexers (WADMs), 166–67 wavelength counters, 165–66 wavelength selective switches (WSS), 165 See also Dense wavelength division multiplexing (DWDM) Wavelength selective switches (WSS), 165 Wave-particle duality, 6 Wave theory, 5 XFP modules, 70–71

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