Troubleshooting optical-fiber networks

Troubleshooting Optical-Fiber Networks

Second Edition

Troubleshooting

Optical-Fiber Networks

Understanding and Using Your

Optical Time-Domain Refl ectometer

Second Edition

Duwayne R. Anderson

Principal Engineer

Larry Johnson

President, The Light Brigade, Inc.

Florian G. Bell, Ph.D.

Production Manager, MiniMitter, Inc.

Amsterdam Boston Heidelberg London New York Oxford

Paris San Diego San Francisco Singapore Sydney Tokyo

Elsevier Academic Press

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vii

Foreword............................................................................................ xiii

Chapter 1Early developments..................................................................1

1.0 Introduction ..............................................................................1

1.1 The birth of the OTDR..............................................................2

1.2 Features, functions, and performance improvements ............4

1.3 Summary...................................................................................9

Suggested reading...................................................................11

Chapter 2 Fundamentals of fi ber optics .................................................13

2.0 Introduction ............................................................................13

2.1 Total internal refl ection ..........................................................13

2.2 Fiber attenuation.....................................................................19

2.3 Numerical aperture.................................................................23

2.4 Multipath (modal) dispersion ................................................24

2.5 Chromatic dispersion .............................................................28

2.6 Wavelength-division-multiplexed systems ............................35

2.7 V-parameter, cut-off wavelength, and spot profi le.................36

2.8 Splices and connectors...........................................................42

2.9 Bending loss............................................................................48

2.10 Coherence ...............................................................................52

2.11 Summary.................................................................................55

Suggested reading...................................................................57

Problems..................................................................................57

Chapter 3 Fundamentals of OTDR operation ........................................59

3.0 Introduction ............................................................................59

3.1 OTDR design...........................................................................62

3.2 A typical OTDR waveform .....................................................63

3.3 Multiple-wavelength OTDRs..................................................65

3.4 Optical masking......................................................................67

3.5 Evolution of the OTDR ...........................................................69

3.6 Rayleigh backscatter ...............................................................72

3.7 Dynamic range ........................................................................74

3.8 Differences between multimode and single-mode OTDRs ...78

3.8.1 Physical differences...........................................................81

3.8.2 Refl ectivity differences between single-mode and

multimode fi ber connectors ..............................................81

3.8.3 Differences in backscatter levels.......................................82

3.8.4 Optical-power differences .................................................83

Contents

viii Contents

3.8.5 Pulse-width differences .....................................................83

3.8.6 Operating-wavelength differences ....................................84

3.9 Echoes and ghosts...................................................................87

3.10 Other types of OTDR confi gurations......................................91

3.10.1 Standard OTDR with an EDFA..........................................91

3.10.2 Coherent-detection OTDR .................................................93

3.10.3 Correlation OTDR ..............................................................95

3.10.4 Short-coherence-length coherent OTDRs (multimode)....96

3.10.5 Photon-counting OTDRs....................................................98

3.11 Summary.................................................................................99

Suggested reading.................................................................100

Problems................................................................................101

Chapter 4 Performance characteristics of OTDRs...............................103

4.0 Introduction ..........................................................................103

4.1 Figures of merit.....................................................................103

4.2 Historical fi gures of merit.....................................................105

4.3 Detailed fi gures of merit .......................................................107

4.3.1 Refl ective dynamic range ................................................108

4.3.2 Scattering dynamic range................................................109

4.3.3 Derivation of the noise fl oor............................................115

4.3.4 Dynamic-range margin ....................................................121

4.3.5 Usable dynamic range .....................................................123

4.3.6 Measurement range .........................................................124

4.3.7 How offset errors affect dynamic-range and

measurement-range calculations.....................................125

4.3.8 Event resolution...............................................................135

4.3.9 Loss-measurement resolution .........................................137

4.3.10 Return loss .......................................................................139

4.3.11 Linearity ...........................................................................140

4.3.12 Data resolution.................................................................141

4.3.13 Clock accuracy.................................................................143

4.3.14 Cursor resolution .............................................................144

4.3.15 Refractive-index uncertainty...........................................144

4.3.16 Speed of measurement ....................................................146

4.3.16.1 Fiber length ..............................................................146

4.3.16.2 Interleaving ..............................................................147

4.3.16.3 Data-processing speed .............................................147

4.3.16.4 Display speed...........................................................148

4.3.17 Event-detection accuracy ................................................148

4.4 Standards ..............................................................................149

4.5 Summary...............................................................................150

Suggested reading.................................................................151

Problems................................................................................151

Contents ix

Chapter 5Measuring nonrefl ective events ..........................................153

5.0 Introduction ..........................................................................153

5.1 Sources of nonrefl ective events............................................153

5.2 Cursor placement for manual loss and

distance measurements ........................................................156

5.3 Distance-measurement errors of nonrefl ective events ........158

5.3.1 Distance-measurement errors caused by waveform

noise .................................................................................158

5.3.2 Distance-measurement errors caused by fi ltering ..........167

5.3.3 Other contributions to distance-measurement errors ....170

5.4 Summary ..............................................................................176

Suggested reading.................................................................177

Problems................................................................................178

Chapter 6 Loss-measurement error......................................................179

6.0 Introduction ..........................................................................179

6.1 Loss-measurement errors caused by waveform noise .........179

6.2 Loss-measurement errors due to mismatch of

single-mode fi bers.................................................................185

6.3 Loss-measurement errors on multimode fi ber ....................190

6.4 Bending loss and stress loss in single-mode fi bers..............196

6.5 Wavelength-dependent loss in fusion splices and

connectors between single-mode fi bers ..............................198

6.6 Intrinsic loss in single-mode fusion splices.........................200

6.7 Summary...............................................................................205

Suggested reading.................................................................206

Problems................................................................................206

Chapter 7 Measuring refl ective events.................................................209

7.0 Introduction ..........................................................................209

7.1 Background ...........................................................................209

7.2 Causes of refl ection...............................................................212

7.2.1 Refl ections from angled cleaves ......................................212

7.2.2 Refl ections from connectors............................................214

7.2.3 Refl ections from mechanical splices...............................220

7.3 Measuring refl ective events using an OTDR........................221

7.4 Effects that can reduce the accuracy of

refl ectivity measurements ....................................................226

7.5 Calibrating the OTDR’s backscatter coeffi cient ...................229

7.5.1 Calculating the backscatter coeffi cient ...........................230

7.5.2 Measuring the backscatter coeffi cient.............................233

7.5.2.1 Measuring the backscatter coeffi cient with a

calibrated refl ector...................................................233

x Contents

7.5.2.2 Measuring the backscatter coeffi cient using

a CWR ......................................................................235

7.5.2.3 Measuring the backscatter coeffi cient

using an OTDR.........................................................237

7.6 Integrated return loss............................................................240

7.7 Summary...............................................................................245

Suggested reading.................................................................247

Problems................................................................................247

Chapter 8 Complications caused by refl ective events.........................249

8.0 Introduction ..........................................................................249

8.1 Refl ections and the dead zone..............................................249

8.2 Improving the dead zone by optical masking......................252

8.3 Impact of refl ections on transmitters and receivers ............257

8.4 Dead zone box.......................................................................263

8.5 Summary...............................................................................265

Suggested reading.................................................................265

Problems................................................................................265

Chapter 9 Measuring the numerical aperture and

mode-fi eld diameter of single-mode fi ber...........................267

9.0 Introduction ..........................................................................267

9.1 The far-fi eld scanning method of determining

numerical aperture ...............................................................268

9.2 Measuring the numerical aperture of single-mode fi ber

using an OTDR......................................................................272

9.3 Measuring the cut-off wavelength of a single-mode fi ber

using an OTDR......................................................................275

9.4 Summary...............................................................................277

Suggested reading.................................................................277

Problems................................................................................278

Chapter 10 Analyzing passive networks containing

splitters and couplers ..........................................................279

10.0 Introduction ..........................................................................279

10.1 Determining the locations of breaks in systems

containing splitters ...............................................................284

10.2 OTDR requirements for testing networks with splitters .....287

Suggested reading.................................................................289

Problems................................................................................290

Chapter 11 Automatic event-marking algorithms and calibration ......291

11.0 Introduction ..........................................................................291

11.1 Types of event markings .......................................................291

Contents xi

11.2 Functionality of different types of event markings..............293

11.3 Optimizing acquisition parameters......................................298

11.4 Measuring individual events................................................304

11.5 Testing event-marking software ...........................................312

11.5.1 Test-fi ber calibration........................................................312

11.5.2 Repeatability testing ........................................................314

11.5.3 Summary of testing event-marking software..................317

11.6 Event-marking features.........................................................317

11.7 Remote OTDRs for monitoring networks.............................321

11.8 Summary...............................................................................325

Suggested reading.................................................................326

Problems................................................................................327

Chapter 12 Test fi xtures...........................................................................329

12.0 Introduction ..........................................................................329

12.1 Dead zone fi xture..................................................................329

12.2 Fiber circulator......................................................................330

12.3 External-source test fi xture ..................................................337

12.4 Loss calibration with fi ber standard.....................................338

12.5 Summary...............................................................................340

Suggested reading.................................................................341

Problems................................................................................342

Chapter 13 Polarization mode dispersion .............................................343

13.0 Introduction ..........................................................................343

13.1 Measurement techniques .....................................................346

13.2 PMD model of optical fi ber...................................................349

13.3 Mathematical model of a polarization OTDR......................353

13.4 Summary...............................................................................357

Problems................................................................................358

Chapter 14 Dispersion in optical fi bers .................................................359

14.0 Introduction ..........................................................................359

14.1 Intermodal dispersion...........................................................361

14.2 Intramodal dispersion ..........................................................364

14.2.1 Material dispersion..........................................................364

14.2.2 Waveguide dispersion......................................................366

14.3 Dispersion reduction in optical fi bers..................................368

14.4 Measuring dispersion using an OTDR .................................369

14.5 Measuring chromatic dispersion using

multiple-wavelength OTDRs................................................372

14.6 Summary...............................................................................375

Suggested reading.................................................................376

Problems................................................................................377

xii Contents

Chapter 15 Considerations when selecting an OTDR ...........................379

15.0 Introduction ..........................................................................379

15.1 Durability ..............................................................................380

15.2 Display and controls.............................................................382

15.3 Human interface ...................................................................386

15.4 Optical port ...........................................................................387

15.5 Accessories, options, and features .......................................394

15.6 Safety.....................................................................................396

15.7 Performing a fi ber-acceptance test .......................................399

15.7.1 Sequential steps of an acceptance test............................400

15.8 Measuring the splice attenuation.........................................402

15.8.1 Corrective actions ............................................................403

15.9 When should OTDR traces be taken? ..................................404

15.10 Span measurements..............................................................405

15.10.1 What measurements are required? ................................405

15.11 Field technician’s top-ten list ...............................................406

15.12 Summary...............................................................................407

Problems................................................................................407

Glossary of Terms...............................................................................409

Mathematical Glossary ......................................................................425

Answers to Problems .........................................................................431

Index...................................................................................................433

xiii

Two of us (Anderson and Bell) wrote the fi rst edition of this book,

titled Optical Time-Domain Refl ectometry, published in 1997. At the time

we were engineers at Tektronix, in the company’s Cable and Network

Analysis Division. In 1997, Tektronix was a major producer of optical

time-domain refl ectometers (OTDRs), and our book was the fi rst single￾source handbook describing OTDRs. Tektronix sponsored our work in

writing the book, and they published it and distributed it worldwide.

At the time, worldwide investment in fi ber-optic networks was

expanding at double-digit rates, and Tektronix was one of the top

manufacturers of OTDRs, responsible for both their early innovation

and the development of several key technologies. Neither of us could

have anticipated the dramatic changes that would take place in the next

six years. While fi ber optics proceeded at a steady and respectable pace

during most of the 1990s, the industry exploded in irrational exuberance

near the turn of the century as part of the telecom bubble, only to

be followed by a deep depression that would see many companies,

including Tektronix, abandon key business sectors devoted to optical

telecom by the end of 2003.

Through these tumultuous times, the OTDR continued its

remarkable evolution, resulting in today’s machines, which are

considerably smaller, faster, more capable, and less expensive than their

predecessors. This evolution has been driven largely by the widespread

use and deployment of optical fi ber and the need to put high-powered

test capability into the hands of the optical technicians that engage daily

in installing, maintaining, and repairing these networks. The days when

optical engineers do fi eld tests with bulky $35,000 OTDRs are gone

forever.

It is because of the dramatic changes in the markets and the

science of OTDRs that this second edition is needed. Furthermore, the

widespread use of OTDRs necessitates publishing and distributing the

book in a manner that makes it more widely available to fi eld technicians

as well as engineers. These changes also demand a new title, one that

refl ects a broader theme and goes beyond simply testing with OTDRs

to address additional issues surrounding fi ber-optic networks in general

and how to keep them working properly.

Foreword

xiv Foreword

Because OTDRs have evolved from diffi cult-to-use, specialized

equipment to ubiquitous mainstay status, we found the book required

contributions from a third author. The obvious choice was Larry Johnson,

owner and founder of The Light Brigade. Larry’s company is a leading

training organization with nearly two decades of experience teaching

technicians how to use OTDRs in the fi eld to install, maintain, and repair

fi ber-optic networks. He has an intimate knowledge of the challenges

and needs of the technician that are unmatched in the industry, as well

as a high degree of familiarity with different OTDR manufacturers, their

products, and related fi ber-optic standards.

As with the fi rst edition, our objective in writing this book is to go

beyond a simple handbook that discusses operational procedures such

as cursor placement and waveform interpretation. While we do cover

these topics, we also explain more esoteric subjects and offer insight

into the technical nuances of OTDRs that enable nontraditional OTDR

measurements, such as mode-fi eld diameter, cut-off wavelength, and

polarization mode dispersion. We also give considerable attention to

measurement errors and how to estimate them. This, especially, is a

subject with which all OTDR users should be quantitatively familiar. As

a part of this, we have devoted an entire chapter (11) to event-marking

software, which was the key innovation that resulted in OTDRs being

readily used by fi eld technicians.

Our target audience comprises practicing engineers, system

technicians, and fi eld technicians. We have constructed each chapter

so that, for the most part, simpler concepts and ideas are discussed fi rst,

followed by concepts of increasing diffi culty as the chapter progresses.

The level of diffi culty varies. When this happens, we try to warn the

reader that the level of diffi culty may rise for a while but subside in

succeeding sections. We have also added a short quiz at the end of each

chapter so that the book can be used in the classroom as a text or for

self-study.

We make no apologies for equations and mathematical detail,

which are used liberally throughout. These are intended to provide

quantitative support for many of the book’s central points. Though we

use them unabashedly, most of the equations are summaries, with few

derivations, and involve only algebra or elementary calculus. Where

appropriate, we use diagrams and text that illustrate ideas embedded

within the equations so that full command of the mathematics is not

necessary to obtain an intuitive feel for the subject being discussed.

Foreword xv

For the most part, the book can be read and the key points understood

without having to understand the mathematical equations. For those

who want to delve a little deeper, however, the mathematics opens the

door to understanding OTDRs at a level of detail that should appeal to

design engineers.

We hope this book serves as a useful desk reference and provides

value even if not read cover to cover. Toward this end, we have included

a detailed table of contents, a glossary of terms, a mathematical glossary,

and a detailed index.

One of the most diffi cult aspects of writing this book was

researching the appropriate references. OTDRs are a relatively new

type of test instrument, and references abound regarding their early

development and use. The diffi culty has primarily been in searching

vast databases in an attempt to determine fi rst usage. Additionally,

the proprietary nature of much of the information makes it diffi cult to

assign credit to the originators. Although we have made a concerted

effort to do so, there are undoubtedly instances where we have failed to

reference the original author. In such cases, we offer our apologies, and

we welcome suggestions and comments that might be included in future

editions of this book.

Too many people have helped us write this book to name them

all here, but some have made contributions for which particular thanks

and recognition is warranted. We are especially grateful for the technical

reviews provided by Mark Lund, William Trent, Mark Marineau, Mehrdad

Givehchi, Raza Ahmed, Ronald Larrick, Kenneth Ditto, Matthew

Harcourt, Morris Anderson, Brandon DuRette, Robert Cook, Robert Jahn,

Peter Schweiger, and Peter Lovely. Without the technical advice and

excellence of the engineering staff at Tektronix, CNA, this book would

not have been possible. Thanks especially to those who spent endless

hours in technical conversation about the details of OTDR operation:

Richard Lane, Frank Borden, Glenn Bateman, Kevin McDonald, Bob

McMahon, Doug Rasmussen, and Ken Coulson. As with any book,

responsibility for technical accuracy lies with the authors. Without the

association of these very talented scientists and engineers, however, this

task would have been unapproachable. In addition, special thanks to Dr.

Mel Holzman, Dave Bartlett, and the late Ellis Dupuy for opening many

doors with their mentoring and early work in fi ber optics. Special thanks

also to Harvey Jauvtis, who provided much of the material for chapter

10, and to Matthew Diessner, who has added to our insight regarding

xvi Foreword

customer applications, and Gina Lynd, who organized and coordinated

the content of the book for editing.

Finally, we thank our publisher and especially Charles Glaser for

his help, encouragement, and professional demeanor in bringing this

project to fruition.

Duwayne Anderson

Larry Johnson

Florian Bell