Time domain reflectometry techniques for optical communications

UNIVERSITY OF SOUTHAMPTON DEPARTMENT OF ELECTRONICS

TIME-DOMAIN REFLECTOMETRY TECHNIQUES FOR OPTICAL COMMUNICATIONS

BY

ALLEN JAMES CONDUIT

A thesis _submitted for the degree of Doctor _{of-Philosophy}

UNIVERSITY OF SOUTHAMPTON ABSTRACT

FACULTY OF ENGINEERING & APPLIED SCIENCE ELECTRONICS

Doctor _of Philosophy

TIME-DOMAIN REFLECTOMETRY TECHNIQUES FOR OPTICAL COMMUNICATIONS

Time Domain Reflectometry Techniques _{are applied} to measurements in optical fibres. The study covers broadly

two areas of interest.

Firstly, the Backscatter Technique for _attenuation

measurements is considered. The resolution requirements of instrumentation _{are studied and a new 2-channel approach to} backscatter _{waveform analysis} is _{proposed. An optimum}

operating strategy is described such that the sensitivity

and range are maximized. The fundamental accuracy of the backscatter _measurement is determined _{by a comparison with} the _{more-standard cutback} technique _{over a wide spectral}

range and variations in OH- impurity _along the length of a fibre _{are tracked by measurements at different wavelengths.}

The effects on the backscattered _power of variations in longitudinal fibre _{parameters are also} demonstrated. In

particular, backscatter-loss _{signatures are} presented which clearly _{show correlation with programmed} fibre defects and

indicate _{that in many cases diameter variations are the}

cause _of the _{previously unidentified} features in some back- scatter waveforms. In addition, the backscatter method is used for the first time to track the _{state of polarization}

of _{a pulse propagating} in _{a monomode} fibre.

The second _{application of Time Domain} Reflectometry is to the _{assessment of pulse delay stability against variations}

in temperature _{and external stresses. Data} from _measurements on unjacketed fibres is _{used to analyse} the time delay

variations found in jacketed fibres. It is _{shown that} the application _{of a close-fitting plastic} jacket results in a level _{of residual compressive stress} in _the fibre _{and the}

ACKNOWLEDGMENTS

I would like to _sincerely thank Professor Gambling _and the _{members of the} Optical Communications _group for the _warm welcome which was afforded me as a visitor to the U. K. and I am grateful for the support and encouragement received

throughout the _{course of my studies.}

I would especially like to thank Dave Payne for the many discussions, _{suggestions and constructive criticisms of} the

work. Also to Arthur Hartog with whom I shared measurements with the dye-laser system and the experiment on pulse delay

stability. Arthur's contribution is very much appreciated.

Many thanks _are due also to John Hullett from the University

of Western Australia for his collaboration in early work on instrument limitations.

Thanks _{also to Francis Sladen, Steve Norman, Bob Mansfield,} Max Hadley _{and Eleanor Tarbox} for _{the many discussions and/or}

their _efforts in'fabricating fibres for these _experiments.

I am grateful to Jan Ditchfield _{and Nicky} Pink for their many pages of typing _{and to Chris} Nash for his immaculate

draughting. _{I also wish to thank Roy Came and the mechanical} workshop _staff for their _{advice and craftmanship.}

I wish _{to acknowledge} the _{support of my parents and family} over _{many years.} Special _{thanks are} due to Sheila for her

help _{and encouragement (and most of all for just being there!} ). Finally _{I am grateful to the British Council and the}

Association _{of Commonwealth Universities} for _{the scholarship}

CONTENTS

page no.

Chapter 1 _Introduction 1

1.1 _{The Backscatter Technique 2}

1.2 _{Pulse Delay Measurements 4}

1.3 _{Notes on General Layout of Thesis} 4

Chapter 2 Theoretical Background _of the Backscatter

Technique 6

2.1 _{Loss Mechanisms} in _Optical 2.1.1 _Absorption

2.1.2 _Radiation

2.2 _{Rayleigh Scattering}

2.2.1 _{Rayleigh attenuation} 2.2.2 Origin _{of refractive}

2.2.3 Rayleigh _{radiation p,}

Fibres 7

7 8 8 10

index fluctuations 11

attern 12

2.3 Differential _{Mode Attenuation (DMA) and the}

Equilibrium _{Mode Distribution} (EMD) 13

2.4 The Backscatter-Characteristic ₁₄

2.4.1 _{Impulse response - 14}

2.4.2 _{Response to finite input pulse 16}

2.4.3 _{Relative backscatter power 17}

2.5 _{Determination of Local Fibre Attenuation 17}

2.6 _{Determination of Loss at a Fault Point 19}

2.7 _{The Backscatter Factor for Multimode Fibres 20}

2.7.1 _{Approximation 20}

2.7.2 _{Generalization 21}

2.7.3 _{Assignment of typical radial characteristics}

to fibre _parameters 21

2.7.3.1 _{Core grading 22}

2.7.3.2 _{Modal energy distribution 22}

2.7.3.3 _{Local numerical aperture} 23

2.7.3.4 _{Average scattering loss} 23

2.7.4 _{Backscatter factor for power law profiles 24} 2.8 _{Errors in Attenuation Measurement Caused by}

Variations _{in the Backscatter Factor 25}

2.8.1 _{Dependence on NA and scattering loss for}

germano-silicate fibres 25

2.8.2 _{Long wavelength measurements 26}

2.8.3 _{Effect of non-linear scattering mechanisms 26}

2.9 Su=ary ₂₇

Figures 2.1-2.4

Chapter 3 _{Theory of Measurement Limitations and the}

Ultimate _{Range 28}

3.1 _{Single Channel Sampling, Boxcar Integration and}

Numerical Averaging ₂₈

3.2 _{Two Channel Sampling 30}

3.3 _{The Optimum Operating Strategy and Measurement}

Accuracy Requirements ₃₂

3.3.1 _{The attenuation measurement error 32}

'3.3.2 _{optimum sample separation 33}

3.3.3 _{Dynamic range utilization 34}

3.3.4 _{Penalties for non-optimal operation 35}

page no. 3.5 _{Bandwidth Requirements and}

3.5.1 _Bandwidth for local 3.5.2 _Bandwidth for fault 3.5.3 _{The minimum} distance

Distance Resolution

attenuation measurement location resolution 37 37 37 38 3.6 _{Noise Averaging}

3.7 _{Effects of Noise} Clipping 3.8 _{Non-Linearities}

3.9 _{Range Performance}

3.9.1 _{The decay} in _{signal-to-noise ratio} 3.9.2 _{Design example}

3.9.2.1 Limits _{to range}

3.9.2.2 _{Range at optimum sample separation} 3.9.2.3 _{Extension of range}

3.10 _Measurement Time

3.11 _{Multi-Channel} Sampling

3.11.1 Maximum _span (Tmax)

3.11.2 _{Range for m-channel sampling} 3.12 _Fault Location

3.13 Extension _{to Long Wavelengths} 3.14 Summary

Table 3.1

Figures 3.1-3.14

39 40 42 45 45 46 47 48 49 49 50 51 52 53 54 55

Chapter 4A 2-Channel _{Implementation with Boxcar}

Averaging _{and a Dye-Laser} 57

4.1 _{Basic Experimental Arrangement 57}

4.2 _{Dye-Laser and Optical-Parametric-Oscillator (OPO) 58}

4.2.1 _{Dye-laser 58}

4.2.2 _{Optical-parametric-oscillator (OPO) 59}

4.3 _{Pulse Slicing 59}

4.4 _{Optical Pedestal 60}

4.5 _{Anti-Reflection Cell 60}

4.6 _{Mode-Scrambler 62}

4.7 _{Amplifier 62}

4.8 2-Channel _{Sampling with Boxcar Integrators 64}

4.8.1 _{Aperture duration 65}

4.8.2 _{Analogue averaging 65}

4.8.3 _{Digital averaging 66}

4.9 _{Alignment of a 2-Channel Boxcar System 66}

4.9.1 _{Gain correction 66}

4.9.2 _{Offset compensation 67}

4.9.3 _{Gate triggering 68}

4.9.4 _{Gate non-linearity}

-68 4.10 _{Spark-Gap Instabilities and Electrical}

Interference ₆₈

4.11 _{Computer Control 69}

4.12 _{Summary 71}

page no. Chapter 5A 2-Channel _{Implementation with Fast}

Sample-Holds, Fast Analogue-Digital

Converters _{and a Semiconductor} Laser 73

5.1 Basic Experimental Arrangement 73

5.2 _{Pulse Laser} 74

5.3 _{Launching and Reception optics} 74

5.4 _Amplifier 75

5.4.1 _{Pre-amplifier} 75

5.4.2 _{Automatic gain control} 75

5.5 _{Computer Interfacing} 76

5.6 _{Sampling System} 76

5.6.1 Linearity _{of sampling system} 77

5.6.2 _{Dynamic performance of sampling system} 77

5.7 _Computer Control 78

5.7.1 Number _{of samples} 79

5.7.2 Dynamic _{range utilization} 79

5.8 Summary 79

Figures 5.1-5.10

Chapter 6 Cutback Method for Spectral Loss

Measurements 81

6.1 Experimental _{Arrangement 81}

6.2 Confidence _{of Measurement 82}

6.3 Measurements _{on a Plastic-Clad Silica Fibre (PCS) 83}

6.3.1 _Microbending losses ₈₃

6.3.2 _Launch dependent losses ₈₄

6.4 _{Measurements on a MCVD Fibre 84}

6.4.1 _{Effect of the} 'short length' ₈₅

6.4.2 _{Effect of drift 86}

6.5 _{Summary 86}

Figures 6.1-6.4

Chapter 7 _{Spectral Backscatter Attenuation}

Measurements _{and a Comparison with the} More-

Standard Cutback Method 88

7.1 _{Fabrication Details of Fibre VD277/L 88}

7.1.1 _{Refractive index profile 89}

7.1.2 _{Diameter control details 89}

7.2 Length-Dependent _{Loss of VD277/L at} 1.0 _{PM 90}

7.3 _{Effect of Launchinq Conditions on the} Backscatter _Method

7.4 _{Backscatter Spectral Measurements on Fibre} VD277/L

7.4.1 _{Attenuation of the section 500-1500 m}

7.4.2 _{Attenuation of the sections 500-1000 m and}

1000-1500

_m

7.5 _{Evolution of OH- Along the Length of} VD277/L

7.5.1 _{Steroscan studies of B20}

3/S'02 7.5.2 _{Thermal history}

7.6 _{Absolute Accuracy of the Backscatter} by a Cutback Comparison

91 92 92 93 Fibre 94 buffer-layer 95 96 Technique

/. b. 1 Formulation _{of a method} for _comparison 7.6.2 _{Spectral. comparison number 1}

7.6.3 _{Spectral comparison number 2} 7.7 _Summary

page no. Chapter 8 _{Interpretation of Features in Backscatter}

Waveforms 102

8.1 _{Fluctuations in Length-Dependent Loss} 102

8.1.1 _{Correlated features} 103

8.1.2 Scatter _centres 103

8.1.3 _{Anti-correlated} features 103

8.1.3.1 _{origin of variations in numerical}

aperture 105

8.1.3.2 _{Origin of variations} in diameter 105 8.2 _{Effect of Programmed Diameter Changes} in _Fibre

VD277/L 106

8.2.1 _Programmed diameter _profile 106

8.2.2 _{Identification of backscatter} features _and

correlation with programmed defects 107

8.2.3 Single-channel _measurements 108

8.2.4 High _{resolution measurements} 109

8.3 _Modelling the Effect _{of Diameter Variations} ill

8.3.1 _{Expanding taper ill}

8.3.2 Contracting _{taper with differential mode}

attenuation (DMA) 112

8.3.3 _{Contracting taper-with cladding effect 113}

8.3.4 _{Computer simulation 114}

8.3.4.1 _{Effect of step changes in fibre}

diameter ₁₁₅

8.3.4.2 _{Effect of cladding loss 116}

8.3.4.3 _{Effect of taper slope 116}

8.4 _{Comparison of Model and Experimental Results 117}

8.4.1 _{Magnitude of} features ₁₁₈

8.4.2 _{Effect of DMA on contracting tapers 119}

8.4.3 _{Verification of coupling from cladding}

modes 120

8.4.4. _{Effect of taper slope 122}

8.5 _{Oscillatory Diameter Variations 122}

8.6 _{Length-Dependent Measurement of Microbending Loss 123}

8.7 _{Summary 124}

Figures 8.1-8.22

Chapter 9 _{Variation of Pulse Delay with Stress and} Temperature in _{Jacketed and Unjacketed}

Optical _{Fibres 127}

9.1 Theory ₁₂₈

9.2 _{Experimental Procedure 129}

9.3 _{Experimental Results 131}

9.3.1 Effect _{of applied tension on unjacketed}

fibres ₁₃₁

9.3.2 Effect _{of temperature on unjacketed fibres 133} 9.3.3 _{Residual fibpe stress introduced by}

jacketing _{with Nylon 6 133}

9.3.4 _{Delay stability-in jacketed fibres 134}

9.3.5 _{Effect of water ingress on jacketed fibres 136}

9.4 _{Summary 138}

page no.

Chapter 10 Conclusion 140

10.1 _{The Backscatter Technique} 140

10.1.1 _{Optimization of measurement} 140

10.1.2 _Developments in instrumentation 141

10.1.3 _{Accuracy of the method} 142

10.1.4 _{Analysis of backscatter} loss _signatures 142

10.1.5 _{Single mode measurements} 143

10.1.6 Current _{status and future prospects} for

the backscatter technique 144

10.2 _{Pulse Delay Measurements} 145

10.3 _{Closing Remark} 146

Appendix I Measurement Errors Due to Noise Clipping 147

Appendix II Computer Simulation _of Expanding Fibre

Section 150

Appendix III Computer Simulation _{of Contracting} Fibre

Section 151

Table III. 1

Appendix IV Backscatter Measurements in Single Mode

Fibres 153

Figures 1-3

Appendix V Stress-Optical Effect 157

V. 1 Stress Optical Effect in Bulk Glass 157

V. 2 Stress Optical _{Effect in Glass Fibres} 159

Figure V. 1

Appendix _{VI Publications, Conference Papers and}

160 Exhibitions

1

CWýPTER 1

INTRODUCTION

The technology _{of sources,} detectors _{and multimode} fibres for _{operation in the wavelength region 0.8-0.9 pm has}

matured sufficiently for optical communications to gain 166

acceptance and assure its commercial exploitation Low

losses, high bandwidths, _{small size and immunity to electro-} magnetic interference are particular advantages of the

optical fibre.

Currently, the introduction _of fibres is _{on a small scale} and is mainly in high capacity trunk telephony _{and video}

circuits where their _presence is largely transparent to the 167

subscriber However, the _{potential of} fibres is to

168 provide a wide range of new services at the subscriber level

and this prospect _{presents an exciting challenge} from both technological _{and sociological viewpoints. The rate of}

introduction _{of fibres for telecommunications will depend on} economics _{and to a large extent on demand} for digital _services and any moves towards _{all-digital networks.} It is _expected

that _{costs will} decrease _{dramatically as production} quantities increase.

In parallel _with the developments in fibre fabrication 169

have been developments _{in measurement techniques} Now,

with the _{move towards mass production and field applications}

of fibres, laboratory _measurement techniques _must be adapted to the factory _{and field environment and in some cases new} and more suitable _practices developed.

This thesis _{considers a measurement technique called} Optical Time Domain _{Reflectometry (OTDR) which} is _the

analogue of the conventional Pulse Echo Tester (PET) 170

used in coaxial cable _systems. The study _covers broadly two areas _of interest. Firstly, the Backscatter Technique 2-7 for measuring fibre attenuation is _{analysed and constitutes} the

bulk _{of the thesis (Chapters 2-8 and Appendices I-IV).}

Secondly, _{a modification of the Shuttle Pulse Technique} 171 is _{used to m' easure the dependence of-transit time on stress} and temperature in jacketed _{and unjacketed optical} fibres

2

1.1 _{The Backscatter Technique}

The backscatter technique is _{a powerful measurement method} first demonstrated _{by Barnoski} 2,3 _{and may be used to determine}

the _overall fibre length, _{to locate} faults _{and to measure the} local _{fibre loss (i. e. attenuation as a function of length).}

The principle is to launch _{a short pulse of} light into _a

fibre _{and analyse'the characteristics of reflected arscattered} radiation returning to the source in the ensuing interval.

A major attraction over the more-standard cutback and insertion-loss _measurements 148-150 _{is that it is non-}

destructive _{and access to only one end of the fibre is}

required. This is particularly important in. f ield applications where the fibres cannot be cut for _{source compensation}

measurements (as is necessary for the _{cutback method) and} where the opposite _{end of the cable may be many kilometres}

away. In addition, the local fibre _{attenuation cannot} be

determined _{by the cutback and insertion-loss methods. This} valuable facility _provided by the backscatter technique _may

be utilized for _{the monitoring of fibre quality and for fault} location in _{the field.}

At the time _{of commencement of this study a number of} 2-4

other _researchers had developed the technique further and the first _{commercial instruments, although primitive, were}

172,173

soon to appear _{on the market} However, there had been

little _{consideration given to the absolute accuracy of the} backscatter _{attenuation measurement or to the accuracy}

requirements _of the instrumentation _{or to the potential range.} The prime objective _of this thesis is _{to address} these

problems. Firstly, the theory _of the backscatter _process is discussed _{with particular emphasis on the assumptions which}

must be valid for the _{success of} the technique. A consider- ation of the accuracy _{requirements of} the instrumentation

shows that an optimum _{operating strategy exists} in terms _of the _{choice of system parameters such that the accuracy with}

9a which the local fibre _{attenuation can be measured} is _maximize

A new technique called 'The Two-Channel Method' _was

developed _{for backscatter waveform analysis} 93 _{and was shown} to be capable of measuring local fibre _attenuation in much

3

of a 2-channel technique are numerous and make this approach- a very attractive _proposition for commercial exploitation.

This development _opened the _{way for an accurate comparison} between _the backscatter _{and more-standard cutback method.}

In particular, the first _{comprehensive} backscatter _spectral measurement was performed over a wide wavelength range 94

and produced an excellent agreement when compared to the cut- back method. This _result lends _{a considerable confidence} to

the _{efficacy of} the backscatter technique.

The 2-channel _{approach was also applied} to the _measurement of fluctuations in local fibre losses. The measurement _of

the length dependence _{of attenuation at different wavelengths} is demonstrated. _It is _{shown that the evolution of an}

absorbing impurity such as OH- can be tracked along the length 94

of a fibre This _measurement is important _since it _may enable the source _{of a contamination} to be identified _and lead _{to modifications and improvements in fibre fabrication}

methods.

The effects _{on the backscattered power of non-uniformities}

in _{longitudinal fibre parameters is also considered. It is} shown that depending _{on the} fibre _{quality, plots of local}

attenuation _with length (the backscatter-loss _signature)

frequently _{exhibit severe fluctuations and anomalies such as} negative _{values. In particular,} backscatter-loss _signatures

are _{presented which clearly show correlation with programmed} fibre defects _{and indicate, in contrast to conclusions in}

the literature 7 _{that in many cases diameter fluctuations are} the _{cause of unidentified features in backscatter waveforms.}

Moreover, it is _{possible to infer the nature of the} 95-98

fluctuation from its _{loss signature}

Theoretical _work 8 ýas _{shown that the} backscatter _technique is _{also applicable to single-mode fibres, although prior}

to the _{commencement of} these _studies it _{had only been used} 125

for _fault location _{in this type of fibre. It was recently} pointed _out that the _{scatter return} in monomode fibres

contains _{not only} the _usual intensity information from _which the length-dependent fibre loss _{may be inferred, but also}

polarization information, _since the backscattered light

4

pulse. A new technique te=ed Polarization Optical Time

Domain Reflectometry (POTDR) has been proposed in which the scatter return from monomode fibres is analysed to determine

55,56

the _{SOP along} the length Results _presented in

Appendix _{IV represent} the first _measurements of POTDR and 57,58 clearly-demonstrate the feasibility of the technique

1.2 _{Pulse Delay Measurements}

Measurements _{on pulse} transit time _presented in Chapter 9 were performed with a dual objective. Firstly, the

stability of the time delay is evaluated for variations in temperature _{and axially applied stress} in order to provide

data for _the design _{of optical} delay lines. _{Both unjacketed}

and plastic-clad fibres have been tested and it is shown that the _{application of a secondary coating of} Nylon _considerably

degrades the delay _stability.

A second objective _results from the _observation that the pulse delay is dependent on fibre tension, suggesting the

use of the measurement _{as a diagnostic} test for residual

stress levels in optical _cables. The latter _measurement is

of some-importance to cable designers, _since ideally the fibre should be neutrally _stressed in order to avoid static fatigue

and microbending effects. Refinements to the previously

reported delay-measurement technique 42, based _{on a time-} domain _{reflectometry principle, have resulted} in improved

long-term _{stability and this has permitted experiments} to be undertaken to determine the _{residual compressive} stress level

in a fibre following _{the application of a Nylon} 6 over-jacket. In addition, the _{effect of water} ingress _{on the cable} has

200-202 been assessed

1.3 _{Notes on General} Layout _of Thesis

The _{study of} the backscatter technique _spans Chapters 2- 8 and Appendices I-IV _{and comprises} the bulk _of this thesis.

Measurements _{of pulse} delay _{stability are presented} in Chapter _{9 and Appendix} V.

Chapters _{2 and 3 cover} the theory _of the backscatter

5

ultimate _range. Chapters 4,5 _{and 6 describe} the

development _{and evaluation of the} instrumentation. _Chapters 4 and 5 detail the backscatter _apparatus for _{multi- and}

single-wavelength measurements respectively and Chapter 6 describes _{the more-standard cutback technique. Following,}

in Chapters _{7 and 8 and Appendix IV are the experimental}

results. Chapters 7 and 8 cover measurements _{on multimode}

aim

6

CHAPTER 2

THEORETICAL BACKGROUND OF THE BACKSCATTER, TECHNIQUE

The basic _{experimental arrangement} for _{a backscatter}

measurement is shown in figure 2.1. A short pulse of light

is _launched into _{an optical} fibre _{and scattering and absorption}

cause attenuation of the pulse as it propagates. A fraction of the _{scattered radiation} falls _within the fibre _numerical

1

aperture (NA), and is itself guided In fact, a backward wave returns to the input after a delay and may be directed

2 to an optical detector

The power backscattered at any point in the fibre depends on the energy of the forward travelling _pulse. Thus, _{as the} amplitude of the forward _pulse diminishes, _{so the received}

2-4

backscattered _power decays _{with time For a fibre} having a constant loss along its length, _{an exponential} backscatter

waveform is observed (see figure 2.2(a)) _{and at a fault point} or joint, the _{attenuation of'the} forward-and backward-

travelling light _{causes a sudden drop in backscattered}

power (see figure 2.2(b)). Reflecting discontinuities _are indicated _{by spikes which are also often associated with a} drop in _{the backscatter level thereafter.}

The backscatter _{technique relies on the existence of a} mechanism _{which scatters} light in _{strict proportion} to the

energy in the forward _{propagating pulse. Rayleigh}

scattering3-7 _{, to a first order, provides such a mechanism.}

There _are, however, _{many factors which may cause changes} in scattering,. _{and these result} in distortion _of the ideal

backscatter _{waveforms described above; both the cause and the} effect of these _{variations are} described in the following

sections. Firstly, however, the fundamental _{mechanisms of} attenuation are considered.

2.1 _{Loss mechanisms in optical fibres}

The attenuation in optical fibres is determined _{by a} 8-11

7

2.1.1 _Absorption

For _{silica, molecular vibrations with} broad _absorption

peaks centered at 9,12.5 _{and 21 Um in} the infrared region (IR) 9,12,13 _{have tails which extend down to about 1.5 pm}

and contribute significantly to the loss _at longer _wavelengths? Dppant _materials (e. g. Ge02' P205' _{B203, F20 and TiO 2) are}

used in fibre fabrication to produce the _{optical guiding}

structure 10,11 and may contribute to IR absorption 9. The dopant _B203 14

, in particular, causes a shift of the absorption tail _{to shorter wavelengths and is therefore not used} in

optical fibres designed for operation in the 1.3-1.55 _pm 9

range

The electronic absorption levels associated with the Si-O bonds, _{on the other hand, are in the ultra-violet (UV). The} valence-conduction bandgap is _{, A. 9 eV and corresvonds} to. a

15

wavelength of 140 nm. The tails of the absorption extend into _{the IR, however, the losses at wavelengths longer than} A, 700 nm. are negligible in comparison to those _{due to impurity}

8,9,16 absorption _{and scattering}

Impurities _{such as the} hydroxyl ion (OH _commonly referred to as 'water' 17 _{, and} transition _metal ions 18

, may also contribute to absorption _{at certain wavelengths.} The-

19-21 water has proven _{a particularly} difficult-one to _eradicate

Typically, _{1 ppm, OH produces absorption peaks of 1,2 and} 40 dB km-l _{at wavelengths of 0.95,1.24 and 1.38 Um,}

respectively*.

The incorporation _{of aB203 buffer-layer in the cladding} of MCVD fibres limits OH_ diffusion into the _core from _the outer substrate tube _{and reduces} the _{absorption at} 0.95 _um

14

to negligible levels For long _{wavelength operation,}

however, B2 03 is _{not suitable due to IR absorption} 11 _{and new} techniques _must be developed _{to further reduce the loss at}

1.38 _{pm. Recently reported dehydration techniques using}

The fundamental _{OH- vibration is at 2.72 um and harmonics} appear _at 1.38,0.95 _{and 0.72 Um.} Combinations _{with the}

8

'gettering' _agents look _{promising and some extremely} low 22

water content fibres have been produced

2.1.2 _Radiation

Radiative losses _{may result} from _{an intrinsic} property of the material or from waveguide effects.

Rayleigh _scattering 23,24 _{, caused} by fluctuations in the molecular fine-structure of the glass, is the most important

and is the dominant loss mechanism in high-quality optical 9

fibres _{For a high-grade} fibre, _{operating over} the _wavelength range 11,0.7-1.4 um, the total loss a, is approximately equal

to the Rayleigh _scattering loss _{as, except} in bands associated with an absorbing impurity. That is,

a 2: CL 0.7 <X :5 -1.4 pm

The importance _{of Rayleigh scatterinq,} both in determining

the _ultimate fibre loss 9 _{and providing a basis} for _{an optical} time domain _{reflectometer} 2, _{make it worthy of} further

consideration in the next section.

Further linear _{scattering may result} from _waveguide effects or imperfections. For example, high-order modes close to cut-

25-27 28,29

off _{and leaky modes are} lossy In addition, core

30 _31,32 32-34

ellipticity _{, variations} in diameter _{, microbending}

bubbles, _{point defects} 35 _{and drawing-induced colouration} 36 may also _contribute to losses. These _effects, however, are

normally negligible in good quality _optical fibres and cables. Non-linear _{processes such as Brillouin and Raman scattering}

(37-40)

may also contribute to fibre loss These are

characterized by onset-thresholdý which are usually above the power levels considered for optical communication 38 _{, and are} therefore _{not normally} important. However, in backscatter

measurements, high optical power levels are often desirable

to obtain increased range or resolution and, particularly in 38

the _{case of monomode fibres ,} these _{effects should} be considered.

2.2 _{Rayleigh scattering}

The _{origin and} features _{of Rayleigh scattering are}

described _{here in brief. Further details may be found in the} 23,24

9

Consider _{a dielectric medium of mean permittivity} I

exhibiting a variance Ac". Assume that the spatial width of the _permittivity fluctuations _{is much smaller than the wave-}

2= -

length _of light X/n, _{where n is the refractive} index (n _C). If the _medium is free _{of current sources} 41

VxH= jwcE _(2.2)

For _{a small volume element} 6V, however, _{the refractive} index change Ae, results in the _production of an apparent current

source J, _{and equation} (2.2) becomes

where

VxH= jwFE _+J... (2.3)

J= jw(Ac)E

... (2.4)

This fictitious _{current extracts power} from _the incident _wave and re-radiates. The radiation or scattering pattern produced

41 is _{that of a linear current dipole}

Consider _{a set of cartesian coordinates x, y and z in}

which the light _{propagates along} the _z-axis. If the incident radiation _of intensity Iox, is _{polarized with electric vector} parallel to x, then the intensity _of the _scattering from the

small volume element is 24

Tr 2 6V 2 _Ae

sin 2e (2.5)

ox ,X42x

where r is the distance from _{the volume element and 9x is} the angle of the _{scattering relative to the x-axis.}

Similarly, _{for incident radiation of intensity I}

oy , with polarization _parallel to the _y-axis, the intensity _of the

scattering is 24

7T 2 6v 2 --2 2

Iy

oy . X4 r2 Ac sin 9y... (2.6)

where 9y is the angle _of the _{scattering relative} to the _y-axis. For natural or unpolarized light _of intensity ₁₀

ox

_oy

10

Tr 2 6V 222

0 2X 4r2 Ae (sin E) x+ sin E) Y] (2.8)

Let ez be the _{angle of} the _{scattering relative} to the _z-axis and with ex and 9y as before

Cos 29x+ Cos 2ey+ Cos 2ez... (2.9)

Substituting _equation (2.9) into (2.8) _gives

7T 2 6v 2 --T 2

0 2X 4r2 Ac (i + Cos ez] ... (2.10)

The total _scattered intensity from the _{whole volume,} V, assuming that adjacent scattering elements are independent,

can be found by multiplying _equation (2.10) by V/SV. Thus,

7r 2 vsv --T 2

T0

2X 4r2

Ac (i _{+ Cos ez]} ...

(2.11)

2.2.1 _{Rayleigh attenuation}

Scattering _{causes a loss of power} in _{the propagating wave} and the _{attenuation coefficient} is 24

3 --T

8

3 -X4

c2

(2.12)

The strong dependence _{of loss on wavelength} (X-4) is _of

particular _significance in-optical _{communications where} fibre loss _{should be minimized. This is, in part, the reason for}

the large _{research effort into development of} fibres _and systems for the long _wavelengths 1.3-1.55 _{um, where} losses

9

are lower* For _applications to OTDR, the decrease in back- scatter power at long _wavelengths is _{also significant as it}

affects instrument _{accuracy and range} (see _section 3.13)

11

2.2.2 Origin _{of refractive} index fluctuations

The Rayleigh Law, described in the _{above equations,} is valid for any medium in which the correlation-distance of

the local _refractive index _variations is _{small compared} to 23,24

the _{wavelength of} light*

The refractive index fluctuation, Ae, can be expressed in general as a function of two independent thermodynamic

variables such as the density p, and the temperature T, and

22ý24 in _{the case of a multicomponent medium, the concentration c}

Ac (p, T, C) (ap Ap + ý-aT); AT + Ac (2.13)

IC)

If the fluctuations in density, temperature _{and concentration} arise independently of each other and are statistically

.2

independent, _{then the variance} Ad is _given by 2e 2

c

at .

ýC2 2a2Q

Ac ( P, T, C) + 5)p _... (2.14)

PjT pit T

In the _{case of silica, microscopic density fluctuations are}

43-45 frozen _{into the solid below the glass transition-temperature}

This is _{an intrinsic feature of glass, independent of any}

waveguide _{structure and results in a fundamental lower} limit 45

to fibre loss _{due to Rayleigh scatter}

The dopant _{materials (e. g. GeO ,BF0 and TiO} 10,11

2 203' P205 22

used in fibre fabrication _{may also contribute to Rayleigh}

scattering9; _{random variations} in dopant _{concentration within} the _{silica contribute to small index fluctuations (see}

equation (2.13))- _{Germania, in particular, causes significant}

additional Rayleigh _scatter 9,46and the power loss in a germano- silicate glass**is _{given approximately} by 47,48

ct

s =1.8 x 10-28X-4 (1 + 100A) npm-1 ... (2.15) where the refractive index difference

NA2 n1

2_

n22n _{1- n2}

A=- _{Tn (2.16)}

12 2n _12n1...

*If _{the correlation distance of the refractive index}

fluctuations _{is comparable to the wavelength, the effect is} known as Mie scattering24.

"The _{core is Geo}

12

and NA =In _12- n2 2 is the numerical aperture of the-fibre.

The effect on backscatter measurements of fluctuations in the _scattering loss due to changes in germania _{concentration,}

is _analysed in _section 2.8.

It _should be noted that _{certain additives can cause a}

reduction in scattering 49. This is due to-24ther an improve- ment in the lattice order or a reduction in glass fictive

temperature, however, these _{effects are yet} to be fully clarified.

In addition to the _{above sources of refractive} index

variation, _other dielectric imperfections can be caused by

the _{presence of phase} boundaries, _{partial crystallization and} voids 35 _, but _good fabrication techniques can minimize their

contribution to the loss.

2.2.3 _{Rayleigh radiation pattern}

The radiation pattern of the Rayleigh scattered light is described _{by equations} (2.5), (2.6) _and (2.10). _{In the case}

of propagation of a beam polarized in the x-direction, the

angular dependence _of the _radiation is sin 29x which describes a toroid in the _{y-z plane} (see equation (2.5)). If, _{on the}

other hand, the beam has no defined _polarization then the angular dependence _of the _scattering is described by the

surface (1 + cos 29z) (see equation (2.10)).

In the _{case of multimode optical} fibres it has been _shown that _{plane polarized} light becomes de-polarized, _{even in very}

short lengths 50. For _{a weakly-guiding} fibre _{with electric} vectors _{almost perpendicular} to the _axis 25 the _radiation

pattern is therefore _of (1 + cos 2 E)z ) dependence. Consider _a small _{volume element within} the fibre _{core and let} dEv be

the _{total scattered energy from the element. Then the energy} dEQ

, radiated into a solid angle Q, is given by 5

dE (r, z, 9) 3 _{dE (z, r)(1 + cos} 2 9] dil

... (2.17)

01 67r _v

where _{r, z, 9 define a cylindrical coordinate system and the} factor _{3/167 is a normalizing constant}

such that

3f(,.

_+ co s2e)dn

32 Tr 7T ₂

16,

f

(1 + cos 9)sin 9 d9 dO fo

13

Single-mode fibres, _{on the other} hand, _{can sustain} the polarization properties of the incident beam over a

considerable distance 51-54 and therefore the radiation pattern, assuming excitation of only one x-orientated

polarization, is given by

32

dE (z, r, 9) _{= --g- dEv (z, r) sin} 9 dQ

7T (2.19)

Q _{7r x}

where 9x is the angle between the electric vector and the incident _{wave, and the constant of proportionality} is

determined from _{the condition,}

323 f 2Tr Tr 22

sin GxdQ = -ýTr _{- sin} 9cos O)sined9do 8 Tr _,rf

f(l

47 00... (2.20)

In practice, birefringence in the _{glass results} in different

* 51-54 speeds _{of propagation} of the x7 and y-orientated fields

Thus, in _{general, the state of polarization varies along} the length _{of a single-mode} fibre. _{The Rayleigh scattered} light

at _{each point} in the fibre also 'mirrors' the polarization 55

of the _{primary propagating} beam This phenomenon may be exploited in a new instrument called a 'Polarization Optical

55-58

Time-Domain _{Reflectometerl} (POTDR) _{The first}

demonstration _{of this technique} 57,58 is _{described briefly} in Appendix _IV.

The discussion _{of Rayleigh scattering} is _{extended to the} application _of time domain _{reflectometry} in multimode _optical

fibres in _{section 2.4. Firstly, however, the effect known as} differential _{mode attenuation (DMA) is described.}

2.3 _{Differential Mode Attenuation (DMA) and the Equilibrium} Mode Distribution (EMD)

Power _propagating in _{a multimode} fibre is _usually

distributed _{amongst several hundred modes, each being an,} eigenvalue _solution derived from Maxwell's _{equations, with}

waveguide boundary _conditions. In _general, fibre loss depenas on modal distribution 59 _{; high-order modes suffer} increased

losses _{due to proximity to cutoff, lossy claddings, finite} 60,61

14

dip _{in the} index _{profile commonly observed in MCVD fibres} 62,63

1 or due to the higher dopant concentrations near the core-centre

in graded-index fibres (see _section 2.2.2).

As a consequence, the _{mode distribution varies along} the 64,65

length _{of a fibre Microbending and waveguide} irregular-

ities, _{however, cause coupling} between _modes 66-68 _{and after a} distance _{Lc, known as the coupling} length, _{the propagating}

69,70 power stabilizes into an equilibrium mode distribution (EMD)

Coupling lengths _{range, typically,} from tens _{of metres to a} 71,72

few kilometres _{and depend on fibre/cable quality}

The attenuation of the equilibrium mode set is a useful

quantity in system design in that, if known, it can be extra- 73

polated to predict the attenuation of long fibre links A similar _{situation arises} with fibre dispersion, due to

differential _{mode delay} (DMD), but _{unfortunately there} is somewhat less certainty in prediction of overall link

73-76 bandwidth

Thus, in the _{measurement of} fibre _{propagation character-} istics _{careful consideration must be given to the mode}

distribution _{and in particular, the} launching _conditions.

This is _{a subject of} further discussion in _{later sections.}

2.4 _{The Backscatter Characteristic}

The theory _of the backscatter technique, _originally

developed _{by Barnoski} 3 _{and Personick} 4 _{has been generalized} 5,6

and extended by Neumann A summary is presented here in order to create _{a coherent picture of} the theory _{and to}

emphasise the basic _assumptions. The fundamental _{accuracy of} the _{technique and the specifications} for _{the measurement}

apparatus _{are shown to rely} implicitly _{on the characteristics} of the backscattered _{power and the validity of these}

assumptions.

2.4.1 _{IMpulse response}

Consider _{an impulse of one joule energy, launched at time} t=O, into _{a fibre at position z=O. The pulse will attenuate}

15

(Z) 1Z

(2.21) where a'(z) is the average attenuation up to the point z and

is _calculated from _the local _{attenuation al(z), as}

Z

CL,

(Z) =i

Z. f

0 a'(z)dz ...

(2.22)

(Note _{that, throughout this thesis, the units of attenuation} are np. m-l unless specified otherwise. )

The light _{energy scattered} in the backward direction from an element of length dz at position z is given by

dEb ₌ S(Z). E(z). dz

... (2.23) where S(z), known as the 'backscatter factor', is the fraction

of _{energy scattered} in the backward directiQn. (The properties of S(z) are described in section 2.7).

The energy propagating in the backward direction undergoes attenuation _{of average} magnitude cx"W, in returning to the

fibre input, _where

z

a" (Z) Lf a 11 (z) dz... (2.24)

z0

and a"(z) is the local backward loss.

In general, al(z) _{34 all(z),} since the mode distributions

are _not identical in the forward and backward direction _and therefore _the loss is _affected by differential _{mode attenuation}

(see _{section 2.3). Furthermore, fluctuations in fibre diameter,} numerical _{aperture and profile are expected} to influence the

forward _{and backward losses differently. (These effects are}

neglected for the _moment, but _{are analysed} in later _sections. ) The pulse transit time _{T, in a fibre of} length _{L, may be}

found _{approximately from}

N,

... (2.25) where _{c is} the _{speed of} light in vacuo and

dn

1

N1=n1- X-

... (2.26) dX

is _{the material group index. Here}

16

the _{average group velocity, v9} forward _{and backward wave, the} arrives _over the time interval

fibre input _{for each scatterim}

= cIN1, is identical for the energy from the element dz

2dz/v

9, and the power at the point z, is

dE

i (Z) 2dz/v

9

= hv

9 S(z). e

-[&I(z)+3"(z)lz

... (2.27a)

Using _{the variable transformation t=} 2z/v

9 where-the factor

121 accounts for forward _{and backward travel, equation (2.27a)} reduces to

t _-[-o (t/2)+3"(t/2)lv t Pi (t)

9 S(l). e-

a9

... (2.27b)

Equations (2.27a _{and b) represent the} impulse _{response of}

the fibre. _{In the latter equation, It/21 has been substituted}

for 1z' in _{the attenuation and scattering terms. This implies} that _{the backscattered power arriving at the input at time t,} results from _scattering in the fibre _{at position z, at time}

t/2. _{The following discussions will assume a 1z' or It/21} dependence _{and it will not be shown explicitly.}

2.4.2 _{Response to finite input pulse}

The response to _{a rectangular} input _{pulse of} finite _width

W(seconds), _{and a finite power Po(watts), can be found by} convolution _with the impulse _{responses of equations} (2.27a and b). Thus,

P (Z) ₌

SPO

1+ _eh

[a4

+a-, lwv

. e- [ý 43 -] Z _... (2.28)

jr-+-co,

_*

1-

91

or _equivalently

p (t) SP ₀

.

1-1

+ e+l'+cl"lWv9 _e-ý[@+3-, jv _9t (2.29) al+a"

I

Now if

1

W << _(al+all)v

9

17

+ _+a"lz

(2.31) P

_{(Z) = hpowsv}

9e

ce

...

and

p (t) ₌ lip

0 wsv 9e

ce+ýa-lvgt

... (2.32) Thus, _{according to equation} (2.30), _{provided negligible}

attenuation _{occurs over} the width _{of the} input _{optical pulse,} or equivalently provided the width of the input _pulse is far

less _{than the time constant of the} backscatter _{curve, then the} response due to a finite _{pulse of power} P0, and width W, is

identical _{to that for an impulse of energy P0W.}

2.4.3 _{Relative backscatter power}

The backscatter factor for _{a multimode} fibre is _analysed in _{section 2.7.4. It is shown that a 0.2 NA graded-index}

multimode fibre _{with a scattering} loss of 3dB. km-l (6.9 x 10-4 _{np. m-'), has a backscatter} factor _{S'ý, 3.2 x 10-} 6 _{M-1 .}

Thus, _{with an input pulse width W= 100 ns, the backscattered} power _{at any point,} from _equations (2.31) _or (2.32), is

1ý45 dB below the _power in the forward travelling _{pulse or,} equivalently, _*'%, 31 dB below the 4% Fresnel _{reflection which} occurs _{at a perfect} fibre break.

The power of typical launched _pulses is 0.1-1.0 _{W and}

thus it is _{clear that an amplifier must be incorporated} into a backscatter instrument for _{adequate signal recovery.}

2.5 _{Dete=ination of Local Fibre Attenuation}

Equation (2.32) _{shows that the backscattered power depends} on the fibre _attenuation. The time _{constant of the back-}

scatter _waveform is

d

lnp (t) _{= -W} [F'+3-1 _-Wz+W ! L(ln _S)

iff _{p0w. 9g dz cl}

g dz

... (2.33) Using _equations (2.22) _{and (2.24), this reduces to}

d

lnp (t) _{=- hvg [a I (z) +a " (z) ]+ liv} 1 dS

... (2.34)

ZEE _{p0w9S dz}

Thus, the time _constant is determined _{by both the local} attenuation _and the length dependence _{of the backscatter}