Optical Fibers and RF: A Natural Combination

275 Pages·2004·3.422 MB·English

by  Malcolm Romeiser

Optical Fibers and RF:
A Natural Combination
Malcolm Romeiser
Noble Publishing Corporation
Atlanta, GA
Library of Congress Cataloging-in-Publication Data
Romeiser,Malcolm
Optical fibers and RF :a natural combination / Malcolm Romeiser.
p.cm.
Includes index.
ISBN 1-884932-34-7
1.Optical communications. 2.Optical fibers. I.Title.
TK5103.592.F52R65 2003
621.382'75—dc21
2003051041
Copyright 2004 by Noble Publishing corporation.
All rights reserved.No part of this book may be reproduced in any form or
by any means without prior written permission of the publisher.
Printed in the United States of America
ISBN1-884932-45-2
ix
Preface
I will use the first person in writing this preface because it will contain
many of my thoughts on this technology, how it has grown, and how it
impacts society.The remainder of this book is in the third person,which is
the usual way to present technical material.
This book was written with three objectives in mind: first, and fore-
most,as a teaching text;second,to present updated information on optical
fiber technology in a practical context useful to students or professionals;
and third,to provide a platform for presenting different insights and analy-
ses on certain optical fiber system design issues.The ability to achieve the
first comes from my experiences through the last decade while teaching
undergraduates at a university and as a consultant.The third comes from
my earlier years in industry, where I helped research, design and develop
transmission systems that used radio,wire,coax,and optical fibers.From
this broad base of practical and theoretical knowledge I gained an ability to
analyze and simplify telecommunications transmission systems issues.The
second objective is a blend of the other two.I have noticed that very useful
information and insights are lost when too much technical detail is pre-
sented. Having experienced the growth of the technology and also having
developed an understanding of how it can best be taught,I feel compelled
to share these experiences.
The way I approach optical fiber technology in this book probably owes
more to my understanding of today’s undergraduates than to my industrial
experiences.There was a great temptation in writing this book to do what
most authors do:present many equations without derivation or explanation
so as to cover more details in the allotted space.Unlike earlier generations,
x |PREFACE
however, present-day engineering students are not comfortable with this
approach. For whatever reasons, they find it hard to “go learn it on your
own”. Some amount of forced independent analysis is unavoidable, but
hopefully I have kept it to a minimum.Like my college students,the read-
er is expected to have a working knowledge of differential calculus and basic
physics.
The book’s title was inspired by a recognition that radio technology and
optical fiber technology have much in common.I believe optical fibers should
be viewed as the latest technology, in a century long progression of tech-
nologies, that allow the efficient transmission of information. One obvious
link between the two is that transmission is achieved using the same three-
step sequence: carrier modulation at a transmitter, propagation through a
media,and demodulation at a receiver.The carrier frequencies and devices
differ between technologies but many of the transmission techniques are
similar.Since optical systems transmit radio frequencies,there is also ongo-
ing synergy between RF and optical fiber research and development.
Another inspiration for the title came from a study of the history of
radio development. Radio frequency technology began about 100 years
before the beginnings of optical fiber technology.Optical fiber transmission
depends on advances made by early radio researchers.One example is the
fundamental research on electromagnetic (EM) wave propagation per-
formed by C.Maxwell and W.T.Kelvin in the late 19thCentury.This proved
essential in achieving transmission of information first over long and short
wave radio, then wire pairs, coaxial cables, line-of-sight microwave radio,
and finally optical fibers.Each of these media in turn provided increasing
amounts of bandwidth, increased information capacity, and greater trans-
mission distances.Many other connections from radio system development
to optical fiber system development exist.Some examples are:the modula-
tion and demodulation of signals on carriers,the filtering and separating of
these signals at high frequencies,and the frequency division multiplexing
of many independent signals.
Optical fiber technology has grown rapidly in the period from1970 to
the early 2000’s.Current popular opinion holds that the pace of adoption of
optical fiber technology has been unprecedented.This is actually not true
when compared to the historical development of other innovative technolo-
gies like radio, transistors, integrated circuits, plastics, etc. Probably the
most surprising parallel between RF and optical fiber development is that
both required about an equal length of time to progress from initial theo-
retical studies to significant commercial use,a period of about 35 years.
At the time of discovery,both technologies were too impractical to draw
much attention.At the turn of the 20th Century G.Marconi,the generally
acknowledged father of radio,had great difficulty interesting anyone in its
commercial possibilities. As for optical fibers, this author can personally
PREFACE | xi
attest to an early lack of commercial interest.I organized a conference ses-
sion on optical fibers in the early 1970s that I was convinced would be very
well attended.After all, this new technology was going to be the greatest
thing since sliced bread. Only about 10 people showed up. I realized that
even the most promising technology is of little interest if it has no immedi-
ate commercial value.
For both technologies,rapid commercialization ensued once reasonably
priced components became available. In both cases this took about ten
years from the time of the seminal work.The fundamental theories under-
lying radio transmission were developed in the late 1800s. The enabling
technologies were developed soon after the turn of the century.By the late
1930’s there was widespread use of radio for communications.The funda-
mental optical fiber work began in the mid-1960s.The enabling technolo-
gies were first available commercially in the 1970s.By 2000 the technology
was in widespread use.
More detailed comparisons can be drawn at the component level.In the
very early years,radio transmission used telegraph signaling and spark-gap
transmitters.The RF frequencies were low (60 kHz) which resulted in high
atmospheric attenuation and limited transmission distances.The transmit-
ted RF spectra were broad and the detectors used intensity sensitive metal
filings that changed resistance according to the received EM energy. The
antennae were nondirective which meant the media was used very ineffi-
ciently.Compare this to early work with optical fibers.The only light sources
available were at 800 nm.These wavelengths are in the lower,high attenu-
ation range of useable wavelengths for glass fibers.As a result,transmission
distances were limited.The light sources had broad spectra,and the detec-
tors were sensitive only to intensity variations.As an aside, the detectors
used in modern systems still use intensity detection.The fibers were large-
core,multimode,which resulted in inefficient media usage.
For radio, the most important component advance was probably the
invention of the thermionic valve (vacuum tube triode) in 1908.This allowed
signal amplification and more advanced modulation/detection methods.A
comparable advance with optical fiber technology was the development of
the room temperature, 850 nm semiconductor laser and the etched well
LED in the early 1970s.
When a baseband signal is modulated on an RF carrier the resulting
spectra can be wide or narrow.The type of modulation determines the spec-
tral width,as well as the sensitivity of the signal to noise,distortion,and
interferences. If the spectra are narrow the transmission media can be
used more efficiently but, in general, the system costs will be higher.The
spark-gap/telegraph transmission used for the first radio systems was inef-
ficient and was replaced by amplitude-modulated high frequency carriers
once vacuum tubes were developed. Another important advance was the
xii | PREFACE
development of single-sideband suppressed carrier (SSBSC) transmission
in the 1920s. SSBSC transmission basically allowed the RF energy to be
concentrated in the signal and not in a carrier, thus improving transmis-
sion quality and efficiency.The ability to generate high frequency,coherent
carriers also allowed accurate, predictable and reliable signal filtering to
be achieved. Also, longer transmission distances and greater total band-
widths were possible through the use of higher frequencies.
With SSBSC, independent (modulated) carriers are transmitted in
closely packed adjacent bands. The term applied to this approach is fre-
quency division multiplexing (FDM). With optical fibers the comparable
technique is called wavelength division multiplexing (WDM).WDM became
realizable and economically attractive only with the availability of longer
wavelength sources and detectors.At longer wavelengths glass fibers have
less attenuation and distortion.Widespread use of WDM began in the early
1990s,about 15 years after the beginning of fiber system commercialization.
The more advanced WDM systems use nearly coherent (single frequency)
laser sources and intensity modulation and detection. This modulation
approach produces a double sideband amplitude modulated signal like that
used by radio before SSBSC.At some future date optical fiber systems will
undoubtedly adopt more efficient types of modulation like SSBSC.
Why dwell on these comparisons? As I mentioned, my view of optical
fiber technology is that it is just another step in a long progression of
telecommunications technologies. I began my telecommunications career
working on analog microwave radio.I have had the advantage of working
on many technologies since then:analog satellite,digital wire pair,digital
coax,digital microwave,analog cellular radio,digital optical fiber systems.
For each of these technologies the media and required hardware are
unique, but the underlying communications principles are the same. In
writing this book I have attempted to concentrate on what makes optical
fiber transmission unique, while at the same time, trying to position the
technology as just another step on this evolutionary path.
Optical fiber technology is both broad and deep. Understanding the
technology thoroughly requires that knowledge be obtained from many dis-
ciplines:physics,engineering,optics,EM wave propagation,semiconductor
theory,electronic circuits,and communication theory.Because the intent in
this book is to present the technology in an overview,it has been necessary
to leave out many details.Unfortunately some important topics are treated
very briefly. The reader is encouraged to seek additional information. To
that end a limited list of references has been appended.
This book begins with a discussion of underlying physical concepts
important to optical fiber systems.Chapter 2 discusses the construction and
propagation of light in the two basic types of cylindrical glass fibers,multi-
mode and single-mode.Non-cylindrical optical waveguides are used in opti-
PREFACE | xiii
cal integrated circuits and some optical sensors.This use is mentioned in
Chapter 6,without a derivation of the waveguide characteristics.Chapter
3 presents the performance characteristics of cylindrical fibers.Their abil-
ity to transmit information is dictated by two parameters,dispersion and
attenuation. Both play a role in determining the available system band-
width and the maximum transmission distance.The devices used for gen-
erating and detecting light signals are discussed in Chapters 4 and 5.An
important characteristic of the light, coherence, is covered in Chapter 4.
Radio signals generated to carry modulation or help in the detection
process at a receiver are very coherent. This allows the RF bands to be
used with a high degree of spectral efficiency.Optical carrier sources still
have significant coherence limitations. Chapter 6 discusses the optical
components needed to assemble and test an end-to-end system.These com-
ponents provide many important functions: connecting fibers, controlling
levels and reflections of light energy, separating and combining signals,
switching,and modulating.Chapters 7 and 8 combine the concepts devel-
oped in Chapters 1–6 Chapter 7 presents two basic system designs,a short
distance analog video link and a long distance digital link.Chapter 8 deals
with the current trends in optical fiber use and where the technology
might be headed in the future.The spread of Internet packet switching is
probably the most important factor in how this technology will be used in
the telecommunications arena.
Finally,I would like to acknowledge the encouragement received from
my family and friends. Especially I appreciated the support and patience
of my wife,Sonja.
Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
Chapter 1 From RF to Optical Fibers
1.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2. Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.1 Frequency/Wavelength/Bandwidth . . . . . . 2
1.2.2 Power. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.3 Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. An Optical System. . . . . . . . . . . . . . . . . . . . . . . . 6
1.4. Optical Fiber History . . . . . . . . . . . . . . . . . . . . . 8
1.5. Units and Constants. . . . . . . . . . . . . . . . . . . . . 10
Chapter 2 Optical Fiber Characteristics
2.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 Rays & Waves . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2.1 Polarization . . . . . . . . . . . . . . . . . . . . . . . 16
2.2.2 Step Index Fiber . . . . . . . . . . . . . . . . . . . 17
2.2.3 Total Internal Reflection . . . . . . . . . . . . . 18
2.2.4 Numerical Aperture. . . . . . . . . . . . . . . . . 21
2.2.5 Velocities . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.3 Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.3.1 Step-Index Modes . . . . . . . . . . . . . . . . . . 25
2.3.2 Cutoff Wavelength. . . . . . . . . . . . . . . . . . 27
2.3.3 Single Mode Fiber . . . . . . . . . . . . . . . . . . 29
2.3.4 Step-Index Single Mode Radius. . . . . . . . 30
2.3.5 Shaped SM Fiber Cores. . . . . . . . . . . . . . 31
2.3.6 Graded Index Multimode Fiber. . . . . . . . 32
2.4 Optical Fiber/Cable Manufacture . . . . . . . . . . . 33
Chapter 3 Optical Fiber Performance. . . . . . . . . . . . . . . . . . . . . 39
3.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.2 Attenuation. . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.2.1 Material Absorption. . . . . . . . . . . . . . . . . 44
3.2.2 Scattering Losses. . . . . . . . . . . . . . . . . . . 45
3.2.3 Bending Losses . . . . . . . . . . . . . . . . . . . . 45
3.3 Dispersion (Arrival Time Distortion). . . . . . . . . 46
3.2.1 Delay,Bandwidth,and Risetime . . . . . . . 48
3.2.2 Modal Dispersion. . . . . . . . . . . . . . . . . . . 51
3.2.3 Chromatic Dispersion . . . . . . . . . . . . . . . 53
3.4 Polarization. . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.5 Non-Silica Fibers. . . . . . . . . . . . . . . . . . . . . . . . 60
Chapter 4 Optical Sources for Fibers. . . . . . . . . . . . . . . . . . . . . 63
4.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.2 Optical Semiconductors. . . . . . . . . . . . . . . . . . . 64
4.3 Light-Emitting Diodes. . . . . . . . . . . . . . . . . . . . 68
4.3.1 LED Construction . . . . . . . . . . . . . . . . . . 68
4.3.2 LED Modulation Bandwidth . . . . . . . . . . 70
4.3.3 LED Power Output and Modulation . . . . 72
4.3.4 LED Spectral Emission
and Coherence. . . . . . . . . . . . . . . . . . . . . 73
4.3.5 LED Coupling to Fibers. . . . . . . . . . . . . . 75
4.3.6 LED Packaging and Reliability. . . . . . . . 77
4.4 Semiconductor Lasers. . . . . . . . . . . . . . . . . . . . 78
4.4.1 ILD Construction. . . . . . . . . . . . . . . . . . . 78
4.4.2 ILD Modulation Bandwidth. . . . . . . . . . . 81
4.4.3 ILD Power Output and Modulation. . . . . 82
4.4.4 ILD Spectral Emission and Coherence . . 84
4.4.5 ILD Coupling to Fibers . . . . . . . . . . . . . . 86
4.4.6 ILD Packaging and Reliability. . . . . . . . . 87
4.4.7 Vertical Cavity Surface
Emitting Lasers (VCSEL) . . . . . . . . . . . . 88
4.5 Optical Source Coherence . . . . . . . . . . . . . . . . . 90
4.6 Erbium-Doped Optical Amplifiers (EDFA) . . . . 93
Chapter 5 Optical Detectors and Receivers. . . . . . . . . . . . . . . . 97
5.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
5.2 Optical Detection. . . . . . . . . . . . . . . . . . . . . . . . 98
5.3 Photodiode Construction. . . . . . . . . . . . . . . . . 100
5.4 Photodiode Performance . . . . . . . . . . . . . . . . . 104
5.4.1 Spectral Response and Quantum
Efficiency. . . . . . . . . . . . . . . . . . . . . . . . 104
5.4.2 Sensitivity and Additive Noise. . . . . . . . 108
5.4.3 Frequency and Pulse Response . . . . . . . 112
5.4.4 Photodiode Summary. . . . . . . . . . . . . . . 114
5.5 Preamplifiers. . . . . . . . . . . . . . . . . . . . . . . . . . 114
Chapter 6 Optical Components . . . . . . . . . . . . . . . . . . . . . . . . 117
6.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . 117
6.2 Connectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
6.2.1 Multimode Connectors. . . . . . . . . . . . . . 120
6.2.2 Single-Mode Connectors. . . . . . . . . . . . . 125
6.3 Splices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
6.4 Couplers and Splitters. . . . . . . . . . . . . . . . . . . 130
6.5 Wavelength Division Multiplexers (WDM)
and Filters. . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
6.6 Circulators. . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
6.7 Isolators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
6.8 Attenuators . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
6.9 Switches. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
6.10 Modulators. . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
Chapter 7 Optical Fiber Systems. . . . . . . . . . . . . . . . . . . . . . . 143
7.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . 143
7.2 System Signals,Noise and Distortion. . . . . . . 145
7.2.1 Baseband Signals. . . . . . . . . . . . . . . . . . 145
7.2.2 Performance Objectives . . . . . . . . . . . . . 148
7.2.3 Digital Line Coding . . . . . . . . . . . . . . . . 151
7.2.4 Noise,Distortion,and
Nonlinearity. . . . . . . . . . . . . . . . . . . . . . 155
7.2.4.1 Intersymbol Interference
(ISI) and Dispersion . . . . . . . . . 158
7.2.4.2 Optical Source Induced
Noise . . . . . . . . . . . . . . . . . . . . . 159
7.2.4.3 Optical Amplifier Noise. . . . . . . 162
7.2.4.4 Polarization Mode Dispersion
(PMD) . . . . . . . . . . . . . . . . . . . . 163
7.2.4.5 Nonlinear Fiber
Performance. . . . . . . . . . . . . . . . 164
7.2.4.6 Crosstalk . . . . . . . . . . . . . . . . . . 166
7.2.4.7 Solitons . . . . . . . . . . . . . . . . . . . . 67
7.3 Link Budgets. . . . . . . . . . . . . . . . . . . . . . . . . . 168
7.3.1 Analog Video System. . . . . . . . . . . . . . . 170
7.3.1.1 Analog Link Power Budget . . . . 171
7.3.1.2 Analog Link Bandwidth
Budget. . . . . . . . . . . . . . . . . . . . 173
7.3.2 Digital Link . . . . . . . . . . . . . . . . . . . . . . 174
7.3.2.1 Digital Link Power Budget . . . . 179
7.3.2.2 Digital Link Rise Time
Analysis. . . . . . . . . . . . . . . . . . . 182
7.4 Test and Measurement . . . . . . . . . . . . . . . . . . 184
7.4.1 Visual Tests . . . . . . . . . . . . . . . . . . . . . . 187
7.4.2 Power/Loss Tests . . . . . . . . . . . . . . . . . . 187
7.4.3 Dispersion/Spreading Tests . . . . . . . . . . 189
7.4.4 End-to-End Tests. . . . . . . . . . . . . . . . . . 190