Table Of ContentOptical Fiber Communications
Edited by
Tingye Li
AT&T Bell Laboratories
Crawford Hill Laboratory
Holmdel, New Jersey
A table of contents for Volume 1 appears at the end of this volume.
Topics in
Lightwave
Transmission
Systems
Edited by
TINGYE LI
AT&T Bell Laboratories
Crawford Hill Laboratory
Holmdel, New Jersey
AT&T
ACADEMIC PRESS, INC.
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LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA
(Revised for volume 2)
Optical fiber communications
Vol. 2- has imprint: Boston : Academic Press.
Includes bibliographies and index.
Contents: v. 1. Fiber fabrication—[v. 2] Topics
in Lightwave Transmission Systems
1. Optical communications. 2. Optical fibers.
I. Li, Tingye.
TK5103.59.0675 1985 621.3827 84-9232
ISBN 0-12-447301-6 (v. 1 : alk. paper)
ISBN 0-12-447302-4 (v. 2: alk. paper)
PRINTED IN THE UNITED STATES OF AMERICA
91 92 93 94 9 8 7 6 5 4 3 2 1
CONTRIBUTORS
Numbers in parentheses indicate the pages on which the authors' contributions begin.
Andrew R. Chraplyvy (267), AT&T Bell Laboratories, Crawford Hill
Laboratory, Holmdel, New Jersey 07733
M. Dixon (1), AT&T Bell Laboratories, Breinigsville, Pennsylvania 18103
J. L. Hokanson (1), AT&T Bell Laboratories, Breinigsville, Pennsylvania
18103
Tatsuya Kimura (151, 203), NTT Basic Research Laboratories 3-9-11,
Musashino-shi, Tokyo, 180 Japan
Soichi Kobayashi (151), Phototonic Integration Research, Inc. (PIRI), 1375
Perry Street, Columbus, Ohio 43201-2693
Shigeru Saito (203), NTT Basic Research Laboratories, Musashino-shi,
Tokyo, 180 Japan
Gareth F. Williams (79), NYNEX Corporation, 500 Westchester Avenue,
Room 2F7, White Plains, New York 10604
Yoshihisa Yamamoto (151, 203), NTT Basic Research Laboratories 3-9-11,
Musashino-shi, Tokyo, 180 Japan
vu
PREFACE
Twenty years have passed since the attainment of the first low-loss optical fiber
intended for communication applications. The rapid research progress in the
lightwave field that ensued has led to the widespread application of optical fiber
communication throughout the telecommunications industry. Optical fiber
transmission systems are now in commercial use under the ocean to link
continents, on land to join cities, and in metropolitan areas to connect telephone
central offices and to distribute broadband services to the customers. Indeed,
optical fiber has emerged as the undisputed transmission medium of choice in
almost all areas of telecommunication, mainly because it offers unrivaled
transmission capacity at lower cost.
Research continues today on many fronts to explore the vast potential
bandwidth of low-loss single-mode fibers for long-distance transport as well as for
local networks and local distribution. Monolithic integration of photonic and
electronic devices is being pursued for both transmission and switching, to
enhance subsystem performance and functionality, and to reduce overall system
cost. The continuing interest in increasingly higher speed devices and systems is
shown by demonstrations of transmission experiments at data rates of up to 20
Gb/s. Coherent detection offers high receiver sensitivity and efficient means for
channel selection in a densely packed wavelength-multiplexed system, but
noncoherent techniques involving optical amplifiers and tunable optical filters
may pose a serious challenge. Various coherent and noncoherent experimental
systems have been demonstrated for transmission and networking. Recent
developments in erbium-doped fiber amplifiers are evoking renewed excitement in
the field as optical amplifiers, operated as non-regenerative repeaters, make
wavelength multiplexing attractive and therefore promise significant benefits in
operational flexibility and overall cost for both terrestrial and undersea long-haul
transmission. Nonlinear effects in the transmission fiber are therefore of great
interest, as these effects may impair signal transmission when distances between
regenerative repeaters become very large.
This book is a second volume of a treatise on optical fiber communications that
is devoted to the science, engineering, and application of information transmission
via optical fibers. The first volume, published in 1985, dealt exclusively with fiber
fabrication. The present volume contains topics that pertain to subsystems and
IX
X PREFACE
systems. Transmitters and receivers, which are basic to systems now operating in
the field, are treated in the first two chapters. The next two chapters cover topics
relating to coherent systems: frequency and phase modulation of the optical
carrier, and systems considerations and experiments. The last chapter reviews the
fundamentals of nonlinear effects in optical fibers and considers how systems are
affected by various nonlinear phenomena.
Tingye Li
Optical Transmitter Design
M. DIXON AND J. L. HOKANSON
AT&T Bell Laboratories
Solid State Technology Center
Breinigsville, Pennsylvania
I. Introduction 1
II. System Constraints on Transmitter Design 3
A. Modulation Properties of Sources 4
B. Modulation Format 8
C. Source Wavelength and Line width 10
D. Modulation Rate 14
III. Optical Properties of Sources for PCM Transmitters 18
A. Optical Pulse Requirements 18
B. Direct Current Modulation of Lasers and LEDs 19
C. Summary of Source Properties 25
IV. Circuit Strategies for PCM Transmitters 26
A. Modulator Circuits 27
B. Biasing Circuits for LEDs 29
C. Biasing Circuits for Lasers 31
D. Source Temperature Control 40
E. Equivalent Circuit Models 41
V. Practical PCM Transmitters and Their Performance 44
A. LED Transmitters 44
B. Laser Transmitters 48
C. Testing of Transmitters 64
VI. Multigigabit-per-Second Transmission Systems 66
A. System Overview 66
B. System Performance Limits 69
C. FT Series G 1.7 GB/s 1.55-jim DFB WDM Channel 69
VII. Conclusion 71
A. Summary of Presenter Transmitter Design 71
References 72
I. INTRODUCTION
The initial driving force for lightwave technology has been in the interoffice
and intercity trunking and loop feeder network of the telecommunications
system. The technology has expanded to capture both the terrestrial and the
subcable long-haul transmission networks, and it is moving rapidly to capture
the last few kilometers to the home markets. Exciting advances in optical
technology have been made over the last decade, with performance as
1
Topics in Lightwave Copyright © 1991 by American Telephone and Telegraph Company.
Transmission Systems All rights of reproduction in any form reserved.
ISBN-0-12-447302-4
2 M. DIXON AND J. L. HOKANSON
measured by the bit rate x distance product almost doubling each year. The
development of very high frequency, long-wavelength, single-frequency semi-
conductor lasers, wideband photodetectors, and microwave electronic cir-
cuits in both silicon and gallium arsenide, along with the concurrent reduc-
tion of fiber loss toward the Rayleigh scattering limit, has brought about this
phenomenal growth. As a result, large quantities of digitized information can
be transmitted over long distances and provide the foundation for the
envisioned future broadband integrated services digital networks, B-ISDN
(Catania, 1986). Such lightwave communication systems are configured pres-
ently, as shown in Fig. 1, to be compatible with existing communication
networks. It is seen immediately that optical transmitters play an integral
role in such systems. However, as a result, the choice of optical sources,
modulation schemes, and even optical performance are not dictated solely
by the transmitter designer. At a time when optical transmitter design is it-
self evolving, transmitter design decisions are often made to accommodate
advances in, or limitations of, other parts of a rapidly changing system.
For these reasons we shall initially discuss in Section II those system
considerations that have limited the majority of viable present-day optical
transmitter designs for long-haul communications to the direct-current
modulation of semiconductor light-emitting diodes (LEDs) and lasers. This
will include a brief description of optical sources, their available power,
wavelengths, linewidths, bandwidth, transmission characteristics of the fiber
medium, modulation format (analog or digital), the physical properties of the
sources available for modulation, and the modulation rate. These will be
discussed in the context of the technical considerations affecting their choice.
Following that, Section III will review the properties of optical sources
that are important for the most widely adopted of the modulation schemes,
INFORMATION INFORMATION
SOURCE RECEIVER
ï ï
CODER DECODER
ï
Ï
OPTICAL FIBER
OPTICAL OPTICAL
MODULATOR DEMODULATOR
TRANSMISSION MEDIUM
FIG. 1. Schematic of an optical fiber transmission system.
OPTICAL TRANSMITTER DESIGN 3
viz., pulse code modulation. This will cover, in a general manner, the desirable
optical pulse characteristics imposed by lightwave system constraints. It will
be followed by a description of the dynamic pulse response characteristics of
semiconductor LEDs and lasers, and their limitations in meeting such system
objectives. As the modulation rates cascade into the multigigabit range and
large fiber chromatic dispersion is encountered, system limitations brought
about by the effect of direct modulation on the dynamic spectral character-
istics of the laser source (Linke et al, 1985) has led to successful experiments
with external modulators (Korotky and Alferness, 1988). In this scheme, the
laser is operated in a cw mode, and its light is modulated using the external
modulator.
Section IV will discuss the modulation and feedback control strategies that
have been suggested to produce and to retain adequate transmitter perfor-
mance with both time and environmental changes of the direct current
modulated semiconductor sources and their packaging. Topics include new
ICs in Si-bipolar or GaAs MESFET for electrical-to-optical conversion;
microwave interconnections between components in the data path; and for
long-wavelength laser sources, thermoelectric heat pump temperature control.
The section will conclude with a discussion of computer modeling of laser
diodes.
Section V will present a more detailed discussion of practical transmitter
design. No attempt at completeness will be made, as this section represents
only our combined transmitter design experiences for real-world systems.
However, it will serve to illustrate the problems encountered, circuit strategies
adopted, and performance obtainable with pulse modulated transmitters up
to a few hundred Mb/s with LED sources and extending into the multigigabit
range with laser sources. Both short-wavelength (0.8-0.9 μπι) and long-
wavelength (1.3 μτή) devices will be discussed.
Section VI will discuss the successful laboratory demonstrations aimed at
the next generation of very large capacity systems with long repeater spans.
The maximum bit rate attainable in present experimental systems is limited by
laser modulation speeds and dynamic optical spectra; by external modulator
capabilities; by detector and receiver bandwidths; and finally, by the per-
formance of electronic amplifiers and circuits (Li and Linke, 1988).
Finally, the concluding Section VII will summarize the status of present-
day transmitter design.
II. SYSTEM CONSTRAINTS ON TRANSMITTER DESIGN
The optical transmitter in a lightwave system, of the type shown in Fig. 1,
performs the function of electro-optic transducer, as well as that of signal
4 M. DIXON AND J. L. HOKANSON
modulator. Some choices in the transmitter design are strongly influenced by
the system environment in which it is embedded. These include (1) modulation
properties of the source, (2) modulator format, (3) source wavelength and
linewidth, transmission characteristics of the fiber medium, and (4) modula-
tion rate. We will outline the significance of these properties for present
transmitter design.
A. Modulation Properties of Sources
A number of physical properties of optical sources potentially could be
modulated (Kaminow and Li, 1979). They can be seen clearly in Fig. 2.
However, our current choices from this list are reduced by limitations in the
state of the art of fibers and detectors, as well as of the sources themselves.
1. Fibers
Optical loss and its wavelength dependence, optical dispersion in its three
forms of material, waveguide, and intermodal dispersion, and optical depo-
larization caused by birefringence and fiber dimension variations are all
encountered during the transmission of a modulated signal. The difficulty of
producing fibers that retain a fixed polarization state (Rashleigh and Marrone,
1983) has so far eliminated modulation of the polarization vector.
INTENSITY
POLARIZATION
FREQUENCY
MODULATED, \J
OPTICAL
SOURCE DIRECTION
PHASE /
/Λ
/
FIG. 2. Optical wave properties for modulation.
