Semiconductor Optical Amplifiers

171 Pages·2004·8.23 MB·English

by  Michael J. Connelly (auth.)

SEMICONDUCTOR OPTICAL AMPLIFIERS
Semiconductor
Optical Amplifiers
by
Michael J. Connelly
University of Limerick, Ireland
KLUWER ACADEMIC PUBLISHERS
NEW YORK,BOSTON, DORDRECHT, LONDON, MOSCOW
eBookISBN: 0-306-48156-1
Print ISBN: 0-7923-7657-9
©2004 Kluwer Academic Publishers
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Print ©2002 Kluwer Academic Publishers
Dordrecht
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No part of this eBook maybe reproducedor transmitted inanyform or byanymeans,electronic,
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For my parents Michael and Margeret
and brother Brendan
Contents
Preface ix
INTRODUCTION 1
BASIC PRINCIPLES 7
STRUCTURES 21
MATERIALS 43
MODELLING 69
BASIC NETWORK APPLICATIONS 97
FUNCTIONAL APPLICATIONS 127
Index 167
vii
Preface
Communications can be broadly defined as the transfer of information from
one point to another. In optical fibre communications, this transfer is
achieved by using light as the information carrier. There has been an
exponential growth in the deployment and capacity of optical fibre
communication technologies and networks over the past twenty-five years.
This growth has been made possible by the development of new
optoelectronic technologies that can be utilised to exploit the enormous
potential bandwidth of optical fibre. Today, systems are operational which
operate at aggregate bit rates in excess of 100 Gb/s. Such high capacity
systems exploit the optical fibre bandwidth by employing wavelength
division multiplexing.
Optical technology is the dominant carrier of global information. It is also
central to the realisation of future networks that will have the capabilities
demanded by society. These capabilities include virtually unlimited
bandwidth to carry communication services of almost any kind, and full
transparency that allows terminal upgrades in capacity and flexible routing
of channels. Many of the advances in optical networks have been made
possible by the advent of the optical amplifier.
In general, optical amplifiers can be divided into two classes: optical fibre
amplifiers and semiconductor amplifiers. The former has tended to dominate
conventional system applications such as in-line amplification used to
compensate for fibre losses. However, due to advances in optical
semiconductor fabrication techniques and device design, especially over the
last five years, the semiconductor optical amplifier (SOA) is showing great
promise for use in evolving optical communication networks. It can be
utilised as a general gain unit but also has many functional applications
including an optical switch, modulator and wavelength converter. These
ix
x Introduction
functions, where there is no conversion of optical signals into the electrical
domain, are required in transparent optical networks.
It is the intention of this book to provide the reader with a comprehensive
introduction to the design and applications of SOAs, particularly with regard
to their use in optical communication systems. It is hoped that the book has
achieved this aim.
Chapter 1
INTRODUCTION
In this chapter we begin with the reasons why optical amplification is
required in optical communication networks. This is followed by a brief
history of semiconductor optical amplifiers (SOAs), a summary of the
applications of SOAs and a comparison between SOAs and optical fibre
amplifiers (OFAs).
1.1 THE NEED FOR OPTICAL AMPLIFICATION
Optical fibre suffers from two principal limiting factors: Attenuation and
dispersion. Attenuation leads to signal power loss, which limits transmission
distance. Dispersion causes optical pulse broadening and hence intersymbol
interference leading to an increase in the system bit error rate (BER).
Dispersion essentially limits the fibre bandwidth. The attenuation spectrum
of conventional single-mode silica fibre, shown in Fig. 1.1, has a minimum
in the 1.55 wavelength region. The attenuation is somewhat higher in the
1.3 region.
The dispersion spectrum of conventional single-mode silica fibre, shown
in Fig. 1.2, has a minimum in the 1.3 region. Because the attenuation and
material dispersion minima are located in the 1.55 and 1.3
‘windows’, these are the main wavelength regions used in commercial
optical fibre communication systems. Systems operating in the 830 nm
region are also utilised, mainly for short-haul links at moderate bit rates
which do not usually require optical amplification.
Because signal attenuation and dispersion increases as the fibre length
increases, at some point in an optical fibre communication link the optical
signal will need to be regenerated. 3R (reshaping-retiming-retransmission)
1
2 Semiconductor Optical Amplifiers
regeneration involves detection (photon-electron conversion), electrical
amplification, retiming, pulse shaping and retransmission (electron-photon
conversion).
This method has a number of disadvantages. Firstly, it involves breaking
the optical link and so is not optically transparent. Secondly, the regeneration
process is dependent on the signal modulation format and bit rate and so is
not electrically transparent. This in turn creates difficulties if the link needs
to be upgraded. Ideally link upgrades should only involve changes in or
replacement of terminal equipment (transmitter or receiver). Thirdly, as
regenerators are complex systems and often situated in remote or difficult to
access location, as is the case in undersea transmission links, network
Introduction 3
reliability is impaired. In systems where fibre loss is the limiting factor, an
in-line optical amplifier can be used instead of a regenerator. As the in-line
amplifier has only to carry out one function (amplification of the input
signal) compared to full regeneration, it is intrinsically a more reliable and
less expensive device.
Ideally an in-line optical amplifier should be compatible with single-mode
fibre, impart large gain and be optically transparent (i.e. independent of the
input optical signal properties).
In addition optical amplifiers can also be useful as power boosters, for
example to compensate for splitting losses in optical distribution networks,
and as optical preamplifiers to improve receiver sensitivity. Besides these
basic system applications optical amplifiers are also useful as generic optical
gain blocks for use in larger optical systems. The improvements in optical
communication networks realised through the use of optical amplifiers
provides new opportunities to exploit the fibre bandwidth.
There are two types of optical amplifier: The SOA and the OFA [1-6]. In
recent times the latter has dominated; however SOAs have attracted renewed
interest for use as basic amplifiers and also as functional elements in optical
communication networks and optical signal processing devices.
1.2 BRIEF HISTORY OF SEMICONDUCTOR OPTICAL
AMPLIFIERS
The first studies on SOAs were carried out around the time of the invention
of the semiconductor laser in the 1960’s. These early devices were based on
GaAs homojunctions operating at low temperatures. The arrival of double
heterostructure devices spurred further investigation into the use of SOAs in
optical communication systems. In the 1970’s Zeidler and Personick carried
out early work on SOAs [7-8]. In the 1980’s there were further important
advances on SOA device design and modelling. Early studies concentrated
on AlGaAs SOAs operating in the 830 nm range [9-10]. In the late 1980’s
studies on InP/InGaAsP SOAs designed to operate in the 1.3 and 1.55
regions began to appear [11].
Developments in anti-reflection coating technology enabled the
fabrication of true travelling-wave SOAs [12]. Prior to 1989, SOA structures
were based on anti-reflection coated semiconductor laser diodes. These
devices had an asymmetrical waveguide structure leading to strongly
polarisation sensitive gain.
In 1989 SOAs began to be designed as devices in their own right, with the
use of more symmetrical waveguide structures giving much reduced
polarisation sensitivities [13]. Since then SOA design and development has