Optical Fiber
`Communication
`Systems
`
`PNET etbio
`
`Leonid Kazovsky
`Sergio Benedetto
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`Library of Congress Cataloguing-in-Publication Data
`Kazovsky, Leonid G.
`
`Optical fiber communication systems/Leonid Kazovsky, Sergio Benedetto, Alan Willner.
`
`p.
`cm.
`Includes bibliographical references and index.
`
`ISBN 0-89006-756-2 (alk. paper)
`
`1. Optical communications. 2. Fiber optics. 3. Digital communications.
`
`Sergio. II. Willner, Alan E. II. Title.
`
`TK5103.59.K39
`1996
`621.382°75—de20
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`
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`
`
`I. Benedetto,
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`96-27860
`cIP
`
`British Library Cataloguing in Publication Data
`Kazovsky, Leonid
`Optical fiber communication systems
`1.' Optical fibers 2. Fiber optics 3. Optical communications I. Title Il. Benedetto, Sergio
`Ill. Willner, Alan
`621.3°8275
`
`ISBN 0-89006-756-2
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`Cover design by Jennifer Makower=-
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`© 1996 ARTECH HOUSE,INC.
`685 Canton Street
`Norwood, MA 02062
`
`All rights reserved. Printed and bound in the United States of America. No part ofthis book
`maybe reproducedorutilized in any form or by any means,electronic or mechanical, includ-
`ing photocopying, recording, or by any information storage andretrieval system, without per-
`mission in writing from the publisher.
`All terms mentionedin this book that are known to be trademarks or service marks have been
`appropriately capitalized. Artech House cannotattest to the accuracyof this information. Use
`of a term in this book should not be regarded as affecting the validity of any ademark or
`service mark.
`
`International Standard Book Number: 0-89006-756-2
`Library of Gongress Catalog Card Number: 96-27860
`
`1098765 4 3 2
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` The Basic Binary Optical Communication System
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`Figure3.1BlockdiagramofthebasicIM-DDopticalbinarycommunicationsystem.
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`
`
`
`
`
`
`simpler and ms) consists
`Information
`
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`Amplifier|°
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`8 RECEIVER
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`(PIN/Avaianchediode)
`
`Photodetector
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`
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`
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`as shown in
`itted into an
`odulation of
`in Fig. 3.1).
`the emitted
`uency chirp
`' affects the
`
`ing, may be
`3d. The most
`10].
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`ptical beam
`nuation and
`ength of the
`y transmit a
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`or an APD
`ronic signal.
`fication and
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`le threshold.
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`and expands
`voted to the
`:
`1 noises and
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`focusing on
`'s of system
`il noise, and
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`ary symbols
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`
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`Coherent Systems
`
` ification, which causes
`
`the information channels in the frequency domain,a key feature for the exploitation
`retical quantum noise
`of the huge bandwidth ofoptical fibers, particularly in the context of communication
`networks, where the frequencyselectivity of conventionalradio systems combined with
`the immense bandwidthof optical fibers opens up a wide range of new applications
`for telecommunications. Moreover, coherent technology has the potential of reducing
`by a factor of up to 4-5 the bandwidth through the use of multilevel transmission.
`Further benefits have to do with the possibility of using constant envelope
`modulation schemes, like PSK and FSK. The former requires an external modulator
`butyields a reduced impactofstimulatedBrillouin scattering (from 3 dBm to 30 dBm),
`whereasthelatter can be obtained by direct modulation of the laser source,like for
`direct detection ASK modulation. With respect to direct detection, however,it has the
`advantage of significantly reduced chirp effects.
`Optical coherent communication usesthe opticalfield as a very high frequency
`carrier whose amplitude, phase, frequency, or polarization may be modulated by the
`information-bearing signal. Althoughthis is very much the sameas is commonly done
`for electromagnetic fields at lower frequencies, the big difference between the carrier
`frequency and the information signal bandwidth poses in the optical case some
`peculiar technological problems. Even the term coherent hashere a different meaning
`from the standard radio environment. In fact, it is customary in the optical commu-
`nication community to associate the adjective coherent to those systems in which a
`local oscillator lightwaveis added to the incomingsignal, even if subsequent processing
`and demodulation completely ignore the phase and frequency, as is the case of
`envelope detectors. This contrasts with the meaning of coherent systemsintheclassical
`communication literature, which require the recovery and use of the phase and
`frequencyofthe carrier to perform the demodulation and detection.
`The basic configuration of a coherent communication system is shown in
`Figure 4.1. A laser-emitted light possessing a sufficiently stable frequency (quasi-
`monochromatic signal) is used as the carrier wave and modulated (in amplitude,
`frequency, phase, or polarization) by the informationsignal. At the receiversite, the
`
`x
`
`Information Signal
`
`Data Out
`
`electronic components
`ow loss anddispersion,
`£ megahertz or, even,
`dation techniques and
`eater spacing through
`at of coherent optical
`
`eral advantages over
`ison is that, when the
`ns are limited only by
`ntly different from the
`al noise dominate the
`Chapter 3 that several
`ry to achieve an error
`antum noise limit of
`ation schemes brings
`il communications. As
`: modulation requires
`s crease in sensitivity
`n increase in repeater
`compensatefor bridg-
`
`sreat success obtained
`>), which have allowed
`to a point where the
`m is presently a matter
`
`. lies in the increased
`M environment, they
`nicrowavefilters after
`h WDM,must employ
`order of a nanometer
`
`ich denser packing of
`
`
`
`Modulator
`
`Polarization
`
`I
`
`Maintaining Fiber
`
`
`
`Demo -
`dulator
`
`
`
`
`ms, the same term applied
`(FDM).
`GHz) have been reported.
`tt detuning and computer-
`annel is selected.
`
`
`Local
`Oscillator
`
`Circuit
`
`Figure 4.1 Block diagram of a coherent optical communication system.
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`Figure 7.16 Optical WDM channels being demultiplexed by an opticalfilter.
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`filter. In the figure, four channels are input to an optical filter that has a nonideal
`transmissionfiltering function. Thefilter transmissionpeak is centered over the desired
`channel, in this case, As, thereby transmitting that channel and blocking all other
`channels. Becauseof the nonidealfilter transmission function, some optical energy of
`the neighboring channels leaks throughthefilter, causing interchannel, interwave-
`length cross-talk. This cross-talk has the effect of reducing the selected signal’s
`contrast ratio and can be minimized byincreasing the spectral separation between
`channels. Although there is no set definition, a nonstandardized convention exists for
`defining optical WDM, dense WDM, and frequency-division multiplexing (FDM)as
`encompassing a system for which the channel spacing is approximately 10 nm, 1 nm,
`and 0.1 nm, respectively. However, we will not make any distinction among those
`system labels in this book.
`
`Modulated
`Channel
`
`i
`
`JL
`Wavelength
`INPUT
`
`MMED
`
`avelengths.
`t emit light
`nto a single
`il fiber, the
`distributing
`ch receiver
`cach laseris
`ritted along
`is the sum
`am capacity
`itted on one
`sm circum-
`bandwidths
`red require-
`nodulators)
`andwidth is
`
`ig an optical
`
`Multichannel Systems
`
`
`
`SwitchingMatrix
`
`My A2s ony AN
`Wavelength
`
`Ome
`Filter
`
`Spectrum Optical
`Detector
`
`Figure 7.15 Diagram of a simple WDM system.
`
`Optical
`Filter
`oy
`
`¢ véf \4
`1
`
`
`
`_naha.
`
`Tunable Optical
`Filter
`(A3)
`
`OUTPUT
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