Download Fiber-Optic Communication Systems, 4th Edition (Wiley Series in Microwave and Optical Engineering) PDF

TitleFiber-Optic Communication Systems, 4th Edition (Wiley Series in Microwave and Optical Engineering)
File Size39.6 MB
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Document Text Contents
Page 1

Fourth Edition

Govind P. Agrawal
The Institute of Optics
University of Rochester
Rochester, New York



Page 315

Chapter 7

Loss Management

As seen in Chapters 5 and 6, the transmission distance of any fiber-optic communica-
tion system is eventually limited by fiber losses. Until 1995, this loss limitation was
mostly overcome using optoelectronic repeaters, devices in which the optical signal
is first converted into an electric form using a receiver and then regenerated using a
transmitter. Such regenerators become quite complex and expensive for WDM sys-
tems because they require demultiplexing of individual WDM channels. An alternative
approach to loss management makes use of optical amplifiers, which amplify the en-
tire WDM signal directly without requiring conversion of each channel to the electric
domain. Several kinds of optical amplifiers were developed during the 1980s, and the
use of optical amplifiers for lightwave systems became widespread during the 1990s.
By 1996, optical amplifiers became a part of the fiber-optic cables laid across the At-
lantic and Pacific oceans. This chapter is devoted to the management of fiber losses in
long-haul systems. In Section 7.1 we discuss the common technique in which optical
amplifiers are used periodically along a fiber link and identify the two schemes known
as lumped and distributed amplification schemes. Section 7.2 is devoted to erbium-
doped fiber amplifiers, used routinely as lumped amplifiers. Section 7.3 focuses on
Raman amplifiers that have been developed for distributed amplification of lightwave
signals. The signal-to-noise ratio of amplified lightwave systems is considered in Sec-
tions 7.4 and 7.5, whereas Section 7.6 deals with the receiver sensitivity. The impact
of amplifier noise on the transmitted signal is studied in Section 7.7. The final section
focuses on issues relevant for periodically amplified lightwave systems.

7.1 Compensation of Fiber Losses
Fiber losses must be compensated for lightwave systems designed to operate over more
than 100 km or so because their cumulative effects eventually make the signal so weak
that information cannot be recovered at the receiver. In some cases, the use of two
lumped amplifiers, one at the transmitter end and the other at the receiver end, can
extend the system range to up to 400 km. Since long-haul and submarine lightwave
systems extend over thousands of kilometers, fiber losses must be compensated in such


Page 316

296 Chapter 7. Loss Management


Figure 7.1: Schematic of fiber-loss management using (a) lumped or (b) distributed amplifica-
tion schemes. Tx and Rx stand for optical transmitters and receivers, respectively.

systems using a chain of amplifiers that boosts the signal power periodically back to its
original value.

7.1.1 Periodic Amplification Scheme
Until 1990, the only loss-management technique available to the system designer con-
sisted of inserting an optoelectronic regenerator (often called a repeater) within the
fiber link every 80 km or so. A repeater is nothing but a receiver-transmitter pair. In
such a device, the optical bit stream is first converted into the electric domain and then
regenerated with the help of an optical transmitter. This technique becomes quite cum-
bersome and expensive for WDM systems as it requires demultiplexing of individual
channels at each repeater. A better solution to the fiber-loss problem is to make use of
optical amplifiers because they can amplify multiple WDM channels simultaneously.
Figure 7.1(a) shows how amplifiers can be cascaded in a periodic manner to form a
chain, and thus enable one to transmit an optical bit stream over distances as long as
10,000 km, while retaining the signal in its original optical form.

Depending on the amplification scheme used, one can divide amplifiers into two
categories known as lumped and distributed amplifiers. Most systems employ lumped
erbium-doped fiber amplifiers (EDFAs) in which losses accumulated over 60 to 80 km
of fiber lengths are compensated using short lengths (~10 m) of erbium-doped fibers
[l]-[4]. In contrast, the distributed amplification scheme shown in Figure 7.1(b) uses
the transmission fiber itself for signal amplification by exploiting the nonlinear phe-
nomenon of stimulated Raman scattering (SRS). Such amplifiers are known as Raman
amplifiers and have been used for lightwave systems since 2002. Their use for loss
compensation requires that one or more pump lasers at suitable wavelengths inject op-
tical power periodically, as shown in Figure 7.1(b).

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