184 CHAPTER 5. LIGHTWAVE SYSTEMS
Figure 5.1: Point-to-point fiber links with periodic loss compensation through (a) regenerators
and (b) optical amplifiers. A regenerator consists of a receiver followed by a transmitter.
(short haul) to thousands of kilometers (long haul), depending on the specific appli-
cation. For example, optical data links are used to connect computers and terminals
within the same building or between two buildings with a relatively short transmission
distance (<10 km). The low loss and the wide bandwidth of optical fibers are not of
primary importance for such data links; fibers are used mainly because of their other
advantages, such as immunity to electromagnetic interference. In contrast, undersea
lightwave systems are used for high-speed transmission across continents with a link
length of several thousands of kilometers. Low losses and a large bandwidth of optical
fibers are important factors in the design of transoceanic systems from the standpoint
of reducing the overall operating cost.
When the link length exceeds a certain value, in the range 20–100 km depending on
the operating wavelength, it becomes necessary to compensate for fiber losses, as the
signal would otherwise become too weak to be detected reliably. Figure 5.1 shows two
schemes used commonly for loss compensation. Until 1990, optoelectronic repeaters,
calledregeneratorsbecause they regenerate the optical signal, were used exclusively.
As seen in Fig. 5.1(a), a regenerator is nothing but a receiver–transmitter pair that de-
tects the incoming optical signal, recovers the electrical bit stream, and then converts
it back into optical form by modulating an optical source. Fiber losses can also be
compensated by using optical amplifiers, which amplify the optical bit stream directly
without requiring conversion of the signal to the electric domain. The advent of optical
amplifiers around 1990 revolutionized the development of fiber-optic communication
systems [8]–[10]. Amplifiers are especially valuable for wavelength-division multi-
plexed (WDM) lightwave systems as they can amplify many channels simultaneously;
Chapter 6 is devoted to them.
Optical amplifiers solve the loss problem but they add noise (see Chapter 6) and
worsen the impact of fiber dispersion and nonlinearity since signal degradation keeps
on accumulating over multiple amplification stages. Indeed, periodically amplified
lightwave systems are often limited by fiber dispersion unless dispersion-compensation
techniques (discussed in Chapter 7) are used. Optoelectronic repeaters do not suf-
fer from this problem as they regenerate the original bit stream and thus effectively
compensate for all sources of signal degradation automatically. An optical regenera-
tor should perform the same three functions—reamplification, reshaping, and retiming