232 CHAPTER 6. OPTICAL AMPLIFIERS
Figure 6.3: Three possible applications of optical amplifiers in lightwave systems: (a) as in-line
amplifiers; (b) as a booster of transmitter power; (c) as a preamplifier to the receiver.
6.2 Semiconductor Optical Amplifiers
All lasers act as amplifiers close to but before reaching threshold, and semiconductor
lasers are no exception. Indeed, research on semiconductor optical amplifiers (SOAs)
started soon after the invention of semiconductor lasers in 1962. However, it was
only during the 1980s that SOAs were developed for practical applications, motivated
largely by their potential applications in lightwave systems [3]–[8]. In this section we
discuss the amplification characteristics of SOAs and their applications.
6.2.1 Amplifier Design
The amplifier characteristics discussed in Section 6.1 were for an optical amplifier
without feedback. Such amplifiers are calledtraveling-wave(TW) amplifiers to em-
phasize that the amplified signal travels in the forward direction only. Semiconductor
lasers experience a relatively large feedback because of reflections occurring at the
cleaved facets (32% reflectivity). They can be used as amplifiers when biased be-
low threshold, but multiple reflections at the facets must be included by considering a
Fabry–Perot (FP) cavity. Such amplifiers are calledFP amplifiers. The amplification
factor is obtained by using the standard theory of FP interferometers and is given by [4]
GFP(ν)=( 1 −R 1 )( 1 −R 2 )G(ν)
( 1 −G√
R 1 R 2 )^2 + 4 G
√
R 1 R 2 sin^2 [π(ν−νm)/∆νL]