6.2. SEMICONDUCTOR OPTICAL AMPLIFIERS 241
Figure 6.9: (a) Shape and (b) spectrum at the output of a semiconductor optical amplifier with
G 0 =30 dB andβc=5 for a Gaussian input pulse of energyEin/Esat= 0 .1. The dashed curves
show for comparison the shape and spectrum of the input pulse.
consists of a random sequence of 1 and 0 bits. If the energy of each 1 bit is large
enough to saturate the gain partially, the following bit will experience less gain. The
gain will recover partially if the bit 1 is preceded by one or more 0 bits. In effect, the
gain of each bit in an SOA depends on the bit pattern. This phenomenon becomes quite
problematic for WDM systems in which several pulse trains pass through the amplifier
simultaneously. It is possible to implement a gain-control mechanism that keeps the
amplifier gain pinned at a constant value. The basic idea is to make the SOA oscillate at
a controlled wavelength outside the range of interest (typically below 1.52μm). Since
the gain remains clamped at the threshold value for a laser, the signal is amplified by
the same factor for all pulses.
6.2.4 System Applications
The use of SOAs as a preamplifier to the receiver is attractive since it permits mono-
lithic integration of the SOA with the receiver. As seen in Fig. 6.3(c), in this application
the signal is optically amplified before it falls on the receiver. The preamplifier boosts
the signal to such a high level that the receiver performance is limited by shot noise
rather than by thermal noise. The basic idea is similar to the case of avalanche pho-
todiodes (APDs), which amplify the signal in the electrical domain. However, just
as APDs add additional noise (see Section 4.4.3), preamplifiers also degrade the SNR
through spontaneous-emission noise. A relatively large noise figure of SOAs (Fn=5–
7 dB) makes them less than ideal as a preamplifier. Nonetheless, they can improve the
receiver sensitivity considerably. SOAs can also be used as power amplifiers to boost
the transmitter power. It is, however, difficult to achieve powers in excess of 10 mW
because of a relatively small value of the output saturation power (∼5 mW).
SOAs were used as in-line amplifiers in several system experiments before 1990.
In a 1988 experiment, a signal at 1 Gb/s was transmitted over 313 km by using four