264 CHAPTER 6. OPTICAL AMPLIFIERS
Figure 6.20: Receiver sensitivity versus optical-filter bandwidth for several values of the noise
figureFnwhen an optical amplifier is used for preamplification of the received signal.
EDFAs to boost the signal power from−8 to 15.5 dBm (about 35 mW). This power
level is large enough that SBS becomes a problem. SBS can be suppressed through
phase modulation of the optical carrier that broadens the carrier linewidth to 200 MHz
or more. Direct modulation of lasers also helps through frequency chirping that broad-
ens the signal spectrum. In a 1996 experiment, a 10-Gb/s signal was transmitted over
442 km using two remotely pumped in-line amplifiers [117].
6.5.2 Noise Accumulation in Long-Haul Systems..........
Optical amplifiers are often cascaded to overcome fiber losses in a long-haul lightwave
system. The buildup of amplifier-induced noise is the most critical factor for such
systems. There are two reasons behind it. First, in a cascaded chain of optical amplifiers
(see Fig. 5.1), the ASE accumulates over many amplifiers and degrades the optical SNR
as the number of amplifiers increases [118]–[121]. Second, as the level of ASE grows,
it begins to saturate optical amplifiers and reduce the gain of amplifiers located further
down the fiber link. The net result is that the signal level drops further while the ASE
level increases. Clearly, if the number of amplifiers is large, the SNR will degrade so
much at the receiver that the BER will become unacceptable. Numerical simulations
show that the system is self-regulating in the sense that the total power obtained by
adding the signal and ASE powers remains relatively constant. Figure 6.21 shows this
self-regulating behavior for a cascaded chain of 100 amplifiers with 100-km spacing
and 35-dB small-signal gain. The power launched by the transmitter is 1 mW. The
other parameters arePouts =8mW,nsp= 1 .3, andG 0 exp(−αLA)=3, whereLAis the