y allowing the signal power to fall to low values. The compromise that system engineers reach is influenced by other factors, such as the logistics of amplifier placement and their associated electrical powering; the desire to make the periodicities of the amplifiers and the dispersion maps coincide; and, crucially, the cost of using many amplifiers. When using discrete amplifiers, such as EDFAs, the opti- mized solutions tend to favor a 60- to 80-km spacing for terrestrial systems and 40 to 50 km between submerged transoceanic repeaters. Benefits of Distributed Raman Amplification As argued earlier, the use of Raman amplification is very effective in providing a compro- mise between the opposing requirements of maintaining low overall noise figures and minimizing nonlinear crosstalk. Distributing the gain prevents the signal powers from descending to very low values, and SO we have the flexibility to launch lower channel powers and/or to extend the length of the amplifier stages. A simple simulated example that illustrates the longitudinal power variation in a 75-km link is shown in Figure 6.32. It is for a backward-pumped small-signal fiber Raman amplifier based on G.652 fiber. Three pump powers are selected to provide net gains of -3 dB, 0 dB (i.e., the transparency condition), and +3 dB; they are compared with an unpumped fiber. Operators sometimes prefer to have a small net gain (e.g., +3 dB, as shown in Figure 6.32) to compensate for any additional components such as isolators and filters. Various experi- mental studies have demonstrated good performance by placing the Raman pump lasers up to 100 km apart, even in very-long-haul communications (Ritwitt, 2005; Islam, DeWilde, & Kuditcher, 2004). Unfortunately, distributed FRAs cause problems that result from their low pumping efficiencies (in terms of dB gain per km of fiber). The necessary total launched powers can be 1 to 3 watts in some cases, which raises concern regarding harm to the eye and skin Gnet: 3 dB Fig. 6.32 Longitudinal gain dependence of a sm