scattering and allows forward and backward pumps to be effectively separated. Today Raman amplifiers are not considered as possible substitutes of EDFAs, even if this would be not impossible at least in high-performance systems, but are adopted as complementary amplification besides EDFAs. An exception of course is the case in which transmission in S band is needed, a case in which Raman amplifiers are the only solution. 4.3.4 Hybrid Raman-EDFA Amplifiers Erbium-doped and Raman amplifiers can be usefully combined in very high-performance transmission systems. In this case, distributed Raman amplification can be used either to extend the bandwidth of EDFA amplifiers or to partially compensate the span loss via distributed amplification [60,73]. In the first case, the amplifier scheme is that shown in Figure 4.41. The optical signal divided into bandwidth, C and L bands are amplified via specialized EDFAs, while the other bandwidths are amplified through concentrated Raman amplifiers. This is a promising solution for ultra-high-capacity systems, even if balancing problems among amplifiers in the different bands can be difficult to solve. The other application, however, is more than promising, and ultra-long-haul systems using this solution are already on the market. The scheme of the hybrid amplifier is reported in Figure 4.42, while in Figure 4.43 the power profile along a link adopting hybrid amplifiers is compared with the power profile along a link adopting only concentrated EDFA amplifiers. From Figure 4.42, some key advantages of using hybrid amplifiers can be seen immedi- ately. From the fact that the Raman amplifier works as a low-noise preamplifier (it is dis- tributed; thus, the noise figure is small) and from the general theory of cascaded amplifier, it is immediately deduced that the hybrid amplifier shall have a better noise figure with respect to an equivalent concentrated amplifier. Thus, either a lower signal powers can be used or a higher loss can be tolerated. Alternatively, a longer transmission 