is again expressed by an equa- tion like (6.92) with the parameters of the SSFM and the power budget equations becomes P1 - AT - A' Arec (all dB) (6.96) 240 km SSMF Raman Raman pumps pumps FIGURE 6.34 Architecture of a single span DPSK system using all Raman amplification both in the DCF and in the transmis- sion SSMF. TX, transmission transponders; RX, receiver transponders; ISO, isolator; DCF, dispersion compen- sating fiber; GFF, gain flattering filter (if needed); PEF, pump extraction filter; AWG, array waveguide; SSFM, standard single mode fiber. Signal power (dBm) Link length (km) FIGURE 6.35 Single-channel optical power profile in the Raman amplified single span system depicted in Figure 6.34. Transmission Systems Architectures and Performances Pmax = GRI + P - A (all dB) (6.97) where the symbols are those of the previous section but GR3 that is the gain of the new Raman section and A' that now refers to the passive part of the SSFM fiber, that is, L-L3 if the Raman pumps are stopped at a distance L3 from the end of the SSMF. The gain in the SSMF is used differently from that in DCF. As a matter of fact, the pumps are stopped after a length when the power is the same it has to be at the output of the line (compare Figure 6.32 for the gain behavior with distance). The gain at that point is zero, but it is as if the length was reduced of L3 since in this part of the link the effective loss is zero. Naturally, ASE is generated, and in order to evaluate the generated ASE this piece of fiber has to be considered as a distributed amplifier adding the noise term to the Raman equations, linearizing the equations around the noiseless solution and estimating the lin- earized variance of the amplified field amplitude. This is an easy but cumbersome procedure and we do not detail it here, even because the relative noise contribution is not very important. In the case of the DPSK the maximum power per channel changes too, due to the greater sensitivity to nonlinear phase noise becoming 15.5mW. The design results are su