an effect is negligible, as far as the peak power per channel along the link is limited at 17.5 mW, as it can be demonstrated by evaluating it starting from Raman gain curves. Thus, we will deploy two sections of DCF, one at the transmitter and one at the receiver, and we will optimize their length and dispersion coefficient to have the most efficient Raman gain. The peak power per channel on the link, that is, the power at the output of the Raman amplifier at the transmitter, is maintained at 17.5mW per channel, regulating the power of the Raman pumps. A scheme of the resulting link architecture is plotted in Figure 6.30. It is to be noted that in this case the DCF has a double role. It provides dispersion com- pensation and enhances the Raman gain. The two Raman amplifiers can be dealt with independently. Solving the Raman amplifier equation (see Chapter 4) for the amplifier at the receiver we obtain the following expression of the gain: (6.84) where (6.85) and the symbols have the following meaning: 82(w) is the microscopic Raman gain in the DCF located at the receiver. Vs is the attenuations of the signal in the DCF. P1 and P are the pumps powers injected at the DCF far end. L2 is the length of the DCF located at the receiver. LeD is the DCF effective length. Analogously for the Raman amplifier at the transmitter the gain results to be (6.86) 240 km SSMF Raman Raman pumps pumps FIGURE 6.30 All Raman amplification single span IM-DD system architecture. TX, transmission transponders; RX, receiver transponders; ISO, isolator; DCF, dispersion compensating fiber; GFF, gain flattering filter (if needed); PEF, pump extraction filter; AWG, array waveguide; SSFM, standard single mode fiber. Telecommunication Networks with the corresponding meanings of the symbols. The Raman gains have to be optimized both for flatness and for value. Flatness is achieved by placing the pumps suitably as seen in Chapter 4. As far as the gain is concerned, it has to be set equal to the required value by setting the values of the pumps po