wers. If the APD sensitivity is -28 dBm, it is reasonable to fix an amount of power Prec on the APD greater than the sensitivity, to be sure that the electrical noise will be negligible. Moreover, due to the performance required to the system, we will adopt a laser with an emitted power P=20 dBm; it is a DBR tunable laser with an integrated semiconductor optical amplifier (SOA) providing a gain of 8 dB that brings the power from 12 to 20 dBm. Calling Pmax=17.7mW the maximum peak power per channel at the Raman amplifier at output of the transmitter, the power budget equations write Prec=GR1+GR2P1-AT-A-Arec - - (alldB) (6.87) (6.88) where AT, A, and Arec are the losses of the transmitting chain, the transmission fiber, and the receiving chain. In the term A, there is the link length. A fast evaluation of the param- eters versus the link length suggests fixing it on the order of 300 km. Solving these equations, the target values of the gain of the two Raman amplifiers determined and, from the gain expression, the needed pump powers are fixed. All the results of the draft design are summarized in Table 6.12. In determining the gain of the amplifier, the DCF length has been optimized at both sides to achieve maximum amplification. To understand the meaning of this optimization, the behavior of the single-channel opti- cal power is reported in Figure 6.31. Observing Figure 6.31, emerges another important feature of Raman assisted transmission that is always present, but in this case is particu- larly evident. During propagation of the signal in the DCF two phenomena occur: Raman gain and attenuation due to scattering and absorption. TABLE 6.12 Results of the Draft Design of the All Raman Single Span Intensity Modulation and Direct Detection System Pump 1 power-receiver amplifier 455 mW Pump 2 power-receiver amplifier 350 mW Pump 1 power-transmitter amplifier 525 mW Pump 2 power-transmitter amplifier 600 mW Raman pumps insertion loss DCF length transmitter DCF length receiver Raman gain transmitter 23.5 dB Raman gain re