ign SO that the com- pensating effect is evident. The best working point is around 2 dBm, which is exactly the chosen working point for our system. However, even at the best point, the link is SO long that the pulse in free propagation completely loses its shape and widens more than 50%. Summarizing, at 2 dBm per channel, the ISI index is 1.5 due to SPM and chromatic dis- persion and 0.25 due to PMD, for a global ISI index of 1.55. From Figure 9.53, we conclude that adopting a 16 states MLSE, the corresponding pen- alty is about 0.2 dB. 9.5.4.2.3 Nonlinear Crosstalk: XPM and FWM Also in this case, following [75], the system is in the SPM-dominated regime due to the adopted chromatic dispersion map. As far as FWM is concerned, the same model used in Chapter 6 can be reapplied here: FWM-induced crosstalk can be considered as an additional noise, due to its variability that is almost random for the effect of the walk-off. The effect of both XPM and FWM are however enhanced by the channel proximity, by the great bandwidth, and by the long distance. Globally, the FWM penalty is 1.3 dB and the XPM penalty is 0.4dB. 9.5.4.2.4 Transmitting Laser and Local Oscillator Phase Noise We will imagine using external cavity tunable lasers and, as a reference, we will assume the lasers parameters values listed in Table 5.5. The linewidth listed in the table is 50KHz, really small with respect to the symbol rate of 25 Gsymbols/s. In our case, Av/Rs = 0.0002%. Looking at Equation 9.46 we see that the phase noise plateau is well beyond 55 dB, mean- ing, that at the required values of SN, the phase noise induces only a small power penalty that from Equation 9.78 can be evaluated in 0.1 dBm. 9.5.4.2.5 Depolarization due to PMD and Nonlinear Propagation The analysis of the behavior of SPOL where a WDM signal propagates through the fiber has been carried out in [65,77]. In particular, applying the methods developed in [65] to the present case, the plot reported in Figure 9.64 is obtained for the penalty due to the depolarization. In the 