compensator. For their nature, all electronics adaptive dispersion compensators can be optimized SO to face also PMD. The great majority of them does not distinguish pulse broadenings due to different physical causes SO that, naturally, they try to compensate any distortion. However, linear compensators like FFE-DFE suffer in terms of performances of the non- linear nature of the communication channel. Completely different is the situation for adaptive nonlinear compensator that reaches a very good level of compensation (see Figure 6.11). Up to now, we have evidenced the penalty caused by linear and nonlinear propagation effects on the system performances. Now we pass to consider another important element of the transmission chain: the detector. A PIN photodiode can be substituted with an APD, SO to exploit the detector gain to increase the SN that is dominated by the receiver thermal noise. From the previous section, we have all the elements to analytically estimate the BER in this case. There is only one important observation to do. Looking at Equations 6.27 through 6.29 we can see that the overall noise power is com- posed of two terms. The thermal noise that does not depend on the APD gain and the sum of shot noise and dark current noise that depends on the gain. For a great value of Gm the shot term prevails and increasing the gain the SN decreases; for small values of Gm the APD behaves like a PIN with a small gain SO that increasing the gain the SN increases. This behavior indicates the presence of an optimum gain that maximizes the SN The optimum gain naturally depends on the parameters of the APD. Just for an example, the parameters of a medium performance InGaAs APDs are reported in Table 6.1. The Telecommunication Networks Sensitivity at 10 Gbit/s (dBm) -31.0 -32.0 -33.0 -34.0 APD gain (dB) FIGURE 6.13 Sensitivity (input optical power needed to attain a BER of 10-12 at 10 Gbit/s) versus APD gain in the absence of dispersion and nonlinearity. The parameters are reported in Table 6.1. optimum gain at 10 