he differential group delay (DGD) At and then recombined. The delay is generally obtained by rotating the PSPs to match the axes of a tunable birefringence element. Both the PSP decomposition and the DGD have to be adjusted via an adaptive algorithm to match the slowly varying transmission link characteristic [11-14]. Several algorithms have been devised to simplify the apparent complexity of the PMD compensator and several designs have been presented that are quite effective in perfectly compensating the PMD in the central frequency of the signal spectrum. Obviously, to compensate a whole WDM comb, it has to be demultiplexed and compen- sated channel by channel. Thus, a first order PMD compensator like that shown in Figure 9.13 is generally designed to be deployed immediately before detection. A first order PMD compensator has two main limitations. The first is related to the non- linear evolution of the signal along very long fiber links. When the linear and nonlinear polarization evolutions are mixed, it is not yet possible to distinguish them and a linear compensator is not yet able to correctly recover the signal [11]. This means that in order to use effectively such compensators, the propagation has to be maintained as linear as pos- sible (but for the soliton case that is considered in detail in [12]). More important, by increasing the bit rate, the bandwidth of the PSPs starts to be of the order or even smaller than the optical bandwidth of the signal. As a matter of fact, from Chapter 4, we derive that the PSP bandwidth in a Standard Single Mode Fiber (SSMF) if of the order of 100 GHz, while the bilateral optical bandwidth of a 40 Gbit/s NRZ signal is 80 GHz and that of a 100 Gbit/s NRZ signal is 200 GHz. In such a condition, intrachannel variations of the PSP causes a relevant signal distortion even if a first order PMD compensator is used. A more accurate compensation can be attained by approximating the PMD with a sec- ond order expression in W instead of a first order one [15] and designing a second 