order PMD compensator. A possible scheme of a second order PMD compensator is reported in Figure 9.14 [16,17]. The effectiveness of second order compensation is quite improved for high bit rate sig- nals with respect to first order compensation as shown in [18]. A comparison between dif- ferent first order and second order PMD compensators based on various control algorithms and compensating elements is reported in Figure 9.15 [18] where the outage probability P Telecommunication Networks Polarization Polarization controller controller Optimization algorithm FIGURE 9.14 Block scheme of a second order PMD compensator. is reported as a function of the design average PMD penalty ASN® (dB) for a bit rate of 100 Gbit/s, NRZ transmission, and an average PMD-induced DGD of 3.8 ps (deriving, e.g., from a 100km long link with DPMD = 0.38ps/km ¹/2). Comparing Figure 9.15 with literature, it is to take into account that the penalty is fre- quently expressed as instantaneous Eye Opening Penalty (EOP). Since evolution of the PMD is much slower than the bit-time, the PMD DGD can be considered almost con- stant for a great number of consecutive bits. In this situation, there is a direct relationship between the instantaneous EOP and the SN penalty once the link characteristics are fixed. As a matter of fact, indicating with I and I the average values of the photocurrent decision samples corresponding to the two transmitted bits, and with Ioo and I the same values in the absence of PMD, we can write (see Chapter 6) Outage probability (log10(Po)) No compensation Average penalty (dB) FIGURE 9.15 Comparison between different first order and second order PMD compensators based on various con- trol algorithms and compensating elements. (After Heismann, F., IEEE Photon. Technol. Lett., 17(5), 1016, 2005.) (a) Conventional FO-PMDC with variable differential phase delay. (b) PMDC based on the Kogelnik- Bruyère model described in [19]. (From Kogelnik, H. et al., Opt. Lett., 25(1), 19, 2000.) (c) PMDC based on the truncated EEF described 