ticular, let us assume that it is 20% smaller than the speed needed to span all the possible states of polarization. The resulting outage probability is also reported in Figure 9.13 [8] and the result is that, even if the scrambling is much less effective, the outage probability is in any case much smaller than the case without scrambling if a sufficient number of scramblers is used. The fact that PMD fluctuations induce outage also suggests the fact that errors due to these fluctuations form bursts. In case of transmission channels causing error bursts, the use of specific burst correcting forward error correcting codes (FECs) is quite effective. Systems using specific FECs and fast polarization scrambling can be optimized to greatly reduce PMD penalty fluctuations up to very low values of the outage probability [9,10]. If the average penalty due to PMD is acceptable from a system design point of view, a reduction of the outage probability below a required lower limit is enough to assure a cor- rect system working. scramblers FIGURE 9.12 PMD related outage probability versus the num- Ideal ber of in line scramblers, both ideal and slow scramblers scramblers (see text) are considered. (After Lid, X. et al., Multichannel PMD mitigation through forward-error correction with distributed fast PMD scrambling, Optical Fiber Communication Conference (OFC 2004), Los Angeles, CA, IEEE, s.l., 2004, Vol. 1, pp. WE2 1-3.) Number of scramblers Next Generation Transmission Systems Polarization controller FIGURE 9.13 Optimization algorithm Block scheme of a first order PMD compensator. If the average PMD penalty is too high, an effective PMD compensation is needed that is able to reduce both the outage probability and the average penalty. The scheme of a simple PMD compensator, a so-called first order compensator, is reported in Figure 9.13. At the transmission fiber output a Polarization Controller is set to split the signal between fast (p_) and slow (pt) PSP components. The components along the PSPs are mutually delayed by t