PMD mitigation by optic and electronic signal processing

We have seen that for electronic PMD equalization powerful nonlinear signal processing schemes and efficient and fast adaptation procedures exist. Nevertheless, at strong PMD distortion residual penalty below 4 dB seems not to be impossible in the presence of optical noise. Though optical compensation theoretically permits zero penalty operation, its limits seems to be posed by the lack of continuously and fast tunable filters (group delay lines, dispersion) which allow to avoid trapping in relative adaptation maxima, and by the availability of only one scalar feedback signal, which prohibits fast adaptation of more effective optical processing schemes.