ly this technique if an external modulator is used for signal modulation. In this case, pre-chirp can be achieved by simply modulating the transmitting laser bias current or using the pre- chirp embedded functionality that is present in many external modulators. It is also to be noticed that pre-chirping does not conflict with dithering for Brillouin removal. This last modulation does not have requirements in terms of pulse shape, but only in terms of residual carriers' bandwidth width. Pre-chirp has to be optimized for the dispersion of the particular link and generally this optimization is carried out by an automatic control loop driving the chirp parameters in order to maximize either the EOP that is detected at the receiver or, when available through an error correcting code, the estimated BER. In real systems, the effectiveness of pre-chirp is limited essentially by nonperfect lin- earity of the phase modulation, by the spurious intensity noise that always comes with a semiconductor laser phase modulation, and by the jitter coming from imperfect synchro- nization of the phase modulation with the information bearing intensity modulation. In Figure 6.9, the dispersion related power penalty of a 10 Gbit/s system using a SSMF fiber and optical pre-chirping is shown, evidencing both the effectiveness and the limits of pre-chirping techniques. An advantage of the use of pre-chirping is that it can be combined with almost all the other dispersion compensating techniques with a good result and that it can be used also in the presence of SPM. Also in this last case, pre-distortion parameters have to be opti- mized for the specific case. Another method to compensate for channel distortions is electronic compensation. As discussed in Chapter 5, electronic compensation has two important characteristics: it potentially compensates for every kind of channel distortion, independently from its causes, SO that it can be useful also for management of nonlinear effects and, in the case of fast adaptive algorithms, also for PMD