FIGURE 9.56 Normalized ISI index due to the interplay between SPM and chromatic dispersion in our 8-PolSK trans- Maximum transmitted power (dBm) mission example. Telecommunication Networks 8-PolSK FIGURE 9.57 Normalized ISI index due to tight optical filtering in 8-PolSK assuming Gaussian filtering. (After Jiang, Z. et al., Nat. Photon., 1, 463, 2009.) Inverse normalized link optical bandwidth (2R / Bo) The only possible solution is an analog per channel compensation. A very interesting method that can solve this kind of problems has been presented in [80]. The idea is to use an optical pulse shaper to predistort the optical signal at the transmitter in order to obtain the wanted pulse shape. An AWG is used as spectrum analyzed SO that, once the spectral components of the transmitted signal are separated, they can be filtered with an amplitude and phase mask to create an approximation of the Fourier transform of the wanted pulse. After that, the spectral components are recombined with another AWG. If only small changes in the channel characteristics has to be compensated, the intensity and phase masks can be tuned, for example, by temperature controller, if fast channel estimation and tracking is needed, they can be substituted with modulators controlled by a DSP. The scheme of this optical per channel compensator is presented in Figure 9.58 [81]. The entire pulse shaper has been also realized in a single InP-based monolithic chip, demon- strating the potentiality of this technique [82]. In principle, the chosen solution can compensate whatever amount of ISI can be gener- ated; in practice the amount of ISI that can be compensated is limited by both component and system limitations. The main limitations to the compensation method efficiency are the imperfect AWG working that causes the produced set of fields to deviate from the components of the Fourier transform of the input field, the needed approximation intrinsic in working with a discrete number of spectral components, and the presence of nonlinear propagati