ned is to detect the Stokes parameters to be able to operate processing and decision in the Stokes space. The block scheme of the optimum M-PolSK receiver is presented in Figure 9.33, and it is composed of a front end producing four photocurrents that are proportional to the Stokes parameters and by an electronic processing section that is in charge to compensate polar- ization fluctuations, in the coherent case to drive the LO frequency locking and to perform symbol decision. The block scheme of the coherent front end is reported in Figure 9.34; it is a combination of an heterodyne optical receiver and of square law type electronic demodulation that allows the Stokes parameters of the incoming field to be detected. Polarization rotation estimator Front Polar. Decision tracking device FIGURE 9.33 Local oscillator Functional scheme of the optimum M-PolSK frequency tracking driver receiver. Filter Received field Filter Local oscillator Filter FIGURE 9.34 Coherent front end of a PolSK receiver; BS, Beam Splitters; PBS, Polarization Beam Splitters. Next Generation Transmission Systems As far as Direct Detection is concerned, implementing an optical front end that is able to detect the Stokes parameters is equivalent to search the structure of a direct detec- tion polarimeter. Several such polarimeters are used in practice, based on different principles, and the optical front end scheme of choice depends essentially on practical considerations. An example of direct detection front end is reported in Figure 9.35: this structure is based on the analysis of the incoming field by means of quarter and half wave plates and polarization beam splitters. The input electrical field is divided by means of a polariza- tion beam splitter into its linear polarization components and then these are combined by means of a couple of standard beam splitters of splitting ratio E1, a couple of standard beam splitters of splitting ratio E2, and a couple of polarization beam splitters as shown in the figure. If the optical path is accuratel