y controlled, the two polarization components are combined in phase (technically means with the same phase) before detection on the second photodiode and in quadrature on the third one. This structure is difficult to be realized via discrete components, due to the need of a very accurate control of the length of the light paths. On the other hand, it is quite suitable to be implemented by hybrid integration, where all the splitting structure shown in gray the figure is integrated in a single silicon on silica chip. In this case, the phase control can be implemented by thermal phase controller and the structure results quite stable. At the output of the interferometric polarimeter, the photocurrents results to be a lin- ear combination of the Stokes parameters, thus inverting this relation, an estimate of the Stokes parameters is obtained. Another interesting example of direct detection polarimeter is reported in [57] and it is based on the analysis of the incoming field by means of half and quarter wave plates. This structure does not require optical path length control and it is promising for discrete components implementation. Finally, a polarimeter can be designed based on the fact that partially reflecting mirrors reflects the orthogonal linear polarization in different ways. Its schematic block diagram is shown in Figure 9.36. Received field 7/1- 2 7/1 E2 Possible siO/si chip FIGURE 9.35 Direct Detection front end of a PolSK receiver based on half and quarter wave plates: BS indicates unbalanced Beam Splitters; PBS, Polarization Beam Splitters; DL, Delay Line; A is the matrix relating the photocurrents to the Stokes parameters. Telecommunication Networks Received field FIGURE 9.36 Direct Detection front end of a PolSK receiver based on partially reflective coating of four photodiodes. Three photodetectors are coated with a reflection coating characterized by reflection parameters 4k Pkr and Kk (k = 1, 2, 3) while the fourth photodetector is antireflection coated SO that all the optical power incident on it i