o detect the transmitted quadrature vector for each symbol interval and com- pare it with the constellation in the symbol space to produce an estimate of the transmitted symbol. The receiver is divided into two sections: an optical front end (depicted in Figure 9.29a), whose scope is to detect the received field quadratures, and an electronic processing stage (whose architecture is reported in Figure 9.29b), whose scope is to invert the effect of fiber propagation to reconstruct the transmitted field. A key component in the optical front end is the /2 optical hybrid that is used, after the separation of the incoming field linear polarization components, both to insulate the quadratures of each polarization and to combine the LO with the incoming field before balanced detection. Optical hybrids has been produced with several technologies [48-50] and their perfor- mances are quite good and stable. The four fields É (j = 1, 2, 3, 4) obtained after polarization splitting and branching with /2 hybrids can be written as (9.33) where the information bearing quadratures are indicated with 5;(t) and the ASE optical noise quadratures with nj(t). The linear polarization unitary polarization vectors directed along the quadrature space reference axes are indicated with si(j = 1,2,3,4) while the ASE polarization vector, that is a fast function of time due to ASE complete depolarization, is indicated with k(t). Finally indicates the scalar product. The field emitted by the LO at /4 with respect to the local receiver axis is divided into its polarization components by a polarization beam splitter and coupled with the received field components by a set of four /2 optical hybrids. In a practical receiver, the set of hybrids can be substituted by a single monolithic hybrid realized in silica on silicon technology. A possible architecture for such a hybrid is shown in Figure 9.30. Also the set of eight PIN Photodiodes that are needed to realize four balanced receivers can be realized with a diode bar in a single package SO that the 