er, that it can be realized by using hybrid integration with two Silica on Silicon chips and a modulator array composed of three phase modulators (let us remember that the modulator of Figure 9.21 is only apparently simpler, since each amplitude modulator is in reality a couple of phase modulators on the M-QAM symbol stream Multilevel encoder Amplitude e1 (t) modulator Transm. laser Amplitude modulator e2(t) Dual amplitude modulator FIGURE 9.20 Block scheme of an M-QAM modulator. Telecommunication Networks Phase modulator 1 x 3 splitter Phase modulator coupler FIGURE 9.21 Architecture of a hybrid modulator for Phase 64-QAM realized with a parallel of three phase modulator modulators. Phase Electrodes Thermo optics modulator phase equalizers FIGURE 9.22 Scheme of the layout of a hybrid Si-SiO2 and Lithium Niobate-integrated opti- cal circuit for the implementation of the modulator represented in Figure 9.20. Silica on silicon Lithium Silica on silicon (From Yamazaki, H. et al., IEEE Photon. (14 X 7 mm) niobate (34x71 mm) Technol. Lett., 22(5), 344, 2010.) (64x6 mm) branches of an interferometer). The scheme of the hybrid integrated chip is reported in Figure 9.22 where the Silica on Silicon chips are in white and the Lithium-Niobate chip is in gray. With the same technology, also other kinds of multilevel modulators has been designed and prototyped, like a 16-QAM modulator [33] and a 4-DPSK modulator [34] demonstrat- ing its flexibility. 9.3.2.3 Multilevel Polarization Modulation (M-PolSK) This modulation format exploits the property of the optical field to be polarized to switch the field polarization among a certain number of states that represents the transmitted symbols. In order to represent optical field states with the same power and phase, but with different polarization in the quadrature space it is needed to represent the surface defined by the following equations: e2(t) = JP cos(a)sin(0) e3(t) = VP sin(a)cos(0+()) (9.20) e4(t) = JP sin(a)sin(9+()) Next Generation Transmission Systems where the three ang