ense of that along the orthogonal one, the signal could be transmitted along the low ASE polarization thus improving the SNo. Unfortunately, the resulting quantum state of the optical field at the output of such an amplifier is not a coherent state and such nonclassical field states tend to asymptotically transform into a coherent state during a lossy propagation due to the combination with the void state (see Chapter 4, Basic Theory of Optical Amplifiers section for the quan- tum model of the power loss). Thus this solution, which could be very attractive for other applications, is not practical for telecommunications where long distance and thus high propagation loss is required. The only possible solution is to try to shrink the transmitted signal bandwidth by using suitable transmission formats. As a matter of fact, if the transmitted symbols are M instead that two, that is, if multilevel transmission is used, the bandwidth of the transmitted signal with a given bit rate results smaller with respect to that of a binary signal. With a good approximation, if Nonreturn to Zero (NRZ) transmission is used and the transmitted symbols are statistically independent, M-levels transmission has a unilateral optical bandwidth of the order of R/log2(M) almost independently from the modulation for- mat (excluding a few cases of frequency modulation). In order to gain 10 dB with multilevel transmission, 210 = 1024 symbols are needed, a thing quite difficult with current technology. If the target is to gain 4dB, to perform from an ASE point of view like a 40 Gbit/s trans- mission, it is sufficient to transmit 23 = 8 symbols, which is an approachable task, even if not easy. Thus, multilevel modulation is one of the main design strategies to arrive to a practical 100 Gbit/s long distance Wavelength Division Multiplexing (WDM) system whose perfor- mances would be comparable with a 40 Gbit/s or even with a 10 Gbit/s. Another possible strategy is using coherent detection. In a standard Intensity Modulation- Direct Detection (IM-D