lation The unilateral bandwidth of the signal generated after 8-PolSK modulation is around 33 GHz, against the 20 GHz available on the line (the system is designed with = 2 for a 10 Gbit/s channel). 9.5.3.1.1 Ideal SN and SN, In this case, the required SN to reach an error probability of 10-12 can be deduced from Figure 9.43 resulting to be 20.7 dB. Also in this case, the corresponding SNo can be obtained in the usual way since is smaller than one, but with greater than 0.5 (in a very first approximation the ratio between the optical bandwidth and the electrical one is 0.61). As a matter of fact, beyond 0.5 the equation we are using to relate the optical and the electrical SN is no more valid. The requirement in terms of ASE-limited SNo results to be 25.8 dB. 9.5.3.1.2 Power Budget and Polarization Dynamics In order to try a design using Direct Detection, we will select interferometric receiver. The relevant characteristics of receiver components are summarized in Table 9.13; all the power budget equations are fulfilled as can be easily derived. Next Generation Transmission Systems TABLE 9.13 Transmitter and Receiver Data for 8-PolSK Designed to Use a 10 Gbit/s Line Transmitting Chain Source laser maximum emitted power 13 dBm Laser linewidth (external cavity) 500kHz Mach-Zehnder Lithium Niobate modulator loss Multiplexer (multistage AWG X 90) loss Patchcords, connectors, monitoring taps loss Detection Chain High efficiency APD sensitivity at 33 Gsymbols/s -19.5 dBm Demultiplexer (multistage AWG x90) loss 90° Hybrid loss Splitters loss Delay line loss (low loss component) Patchcords, connectors, monitoring taps loss The situation would be very similar if the coated diodes receiver would be chosen. The ASE power in the signal bandwidth is -15.4 dBm SO that SNo = 17.44 dB. This value, even after the addition of 11 dB gain of a super-FEC, is only a bit higher than the required 25.8 dB leaving no sufficient margin for the penalties allocation. As a matter of fact we have to expect at least 6 dB of penalties and margin