th coherent homodyne receiver. The value of the electrical signal to noise ratio allowing to reach a BER of 10-12 is in this case SN = 17.5 dB, really a low value for a 16 levels modulation format. The unilateral optical bandwidth of the signal is 25 GHz while the overall channel bandwidth as seen by the single channel is 38 GHz SO that = 1.5 and the required optical signal to noise ratio is SN, = 18.7. Due to the fact that the signal has a 25 GHz large spectrum on each side of the carrier, the channel spacing is selected to be 100 GHz, SO that 45 channels are transmitted in the extended C band. Precompensation is adopted, placing the DCF at the transmitter, and a residual disper- sion of 1000 ps/nm is left to compensate channel by channel to partially counterbalance nonlinear effects during propagation. Transmission Isolator fiber FIGURE 9.61 Block scheme of the line site of the Ultra Long Haul 100 Gbit/s system presented in this chapter. Raman pumps Next Generation Transmission Systems TABLE 9.15 Transmitter and Receiver Data for an Ultra Long Haul System at 100 Gbit/s Adopting Constant Power 16-4QAM with Coherent Detection Transmitting Chain Source laser maximum emitted power 16 dBm Laser linewidth (external cavity) 250kHz Mach-Zehnder Lithium Niobate phase modulator loss Mach-Zehnder Lithium Niobate Polarization modulator loss Multiplexer (multistage AWG X 90) loss Patchcords, connectors, monitoring taps loss Detection Chain High efficiency APD sensitivity at 25 Gsymbols/s -20.5 dBm Demultiplexer (multistage AWG X 90) loss 90° Hybrids + splitters + insertion of an integrated chip 10 dB Patchcords, connectors, monitoring taps loss 9.5.4.1.1 Power Budget and SN The main parameters of the components at the transmitter and at the receiver are reported in Table 9.15. With 16 dBs emitted power and a transmitter loss of 20 dB, the power at the booster input is 11.5 dB that is sufficient to saturate the booster without damaging it. The booster gain results to be dB and at the booster output and at the input of every 