etected at the decision level. This explains why in the plot the SN increases, decreasing the electrical bandwidth. Electrical SN (dB) Moe= 1 FIGURE 5.71 Relation between the optical signal to noise ratio (SNo) and the electrical signal to noise ratio (SN) for different values of the ratio between the optical bandwidth and the electrical unilateral bandwidth Optical SN (dB) (B /B ). Technology for Telecommunications: Integrated Optics and Microelectronics Optical power gain (dB) (electrical noise dominant) RS 16 (255,247) (255,239) RS 16 (255,223) FIGURE 5.72 SNo gain of three different RS codes in an unampli- Log10(BER) after FEC operation fied system. In Figure 5.72 the gain of a few RS codes is plotted versus the target value of the BER (logarithm of the target BER after FEC operation) when no optical noise is present. The greater the code overheard, the greater is the code gain, even if in this case there is a mod- est gain due to the fact that SN depends on the square of the optical power while the noise is added after square law detection. The FEC gain curve is shown in Figure 5.73 for the case in which the optical noise is dominant. In this case the FEC allows higher gains to be achieved. It is to be underlined that the FEC gain decreases at the increase of . This is due to the fact that in this case the plot is made maintaining constant the BER, and thus the SN. Inverting (5.53) we obtain (5.54) Thus it is evident that increasing M oe while SN is constant means decreasing SNo, making clear the behavior of the curves of Figure 5.72. From Figure 5.72 it is also derived that in the reference case M22, when the standard code RS (233, 249) (indicated with RS8 in this Figure) is used to reach a BER of 10-12, a cod- ing gain of 6 dB is attained. Optical SN gain (dB) (optical noise dominant) RS 16 FIGURE 5.73 SN, gain of three different RS codes in a system with in- line amplifiers SO that the performances are dominated by the optical noise. RS (233, 249) is indicated with RS8. Log10(BER) after FEC operation Tele