 fiber with a residue chromatic dispersion after compensation of 0.5 ps2/km and a PMD parameter of 0.4ps/km ¹/2. The PMD induced penalty is shown in Figure 6.12 versus distance for 40 and 100 Gbit/s. It is clear that great areas of the figure do not correspond to links that can be realized with unamplified systems, but the graphic is quite important since, due to the linear nature of PMD, the effect is independent of the transmitted power. This means that Figure 6.12 approximately holds also for amplified systems (but for the fact that an accurate estimation of the penalty in that case has to take into account the different noise distribution). Figure 6.12 demonstrates that PMD is an important impairment in long-reach, high bit rate systems, and in a few important cases, for example, when mixed erbium-doped optical Optical signal to noise ratio (dB) +3 dBm Penalty 2 dB Link length (km) FIGURE 6.11 Performance of a mix look-up table and linear electronic equalizer: a maximum distance of more than 500km is foreseen with only signal pre-distortion in pure linear propagation. System parameters are reported in Table 6.1 but for fibers parameters that are reported in Table 4.1. (After Killey, R.I. et al., IEEE Photon. Tech. Lett. 17(3), 714, 2005.) Transmission Systems Architectures and Performances Penalty (dB) 100 Gbit/s 40 Gbit/s Link length (km) FIGURE 6.12 PMD induced penalty versus distance for 40 and 100 Gbit/s. The parameters of an SSMF fiber with a residue chromatic dispersion after compensation of 0.5ps2/km and a PMD parameter of 0.4ps/km ²/2 are considered. The system parameters are reported in Table 6.1. fiber amplifier (EDFA)-Raman amplification is used to maintain the optical power low and avoid nonlinear effects, it can be the ultimate limit to the system reach. PMD compensation is much harder with respect to chromatic dispersion compensation due to the fact that PMD not only is time varying, but it is also a random process. This means that the only possible way for compensation is to devise an adaptive 