ectly linear transmission of a WDM 100 Gbit/s NRZ signal through a Normalized pulse amplitude L = 5 km L=1 km FIGURE 9.1 NRZ pulse at 100 Gbit/s after linear propagation without any PMD. The effect of dispersion slope is evident in the asymmetric form of the pulse Time (ps) after propagation. Normalized pulse amplitude L 50 km L 200 km FIGURE 9.2 NRZ pulse at 100 Gbit/s after linear propaga- tion without any PMD. The spectrum is cen- tered on the dispersion zero of the used DS Time (ps) fiber to show the effect of dispersion slope. Telecommunication Networks link of DSF fiber is reported in Figure 9.1 without polarization mode dispersion (PMD). The WDM comb is composed of 16 channels at 200 GHz spacing and the dispersion zero is exactly in correspondence with the carrier of the eighth channel. In particular, the pulse shape of the 16th channel is reported for a dispersion slope of 0.05ps3/ km and different link length. All the pulses are normalized to their maximum amplitude. The pulse distortion due to the dispersion slope is quite evident, and after 5 km propaga- tion, the greatest part of the transmitted energy is out of the bit interval while the pulse shape is completely destroyed. This means that dispersion compensation has to be performed channel by channel, tak- ing into account the exact dispersion value in the channel spectral position. But this is not enough, as shown in Figure 9.2. In this figure, similar to Figure 9.1 for construction, the center channel of the WDM comb is considered, having the dispersion zero in its center wavelength. Nevertheless, the fact that the channel spectrum is SO wide causes a pulse distortion due to the third order dispersion even if the average dispersion in the channel bandwidth is zero. The intersymbol interference (ISI) caused by third order dispersion is already impor- tant after 50 km propagation and after 200 km propagation a relevant quantity of the pulse energy is out of the bit interval. The need of per channel compensation depends from the width of the NRZ spec