chronous and CO polarized with the useful channel (see Equation 6.41) [19]. With all these approximations the photocurrent writes t<RpG(0(() (6.42) and we can use the tools used in the previous sections to evaluate the BER. In Figure 6.16, the worst case linear interference penalty is plotted versus the chan- nel spacing at 10 Gbit/s and for different optical filters. Perfect dispersion compensation and absence of relevant PMD and SPM are assumed. The results are plotted searching the worst case with respect to the relative phase of the carrier of the considered and the interfering channel. This assumption is important for small channel spacing, where only few oscillations of the relative interfering carrier (with angular frequencies A@) are com- prised into the bit interval. In normal situations, where there is at least a factor 5 between the bit interval and the interfering carrier relative frequency this is not an important assumption. Gaussian filters are considered in Figure 6.16a while ideally rectangular filters are used to plot Figure 6.16b. Moreover, the optical bandwidth is unilateral and this convention will be always assumed in this book. Thus, the minimum optical bandwidth that catch the entire signal spectrum main lobe is Bo= = and the Nyquist bandwidth is B = R/2. Sensitivity penalty (dB) Sensitivity penalty (dB) Gaussian filter Bo= R Rectangular filter Bo=6R Bo=6R Optical channel spacing (Aw/(2R)) Optical channel spacing (Aw/(2R)) FIGURE 6.16 Worst case linear interference penalty versus the channel spacing for different optical filters: (a) shows the results for Gaussian filters (where the bandwidth is half width at 1/e) and (b) for rectangular filters. Transmission Systems Architectures and Performances The impact of the optical filter shape is clear, since the amount of interference power depends on the filter shape. From the figures, also the oscillating nature of the interference can be noted, that depends on the channels spectrum shape. 6.2.6.2 Nonlinear Interference in Wavelength Division M