s orthogonal to that of the AWG and the two dispersion axes of the LCOS are parallel and orthogonal to the AWG axis, it is possible to combine the dispersion contributions to have different dispersions for the different channels. Next Generation Transmission Systems Transmission fiber Grating Circulator FIGURE 9.8 Transmission fiber Discrete optics TODC based on LCOS technology. -800 ps/nm; -800 ps/nm 400 ps/nm 400 ps/nm FIGURE 9.9 0 ps/nm Differential delay of the LCOS based TODC. (After Seno, K. et al., 50-Wavelength chan- nel-by-channel tunable optical dispersion compensator using combination of arrayed- waveguide and bulk gratings, Conference on Fiber Communication (OFC), Collocated National Fiber Optic Engineers Conference, 2010 (OFC/ 1587.1 1587.2 1587.3 NFOEC), San Diego, CA, IEEE/OSA, s.l., 2010, Wavelength (nm) pp. 1-3.) In the paper where this architecture is proposed [5] the TODC design is performed for application in the L band at 40 Gbit/s. The measured dispersion performances for a single channel among the 50 that can be simultaneously processed by the device are reported in Figure 9.9 for different driver volt- ages of the LCOS. The ripple that can be observed in the experimental characteristics is attributed to the imperfect alignment of the LCOS device, a hypothesis that is reinforced by the presence of an analogous ripple in the amplitude transmittance [5]; in any case, the ability of the device to attain a range of dispersion between -800 and 800 ps/nm results clearly from the figure. 9.2.3 Fiber Polarization Mode Dispersion 9.2.3.1 Impact of Polarization Mode Dispersion on 100 Gbit/s Transmission A first evaluation of the impact of PMD on transmission of high speed channels has been carried out in Chapter 6 and an example of performance evaluation is reported in Figure 6.12 for channels at 40 and at 100 Gbit/s. Using the parameters of Figure 6.12 (see Table 6.1) it is possible to evaluate the trans- mission length at which the PMD-induced penalty is 2 dB, that we can call maximum PMD transmiss