trum that is about 200 GHz. Since multilevel modulation shrinks the spectrum of the transmit- ted signal, it helps to attenuate the penalty dependence on third order dispersion. As an alternative, or even to integrate the effect of spectrum width reduction, per chan- nel compensation can be applied. The cheaper method to operate per channel dispersion compensation is to use an elec- tronic equalization circuit. The exact compensation of a precise wavelength behavior of the dispersion coefficient is more difficult if electronic dispersion compensation is used conjunction with Direct Detection since the nonlinear detector characteristic has to be taken into account. This difficulty is removed if coherent detection is used, due to the com- plete proportionality of the photocurrent to the optical field, both in amplitude and phase. In this condition, if linear fiber propagation can be assumed, the propagation channel is linear even considering the detector characteristic, thus allowing far more efficient disper- sion compensation to be applied. 9.2.2.2 Tunable Optical Dispersion Compensator A great amount of research has been devoted also to Tunable Optical Dispersion Compensators (TODC) for application at 40 and 100 Gbit/s. A TODC is an optical transpar- ent component that is able to compensate a selectable amount of dispersion (generally with a selected slope) on one or more high speed channels. TODC are divided into single channel and multiple channels devices, where the first are suitable to compensate a single channel, while a whole WDM comb can be simultaneously compensated by the latter. Single channel TODC [1] are mainly studied for intermediate reach, very high speed systems, where electronic dispersion compensation is not enough due to the nonlinear characteristics of the photodiode, but cost constraints or other considerations (related for example to real estate or power consumption) render unsuitable coherent detection. On the other hand, the advantage expected by multichannel TODC is the ability to com- 