Next Generation Transmission Systems Thus, multichannel TODCs are complementary to the electronic compensation and the most probable solution to deal with chromatic dispersion of large bandwidth optical sig- nals is a contemporary use of TODC for in-line compensation plus electronic fine equaliza- tion at the receiver. This solution, based on periodic compensation along the optical transmission line, has the advantage of avoiding combination between dispersion and nonlinear distortion that unavoidably occurs if all equalization is placed at the receiver and that renders electronic equalization much more difficult and expensive. In order to be used in this way, the multichannel TODC have to be simple from a design point of view and must be able to compensate the typical amount of dispersion occurring in a long haul system span. One of the more direct ways to realize a multiple channel TODC is shown in Figure 9.3 and consists in dividing the incoming optical channel between the two branches of an interferometer where two different dispersion elements are placed. The amount of optical power sent in each branch is regulated by a selectable directional coupler [2]. The transfer function of the system is given by (9.2) where ki indicates the coefficient of dispersion of the dispersive elements Ti is the overall delay along the two interferometer branches a is the tunable splitting ratio, while the factor 1/2 takes into account the unavoidable loss of the beam splitter The module of the transfer function writes (9.3) where kk1-k2and12 = The module of the spectral response of the interferometer is a periodic function, as obvi- ous, whose period is frequency dependent due to the quadratic behavior of the object of the cosine function. The optical bandwidth depends on the frequency dependent term and it is maximum where this term is zero, that is, when a = 0 or a = 1. On the other hand, the optical bandwidth is minimized when the third term in Equation 9.3 is maximum, that is, a = 1/2. In order to determine the group delay