quation 5.32 it is evident that the frequency response depends, at least in the absence of important defects of the chip, from the microwave circuit conveying the modu- lating signal. In Figure 5.27 the frequency response of a dual-drive LiNbO3 modulator is shown whose transfer function is quite flat in a bandwidth compatible with 10 Gbit/s modulation (requir- ing at least 12 GHz bandwidth due to overhead, see Chapter 3). A second important effect limiting the response of Mach-Zehnder modulators, espe- cially those built on LiNbO3 for the greater length of the interferometer's branches, is the different propagation speeds of the microwave and optical fields. As a matter of fact, this difference creates a misalignment between the modulating func- tion and the carrier to be modulated that depends on the distance along the waveguide, thus inducing signal distortion. This problem has to be solved with a very careful design of the microwave contacts that convey the microwave signal along the optical waveguide path. Telecommunication Networks Response spectrum (dB) Modulator Microwave circuit FIGURE 5.27 Frequency response of a Mach-Zehnder modulator and corresponding response of the microwave cir- cuit conveying the signal. Frequency (GHz) The design can be conceived in such a way as to use the waveguide contribution to the microwave group velocity to compensate, at least partially, the group velocity mismatch. Considering overall modulator performances, LiNbO3 modulators have the better characteristics in terms of modulation bandwidth, chirp-free or chirped operation, and dynamic range. Due to the values of the electro-optic constants of the LiNbO modulators built out of this material are bigger than those built out of InP and need much higher microwave voltage. Performances of InP-based modulators are improving fast and it is possible that in not SO long a time the performance difference will almost vanish [38]. At present, LiNbO 3 modulators are used in high-quality DWDM interfaces, when compromises are not possibl