d on LiNbO3 and on InP. By applying a voltage on the interferometer branches through suitable electrodes, the optical signal in each path is phase-modulated as the optical path length is altered by the variable microwave electric field. Injecting an optical field in the interferometer and combining at its output the fields coming from the two branches of the phase modulation it is converted into amplitude modulation due to interference. If the phase modulation is exactly equal in each path but different in sign, the modulator is chirp-free; this means that the output is only intensity-modulated without incidental phase (or frequency) modulation. In Figure 5.25 the schematic of a Mach-Zehnder modulator is shown with a particular reference to a LiNbO3 modulator. Mach-Zehnder modulators can be either dual-drive or single-drive. In a single-drive modulator a single electrode driven by a signal proportional to the data induces phase modulation on one interferometer branch. This phase modulation is designed SO that, at the output of the interferometer, the modulated field combines with the unmodulated one either in phase or in quadrature, SO as to produce no chirp and the maximum dynamic range in the intensity-modulated optical field. Telecommunication Networks Optical signal modulated Optical signal Microwave in FIGURE 5.25 Scheme of a Mach-Zehnder modulator. The dual-drive configuration uses a data-driven signal on one branch and a signal driven by the inverted data on the other branch to induce modulation on the two branches of the interferometer. The dual-drive configuration is more stable and resistant to changes in the driving sig- nal and in the environment. The transfer function of a Mach-Zehnder modulator is shaped as an interferometer characteristic response like 0.5[cos(x) + 1]. This means that the modulator cannot be driven directly with the signal proportional to data, otherwise the nonlinear transfer function will completely distort the signal. It is needed to bias the modulator with a continuous voltage 