r third transmission window versus the applied voltage. The structure of an EA modulator is based on a p-i-n junction built using suitable III-V alloy (e.g., InGaAsP) in order to work at the required frequencies. Since the modulation mechanism is essentially based on absorption coefficient modula- tion, the longer the modulator, the greater is the extinction ratio, since the loss in the off state is greater and the signal is better suppressed. On the other hand, a longer modulator has two issues to solve: the loss is high also in the on state, increasing the modulator insertion loss. Moreover, the longer the modulator, the greater are both the junction and the parasitic capacities, and thus the smaller the modula- tor bandwidth. It is clear that there is an unavoidable trade-off between the extinction ratio on the one hand and the insertion loss and bandwidth on the other. Optical power transmission (dB) 1570 nm 1560 nm 1550 nm Reverse bias voltage (V) FIGURE 5.28 Characteristic curve (transmission versus applied voltage) of an EA modulator realized in InGaAsP for different wavelength and polarization of the input carrier. (After Yamanaka, T. and Yokoyama, K., Design and analy- sis of low-chirp electroabsorption modulators using bandstructure engineering, 2nd Edn., Second International Workshop on Physics and Modeling of Devices Based on Low-Dimensional Structures, 1998.) Telecommunication Networks In order to try to remove this limitation, which also constrains the selection of the material with which the modulator is fabricated, traveling-wave EA modulators have been proposed. Figures 5.29 and 5.30 show the different structures between a lumped and a traveling- wave EA modulator. In the lump configuration, the microwave signal is applied to the center of the wave- guide. At high frequency, due to the strong microwave reflections at both ends, the device speed is strongly dependent on the total resistance capacity (RC) circuit time constant. As shown in the figure, the lumped EA modulator can be modeled as p-i-n