 junction capacitance C and the differential resistance Rd, which represents the resistivity of p- and n-semiconductor materials and metal contact. In the traveling-wave configuration (Figure 5.30) the device is designed to have micro- wave co-propagation with optical waves. By matching the output termination, the electro-optic conversion is dominated by the distributed interaction between microwave and optical wave. The electrical equivalent model of the traveling-wave modulator, as shown in the figure, is a transmission line. The microwave experiences distributed junction capacitance and inductance along the waveguide; thus the limitation is not the total capacitance, but the microwave characteristics, termination and the velocity matching, somehow similar to what happens in the Mach-Zehnder modulator. Due to the large amount of studies done on Mach-Zehnder modulators and many other microwave circuits, there are several possible solutions to adapt microwave and optical propagation; thus it is possible to design the traveling-wave structure SO as to have better performance with respect to the lumped architecture [41]. However, in both the configurations, the more important property of EA modulators is that they can be integrated with other InP-based components. There are several commercial devices that are built as a laser plus an integrated EA modulator or even a set of DFBs, an EA modulator, and an SOA. These integrated components are instrumental in the design of small and low- consumption optical interfaces like high-performance XFP transceivers. p Contact n-InP n Contact Junction Voltage source FIGURE 5.29 Scheme and equivalent circuit of a lumped EA modulator. Rp, junction resistance; Cj, junction capacity; Cp, parasitic capacity. Technology for Telecommunications: Integrated Optics and Microelectronics p Contact n Contact n Contact Waveguide for traveling wave effect Voltage source FIGURE 5.30 Scheme and equivalent circuit of a traveling-wave EA modulator. (After Zhang, S.Z. et al., IEEE Photon. Technol. 