Design and fabrication of traveling wave electroabsorption modulator

Semiconductor electroabsorption modulator (EAM) is a promising alternative to lithium niobate modulator for digital and analog fiber optic links due to its inherent small size. high modulation efficiency, and potential of monolithic integration with other electronic and optoelectronic components. For high-speed application, the bandwidth of the lumped element EAM is known to be PC-time limited. To achieve an ultra large bandwidth in lumped element EAM, the modulation efficiency has to be greatly sacrificed. This is especially critical in analog operation where RF link loss and noise figure must be minimized. To overcome the RC bandwidth limit and to avoid significantly compromising the modulation efficiency, the traveling wave electroabsorption modulator (TW-EAM) has been proposed and experimentally investigated by several authors [1-5]. In our previous work [5], detailed theoretical analysis and numerical calculations have been carried out for ultra high-speed (>50 GHz) TW-EAM, including effects of velocity mismatch, impedance mismatch and microwave attenuation. It was found that due to the optical propagation loss of the waveguide, the TW-EAM waveguide length for maximum RF link gain is limited to 200 similar to 300 mu m A quasi-static equivalent circuit model was used to examine the TW-EAM microwave properties, including the effect of photocurrent. Three TW-EAM design approaches were discussed: low impedance matching; reducing the waveguide capacitance; and distributing the modulation region. Following the previous analysis, we have designed and fabricated TW-EAM devices using low impedance matching approach. figure 1 shows the cross section of the device. The TW-EAM optical waveguide is similarly designed as that of the lumped element EAMs [6]. Its microwave design is more critical and deserves a more thorough discussion.