Ultra Wide Bandwidth Traveling Wave Modulators In GaAs/AlGaAs

Abstract

Optical modulators with very wide electrical bandwidths are essential components for optical control of microwaves and millimeter waves as well as high speed optical communication systems. It is possible to use different technologies to realize such components. LiNbO3 offers mature technology but suffers from a large index difference between optical and microwave frequencies, drift and low optical damage threshold. Polymers are promising canditates but their technology is immature and they have temperature stability difficulties. On the other hand compound semiconductors offer a mature material and processing technology. Among the compound semiconductor modulators electroabsoption modulators have demonstrated electrical bandwidths approaching 50 GHz with low drive voltages. For such devices electrical bandwidth is limited by the capacitance of the device. For low voltage operation upper limit seems to be around 40 GHz. For higher bandwidths electmoptic modulators utilizing traveling wave designs are the most promising candidates [ 11. In such a design electrode is designed as a transmission line. Therefore, electrode capacitance is distributed and does not limit the modulator speed. Modulating electrical signal on the electrode travel in the same direction as the modulated optical signal. If they travel with the same velocity the phase change induced by the electrical signal is integrated along the length of the electrode. Since the electrode capacitance is not the bandwidth limit one can make the electrode very long, typically thousands of wavelengths. This allows even a very small phase change over a wavelength to accumulate to an appreciable value. Therefore, drive voltage requirements can be significantly relaxed without sacrificing electrical bandwidth.