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UBC Theses and Dissertations

Electrooptic mach-zehnder modulators in gallium arsenide Tsou, Benny Pen-Cheng


External travelling-wave modulators, utilizing the electrooptic effect, are one class of devices currently being investigated for converting electrical signals to optical signals in applications involving high-speed data transmission. Modulators fabricated on semiconductor substrates, such as GaAs and InP, are particularly attractive in that there exists the possibility of monolithic integration of these devices with other optoelectronic components. The bandwidths of this type of modulator are limited by the velocity mismatch between the microwave and the optical wave, where, in these compound semiconductors, the microwave has the greater phase velocity. To eliminate this problem, a slow-wave electrode structure has been developed in our laboratory in which the phase velocity of the microwave is reduced to match the phase velocity of the optical wave. In this thesis, modulators, based on the integrated optics version of the Mach-Zehnder interferometer, using GaAs substrates, were modelled and fabricated for use with these slow-wave electrodes. The variational method, effective-index method, and beam propagation method were used to model the propagation of light in these devices, and, hence, obtaining the necessary design parameters for these devices. Based on these numerical simulations, modulators were designed and fabricated in AlGaAs epitaxial layers, grown on GaAs substrates by the metal organic chemical vapour deposition technique. Optical attenuation in the waveguides was measured. Parameters such as the half-wave voltage, intrinsic bias, extinction ratio, and microwave indices of the electrodes were measured. The extinction ratio, for most devices, was greater than 99%. The half-wave voltage and the microwave indices, which could be calculated theoretically, were found to be in good agreement with the measured values.

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