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Slow-wave electrode structures for III-V semiconductor based electro-optic travelling-wave modulators Lee, Zachary Ka Fai

Abstract

Electro-optic modulators are important devices in high-speed optical communications and signal processing. These devices are particularly useful if they are fabricated on semiconductor materials since they may be monolithically combined with other electronic and/or optoelectronic devices. Conventional travelling-wave electro-optic modulators fabricated using III-V semiconductor materials such as gallium arsenide and indium phosphide suffer from the problems created by the velocity-mismatch between the microwave modulating signal and the optical signal (or equivalently, the mismatch between the effective refractive indices of the microwave and the optical wave). These problems ultimately limit the modulator bandwidth and require large amounts of modulating microwave power to obtain useful modulation depths. It was our objective to design coplanar slow-wave electrode structures allowing the fabrication of wide-band modulators with reasonable demands of modulating power levels. This objective was achieved. In this thesis slow-wave coplanar electrode structures for use in these modulators are described. They are periodically loaded with narrow capacitive fins. Small pads may also be added on the ends of the fins to further increase the capacitance. If the fins are narrow, the capacitance per unit length can be increased substantially due to the fringing electric fields about the fins (and pads), while the effect on the inductance per unit length can be relatively small. This increase in capacitance per unit length results in the slowing of the microwave. By carefully choosing the dimensions, slow-wave electrodes having a prescribed microwave effective refractive index as well as 50 or 75 ohm characteristic impedances can be designed. A dielectric superstrate may also be used to help slow down the microwave somewhat, potentially enabling fine tuning, as well as to protect the electrodes. The electrodes may also be partially buried in the substrate to achieve some additional slowing of the microwave. Design formulas have been derived. Design curves for 50 and 75 ohm slow-wave electrode structures fabricated on gallium arsenide and indium phosphide based materials are presented. In order to verify the theory, a large number of slow-wave electrodes as well as a conventional coplanar strip electrode have been fabricated on semi-insulating gallium arsenide. The fabrication was straight forward, with the whole electrode being formed in a single layer of metallization using a single-step photo-resist patterning and lift-off technique. Measurements of the microwave effective refractive indices of the electrodes fabricated clearly indicate that these electrodes are capable of velocity-match in modulators fabricated using gallium arsenide based materials. The measured values range from 2.84 to 3.46. Good agreement was found between the measured results and the theoretical predictions. The electrodes were also found to have very low microwave losses of a few tenths of a decibel per centimetre, and very low dispersion at frequencies at least up to the 20 GHz limit of the microwave source. Slow-wave electrode structures, capable of matching the velocities of microwaves to those of optical waves in III-V semiconductor travelling-wave electro-optic modulators, that offer low loss, low dispersion, flexible dimensions, and ease of fabrication have been designed, fabricated, and tested.

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