Fabrication and Analysis of Integrated Optic Modulators in Multiple Quantum Well Gallium Arsenide

This is the first report of single mode waveguide fabrication in multiple quantum well gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) using reactive-ion etching with a CCl_2F_2/O _2 (1:2) plasma. Also, two techniques have been developed to perform this etching with a self -aligned electrode mask. Reactive-ion etched waveguides are compared to chemically etched waveguides through propagation loss measurements and guided mode structure as a function of waveguide geometry. A numerical waveguide model based on the effective index method for 3-D waveguides is developed to yield optical field profiles and propagation constants. This model is compared to the experimental data. This is also the first report of integrated optic waveguide Mach-Zehnder modulators and directional coupler modulator/switches in this same multiple quantum well material system. The high bulk index and strong excitonic electrorefraction of this material results in compact, high efficiency devices. The Mach-Zehnders are fabricated by chemical etching and are operated in single drive and push-pull mode, yielding a voltage-length product as low as 4.4 Vcdot mm. The directional couplers are fabricated by reactive-ion etching with a novel step-etch masking technique which enhances evanescent coupling without increasing bending loss in the branching waveguides. The passive waveguide model is modified to include the electro-optic effect, and from this, models are developed for the Mach-Zehnder and the directional coupler to predict the modulation transfer curves as a function of applied voltage.