High-Speed Directional Coupler Switch Modulator

In this thesis, the design and characterization of high-speed directional coupler modulator switches at 0.83 μm are investigated. A comprehensive simulation model of optical and electrical characteristics necessary for the design has established. The semi-vectorial finite difference technique was adopted for determining the odd and even modes propagation in the AlGaAs directional coupler waveguide structure, and a quasi-static finite difference method was used to obtain the RF characteristics of traveling-wave electrode of the modulators. In the design of high-speed performance, the asymmetric coplanar waveguide electrode structure was adopted for its low RF loss characteristics. Two types of slow-wave electrodes were designed and tested, the Z-shaped resonant electrode employing a resonant structure for reducing the phase velocity of microwave signal, and the capacitively loaded electrode structure using the capacitance loading for slow-wave effect. The DC and RF performance of the modulators were experimentally determined and compared with that calculated by the simulation. The measured intensity responses of the modulators vary between 22V and 27V, which is less than 8% from the theoretical value of 25V. A prototype design for the resonant slow -wave electrode structure has shown that the phase velocity mismatch between the optical and electrical signals may be reduced by adjusting the resonator spacing. Less than 2 dB insertion loss in the range of 45 MHz and 10 GHz was obtained for the capacitively loaded electrode at 3600 μm length. The 3dB bandwidth of 17 GHz, the highest coupler response in AlGaAs measured to date, has obtained for the capacitive-loaded directional coupler modulator, with 2500 μm interaction length, and is in good agreement with the predicted result.