High-speed guided-wave electro-optic modulators and polarization converters in III-V compound semiconductors
Creators: name: Rahmatian, Farnoosh
Publisher: name: University of British Columbia
Year: 2009

description: In the last few decades, the need for electronic communication has increased by several orders of magnitude. Due to the rapid growth of the demand for transmission bandwidth, development of very high-speed communication systems is crucial. This thesis describes integrated-optic electro-optic modulators using travelling-wave electrodes in compound semiconductors for ultra-high-speed guided-wave optical communications. Both Mach-Zehnder (MZ) interferometric modulators and polarization converters (PC) have been studied with particular emphasis on the latter ones. Slow-wave travelling-wave electrodes in compound semiconductors have previously been proposed and demonstrated. Here, a study of slow-wave, travelling-wave electrodes on compound semiconductors has been performed in order to significantly improve their use in ultra-wide-band guided-wave electrooptic devices. The most important factors limiting the high frequency performance of such devices, in general, are the microwave-lightwave velocity mismatch and the microwave loss on the electrodes. Based on the deeper understanding acquired through our study, we have designed, fabricated, and tested low-loss, slow-wave, travelling-wave electrodes on semiinsulating GaAs (SI-GaAs) and AlGaAs/GaAs substrates. Microwave-to-lightwave velocity matching within 1% was achieved using slow-wave coplanar strip electrodes; many of the electrodes had effective microwave indices in the range 3.3 to 3.4 (measured at frequencies up to 40 GHz). For the electrodes fabricated on SI-GaAs substrates, microwave losses of 0.22 Np/cm and 0.34 Np/cm (average values at 40 GHz) were measured for the slow-wave coplanar strip and the slow-wave coplanar waveguide electrodes, respectively. For the electrodes fabricated on the AlGaAs/GaAs substrates containing the modulators, the corresponding losses were, on average, 0.17 Np/cm higher at 40 GHz. For the first time, ultra-wide-band polarization converters using slow-wave electrodes have been designed, fabricated, and tested. A detailed analysis of the use of the slow-wave electrodes together with optical ridge waveguides as polarization converters has been provided. The effects of the modal birefringence of the optical waveguides, the microwave loss on the electrodes, and the residual microwave-lightwave velocity mismatch have all been taken into account in our study. Low frequency optical measurements showed very good qualitative agreement between the measured and the predicted results as regards the effect of the modal birefringence; it was also shown that the modal birefringence has to be kept to very small values to keep the efficiency of such modulators high. High-speed optical measurements were performed at frequencies up to 20 GHz (limited by the equipment bandwidth); the 3-dB optical bandwidths exceeded 20 GHz for both the MZ type and the PC type devices. The MZ modulators, however, had significantly larger half-wave voltages, -25 V, and their electrodes were significantly "over-slow" (by -15%). Evidence acquired through this study suggests that reducing the half-wave voltages below 5 volts and keeping the bandwidth in excess of 40 GHz is extremely difficult for these MZ type devices. The PC type devices using slow-wave coplanar strip electrodes, on the other hand, had lower half-wave voltages, as low as 7 V was measured, and had very good microwave-tolightwave velocity matching, within 1%. From this study we conclude that these devices can be designed to have bandwidths in excess of 100 GHz and half-wave voltages less than 2 V.