| Summary: | 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.
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