Wavelength dependence of GaAs directional couplers and electrooptic switches.

The characteristics of GaAs directional couplers and electrooptic switches have been studied at 0.92 microm, 1.06 microm, and 1.15, microm. The devices were formed from pairs of closely spaced low-loss (alpha~1 cm(-1) at 1.06 microm) single-mode p(+)n(-)n(+) channel-stop strip guides. It was found that the coupling length L(c) varies faster than inversely with wavelength, especially near 0.92 microm, and that for each guide width and spacing, L(c) increases as the guide thickness is increased. Modeling the couplers to first order as coupled slab waveguides, the wavelength-dependent results can be explained by a variation with wavelength of the effective guide index. This model also predicts a minimum in L(c) as a function of the difference in effective index between the guides and their surrounding regions and is in agreement with the experimental increase of L(c) with increasing guide thickness (i.e., decreasing confinement). Switching performance at each wavelength was evaluated using devices cleaved to their 1.06-microm coupling length. The zero-bias power isolation obtained at 1.06 microm was 17 dB. At optimum switching bias, i.e., power confined to the input guide, power isolation of 16 dB at 1.15 microm, 17 dB at 1.06 microm, and 18 dB at 0.92 microm was obtained. Total power output is constant to </=0.2 dB between zero bias and the switching bias at 1.06 microm and 1.15 microm. At 0.92 microm, a power decrease of 1.7 dB at the switching bias was observed. This effect could be attributable to electroabsorption effects.