Vertically Stacked and Directionally Coupled Cavity-Resonator-Integrated Grating Couplers for Integrated-Optic Beam Steering

Combination of an integrated-optic chip launching a light beam from variable position on a waveguide surface and a Fourier transform lens will provide a microoptic beam-steering device. An array of switching grating couplers in a channel waveguide is a possible candidate for varying the beam launching position with miniaturized size. Utilization of a cavity-resonator-integrated grating coupler is discussed theoretically. A resonator waveguide with a grating coupler is stacked on a bus waveguide. Vertical directional coupling between the two waveguides occurs only when a resonance wavelength coincides with that of an incident guided wave. Vertically transferred optical wave in the resonator is coupled out by the grating coupler. The vertical directional coupling can be electrically tuned by utilizing electrooptic or thermooptic effects. A design model was developed on the basis of the coupled mode analysis. Coupling characteristic of design examples using silicon waveguides were discussed. Selective coupling was predicted with the radiation efficiency of 30% and the FWHM of 1.4 x 10-3 in the effective refractive index of the cavity waveguide. Difference between neighboring peaks of radiation efficiency was predicted to be 5.2 x 10-2 indicating the resolution power of 37 for cavity length of 15 microns. These characteristics show good agreement with simulation results by the finite-difference time-domain method.