Distributed Bragg deflector coupler for on-chip shaping of optical beams
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In integrated optical circuits light typically travels in waveguides which provide both vertical and horizontal confinement, enabling efficient routing between different parts of the chip. However, for a variety of applications, including on-chip wireless communications, steerable phased arrays or free-space inspired integrated optics, optical beams that can freely propagate in the horizontal plane of a 2D slab waveguide are advantageous. Here we present a distributed Bragg deflector that enables well controlled coupling from a waveguide mode to such a 2D on-chip beam. The device consists of a channel waveguide and a slab waveguide region separated by a subwavelength metamaterial spacer to prevent uncontrolled leakage of the guided mode. A blazed grating in the waveguide sidewall is used to gradually diffract light into the slab region. We develop a computationally efficient strategy for designing gratings that generate arbitrarily shaped beams. As a proof-of-concept we design, in the silicon-on-insulator platform, a compact ×75 Gaussian beam expander and a partial beam deflector. For the latter, we also demonstrate a prototype device with experimental results showing good agreement with our theoretical predictions. We also demonstrate via a rigorous simulation that two such couplers in a back-to-back configuration efficiently couple light, suggesting that these devices can be used as highly directive antennas in the chip plane.
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