Distributed Bragg deflector coupler for on-chip shaping of optical beams

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.