Low-loss chirped grating for vertical light coupling in lithium niobate on insulator

We report a vertical chirped grating coupler with high coupling efficiency (CE) fabricated on lithium niobate on insulator (LNOI). According to finite difference time domain simulations, loss in a uniform grating in LNOI comes from downward diffracted light, internal reflections, and mode mismatch between the single-mode fiber and the out-coupled light from the grating. A chirped grating coupler, whose parameters were optimized to reduce the mode mismatch loss and internal reflections, was fabricated with integrated waveguides through a single etch-step and exhibited insertion losses as low as 3.6 dB, 1.9 dB lower than a uniform grating fabricated in the same process. The demonstrated grating features high CE, ease of manufacturing, and the ability to vertically couple light—a fundamental characteristic required to facilitate the integration of on-chip laser sources and detectors. (paper)