Inverse design of high-efficiency lithium-niobate-on-insulator grating couplers with user-defined apodization.

Grating couplers typically encounter minimum feature size (MFS) constraints in fabrication. To address this issue, we incorporated user-defined apodization profiles into the inverse design process to enhance conversion efficiency (CE). We optimized grating parameters based on lithium-niobate on insulator (LNOI) platforms. With a single-step lithography and etching process, we achieved a remarkable peak CE of -1.97 dB at an MFS of 200 nm in the telecom C-band. This result surpasses previous LNOI designs that lack complex structures, such as bottom metal reflectors or overlaid materials, offering a more straightforward path for developing high-efficiency LNOI grating couplers. Leveraging this design strategy, we also demonstrated a broadband grating coupler with a peak CE of -3.22 dB and 3-dB bandwidth over 106 nm.