High efficiency grating coupler on thin-film lithium niobate for visible light

This work demonstrates high-efficiency grating couplers (GCs) designed for single-mode fiber coupling in the visible spectrum (centered at 532 nm) on an X-cut thin-film lithium niobate (TFLN) platform. By systematically optimizing the device architecture, the lithium niobate (LN) layer thickness was reduced to 150 nm, effectively enhancing coupling efficiency (CE). A hybrid optimization approach, combining commercial finite-difference time-domain (FDTD) simulations with a particle swarm optimization (PSO) algorithm, was employed to determine the optimal geometric parameters. This strategy yielded a simulated CE of 65% (−1.8 dB loss for non-uniform GC) and a measured CE of 45.2% (−3.45 dB loss for uniform GC). These results represent the highest, to the best of our knowledge, reported CE in the visible band for TFLN-based GCs. The demonstrated high-performance GC configuration serves as a critical photonic component for advancing integrated optical systems in visible-light communications, quantum photonic processing, and precision frequency metrology applications.