Double-layer metasurface-based edge coupler for low-loss coupling between optical fiber and thin-film lithium niobate photonic chip.

Thin-film lithium niobate (TFLN) has recently emerged as a promising platform for next-generation photonic integrated circuits, owing to its exceptional electro-optic, nonlinear, and optical properties. Despite their advantages, efficient light coupling between standard micron-sized single-mode optical fibers and submicron-sized TFLN waveguides remains a critical challenge, primarily because of the mode size mismatch and limited tolerance over broad wavelength ranges. In this paper, we propose and theoretically investigate a novel edge coupler based on an all-dielectric double-layer metasurface (DLMS), designed to overcome these limitations. The DLMS structure, inspired by the optical function of two orthogonal cylindrical lenses, reshapes a Gaussian beam emitted from an optical fiber into an elliptical profile that closely matches the guided mode of rib-type TFLN waveguides. Finite-difference time-domain simulations confirm that the coupler achieves a coupling loss (CL) below 1.9 dB/facet across the full telecommunication band (1460-1630 nm), reaching a minimum CL of 0.6 dB/facet at 1550 nm. Moreover, the device offers relaxed alignment tolerances of ± 1 μm vertically and ± 2.5 μm laterally, facilitating practical implementation. The proposed DLMS-based edge coupler offers a compelling solution for low-loss broadband fiber-to-chip interfaces in TFLN photonic systems.