Ultra-Compact Sub-Wavelength Grating Edge Coupler on a Thin-Film Lithium Niobate Platform

Photonic integrated circuits (PICs) based on lithium niobate on insulator (LNOI) platform have significantly advanced in optical communication, nonlinear photonics, and optical computation. Couplers are essential components for connecting optical fibers to PICs. Achieving compact couplers is crucial for realizing high-density integration in PICs. In this Letter, we propose an ultra-compact sub-wavelength grating (SWG) edge coupler on LNOI, reducing the footprint to just $18.8~\mu $ m, 1-2 orders of magnitude smaller than traditional edge couplers while simplifying fabrication by requiring only a single etching step. Experimental results demonstrate coupling losses of 3.8 dB/facet for TE polarization and 3.38 dB/facet for TM polarization at 1550 nm, with stable coupling loss across the S+C band. This design offers a highly compact solution for efficient light coupling and benefits the development of photonic integration on LNOI.