Focusing Polarization‐Splitting Grating Coupler in Lithium Niobate on Insulator Photonic Platform

Lithium niobate‐on‐insulator (LNOI) is a rapidly emerging platform for integrated photonics, offering high‐speed modulation, efficient frequency conversion, and scalable quantum circuits. However, polarization‐diverse fiber‐to‐chip interfaces remain challenging, as efficient polarization‐splitting grating couplers (PSGCs) are demanding on LNOI due to its lower refractive‐index contrast and scattering efficiency compared to silicon. This work demonstrates a compact, focusing PSGC operating in the telecom C‐band with low (<1 dB) polarization‐dependent loss (PDL) and fabricated in an LNOI foundry using a process design kit (PDK). The device features a two‐dimensional elliptical grating that splits input polarizations into two TE‐polarized waveguides, with a compact ~75  × 75  µm ² footprint compatible with fiber‐array packaging. Experimental results show a PDL of ~0.2 dB @1550 nm and a 1‐dB PDL bandwidth of ~35 nm, comparable to a high‐contrast silicon photonics platform. The elevated insertion loss (IL ~ 11.85 dB) is attributed to fabrication tolerances and PDK limitations, including scatterer shape distortion, partial etching, and substrate leakage. Simulations suggest that increasing the etch depth and adding a back reflector could reduce IL to ~3 dB. To our knowledge, this is the first demonstration of a broadband, low‐PDL focusing PSGC on LNOI, offering a proof‐of‐concept for polarization‐diverse fiber interfaces in LNOI photonics.