Low-loss and polarization-insensitive double-tip edge coupler with etched facet on a lithium niobate on insulator platform

In this Letter, we propose and demonstrate a low-loss and polarization-insensitive edge coupler based on an etched facet double-tip inverse taper structure in lithium niobate on insulator (LNOI) across the C- and L-bands. Using a simple dry etching process, we have obtained a flat edge facet for fiber coupling, eliminating the need for chip edge polishing. The fabricated edge coupler exhibits a fiber-to-chip coupling loss of 1.28/0.67 dB per facet for TE/TM mode at 1550 nm. The measured coupling loss is lower than 1.5/1.0 dB per facet for TE/TM mode, and the polarization-dependent loss is below 0.85 dB over the 1530-1630 nm wavelength range. This scheme is suitable for wafer-scale production and provides a cost-effective and efficient solution for fiber-to-chip coupling, which will find applications in the LNOI photonic integrated circuits.