Ultra-low loss edge coupler based on thin-film lithium niobite

Lithium niobate (LN) devices have been used in optical communication and nonlinear optics widely due to its impressive optical properties. Thin-film lithium niobate on insulator (LNOI) improves performances of LN-based devices further. But a high-efficient fiber-chip optical coupler for the LNOI-based devices for practical applications is still absent. In this article, we demonstrate a wide-band, highly efficient and polarization independent edge coupler based on LNOI fabricated by planar semiconductor process. The measured ultra-high numerical aperture fiber (UHNAF)-to-chip optical coupling loss at 1550 nm is 0.54 dB/facet (0.59 dB/facet) for TE(TM). The coupler has the coupling loss lower than 1dB/facet for both TE and TM light at wavelengths longer than 1527nm.