  Lithium niobate (LN) devices have been widely used in optical communication
and nonlinear optics due to its attractive optical properties. The emergence of
thin-film lithium niobate on insulator (LNOI) improves performances of LN-based
devices greatly. However, a high-efficient fiber-chip optical coupler is still
necessary for the LNOI-based devices for practical applications. In this paper,
we demonstrate a highly efficient and polarization-independent edge coupler
based on LNOI. The coupler, fabricated by standard semiconductor process, shows
a low fiber-chip coupling loss of 0.54 dB/0.59 dB per facet at 1550 nm for
TE/TM light respectively, when coupled with ultra-high numerical aperture fiber
(UHNAF) of which mode field diameter is about 3.2 micrometers. The coupling
loss is lower than 1dB/facet for both TE and TM light at wavelengths longer
than 1527nm. A relatively large tolerance for optical misalignment is also
proved. The coupler shows a promising stability in high optical power and
temperature variation.
