  We present the design, fabrication and characterization of LNOI fiber-to-chip
inverse tapers for efficient edge coupling. The etching characteristics of
various LNOI crystal cuts are investigated for the realization of butt-coupling
devices. We experimentally demonstrate that the crystal cut limits the
performance of mode matching tapers. We report a butt-coupling loss of 2.5
dB/facet and 6 dB/facet by implementing 200 nm tip mode matching tapers in
$+Z$-cut LNOI and $X$-cut MgO:LNOI waveguides with low propagation loss. We
anticipate that these results will provide insight into the nanostructuring of
LNOI and into the further development of efficient butt-coupling in this
platform.
