High coupling efficiency grating couplers on lithium niobate on insulator

We demonstrate monolithically defined grating couplers in $Z$-cut lithium niobate on insulator for efficient vertical coupling between an optical fiber and a single mode waveguide. The grating couplers exhibit $\sim 44.6\%$/coupler and $\sim 19.4\%$/coupler coupling efficiency for TE and TM polarized light respectively. Taperless grating couplers are investigated to realize a more compact design.