  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.
