  A grating coupler on 700-nm-thick Z-cut lithium-niobate-on-insulator platform
with high coupling efficiency, large bandwidth, and high fabrication tolerance
is designed and optimized by inverse design method. The optimized grating
coupler is fabricated with a single set of e-beam lithography and etching
process, and it is experimentally characterized to possess peak coupling
efficiency of -3.8 dB at 1574.93 nm, 1-dB bandwidth of 71.7 nm, and 3-dB
bandwidth of over 120 nm.
