Inverse Designed Grating Coupler With Low Loss and High Bandwidth on LNOI Platform

A grating coupler structure on a X-cut lithium niobate on insulator (LNOI) platform is designed and demonstrated. The utilized inverse design method is based on gradient-based adjoint optimization method, which is time saving and with more optimization degrees of freedom. For TE polarization, the high coupling efficiency of -3.3 dB is experimentally obtained. The maximum coupling efficiency is still -3.9 dB with the broadband 3-dB bandwidth of 90 nm. The grating coupler of high performance is realized by only single step etching and without bottom metal reflection layer.