Inverse design and fabrication of high-efficiency perfectly vertical LNOI grating couplers.

A perfectly vertical grating coupler (GC) on X-cut lithium niobate on insulator (LNOI) platform is designed and demonstrated. The optimized random grating structure of GC is designed by an inverse design method and exhibits a simulated coupling efficiency of approximately -2.52 dB and measured coupling efficiency of approximately -3.18 dB at a communication wavelength of 1550 nm. This is the highest, to the best of our knowledge, measured result yet reported compared to the coupling efficiency of LNOI grating couplers without reflector in existing literature, regardless of the angle of incidence and the thickness of lithium niobate. The fabrication tolerance is investigated, and the discrepancy between simulation and experiment is analyzed.