Design and demonstration of high efficiency perfectly vertical grating couplers with a random structure.

Utilizing an automated optimization method, we propose a perfectly vertical grating coupler (PVGC) characterized by random structure, superior performance, simplified fabrication process, and increased minimum feature size (MFS). Within the range of MFS from 60 to 180 nm, the optimized PVGC exhibited a simulated coupling efficiency of approximately -2.0 dB at 1550 nm with a 34 nm 1-dB bandwidth. Experimental results for the PVGCs fabricated by electron beam lithography (EBL) demonstrated coupling efficiencies ranging from -2.5 to -2.8 dB with a 32 nm 1-dB bandwidth while maintaining high manufacturing tolerances. This represents the most outstanding experimental outcome to date regarding the coupling performance of a PVGC fabricated on a 220 nm silicon on insulator (SOI), without requiring any complex processes as reported in the existing literature.