Optimized shift-pattern overlay for high coupling efficiency waveguide grating couplers

We propose and validate a new, to the best of our knowledge, approach for increasing the coupling efficiency of waveguide grating couplers by introducing an optimized shift-patterned polysilicon overlay above the silicon grating structure. After optimizing the shifts in position and duty cycles of each period in the polysilicon overlay and silicon grating, the silicon grating and polysilicon overlay can form composite subwavelength structures which improve both the mode matching and the directionality of the grating coupler, and enable the design of a high-efficiency perfectly vertical grating coupler (PVGC) with -0.91 dB simulated coupling efficiency. The devices are fabricated using photolithography in a standard commercial multi-project wafer fabrication service by IMEC and have a measured coupling loss of approximately 1.45 dB. (C) 2022 Optica Publishing Group

Automated summary

We propose a new approach to increase the coupling efficiency of waveguide grating couplers by introducing an optimized shift-patterned polysilicon overlay. The composite subwavelength structures formed by the polysilicon overlay and silicon grating improve the mode matching and directionality of the grating coupler, resulting in a high-efficiency perfectly vertical grating coupler with -0.91 dB simulated coupling efficiency. The devices are fabricated using photolithography and have a measured coupling loss of approximately 1.45 dB.