Inverse design of efficient perfectly vertical grating couplers on 220-nm SOI platform based on adjoint optimization

Grating couplers (GCs) are extensively employed in silicon photonics platforms as the I/O interfaces for data transmission between the chips and fiber systems. Perfectly vertical grating couplers (PVGCs) can further enhance optical packaging convenience and increase the spatial I/O density of optical links. In this study, we present an inverse design approach to develop high-performance PVGCs on a 220 nm silicon-on-insulator (SOI) platform. The gratings are designed for fabrication via a 70 nm shallow etch lithography step, ensuring compatibility with the standard multi-project wafer (MPW) runs. Through adjoint optimization, the PVGC achieves a simulated coupling efficiency of -2.4 dB at 1550 nm, with a 3 dB bandwidth of 40 nm. Experimental results show a peak coupling efficiency of -3.98 dB and a 3 dB bandwidth of 35 nm. To further broaden the bandwidth, a bidirectional perfectly vertical grating coupler (BPVGC) was designed using a similar optimization approach. Simulations predict a coupling efficiency of -1.95 dB at 1550 nm, featuring a relatively flat transmission spectrum and a 3 dB bandwidth of 96 nm. This work provides a practical pathway for the design of efficient on-chip interfaces.