Ultra-compact vertical grating coupler for high-density photonic integration

We report an ultra-compact and process-compatible vertical grating coupler on a silicon-on-insulator platform synthesized using a cascaded inverse-design framework. Our methodology employs a joint optimization strategy that combines the beam propagation impact of a hybrid L-shaped and strip grating. By integrating a boundary-scaling algorithm with explicit fabrication constraints into the gradient-based loop, we ensure strict adherence to an 80 nm or 100 nm minimum feature size while achieving structural robustness independent of initial configurations. Comparing with the metalens approach, which has its size to be physically constrained by its constitutive unit-cell dimensions, the proposed method scaled the focusing length down to 2.5 µm for a total footprint of 12.5 × 14.14 µm². The resulting design exhibits promising performance across the C- and L-bands, yielding a peak efficiency of -3.36 dB and a 1-dB bandwidth of 59 nm with 80 nm minimum feature size. This work provides a practical and scalable pathway for high-density photonic integration.