Compact inverse-designed ultra-broadband and low-loss waveguide crossing for photonic integration

Waveguide crossing is the indispensable building block for large-scale photonic integrated circuits. However, current existing silicon waveguide crossing still suffers from excessive insertion loss, limited bandwidth, and large footprint, which severely restricts on-chip photonic circuit scalability. Here, we propose an ultra-compact waveguide crossing that achieves a 400-nm bandwidth (1250-1650 nm) with average insertion loss below 0.19 dB and crosstalk below -17 dB, using a novel, to the best of our knowledge, root-mean-square transmittance figure of merit (RMS-FOM) methodology. Meanwhile, the device maintains an ultra-compact footprint of 4 × 4 μm². Comprehensive tolerance analysis reveals that the optimized devices sustain insertion losses below 0.4 dB at 1310 nm and 0.35 dB at 1550 nm across manufacturing deviations of up to ±100 nm, confirming the robustness and reliability of our device under realistic fabrication. Large-scale experimental validation demonstrates that the fabricated device exhibits low crosstalk of below -18.8 dB and -20 dB and insertion losses of 0.32 dB and 0.35 dB in two operating bands (1250-1350 nm and 1500-1600 nm). We believe the device establishes a new benchmark for silicon waveguide crossings by integrating optimized overall performance with an ultra-compact footprint, broadband, and fabrication tolerance, thereby enabling high-density on-chip optical interconnects in communication, data centers, and quantum technologies.