Compact polarization-insensitive silicon nitride waveguide crossing via adjoint-based inverse design.

We demonstrate a compact, polarization-insensitive silicon nitride (SiN) waveguide optical crossing optimized via adjoint-based inverse design. By co-optimizing the fundamental transverse electric (TE0) and transverse magnetic (TM0) modes within a 16.15 × 16.15 μm2 footprint, the device achieves measured insertion losses of 0.25 dB and 0.35 dB, respectively. Furthermore, the measured crosstalk is maintained below -28 dB and -31 dB across the 1500 nm to 1600 nm wavelength band, reaching minimum levels of -55 dB and -46 dB. Fabricated using standard foundry processes, this device serves as a robust and scalable building block for next-generation photonic integrated systems.