Inverse design of an ultra-compact polarization beam splitter

An ultra-compact polarization beam splitter (PBS) is designed on the silicon nitride (Si₃N₄) platform based on an inverse design with a footprint of only 2.4×4.8µm². The simulated PBS device is designed using a nonlinear direct binary search (DBS) algorithm, enabling one-step lithography fabrication. We analyzed the initial structure, the design region footprint, the fabrication tolerance, and the optical field distribution. The simulation results exhibit that the insertion loss of the designed device is less than 0.89 dB (1.21 dB) for TE (TM) polarization within a bandwidth wavelength of 50 nm (1525-1575 nm). The average extinction ratios for TE and TM polarizations are 13.24 dB and 14.15 dB, respectively. Additionally, the device exhibits high fabrication tolerance, which is up to ±50nm in etch depth and ±8nm in etch diameter.