Silicon nitride directional coupler-based polarization beam splitter utilizing shallow ridge waveguides for improved fabrication tolerance

This study presents the design and fabrication of a silicon nitride (Si₃N₄) asymmetrical directional coupler (DC)-based polarization beam splitter (PBS) utilizing a shallow ridge waveguide structure. The use of Si₃N₄ with a moderate refractive index contrast and the design of a shallow ridge waveguide enables the DC structure to have a wider coupling gap of approximately 600 nm, which can be readily achieved with standard I-line lithography. The simulation results indicate that the polarization splitting is highly efficient with minimal insertion loss. This is corroborated by the experimental measurements, which show consistent broadband performance. The fabricated polarization beam splitter (PBS) exhibits a high polarization extinction ratio (PER) of over 20 dB for both transverse electric (TE) and transverse magnetic (TM) modes across a broad bandwidth of 120 nm (1500–1620 nm). This work demonstrates the potential of shallow ridge Si₃N₄ waveguides to enhance the fabrication tolerance and performance of integrated photonic devices.