Monolithic low-birefringence silicon nitride platform for polarization-insensitive photonic integrated circuits.

The scalability of integrated photonic systems for next-generation datacom and sensing applications is fundamentally limited by the strong polarization dependence of high-confinement waveguides. While silicon nitride (Si3N4) has emerged as a premier platform for low-loss, broadband passive functionalities, mitigating polarization sensitivity typically requires complex polarization diversity schemes that increase footprint and insertion loss. We demonstrate a monolithic, polarization-insensitive Si3N4 platform that eliminates the need for polarization splitters or rotators by engineering the waveguide geometry to suppress birefringence at the fundamental mode level. By utilizing a near-square cross-section (800 nm × 800 nm), we achieve nearly identical effective and group indices for TE and TM modes across a broad spectral range. Leveraging this platform, we present two key devices: a broadband 1 × 2 optical switch and a 1 × 4 coarse wavelength division multiplexing (CWDM) demultiplexer. The optical switch operates across the C and L bands (1500-1620 nm) with insertion losses below 1.52 dB and polarization-dependent losses (PDL) below 1.53 dB. The CWDM device exhibits superior performance with insertion losses below 1.14 dB and PDL below 0.75 dB. This approach offers a robust, low-complexity pathway for polarization-agnostic photonic processing in high-capacity optical networks.