Polarization‐Independent Complex Bragg Grating Filters on Silicon Nitride

Integrated optical filters are essential components of on‐chip multiplexers, which play a critical role in enhancing data throughput for high‐demand networks. Since optical multiplexers are typically connected through fiber‐optic links where the polarization state of light fluctuates randomly, polarization insensitivity is an important prerequisite for integrated optical filters. In this work, polarization‐insensitive optical filters are demonstrated on the silicon nitride platform operating in the datacom O‐band. The filters are based on cladding‐modulated Bragg gratings, which offer improved fabrication tolerances and reduced losses compared to conventional sidewall gratings. By judiciously designing grating dimensions, filters are demonstrated with single‐band, dual‐band, and four‐band spectral characteristics. The fabricated filters achieve an insertion loss below 1.0 dB and a band rejection greater than 25 dB for spectral bands that are 3 nm wide and 9 nm apart.