Compact and fabrication-tolerant CWDM filter based on cascaded silicon nitride Mach–Zehnder interferometers

This work presents a compact, fabrication-tolerant coarse wavelength-division multiplexing (CWDM) filter for co-packaged optics (CPO) on a 400 nm thick silicon nitride (SiN) platform. The device monolithically integrates Bézier bends within a Mach-Zehnder interferometer architecture, achieving broadband operation across the O band with precise splitting ratios. By adopting Bézier-waveguide curves with an effective radius of 25 μm, the design reduces the bend footprint by 50% while maintaining an ultra-low loss of 0.04 dB per 90° turn-a fivefold improvement over conventional circular arcs. The fabricated filter exhibits an average insertion loss of 2.2 ± 0.3 dB and crosstalk below -19.3 ± 1.4 dB. Wafer scale characterization demonstrates exceptional uniformity, with a record low center-wavelength standard deviation of 0.27 nm, highlighting the potential of Bézier-based SiN photonics for high-density, low-loss, and fabrication-robust WDM components in next-generation CPO systems.