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

Area of Science:

• Photonics
• Materials Science
• Optical Engineering

Background:

• Co-packaged optics (CPO) demand compact and efficient wavelength-division multiplexing (WDM) filters.
• Traditional Mach-Zehnder interferometer (MZI) designs face limitations in footprint and loss.
• Silicon nitride (SiN) photonics offers a promising platform for integrated optical devices.

Purpose of the Study:

• To develop a compact and fabrication-tolerant CWDM filter for CPO applications.
• To leverage Bézier curves for reduced footprint and improved performance in SiN photonic devices.
• To demonstrate broadband operation with precise splitting ratios in the O-band.

Main Methods:

• Integration of Bézier bends within an MZI architecture on a 400 nm thick SiN platform.
• Design optimization focusing on effective radius of 25 μm for Bézier curves.
• Fabrication and characterization of the CWDM filter, including insertion loss and crosstalk measurements.

Main Results:

• Achieved a 50% reduction in bend footprint compared to circular arcs.
• Demonstrated ultra-low loss of 0.04 dB per 90° turn.
• Fabricated filter showed average insertion loss of 2.2 ± 0.3 dB and crosstalk below -19.3 ± 1.4 dB.
• Wafer-scale characterization revealed a record low center-wavelength standard deviation of 0.27 nm.

Conclusions:

• Bézier-based SiN photonics provide a viable solution for high-density, low-loss WDM components.
• The developed CWDM filter is suitable for next-generation CPO systems.
• The design exhibits high uniformity and fabrication tolerance, crucial for mass production.