Ultra-low loss optical delay lines based on silicon nitride SWG technology

Optical delay lines are essential for microwave photonics and optical signal processing applications. This work presents the design, optimization, and experimental demonstration of compact optical delay lines using subwavelength grating (SWG) structures on a silicon nitride platform. By leveraging the low-loss properties of silicon nitride, our approach reduces the insertion loss compared to silicon-based alternatives. We propose optimized SWG tapers with a loss of 0.04 dB per taper for a 15 µm taper length and SWG bends achieving 0.82 dB loss per 90° bend using 20 µm radius. Our results show a linear relationship between group delay and the SWG duty cycle, offering a tunable delay mechanism without increasing the waveguide length. The fabricated delay lines demonstrate a 1.6 dB/cm loss and significantly improved performance over previously reported silicon-based SWG delay lines. These findings highlight the potential of silicon nitride SWG structures for high-performance, compact optical delay lines in photonic integrated circuits.