True-time delay and mode-matching in Archimedean spiral silicon photonic waveguides

We report a comprehensive analysis and design optimization of true-time delay (TTD) photonic waveguides using compact Archimedean spiral geometries, with a focus on low-loss silicon nitride (SiN) and silicon-on-insulator (SOI) platforms. Using a combination of finite element and finite-difference time-domain simulations at 1550 nm, we evaluate the propagation characteristics, mode-matching behavior, and bending losses of various waveguide designs. Our results demonstrate that Si 3 N 4 rectangular waveguides exhibit superior performance for long optical paths, achieving minimal radiation and scattering losses, while SOI is more suitable for compact integration. We further investigate curvature-engineered bends-Euler, Bezier, and circular-and identify Quartic Bezier bends as optimal for tight-radius, low-loss guiding. By minimizing crosstalk through tailored waveguide gaps and carefully engineered S-bend transitions, we present a scalable, low-loss TTD architecture. These findings provide critical insights for the integration of ultra-low-loss, material-efficient delay lines into photonic circuits for applications in signal processing, LiDAR, phased-array antennas, and neuromorphic computing.