On-chip wide-FSR silicon optical filter with high extinction ratio and compact footprint

Optical filters play a crucial role in advanced optical computing, communication, and sensing applications, and those featuring an ultra-large free spectral range (FSR) are particularly significant. These filters offer access to numerous finely tuned wavelength channels, significantly enhancing the optical transmission capacity and performance of sensing applications. However, the existing wide-FSR filter designs are hampered by limitations in cavity length, fabrication complexity, or the need for additional active-tuning controls. To address these challenges, we present what we believe to be a novel tapered-cavity-coupled asymmetric-gratings that leverages an inter-mode coupling phenomenon, enabling wide-FSR operation with nanometer-scale full width at half maximum (FWHM) in a compact on-chip silicon device. Our approach experimentally demonstrates a single resonant peak of 1.2 nm FWHM and 14 dB extinction ratio, alongside a theoretically predicted FSR exceeding 200 nm, all within a highly miniaturized footprint of 18 µm². This compact and fabrication-tolerant architecture offers a scalable route for implementing high-performance single-channel filters across a broad spectral range, making it highly relevant for next-generation optical communication, quantum photonics, and lab-on-chip sensing platforms.