A Coupled Resonator Optical Waveguide-Based Refractive Index Sensor Employing Sagnac Loop Reflectors

This work presents a silicon-on-insulator (SOI) refractive index sensor based on a coupled resonator optical waveguide (CROW) architecture employing two inversely coupled Sagnac loop reflectors (SLRs) connected through a self-coupled feedback waveguide. The structure exploits bidirectional propagation and discrete–continuum interference to produce sharp Fano-type asymmetric resonances with steep spectral slopes, enabling enhanced wavelength sensitivity. Numerical analysis demonstrates that tuning the loop radius, directional-coupler length, coupling gap, and feedback-path length provides precise control over free spectral range (FSR), resonance asymmetry, and spectral sharpness. The sensor exhibits stable and reproducible resonance shifts for refractive index variations from 1.33 to 1.36, achieving sensitivities between 106 and 120 nm/RIU for the ridge-feedback configuration. Sensitivity is further improved by introducing a subwavelength-grating (SWG) segment into the feedback waveguide, which enhances evanescent-field interaction and increases the overlap factor without compromising compactness or Fano asymmetry. The SWG-assisted design attains sensitivities of 185.8–212.2 nm/RIU, nearly doubling responsivity. The proposed coupled-SLR CROW exhibits strong fabrication tolerance, high Q-factor, and compact footprint, outperforming conventional microring and cascaded resonator sensors in robustness and tunability. These characteristics establish the coupled-SLR and SWG-enhanced CROW as a promising platform for high-resolution, low-limit-of-detection photonic refractive index sensing applications on SOI.