Fano-Resonant Coupled Sagnac Loop Resonator for Enhanced Integrated Refractive Index Sensing

Introduction: Integrated photonic refractive index sensors are increasingly important for chemical and biological sensing due to their high sensitivity, compact footprint, and compatibility with silicon-based fabrication technologies. Resonant photonic structures that support sharp spectral features are particularly attractive, as they enable enhanced detection of small refractive-index variations through precise wavelength tracking.

Methods: In this work, a silicon-on-insulator (SOI) refractive index sensor based on a coupled-resonator optical waveguide (CROW) architecture is presented. The design employs two inversely coupled Sagnac loop reflectors connected by a self-coupled feedback waveguide, generating Fano-type asymmetric resonances through discrete–continuum interference. Numerical simulations based on the finite-element method were used to analyze the influence of structural parameters, including loop radius, coupling gap, and feedback length. Refractive index sensing was evaluated over the range of 1.33–1.36 by monitoring resonance wavelength shifts.

Results: The ridge waveguide configuration exhibited stable asymmetric resonances with sensitivities between 106 and 120 nm/RIU. By incorporating a subwavelength-grating segment into the feedback waveguide to enhance evanescent field interaction, the sensor sensitivity was increased to 185.8–212.2 nm/RIU while preserving resonance asymmetry and spectral stability.

Conclusion: The proposed coupled Sagnac loop CROW sensor demonstrates high sensitivity, tunability, and fabrication tolerance in a compact footprint. These characteristics highlight its strong potential for integrated biochemical and environmental sensing applications.