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

## TL;DR

This paper introduces a new optical sensor using a coupled resonator waveguide design that improves sensitivity for detecting refractive index changes.

## Contribution

A novel coupled resonator optical waveguide sensor using Sagnac loop reflectors and subwavelength gratings to enhance refractive index sensitivity.

## Key findings

- The sensor achieves sensitivities of 106–120 nm/RIU with ridge feedback and 185.8–212.2 nm/RIU with subwavelength grating enhancement.
- Sharp Fano-type resonances are produced through bidirectional propagation and interference, enabling high spectral sensitivity.
- Geometric parameters like loop radius and coupling gap allow precise control over resonance characteristics.

## Abstract

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 consistent and monotonic resonance shifts for refractive index variations from 1.33 to 1.36, with sensitivities ranging from 106 to 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 sensitivity. The proposed coupled-SLR CROW provides a compact footprint, high-Q resonances, and flexible spectral engineering through accessible geometric parameters. These characteristics highlight the potential of the coupled-SLR and SWG-enhanced CROW as a promising platform for high-resolution, photonic refractive index sensing applications on SOI.