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A Coupled Resonator Optical Waveguide-Based Refractive Index Sensor Employing Sagnac Loop Reflectors.
Muhammad A Butt1, Bartosz Janaszek1
1Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland.
Sensors (Basel, Switzerland)
|March 14, 2026
Summary
This study introduces a novel silicon-on-insulator refractive index sensor using coupled resonators for enhanced sensitivity. The design achieves nearly double the sensitivity with a subwavelength grating, promising high-resolution photonic sensing.
Area of Science:
- Photonics
- Nanotechnology
- Materials Science
Background:
- Refractive index sensors are crucial for various applications, including environmental monitoring and medical diagnostics.
- Existing silicon-on-insulator (SOI) based sensors face limitations in sensitivity and resolution.
- Coupled resonator optical waveguide (CROW) architectures offer potential for enhanced sensing performance.
Purpose of the Study:
- To develop a high-sensitivity refractive index sensor on SOI using a coupled Sagnac loop reflector (SLR) CROW architecture.
- To investigate the impact of geometric parameters on sensor performance, including free spectral range (FSR) and resonance sharpness.
- To enhance sensor sensitivity by incorporating a subwavelength grating (SWG) into the feedback waveguide.
Main Methods:
- Numerical analysis of a CROW structure comprising two inversely coupled SLRs connected by a feedback waveguide.
- Simulation of Fano-type asymmetric resonances and their dependence on geometric parameters (loop radius, coupler length, coupling gap, feedback path length).
- Integration of an SWG segment into the feedback waveguide to boost evanescent-field interaction and sensitivity.
Main Results:
- The coupled-SLR CROW design produces sharp Fano resonances with steep spectral slopes, enabling high wavelength sensitivity.
- Sensitivities of 106-120 nm/RIU were achieved with the basic ridge feedback configuration for refractive index variations from 1.33 to 1.36.
- The SWG-assisted design significantly improved sensitivity to 185.8-212.2 nm/RIU, nearly doubling the performance.
- The sensor demonstrated consistent and monotonic resonance shifts with refractive index changes.
Conclusions:
- The proposed coupled-SLR CROW platform offers a compact footprint and high-Q resonances for photonic refractive index sensing.
- The SWG enhancement provides a viable route to achieve ultra-high sensitivity in SOI-based sensors.
- This technology holds significant promise for high-resolution, label-free biosensing and chemical detection applications.
