Linear and passive silicon-on-insulator refractive index sensor utilizing Bragg grating-assisted Michelson interferometer

In this work, we present a design for a linear and entirely passive refractive index (RI) sensor based on silicon-on-insulator (SOI) technology, addressing the need for sensors capable of monitoring continuously changing gas concentrations. Conventional RI sensors, based on interferometers, exhibit a nonlinear response, limiting their effectiveness for applications requiring consistent performance across a wide range of refractive index changes. The proposed configuration utilizes a Michelson interferometer and two long Bragg gratings to linearize the interferometer response, ensuring a constant figure of merit (FOM) across the entire range of changes in the cover medium's refractive index. A slotted Bragg grating is employed in the sensing arms to enhance the interaction between the light inside the waveguide and the surrounding medium. We demonstrate the sensor's operation and design using finite-difference eigenmode (FDE) and finite-difference time-domain (FDTD) simulations, followed by a theoretical analysis of its performance based on key design parameters, including grating properties, waveguide sensitivity, and phase delay. With a slot width of 80 <i>nm</i> and assuming a sensing arm index modulation depth (<i>δn_sen</i>) of 0.01, the design achieves a waveguide sensitivity of ∼ 0.72 and a sensor with a constant FOM of ∼ 113 RIU^-1. We fabricated several Bragg grating devices and Michelson interferometers based on slot and strip waveguides with oxide cladding, measured their responses, and compared these with simulations to validate the proposed design's functionality. The proposed sensor can be entirely exposed to air, simplifying the fabrication process by eliminating the oxidation step and the need for additional patterning of a window for the sensing area. This is made possible by the strong desensitizing factor (<i>S_refδn_sen/S_senδn_ref</i>) of the reference arm compared to the sensing arm. Consequently, the proposed sensor is well-suited for low-cost fabrication and mass production.