High Sensitivity Refractive Index Sensor Based on TiO₂-Ag Double-Layer Coated Photonic Crystal Fiber

In this study, a D-type photonic crystal fiber (PCF) refractive index (RI) sensor based on surface plasmon resonance (SPR) is designed. The plasma material is silver (Ag), and the titanium dioxide (TiO₂) is selected to cover the silver film to protect silver from oxidation and enhance the SPR effect. This design enables the sensor to effectively detect the RI change of the analyte. Utilizing the finite element method (FEM), we elaborately researched and optimized various structural parameters and analyzed the influencing factors of the sensor performance. The operating wavelength range of the sensor is from 1000 to 2000 nm. In the RI sensing range of 1.345 to 1.405, the designed PCF sensor possesses an extraordinary maximum wavelength sensitivity of 32,000 nm/RIU, the outstanding figure-of-merit (FOM) of 584.59 RIU⁻¹, and a maximum resolution of 3.125 × 10⁻⁶ RIU. The results concise indicate that the proposed sensor exhibits predominant sensitivity and resolution to the changes of RI of analyte through the SPR effect. The sensor has significant advantages such as ultra-high sensitivity, small size, and low manufacturing complexity.