Dual-Parameter Optical Fiber Sensors for Refractive Index and Temperature Measurements Based on a Cascaded SNS–FBG Structure

A cascaded dual-parameter fiber-optic sensor is presented, in which a single-mode–no-core–single-mode (SNS) multimode-interference (MMI) interferometer is integrated with a fiber Bragg grating (FBG) to achieve simultaneous refractive index (RI) and temperature sensing. The SNS segment, fabricated by fusion splicing a no-core fiber (NCF) between two single-mode fibers, exploits MMI to generate spectral features that are highly responsive to ambient RI changes. Meanwhile, the FBG serves as an independent temperature reference, owing to its negligible RI sensitivity. Based on the beam propagation method, the MMI characteristics in the NCF are analyzed. By combining the simulation results with the experimental spectra, the NCF length is optimized by comprehensively considering the interference-fringe visibility, free spectral range, and spectral separation from the FBG wavelength. Experimental results show that the maximum RI sensitivity of the SNS interferometric structure reaches 136.29 nm/RIU, with a corresponding temperature sensitivity of 9.14 pm/°C. The FBG exhibits a temperature sensitivity of 9.83 pm/°C while remaining virtually unresponsive to surrounding RI variations. By establishing a dual-parameter sensitivity matrix, RI and temperature variations can be simultaneously demodulated, enabling effective temperature compensation for RI sensing. The sensor requires no tapering, etching, or surface modification and can be fabricated using only conventional fiber-cleaving and fusion-splicing processes. With its simple fabrication, compact structure, low cost, and good mechanical stability, the sensor shows promising potential for temperature-compensated RI sensing, biochemical detection, liquid-concentration monitoring, and environmental sensing.