Highly Sensitive Fiber Optic Sensor for Simultaneous Refractive Index and Temperature Measurement Using Suspended Core Fiber

Abstract

A novel fiber optic sensor has been developed using suspended core fiber (SCF) to simultaneously measure the refractive index (RI) and temperature of liquids. The innovative design comprises an SCF segment spliced between two tapered single-mode fibers (SMFs). The SCF allows the transmission of multiple modes, thus enabling multimode interference. Importantly, the three internal holes of the SCF function as microfluidic channels, providing direct access to the evanescent field of the micrometer-scaled core. This direct interaction significantly enhances the sensor’s sensitivity by intensifying the light-sample interaction. Each pair of interfering optical modes exhibits distinct sensitivity to both temperature and RI. By performing Fourier analysis, the interference spectrum for each mode pair can be extracted. High sensitivity is achieved, with the values of 1560 and 1217 nm/RIU for RI within the range 1.3309–1.3350 and −0.25 nm/°C and −0.31 nm/°C for temperatures ranging from 20 °C to 40 °C. The resolutions of RI and temperature were $1.3 \times 10^{-5}$ RIU and 0.06 °C, respectively. The transfer matrix method effectively eliminates temperature interference during RI measurement, ensuring reliable and accurate RI readings. The sensor is shown to have a detection limit as low as $6.1 \times 10^{-5}$ RIU and 0.3 °C. Combining its desirable characteristics of good stability, a simple structure, and high sensitivity, this novel sensor holds significant promise for diverse applications in environmental monitoring, medical testing, and biological sensing.