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• A compact concentration sensor based on the modified thin-diameter polarization-maintaining fiber is demonstrated. • The relationship between the measurement sensitivity and the length and corroded time of the TDPMF is discussed. • The SC concentration sensor exhibits a high linear sensitivity response up to 4.65 nm/%, and the temperature sensitivity is 0.08179 nm/°C. • The MMF-MTDPMF-MMF structure sensor has excellent repeatability and stability in SC concentration sensing. Sodium citrate, as strong alkali and weak acid salt, play an essential role in the food and pharmaceutical industries. However, lack of detection method selectivity, limited biocompatibility and complexity of the manufacturing process are challenges that hinder the practical application of the sensors. In this paper, a improved-sensitivity sodium citrate concentration sensor with simple and compact interrogation setup is proposed. The sensor is fabricated by fusion splicing the sandwich structure of modified thin-diameter polarization-maintaining fiber, two pieces of multimode fiber, and single-mode fiber. In theoretical modeling, the properties of the structure are investigated. In the experiment, the evanescent field generated by the modified thin-diameter polarization-maintaining fiber plays a vital role in the detection of different sodium citrate concentrations. The sodium citrate concentration sensor exhibits a high linear sensitivity response up to 4.65 nm/%. The sensor provided refractive index sensitivity of 3173.2 nm/RIU. In addition, the response, repeatability, and stability are also evaluated by the experiments. Our results show that the sodium citrate concentration sensor holds great promise for serving as a soft and biocompatible probe for monitoring sodium citrate in situ. [ABSTRACT FROM AUTHOR]

• Recycling fused optical fiber to develop a miniature self-referred Fabry-Perot microcavity sensor. • The physical theory and Finite Element Method (FEM) of the Fabry-Perot microcavity were considered to fabricate an efficient sensor. • This sensor simultaneously measurements the refractive index and temperature. • This sensor has the advantages of low cost, high repeatability, and small dimension, making it a desirable sensor for chemical and biomedical applications. In this paper, a fiber-optic Fabry-Perot microcavity sensor is presented based on splicing a catastrophic fused fiber (CFF) to a single-mode fiber (SMF) for the simultaneous measurement of the refractive index (RI) and temperature. CFF formed by the catastrophic fuse effect consists of a periodic void in the fiber core that creates a teardrop shape or elliptical air microcavity after splicing. By simulation, the elliptical microcavity sensor is selected to measure external RI changes by Fast Fourier Transform (FFT) analysis. The RI sensitivity was obtained to be −2.5 ± 0.0074/RIU with a resolution of about 4 × 10−5 RIU. Also, the experimental results showed that temperature is obtained independently from the FFT parameter without being affected by the RI. The sensor temperature was determined by tracking the wavelength shift of the interference spectrum. Consequently, the temperature sensitivity of 7 pm/°C in the range of 20 °C to 95 °C was achieved. Therefore, the RI and the temperature were simultaneously measured by analyzing the spatial frequency and wavelength shift. The proposed dual-microcavity FP sensor can compensate for any power fluctuation in the input light source. [ABSTRACT FROM AUTHOR]