High-sensitivity and fast-response fiber-optic micro-thermometer based on a plano-concave Fabry-Pérot cavity filled with PDMS

A high-sensitivity and fast-response fiber-optic micro-thermometer based on a liquid-filling plano-concave Fabry-Pérot (FP) cavity was proposed in this paper. The proposed sensor was fabricated by splicing a short section of hollow silica capillary (HSC) on the end of the single mode fiber (SMF). Liquid polydimethylsiloxane (PDMS) was injected from the open end of the HSC, and the air in the HSC was compressed by the PDMS to form a plano-concave air cavity. The open end of the HSC was sealed by a polymethylmethacrylate (PMMA) coating to prevent liquid PDMS leakage. Owing to the high thermal expansion coefficient (TEC) of PDMS, the length of air cavity in the HSC changed with temperature, which caused the dips to move in the reflection spectrum. The total length of the sensing probe was only 120 μm, and a high temperature sensitivity of 1.509 nm /°C was obtained in the range of 35 °C–45 °C. The proposed structure exhibited rapid response time, good repeatability, and biocompatibility in body temperature testing.