High sensitivity fiber optic temperature sensor composed of two parallel FPI and enhanced harmonic Vernier effect

A high-sensitivity fiber optic temperature sensor based on the enhanced harmonic Vernier effect (HVE) is proposed, which consists of two Fabry-Perot interferometers (FPI) that are sensitive to temperature and connected in parallel. FPI₁ is a polydimethylsiloxane (PDMS) cavity formed by filling a ceramic ferrule with PDMS, and FPI₂ is an air-cavity formed by inserting a single-mode fiber into a ceramic ferrule coated with PDMS film on the end face. FPI₁ and FPI₂ have opposite temperature responses and an approximate 2-fold free spectral range (FSR) relationship. As the temperature rises, the interference spectrum of FPI₁ gradually red-shifts, while the interference spectrum of FPI₂ gradually blue-shifts, resulting in an enhanced HVE. Its temperature sensitivity is much higher than that of a single FPI, and the amplification rate is significantly higher than that of ordinary Vernier effect. Two enhanced HVE sensors S₁ and S₂ are developed using this method, but there is a certain difference in their FSR detuning. The experimental results reveal that within the temperature range of 30-35 °C, the temperature sensitivity of S₁ and S₂ reach - 44.39 nm/°C and - 23.14 nm/°C, respectively. Both S₁ and S₂ have extremely high temperature sensitivity, but FSR detuning has a significant impact on sensitivity amplification. Additionally, the proposed enhanced HVE sensor has good repeatability and stability in measuring temperature.