Ultra-sensitive fiber-optic temperature sensor based on UV glue-based FPI and Vernier effect.

• The compact cascaded FPI fiber optic temperature sensors are proposed. • The sensor based on UV glue and HVE exhibits high sensitivities of 2.24 nm/°C. • Increasing the harmonic order of HVE improves the sensor's temperature sensitivity. • The sensors exhibits good linear response, excellent repeatability and stability. In this paper, compact cascaded Fabry-Perot interferometers (FPI) for fiber-optic temperature sensors are proposed and verified. The sensors are prepared by combining the properties of temperature-sensitive material ultraviolet glue (UV glue) and the basic principle of the traditional Vernier effect (TVE), first-order harmonic Vernier effect (HVE), and second-order HVE. The sensing cavity FPI s is made up of UV glue filling the gap between two single-mode fibers (SMF) with flat cut ends. The reference cavity FPI r is composed of SMF-capillary-SMF structure. The temperature sensitivity of the FPI s is up to 1.01 nm/°C by the high thermal expansion coefficient of the UV glue. By changing the free spectral range (FSR) of the FPI r , high detection sensitivities achieved are −10.14 nm/°C, 15.22 nm/°C, and 22.24 nm/°C, corresponding the situation of TVE, first-order HVE, and second-order HVE, respectively. The experimental results indicate that temperature sensors based on UV glue possess a simple and compact structure and strong resistance to electromagnetic interference, making them suitable for temperature measurement in various environments. [ABSTRACT FROM AUTHOR]