Ultrashort Fiber Optic Temperature Sensor for the Small-Scale Heat Sources

Fiber optic temperature sensors for small-scale heat sources have an urgent need in many industrial fields, for example, the temperature field of the laser heat source, mini LED, microchip, and so on. To the best our knowledge, we first propose and demonstrate an ultrashort fiber optic temperature sensor based on a Fabry–Perot (FP) interferometer which combines a low reflection fiber Bragg grating (FBG) with Fresnel reflection (i.e., ~4%) on the fiber end face. This unique FBG exhibits a broad Gaussian spectrum with a full-width at half-maximum (FWHM) bandwidth of 12.4 nm. This FBG-FP cavity structure was fabricated by a femtosecond laser line-by-line (LbL) scanning technology, and it has stronger mechanical strength due to no damage to the fiber material compared to other fiber optic FP interferometers. The free spectrum range (FSR) of the FBG-FP cavity is about 1.56 nm, corresponding to the cavity length can be calculated to be about $530 \mu \text{m}$ . The selected dip wavelength of the FBG-FP cavity sensor changes linearly with the temperature increase, corresponding to the temperature sensitivity of 10.8 pm/°C. Moreover, the FBG-FP cavity sensor exhibits a high spatial resolution of $580 \mu \text{m}$ and it can be used to measure the temperature of the small-scale laser spot due to its compact structure. The experimental results show the temperatures at different positions of 1064 nm laser exposure area are 138.4 °C, 111.5 °C, and 74.4 °C, respectively.