Overview: Optical fiber temperature sensors have received widespread attention because of its high sensitivity, fast response, anti-electromagnetic interference, ultra-high voltage insulation, anti-combustion and anti-explosion. However, the temperature sensing probes of the available interferometric optical fiber temperature sensors are generally made of optical fibers, and most of them use the white light interference measurement technology. Because the thermal expansion coefficient of optical fiber is not high, the sensitivity of temperature measurement needs to be further optimized. In addition, because the optical fiber is relatively fragile, especially at higher temperature, the loss of coating-protected fibers can be embrittled in contact with air, which will affect the reliability of the temperature sensor. The white light interferometry uses a broadband light source and a spectral analysis device to collect the reflection or transmission spectrum. White light interference solves the optical path difference from the interference beam by spectral information, but its measurement range is limited, and requires a sophisticated spectral analysis device or module, which is relatively high in cost.
This paper presents an extrinsic Fabry-Perot (F-P) cavity optical fiber temperature sensor, which is based on the frequency-modulated continuous-wave (FMCW) laser interference. Compared with the traditional laser interferometry technology, since the signal of optical FMCW is a dynamic signal (i.e., a time continuous function), to calibrate the fractional phase, distinguish the phase-shift direction and count the number of full periods is quite easy. The proposed FMCW laser interference temperature sensor mainly composed of an optical system, a modulation signal generation system and a signal acquisition and processing system. The temperature sensor uses a stainless-steel tube with a higher expansion coefficient and better chemical stability as the F-P cavity. The extrinsic fiber F-P cavity temperature sensing probe is fabricated by coupling the cavity with the fiber using a single mode fiber collimator with output direction is strictly perpendicular to the two mirrors of the F-P cavity. The temperature is determined by measuring the change in cavity length caused by thermal expansion of the F-P cavity, and the cavity change is found out by using the frequency-modulated continuous wave interferometry. In this experiment, the F-P cavity is a stainless-steel tube, and the temperature probes with lengths of 100 mm and 200 mm (the length of the Faber cavity are 80 mm and 180 mm) are used for the experiment. The experimental results show that the temperature measurement resolution of the optical fiber temperature sensor reached of 0.0002 ℃ and the temperature measurement sensitivity reached 3022 nm/℃.