Miniature Fizeau Interferometric Thermometer

Abstract

Precise measurement of temperature is vital in a variety of industries and basic sciences. Numerous techniques have been proposed for temperature measurement, such as fiber Bragg gratings and Fabry–Perot interferometers. Despite several important accomplishments, such optical thermometers are still limited in their performances. Two major limitations are the low operational temperature and the paradox between a large dynamic range and a high resolution. Here, we propose a fiber-optic thermometer based on fiber-tip microspherical Fizeau interferometer made from high temperature-resistant material. The sensor is fabricated by inserting a section of single-mode fiber into a spherical microcavity to form a Fizeau interferometer. Thanks to the thermo-optic effect and thermal expansion effect, the variation of ambient temperature modifies the refractive index and the interferometric length of the microcavity, and then a corresponding change in the interferometric spectrum can be observed. The demodulation method is based on the combination measurement of interferometric fringe shift and free spectral range variation, which is capable of providing high resolution and wide dynamic range. Experimental results show that by using a simple wavelength tracking demodulation method, a sensitivity of 22 pm/°C with a high resolution and a dynamic range over 1000 °C can be achieved. Since the sensor is of small size, it exhibits fast response and recovery time which mean it can monitor ambient temperature variation in real time. Additionally, the structure is robust and compact which makes the technology an interesting alternative to conventional thermometers for temperature measurement.