Low-temperature-crosstalk humidity fiber sensor based on a thermal expansion-resistant Fabry-Perot interferometer

A low-temperature-crosstalk fiber-optic humidity sensor based on a thermal expansion-resistant Fabry-Pérot interferometer (FPI) has been proposed. This sensor is constructed by cascading a single-mode fiber, a hollow core fiber, an air-clad photonic crystal fiber (APCF), and a polyimide microrod. The two mirrors of FPI are composed of the end faces of single-mode optical fiber and polyimide microrod (located on the end face of APCF). Thus, hollow core fiber thermal expansion will offset the change in FPI length caused by polyimide microrod thermal expansion, thereby achieving low temperature sensitivity. In contrast, an increase in humidity causes the polyimide microrod to absorb moisture and swell, resulting in a shortening of the FPI. Experimentally, the sensor’s humidity sensitivity between 30 % RH and 80 % RH and temperature sensitivity between 20 °C and 200 °C are 203.1 pm/% RH and 0.95 pm/°C, respectively. With a temperature cross-sensitivity as low as 0.0047 % RH/°C. This article provided a novel solution for achieving low temperature cross sensitivity in humidity sensors.