Resonant micromechanical fiber optic sensor of relative humidity

This paper presents a novel type of humidity sensor which is based on silicon micromechanical resonant structure with an attached silica gel granule. The mechanical oscillations of microstructure are interrogated with the aid of fiber-optic Fabry-Perot interferometer. The output signal of such a sensor in the frequency form is not sensitive to long-term variations of the optical power and it can be easily digitized by counting the periods. Two-channel PLL was used for the first time to track the resonant frequency at any interferometer working point position. The design of a sensor turned out to be rather successful: the mass of the air moisture absorbed by silica gel granule turned out to be dependent linearly on the relative humidity in the range from 0% to 75% that simplifies the calibration of the sensor; and the high quality factor of the mechanical resonator allowed us to resolve the resonant frequency with precision about 0.1 Hz, which corresponds to sensor threshold sensitivity in 0.02% of relative air humidity.