High Sensitivity Interferometric Optical Fiber Humidity Sensor Based on a Moisture Sensitive Film

A novel interferometric humidity sensor constructed by a pair of fiber optic collimators and a piece of inserted plate with moisture sensitive film on its surface is presented in this paper. The moisture sensitive film was made by spreading a layer of optical adhesive from Norland Company (NOA61) and drying it in air. Owing to the porous structure of the polymer film made by NOA61, its refractive index will change with the change of ambient moisture composed by the water molecules, which will result in the phase change of the light passing through the film. In our experiments, we let only half of the optical beam exiting from the first collimator pass through the moisture sensitive film, and the other half of the beam go through without the film. The two parts of the beam was collected by the second collimator, combining and focusing on to the end of the receiving fiber. Consequently, a Mach-Zehnder interferometer was constructed and periodical change in the transmission spectrum of this optical fiber collimator pairs could be observed clearly. Since the phase difference between the two parts of the transmission beam of the collimators will be affected by the surrounding moisture, one can detect the environmental humidity from the wavelength shift of the peaks or dips in the spectrum. In the experiments, the relative humidity (RH) was changed from 30% to 90% as the temperature kept at 50°C, and with film of 0.08 mm in thickness, the wavelength shift can be as large as 9.01 nm, which corresponding to a humidity sensitivity of 151.39 pm/%RH. A cycling test indicates the sensor has good repeatability and stability. The proposed sensing scheme has advantages of high sensitivity, compactness, low cost, etc., which make it promising in practical applications.