Dual-Cavity Fiber Fabry-Perot Interferometer Coated With SnO2 for Relative Humidity and Temperature Sensing

An optical fiber tip interferometer for the measurement of relative humidity (RH) and temperature is proposed. The optical fiber structure used, a dual-cavity optical fiber Fabry-Perot interferometer (DFFPI), is simply-to-fabricate, compact, and robust. The reflectance (R-DFFPI) of the interferometer is sensitive to the refractive index (RI) and temperature of the external medium. Consequently, when the cross-section of the fiber tip was coated with a SnO2 thin film, whose RI changes according to the humidity of the surrounding ambient, the measurement of the RH was possible. An increment of the RH produced a decrement of RI of the SnO2 thin film, then the reflectance of the fiber tip end-face diminished, and this produced a decrement of the visibility of the interference fringes. The analysis of the R-DFFPI was carried out in the Fourier domain, using a novel processing method it was possible to establish that the amplitude of two peaks of Fourier spectrum changed at a ratio of 39.49 x 10(-3) %RH-1 in the range of 40 to 90 RH%. On the other hand, the temperature of the humidity chamber was monitored, from 25 to 60 degrees C at a fixed RH%, by analyzing the phase shift of the interference pattern produced by the changes in the optical path length of the cavities. The good sensitivity, stability, reproducibility, and compactness of the fiber tip RH sensor make this proposal very appealing in a wide range of applications.