Agarose Filled Fabry–Perot Cavity for Temperature Self-Calibration Humidity Sensing

An agarose-based fiber-optic Fabry-Perot (F-P) sensor is presented for relative humidity (RH) measurement. The reflectors are formed by two identical fiber Bragg gratings (FBGs), and the cavity is filled with the agarose gel. Because of the hydrophilic characteristics of the agarose, the humidity variation will induce the expansion and refraction index change of the agarose gel, which will lead to the shift of the F-P resonance peak. The FBG spectrum can be recovered by applying a fitting algorithm to the spectrum of the F-P cavity, and is used for temperature self-calibration to resolve the temperature cross-sensitivity problem. The average sensitivity is 22.5 pm/%RH. Benefiting from the tiny size of the RH sensitive element, the rise time and the fall time are tested to be 5.11 and 10.35 s, respectively.