Optical Fiber Interferometric Humidity Sensor by Using Hollow Core Fiber Interacting with Gelatin Film

An optical fiber Fabry-Perot interferometer (FPI) is constructed for relative humidity measurement by fusion splicing a short hollow core fiber (HCF) to the end of a single-mode fiber and coating the tip of the HCF with a layer of gelatin. The thickness of the gelatin film changes with ambient humidity level and modulates cavity length of the FPI. Humidity measurement is therefore realized by measuring the wavelength shift of the interreference fringe. RH sensitivity of 0.192 nm/%RH is achieved within a measurement range of 20-80%RH. Dynamic measurement shows a response and recovery time of 240 and 350 ms, respectively. Sensor performance testing shows good repeatability and stability at room temperature but also reveals slight dependence of the RH sensitivity on environmental temperature. Therefore, a fiber Bragg grating is cascaded to the FPI sensing probe to monitor temperature simultaneously with temperature sensitivity of 10 pm/degrees C.

Automated summary

An optical fiber Fabry-Perot interferometer (FPI) is used for relative humidity measurement by coating a short hollow core fiber (HCF) with gelatin and fusion splicing it to a single-mode fiber. The gelatin film thickness changes with humidity, which modulates the FPI cavity length. The humidity measurement is achieved by measuring wavelength shift. The FPI-HCF sensor can also monitor temperature using a cascaded fiber Bragg grating with temperature sensitivity.