Plastic fiber optic humidity and gas sensor

Abstract

In this thesis, a simple and low cost plastic optical fiber (POF) sensor with a humidity sensing range from 13 % to 95 % relative humidity (RH) is described. The sensing mechanism of POF humidity sensor is based on the alterations of refractive index of a polymeric thin film which is coated directly onto a polished segment of plastic optical fiber as cladding layer. When the refractive index of the cladding layer of POF changes, due to the relative humidity, fiber output voltage changes significantly. Gammaisocyanatopropyltriethoxysilane end-capped polyethylene glycol (PEG-Si), which is a highly hydrophilic polymer, is used as the coated polymer film. At low humidity levels, PEG-Si has a semicrystalline form; therefore, its refractive index slowly decreases with respect to increase in the amount of absorbed water molecules. As a result, the fiber output voltage decreases linearly up to 80 % RH; then, it shows a turning point and fiber output voltage starts to increase. Since PEG-Si melts from the semicrystalline form to a gel form, its refractive index shows a sudden decrease. To overcome this instability problem, PEG-Si polymer layer is enriched with hydrogen. The turning point in the hydrogenated PEG-Si shifts to higher values of relative humidity and the sensor output gives a linear and repeatable response to the humidity. The response of PEG-Si to various gases such as acetone, methanol, hexane, benzene, and toluene is also studied. It is observed that when acetone, methanol, and hexane vapors are increased, the fiber output voltage increased; whereas toluene and benzene decreased the output voltage due to their ring structure and their high refractive index.