Single mode modified tapered structure with graphene oxide/polyvinyl alcohol composite layer for relative humidity sensing

Abstract

A moisture-sensitive sensor based on tapered optical fiber coated with graphene oxide (GO)/polyvinyl alcohol (PVA) nanocomposite film was studied and experimentally demonstrated. Modification on waveguide structure has been made by reduction of the waist diameter to promote evanescent wave for sensing purposes. In this study, standard silica single mode optical fiber with diameter of 125 microns was successfully tapered down to a smaller beam waist around 9 microns by flamebrushing technique. The tapered section of the fiber was then functionalized with additional deposition of GO/PVA nanocomposite layers through dip coating technique. The sensing principle is based on ability of the moisture-sensitive coatings to undergo changes in refractive index and modulates the intensity of light propagating through the fiber. Three different concentrations of nanocomposites comprise of GO/0.2 g PVA, GO/0.3 g PVA and GO/0.4 g PVA were prepared and characterized using field emission scanning electron microscope (FESEM) and Raman spectroscopy. The presence of oxygen functional groups (such as -OH, COOH) on the GO structure creates an invitation for the attachment of PVA through hydrogen bonding as evidenced by the FTIR analysis, thus providing strong adhesion between the GO and PVA layers. The performance of the sensor was investigated over a wide range of 20.0 to 99.9% relative humidity (RH), where the sensor showed good sensitivity as its signal increased linearly with respect to the surrounding RH. Tapered optical fiber sensor with coating of GO/0.3 g PVA recorded the highest sensitivity, which was found at 0.5290 % absorption/% RH. The sensor shows good repeatability with precision error of 3.11%. The sensor acquired good reversibility in both increasing and decreasing relative humidity with close points recorded at the same % RH. Stability of the output signals suffers from high fluctuation at extreme surrounding condition of 99.9% RH with fluctuation rate of 0.009/90 minutes. Finally, response time of 141 seconds corresponding to full range of RH spread from 20.0 to 99.9% has been experimentally achieved.