Polyvinyl Alcohol Coated Thin Core Optical Fiber with Different Concentrations and Dipping Numbers for Humidity Sensor Applications

A polyvinyl alcohol (PVA) film-coated single-mode optical fiber was studied, and its capability as a humidity sensor was proven. When designing an optical fiber-based humidity sensor, a critical problem lies in determining the optimal concentration and thickness of the coating layer to achieve high sensitivity, accuracy, and stability in detecting humidity variations, while maintaining the optical performance of the fiber for practical sensing applications. Different PVA concentrations as a coating material on optical fiber were used to investigate the most suitable and high-quality sensors for humidity sensing purposes. The PVA solution was prepared by dissolving 0.28 g, 0.57 g, 0.89 g, and 1.23 g of PVA granules for 3 wt.%, 6 wt.%, 9 wt.%, and 12 wt.% concentrations, respectively. FESEM was used to obtain the morphology and thickness of the PVA coating layer. It was discovered that the thickest coating is 12 wt.% sample, which is 4.18 µm. Higher concentrations of PVA produced higher viscosity of the prepared PVA solution and subsequently caused the coating to become thicker. The performance of the sensor was examined inside the humidity chamber with relative humidity (RH) from 40% to 90%. Optical fiber with 6 wt.% and 9 wt.% PVA coating exhibited the highest intensity indicating these concentrations are the optimum concentration. However, 6 wt.% concentration is more suitable for a higher %RH level, while a 9 wt.% concentration is more applicable for a lower %RH level. On the other hand, the core of the fiber optics has also been coated with different dipping numbers (2, 4, 6, 8, and 10 times) to determine the optimum number for sensing performance. The intensity spectrum of coated optical fibers was increased linearly with respect to the dipping number.