Development of fiber Bragg grating pH sensors for harsh environments

Fiber Bragg gratings (FBGs) are inherently sensitive to temperature and mechanical deformation. Coating and packaging the fiber by particular materials which are responsive to certain parameters can extend the range of sensing capabilities of FBG-based fiber optic sensors. In this study, a stimuli responsive polymeric material is developed to behave reversibly when exposed to environments with different pH concentrations. Protonation and deprotonation of acidic or basic pendant groups on the polymer cause a pH-dependent osmotic pressure difference which leads to the swelling and deswelling of the polymer relative to the external conditions. This propensity to swell can be translated into a strain which is detected by the FBG. In this work, the FBG section of a fiber optic is coated with a custom designed and nanostructured polymer materials. Various super porous polymers have been developed by tuning the micro and nanostructure of the custom-designed polymer to explore the relationship between the polymer mechanical properties and the strain induced on the FBG and investigate optimal formulations with sufficient sensitivity. It was observed that changing the concentration of porosity in the polymer leads to different time scales for swelling and consequently, sensor response time. The optimized super-porous polymer coated on the fiber showed a reversible response to pH over a wide range (3 to 8). The as-developed quasi-distributed FBG pH sensor cable can be used for real-time monitoring of chemical substances in harsh environments such as chemical and wastewater treatment plants, and also in smart greenhouses.