Small and Robust All-Polymer Fiber Bragg Grating Based pH Sensor

The smallest all-polymer optical fiber Bragg grating based transducer element for pH sensing is presented. We show that, considering it's size and robustness, it out-performs similar state-of-the-art fiber Bragg grating based pH sensors regarding both sensitivity and response time. A 5-10 \mum thick pH sensitive hydrogel coating is placed on a PMMA-based microstructured polymer optical fiber Bragg grating. The hydrogel expands or contracts depending on the pH and thus changes in pH are monitored by following the fiber strain induced changes in the reflected Bragg wavelength \boldsymbol{\lambda _B}. Prior to applying the hydrogel coating the mPOF is etched from 150 to 80 \mum to enhance sensitivity and surface crazing is introduced with a 50/50 vol% solution of acetone and methanol to enhance spreading of the hydrogel during the application and adhesion after cure. With this design, we achieved a sensitivity of { \Delta \mathbf {\lambda }_B=\text{73 pm/pH} \pm \text{2 pm/pH}} and response times below 4.5 mins. for pH 5-7 and 4-7, respectively, and a thermal cross sensitivity of \text{31.4 pm}/^{\circ}\text{C} \ \pm \text{0.4 pm}/^{\circ}\text{C}.