A fiber sensor tip was proposed for measuring temperature and liquid refractive indices (RI). The sensor tip was fabricated by dipping a solidification ultraviolet (SU-8) photoresist onto the end surface of a simple-mode optical fiber. Then the SU-8 photoresist was cured by an ultraviolet source. A Fabry-Perot (FP) cavity was formed on the end faces of the SU-8 and optical fiber, and it showed a perfect FP interference spectrum. The temperature and liquid RI can be independently measured by monitoring the shift and the loss variation of interference peaks. The liquid RI sensitivity is 96.07232 dB/RIU in the RI range of 1.333 to 1.3714, and the temperature sensitivity is 0.38052 nm/°C within the temperature range from 25 to 55°C. The cross-sensitivities of temperature and liquid RI can be ignored. The sensor tip can be used in harsh environments, including strong acid, strong alkali, and violent vibration liquids, as cross-linking SU-8 has excellent chemical corrosion resistance capacity and mechanical strength.
"Fiber-optic sensor tip for measuring temperature and liquid refractive index," Optical Engineering 53(11), 116110 (13 November 2014). https://doi.org/10.1117/1.OE.53.11.116110