Simultaneous Measurement of Temperature and Humidity Based on Phase-Shifted Fiber Bragg Grating and Fabry–Perot

A dual-parameter sensor for temperature and humidity based on phase-shifted fiber Bragg grating (PS-FBG) with polymethylmethacrylate (PMMA) microsphere Fabry–Perot (FP) cavity is designed. The sensor consists of PMMA microspheres forming the FP cavity in cascade with PS-FBG. The results show that the FP cavity exhibits a high sensitivity of 83.38 pm/%RH with an excellent linearity of 0.9982 in the range of 35%RH–85%RH, while the PS-FBG humidity response is zero. The temperature sensitivities of the FP cavity and PS-FBG are −87.26 pm/°C and 12.05 pm/°C, respectively, when the temperature increased from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$25~^{\circ }$ </tex-math></inline-formula>C to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$50~^{\circ }$ </tex-math></inline-formula>C. The 3-dB ultranarrow bandwidth characteristic (0.022 nm) based on the PS-FBG realizes the high-precision temperature measurement, which is conducive to reducing humidity measurement error during the dual-parameter measurement. Moreover, the scheme of utilizing the sensitivity coefficient matrix to realize the simultaneous measurement of temperature and humidity is experimentally verified, and the maximum temperature and humidity errors are, respectively, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.21~^{\circ }$ </tex-math></inline-formula>C and 1.98%RH, respectively. The sensor combines the features of high sensitivity, high-precision temperature compensation, simple structure, and excellent repeatability.