Compact fiber Fabry–Perot sensors filled with PNIPAM hydrogel for highly sensitive relative humidity measurement

• Published in: Measurement : journal of the International Measurement Confederation 2022-09, Vol.201, p.111781, Article 111781
• Main Authors: Xia, Ru-Lei, Liu, Juan, Shi, Jiulin, He, Xing-Dao, Yuan, Jinhui, Pike, Andrew R., Chu, Liang, Wu, Qiang, Liu, Bin
• Format: Article
• Language: English
• Subjects: Fabry-Perot interferometer (FPI); Fiber-optic sensor; PNIPAM hydrogel; Relative humidity (RH)
• ISSN: 0263-2241; 1873-412X
• DOI: 10.1016/j.measurement.2022.111781

•First propose using PNIPAM hydrogel for relative humidity detection by filling fiber Fabry–Perot interferometer structure.•Ultra-high sensitivity of 1.634 nm/%RH with good repeatability.•Realizes the application for effectively respiratory monitoring with response and recovery time of 0.91 s and 1.91 s, respectively. A fiber-optic Fabry–Perot (FP) based relative humidity (RH) sensor was proposed and experimentally investigated by using Poly(N-isopropylacrylamide) (PNIPAM) hydrogel to connect two ends of single-mode optical fiber (SMF) to formulate a PNIPAM FP cavity. The water vapor will be absorbed by the PNIPAM hydrogel, which will alter both the refractive index (RI) and dimension of the PNIPAM FP cavity, resulting in the change of output resonance intensity in the reflective spectrum. The measure RH sensitivity is 1.634 nm/%RH within RH range from 45 to 75 % with good linearity of 0.9897 and excellent repeatability. The influence of PNIPAM hydrogel concentration on the RH sensitivity has been investigated - the higher the concentration of PNIPAM hydrogel, the higher the RH sensitivity of the sensor. The developed sensor can effectively monitor human respiratory with response and recovery time of 2.29 s and 1.90 s, respectively.