A Parallel Optical Fiber Fabry–Perot Interferometer for Simultaneous Measurement of Relative Humidity and Temperature

A parallel optical fiber Fabry–Perot interferometer (FPI) and Vernier effect sensor for simultaneous high-sensitivity measurement of relative humidity (RH) and temperature is proposed and verified by experiments. Two sections of a single-mode fiber (SMF) are fixed on the precision displacement platform, aligning them and leaving a small gap between them, and then filling the gap with polydimethylsiloxane (PDMS) and polyimide (PI), respectively, to form two Fabry–Perot (F–P) cavities. To produce a Vernier effect when two F–P cavities are connected in parallel, the length of two F–P cavities must be accurately controlled to ensure that the free spectral range (FSR) of the two F–P cavities is similar. It is found that the F–P cavity filled with PI is sensitive to RH and temperature, while the F–P cavity filled with PDMS is only sensitive to temperature. Due to the different characteristics of the two F–P cavities, the simultaneous measurement of RH and temperature is realized by constructing the sensitivity measurement matrix. If they are connected in parallel to produce the Vernier effect, the RH and temperature sensitivities of the parallel structure can reach −11.388 nm/%RH and 18.436 nm/°C, respectively. The sensor has potential applications in the fields of biopharmaceuticals, environmental monitoring, food processing, and microbial sensing.