Micro-Conical Cascaded Dual-Channel Fiber Optic SPR Sensor with Integrated Polymer Film for High-Sensitivity Relative Humidity and Temperature Measurement

A dual-channel fiber optic surface plasmon resonance (SPR) sensor featuring a micro-conical design is developed to enable highly sensitive and concurrent sensing of relative humidity (RH) and temperature. The humidity-sensing channel is constructed relying on the micro-conical fiber structure, deposited with a gold layer as well as a nanoscale-thickness polyvinyl alcohol (PVA) film, which can undergo rapid volume changes in response to humidity. The conical fiber tip enhances the evanescent field, thereby increasing the interaction between surface plasmon polaritons (SPPs) and the nanoscale PVA film, and realizing high-performance RH sensing. Cascaded with the micro-conical structure is the temperature-sensing channel, which is built using a twin-core dual-hole fiber (TCDHF) combined with polydimethylsiloxane (PDMS), aiming to achieve simultaneous temperature measurement. Simulation and preliminary experimental results showed that within a certain range, a thicker nanoscale PVA film leads to a longer resonant wavelength and a higher response to humidity. Further dual-parameter experimental results indicated that when three layers of PVA are integrated on the surface, the developed dual-channel SPR probe exhibits outstanding RH and temperature sensing performance. It can provide an RH sensitivity up to 2.80 nm/%RH and –2.66 nm/°C for temperature. Meanwhile, dynamic response tests verify that the sensor has reliable cyclic consistency, achieving fast response times of 7 s and 12.67 s for the RH increasing and decreasing processes, respectively. Additionally, with its reusability and compact design, the sensor is expected to provide a promising solution for the rapid and reliable humidity and temperature monitoring in confined spaces.