Optical fiber-based surface plasmon resonance sensor for simultaneous measurement of relative humidity and temperature

In this work, an optical fiber sensor based on surface plasmon resonance (SPR) is proposed for simultaneous measurement of relative humidity (RH) and temperature, which is constructed by connecting two segments of thin-core fibers (TCFs) using a multimode fiber (MMF). The gold film was coated on TCFs to excite the SPR effect. One was further coated with carboxymethyl cellulose (CMC) to sense RH (Channel I), and the other was coated with gold nanoparticles (AuNPs) and polydimethylsiloxane (PDMS) to sense temperature (Channel II). The detection was performed via the resonance wavelength shift induced by the refractive index (RI) change of CMC and PDMS in relation to RH variation and temperature variation, respectively. The incorporation of gold nanoparticles enhances the sensitivity of the SPR sensor and shifts the resonance wavelengths, allowing for more outstanding demarcation of the SPR dip between the two portions inside the equivalent spectrum. Experimental results show that the sensor has an RH sensitivity of -1.053<i>n</i><i>m</i>/<i>%</i><i>R</i><i>H</i> and a temperature sensitivity of -2.306<i>n</i><i>m</i>/^∘<i>C</i>. Moreover, the cross-sensitivity issue present in RH and temperature measurements was resolved by introducing a dual-wavelength matrix method. The proposed sensor offers high sensitivity, a simple structure, and ease of manufacturing, making it broadly applicable in fields such as food safety, biotechnology, and manufacturing.