Simultaneous Measurement of Humidity and Temperature Using a Hybrid Plasmonic Waveguide Bragg Grating

A hybrid plasmonic waveguide Bragg grating (HPWBG) consisting of a polyvinyl alcohol (PVA) microfiber coupled with metal strips oriented at right angles is reported for simultaneous measurement of humidity and temperature. Due to the convenient adherence of PVA microfiber to metal gratings and its easy connectivity to silica fiber tapers, the hybrid plasmonic waveguide sensor exhibits high flexibility. Numerical calculation reveals that TM polarization shows more prominent Bragg reflection owing to hybrid plasmonic modes exciting. Two hybrid plasmonic TM modes, namely the fundamental mode and the high-order mode, are excited, resulting in two separate reflection peaks. As the two reflection peaks exhibit different behaviors in response to changes in humidity and temperature, simultaneous measurement of humidity and temperature can be realized through a sensitivity coefficient matrix. The experimental results show that the humidity sensitivity reaches −1.98 nm/%RH in the range of 30%–90%RH and the temperature sensitivity is −0.64 nm/°C within a temperature range of 20–90 °C for a PVA microfiber diameter of 1.57 μm. The excellent sensing performance, compactness, and flexibility of the hybrid plasmonic waveguide make it an encouraging choice for humidity and temperature sensing applications.