Spatially confined liquid temperature and alcohol concentration sensing based on forward Brillouin scattering in highly nonlinear fiber

We propose and experimentally demonstrate a dual-function forward Brillouin scattering (FBS) sensor for liquid temperature and alcohol concentration measurements in spatially constrained environments. The sensor employs a modified Sagnac loop design with a 25-m highly nonlinear fiber (HNLF) coiled into compact optical fiber coils as the primary sensing element, enabling efficient utilization of limited test space. This configuration integrates an external polarization controller to facilitate rapid switching between liquid temperature and alcohol concentration detection, thereby supporting dual-functionality operation. By adjusting the curvature of the fiber winding, stronger stress birefringence is introduced, although this modification increases micro-bending loss, it does not significantly compromise the temperature sensing sensitivity while enhancing the alcohol concentration sensing sensitivity. Under identical conditions, the temperature sensing sensitivity of the selected R_{0, 17} mode exceeds that of the TR_{2, 33} mode, making the R_{0, 17} mode the preferred choice for temperature sensing. The frequency shift-temperature coefficients of the R_{0, 17} mode using HNLF coils with a large-radius and small-radius are 73.8 kHz/°C and 72.8 kHz/°C, respectively. When the temperature of the alcohol solution is fixed at 25 °C, the linewidth concentration coefficient for HNLF coils with a large radius is 8.3 kHz/%, whereas that for HNLF coils with a small radius is 19.5 kHz/%. Comparative analysis indicates that the linewidth of the TR_{2, 27} mode, when utilizing HNLF coils with a small radius, exhibits more pronounced variation and is thus suitable for alcohol concentration measurement. Consequently, this dual-function FBS sensing scheme demonstrates significant potential for process monitoring in food and beverage processing, as well as in chemical and pharmaceutical manufacturing industries where multi-parameter sensing in confined spaces is required.