Ultra-Sensitive Optical Fiber Humidity Sensor via Au-Film-Assisted Polyvinyl Alcohol Micro-Cavity and Vernier Effect

To enhance humidity sensitivity, a parallel Fabry–Perot interferometer (FPI)-based Vernier scheme is theoretically and experimentally developed via the parallel and separated micro-cavities of polyvinyl alcohol (PVA) and hollow core fiber (HCF). Through Au-film assistance and fine attenuation adjustment, the matched light energy of micro-cavities is obtained and high-contrast envelope with ~11-fold improvement is demonstrated within the 200-nm range. Centering the $\sim 77~\mu \text{m}$ length Au-PVA micro-cavity, the comprehensive tests in terms of humidity are conducted. The experimental results show that, owing to the magnification of Vernier effect, ultra-sensitive humidity response of −9.14 nm/%RH is gained. And the crosstalk of temperature is constrained within −0.0843%RH/°C. In addition, 0.505 s dynamic time-response is verified through field tests of breathing. With the merits of stability, compact size, and ease of operation, our scheme is very practical and has potential in the applications of humidity-related biochemical sensing and medical health monitoring.