ZnO–PVA Filled in Hollow Core Bragg Fiber for Soil Moisture Sensing

A novel optical fiber sensor based on Zinc Oxide (ZnO)-Polyvinyl alcohol (PVA) filled into the core of a brief hollow core Bragg fiber (HCBF) for soil moisture sensing is proposed and experimentally validated. The HCBF is employed to improve the transmission spectrum’s visibility and the sensor’s reliability, while the ZnO-PVA compound constitutes a Fabry-Perot (FP) cavity that directly senses the ambient humidity. The sensor holds an ultra-high air humidity sensitivity of -3.1717 nm/%RH in the range of 44-71 %RH, and a high soil moisture sensitivity of -2.4234 nm/%RH in the soil range of 44.1-70 %RH. Further soil moisture experiments reveal that the soil temperature sensitivity is 0.2286 nm/°C in the soil temperature range from 27.1°C to 44°C, and a low instability is 0.0215 %RH. The sensor is expected to offer a practical strategy for soil moisture monitoring in harsh environments benefiting from compact design, ease of manufacturing, superior sensitivity, good stability, and low cost.