All-optical active sensing platform for continuous and sustainable soil water content monitoring

The determination of the amount of moisture in soil is crucial in several application fields such as agricultural, geotechnical, hydrological, and environmental engineering. In this paper, we propose the use of an advanced all-optical self-heated sensing platform for highly sensitive and repeatable monitoring of the soil volumetric water content (VWC). The proposed platform is realized by integrating, inside a standard metallic needle, a fiber optic heating device integrated with a fiber Bragg grating sensor (FBG) acting as thermal monitor. The all optical active heating device consists of two main components: i) a core offset fusion splice between two single mode optical fibers to spread light from the core to the cladding and ii) a metal-coated region of the fiber where the FBG is inscribed. A full characterization of the proposed device has been performed for different VWC values from 9 to 36 %VWC in the soil temperature range between 5 and 40 °C. Collected results demonstrate that the proposed platform is able to perform highly stable, fast, robust and energy efficient measurements of the soil water content, with a repeatability error of 1.5 %VWC, time response ≤ 10 s and resolution ≤ 0.25 %VWC. The possibility of tuning the value of the injected power according to the application makes the proposed sensor extremely flexible and versatile for its exploitation in large areas and/or long-distance monitoring applications with relevant advantages in terms of energy savings, costs and system complexity.