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.

中文翻译：

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用于连续、可持续土壤水分监测的全光学主动传感平台

土壤水分含量的测定在农业、岩土工程、水文和环境工程等多个应用领域至关重要。在本文中，我们建议使用先进的全光学自加热传感平台对土壤体积含水量（VWC）进行高灵敏度和可重复的监测。所提出的平台是通过在标准金属针内集成光纤加热装置来实现的，该光纤加热装置与充当热监视器的光纤布拉格光栅传感器（FBG）集成。全光学有源加热装置由两个主要组件组成：i) 两根单模光纤之间的纤芯偏置熔接点，用于将光从纤芯传播到包层；ii) 刻有 FBG 的光纤金属涂层区域。在 5 至 40 °C 的土壤温度范围内，对 9 至 36 %VWC 的不同 VWC 值对所提出的装置进行了全面表征。收集的结果表明，该平台能够对土壤水分含量进行高度稳定、快速、稳健和节能的测量，重复性误差为1.5%VWC，时间响应≤10s，分辨率≤0.25%VWC。根据应用调整注入功率值的可能性使得所提出的传感器极其灵活和通用，适用于大面积和/或长距离监测应用，在节能、成本和系统复杂性方面具有相关优势。