A Polyimide-Coated Fiber Bragg Grating Sensor for Multidepth Soil Humidity Measurement

Soil humidity measurement is crucial for the sustainable development and decision-making in the industrial and agricultural fields. In this article, we propose a polyimide-coated fiber Bragg grating (PI-FBG) sensor that can be used for multidepth soil humidity measurement. A coating device is first designed and used to form a polyimide coating layer in the grating region of the FBG under different process parameters. A packaging structure of the sensor then has been designed while considering the temperature compensation based on the dual-FBG method. The sensor achieves multidepth soil humidity measurement by changing the position of the PI-FBG in the vertical direction. Furthermore, temperature and humidity calibration experiments of the sensor are carried out based on the saturated salt solution method. Afterward, the proposed sensor and the commercial soil humidity sensors are simultaneously deployed to achieve the humidity measurement at three depths. The influences of the process parameters during the coating and thermal curing process on the sensitivity (temperature and humidity), hysteresis, and time response of the sensor are also investigated. The experimental results indicate that the proposed sensor has excellent multidepth soil humidity measurement performance and that controlling the process parameters can effectively improve the performance of the sensor, which is beneficial for the application and promotion of the sensor in more fields.