Can optical fibers measure soil moisture?

OFSCN Global Technical Center OFSCN Technical Knowledge Base
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In agricultural irrigation, how do fiber optics know if it’s dry underground?

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In agricultural irrigation and geological hydrological monitoring, optical fibers themselves (silica quartz glass material) do not have natural selective absorption for water molecules, and pure optical fibers cannot directly “read” soil moisture content.

In engineering and scientific research practices, the detection of underground soil wetness using optical fibers is mainly based on the following general physical and optical principles:

1. Active Distributed Thermal Pulse Method (A-DTS, the most mainstream underground monitoring solution)

This is the most practical technical path for monitoring multi-depth soil moisture in large-area farmland or irrigation networks.

  • Physical Mechanism: The thermophysical parameters of water, soil particles, and air differ significantly. Water has a much higher specific heat capacity (approx. 4.18\times 10^{3}\ \text{J}/(\text{kg}\cdot\text{K}) ) and thermal conductivity than dry soil skeleton and air.
  • Working Process:
    1. Bury optical cables with integrated heating elements (such as resistive heating wires or metal reinforcement layers) underground.
    2. The control system inputs constant electrical power to the heating element for a short period to generate a thermal pulse.
    3. Use a Distributed Fiber Optic Temperature Sensing (DTS, based on Raman or Brillouin scattering effects) system to continuously record the temperature response curve (temperature rise amplitude and cooling rate) along the optical cable.
  • Water Content Inversion:
    • Moist Soil: High thermal conductivity and heat capacity lead to rapid heat dissipation. The measured maximum temperature rise amplitude \Delta T is smaller, and the cooling process is fast.
    • Arid Soil: High air void content and poor thermal conductivity cause heat accumulation. The measured temperature rise amplitude \Delta T is significantly larger, and heat dissipation is slow.
    • By establishing an inversion model of temperature response to soil thermal conductivity and volumetric water content (VWC) using the heat conduction equation, the wetness state at each point along the entire cable path can be calculated.

2. Wet-Sensitive Material Strain Coupling Mechanism (Point-type FBG Solution)

This method utilizes Fiber Bragg Gratings (FBGs) as the sensor core, coupled with functional materials exhibiting “swelling when wet and shrinking when dry” for conversion and measurement.

  • Physical Mechanism: Composite hygroscopic polymer materials (such as polyimide, hydrogels, or porous ceramic substrates) are applied to the surface or substrate of the fiber Bragg grating.
  • Working Process:
    • When the surrounding soil moisture increases, the sensitive material absorbs water molecules and expands in volume; when the soil dries, the material loses water and shrinks.
    • The volume change of the material is converted into axial strain \varepsilon applied to the fiber Bragg grating.
    • The reflection center wavelength of the Bragg grating shifts, and the wavelength shift is linearly related to the strain:
      \frac{\Delta\lambda_B}{\lambda_B} = (1 - p_e)\varepsilon + (\alpha_\Lambda + \alpha_n)\Delta T
    • By introducing a reference temperature grating unaffected by swelling strain for differential temperature compensation to eliminate environmental temperature drift, the local soil matrix potential or water content state can be resolved through an optical interrogator.

3. Evanescent Wave and Interface Refractive Index Modulation Mechanism (Laboratory/Specialty Fiber Solution)

  • Physical Mechanism: Partial removal of the fiber cladding through chemical etching, or the use of tapered micro/nano-fibers or D-shaped polished fibers.
  • Working Process: Total internal reflection of light propagating in the core generates an evanescent field at the interface with a depth of a few micrometers. Water (refractive index n \approx 1.33) and air (refractive index n \approx 1.0) have distinct refractive index differences. When the ratio of water to air in the porous medium surrounding the fiber changes, the evanescent field loss or interference phase changes, thus reflecting the moisture content. This method has a very small spatial influence radius and is mostly used for micro-pore flow monitoring at close range.

Official Product Description

Beijing Dacheng Yongsheng Technology Co., Ltd.'s (OFSCN®) core product series focuses on: high-reliability Fiber Bragg Grating (FBG) temperature sensors, strain sensors, stress sensors, 3D force sensors, and specialty optical cables that are resistant to high temperatures/harsh environments.

Currently, OFSCN®'s product catalog does not include complete sets of soil moisture/humidity sensors directly for agricultural irrigation. In projects involving soil and geological monitoring, OFSCN® primarily provides fiber optic sensing hardware for environmental temperature monitoring and monitoring of stress/strain in diaphragm walls/pile foundations. Soil moisture monitoring systems typically require system-level development by integrators, incorporating the heating and physical inversion schemes mentioned above.