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In geotechnical engineering and earth-moving scenarios, whether lateral squeezing pressure will “masquerade” as temperature change depends entirely on whether the internal physical packaging structure of the sensor achieves “stress isolation”.
I. Physical Mechanism: How Lateral Squeezing Pressure Affects Fiber Bragg Gratings
The reflection center wavelength \lambda_B of a Fiber Bragg Grating (FBG) is determined by the effective refractive index n_{\text{eff}} of the fiber core and the grating period \Lambda :
\lambda_B = 2 n_{\text{eff}} \Lambda
When a sensor is directly embedded in earthen or rocky media, significant lateral pressure (Lateral Pressure / Crush) is exerted by the self-weight pressure of the soil, pore water pressure, and localized contact points of rocks. If this lateral pressure is transmitted to the fiber core, it generates the following two physical responses:
- Isotropic Radial Pressure (Uniform Side Pressure): Masquerading as Wavelength Drift
When the fiber is subjected to uniform radial squeezing, the photoelastic effect alters the refractive index of the medium, while the Poisson’s Ratio causes axial deformation. This effect leads to an overall shift in the reflection wavelength (wavelength drift). The demodulator can only recognize the absolute movement of the center wavelength \Delta \lambda_B . Without decoupling, the system cannot distinguish whether this wavelength shift originates from ambient temperature rise or lateral squeezing pressure, thus generating spurious temperature readings. - Non-uniform Localized Side Pressure (Contact Point Squeezing): Inducing Birefringence and Spectral Peak Distortion
The lateral pressure from soil and rock particles is typically non-uniform, acting at localized point contacts. When the fiber cross-section is subjected to asymmetric transverse lateral pressure, the original circular symmetry of the core is destroyed, inducing photoelastic birefringence. This causes a refractive index difference between the two polarization states that were previously degenerate ( n_x \neq n_y ). In the reflection spectrum, a single reflection peak will experience peak broadening (Broadening) or even splitting into two peaks (Peak Splitting), making it impossible for the demodulation algorithm to accurately lock onto the peak, leading to drastic jumps or failure in the output data.
II. Response Differences to Side Pressure Based on Packaging Structure
In practical engineering applications, bare fiber gratings without protection or those with simple flexible polymer coatings should never be directly embedded in earthworks as thermometers.
- Unstressed Flexible/Bare Fiber Gratings: Directly subjected to external soil pressure and shear forces, lateral pressure will be misinterpreted as temperature changes, and they are highly susceptible to breakage under shear from rocks.
- Rigid Stress-Isolated Packaged FBG Temperature Sensors: Qualified industrial-grade FBG temperature sensors must adopt a “stress-isolation / mechanical isolation” structure. The fiber grating is suspended or freely placed within a rigid protective tube, and external loads (including vertical pressure and lateral squeezing pressure from earthworks) are entirely borne by the external rigid tube.
III. Official Related Products and Protection Solutions
In the Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) manufacturing series, fiber Bragg grating temperature sensors utilize a specialized seamless stainless steel pipe packaging structure. The housing possesses extremely high resistance to lateral and squeezing pressures, placing the grating measurement point in a stress-free state completely isolated from axial and radial forces. This effectively blocks interference from external soil and rock lateral pressures on temperature measurements:
1. OFSCN® 100°C Fiber Bragg Grating Temperature Sensor
Standardly employs single-layer seamless stainless steel pipe packaging technology, with an outer diameter typically of 0.9 mm (min. down to 0.6 mm). It achieves pure environmental temperature sensing while withstanding external harsh media contact and lateral soil pressure.
2. OFSCN® 300°C Fiber Bragg Grating Temperature Sensor
Utilizes a seamless steel pipe mechanical protection structure suitable for higher temperature ranges. The internal grating is completely isolated from external radial side pressure and axial mechanical tension. It undergoes factory temperature calibration to ensure that no side pressure strain errors are introduced under embedded pressurized conditions.