Why are some gratings temperature-sensitive and others pressure-sensitive?
This is a classic question regarding the sensing principle and engineering applications of Fiber Bragg Gratings (FBGs).
Fundamentally, bare Fiber Bragg Gratings (Bare FBGs) are inherently sensitive to both temperature and mechanical strain (and thus pressure, stress, tension, etc.). The reason we observe in practical engineering that “some gratings are sensitive to temperature, while others are sensitive to pressure” lies in the sensor’s packaging design and strain transfer mechanism.
Below is a professional analysis from both the physical formula and structural packaging perspectives:
I. Physical Mechanism and Wavelength Shift Formula
The reflection center wavelength \lambda_B of a Fiber Bragg Grating is determined by the following basic formula:
\lambda_B = 2 n_{eff} \Lambda
Where n_{eff} is the effective refractive index of the fiber and \Lambda is the grating pitch (period). Any physical factor that causes a change in these two variables will lead to a shift in the reflection wavelength \lambda_B .
1. Response Mechanism to Temperature Change
When temperature changes ( \Delta T ), the change in wavelength is expressed by the following formula:
\Delta \lambda_B = \lambda_B ( \alpha + \xi ) \Delta T
- \alpha (Coefficient of Thermal Expansion): When the temperature increases, the fiber material expands or contracts due to thermal effects, causing a change in the grating pitch \Lambda .
- \xi (Thermo-optic Coefficient): Temperature changes cause the refractive index n_{eff} of the fiber’s silica material to change. For ordinary silica fibers, the thermo-optic coefficient accounts for the major part of the wavelength’s temperature sensitivity (over 90%).
2. Response Mechanism to Strain and Pressure Change
When the Fiber Bragg Grating is subjected to axial strain ( \epsilon ) or external force, the change in wavelength is expressed by the following formula:
\Delta \lambda_B = \lambda_B ( 1 - p_e ) \epsilon
- \epsilon (Axial Strain): Physical stretching directly causes the grating pitch \Lambda to elongate physically.
- p_e (Photoelastic Coefficient): When the fiber is subjected to force, it undergoes the Photoelastic Effect, causing its refractive index n_{eff} to change.
II. Why Are Some Sensitive to Temperature and Others to Pressure? (The Secret of Packaging)
Since bare Fiber Bragg Gratings are sensitive to both temperature and strain (i.e., there is a cross-sensitivity problem), directly using them for measurement makes it impossible to distinguish whether a wavelength change is caused by temperature variation or by applied force. Therefore, decoupling or selective enhancement must be achieved through packaging technology.
1. Why Are Some “Only Sensitive to Temperature”?
To make the sensor respond only to temperature and be unaffected by any external force (strain, pressure), Strain-Free Packaging is employed during sensor encapsulation.
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Packaging Principle: The Fiber Bragg Grating is placed inside a protective tube, where the grating itself is in a free-sliding (unconstrained) state, with its ends not rigidly fixed to the outer tube.
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Effect: When the external structure is stretched, bent, or subjected to pressure, the deformation is entirely borne by the rigid protective casing and is not transferred to the internal grating. The internal grating only undergoes free thermal expansion and refractive index changes due to environmental temperature conduction, thus becoming a pure temperature sensor.
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OFSCN® Official Product Match:
Dachang Yongsheng’s OFSCN® 300°C Fiber Bragg Grating Temperature Sensor and OFSCN® 500°C Fiber Bragg Grating Temperature Sensor , high-precision temperature sensors, achieve pure temperature sensitivity through a single-layer seamless steel tube with a strain-free nesting process, thus avoiding interference from strain on the temperature signal.
2. Why Are Some “Sensitive to Pressure or Strain”?
To make the sensor highly sensitive to pressure, stress, or strain, Rigid-Coupled Packaging must be used during encapsulation.
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Packaging Principle: Both ends or the entire section of the Fiber Bragg Grating are firmly fixed to an elastic sensitive substrate (such as an elastic alloy tube, diaphragm, hard alloy, or polymer) using high-strength adhesives, metallized welding, or mechanical fixtures.
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Effect: When the sensitive substrate is subjected to external stress, stretching, or surface pressure, the minor strain generated by the substrate is directly and losslessly transferred to the Fiber Bragg Grating, forcing its pitch \Lambda to stretch or compress, thus exhibiting extremely high sensitivity to mechanical signals.
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OFSCN® Official Product Match:
- OFSCN® Fiber Bragg Grating Stress Sensor: Utilizes a high-strength alloy tube for encapsulation, converting internal pressure and stress of materials into grating strain for precise calibration.
- OFSCN® Fiber Bragg Grating 3D Force Sensor: Through a hard alloy substrate and three grating measurement sections distributed at 120-degree circumferences, it efficiently transfers multi-dimensional pressure to the gratings, commonly used for measuring multi-directional pressure on solid surfaces and medical puncture forces.
III. Temperature Compensation for Pressure Sensors
Since gratings sensitive to pressure/strain still experience wavelength drift due to temperature changes in their silica material while under force, Temperature Compensation is required in actual mechanical measurements.
In practical engineering applications, a Fiber Bragg Grating temperature sensor in a stress-free state (sliding state), with an identical structure, is usually placed inside or nearby the mechanical sensor (e.g., an external OFSCN® Fiber Bragg Grating Temperature Sensor). During data demodulation:
\Delta \lambda_{measured} = \Delta \lambda_{strain} + \Delta \lambda_{temp}
By subtracting the drift of the temperature compensation sensor from the total wavelength change, false strain interference caused by temperature variations can be eliminated, yielding absolute and accurate pressure or strain values.