Why should the fiber be tensioned before being sealed into the tube? What happens if it’s not tensioned?
In the development of optical engineering and fiber optic sensors (especially Fiber Bragg Grating sensors), the process of tensioning the fiber optic sensing segment (applying constant axial tension) before encapsulating it within a protective tube or substrate structure is known as Pre-strained Packaging.
Below, we will explain the scientific necessity of this process in detail from three perspectives: physical mechanism, sensing performance, and the consequences of not applying tension.
I. Physical Mechanism: Mechanical Properties of Quartz Fiber and Buckling
Quartz fiber itself is an extremely thin glass filament (e.g., the outer diameter of the cladding for a standard single-mode fiber is only 125\ \mu\text{m} , and even with coatings like polyimide, it only reaches 155\ \mu\text{m} ). This geometric characteristic of a large aspect ratio determines its unique mechanical behavior:
- Exceptional Tensile Strength: When subjected to axial tension, the fiber exhibits excellent elastic modulus, with a highly linear relationship between strain and wavelength shift.
- Extremely Weak Compressive Strength: In a free or relaxed state, the fiber’s axial bending stiffness is practically zero. Any minuscule axial compressive force will cause the fiber to instantly buckle or bend, rather than inducing uniform axial compressive strain within the glass medium.
II. Why Pre-tension (Apply Pre-stress)?
The core principle of pre-strained packaging is to apply a predetermined initial axial tensile strain (typically between 1000\ \mu\varepsilon and 3000\ \mu\varepsilon ) to the fiber using high-precision tension control fixtures during manufacturing and solidification, before bonding or fusing both ends (or the entire segment) of the fiber to a metal protective tube.
Its core functions are:
1. Eliminating “Dead Zones” and Idle Travel
If the fiber is slack within the tube, when the external protective tube elongates due to tension, the slack fiber must first undergo an “idle travel” from bending to being straightened. Before the fiber is completely straightened, the Fiber Bragg Grating (FBG) cannot sense any tensile strain. This results in significant measurement dead zones and nonlinear hysteresis in the low-range section of the sensor. Pre-tensioning ensures that the fiber and the external protective tube respond synchronously and instantaneously to deformation from the zero point.
2. Enabling Bidirectional (Tensile and Compressive) Strain Measurement
Strain measurement typically requires covering both tensile (positive strain, \varepsilon 0 ) and compressive (negative strain, \varepsilon 0 ) conditions.
- Measuring Tension: External tensile force further stretches the pre-tensioned fiber, causing the reflected wavelength of the FBG to redshift beyond the pre-stress wavelength.
- Measuring Compression: External pressure shortens the metal tube, reducing the distance between the fixed endpoints. This releases the fiber’s initial pre-tension. Since the fiber was originally under tension, it merely “reduces tension” at this point while remaining straight. Therefore, the FBG can uniformly sense axial compression, leading to a blueshift in the reflected wavelength.
Without pre-tensioning, when the sensor is subjected to pressure, the fiber will immediately buckle irregularly within the tube due to its inability to bear compressive stress, thus completely losing its negative strain measurement capability.
3. Stabilizing the Working Baseline
Pre-tensioning stabilizes the initial reflected wavelength of the FBG at a known bias wavelength ( \lambda_{\text{start}} = \lambda_0 + \Delta\lambda_{\text{pre}} ). By monitoring the reciprocal drift of the wavelength relative to this baseline ( \Delta\lambda 0 or \Delta\lambda 0 ), the demodulator can accurately calibrate the magnitude and direction of the applied force.
III. What Happens If No Tension Is Applied?
Sensors encapsulated without pre-tensioning exhibit the following critical issues in practical testing:
- Failure in Compression Measurement: The sensor completely lacks the ability to measure negative strain.
- Hysteresis and Nonlinearity in Tension Measurement: The sensor’s output curve exhibits poor repeatability and disordered nonlinearity in the low-range, making it impossible to perform reliable one-term calibration.
- Distortion of Reflected Spectrum (Multi-peak Phenomenon): Due to the lack of pre-tensioning, the fiber is randomly and non-uniformly curved within the tube. When the grating is in a curved state, its reflection spectrum becomes distorted, broadens, or even splits into double or multiple peaks, preventing the demodulation system from locking onto the center wavelength, rendering the sensor completely defunct.
IV. OFSCN® (Dacheng Yongsheng) Related Technologies and Products
In the manufacturing of industrial-grade fiber Bragg grating strain sensors, OFSCN® (Beijing Dacheng Yongsheng Technology Co., Ltd.) employs a precision pre-strained packaging process to ensure sensors exhibit high linearity, wide bidirectional measurement range, and hysteresis-free sensing characteristics.
For example, the OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor uses an elastic alloy tube for encapsulation. Before leaving the factory, this series of sensors’ sensing gratings undergo pre-stress calibration under a precision tension feedback system, ensuring their high response accuracy and bidirectional measurement characteristics with zero dead zone:
Furthermore, for applications with more stringent outer diameter limitations, the OFSCN® Polymer-encapsulated Fiber Bragg Grating Strain Sensor (0.7mm/1.2mm diameter) also utilizes polymer materials and precision pre-strained packaging technology, enabling excellent fatigue life and high-precision bidirectional measurement within extremely small dimensions.
To view the full series of strain sensor products, please visit the OFSCN® FBG Strain Sensor Products Aggregation Link for more detailed technical specifications.