If the packaged tube is twisted, will the grating signal jump erratically?
In Fiber Bragg Grating (FBG) sensing engineering, whether the grating signal will “jump” or drift after the packaging tube is subjected to torsion (twisting) primarily depends on the sensor’s packaging structural design (tightly coupled vs. loosely coupled, stress-free) and the physical effects of torsional force on the fiber core.
I. Physical Mechanisms and Wavelength Drift Principles
The formula for the Bragg central reflection wavelength of an FBG is:
\lambda_B = 2 n_{\text{eff}} \Lambda
Where:
- n_{\text{eff}} is the effective refractive index of the fiber core;
- \Lambda is the grating period.
When the external packaging tube is subjected to a torsional torque, the signal’s fluctuation is determined by the following mechanical and optical mechanisms:
1. Photoelastic Effect and Birefringence
Torsion introduces non-axisymmetric shear stress on the fiber’s cross-section. Through the photoelastic effect, this stress disrupts the isotropic refractive index distribution of the fiber core, inducing optical birefringence. Consequently, the originally superimposed reflection peaks for the two polarization states split (Peak Splitting). If the interrogator’s peak-finding algorithm identifies and jumps between these split double peaks, it will manifest as data display “jumping”.
2. Axial Strain Component
For tightly coupled or cured packaging structures, the torsional deformation of the tube typically involves a slight axial elongation or stretching component. This axial strain is directly transmitted to the fiber core, altering the period \Lambda and causing a significant drift in the central wavelength \lambda_B.
3. Optical Power Attenuation and Microbending Loss
If torsion causes microbending or excessive compression of the fiber within the packaging tube, the optical transmission loss increases sharply, degrading the signal-to-noise ratio (SNR) of the reflection spectrum. When the reflection peak intensity falls below the interrogator’s threshold, the demodulated data may exhibit severe jitter or frame loss.
II. Actual Performance in Different Packaging Structures
1. Strain Sensor (Tightly Coupled Cured Structure) — Signal Jumps or Drifts Continuously
In strain sensors, the grating is tightly coupled and cured within the packaging tube (e.g., elastic alloy tube or polymer material).
- Torsional Effect: Twisting the packaging tube directly transmits shear and torsional stress to the fiber core. If uneven forces are applied during twisting or if the tube undergoes plastic micro-deformation, the optical wavelength signal will exhibit significant fluctuations, jumps, and may not recover even after the torque is released.
For example, the OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor, which uses an elastic alloy tube to tightly encapsulate the grating for precise transmission of minute strains:
2. Temperature Sensor (Stress-Free Loosely Coupled Structure) — Signal Remains Relatively Stable, but Excessive Torsion Can Still Have an Impact
In temperature sensors, the grating typically adopts a stress-free structure, fixed at one end and suspended within the tube.
- Torsional Effect: When the outer seamless steel tube is slightly twisted, the stress on the tube wall does not directly transmit to the internally suspended fiber, and the grating wavelength usually remains stable.
- Abnormal Situations: If the twisting angle is too large, causing local bending or distorted compression of the fiber inside the packaging tube, which leads to the fiber core being stressed or experiencing polarization changes, the reflection waveform may still distort and the signal may fluctuate.
For example, the OFSCN® 300°C Fiber Bragg Grating Temperature Sensor, which uses a single-layer seamless stainless steel tube for stress-free protective packaging:
III. Engineering Recommendations
- Avoid Applying Pure Torsional Torque: Fiber Bragg Grating sensors are primarily designed for measuring axial tensile/compressive strain or temperature. Torsion represents a complex stress in a non-design dimension. During installation, use specialized fixtures to prevent the sensor from twisting.
- Diagnosing Signal Jitter: If the signal jitters after twisting, observe the Reflectance Spectrum using the interrogator. If the reflection peak shows double peak splitting or a significant decrease in peak intensity, it confirms that the packaging structure has been subjected to torsional stress or fiber compression damage.