Paketlemenin düzensiz olması nedeniyle tek bir tepe ikiye ayrıldı, bu sensör hala kullanılabilir mi?
Fiber Bragg Gratings (FBGs) ideally have a reflection center wavelength that satisfies the Bragg condition:
\lambda_B = 2 n_{\text{eff}} \Lambda
where n_{\text{eff}} is the effective refractive index of the core fundamental mode, and \Lambda is the grating period. Under conditions of isotropy and uniform axial strain, the two orthogonal polarization states of the single-mode fiber core (x-axis and y-axis) are degenerate ( n_x = n_y = n_{\text{eff}} ), and the reflection spectrum appears as a single symmetric peak.
When uneven packaging causes the reflection peak to split into “double peaks,” it is usually due to the following physical mechanisms. Its engineering usability needs to be evaluated based on the specific demodulation method and application scenario:
I. Physical Mechanisms Causing the “Double Peak Phenomenon”
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Stress Birefringence Introduced by Transverse Non-uniform Stress
- Mechanism: Uneven curing shrinkage of the packaging, eccentric colloid, or asymmetric clamping by the mechanical structure causes non-axisymmetric transverse strain differences ( \sigma_x \neq \sigma_y ) on the grating core cross-section. Through the Photoelastic Effect, the effective refractive indices of the two orthogonal polarization modes separate ( n_x \neq n_y ).
- Manifestation: The reflection peak splits into two polarization-dependent sub-peaks, corresponding to \lambda_{B,x} = 2 n_x \Lambda and \lambda_{B,y} = 2 n_y \Lambda , respectively. The distance between the two peaks is proportional to the transverse stress difference.
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Axial Strain Gradient in the Grating Region (Strain Gradient / Chirp Effect)
- Mechanism: Within the grating region length (e.g., 5\text{ mm} \sim 10\text{ mm} ), the adhesion or transfer stiffness of the packaging material is non-uniform along the axis, causing local tension/compression in the grating region while other parts are relaxed or in a different strain state.
- Manifestation: This is equivalent to cascading two gratings with different effective periods \Lambda_1 and \Lambda_2 , causing the originally single reflection peak to split into two separated peaks or exhibit severe asymmetric broadening and secondary peaks.
II. Can a Sensor Exhibiting Double Peaks Still Be Used?
1. Under Conventional Engineering Applications and Standard Demodulators: Generally not recommended for continued use (considered failed/defective)
- Peak Hopping and Centroid Drift: Mainstream commercial FBG demodulators typically lock onto a single peak position based on Gaussian fitting, thresholding, or the Centroid Algorithm. When the reflection spectrum shows double peaks of similar energy, external fiber perturbations (polarization state rotation) or slight temperature variations can cause drastic fluctuations in the energy distribution between the two peaks, making the demodulator prone to hopping between the two peak tops, resulting in significant spurious wavelength jumps (possibly tens to hundreds of \text{pm} ).
- Non-linear Distortion: For peak splitting caused by axial strain gradients, the strain transfer during loading is non-linear in different regions, which can lead to the failure of calibration coefficients.
- Conclusion: In standard temperature or strain engineering monitoring, packaging-induced peak splitting represents severe stress concentration or eccentric loading, lacking reliable engineering measurement accuracy.
2. Under Specific Algorithms or Scientific Research Scenarios: Limited usability exists
- Transverse Load/Strain Sensor: If the double peaks are caused purely by birefringence due to transverse forces, and the demodulator supports high-resolution full-spectrum fitting:
- By extracting the peak spacing \Delta \lambda = |\lambda_{B,x} - \lambda_{B,y}| , the magnitude of the transverse load/pressure can be inferred.
- By extracting the average center wavelength \bar{\lambda} = \frac{\lambda_{B,x} + \lambda_{B,y}}{2} , the influence of transverse stress on temperature or axial strain measurements can be compensated.
- Dual Wavelength Independent Tracking: If the demodulation software has the capability for independent multi-peak tracking, and the peak spacing remains greater than the instrument’s resolution limit and does not merge throughout the entire range, it can be treated as two independent weak reflection peaks for separate calibration.
III. Summary and Recommendations
If the sensor is intended for conventional uniaxial strain measurement or temperature monitoring, double peaks caused by uneven packaging are typical packaging stress defects. Replacement or repacking is recommended. If it is used for birefringence load research or in a laboratory environment with custom spectral fitting algorithms, it can be utilized specifically based on the spectral splitting interval.