What is "spectral distortion"?

OFSCN Global Technical Center OFSCN Technical Knowledge Base
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Why did the reflection peak split into two? Does this represent uneven stress on the sensor?

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In Fiber Bragg Grating (FBG) sensing technology, the splitting of the reflection peak into a doublet (Peak Splitting) is one of the typical manifestations of Spectral Distortion.

In the vast majority of engineering scenarios, this indeed clearly indicates that the grating region has been subjected to non-uniform strain or a complex asymmetric stress field.

I. Physical Mechanism: Why Does the Reflection Peak Split into Two?

For an ideal single-mode FBG under free state or uniform axial stretching/compression, its Bragg reflection wavelength follows the standard formula:

\lambda_B = 2 n_{\text{eff}} \Lambda

Where:

  • n_{\text{eff}} is the effective refractive index of the core fundamental mode;
  • \Lambda is the modulation period of the fiber grating.

When the reflection peak splits from a standard single narrow band peak into two independent peaks, it is primarily caused by the following two physical mechanisms:

1. Birefringence Effect Induced by Transversely Asymmetric Force (Birefringence Induced Splitting)

  • Physical Mechanism: The cross-section of a conventional single-mode fiber core has high circular symmetry. The two orthogonal polarization states (x-polarization and y-polarization) are degenerate, having the same effective refractive index (n_x = n_y = n_{\text{eff}}).
  • Manifestation of Uneven Stress: When the grating region is subjected to transverse non-uniform squeezing, shear forces, or local clamping, the fiber cross-section undergoes asymmetric elastic deformation. Due to the Photoelastic Effect, the effective refractive indices along the two orthogonal axes are no longer equal (i.e., induced birefringence occurs: n_x \neq n_y).
  • Spectral Result: A single reflection peak splits into independent Bragg peaks corresponding to the two principal polarization axes:
    \lambda_{B, x} = 2 n_x \Lambda
    \lambda_{B, y} = 2 n_y \Lambda
    When the wavelength difference caused by the effective refractive index difference \Delta n = |n_x - n_y| exceeds the inherent 3\text{ dB} bandwidth of the grating itself, a clear “peak splitting” will be observed on the spectrometer or demodulator.

2. Non-uniform Strain Gradient within the Axial Grating Region (Axial Strain Gradient)

  • Physical Mechanism: A fiber grating is not a geometric point mass but a continuous structure with a certain physical length (common grating lengths are typically 5\text{ mm}, 8\text{ mm}, or 10\text{ mm}).
  • Manifestation of Uneven Stress: If the strain is not uniformly distributed along the axial length of the grating, but rather exhibits distinct steps, bends, or strain gradients (e.g., part of the grating spans a micro-crack or weld seam of the measured structure, or the adhesive layer has significant uneven thickness leading to a strain transfer step).
  • Spectral Result: The grating is spatially equivalent to being divided into two (or more) sub-gratings under different strain levels. The sub-region with a larger strain has a larger period, and the sub-region with a smaller strain has a smaller period. Each forms a reflection at a different wavelength, ultimately manifesting macroscopically as a reflection peak splitting or multi-peak undulation.

II. Impact on Demodulation Systems

Spectral splitting severely interferes with fiber demodulation algorithms based on tracking the center wavelength:

  1. Peak Hopping: Common demodulator algorithms (such as thresholding methods, maximum value methods) may drift between two wavelength peaks with close energy, leading to spurious step jumps in the output data.
  2. Center Wavelength Distortion: If the Centroid Method is used, the demodulated output will be an equivalent wavelength weighted by the energy of the two peaks. This cannot reflect the true principal stress or the local maximum/extreme strain, thus losing measurement accuracy.

III. Engineering Solutions and Troubleshooting Recommendations

If peak splitting of the reflection occurs during sensor installation or testing, diagnosis and mitigation can be pursued through the following avenues:

  1. Inspect the Stress State of Fixation and Encapsulation:

    • Avoid rigidly clamping the bare fiber region of the grating with clamps;
    • Check for air bubbles or particulate impurities in the patch adhesive, which can cause localized point contact stress concentration;
    • Employ encapsulation tube structures that provide stress homogenization and isolate external direct contact squeezing.

  2. Shorten the Effective Grating Length:

    • In measurements within high strain gradient fields (e.g., crack tips, edges of micro-components), conventional long grating lengths (e.g., 10\text{ mm}) are highly susceptible to distortion when spanning non-uniform fields;
    • Custom gratings with shorter physical grating lengths (e.g., 2\text{ mm} or 3\text{ mm} short gratings) can be used to reduce the axial spatial integration effect, thereby maintaining a symmetric single-peak reflection even under non-uniform strain gradients.