If a grating splits into two peaks, how will the software handle it?
In Fiber Bragg Grating (FBG) sensing applications, if the reflection spectrum of an individual grating exhibits “Peak Splitting,” it is typically caused by birefringence effects introduced by non-uniform transverse stress (separation of effective refractive indices along the two orthogonal polarization axes of the fiber: n_{\text{eff},x} \neq n_{\text{eff},y}) or non-uniform axial strain/temperature gradients along the grating region.
The processing method by the interrogator software upon detecting a split peak depends on the peak-finding algorithm embedded in the software and the identification mode set by the system:
1. Software Peak Finding and Processing Logic at the Algorithm Level
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Max Peak Tracking / Peak Detection Method:
- The software selects the extremum peak with the higher optical power intensity (reflectivity) as the wavelength value for that measurement point.
- Potential Phenomenon: If the heights of the two peaks fluctuate alternately with external force changes (similar intensity), the target peak being tracked by the software may jump between the two, leading to stepped jump noise in the demodulated wavelength values.
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Centroid / Center of Gravity Algorithm:
- The software performs intensity-weighted integration calculation of the reflection spectrum curve within a defined detection wavelength window.
- Potential Phenomenon: If the grating spectrum splits into two peaks, the center wavelength calculated by the centroid method will usually fall at the dip between the two peaks, failing to accurately reflect the physical quantity change of any specific polarization state or location.
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Multi-peak Identification / Absolute Peak List Mode:
- If the interrogator software has the full-spectrum multi-peak identification function enabled, as long as both split peaks exceed the set power threshold (Threshold) and prominence (Prominence), the system will identify them as two independent wavelength peaks.
- Potential Phenomenon: If the system defaults to “mapping sensor ID by wavelength ascending order,” an extra wavelength peak within a channel will cause a misalignment of the corresponding serial numbers for all subsequent fiber gratings (Channel Misalignment).
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Fixed Window Tracking Mode (ROI / Range Filtering):
- If the software allows users to define a fixed wavelength tracking range for each sensor (e.g., setting a measurement point to search only within 1549.0\text{ nm} \sim 1551.0\text{ nm}):
- When more than 1 peak appears within the range, the software will typically trigger a “multi-peak anomaly alarm,” or extract only the single peak with the maximum energy within the range or the one closest to the initial wavelength.
- If the software allows users to define a fixed wavelength tracking range for each sensor (e.g., setting a measurement point to search only within 1549.0\text{ nm} \sim 1551.0\text{ nm}):
2. Countermeasures and Recommendations in Engineering Applications
- Structural and Packaging Optimization: During sensor installation and packaging, try to avoid point-like compression or non-uniform lateral pressure on the grating region. Use appropriate packaging techniques (such as metal micro-tube packaging) to maintain uniformity of stress on the grating region.
- Algorithm and Threshold Adjustment: In the data acquisition software of devices like the OFSCN® Fiber Bragg Grating Interrogator, the spectral identification power threshold (Threshold) can be reasonably increased to filter out secondary split peaks, or switch to a fixed wavelength window locking mode to avoid measurement point misalignment.
