If the stress within a single grating segment is uneven, how does the spectrum change?
When the stress (or strain) within a single Fiber Bragg Grating (FBG) segment is uneven, its reflected spectrum undergoes significant changes.
Specifically:
- Spectral Broadening: The most immediate effect is that the reflected spectrum will broaden. This is because different parts of the grating experience different amounts of strain, leading to varying Bragg wavelengths along the grating length. These different wavelengths are all reflected simultaneously, resulting in a wider spectral bandwidth.
- Distortion and Asymmetry: Instead of a clean, sharp peak, the spectrum will become distorted and may appear asymmetrical. The shape will depend on the specific strain profile along the grating.
- Multi-peaked Structure: In cases of severe non-uniformity, especially with distinct strain gradients, the spectrum might even split into multiple peaks. Each peak would correspond to a region within the grating experiencing a relatively uniform strain.
While the centroid or peak wavelength of the broadened spectrum might still shift in correspondence to the average strain, the non-uniformity makes precise single-point strain measurement challenging and affects the accuracy and resolution of the sensor. Therefore, proper installation and ensuring uniform strain distribution across the FBG sensing region are crucial for accurate measurements.
Here is an image of a typical FBG strain sensor:
You can find more technical details on FBG strain sensors here: OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor
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