Why do reflection peaks sometimes broaden, split, or disappear?
Hello frostweaver,
The spectral distortions you describe—broadening, splitting, or disappearance of FBG reflection peaks—are important indicators of various physical phenomena or issues affecting the Fiber Bragg Grating (FBG). As an experienced FBG application engineer, I can explain the common reasons behind these observations:
1. Broadening of Reflection Peaks:
This typically occurs when the FBG experiences a non-uniform distribution of strain or temperature along its length. Different sections of the grating will then reflect light at slightly different Bragg wavelengths, leading to an overall broadened spectral response. Another cause can be a high-order mode coupling within the fiber, although less common in standard sensing. In some specialized applications, an intentionally designed “chirped FBG” will also show a broadened spectrum.
2. Splitting of Reflection Peaks:
Peak splitting often points to an induced or inherent birefringence in the optical fiber at the grating location. This happens when:
- Asymmetric Stress: The fiber is subjected to non-uniform transverse stress (e.g., side-loading), causing the refractive index to differ for orthogonal polarization states. Each polarization state then sees a slightly different grating period, resulting in two distinct reflection peaks.
- Internal Phase Shifts: Unintended phase shifts within the FBG structure itself, or strong localized perturbations (strain/temperature gradients), can effectively create two “sub-gratings” or significant phase changes that lead to a split peak.
3. Disappearance of Reflection Peaks:
The disappearance of an FBG reflection peak can be due to several critical factors:
- Fiber Breakage: The most straightforward reason; if the optical fiber is physically broken within or adjacent to the FBG, the light path is interrupted, and no reflection can occur.
- Grating Erasure (Annealing): Prolonged exposure to temperatures exceeding the FBG’s manufacturing or operational limits can permanently erase the grating structure. The photosensitivity that creates the refractive index modulation in the fiber core can degrade, causing the grating to vanish. Standard polyacrylate-coated FBGs are limited to around 100°C; OFSCN® Polyimide Fiber Bragg Gratings can withstand up to 300°C, and OFSCN® Gold-Coated Fiber Bragg Gratings up to 700°C, offering solutions for high-temperature environments.
- Exceeding Strain Limits: Excessive strain, beyond the elastic limit of the fiber or sensor packaging, can permanently deform or damage the grating, leading to its disappearance or shifting the Bragg wavelength out of the interrogator’s detection range.
- Sensor Debonding/Failure: For packaged FBG sensors, debonding from the host structure can lead to a loss of strain transfer or mechanical damage to the FBG element itself, resulting in a lost signal.
For applications requiring robust performance under high temperatures or demanding mechanical conditions, special packaging and fiber types are crucial. For example, the OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor is designed with full metal seamless tube encapsulation to eliminate creep issues of adhesives and is suitable for long-term high-temperature welding applications.
Here is an example of an alloy tube packaged FBG strain sensor:
You can find more detailed technical parameters on our website:
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