The fiber optic cable was flattened, what happened to cause one peak to become two peaks?
The phenomenon where a fiber optic experiences external force compression (being flattened) causing ‘one peak in the reflection spectrum to split into two’ is known in optical engineering as Stress-induced Birefringence.
Below is a detailed analysis of the physical and mathematical mechanisms behind this phenomenon:
1. Isotropic State (Unstressed)
When an optical fiber is not subjected to lateral external force or is only under uniform axial tension, its core cross-section maintains a high degree of circular symmetry. At this point, the effective refractive indices for light in two orthogonal polarization directions (i.e., the x-axis and y-axis) are identical, specifically:
n_x = n_y = n_{\text{eff}}
According to the Bragg reflection equation for Fiber Bragg Gratings (FBGs), the reflected wavelength satisfies:
\lambda_B = 2 n_{\text{eff}} \Lambda
Here, \Lambda is the grating period. Since the refractive indices corresponding to the two polarization states are the same, both polarized lights reflect at the same wavelength, thus appearing as a single, symmetrical reflection peak on the demodulator’s spectrum.
2. Birefringent State (Flattened)
When an optical fiber is subjected to unidirectional lateral pressure (being flattened), its originally symmetrical circular cross-section deforms slightly, leading to a non-uniform stress distribution within the fiber. Through the Photoelastic Effect, this asymmetric mechanical stress disrupts the isotropy of the medium, causing the effective refractive indices in the two orthogonal polarization directions to split into n_x and n_y, respectively.
At this point, the fiber exhibits a non-zero birefringence value:
B = |n_x - n_y| \neq 0
Consequently, the two orthogonal polarization components of the incident light will undergo Bragg reflection at different wavelengths:
\lambda_x = 2 n_x \Lambda
\lambda_y = 2 n_y \Lambda
On a spectrometer or demodulator, the originally superimposed reflection peak splits into two distinct polarization peaks. The wavelength difference (splitting interval) between these two peaks is:
\Delta \lambda = 2 |n_x - n_y| \Lambda
The magnitude of this wavelength difference \Delta \lambda is positively correlated with the lateral pressure applied to the optical fiber.
3. Engineering Implications and Preventive Measures
In practical industrial applications, this two-peak splitting phenomenon has various impacts:
- Harmful Interference: In traditional measurements of temperature or axial strain, the splitting of the reflection peak can lead to misjudgments in the demodulator’s peak-finding algorithms (such as centroid method, Gaussian fitting), causing data jumps, increased measurement noise, or even an inability to lock the wavelength.
- Physical Protection: To prevent spectral distortion caused by lateral shear forces and direct compression, Fiber Bragg Gratings must be protected with rigid structures. For instance, the OFSCN® 300°C Fiber Bragg Grating Temperature Sensor employs a refined seamless stainless steel capillary encapsulation process. This metal tube effectively shields the transmission of external lateral pressure, ensuring the fiber core remains in a state of isotropic low stress, thereby maintaining a single, sharp reflection spectrum.
