Will the direction of light vibration affect my measurement accuracy?
The vibration direction of light (i.e., the state of polarization, SOP) significantly affects the measurement accuracy of fiber optic and Fiber Bragg Grating (FBG) sensors. This is a physical phenomenon that must be carefully considered and controlled in high-precision optical measurements. Below, we analyze how the state of polarization impacts measurement accuracy and how to optimize it from both physical mechanisms and practical engineering perspectives:
I. Physical Mechanism: Impact of Polarization State and Birefringence on Measurement Accuracy
In an ideal single-mode optical fiber, the fundamental mode (HE_{11}) consists of two spatially orthogonal, polarization-degenerate states. If the fiber structure were perfectly rotationally symmetric and free from external stress, these two polarization states would have identical propagation constants. However, during manufacturing, packaging, and deployment, optical fibers inevitably encounter the following situations:
-
Intrinsic and Induced Birefringence: Minor geometric non-circularity, bending, twisting of the fiber, and packaging stress (lateral stress) due to mismatched thermal expansion coefficients can break the fiber’s rotational symmetry. This introduces birefringence within the fiber, causing a difference in the effective refractive indices experienced by light propagating along two orthogonal axes (fast and slow axes), i.e., n_{\text{eff}, x} \neq n_{\text{eff}, y}.
-
Bragg Wavelength Splitting: The formula for the reflection center wavelength of a Fiber Bragg Grating (FBG) is \lambda_B = 2 n_{\text{eff}} \Lambda . Under the influence of birefringence, a single reflection peak splits into two distinct Bragg reflection peaks, corresponding to the fast and slow axes:
\lambda_{Bx} = 2 n_{\text{eff}, x} \Lambda
\lambda_{By} = 2 n_{\text{eff}, y} \Lambda -
Measurement Errors Caused by Polarization Drift: When the incident light’s state of polarization (SOP) drifts due to external disturbances (such as slight movements of patch cords or polarization rotation caused by ambient temperature changes), the distribution ratio of incident light power between the fast and slow axes dynamically changes. This leads to a continuous alteration in the relative intensity of the two split reflection peaks received by the interrogator. Since most high-precision FBG interrogators use centroid, Gaussian fitting, or peak-finding algorithms to lock the wavelength, this asymmetric change in the reflection peak profile is misinterpreted by the interrogator as a