Does the plastic coating on the outside of the fiber optic cable affect the grating’s sensing force?
A direct and concise answer is: Yes, significantly.
The outer “plastic jacket” of an optical fiber is known in academic and optical engineering circles as the fiber coating (Coating / Buffer). When using Fiber Bragg Gratings (FBGs) for strain or stress measurements, the coating plays a decisive role in the strain transfer efficiency, measurement accuracy, temperature drift, and response hysteresis.
The specific physical mechanisms and the performance of different materials are as follows:
1. Physical Mechanism: Shear Lag Effect and Strain Transfer
In actual force sensing, the strain caused by external forces cannot directly act on the fiber core. Instead, it must be transferred layer by layer through the path: “External Structure \rightarrow Adhesive (or Encapsulant) \rightarrow Fiber Coating (Plastic Jacket) \rightarrow Silica Cladding \rightarrow Fiber Core (Grating Area)”.
- Modulus Mismatch: The elastic modulus of the silica glass cladding is very high (E_{\text{glass}} \approx 72\text{ GPa}), while the elastic modulus of polymer coatings (plastic jackets) is extremely low (typically only 1\text{ GPa} to 5\text{ GPa}).
- Shear Lag: When external strain is applied, due to the softness and thickness of the coating material, it undergoes significant shear deformation under shear stress. This deformation absorbs and weakens a portion of the strain, resulting in the strain transmitted to the grating core being less than the actual external strain.
- Creep & Hysteresis: Polymer materials exhibit viscoelasticity. Under sustained load or temperature changes, the coating can undergo slow plastic deformation (creep), leading to temperature drift or zero-point drift in the grating’s reflection wavelength, making it impossible to accurately and in real-time track the true stress release and alternating loads.
2. Comparison of Common Coating Materials and Professional Selection
The sensing performance differs fundamentally based on the type and thickness of the coating (plastic jacket) used in fiber Bragg grating sensors:
A. Polyacrylate Coating — Traditional Standard Fiber
- Characteristics: This is the most common coating for standard single-mode fibers (e.g., conventional communication fibers). It has a relatively thick single-side coating, with a typical cladding diameter of 125\ \mu\text{m}, resulting in an outer diameter of 250\ \mu\text{m} or 255\ \mu\text{m} after coating.
- Impact on Sensing Force: Due to the relatively soft nature of acrylate and its considerable thickness, it exhibits low strain transfer efficiency and has a low glass transition temperature. In environments at or above 80^\circ\text{C}, the material rapidly softens, leading to severe shear lag and creep. Therefore, it is unsuitable for high-precision, dynamic, or high-temperature strain and force sensing applications.
- Corresponding Products: OFSCN® Polyacrylate Fiber Bragg Gratings / FBG Strings (Bare)
B. Polyimide Coating — Preferred for High-Precision Sensing
- Characteristics: Polyimide is a high-hardness, high-temperature resistant special polymer. Its coating can be extremely thin (only recoated to 155\ \mu\text{m} outside the 125\ \mu\text{m} cladding, with a single-side thickness of just 15\ \mu\text{m}).
- Impact on Sensing Force: Polyimide has a higher modulus, and the extremely thin coating thickness minimizes the shear lag effect to the extreme, achieving a strain transfer efficiency close to 100\% . Furthermore, it possesses extremely high mechanical stability over a wide temperature range ( -200^\circ\text{C} to 300^\circ\text{C}), with virtually no creep or hysteresis. Consequently, it is the industry standard medium for precision force measurement and aerospace strain monitoring.
- Corresponding Products: OFSCN® Polyimide Fiber Bragg Gratings / FBG Strings (Bare)
C. Uncoated (Bare Glass / Bare Grating)
- Impact on Sensing Force: If, after grating inscription, no recoating is applied and the glass cladding is directly bonded to the object being measured. In this case, strain transfer completely eliminates the polymer intermediate medium, resulting in the highest transfer efficiency and zero hysteresis.
- Defect: It loses the physical protection of the coating. If the bare glass surface comes into contact with air or experiences minor disturbances, its surface micro-cracks will rapidly propagate, making the optical fiber extremely prone to brittle fracture during subsequent operations or tensile testing. In practical engineering, unless extremely specialized metallization or precision protection is employed, bare gratings are typically not used directly.