Can a bare grating be recoated to restore its original strength?
1. Key Conclusion
Not Fully Recoverable.
Re-coating (recoating) the bare grating with a secondary coating cannot completely restore the fiber’s ultimate mechanical strength to its factory state (i.e., the original fiber before stripping the coating, or a grating formed online during the fiber drawing process).
2. Analysis of Physical and Engineering Principles
To understand why the strength cannot be fully recovered, we need to analyze it from the perspectives of fiber Fracture Mechanics and the surface state changes during the grating fabrication process:
① Irreversible Surface Micro-crack Formation (Griffith Theory)
When standard communication fibers are drawn on a fiber drawing tower, the surface of the molten silica (SiO_2) cladding, at extremely high temperatures, is immediately coated with a primary coating (typically polyacrylate) in a clean environment as it cools. This ensures that the fiber surface is virtually free of moisture erosion or physical scratches, resulting in extremely high ultimate tensile strength.
However, during the fabrication of Fiber Bragg Gratings (FBGs), the original coating in the writing area must be stripped (using thermal stripping, mechanical stripping, or chemical stripping). During the coating removal, hydrogen loading, gripping, and ultraviolet laser (UV) writing processes, the exposed silica surface inevitably comes into contact with moisture and micro-dust in the air, leading to micro-physical contact.
According to Griffith’s theory for brittle fracture, the fracture strength \sigma_f of a glass medium is primarily determined by the maximum depth a of internal and surface micro-cracks, satisfying the following relationship:
\sigma_f = \frac{K_{IC}}{Y \sqrt{a}}
Here, K_{IC} is the fracture toughness, and Y is the geometric shape factor. The micro-defects generated during wire stripping and writing increase a, leading to an irreversible and significant decrease in ultimate tensile strength.