What is "coating eccentricity" in optical fiber?

OFSCN Global Technical Center OFSCN Technical Knowledge Base
1

If the glass core is not in the exact center of the coating, is it more likely to break under stress?

2

Yes, if the glass part of the optical fiber (cladding and core) deviates from the geometric center of the protective coating (i.e., there is significant coating eccentricity), the probability of microcrack propagation or even mechanical fracture significantly increases when subjected to mechanical external forces or environmental stresses.

The following provides a detailed analysis from the perspective of optical fiber geometry and mechanical physics mechanisms:

I. What is “Coating Eccentricity” in Optical Fibers?

Standard communication and sensing optical fibers typically consist of three concentric layers:

  1. Core: The central medium for transmitting optical signals (typically about 9\ \mu\text{m} in diameter for single-mode fibers).
  2. Cladding: The total internal reflection light-guiding layer, made of high-purity silica glass (outer diameter typically 125\ \mu\text{m} or 80\ \mu\text{m} for reduced diameter fibers).
  3. Coating: A polymer protective layer applied over the glass cladding (e.g., polyacrylate with an outer diameter of 255\ \mu\text{m}, or polyimide with an outer diameter of 155\ \mu\text{m}).

Coating eccentricity (or Coating Non-Concentricity) refers to the spatial offset between the geometric center of the outer surface of the glass cladding and the geometric center of the outer surface of the coating. International standards (such as ITU-T G.650/G.652) typically strictly require coating concentricity errors to be controlled within a very small range (usually requiring < 12\ \mu\text{m}).

II. Mechanical Mechanisms by Which Coating Eccentricity Makes Optical Fibers More Prone to Fracture

When an optical fiber exhibits significant coating eccentricity, the coating presents an asymmetric state of “extremely thick on one side and extremely thin on the other” circumferentially. This drastically reduces the fracture threshold of the fiber under several common stress conditions:

1. Localized Lateral Compression and Failure of Point Load Buffering (Lateral Pressure & Micro-bending)

One of the primary functions of the optical fiber coating is to act as a mechanical buffer layer, uniformly distributing external point contact forces or lateral pressure onto the entire surface of the glass.

  • When the coating is eccentric, the extremely thin side of the coating loses almost all buffering protection against external mechanical impacts and hard object squeezing.
  • External micro-pressure will directly concentrate on the surface of the quartz glass on the thinner side, causing severe stress concentration and easily inducing the expansion of subsurface microcracks (Griffith Cracks) on the surface of the quartz glass, leading to rapid fracture.

2. Asymmetric Thermally Induced Stress (Thermal Stress Imbalance)

  • The coefficient of thermal expansion (CTE) of polymer coatings (such as polyacrylate, polyimide) is typically 1 \sim 2 orders of magnitude higher than that of quartz glass.
  • When the ambient temperature changes significantly (e.g., high-low temperature cycling), the coating material undergoes intense expansion or contraction.
  • If the coating thickness is asymmetric, the radial/axial contraction stress exerted by the coating on the inner glass cannot be balanced circumferentially, thereby imposing an asymmetric additional bending moment on the glass body. This persistent asymmetric tensile stress accelerates stress corrosion in the silica material, reducing the long-term fatigue life of the optical fiber.

3. Neutral Axis Shift During Bending and Tension

When axial tension is applied to the fiber or it is bent, the mechanical neutral layer of the eccentric composite structure no longer coincides with the physical axis of the glass. Under the same bending radius or tensile load, the quartz surface on the geometrically thinner side will experience peak tensile stress far exceeding that of a theoretically symmetrically designed value, making it more likely to exceed the fracture strength limit of quartz glass.

4. Knife-Edge Mechanical Scoring During Stripping (Stripping Damage)

In practical fiber splicing, end-termination, or sensor fabrication, stripping tools are used to remove the coating. Coating eccentricity makes it extremely easy for the stripper blade to contact and scratch the surface of the quartz cladding on the thinner side. Once micro-level mechanical scratches appear on the silica surface, the optical fiber will undergo brittle fracture under extremely low subsequent bending or vibration stress.

III. Official Related Products and Geometric Specifications

In fiber Bragg grating sensing and industrial optical fiber applications, ensuring strict concentricity between the optical fiber glass cladding and the coating is a key metric for ensuring long-term mechanical reliability and sensing accuracy: