What is "fiber drawing tension"?

OFSCN Global Technical Center OFSCN Technical Knowledge Base
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Does the pulling force applied when laying optical fibers affect their quality?

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In fiber manufacturing engineering, the force applied during fiber drawing (i.e., Fiber Drawing Tension) not only affects fiber quality but is also one of the core process parameters that determine fiber geometry, optical loss, and mechanical reliability.

I. What is Fiber Drawing Tension?

Fiber drawing is typically performed on a drawing tower several tens of meters high:

  1. Melt Deformation: A preform made of high-purity silica glass is fed into a high-temperature drawing furnace (graphite resistance furnace or induction furnace, typically at temperatures between 1900^\circ\text{C} \sim 2100^\circ\text{C}) where it softens, forming a neck-down region.
  2. Drawing into Fiber: A capstan at the bottom pulls downwards at a set speed, stretching the preform (several centimeters in diameter) into a bare fiber with a nominal diameter of 125\ \mu\text{m}.
  3. Tension Definition: The axial tensile force exerted on the fiber in the molten forming region and the cooling section is defined as drawing tension (usually expressed in gram-force, gf, or millinewtons, mN; the common industrial control range is typically between 30 \sim 150\ \text{gf}, depending on drawing speed, cladding diameter, and material formulation).

The drawing tension F is directly related to the viscosity \eta of the glass melt in the neck-down region, the drawing speed v, and the geometric gradient rate:

F \approx 3 \pi \eta r^2 \frac{\partial v}{\partial z}

II. Specific Mechanisms of Drawing Tension’s Impact on Fiber Quality

Excessive or insufficient drawing tension can negatively impact the fiber in multiple dimensions:

1. Geometric Uniformity and Outer Diameter Fluctuation

  • Excessive Tension: Can easily cause capillary instability or rheological instability in the molten region, leading to high-frequency radial jitter in the fiber’s outer diameter.
  • Insufficient Tension: The fiber may sway at low frequencies in the furnace and cooling tube, causing delays in the feedback closed-loop control of the outer diameter. This severely affects the tolerance consistency and roundness of the cladding diameter (125 \pm 0.7\ \mu\text{m}).

2. Microscopic Point Defects and Optical Attenuation (Loss)

  • Excessive Tension:
    • High shear stress can cause bond breaking in the three-dimensional network structure of tetrahedrons in high-purity silica glass (\text{SiO}_2), creating point defects (such as Non-Bridging Oxygen Hole Centers, NBOHC; Oxygen Deficient Centers, ODC; E’ centers, etc.). These color center defects generate additional absorption peaks in specific ultraviolet, visible, and near-infrared wavelength bands.
    • It also increases non-uniformity in the microscopic density of the glass,