How does this needle-like metal protective layer protect the optical fiber inside?
The metal protective layer you mentioned, resembling a “syringe,” is known in optical engineering and fiber optic cable design as Fiber in Metal Tube (FIMT) or Seamless Steel Tube structure. This stainless steel microtube structure (typically with outer diameters ranging from 0.6mm to 0.9mm, and larger ones around 2.0mm to 3.0mm) offers extremely high-strength armored protection to fragile silica glass optical fibers through material science and mechanical design, across multiple physical dimensions.
The physical and engineering principles behind this structure’s protection of internal optical fibers are as follows:
1. Radial Crush Resistance
- Geometric Arch Effect: Seamless stainless steel tubes have a perfect cylindrical shape. According to structural mechanics principles, a ring possesses very high radial stiffness. When subjected to external pressure from heavy objects, crushing, or mechanical squeezing, the tube’s wall thickness and ring structure can create an “arch effect.” This effect transforms concentrated lateral pressure (Point Load) into hoop stress within the tube wall, dispersing it. As a result, the optical fiber, located in the central void of the tube, experiences virtually no direct compression.
- Parameter Performance: The crush resistance of some seamless steel tube armored patch cords can reach >150 MPa or even >200 MPa, enabling them to withstand harsh industrial and outdoor heavy-load environments.
2. Axial Tensile Resistance & Strain Limitation
- Young’s Modulus Dominance: Stainless steel (such as 304, 316L, etc.) has a significantly higher elastic modulus (approx. 190-200 GPa) compared to silica glass or polymer plastics. When the fiber optic cable or patch cord is subjected to axial tension, the steel tube and outer reinforcement members bear almost all the tensile load.
- Fiber Micro-Slack Design: Optical fibers within the seamless steel tube typically have a small amount of Excess Fiber Length (EFL), meaning the actual fiber length is slightly greater than the tube length. When the entire cable is subjected to tensile force, the internal optical fiber simply