Why is there a layer of gold or aluminum on the outside of the glass? Is it for electrical conductivity?
Coating the outer layer of quartz glass fiber with metals such as gold (Gold) or aluminum (Aluminum) does not make electrical conductivity the primary purpose of its design and application.
Although metal coatings possess excellent electrical conductivity, in the fields of optical engineering and sensing, replacing traditional organic polymer coatings with metal coatings is primarily driven by the physical and chemical requirements for resistance to extreme environments, hermetic corrosion protection, and mechanical connection performance.
Here are the core technical reasons for adopting metal-coated fibers:
1. Extreme Temperature Resistance
Standard single-mode optical fibers are typically coated with organic polymer materials, such as acrylate (operating temperatures usually up to 85\ ^\circ\text{C}) or polyimide (operating temperatures usually from -200\ ^\circ\text{C} to 350\ ^\circ\text{C}). At higher temperatures, these organic coatings undergo pyrolysis, carbonization, or combustion, leaving the glass unprotected and extremely brittle.
Metal coatings (like gold or aluminum) have extremely high melting points and thermal stability, significantly expanding the operating temperature limits of the fiber. For instance, gold-coated fibers can operate stably for extended periods within a very wide temperature range of -270\ ^\circ\text{C} to 700\ ^\circ\text{C}.
2. Hermetic Sealing and Protection Against Hydrogen/Moisture Corrosion
When quartz glass fibers (silica) are under high stress or tension and come into contact with trace water molecules (\text{H}_2\text{O}) in the environment, they can experience