How optical fiber is made
A telecom fiber is pulled from a glass rod built layer by layer out of gas, and it ends up about as thin as a hair.
- stations
- 6
- things upstream
- 25
- raw sources
- 12
- steps from the ground
- 6
The short version
Optical fiber starts as a large glass preform made by burning silicon and germanium chlorides with oxygen so fine glass soot settles in layers. The preform is heated to a clear solid, then pulled in a drawing tower into a strand about the size of a human hair. Plastic coatings go on during the draw.
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Burn gas into glass soot
A hollow glass tube about 40 centimeters long turns slowly on a lathe. Silicon tetrachloride and germanium tetrachloride gas flow in with oxygen while a burner outside heats the tube to about 1,600 °C. The gases react and drop a fine soot of glass onto the inner wall.
about 1,600 °C
Goes in -
Layer the core and cladding
The torch travels the length of the tube again and again, melting each pass of soot into a clear layer. Changing the gas mix from layer to layer tunes the refractive index precisely, so light stays trapped in the center.
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Collapse it to a solid rod
When enough layers are down, the preform is consolidated into a transparent, solid rod at about 1,500 °C. Surface flaws on the preform would carry into the fiber, so careful polishing matters.
about 1,500 °C
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Pull it in the tower
The preform hangs in a drawing tower. Its tip is heated, and the fiber is pulled out like a string. The tension on the strand is controlled to keep the thickness uniform.
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Coat the strand
Two layers of UV-cured urethane acrylate go on while the fiber is still being drawn. The inner layer absorbs shocks, and the outer layer guards against knocks and may be colored to tell strands apart in a cable.
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Test the loss
Fiber is proof-tested and measured for light loss. A good modern strand loses so little that, at 0.148 dB/km, a 10 km length still passes nearly 71% of the light.
0.148 dB/km
Worth repeating
- In 1965 Charles Kao and George Hockham proposed that impurities, not physics, were what limited fiber, and the 20 dB/km loss target was first reached in 1970.
- Early high-quality fiber could only be drawn at two meters per second, and by 1983 Corning had raised that to over 50 meters per second.
- A fiber fuse, where damage vaporizes the glass, crawls back toward the laser at 1–3 meters per second.
Back to the ground
Follow every input upstream and optical fiber starts as these 12 raw sources.
Everything it is made of See it on the map
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Sources
Cite this page: "How optical fiber is made." Assembly Line, 2026-11-03. https://assemblyline.org/make/optical-fiber