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Material · Engineered

Optical fibre

Glass so clear you could see through kilometres of it — the material that carries essentially all long-distance communication.

Optical medium Insulator

An optical fibre is a strand of glass with a core of slightly higher refractive index than the cladding around it, so light entering at a shallow enough angle is totally internally reflected and cannot escape. That is the whole principle, and it was demonstrated in the nineteenth century with a jet of water.

What took a century was the glass. Ordinary window glass absorbs so strongly that a signal would be lost in metres; the fibre in the ground today is pure enough that light travels tens of kilometres before losing half its power. Charles Kao argued in 1966 that the loss was impurity rather than a property of glass, which is the insight the whole industry rests on.

Processing

The purity comes from making the glass out of a gas. Silicon tetrachloride and germanium tetrachloride are burned to deposit soot inside or outside a tube, which is then collapsed into a solid preform — a metre-long rod that is a scale model of the finished fibre, core and cladding already in place. Transition metals, which are what absorb the light, simply do not survive the vapour route.

The preform is then drawn: heated at one end and pulled into a fibre a hundred and twenty-five microns across at tens of metres a second, with a polymer coating applied before it can touch anything. A single preform yields hundreds of kilometres.

Germanium is the dopant that raises the core's refractive index, and it is a substantial share of what germanium is used for.

Economic significance

The erbium-doped fibre amplifier is the piece that made the modern internet economic. Before it, a long link needed the signal converted to electricity, amplified and converted back at every repeater — which fixes the data rate at whatever the electronics can do. An erbium amplifier boosts the light directly, at any data rate, and one repeater can carry every wavelength in the fibre at once.

That is why capacity grew faster than the cables did. The glass in the ground has not changed much; what is sent down it has, repeatedly, and every submarine cable in the world depends on a rare earth ion in a few metres of doped fibre.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

contains

  • Silicon — element · as fused silica, and almost nothing else — the fibre is glass made pure by being assembled from a gas
  • Oxygen — element · the other half of the silica
  • Germanium — element · the dopant that raises the core's refractive index above the cladding's, which is the entire mechanism — and a substantial share of what germanium is used for
  • Erbium — element · in the few metres of doped fibre that make up an amplifier, which is what made long-haul optical communication economic

is produced by

  • Vapour deposition — process · the preform is built by burning silicon and germanium chlorides to deposit soot layer by layer, which is why the glass is pure enough to see through kilometres of — transition metals do not survive the vapour route
  • Glass melting — process · drawn from a preform of ultrapure fused silica, at a purity where a kilometre of it is as clear as a window pane

is sourced from

  • Quartz — mineral · as silica, though not as sand: the glass is deposited from silicon tetrachloride vapour, because the transition metals that absorb light do not survive that route

is used as

  • Telecommunications — application · essentially all long-distance communication, and the reason capacity grew faster than the cables did

is an alternative to

  • Copper — element · in communications, and the substitution is essentially complete over distance: fibre carries more information further with less loss, and copper survives where the cost of digging it out exceeds the benefit

is used in

  • Semiconductor manufacturing — industry · the preform and the drawing tower are vapour-deposition equipment, and the industry grew out of the same processes

Sources

  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Optical fibre back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Optical fibre → is produced by (drawn from a preform of ultrapure fused silica, at a purity where a kilometre of it is as clear as a window pane) → Glass melting → takes as input (as the stabiliser, and without it a soda-silica glass would slowly dissolve in water) → Limestone → is sourced from (precipitated from it, mostly by organisms building shells and skeletons out of dissolved calcium and carbonate) → Seawater → is composed of (about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this) → Water
  • Optical fibre → is sourced from (as silica, though not as sand: the glass is deposited from silicon tetrachloride vapour, because the transition metals that absorb light do not survive that route) → Quartz
  • Optical fibre → is produced by (the preform is built by burning silicon and germanium chlorides to deposit soot layer by layer, which is why the glass is pure enough to see through kilometres of — transition metals do not survive the vapour route) → Vapour deposition
  • Optical fibre → is produced by (drawn from a preform of ultrapure fused silica, at a purity where a kilometre of it is as clear as a window pane) → Glass melting → takes as input (the largest ingredient by mass, and it must be low in iron because iron colours glass green) → Sand → is sourced from (the ultimate source of most of it — granite weathers, the feldspar becomes clay and washes out, and the quartz survives as grains) → Granite → is composed of (by volume, alongside feldspar and lesser mica) → Quartz
  • Optical fibre → is produced by (drawn from a preform of ultrapure fused silica, at a purity where a kilometre of it is as clear as a window pane) → Glass melting → takes as input (as the stabiliser, and without it a soda-silica glass would slowly dissolve in water) → Limestone → is composed of → Calcite
  • Optical fibre → is produced by (drawn from a preform of ultrapure fused silica, at a purity where a kilometre of it is as clear as a window pane) → Glass melting → takes as input (as the stabiliser, and without it a soda-silica glass would slowly dissolve in water) → Limestone → is composed of (in young limestone, before conversion to calcite is complete) → Aragonite

These are the most distinct paths back. Optical fibre can be traced through others besides.

Downstream — what it becomes