The Signal light in glass, under the sea

Take it apart

As thick as a garden hose

In early 2025 there were about 570 subsea cable systems in service, with 81 more planned.¹ In deep water a cable is about as thick as a garden hose; near shore, where anchors and trawlers drag, it gets coats of steel armour.

Layer by layer

• Jacket polyethylene
• Armour wires galvanised steel, in shallow water
• Copper sheath carries power out to the repeaters
• Strength wires steel, so the cable can be lowered kilometres deep
• Tube filled with gel to keep water off the glass
• Fibres a handful of pairs of glass threads

Repeaters

Rare earths on the sea floor

Light fades as it travels, so every 60 km or so a repeater boosts it. Inside are lengths of fibre doped with erbium, a rare earth, that amplify the light directly. Power reaches them through the cable's copper, fed from the shore.²

SiGeErCuFeZnGa

One fibre

Light that can't get out

A fibre is a thread of glass thinner than a hair, with a core inside a cladding. Light sent down the core hits the boundary at a shallow angle and bounces back in, again and again, for thousands of kilometres.

The trick

A pinch of germanium

The light stays in because the core bends light slightly more than the cladding. That is done by adding a little germanium dioxide to the core's pure silica, which raises its refractive index without absorbing the light.⁴

Where germanium goes

Fibre optics is the largest use of germanium in the United States, ahead of infrared optics, electronics and solar cells.⁵ Most of it is recovered as a by-product of zinc ores and coal ash.

The last mile

Then into the air

At the end of the glass, the signal often finishes its trip by radio. Many 5G base stations use amplifiers made of gallium nitride, which handle more power in less space than older silicon parts.⁶

The crystal

Gallium and nitrogen click together into a hexagonal crystal, grown layer by layer on a wafer. The same material is in fast phone chargers and in radar.