How a strand of glass carries a signal for a hundred kilometres

There is a short answer about fibre optics and a useful one, and they are not the same. What follows is the useful one.

Light trapped by a change in refractive index

A fibre has a core of glass surrounded by cladding glass with a slightly lower refractive index, and that difference does the work. Light striking the boundary at a shallow enough angle is reflected entirely back into the core rather than passing through it.

Because the reflection is total rather than partial, almost no energy is lost at each bounce along the length of the run. The cladding is not a coating added afterwards; core and cladding are drawn together from one preform of specially prepared glass. The plastic layers outside the cladding are mechanical protection and play no part in guiding the light at all.

Purity is the reason distance is possible

Ordinary window glass would absorb a signal within metres, because impurities and imperfections scatter and absorb light strongly. Fibre glass is manufactured to a purity where a thick block would still look almost perfectly clear from end to end. Even then, attenuation varies with wavelength, and systems deliberately use the narrow bands where the glass happens to be most transparent.

Those bands sit in the infrared rather than in visible light, which is why the light in a working fibre cannot be seen. Absorption by residual water in the glass historically created a gap between usable bands, and manufacturing improvements have narrowed it.

Two kinds of fibre, two different problems

A wide core lets light travel by many different paths at once, and paths of different lengths arrive at slightly different times. That spreading blurs each pulse into the next, which limits how fast and how far such a fibre can usefully carry data. A narrow core allows essentially one path, removing that spreading and leaving distance limited mainly by attenuation instead.

In practice, narrow cores demand more precise alignment at every joint, which makes connectors and splices harder and more expensive to get right. Wide-core fibre survives inside buildings for short runs, while anything crossing a city or a country uses the narrow kind.

Many colours down one strand

Signals at different wavelengths do not interfere with each other, so a single fibre can carry many independent channels simultaneously. Devices at each end combine and separate those wavelengths using optical filters, with no electronics involved in the multiplexing itself. Adding capacity to an installed route therefore often means changing the equipment at the ends rather than laying more glass.

At the protocol level, this is why the cost of long-distance capacity has fallen so much faster than the cost of digging the trenches did.

Channels can be sold or leased separately, so several organisations may be using the same physical strand without any interaction.

Amplifying light without turning it back into electricity

A long route needs the signal boosted periodically, and converting to electricity and back at each point would be slow and costly. Certain rare-earth doped fibre sections amplify passing light directly when energised by a separate pump laser alongside the signal. Because amplification happens optically, all the wavelength channels in the fibre are boosted together in a single component.

The short version: amplifiers add noise as well as gain, so the number of stages a route can tolerate before regeneration is limited. The equipment is protocol-agnostic, which is why an installed route can be upgraded without touching anything in the ground.

How fibre fails and how it is fixed

Glass is strong in tension and unforgiving of tight bends, and a sharp bend lets light escape through the cladding as loss. Cables specify a minimum bend radius, and installers who exceed it create a fault that appears only under close measurement. Joining two fibres properly means aligning cores within a fraction of their width and fusing them together with a controlled arc.

A test instrument sends a pulse and times the faint reflections that return, which locates a break to within a short distance. Most outages are mechanical rather than optical: a digger, a rodent or a damaged duct rather than anything wrong with the glass.

The takeaway

The signal survives because the glass is clear, the angle is shallow and the bend is gentle.

The constraint is almost always physical, and marketing rarely mentions which one.