Optical fibre cable is a cable that carries data as pulses of light through thin strands of very pure glass. It is also called fibre optic cable, or just fibre. Almost all long-distance data in the world, and the broadband in most Indian cities, travels on it for at least part of the way.
Each strand of fibre is about as thick as a human hair, 125 micrometres across. At its centre is a core of glass only about 9 micrometres wide. The glass around the core bends the light back inwards, so the light stays inside the core even when the fibre curves. The light used is invisible infrared. Long routes use a wavelength of about 1,550 nanometres. In modern fibre, the light loses less than 5 percent of its strength per kilometre. That is why a signal can cross 80 kilometres before it needs to be strengthened.
A single strand is too fragile to lay on its own. So a cable bundles many strands, from a few up to several hundred, and in some cables a few thousand, with plastic tubes, strength members and a tough outer jacket. The ends are joined to equipment with connectors, or fused together with a machine that melts two strands into one.
Imagine a crew laying a new 96-fibre cable along a national highway between Nagpur and Raipur, about 290 kilometres. First, the route is drawn with network planning software, which shows where the ducts can go. The crew lays a plastic duct a metre or more underground and blows the cable through it with compressed air. Each reel holds a few kilometres of cable, so the crew fuses the strands at every joint. Then the crew tests each strand with test equipment that sends a pulse of light and measures what comes back. The test shows the loss at every joint, and the exact distance to any fault.
One cable with 96 strands can serve many networks. An operator may use some strands for its optical transport network, each strand carrying many terabits. Other strands may go to the passive optical network (PON) equipment that brings broadband to homes. Spare strands can be rented to other companies.
The biggest problem in India is cuts. Roadworks and digging machines cut fibre cables every day. A cut takes a crew hours to find and fuse. This is why operators lay fibre in rings, so traffic can take the other way round.
Making the glass itself is a specialist job, covered under advanced materials.