Fiber optic communications

Fiber optic communication carries information as pulses of light along a thin strand of very pure glass. A light source, usually a laser or light-emitting diode, is switched on and off to represent data, the light travels down the fibre, and a photodetector at the far end converts it back into an electrical signal. The light stays inside the fibre by total internal reflection: the central core is surrounded by a cladding of slightly lower refractive index, so light striking the boundary at a shallow angle reflects back into the core instead of escaping, guiding it around bends and over long runs.

Fibres come in two main kinds. Single-mode fibre has a very narrow core that allows light to travel essentially one path, which keeps pulses sharp over long distances and gives the highest bandwidth, making it the choice for long-haul links. Multimode fibre has a wider core that supports several light paths at once; it is easier to couple light into and cheaper to use over short distances, but because different paths take slightly different times, pulses spread out as they travel.

Two effects limit how far and how fast a fibre can carry data. Attenuation is the gradual loss of optical power along the fibre from absorption and scattering, eventually requiring the signal to be amplified or regenerated. Dispersion is the spreading of light pulses in time, which can blur fast pulses into one another and so caps the achievable data rate over a given distance. Fibre's freedom from electromagnetic interference, low loss and high capacity have made it the backbone of telecommunications and high-speed networking.

Frequently asked questions

How does light stay inside an optical fibre?

The core is surrounded by cladding of lower refractive index, so light meeting the boundary at a shallow angle reflects back into the core by total internal reflection.

What is the difference between single-mode and multimode fibre?

Single-mode fibre has a narrow core allowing essentially one light path for long-distance, high-bandwidth links; multimode fibre has a wider core supporting many paths, cheaper but limited to shorter distances.

What is dispersion in a fibre?

Dispersion is the spreading of light pulses in time as they travel, which can blur fast pulses together and limits the data rate achievable over a given distance.