(fibre optics, FO, US "fiber", light pipe) A plastic or glass (silicon dioxide) fibre no thicker than a human hair used to transmit information using infra-red or even visible light as the carrier (usually a laser). The light beam is an electromagnetic signal with a frequency in the range of 10^14 to 10^15 Hertz.

Optical fibre is less susceptible to external noise than other transmission media, and is cheaper to make than copper wire, but it is much more difficult to connect. Optical fibres are difficult to tamper with (to monitor or inject data in the middle of a connection), making them appropriate for secure communications. The light beams do not escape from the medium because the material used provides total internal reflection.

AT&T Bell Laboratories in the United States managed to send information at a rate of 420 megabits per second, over 161.5 km through an optical fibre cable. In Japan, 445.8 megabits per second was achieved over a shorter distance. At this rate, the entire text of the Encyclopedia Britannica could be transmitted in one second. Currently, AT&T is working on a world network to support high volume data transmission, international computer networking, electronic mail and voice communications (a single fibre can transmit 200 million telephone conversations simultaneously).

See also FDDI, Optical Carrier n, SONET.