A means of transmitting analogue or digital information using light signals over an optical fibre. An optical fibre is a thin transparent filament made either of glass or, for short distances, special plastics; the diameter of the fibre ranges downward from 125 micrometres, with a number of preferred sizes now being adopted as standard. The information is carried as a light signal, typically in the infrared with a wavelength of about 1200 to 1550 nanometres, and generated by an electrical-to-optical transducer, usually a switchable semiconductor laser. Light of wavelength 1200 nm has a frequency of 250 000 gigahertz (GHz), and is in principle capable of transmitting at bit rates of the order of 100 000 Gbps. Modern fibre-optic networks use wavelength-division multiplexing to achieve speeds of about 14 000 Gbps.

A variety of methods are used to reduce the loss of the optical signal and hence increase signalling distance. The material of which the fibre is made is very pure, and by varying the refractive index of the material across the fibre it is possible to cause light rays at less than a certain angle to the axis of the fibre to be totally internally reflected back into the fibre. This reflection may take place at a discrete boundary between glasses of different refractive index (stepped index fibre), or may take place in a region of gradually varying refractive index (graded index fibre). If the fibre is made very thin, with a diameter of the order of the wavelength of the light, the light rays can only propagate along the fibre (monomode fibre). On a very long path, it is necessary to install amplifiers to regenerate the signal—by converting it from an optical to an electrical form, amplifying the electrical signal, and then reconverting it to an optical signal. Some amplifier designs exploit the nonlinear optical properties of certain glasses to allow direct amplification of the optical signal using a locally powered second laser as the power source.