Fibre Optics

Fibre Optics, branch of optics dealing with the transmission of light through fibres or thin rods of glass or some other transparent material of high refractive index. If light is admitted at one end of a fibre, it can travel with very low loss, even if the fibre is curved.

The principle on which this transmission of light depends is that of total internal reflection: light travelling within the fibre's centre, or core, strikes the outside surface at an angle of incidence greater than the critical angle (see Optics), so that all the light is reflected into the fibre without loss. Thus light can be transmitted over long distances by being reflected thousands of times. In order to avoid losses through the scattering of light by impurities on the surface of the fibre, the optical-fibre core is clad with a glass layer of much lower refractive index; the reflections occur at the interface of the glass fibre and the cladding.

The simplest application of optical fibres is the transmission of light to locations otherwise hard to reach, such as the bore of a dentist's drill. Also, bundles of several thousand very thin fibres, assembled precisely side by side and optically polished at their ends, can be used to transmit images. Each point of the image projected on one face of the bundle is reproduced at the other end of the bundle, reconstituting the image, which can be observed through a magnifier. Image transmission by optical fibres is widely used in medical instruments for viewing the interior of the human body and for laser surgery, in facsimile systems, in phototypesetting, in computer graphics, and in many other applications.

Optical fibres are also used in a wide variety of sensing devices, ranging from thermometers to gyroscopes. The potential of their applications in this field is nearly unlimited, because the light sent through them is sensitive to many environmental changes, including pressure, sound waves, and strain, as well as heat and motion. The fibres can be especially useful where electrical effects could make ordinary wiring useless, inaccurate, or even hazardous. Fibres have also been developed to carry high-power laser beams for cutting and drilling.

One growing application of optical fibres is in communication, because light waves have high frequencies and the information-carrying capacity of a signal increases with frequency. Fibre-optic laser systems are used in communications networks. Many long-haul fibre communications networks providing both transcontinental and transoceanic connections are in operation. One advantage of optical-fibre systems is the great distances that a signal can travel before a repeater is needed to regenerate it. Fibre-optic repeaters are currently separated by about 100 km (about 60 mi), compared to about 1.5 km (1 mi) for electrical systems. Newly developed optical-fibre amplifiers can extend this distance even farther.

Local area networks (LANs) are another growing application for fibre optics. Unlike long-haul communications, these systems connect local subscribers to centralized equipment such as computers and printers. This system increases the utilization of equipment and can easily accommodate new users on a network. Development of new electro-optic and integrated-optic components will further expand the capability of fibre systems.