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A fiber-optic link is a device that sends data between two points by sending light through an optical fiber, which guides the light to the other end of the link. Every fiber-optic link contains the optical fiber itself, a transmitter, and a receiver. Some links also have amplifiers on the fiber between the transmitter and receiver to maintain power over longer links. Fiber-optic links are extremely important in modern communications technology and are used for applications such as telephones, Internet connections, and cable television.
Optical fibers can be used for communication because they are relatively flexible materials that can be used as waveguides, guiding the direction of electromagnetic waves as they travel through the fiber. The fiber has two parts, an outer cladding and an inner core. The core is designed to have a higher refractive index than the cladding, meaning that light in the core travels more slowly. When light hits a boundary between the medium it is passing through and another medium with a lower refractive index, that light will be completely reflected off of the boundary if it strikes at a sufficiently high angle in a phenomenon called total internal reflection. By completely surrounding the core with material with a lower refractive index, light can be confined to the core as it travels the length of the fiber, minimizing attenuation of the signal.
Optical fibers are usually composed of silica (silicon dioxide, or SiO2) glass. Other kinds of glass are also sometimes used, such as fluoride and phosphate glasses, and some fibers are made from crystalline substances such as corundum. The core and cladding are each doped with small amounts of other substances to raise or lower their refractive indexes so that light traveling through the fiber will be kept in the core.
The transmitter in a fiber-optic link is usually a light-emitting diode or laser diode, both of which produce light by running electricity throughout a semiconductor. This light is then released into the fiber at an angle that will cause total internal reflection in the core. Information is encoded in the light through variations in its intensity, phase, or polarization. At the other end of the fiber optic link is a photodetector, a device that detects light and that acts as a receiver. The most commonly used type of photodetector for this purpose is a photodiode, which uses a semiconductor to convert incoming light into electrical signals.
Fiber-optic links are an essential part of modern communications and are widely used due to their light weight, low signal loss, and immunity to electromagnetic interference that can disrupt electrical cables. Their capacity is enormous, and a single fiber-optic link can have hundreds of thousands of channels for telephone connections. They are more costly to make and maintain than electrical connection and so are used primarily for carrying signals for large numbers of long-distance telephone and Internet connections, with electrical transmission used for most shorter links. This is starting to change in some areas, however, with the growth of demand for Internet bandwidth, leading to the creation of high-bandwidth Internet services based on fiber-optic connections that go all the way to the user's home or office.
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