Optical fiber

The optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. Although fibers can be made out of either plastic or glass, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical absorption of glass. The light transmitted through the fiber is confined due to total internal reflection within the material. This is an important property that eliminates signal crosstalk between fibers within the cable and allows the routing of the cable with twists and turns. In telecommunications applications, the light used is typically infrared light, at wavelengths near to the minimum absorption wavelength of the fiber in use.

Fibers are generally used in pairs, with one fiber of the pair carrying a signal in each direction.

Fibers, like waveguides, can have various transmission modes. The fibers used for long-distance communication are known as single mode fibers, as they have only one strong propagation mode. This results in superior performance compared to other, multi-mode fibers, where light transmitted in the different modes arrives at different times, resulting in dispersion of the transmitted signal.

Beause of the remarkably low loss and excellent linearity and dispersion behavior of single-mode optical fiber, data rates of up to 40 Gbit/s are possible in real-world use on a single wavelength. Wavelength Division Multiplexing can then be used to allow many wavelengths to be used at once on a single fiber, allowing a single fiber to bear an aggregate bandwidth measured in terabits per second.

Modern fiber cables can contain up to a thousand fibers in a single cable, so the performance of optical networks easily accommodate even today's demands for bandwidth on a point-to-point basis. However, unused point-to-point potential bandwidth does not translate to operating profits, and it is estimated that no more than 1% of the optical fibre buried in recent years is actually 'lit'.

Recent advances in fiber technology have reduced losses so far that no amplification of the optical signal is needed over distances of hundreds of kilometers. This has greatly reduced the cost of optical networking, particularly over undersea spans where the cost reliability of amplifiers is one of the key factors determining the performance of the whole cable system.

Longer-range systems still have to use optical amplifiers[?].

Advantages of optical fibres over wires

• low loss, so repeater-less transmission over long distances is possible
• large data-carrying capacity (thousands of times greater)
• immunity to electromagnetic interference, including nuclear electromagnetic pulses (but can be damaged by alpha and beta radiation)
• high electrical resistance, so safe to use near high-voltage equipment or between areas with different earth potentials
• light weight
• signals contain very little power

• higher cost
• need for more expensive optical transmitters and receivers
• more difficult and expensive to splice than wires
• cannot carry electrical power to operate terminal devices (Note: current telecommunication trends greatly reduce this concern: availability of cell phones and wireless PDAs; the routine inclusion of back-up batteries in communication devices; lack of real interest in hybrid metal-fiber cables; increased use of fiber-based intermediate systems)

History

In 1966, Charles Kao, born in China, in his PhD thesis estimates that glass fibers need to have an optic signal attenuation of less than 20 dB per kilometer to be useful for long distance communication. The first useful optical fiber was invented in 1970 by researchers Maurer, Keck, Schultz, and Zimar working for British glass maker Corning, Ltd. (http://www.corning.com/) They achieved to make a fiber with 17 dB optic attenuation per kilometer by doping quartz glass[?] with titanium.

The first transatlantic telephone cable to use optical fiber was TAT-8, which went into operation in 1988.

Other uses of optical fibers

• Fibers can be used as light guides[?] in medical and other applications where bright light need to be brought to bear on a target without a clear line-of-sight path.
• In some high-tech buildings, optical fibers are used to route sunlight from the roof to other parts of the building.

See also:

• Glossary of fiber optic terms
• SDH
• Submarine communications cables