In physics, coherence length is the propagation distance from a coherent source to a point where an electromagnetic wave maintains a specified degree of coherence. This concept is commonly used in telecommunication engineering.
In long-distance transmission systems, the coherence length may be reduced by propagation factors such as dispersion, scattering, and diffraction.
In coherence length laser optical communications, the coherence length L is given approximately by L = λ2/(n Δλ), where λ is the central wavelength of the source, n is the refractive index of the medium, and Δλ is the spectral width of the source.
Coherence length is usually applied to the optical regime.
Source: from Federal Standard 1037C in support of MIL-STD-188
The expression above is a frequently used approximation. Due to ambiguities in the definition of spectral width of a source, however, the following definition of coherence length has been suggested:
The coherence length is the optical path length difference of a self interfering laserbeam which corresponds to a 50% fringe x-ray laser coherence length visibility. Where the fringe visibility is defined as V=(Imax-Imin)/(Imax+Imin) and I is the fringe intensity.
Helium-neon lasers have a typical coherence length of 20 cm, while semiconductor lasers reach some 100m. Fiber lasers can have coherence lengths exceeding 100 km.
Electronics Topics
The field of electronics is the study and use of systems that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves and semiconductors. The design and construction of electronic circuits to solve practical problems is part of the fields of electronic engineering, and the hardware design side of computer engineering. The study of new semiconductor devices and their technology is sometimes considered as a branch of physics.
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