 Coherence length

In physics, coherence length is the propagation distance from a coherent source to a point where an electromagnetic wave (or wave_packet, wave_function, etc.) maintains a specified degree of coherence. The significance is that interference will be strong within a coherence length of the source, but not beyond it. This concept is also commonly used in telecommunication engineering.
In longdistance transmission systems, the coherence length may be reduced by propagation factors such as dispersion, scattering, and diffraction.
In radioband systems, the coherence length is approximated by
where c is the speed of light in a vacuum, n is the refractive index of the medium, and Δf is the bandwidth of the source.
In optical communications, the coherence length L is given by
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.
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 can be measured using a Michelson interferometer and is the optical path length difference of a selfinterfering laser beam which corresponds to a 1 / e = 37% fringe visibility,^{[1]} where the fringe visibility is defined as
where I is the fringe intensity.
Multimode heliumneon lasers have a typical coherence length of 20 cm, while the coherence length of singlemode ones can exceed 100 m. Semiconductor lasers reach some 100 m. Singlemode fiber lasers with linewidths of a few kHz can have coherence lengths exceeding 100 km. Similar coherence lengths can be reached with optical frequency combs due to the narrow linewidth of each tooth. Nonzero visibility is present only for short intervals of pulses repeated after cavity length distances up to this long coherence length.
See also
References
 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MILSTD188).
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