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. 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 laserbeam 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 semiconductor lasers reach some 100 m. Fiber lasers can have coherence lengths exceeding 100 km.
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