, coherence length
is the propagation
distance from a coherent
source to a point where an electromagnetic wave
maintains a specified degree of coherence[?]
Note 1: In long-distance transmission systems, the coherence length may be reduced by propagation factors such as dispersion, scattering, and diffraction.
Note 2: In 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.
Note 3: Coherence length is usually applied to the optical regime.
Source: from Federal Standard 1037C and from MIL-STD-188
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