COHERENCE LENGTH

The propagation distance from a coherent source to a point where an electromagnetic wave maintains a specified degree of coherence . ( 188 ) 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 = 2 /( n ), where is the central wavelength of the source, n is the refractive index of the medium , and is the spectral width of the source. Note 3: Coherence length is usually applied to the optical regime.