COHERENCE TIME

For an electromagnetic wave , the time over which a propagating wave may be considered coherent . ( 188 ) Note 1: In long-distance transmission systems, the coherence time may be reduced by propagation factors such as dispersion , scattering , and diffraction . Note 2: In optical communications , coherence time, , is calculated by dividing the coherence length by the phase velocity of light in a medium ; approximately given by = 2 /(c ) where is the central wavelength of the source, is the spectral width of the source, and c is the velocity of light in vacuum. Note 3: " Coherence time " is usually applied to the optical regime.