For an electromagnetic wave, the coherence time is the time over which a propagating wave (especially a laser or maser beam) may be considered coherent. In other words, it is the time interval within which its phase is, on average, predictable.

In long-distance transmission systems, the coherence time may be reduced by propagation factors such as dispersion, scattering, and diffraction.

Coherence time, ?, is calculated by dividing the coherence length by the phase velocity of light in a medium; approximately given by

\tau = \frac{\lambda^2}{c\, \Delta \lambda}

where ? is the central wavelength of the source, ?? is the spectral width of the source, and c is the speed of light in vacuum.

References

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