For an electromagnetic wave
, the coherence time is the time
over which a propagating wave (especially a laser
beam) may be considered coherent
, meaning that its phase
is, on average, predictable.
In long-distance transmission systems
, the coherence time may be reduced by propagation
factors such as dispersion
, and diffraction
The coherence time, usually designated ''τ'', is calculated by dividing the coherence length
by the phase velocity
in a medium; approximately given by
where ''λ'' is the central wavelength
of the source, ''Δν'' and ''Δλ'' is the spectral width
of the source in units of frequency and wavelength respectively, and ''c'' is the speed of light
A single mode fiber laser
has a linewidth of a few kHz, corresponding to a coherence time of a few hundred microseconds. Hydrogen masers have linewidth around 1 Hz, corresponding to a coherence time of about one second.
[ - Precision measurements with atomic hydrogen masers]
Their coherence length approximately corresponds to the distance from the Earth to the Moon.
* Atomic coherence
* Temporal coherence
Category:Radio frequency propagation