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Definition: a measure of temporal coherence, expressed as the time over which the field correlation decays
The coherence time can be used for quantifying the degree of temporal coherence of light. In coherence theory, it is essentially defined as the time over which the field correlation function decays. This correlation (or coherence) function is
where E(t) is the complex electric field at a certain location.
This function is 1 for 
= 0 and usually decays monotonically for larger time delays .
For an arbitrary shape of this function, the coherence time can be defined by
In the case of an exponential coherence decay (as it occurs e.g. for a laser with quantum noise influences only), this is the same as the exponential decay time.
Figure 1: Animated illustration of temporal coherence. An ensemble of 100 oscillators with nominally the same frequency but with same phase noise (corresponding to white frequency noise) are started at t; = 0. The phase space diagram (upper part) shows how they get increasingly out of phase. The thick point shows the complex average amplitude; its oscillations become weaker with time. (This phenomenon is analogous to a reduced interference contrast.) The lower diagram shows the decay of the coherence function with time.
Knowledge of the coherence time (i.e., a single number) can be useful when the shape of the coherence function (or the shape of the Fourier spectrum) is approximately known. Quite obviously, however, the specification of the coherence time (or the linewidth) alone does in no way constitute a full characterization of the coherence.
The coherence time is intimately linked with the linewidth of the radiation, i.e., the width of its spectrum. In the case of an exponential coherence decay as above, the spectrum has a Lorentzian shape, and the (full width at half maximum) linewidth is
The constant factor in this equation is in general different for other shapes of the coherence function (e.g. roughly twice as high for a Gaussian shape). Conversely, the linewidth can be used for estimating the coherence time, but the conversion depends on the spectral shape. In cases where the frequency noise spectrum is not flat but strongly rising toward zero frequency, there can be a significant degree of coherence even for time delays well above the inverse linewidth; this issue is important e.g. in the context of self-heterodyne linewidth measurement.
Instead of the coherence time, it is common to specify the coherence length, which is simply the coherence time times the vacuum velocity of light, and thus also quantifies temporal (rather than spatial) coherence.
Lasers, particularly single-frequency solid-state lasers, can have rather long coherence times, compared with the duration of an optical cycle; values of several milliseconds are possible. A long coherence time is important for many applications (see the article on coherence).
Bibliography
| [1] | B. E. A. Saleh and M. C. Teich, "Fundamentals of Photonics", Wiley, 1991 |
See also: coherence, coherence length, linewidth, speckle
