![]() The coherence time is intimately linked with the linewidth of the radiation, i.e., the width of its spectrum. Obviously, however, the specification of the coherence time (or the linewidth) alone does not constitute a full characterization of the coherence. 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. The lower diagram shows the decay of the coherence function with time. (This phenomenon is analogous to a reduced interference contrast.) The thick point shows the complex average amplitude its oscillations become weaker with time. The phase space diagram (upper part) shows how they get increasingly out of phase. Figure 1:Īnimated illustration of temporal coherence.Īn ensemble of 100 oscillators with nominally the same frequency but with same phase noise (corresponding to white frequency noise) are started at t = 0. 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. ![]() For an arbitrary shape of the coherence function, and in the case of an exponential coherence decay this is the same as the exponential decay time.Ī measurement of coherence time may be based on that equation, where the coherence function is obtained via a path length dependent interference contrast. ![]()
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