Editing Coherence (physics) (section)

=== Measurement of temporal coherence ===
[[File:wave packets.png|thumb|400px|right|Figure 3: The amplitude of a wavepacket whose amplitude changes significantly in time <math>\tau_\mathrm{c}</math> (red) and a copy of the same wave delayed by <math>2\tau</math>(green) plotted as a function of time ''t''. At any particular time the red and green waves are uncorrelated; one oscillates while the other is constant and so there will be no interference at this delay. Another way of looking at this is the wavepackets are not overlapped in time and so at any particular time there is only one nonzero field so no interference can occur.]]
[[File:interference finite coherence.png|thumb|390px|right|Figure 4: The time-averaged intensity (blue) detected at the output of an interferometer plotted as a function of delay τ for the example waves in Figures 2 and 3. As the delay is changed by half a period, the interference switches between constructive and destructive. The black lines indicate the interference envelope, which gives the [[degree of coherence]]. Although the waves in Figures 2 and 3 have different time durations, they have the same coherence time.]]

In optics, temporal coherence is measured in an interferometer such as the [[Michelson interferometer]] or [[Mach–Zehnder interferometer]]. In these devices, a wave is combined with a copy of itself that is delayed by time <math>\tau</math>. A detector measures the time-averaged [[intensity (physics)|intensity]] of the light exiting the interferometer. The resulting visibility of the interference pattern (e.g. see Figure 4) gives the temporal coherence at delay <math>\tau</math>. Since for most natural light sources, the coherence time is much shorter than the time resolution of any detector, the detector itself does the time averaging. Consider the example shown in Figure 3. At a fixed delay, here <math>2\tau</math>, an infinitely fast detector would measure an intensity that fluctuates significantly over a time ''t'' equal to <math>\tau</math>. In this case, to find the temporal coherence at <math>2\tau_\mathrm{c}</math>, one would manually time-average the intensity.
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