Coherent time vs duration time of the wave packet (photon)

[coke]

hi, i really confused about this two term. As i read from a book, ' in the Young's double slits experiment of light, if the time difference (that resulted from the path difference because of the two slits) is longer than the duration of each 'burst' (the photon),the received waves must come from different 'burst', which are unrelated and incoherent.'

i want to know if 'coherent time' = duration of the 'burst' (about nanosecond for white light). if they are equal, i can understanding the above statement. if they are not, can anyone explain clearly, in detail to me ? Thank you very very much!

 

[Andy Resnick]

"Coherence time" is related to the frequency spread (bandwidth) of a beam. Coherence time is most proeprly measured by a Mach-Zender/Michaelson interferometer- the coherence time is related to the difference in path lengths.

"Coherence length", sometimes called the coherence area, is related to the apparent size of a source, and is measured by a Young's double slit interferometer.

For pulsed light, I can see there being competition between the coherence time, the bandwidth, and the pulse duration, and it's not immediately clear what would happen in a Michaelson interferometer, except in the obvious cases of the pulses being coincident or completely missing each other.

Putting pulses through a double-slit interferometer poses no obvious difficulties, since the pulse width and size of the illuminated spot are not related to each other. Perhpas for sub fs pulses, there could be some interesting effects, but it could be hard to distinguish them from changes to a single-slit diffraction pattern.