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If photons are emitted from a thermal source, we get photon bunching. For coherent radiation, the detection probability doesn't change after detecting a photon. For single photon sources, we get anti-bunching.

A LED isn't a thermal source but it isn't coherent either. Does this mean that we still have some amount of bunching?

I didn't find anything in the literature because everybody is all over single photon LEDs.

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  • $\begingroup$ How the one photon from single photon source show bunching? Or you mean that two photons get emitted and they will be redirect into one direction? $\endgroup$ – HolgerFiedler Apr 7 '15 at 15:18
  • $\begingroup$ anti-bunching. In the sense that if you detect a photon from a single photon source, the probability to detect another one immediately afterwards drops. See wikipedia $\endgroup$ – Fritz Apr 7 '15 at 15:47
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One thing that you can be sure of is, for a large enough LED, you will get poisson statistics to a very good approximation. Neither bunching nor anti-bunching. The reason is simple: One photon comes from a certain part of the LED, the next photon is likely to come from a totally different part of the LED and head in a totally different direction. There's no way that either of these photons can influence the other.

The question is, what do I mean by "large enough LED"? 100 microns is definitely large enough. 100 nanometers is probably not. In between those, I don't know. I hope someone else will give a better answer!! :-D

(I'm referring to the size of the active area on the chip, not the size of the package.)

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  • $\begingroup$ +1 Good answer, although I think it may fall down in some instances where e.g. one tries to couple the light into a few mode or one mode fiber. In that case, only emitters near the optical axis have significant amplitude to couple into the fiber's bound mode(s). But this is getting a bit far from the spirit of the question. $\endgroup$ – Selene Routley Sep 29 '15 at 4:13
  • $\begingroup$ BTW I wrote a quantum optic software model of an LED / coupled fiber / fluorphore system some time ago and essentially assumed what your answer states: it does foretell experimental measurements very well for the purposes of the simulation (which was confocal imaging); although there could still be some correlations without disturbing coupled back intensity too much, so this is not the same as saying the model foretold the correct statistics. $\endgroup$ – Selene Routley Sep 29 '15 at 4:18
  • $\begingroup$ Yes, if you have a giant LED but you ONLY look at the 0.1% of photons that are coming out from a particular part of the chip and/or coming out in a particular direction ... then that subset of the photons can have bunching or anti-bunching. $\endgroup$ – Steve Byrnes Sep 29 '15 at 11:15
  • $\begingroup$ Which is exactly what a single mode fiber is doing: anything too off-axis is blocked by tilt aberration. Depending on the fiber, though, this is a region of the order of wavelengths wide, so I suspect any antibunching is very small. As you say, quantitative versions of these statements are much more difficult. $\endgroup$ – Selene Routley Sep 29 '15 at 11:29
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An LED is a collection of discrete atoms. When a current flows, an electron excites one every so often. The excited atom decays after a while and emits a photon. Assuming the current is uniform, variations in intensity come from randomness in arrival times and excited state lifetimes. Some time slices have many photons, some fewer. This distribution is called Shot noise.

An LED with polished ends is a laser. Light is generated by stimulated emission. An excited population of atoms is maintained. The LED is filled with photons all in the same state. Every so often, one will stimulate an excited atom to emit a photon in the same state. If everything else is uniform, this will be subject to the same randomness as an ordinary LED. So again, you get Shot noise.

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  • $\begingroup$ Just to get you right: so both the LED and the laser obey Poisson statistics, i.e. don't show bunching? $\endgroup$ – Fritz Apr 7 '15 at 14:31
  • $\begingroup$ This description of an LED is not correct at all. $\endgroup$ – NLambert Apr 7 '15 at 16:14

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