Talk:Coherence length

message: The article currently (2006-02-14) suggests that semiconductor lasers have longer coherence lengths (100 m) than helium-neon lasers (20 cm). I don't think that's generally correct. Perhaps the article should say 100 microns, rather than 100 m. Opinions?
^I also think these statements are incorrect. Typical coherence lengths of monomode HeNe lasers are 100m to several km, while even a monomode semiconductor laser with a linewidth of about 10pm has a coherence length of about 10cm.
^This statement is technically correct, but is deceptive.  A HeNe laser without frequency stabilization typically has a 20-30 cm coherence length.  A frequency stabilized (monomode, as the previous comment states) HeNe can have a coherence length greater 1 km.  A semiconductor laser can have a coherence length less than 1 mm or greater than 100 m.  Again, the longest coherence lengths are achieved via frequency stabilization.  
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"The expression above is a frequently used approximation. Due to ambiguities in the definition of spectral width of a source, however, the following definition of coherence length has been suggested[citation needed]"
Does anybody have a citation related with the expression of linewidth vs. coherence length?
Some sources refer to a pi factor added in the denominator of the expression while some others do not (this article does not). I did some simple numerical simulations with a sum of planar waves with a lorentzian distribution of the amplitude this gave a visibility of 50% for a path difference around ~3x times smaller than given by the expression in this article. This seems to confirm that pi is needed for a lorentzian linewidth. WB.
:Most that I know use the coefficient of 1, which is close enough for most applications. Otherwise, you have to carefully define line width, and how to measure it. Gah4 (talk) 07:55, 5 April 2018 (UTC)
thread_title: Semiconductor laser coherence length

message: There is a merger template on this article and the article coherence time. I didn't put it there and the person who did has not discussed it. I will remove it in one day unless there is actually a discussion to be had. JHobbs103 (talk) 17:24, 13 June 2010 (UTC)
replies: text: No objection, as long as you remove it from both articles.--Srleffler (talk) 04:11, 14 June 2010 (UTC)
replies: text:  Done. JHobbs103 (talk) 17:40, 16 June 2010 (UTC)
thread_title:  Merger proposal 

message: The article indicates that coherence length is the distance from the source. Now, if that is the way it is used then I suppose, but as I understand it is the distance along the beam, not necessarily from the source. For holography, it needs to be more than, more or less, the distance between the plate (film) and object, which might be a long way from the laser source. Gah4 (talk) 11:46, 2 February 2013 (UTC)
replies: text: You're right. I fixed it.--Srleffler (talk) 19:48, 2 February 2013 (UTC)
thread_title:  distance from source? 

message: From the formula is seems that the coherence length of a single photon is infinite, because the spectral width is zero. But interference only shows with multiple photons, so in practice the visible coherence length depends on the spectral width of all photons together (the same for coherence time). Is that a right? DParlevliet (talk) 09:26, 14 January 2014 (UTC)
replies: text: No, single photons can interfere with themselves. --Srleffler (talk) 06:19, 15 January 2014 (UTC)
replies: text: You are right, but you can only see, so measure the interference when you have more photons. The result is that the wave of a single photon also interferes after a very long distance DParlevliet (talk) 08:33, 15 January 2014 (UTC)

text: A single photon does *not* have zero spectral width, due to the nonzero uncertainty in the photon's energy.  And quantum coherence effects can certainly be observed with a single photon--for example, we will never detect a photon at a double-slit null (even if we're not sure which peak it will land at).  Olawlor (talk) 00:39, 22 January 2014 (UTC)
replies: text: Thanks. I had a bad feeling about that paragraph, but my recollection of this material was too dim to immediately refute it.--Srleffler (talk) 02:28, 22 January 2014 (UTC)
replies: text:  Of course coherence effects can be observed. My edit mentioned that the coherence length was infinite, so interference of a single photon is possible over large distances. There is uncertainty during emitting a photon, and while measuring, but I don't think when it is a photon. But suppose it is, then the spectral width is very small. If I calculate the coherence length would be 7E7 meter. Still very large. DParlevliet (talk) 09:56, 22 January 2014 (UTC)
replies: text:  This is all linear optics, so single photons and beams act identically--a single photon can and will interfere with itself, in things as simple as an anti-reflective_coating.  But this interference just plain stops working if you move beyond the coherence length--this is why a thin film of oil on water shows color interference rings, but a thicker film is dull.  Folks actually do measure the frequency spectrum of individual photons using a Hong-Ou-Mandel interferometer to interfere it with itself; if each photon had zero spectral width (hence infinite coherence length), that bump would be a straight line at minimum amplitude.  (Caveat reader: this isn't my field, the last course I took on it was a dozen years ago, and my copy of Mandel and Wolf is at the office) Olawlor (talk) 03:23, 25 January 2014 (UTC)
replies: text: A thicker oil film is dull because it is absorbing photons or to much/fast variations in phase. Look to Feynmans example of glass, where interference happens even with thick glass. In the Hong-Ou-Mandel interferometer the photon does not interfere with itself, but with another photon (beam). So you can use it to measure the difference in wave length. The article also states that if both photons are identical, there will be a zero dip (but only when they differ 180 degree phase shift!).DParlevliet (talk) 16:40, 26 January 2014 (UTC)
thread_title:  single photon 

message: I moved the statement about an Hg line.  (I suspect 546nm, but didn't check the reference.)  It seems that the way to make it longer is to use isotopic pure .  We could also add some other sources and their coherence length. Gah4 (talk) 07:58, 5 April 2018 (UTC)
replies: text: It seems to be usual to give line widths in wavenumbers, that is, delta reciprocal wavelength. Ignoring small constant factors, that will then be the reciprocal of the coherence length. One source seems to give 0.03 cm-1 for natural Hg, which would be about 30cm coherence length. Gah4 (talk) 20:28, 5 April 2018 (UTC)
thread_title:  Hg