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Originally I was going to put "laser" in the title, but didn't want to restrict wavelength (e.g. masers). I did put "beam" in the title, since there may be highly stable oscillators that are also quite isolated, and I'd like to exclude those for this question.

In this Wikipedia article on Coherence Length it mentions a fiber laser with a coherence length of the order of 100km (yes 100,000 meters, not a typo). The article mentions a bandwidth of a few kHz and that number does get you to that length, and fiber lasers certainly can be collimated to form beautiful beams.

But there is no citation for this measurement in the article as of this moment, and I'm surprised because most laser-related articles used to be curated fairly carefully.

My question "What is the longest coherence length beam demonstrated?" also has the word "demonstrated". Of course it could be a coherence time, or a frequency spread converted to length. They are all roughly the same thing. No not exactly, and please don't open up a long discussion here. If the spread in frequencies has some characteristic width that can be defined in some way, then so will the wavelengths, in this context.

I added "stimulated emission" to the title to be sure to exclude something like a radio transmitter with directional antenna, or it's (in the not so distant future) optical nanoantenna equivalent. I'm looking for beams based on quantum transitions within naturally occurring systems. Thanks!

I thought I remembered seeing HeNe coherence lengths of 1km in catalogues, but now all I see are the 100m values as mentioned in the article. What got me thinking about this is the mention of coherence in this answer.

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  • $\begingroup$ On some level this isn't a very well defined question. The coherence length is proportional to wavelength. The best atomic clocks, at the moment, seem to achieve a relative uncertainty of approx. one part in $10^{16}$. If we would mode-lock a laser to such a clock, then the coherence length would be on the order of $10^{16}$ wavelengths, i.e. somewhere in the region of $10^9$m for visible light. $\endgroup$ – CuriousOne Jul 26 '16 at 4:07
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    $\begingroup$ @CuriousOne It's a well defined question. You just don't like it because you would ask a different question. It's a beam, and its from stimulated emission, but has your "mode lock a laser to a clock" ever been demonstrated to such coherence length. Also you need to be very careful not to mix stability with coherence. That would be a nice - and different - question. That laser could be mode locked, but you haven't talked about the coherence length of the hypothetical laser. $\endgroup$ – uhoh Jul 26 '16 at 5:21
  • $\begingroup$ I've deleted some comments and I have edited those that I left. Attempts to control the behavior of other users—outside of asking them to "Be nice."—are out of line. The purpose of the comments is to improve questions. It's not to force changes down anyone throat and it is not to tell other users how to answer. $\endgroup$ – dmckee♦ Jul 27 '16 at 3:15
  • $\begingroup$ @dmckee Thanks for your interest - I'm here to understand physics better, and maybe some day try to answer a few questions if I can. To me every stackexchange environment is different - it takes some time to get a "feel for the room" each time. You are welcome to look here also. $\endgroup$ – uhoh Jul 27 '16 at 3:23
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I think the coherence length depend on the power that you want from that laser, the frequency profile of it and the loss in your optical fiber. Also you can use repeaters to increase the coherence length. Usually in laser industry to increase the coherence length people using the brag grating. It could help us having single frequency with long coherence length in vacuum but as it decrease the power significantly it's not always welcome by laser industry for the optical fibers.

For long distance quantum communication it is better to use entangled photons sources. If you want to know more about the quantum coherency take a look at this article and some of it's references:

https://arxiv.org/abs/0710.1143

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