Talk:Helium–neon laser

(Older discussion)
I have added references and removed a lot of erroneous material from this article. In particular, the original article described the various electron configurations (2s, 3p, etc...) as states, and talked about transitions INTO the 1s, 2s, and other sub-valence levels. I have removed these incorrect statements and their accompanying figure, and replaced them with proper state descriptions for the 633nm line. I did not have time to look up which states are actually involved in the IR and other transitions, and encourage such additions in the future.

Please visit the NIST atomic spectra database if interested in finding real atomic transitions with appropriate labels. In particular, look up Ne 0, for the NIST energy for the HeNe transition, 2s22p5(2P1/2)3p - 2s22p5(2P1/2)5s).

I have also added scholarly articles and web links where appropriate. Future edits should perhaps move Sam's FAQ link to be featured earlier in the article, due to it's excellent, detailed description of HeNe lasers. Also, the article is getting long and may warrant some organizational editing. Az7997 19:08, 14 August 2006 (UTC)

The references are nice but they will be more appropriate for an article about stabilized lasers. It's on my to-do list if someone else doesn't beat me to it. Paul Koning
 * The references are for stabilized HeNe lasers, and are quite appropriate. The use of stabilized HeNe lasers is so common, mention in this article is warranted.  It is also important to note that HeNe lasers are used for stabilized sources, which considerably improve upon the (already narrow) HeNe frequency spread. Az7997 18:54, 9 October 2006 (UTC)
 * What I meant is that stabilized lasers are a large enough topic to justify an article of its own, in which case the references that talk about stabilized lasers would fit there. Paul Koning 23:00, 9 February 2007 (UTC)

Jan 2007 updates
I fixed what were, according to my references, some errors in the energy level nomenclature. This could be a case of differences between physicist and chemist notation, so let me know and we can do both. I also added some refs and firmed up some of the numbers on the laser construction itself as well as moved the pics around to line up and removed thumb sizing per MOS. --Patrick Berry 19:59, 30 January 2007 (UTC)

Stability
There are many causes of frequency error; temperature change is one of them. The recently added cross-reference to Frequency_drift isn't all that appropriate for "frequency stabilized" because the referenced article doesn't contain anything that applies to stabilized lasers. Paul Koning 21:47, 9 March 2007 (UTC)

The typical 633 nm wavelength red output of a HeNe laser actually has a much lower gain compared to other wavelengths..
The article states:

The typical 633 nm wavelength red output of a HeNe laser actually has a much lower gain compared to other wavelengths such as the 1.15 μm and 3.39 μm lines, but these can be suppressed by choosing cavity mirrors with optical coatings that reflect only the desired wavelengths.

If the red (633nm) emission has lower gain, why HeNe lasers, by default, emit red and not infra-red?

May be, it should be written

".., but these are usually suppressed by selective cavity mirrors.."

dima 00:04, 2 April 2007 (UTC)
 * Coated mirrors (dielectric mirrors) are "selective mirrors", they typically reflect well just around the desired wavelength. Even broadband dielectric mirrors reflect just perhaps 100 nm around the centre wavelength.--Danh 06:51, 3 April 2007 (UTC)

That explanation is correct. The wavelength of operation is dependent on the reflectivity of the mirrors. And the 3.39 micron transition is so strong that it needs to be actively suppressed in longer 633nm lasers.

One other problem though. The 3.39 has very high gain, but I do NOT think this is true for the 1.15 micron line. I think that reference should be removed in this context (unless I am really mistaken).

You are correct. Th gain of the 1.15 micron line is a few percent per meter. —Preceding unsigned comment added by 68.38.140.77 (talk) 20:05, 6 May 2011 (UTC) Interferometrist (talk) 12:34, 2 April 2010 (UTC)

Pressure a function of cavity length?
I'm puzzled by the assertion that the gas pressure depends on cavity length. It doesn't match what I read in my favorite reference (thesis by Schellekens on stabilized lasers) though admittedly he only talks about it briefly and specifically mentions optimal pressure for a particular cavity length. But the numbers he quotes are in the 260 to 400 Pa range for a 13 cm cavity, i.e., substantially lower. I'm also wondering what physical mechanism justifies having pressure proportional to the length. Finally, if that's really true, is it cavity length or discharge tube length? They are quite different for iodine-stabilized lasers. Paul Koning 21:12, 4 June 2007 (UTC)
 * From looking at the reference given, I would say that it is the discharge tube length as thats where the gas is. I think that one could probably find references for a range of 'optimal' pressures, each for different situations. --Chuck Sirloin 21:16, 4 June 2007 (UTC)

Gain bandwidth
The article states: "The gain bandwidth of the laser is dominated by Doppler broadening, and is quite narrow at around 1.5 GHz for the 633nm transition[5][7] lasing on a single longitudinal mode." I am not so sure that the phrase "lasing on a single longitudinal mode" belongs here, because the gain bandwidth should be about 1.5 GHz whether it is lasing on one mode or several. Or are you implying that mode competition will, in effect, change the gain bandwidth? It will turn the smooth gain profile into something else, but I was under the impression that this is not part of the usual definition of the gain bandwidth. Any comments? --JckS 01:40, 19 September 2007 (UTC)

You are absolutely right: the gain bandwidth (that is, the bandwidth of the gain medium) has nothing to do with whether the laser is single longitudinal mode (most He-Ne lasers are NOT) or whether it's even lasing, or for that matter whether it is even a laser: this just has to do with stimulated emission from neon. JckS is right, and I will remove those words. Interferometrist (talk) 12:27, 2 April 2010 (UTC)

Intentional wavelength variation
Can a HeNe laser's wavelength be intentionally modulated for transmitting a simple FM audio frequency signal? I'm looking at gas lasers because it cannot be done with diode lasers, or am I barking up the wrong tree? Roger (talk) 19:59, 7 October 2010 (UTC)

Removal of energy level diagram
I undid the removal of this diagram. If the diagram is indeed incorrect then it should be replaced by an accurate one which the text already refers to.

I am not a spectroscopist and thus I am not qualified to judge the matter. Yes, 1s sounds like the ground state according to my basic knowledge of orbitals, however spectroscopy is much more involved and you can also see a lower state labelled 1s which isn't the ground state in this diagram, found independently:

http://electron9.phys.utk.edu/optics421/modules/m7/images/laser_1.gif

Of course it's possible that both are in error (copied from one another). In that case 132.163.53.125 needs to find one that is correctly labelled. I'm not going to delve into it further, but I DO think that such a diagram is helpful and needed, and there are many versions of this figure around which could (given copyright permission) be used in its place if the 1s notation does not correctly apply to the lower (not ground) state.

Any spectroscopists out there? Interferometrist (talk) 18:16, 25 October 2010 (UTC)

Postscript: I found another diagram:

http://www.fou.uib.no/fd/1996/h/404001/IMG00023.GIF

This one lists the lower state at issue (at 17ev) as 3s, but that is even more confusing because the top level of the 633nm transition is also called 3s. Again, this is an issue for spectroscopists who understand these various notations in detail. Could it be that the levels of more than one electron are involved? Interferometrist (talk) 18:24, 25 October 2010 (UTC)


 * 132.163.53.125 probably has not noticed that the orbitals of Neon are given in Paschen notation, see http://technology.niagarac.on.ca/lasers/Chapter3.html . This is common and useful. I have now included the above link in the article. --Danh (talk) 08:18, 28 October 2010 (UTC)


 * Thank you VERY much! I looked at the webpage and the discrepancy regarding the notation is now clear to me. More than I needed to know in this regard, but at least I'll have a clue next time someone objects to a lower level (other than ground state) being called 1s. Note that someone has removed this figure two separate times, probably due to confusion over this matter in each case.Interferometrist (talk) 11:21, 28 October 2010 (UTC)

The energy levels in the right part of the diagram are wrong. Obvious from the fact that the 2p level is lower than the 2s level (it should be the other way round). 3s,2s,1s should be 5s,4s,3s whereas 3p,2p should be 4p,3p respectively. — Preceding unsigned comment added by 129.94.162.145 (talk) 00:54, 17 September 2013 (UTC)


 * As noticed two comments above the orbitals of Neon are given in Paschen notation, see http://archive.is/pwve . I've now added a note also to the caption of the diagram. I'm not objecting to a change of notation, if done consistently throughout the article. --Danh (talk) 21:48, 22 September 2013 (UTC)

History section
The history section, as presently written, has some very interesting discussion about modes of laser operation, but only the first two sentences and last sentence are actually about the history. Those sentences don't say very much, either. For example, our readers might be interested in who invented the thing. Some of the info about Ali Javan and William Bennett from here would be interesting, if proper sourced: http://inventors.about.com/od/ijstartinventors/p/Ali_Javan.htm I actually have photos of the first HeNe, which served as an ornament in Javan's office at MIT for some time before the Smithsonian took it, and could upload them if I can find them .... Spatrick99 (talk) 21:05, 27 March 2014 (UTC)