I was playing around with a cheap diffraction grating and my set of laser pointers, and I noticed that while the red and the blue pointers produce a single point in the spectrum, my green laser produces three:

        enter image description here       enter image description here       enter image description here

I've googled for green laser pointer spectra and the images I find seem to suggest that I should really see a single peak, not three. This answer also talks about a single frequency. Keep in mind that these are super-cheap lasers, I paid about $20 total for the three.

So is my green laser emitting multiple frequencies, or this some artifact of the diffraction grating I use? If it's really emitting three peaks, what causes this? From my (very basic) understanding of lasers, I thought this isn't supposed to be possible.

P.S. Here's the spectrum of a white LED flashlight taken using the same grating, in case it's relevant to judging my method:

        enter image description here

  • $\begingroup$ That's moderately convincing, but the question is "Why?". And we do have some laser guys among our regulars. $\endgroup$ – dmckee --- ex-moderator kitten Apr 2 '15 at 0:14
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    $\begingroup$ More than one frequency is possible with laser. It's called multi-mode laser. Green laser is probably in fact infrared and frequency doubled. But still I don't have a clue as to why there are three distinct frequencies. Non-linear processes wildly depend on power so there should not be just a bit smaller intensity in the additional beams. It should be orders of magnitude smaller. $\endgroup$ – Jarosław Komar Apr 2 '15 at 7:25
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    $\begingroup$ What a great question and great experimental data to put on this site! This would be great material to put in a first course on laser physics. $\endgroup$ – Selene Routley Apr 2 '15 at 9:03
  • $\begingroup$ +1: Nice question. Does the relative spacing make sense? I.E. does the angular spacing between the three green dots fit within the angular spacing of the green section of your white light picture? $\endgroup$ – Chris Mueller Apr 3 '15 at 15:48
  • $\begingroup$ @ChrisMueller yes, it does; the green dots are very close together compared to the angular distance between the green and the red or blue. $\endgroup$ – Roman Starkov Apr 5 '15 at 16:18

Given that most green pointers are frequency doubled from a 281.8 THz infrared laser ($c$/1064 nm), it's possible that you have a two frequencies $f_1$ and $f_2$ in the original infrared laser (i.e., it is multimode). After passing through the "frequency doubling" nonlinear crystal you see three frequencies: $2 f_1$, $2f_2$, and $f_1 + f_2$.

It looks like your relative intensities are $I(2f_1) < I_(f_1 + f_2) < I(2f_2)$. This means that in the original laser, the original $f_2$ beam was stronger, or at least better concentrated, than the $f_1$ beam.

  • $\begingroup$ +1 Yes, I think that this is almost certainly the answer. $\endgroup$ – Selene Routley Apr 2 '15 at 8:02
  • $\begingroup$ Thanks! The equal spacing between the three dots didn't seem accidental, and this answer explains well why they're equal. With this new keyword I found this video (note, it's a download) showing the spectrum of a laser as the current is increased. It shows multiple peaks at some currents and a single peak at others – fascinating! $\endgroup$ – Roman Starkov Apr 2 '15 at 13:21
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    $\begingroup$ @romkyns This is very typical of any cheap laser. Lasers are cantankerous beasts and the steam engines of optical technology: they are clunky, complicated and pretty much the source of all headaches in any optical project I have worked on. Any laser cavity will support many modes and the trick then is to suppress all but one. Most lasers either have at least a few modes or "hop" chaotically between modes. Someday quantum dot technology might yield lasers that aren't so nonideal. $\endgroup$ – Selene Routley Apr 4 '15 at 1:54

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