I recently used a Michelson interferometer the find the spectrum of a helium neon laser. Initially, I moved the mirror $9\times10^{-6}m$ and the spectrum was as expected, single peak at approx. 633nm.

Then I used the exact same setup but moved the mirror over $2\times10^{-4}m$. The resulting spectrum (image below) was much different, it had three major peaks and two smaller peaks, none of which were at 633nm. In fact, the count at 633nm is almost 0. The interferogram I get is sinusoidal with not a lot of variance.

The spectrum

I would have expected a larger data set to increase spectral resolution and decrease the width of my peak.

My question: Is this the effect of the laser itself, the method of data acquisition of something to do with the mathematics of the Fourier transform?

Please let me know if I need to upload setup/interferograms or give any other information.


1 Answer 1


Depending on the cavity this could be separate longitudinal modes of the laser which you did not spectrally resolve before. Basically a laser is described as "monochromatic" but as you can see here, even the individual lines of the peaks are not monochromatic, and certain cavities can carry several modes with an equal spectral spacing, the envelope these peaks form are shaped according to the gain mediums gain curve.

The distance between them is, if my memory doesn't fail me, called free spectral range (FSR).

Maybe this source can give you a little bit more information. The equations are not hard to derive.

  • $\begingroup$ Thanks for the reply. So what I'm understanding from this is that I have indeed been able to increase the spectral resolution and am now seeing wavelengths emerging that I could not see before. I've read up on longitudinal modes and the free spectral range as you suggested. I'm now only left asking why there is no peak at 633. I'm taking a wild stab saying that for some reason the spectrum has been shifted sideways? And even if that middle peak was the 633nm line I should see, shouldn't it still be dominant? $\endgroup$ Commented Oct 25, 2019 at 14:44
  • $\begingroup$ It is possible that that peak is dominant but for sampling reasons was not the dominant one here, the spectrum may indeed have been shifted for some other reason such as temperature. It is also possible that you measured the same bandwidth before, but you did not have high enough resolution and therefore didn't resolve the individual peaks. @DavidForde $\endgroup$
    – DakkVader
    Commented Oct 25, 2019 at 16:46

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