# Multiple wavelength peaks in HeNe spectrum from FTIR

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.

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.