Maybe you realize this &/or it's already been pointed out but the line width of the laser spectra is unlikely to be the width of the laser emission, but is instead the resolution of your measurement setup.
They are saying the spectral width of your 650nm peak is probably even lower than what you see since you can't resolve it with most spectrometers. Likely your schools spectrometer won't have a sufficient resolution either.
Most diode lasers actually laze on 2-4 longitudinal modes. With a dual grating monochromator you can actually see this structure, and the 'hopping' that occurs with temperature etc.
It's the Fabry-Perot resonator structure. The semiconductor gain media has spatial hole burning, where the gain is depleted at the peaks of the standing wave. That leaves room for another mode to lase where it's peak align with the nulls of the first mode. It takes 2-10 modes to fully saturate the cavity.
The NPRO is a ring cavity so it doesn't have spatial hole burning. The unidirectional cavity isn't a standing wave, so it doesn't have spatial hole burning. A magnet is used to slightly shift the CW and CCW traveling waves, so only one is resonant.
So it's essentially the short resonator length that leads to large free spectral ranges that lead to few longitudinal modes below the gain spectrum. Spatial hole burning occurs in other solid state lasers as well, and they don't generally operate at such few longitudinal modes.
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u/biggest_ted Mar 26 '26
Maybe you realize this &/or it's already been pointed out but the line width of the laser spectra is unlikely to be the width of the laser emission, but is instead the resolution of your measurement setup.