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Re: green HeNe lasers


Does the reflected green light then stimulate more green light emission,
thereby actually altering the emission spectrum of the neon somewhat?



Exactly. The green light is fed back into the laser cavity, and is then
amplified by further stimulated emission. That is, the green photons stimulate
the emission of more green photons. The laser doesn't lase red because that
wavelength isn't fed back into the cavity. The emission spectrum of neon isn't
modified "somewhat," but considerably!

A fun experiment is to view a HeNe laser discharge tube from the side, through
a diffraction grating. You'll see the green and red Ne lines (on either a
green HeNe or a red one) as well as the helium emission lines. I used to have
students view the HeNe discharge mixture, and then a pure Ne discharge, as ask
which gas produces the red output beam. From comparing the spectra, you can
conclude that the neon is doing the lasing.

JEG

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John E. Gastineau john@gastineau.org KC8IEW
900 B Ridgeway Ave. http://gastineau.home.mindspring.com
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