Re: green HeNe lasers

[Herbert H Gottlieb <herbgottlieb@JUNO.COM>]

Kathy Daniels asked how a HeNe laser can produce green light.
Dean Baird is generally correct in his response which I'm reprinting,
followed by some additional details that I'm adding for those interested.

> HeNe produced red (primarily), yellow, and green (to lesser extents).
> Usually, the end mirror filters out all but the red. The expense of the
> greenies is the mirror (dielectric depositing, etc.) to filter out all
but the
> green. Same with yellow. They must also start out with more power, since

> most of the energy goes into producing the red.
<P>Dean Baird, Rio Americano HS, Sacramento

In a HeNe laser the helium energizes the neon which emits photons as
its energized electrons decay to the ground level.  These photons produce
intense lines of infrared and red light and less intense line spectra of
yellow, green, and blue.
All of these lines can be easily observed in a classical spectrograph of
neon.

The end mirrors of the laser tube consist of thin film reflectors of
titanium dioxide and silicon dioxide rather than silvered mirrors. A
transparent thin film will reflect some of the incident light from the
top surface and some from its bottom surface.   If the thickness of the
film is one-quarter the wavelength of green light  (1/4 of 543
nanometers) the reflections from the top and bottom of the thin film will
interfere constructively.  This is true because there is an immediate
reflection (and phase inversion) from the top surface of the titanium
dioxide film. The second reflection from the bottom surface of the film
occurs without a phase inversion. However, it is  is delayed  in arriving
back at the top surface by a half wavelength (one quarter wavelength down
and another quarter of a wavelength back). Therefore with a delay of a
half wavelength coupled with a phase inversion The two reflections are in
phase with each other and  provide constructive interference.

 To produce a HeNe laser that emits green light, there must be not only
be thin film layers to cause constructive interference of the green light
 but there must also be additional layers to cause destructive
interference of the stronger red light .    This is done by adding thin
film layers of TiO2 and SiO2 whose thickness is equal to half the
wavelength of  red light (1/2 of 633 nanometers).

The actual thickness of  each thin-film layer that is deposited on the
end mirrors depends primarily on the wavelength of light that is to be
reinforced or killed. However the index of refraction of the tritanium
dioxide and the silicon dioxide must also be
considered because light travels slower in these materials than it does
in air.

Whereas the output power of a typical red HeNe laser used in schools is
about
a half milliwatt to three milliwatts, the output power of a Metrologic
HeNe green laser is only about 0.04 to 0.05 milliwatt.  The green beam
appears to be much brighter than that because our eyes are very sensitive
to the wavelength of green light.

Herb Gottlieb  from New York City
(Where the thin film of oil on our roads and streets reflects beautiful
colors after a typical April shower)