Re: LASERS

[Herb Gottlieb <herbgottlieb@JUNO.COM>]

 Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU> writes:

n
> many times before they escape from the cavity
> through a semitransparent mirrow. The two mirrors
> must be highly parallel to each other.
In theory. the two end mirrors must be highly parallel to each other.
However, this condition is almost impossible to maintain in practice
because expansion and contraction of the mirror supports occur
as the laser is heated and cooled during normal operation.
Instead of using flat mirrors that are parallel to each other, modern
helium-neon lasers use concave mirrors spaced in a semi-confocal
arrangement.

Also it is important to add that the end mirrors are not the common
silver-backed cosmetic mirrors but are made of transparent glass with
thin film
quarter-wavelength front-surface dielectric coatings that reflect photons

of the desired wavelength and effectively absorb those of other
wavelengths.

For further information on the theory of operation of HeNe and VLD lasers
refer to the internet
        www.metrologic.com

Herb Gottlieb from New York City
(Where VLD lasers are making the HeNe lasers obsolete)