Re: LASERS

[William Beaty <billb@ESKIMO.COM>]

On Mon, 3 May 1999, Herbert H Gottlieb wrote:

> The mirrors in lasers, such as those manufactured by the Metrologic laser
> company, are mounted in a semi-confocal arrangement, i.e., a flat mirror
> at the rear (anode) end of the laser tube, and a concave mirror at the
> front (cathode) where the laser beam emerges. If both mirrors were flat,
> as suggested by David Steyert,  the output power of the laser beam would
> be  greater. However, it is difficult to align flat mirrors so they are
> perfectly parallel to each other  and even more difficult to maintain
> their alignments under thermal stresses as the laser is heated and
> cooled, as well as mechanical stresses as the lasers are handled.

Here's a way to visualize the problems with parallel flat mirrors.  Place
two large, flat mirrors facing each other so an "infinite tunnel" illusion
is created. (Observe the "tunnel" by looking past the edge of one mirror.)
Now adjust one mirror and try to make that "tunnel" straight and
infinitely long.  It's impossible. No matter how perfectly we position the
mirror, the "tunnel" curves off into the distance.  Since the light within
a laser's resonator is moving in a straight line, it essentially is
travelling down the infinite tunnel.  If the tunnel is curved, then the
light will crash into the "walls" formed by the edges of the mirrors.

Since the laser medium only requires that the light take a certain number
of passes through it in order to attain positive gain, the light in our
"tunnel" need not travel an infinite distance.  To arrive at the lasing
threshold, we only need to adjust the mirrors to produce a "tunnel" which
is relatively straight over a certain large distance.

Does anyone remember those "Disco Mirrors" with the tiny light bulbs
around the edge, and the "infinite tunnel" illusion inside?  Those things
are an excellent model for a laser!  They lack only gain.  They give us a
view of a tunnel which is darker and darker as we gaze farther into its
depths.  If we could insert a block of optical-gain material between the
mirrors, then the "tunnel" would appear brighter in its depths.  Off in
the infinite distance, the far end of the tunnel would seem to contain an
infinitely bright star-like point.  Or, from another viewpoint, the
parallel-mirrors Laser would be emitting a plane-wave beam, and the lens
of our eyes would concentrate the beam to a dangerously small spot on our
retinas.

This "Disco Mirror" description helped me immensely in gaining a gut-level
understanding of the nature of lasers.  I hope that this is not just
because I thought of it myself!  :)  I find that sometimes I discover
things that help others understand physics, but other times I discover
things that help only me (and if they appeared in a book, they wouldn't
have nearly the impact that they do when I have the "aha!" experience
without outside help.)


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William J. Beaty                                  SCIENCE HOBBYIST website
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