David Steyert, William Beaty, Kowalski and others question the use of the
term
"confocal" with reference to the distances between the mirrors at the
ends of
a HeNe laser tube.
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.
At the sacrifice of some power, the semi-confocal arrangement features a
non-critical alignment that provides greater stability. The concave
mirror (at the front of the laser tube) focuses light into a cone with
its apex slightly beyond the plane of the flat mirror at the rear end.
As the reflected light returns to the front of the laser tube, it
converges and then diverges, effectively retracing the incident ray path.
Then about 99% of the light is reflected back again to the flat mirror at
the rear to repeat the sequence.
However the remaining perfect of the diverging light is transmitted and
refracted by the glass of the curved mirror to produce an emergent laser
beam whose edges are almost parallel.
For further information and a diagram of the optical paths within the
laser tube refer to the Metrologic Laser Handbook or the Educational
Laser Theory Section of the computer web site ..........
www.metrologic.com
Herb Gottlieb from New York City
(Where parallel mirrors are not always practical)