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Re: LASERS

I think it should be called "concentric" rather than "confocal", since the
mirrors have their center of curvatures in common, not their focal
lengths. If the laser cavity was con-focus, then the output-beam would be
a parallel beam superposed on a sphere-wave beam with origin at the center
of focus (probably not usable for, say, holography). If the mirrors have
a common center of curvature, then the output beam is a perfect
sphere-wave in theory. Add a lens to make it parallel.

Imagine a ruby ball with a reflective coating, then take a "core sample"
through the center of the ball so you end up with a ruby rod with
sphere-mirrors on the end.

The "semi" might refer to using one flat mirror and one curved mirror.
This lets the laser be half as long (basically, the flat mirror folds the
optics.) In the ruby sphere above, we could cut the rod in half and then
mirror-ize the surface of the cut.

Does anybody know why is the "semi-confocal" cavity
less sensitive to mechanical instabilities than the one in
which two mirrors are parallel? That was implied by Herb.

At one time I sat down and figured out what was happening. If you draw a
bunch of ray diagrams for a confocal laser with slight tilting of the
mirrors, you find that, as one mirror is tilted, there is yet a
cone-shaped shaft of rays which repeatedly reflects between the mirrors,
but its diameter is reduced and it doesn't fill the entire area of each
mirror. I assume this means that, as confocal mirrors become misaligned,
the diameter of the re-reflecting "virtual cavity" become smaller, and the
beam diameter becomes smaller. With parallel mirrors, a slight tilt would
make the re-reflecting rays wander to the mirror edge, and the lasing
action would cease. With curved mirrors, the same slight tilting would
simply reduce the beam diameter (I think!)

Don't forget these entries on my "misconceptions" page! :)

http://www.amasci.com/miscon/miscon4.html#phase
http://www.amasci.com/miscon/miscon4.html#lase
http://www.amasci.com/miscon/miscon4.html#coh

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William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://www.amasci.com
EE/programmer/sci-exhibits science projects, tesla, weird science
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