William Beaty wrote:
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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.
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Which caused me to disagree:
My recollection was that resonators of many different types have a role
in lasers. Without wanting to solve problems by appeal to authority, I
will say that a check of my favorite laser book (Principles of Lasers,
by Svelto) confirmed my recollection:
"The disadvantages of a concentric resonator are that:(i) it produces a
very small spot size at the resonator center, which can be a problem for
high-power lasers. [The center of the ruby ball in William's post would
suffer hot spot damage p.d.q. (DWS)](ii) it is rather sensitive to
mirror misalignment. . . . Confocal resonators, on the other hand,
typically give a spot size that is too small for effective use of all
the available cross section of the laser medium. Plane-parallel
resonators can make good use of the cross section. Like concentric
resonators, however,they are rather sensitive to mirror misalingment.
For the various reasons discussed above, the most commonly used laser
resonators make use of either two concave mirrors of large radius of
curvature (say from two to ten times the resonator length) or a plane
mirror and a concave mirror of large radius."
In point of fact, unstable resonators are used for some lasers
(especially the high-gain ones like N2, KrF, etc.) In those cases, the
big goal is to get the light spread out over as much of the population
inversion as possible, because getting a lot of cavity-round-trips out
of the light is not a big concern: getting intensity out is.
Svelto goes into a lot of treatment finding the standing waves that will
survive in a stable cavity, and I suspect that the reason ray diagrams
can mislead us is that we have to consider the wave nature of light, if
not while it's interacting with the medium at least while it's bouncing
around in our cavity.
Hope this helps...
David.
--
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Dr. David W. Steyert steyert_dw@mercer.edu
Department of Chemistry (912)-752-4173
Mercer University
Macon, GA 31207
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