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Path: utzoo!watmath!clyde!floyd!harpo!ulysses!mhuxl!mhuxj!mhuxi!charm!mam
From: mam@charm.UUCP (Matthew Marcus)
Newsgroups: net.physics
Subject: Re: Nature of Photons
Message-ID: <291@charm.UUCP>
Date: Thu, 29-Mar-84 12:51:50 EST
Article-I.D.: charm.291
Posted: Thu Mar 29 12:51:50 1984
Date-Received: Fri, 30-Mar-84 02:57:42 EST
References: <694@ihuxp.UUCP>
Organization: Physics Research - AT&T Bell Labs MH
Lines: 36

ihuxp!b12958 (Pete S.) states that a cavity is not necessary for laser action
and points to the natural 'lasers' discovered in the Martian atmosphere as
example.  He goes on to say that on Earth we use cavities because you get a
bigger path length that way.
	Actually, if you want to get technical, a cavityless 'laser' is really
a 'superradiant device'.  The distinction is that a superradiant device
has the photons only going through once, and thus cannot be single-mode.
It's coherence, divergence, and monochromaticity are much reduced compared
to those of a true laser.  It has been proposed to build a signalling system
to reach the stars by putting huge mirrors in Mars orbit to create a true
laser.  The output power wouldn't be all that impressive, but think of the
narrow beam you'd get from km-diameter mirrors!
	As another example of superradiance vs. lasing, I built a nitrogen
'laser' for an experiment that never materialized.  Nitrogen has a huge gain,
but only sits in its upper laser level for several ns.  Because of this gain,
you get superradiance with a path length of only 6".  With no mirrors, the
output beam at 1m looked like:

            .....
  .........................
.............................
  ........................
            .....

as near as I can recall.  With a rear mirror, but none in front, I got:

	***
	***

showing the advantage of even 1 more pass through the active medium.  Because
the active medium is active only for a few ns, a front mirror is not very effective
at increasing output - the canonical thing here is a 20% reflector, giving
30% (?) more output.  If anybody's interested, I'll send you some references
on how to build this thing.
	Anyway, I hope I've cleared up the role of the cavity in lasing action.
		{BT}!charm!mam  "I whip out my laser pistol and blast 'em!"