Re: Building Lasers
From: Sam Goldwasser (sam_at_saul.cis.upenn.edu)
Date: 07/30/04
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Date: 30 Jul 2004 19:41:58 -0400
jimzotos@yahoo.com (Laserman) writes:
> Sam Goldwasser <sam@saul.cis.upenn.edu> wrote in message news:<6wiscatf92.fsf@saul.cis.upenn.edu>...
> > jimzotos@yahoo.com (Laserman) writes:
> >
> > > Hello all you laser gentle folk out there. A few years ago I was
> > > working for a laser company and they asked me to build this ring
> > > laser. This company had never built a ring laser before and the Phd
> > > assigned to the project had no idea how to do it. My supervisor
> > > handed me 4 lenses and said he wanted the beam 5 inches above the
> > > board. That's it! So I scoured the lab and assembled all the
> > > necessary ingredients and put it together. Next I had to worry about
> > > alignment. This ring laser was to be a double pumped nd:YAG with
> > > input about 30 watts from the power supply and output as much as I
> > > could tweak it. I relized very quickly that if I was to follow normal
> > > procedure for alignment that I might be there for days trying to set
> > > it straight so I carefully alligned the lenses BY EYE, turned on the
> > > power supply, double pumped the nd:YAG and started tweaking the output
> > > lens. I set up the Infrared detector and started looking for a sign.
> >
> > Lenses or mirrors?
> >
> > > nd:YAG puts out 1064 nm infrared cw. About 30 minutes of tweaking and
> > > I saw a green flash across the detector. IT WORKED!! I was so happy
> > > I yelled out, "I GOT A SPARK!" Everyone in the lab came over but the
> > > flash was gone. Ten minutes later I was able to sustain the spark a
> > > few seconds, I knew it was only a matter of time because I had faith
> > > in Einstiens theory of stimulated emission. A half hour later I had
> > > 1064 nm cw so I carefully tightened the lens (the other 3 lenses were
> > > screwed in solid)
> > > I brought the infrared detector closer to the output mirror and saw
> > > that I had 2 green dots. A few minutes of carefull consideration and
> > > I realized what was happening. There were 2 laser beams inside the
> > > optical cavity traveling in opposite directions because tha nd:YAG was
> > > being pumped from opposite sides! I measured both beams with a power
> >
> > Nothing to do with being double pumped (whatever you exactly mean by that!),
> > but rather that without some means of forcing a single direction usch as using
> > an optical isolator or Faraday rotator, there will naturally be two
> > beams traveling in opposite directions, and thus two output beams where
> > the output mirror is at a vertex of the ring.
> >
> > > meter and they were each about 7 watts. I put a power meter on one of
> > > the beams and blocked the other one with a laser mount, the goal being
> > > to "walk the beam around" so as to maximize the power output. I set
> > > up the power meter and started tweaking the lenses. At some point I
> > > wanted to change the position of the power meter because it was
> > > slightly burnt but the laser mount blocking the beam was in the way so
> > > I went around and with my right hand moved the mount that was blocking
> > > the other beam. I was not interfering with any part of the laser
> > > itself. WHAT DO YOU THINK HAPPENED?? If you are interested let me
> > > know. I'll explain in a few days but give you some time to think
> > > about it for now.
> >
> > Not sure what you're leading up to here but if your movement resulted in
> > a reflection back into the laser cavity, quite a lot could have happened.
> You are correct Sam, that is exactly what happened. Imagine my
> surprise when I saw the power meter change readings! I thought that
> the lens mount I was using to block one of the beams was doing just
> that, blocking the beam, but it was also creating feedback into the
> laser cavity. Now the end result of this experiment was that by
> creating this feedback you could minimize power on one of the beams
> and thereby the power output of the other beam was almost doubled and
> as an advantage the other beam is close to SLM (Single Longitudinal
> Mode)because the ring laser is good for that application. Now this
> company had a problem. When building the lasers you optimize the
> power with the laser cavity exposed and then when you think that you
> have tweaked it just right you screw the top on and it is ready for
> shipping. Every time the top was screwed on and then the laser tested
> it would lose about half a watt of power. Moreover, when inserting
> second harmonic and third harmonic generating ccrystals you want
> certain parts of the total beam not to pass through the laser cavity,
> so these other beams were diverted to strike the laser mounts instead
> of passing through the optics. Well to me it became clear what the
> problem was and how to solve it. When you screw the top on because
> of stresses and strains in the equipment the specular reflections off
> the laser mounts were interfering with the lasers dynamics. Now the
> interference is there to begin with but you optimize it before you
> close the laser. When you close the laser the interference has now
> become random which in general results in a loss of power. Most of
> the components of the laser were made of Aluminum. Apparently
> aluminum is highly reflective along the infrared (Can anyone confirm
> or deny this?) I told them that they should build the components out
> of materials that are highly absorbent to the particular wavelengths
> ( suggested ceramic but I'm not sure). You don't want radiation
> bouncing around randomly inside your laser cavity that's for damn
> sure. You would pick up this laser put it on another table and the
> power output would change. Ideally you want the light to pass through
> the optics and come out with as little interference as possible. Let
> me know what you think.
>
> Your's Truly,
> Laserman
>
> P.S. The nd:YAG is a cylindrical crystal(which I call a light
> sponge) about 2 inches long encased in a water cooled housing and I
> was pumping it with 2 laser diodes (from Coherent) on both sides of
> the circular bases. I think that what you are saying is that if I had
> it single pumped it would still produce 2 beams traveling in opposite
> directions? I tend to agree but I did not try single pump. As for
Yes, unless you have some device inside a ring cavity, it will produce
2 beams going in opposite directions. As you found out, this is not
a strong requirement and your accidental feedback was enough to
modify or suppress one of them.
> the optics they were three lenses totally reflective(within
> technological reason, after all, optics are lossy) and one was a
> partially transmitting lens, the output lens. One more thing, I have
But I assume you still really mean mirrors, but curved. :)
> been reading a little about something called Quantum Eraser and am
> wondering if I created something like that accidentally with the
> unwanted feedback. Something like a heavy duty interferometer? Let
> me know please.
Now you're talking heavy duty laser dynamics! A stray reflection back
into the cavity may create a second cavity which results in additional
constraints on the lasing modes. In the case of a ring laser all it
has to do is reduce the gain of one of the beams to result in a large
effect. I'm not sure what could be calculated about this without
more specific information on the setup.
Thanks for the fun. :-)
--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
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