Dear Colleagues,

During the last few months I've been building a couple of laser systems; a
line with a Nd:YAG laser, and another one with a 25W CO2 laser.

Testing of the Nd:YAG system has been successful, and I'm nearly (note that
I say nearly) ready for routine operation.

The CO2 system, however, is giving me some problems, in the sense that I'm
breaking the BaF2 windows now and again.

Tests performed so far allow me to use about 25% power with no problem.
I've been firing the laser through the window for periods over 10 minutes
uninterrupted, with no problem. Both silicates (whole rock powders and Q)
and carbonates were burnt, although somewhat slowly compared to what I
remembered from well stablished, well tested lasers: it takes me minutes,
rather than seconds, to burn out a typical 1 mgr load.

Increasing the power to about 35% results in a broken BaF2 in a couple of
minutes; 50% power breaks the window in seconds, and powers above that,
almost instantaneously.

Some colleagues told me to defocus the beam to avoid breaking the BaF2. A
Vernier spacer in the beam path allows me for about 35 mm focusing range. I
have checked along this range, and no major differences exists as far as
the BaF2 window is concerned.

After noticing that the broken BaF2 windows were hot, I have contacted the
manufacturer (in our case, Crystran, from Dorset, U.K.), and to my
surprise, I was told that BaF2 windows are not advised for the CO2 laser
wavelenght, that they absorb, thereby heating up and breaking.

However, I know that most of you doing laser silicate work use just this,
and in fact, the BaF2 windows were manufactured by Crystran to the exact
specifications of those successfully used at SURRC.

Obviously, I have to be doing something wrong, but after a couple of months
"having fun" breaking the stuff, I cannot work out what.

So far (except for one ocasion I rather forget), all my tests have been on
"blank" reaction chambers (i.e., with no reagent -ClF3-). I hope you will
understand that I rather find out what is going on before I get "into the
real thing".

Anybody can offer some advice? I promish to check all suggestions!

Cheers & thanks,

Clemente



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Dr. Clemente Recio
Stable Isotope Laboratory
Fac. de Ciencias
Univ. de Salamanca
E 37008 - Salamanca
      Spain

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