ADVANCED question, co2 resonator polorization interference

OK, big question. I work on multi KW CO2 LASER cutting tools. I have found in some instances of an unstable mode. Normally my mode is a doughnut shape, little power in the center giving a return spike on a mode shot done in lexan. When power is checked, 2500W for 20sec on a carbonized power meter I can see the glow of the mode and it is useually noce and stable. I have seen now 2 systems where the cut shows signs of mode instability where in a crystalin(sp?) edge shows sparatic 'chatter'. When this happins I nave noticed when 'viewing' the mode on the power meter, I can see 1 quadrent of the mode flickering (this is right at the output of the resonator). I am guessing that since the beam is 'S' polorized within the resonator, there might be some interferance patter being set up in the plasma stream.

OK, OK, I know UBBER teckie question, I have asked some of the field support poeple and even fome of the factory guys this one, but all I get is answers like, change the output mirror, or check gas preasure. I have changed the output optic and the problem is 95% gone, but I really would like a better answer. I looked around yahoo, but this is such a obscure question in a small community of high power CO2 people, that answers are scarce.

If anyone has an idea or knows someone that might know, any help would be helpful.

Jeff

For a tech question like this, try the alt.asers newsgroup, and do a search first of sam's laser faq and the newsgroup.

thanks dr lava, thats what I was thinking. i know there are some CO2 people here that would like a challange.
Jeff

It definitely looks like a problem with your Oscillation Overthruster or Flux Capacitor.

Sorry, this one begged a Buckaroo Bonzai reference.

I really have no idea but I wish you best luck on getting your problem solved.

Any contamination or damage to the cavity optics can cause mode hopping... basically if you have a small area of damage on the optic, it will force the cavity to lase with a different mode due to the reduction in effeciency in that part of the beam. Remember that modes are symmetrical, so if you have a loss on one side of the optic, it can force a mode which will cease lasing on the side of the optic opposite from the damage as well. If the damage or contamination is slight enough that it doesn't totally disrupt lasing, you can end up with a metastable condition in which the beam will pop back and forth between modes when small instabilities in the plasma provide enough gain in the damage area to overcome the mode that has been forced by the reduced efficiency in that area.

YAG... YOU DA MAN.

On the system I was working on thhe PR or OC had 1800Hr on it (recommened to change at 1500Hr). It had a light haze on it, I have seen this haze before but it didn't cause a problem. I changed the PR and the problem went away. I was told that this fixed the one other system that showed that problem. It makes sence in all respects, and totally explaines it. THANKS!!!:rock:

If you dont mind, I will share your answer with the others in my circle.

Jeff

Feel free, glad I could help.

CO2 lasers are very forgiving... you can get quite a bit of contamination in the cavity before they start showing any signs.

How much did the output power come up for the same drive power after you changed the OC? How does the HR look?

Funny enough all the other optics looked good. The beam path is folded in the plasma field in a 'Z' pattern and has the 'S' polorizer and the HR at the back. Oddly, the power was good, actually the cal was WAY off and with a call of 3Kw I was reading 3.7Kw (it is only a 3.5Kw resonator). AND the mode in lexan was good, both cold and after 45secs @3Kw. Useually when there is a problem with the OC there is thermal lensing there. This showed none. Although the mode was a bit more defined after I changed the PR, it was still good before.

And yes, CO2's are by far the most forgiving. I worked at Lumonics on 500-4.5Kw multi-chamber YAG's, and those could be a pain in the arse, especially when you have 4 VERY moody rods in series (coupling and all).

I never got any responce including mode hopping, heck, I have not even heard the term in years... But it sums up the problem perfectally. Now all I need to figure how 'S' polorization works. I know that it works with a bend mirror that is a phase retarder that blocks back reflection off of reflective material, ie brass, copper, and sometimes stainless (depending on cutting conditions), but I cant understand non-linear polorization.

What is your background in the LASER field?

Jeff

I have been a tech in laser entertainment, and a hobby guy for non entertainment lasers for about 12 years... have worked on and played with a lot of different stuff over the years including big CO2.

S and P polarization are just different ways to say horizontal and vertical polarization. I'm not really sure how it would block back reflection except for maybe rejecting some of the reflected light due to it being randomly polarized? http://en.wikipedia.org/wiki/Polarization Maybe there is some info there that can help you with that one... I'm really not sure.