CO2 laser HR curvature, little confused, and some questions.

[Henry]

Hello everyone!

I’m in the build of a sealed CO2 laser and have gathered almost everything i need except for the OC and HR. The HR will be concave and the other flat to get TEM00 mode since i will use a small bore but i was wondering about the curvature of the HR.

Yes, i have read SAMs faq about this where it said that the HR should have a radius of curvature about twice the length of the laser which i was just pretty much only a mouse click away to order. But i thought that i had to check this up a little more first to be sure, since these mirrors are dam expensive, so i would know if there would be any gain in anyway to get one with wider curvature.

I then know that i have seen on more then one place on the net where its mentioned a curvature of anywhere from 5 - 10 meters even for a not that long laser at all and with a quite small bore, so now I’m confused.

The length of my laser isn’t really clear yet but say 800mm and a bore of 8mm ID. What i wonder is if there would be any difference in anything between a RoC of over 2 meters? What would happen if i would use say one at 5 meters or 10 meters? It feels that it has to do something or maybe it doesn’t? Im just so confused..


And btw, since the HR and OC will be dam expensive i would do everything i can to protect them from sputtering from the electrodes. Are there any disadvantage (power supply is not an issue) to use a 3 electrode arrangement so any sputtering would only occur in the center of the bore and thus protect the HR and OC from any type of sputtering? I guess the laser it self wouldn’t have any problems with this and will give a good beam profile and such just as if it was only 2 electrodes?

[mixedgas]

I have a set at home. I bought it for a home made one years ago and ended up with a commercial tube. I'll see what they are and get back to you. In the mean time, download PSST! and play with the resonator modeler click on "launch simulation" and input your mirror curvatures and distances. You need to calculate your resonator "stabilty factor" for a given mirror set.

PSST! does most of what you need except show the limits imposed by the bore diameter

http://www.st-andrews.ac.uk/~psst/so.../free_psst.zip

Resonator stability factor is section 2.3 of this download by Kogelnik and Lee..

http://www.google.com/url?sa=t&rct=j...0WXl-Q&cad=rja

For calculating the OC transmission, you need to know the gain constant... More on that tomorrow perhaps...

God Bless,

Steve

[turbohead]

When you state that the mirrors are "dam expensive" where are you buying your mirrors from? I have seen brand new HR and OC mirrors or eBay for less than $50.

Are sure you are building a sealed CO2 laser? I'm am curious what gas mixture you are using for a sealed CO2 laser. My understanding that the mixture is proprietary and involves some expensive exotic gasses like krypton. A flowing gas laser is less expensive from a gas mixture point of view.

[Henry]

Well its more that i see that a price of 5$ for a mirror would be more what i would like to pay since they are easily produced and I’m also very short of money, nothing else.

Oh I’m very sure that I’m building a sealed laser because of one major thing; the gas needed for a flow laser and i want it cheap.

Where i live premixed laser gas for flow lasers in even the smallest 10l bottle would go for over 600$ and then there is a rent cost of some 50$ a day or so since you cant buy the tube. This is WAY too expensive for just a fun thing so I’m going to mix it my self.

What gas proportion i will use will be something that i have seen on the net and even if it might not be the perfect ratio for this laser it shouldn’t be to far off, and i have seen something about that the ratio could differ up to 10%. The gases i will use will not be near analytic grade but better then at least 99.99% so i will just put in what i have found and if it will lase and i will get some watts out I’m fine with that.

Yes laser gases for sealed lasers can and normally will contain special expensive gases, this to get the efficiency of the laser up a bit more due to better cooling of the gas as i have understood it. I dont need that so its fine for me to mix it my self without any expensive gases.

[mixedgas]

O=rings wont do it for more then a few days. Indium seals....

They are not easily produced...

The classic catalyst is a nickle or gold thin film along the middle of the bore.

The addition of CO, Xe and H20 is a pain in the neck, as the mixes are not disclosed, and H20 attacks the optics.

Would it not just be easier to get the Asian Co2 tubes on Ebay ? Or is this a I'm up to the challenge thing?
Steve

[turbohead]

Have you seen this video:

http://www.youtube.com/watch?v=W_PwEy2FNUs&feature=plcp

It is amazing to me how he is able to build a working laser (with pretty good performance) with ordinary mirrors, helium from a party balloon tank, and CO2 from baking soda.

This is as cheap and ghetto as it gets!

[Henry]

Exactly and it works. I will go about the same approach although with a little cleaner gases but the helium i´m going to use i is actually from a tank to fill balloons. Those i got it from said it was 99.99% pure helium so that sounded good and even if it´s not not analytic grade so would it still be good enough.



O=rings wont do it for more then a few days. Indium seals....

I have a little smaller vacuum chamber but with a lot of o-rings on it for both the chamber, different valves and gauges and such and i pumped it down to 1 micron and now its a 26 Torr. The interesting part is that i pumped this down to 1 micron little over 1 year ago and have never had it opened since so o-rings hold quite good but only as long as the mating surfaces have a high quality shine and that the o-rings are baked. At around 15 Torr which would be about were it will test the laser, this vacuum would hold for more then 1 year and if the pressure after that time have gone up a little then i just fill it up and vacuum it down so will it hold some year again so I’m actually not that very concerned. I have also thought on make it all in glass but it´s just to much work


They are not easily produced...

You are sure right about that.


The classic catalyst is a nickle or gold thin film along the middle of the bore.

I have heard of that before and i will also try some nickle wire or so in the middle.


The addition of CO, Xe and H20 is a pain in the neck, as the mixes are not disclosed, and H20 attacks the optics.

True, but its very small amount needed so there won’t be any bigger harm but it’s absolutely something to check.


[I]Would it not just be easier to get the Asian Co2 tubes on Ebay? Or is this a I'm up to the challenge thing?

Where is the fun in that? Its not really for a challenge or so but i just wanted to build one since i actually saw videos from the person in the link above. Before this i had always thought that gas lasers were inherently complicated or so but as it turned out that wasn’t really the case after all.

[mixedgas]

Where is the fun in that? Its not really for a challenge or so but i just wanted to build one since i actually saw videos from the person in the link above. Before this i had always thought that gas lasers were inherently complicated or so but as it turned out that wasn’t really the case after all.[/QUOTE]


I think you need to make a distinction there. Sealed gas lasers are complicated. Flowing gas systems are easy.

RE the Youtube video, that is a design from a 1970's Scientific American home project. He shortened the tube, so he's doing perhaps 3-4 watts, when its a 6-10 watt design. Not much useful work you can do with less then 10.

Having something run on the bench vs something that has decent output power, beam quality, and long lifetime are TWO very different things. Flowing gas Co2 is easy to get going. Its the only easy one. Making a sealed Co2 that has any decent lifetime is different a issue. Rubber O-rings diffuse helium like mad. The classic 3 gas mix dies from poisoning in a few hours without a flush and refill. In the same tube length where that You-tube video has a few watts, I can get 40 watts, and have done so in the past.

In the 40 watt tube, I got Argon and Co2 with a trace of air to lase, at a watt or two. Simply because I had a tank of TIG welding gas and was out of mix. Lase, yes, lase with quality, no...

I do high vacuum and lasing for a living on a daily basis. I'm well aware that you can get a nasty vacuum to hang around for a year in a Dewer Flask or Bell Jar. Getting something useful to stay around is different. If your going to all the problems of making a Quasi sealed tube, WHY NOT take some professional advice and take it to the next step? Its what, another hour on a lathe to fabricate the indium seals? A day or two of learning to fab the tungsten to 7740 electrode seals? The next logical step is a better sealed system, not a toy.

Hochuli is the guy who did much of the development work on simple sealed Co2s. Perhaps you should read his writings and patents...

Steve

[Henry]

Well this is an experiment to just test and se, the laser will not be built for any practical use what so ever so if the laserbeam would be of bad quality like huge diversion or so i wouldn´t be of any concern at all.

I ofcourse didnt thought about the much higher concentration of helium needed in a CO2 laser compare to ordinary air and thus much more sensitive to helium leaking, thats true but i would use Viton not buna-n but never the less the leakrate is absolutely a concern for a longer seal.

If i didn´t would go on making a flow laser wih o-rings (and a extremely low flow) a plan was otherwise to exclude all o-rings and instead actually softsolder glass to glass and where needed glass to metal by deposit a layer of copper on chromium. Copper only could work as well but copper doesn’t adhere very well to glass but chromium does and copper adhere well to chromium, and then softsolder the glass to each other with silver softsolder to get a physical stronger seal then with lead-tin solder.

Haven’t tried it out yet and experiments will of course be tested first. Just as long as the flux used is thoroughly cleaned off so should it pretty much be as tight as a fused glass to glass joint becasue the leakrate of helium through a metal joint is extremely low. It should only be constructed in such way that the solder joints are not exposed to any plasma in anyway so should it work quite well as i see it but its just a question to construct it right from the beginning to avoid anything like that.

For the electrodes i had planned to use neon sign electrodes and putting them on the side of the tube so they can’t sputter the optics in any way and maybe even a 3-electrode setup to lessen the chance for this even more but i don’t think that is needed if you don’t have the electrodes in the bore so i think i will go with only two.



But beside of all this so if anyone knows i would very much like some input about my first question; will there be any disadvantage to having a HR with a radius of curvature of 5 or even 10 meters for only a 1-meter long tube? What i have just read so does it seem that you could get a little better use of the plasma to generate beam since more of the gas in the periphery is used instead of only the center, but i would like this to be confirmed so i know and could order one asap.

[mixedgas]

Run PSST!, enter the cavity dimensions and radius of the mirrors, and switch to the stability diagram, If the white dot is near the edges of the blue curves, you will find it hard to align and possibly low power.

Technically you have enough radius to lase, the other question is, with such a nearly plano-plano resonator, will you clip the edges of the inside of the tube, resulting in huge diffraction losses. PSST will tell you that too.

Steve