Laser tube output window and dirt or coating problem.

[Mike Lysov]

Hi guys,

Does anybody knows what this part a standard laser tube called?

http://i168.photobucket.com/albums/u180/coasterdynamix/IMAG0636.jpg

It is not my tube picture, just found a sample picture to show what I am talking about.

It is an output window of a laser tube. I have been trying to find diagrams that explains all parts of CO2 laser tubes designs and some of them say it is a mirror.
To me it looks like a lens. Anyway I just need to know its name to explain my problem so I will call it lens below.

One day I found out that there was some residue from smoke left on the lens of one of my laser tubes. I may be responsible for this to happen to my tube because I have been covering the tube output window with the wrong material while I have been trying to align optical path of two tubes.

I did not know about all this residue from the beginning so I was running my laser for at least 300-400 hours before I discovered it. After I found out that the lens was dirty it was very hard to reach it for cleaning it because of the tube position but I managed to clean it with alcohol.

I thought I removed all residue except of a couple of small black dots. So yesterday I moved the tube back a bit, removed the gold colored ring from the tube end and cleaned the lens again. I could remove these two black dots and it looked quite clean but when I inspected the lens with a flashlight I could see there was something wrong. It looks like lens coating burnt showing some spots with rainbow colours. The second tube I have installed in my laser has nothing like this with its lens seems to be completely clean and not showing any other colours on its lens coating.

I do not know if its on outside or on inside side of the lens and I am not even sure if that's a coating problem or lens is dirty from the inside but I believe this problem affects cutting at the shortest distances laser beam traveling from the tubes. The kerf is quite thick there. No problems with cutting if the beam travels extra 30cm or so. It seems it affects the top left corner(the shortest beam path) and a 30cm square area from the top left corner. After that area I have a nice thin kerf.

I do not know if this problem with the lens will shorten the tube life or it just affects the quality of the beam. I was wondering if it is possible to remove the lens assembly from the tube and try to clean the lens from the inside. If it is possible I will either be able to clean it or I will at least know for sure that I have problems with the lens coating.

The tube I have is a GSI 140W model and it looks similar that the one shown on the picture below, The only difference that the ring on my tube is in gold colour. The one on the picture has a silver colored ring.
http://image.made-in-china.com/43f34j00HKuEZvPrJhqT/2013-Hot-140W-Laser-Tube-Water-Cooling-for-CO2-Laser-Cutting-Engraving-Machine.jpg

And on this picture you can clearly see what type of lens I am talking about.

If I can remove it and it happens to be a problem with the lens coating can this part be replaceable by a consumer or it is kind of job can be done by manufacturer only?
If I can do it by myself what are steps involved in order to remove this assembly and to put it back? These four screws look like they are responsible for beam alignment.

[Dan Hintz]

If you see a rainbow after cleaning, it's one of two things... it's either an anti-reflective coating, or you still have some residue left to clean off and the alcohol swab just thinned it out and made a rainbow. Try lightly cleaning again (or twice).

[Mike Lysov]

If you see a rainbow after cleaning, it's one of two things... it's either an anti-reflective coating, or you still have some residue left to clean off and the alcohol swab just thinned it out and made a rainbow. Try lightly cleaning again (or twice).

Thanks for your reply Dan.
I have tried it a few times but the rainbow colours are still there and they are not moving after each cleaning which would have happened if it had to be a dried alcohol/dirt mix. Nothing like this showing after I have done the same cleaning on the other tube. It did not have any smoke residue but it had some dust so I decided to clean it as well. It is perfectly clean with no rainbow showing.

You do not think it is coating problem, do you? This smoke stayed on the lens for a long time. I did not even think I could get some smoke stick to that lens when I covered the tube during alignment. And it is really hard to say whether the dirt/coating problem or whatever it is is on inside or outside of the lens but it looks like it is more of inside problem of the lens.

By "inside" I mean the other side I cannot reach.

[Dave Sheldrake]

You have Halo'd the lens Mike, I did it the other day on a final lens (GaAs) that was dirty then got hit by 180 watts of laser beam (see picture below) :)


It is an output window of a laser tube. I have been trying to find diagrams that explains all parts of CO2 laser tubes designs and some of them say it is a mirror.

It's both, it's an output coupler, a percentage of the beam is reflected back into the tube and a percentage is released to go forwards (the useful beam) that is collimated by the lens.


You do not think it is coating problem, do you?

Yup I do, some couplers are Gallium Arsenide, the halo effect is most noticeable on those compared to Ge or ZnSe, I'd suggest your tube has limited life based on the halo, that could be an hour, it could be a year, it's impossible to say without some exact figures you won't be able to provide.

You end up with something like this effect?

271411

cheers

Dave

[Mike Lysov]

Thank you Dave.

Mine is not as bad as yours shown on the picture and it does not have anything close to that spiral effect, just small spots in different colours which are covering 60-80% of the coupler surface in the middle of it. I would not even call it a halo, it is more like "chicken pox" in its pattern.
It was stupid for me not to take a picture of it. I cannot do it now because the only way I can have some access to it is to move the tube back. If I do it and move the tube back again to its original position after that I have to realign everything again.

But even with the tube moved back and the gold ring removed it is almost impossible to see that coating problem unless you point some bright light at the lens at some specific angle.
It is bad for me that it will affect the tube life.

Is it possible to replace the coupler without shipping the tube to JKLaser? The tube is brand new, with running time 600 hours maximum may be even less. The counter that shows tubes running time in the laser panel has been reset twice so I do not know exactly how many hours they have been fired in total.

[Dave Sheldrake]

If it's a GSI Mike speak to GSI UK about it, they are pretty good for rep[airing tubes and even though their tubes are expensive, their repairs are pretty good :)

I wouldn't remove the coupler, there is a chance you could release or contaminate the gas.

cheers

Dave