Laser power output question

[dwessels]

I am trying to figure out where to set the limit on my DSP controller for the laser power output.
I have a 40w tube and PS from CNColetech. They listed the specs as follows:

Power output(W) 40
Triggering voltage(KV) 22
Operating voltage(KV) 15
Operating current(mA) 18
Power stability ±5%

I have the laser finally up and running and am trying to work out what to set the current limit (power output) to.
I have the Lightobject mA digital panel meter inline. In manual mode (using a 5k pot) when I run the laser, I see upwards of 25mA. With the DSP set to 60% max, I am still seeing 21-22mA.

What should I limit the laser output to?

Thanks.

[J45on]

I will have a look at mine later I have the same parts

EDIT I have mine on 48% which gives me 18ma on my digital meter

[dwessels]

So does that mean that because the spec say 18mA operating current, I should limit it so that it doesn't exceed 18mA?

Does this mean that 18mA = 40w laser output

If you overdrive it, do you get any additional output? Or does it just kill the tube?

Thanks again for everyones help.

[J45on]

Yes you should limit your current to the 18mA specified
18mA should be 40w but without a power meter you will never know
If you overdrive the tube I am unsure if you will get more power but it will definitely shorten the life span of the tube

[dwessels]

Yes you should limit your current to the 18mA specified
18mA should be 40w but without a power meter you will never know
If you overdrive the tube I am unsure if you will get more power but it will definitely shorten the life span of the tube

I finally heard back from CNColetech and they said that the laser should be limited at 18-20mA.
What does everyone think, should I set my limit to 20mA? I wonder if that would give me the max power out of the tube?

Thoughts anyone...

[lasersafe1]

I have a C02 power meter, so I don't have this question. On my laser, I find the 40W power supply is already well matched to the tube, so 100% to the power supply gets me about 35 watts. It did not reach 35 W any earlier. If I had a 60W power supply with my 40W tube, it would be a different story.
So how would I test it without a power meter? I think I would use a book. You can set your laser to do a sweep across a book at some constant speed firing at 50% power, then move down a cm and run it again at 60%, etc. etc. When you are done, open up the book and see how many pages are burned through. If you find that it rolls off to a constant depth at something lower than your 100% setting, then there is no need to ever go higher.
Does this make sense? Perhaps the 40W tube really would put out more, but give a much shorter life, so perhaps I'm wrong. Still, If you have a brand new laser tube and you are happy with the setup, you can do this book test and hold onto it for many months. If you ever suspect your tube is losing power, you can whip out your book and test it.

[r691175002]

Full spectrum engineering recommends running the tubes at 15mA because they did not find a material gain in power from 15 to 18.

The more current you run the shorter the tubes life will be, power will not increase linearly with current. Technically just letting the tube sit around will also reduce power as the helium leaks through the glass and epoxy (which is why helium balloons are made of metal, the gas seeps through almost everything). Technically if you stick it in a box of 100% helium for a few hours it will diffuse enough back in to undo the damage.


The tubes are 200$ imports so the 40W number probably has nothing to do with actual tube output so if you are getting more than 30 I'd imagine you've done pretty well.

[lasersafe1]

I know helium quite well since I worked as an engineer in the cryogenics group at a national lab that has the worlds largest 4 degree Kelvin liquid helium refrigerator. Helium will diffuse just like any other gas from an area of higher pressure to an area of lower pressure, but it will NOT diffuse at any appreciable rate through glass or most epoxies unless it has become a superfluid at 2 degrees Kelvin. At this point, the Bose-Einstien condensate will flow through the pores of the glass as if it was a strainer. The leak rate through most vacuum feedthoughs, including vacuum windows, is rated at < 10-10 mbar·l ·s-1. At this rate it would correspond to a loss of about 1 cubic cm of gas every 300 years. I don't believe helium loss is the cause of tube failure. Also, correct me if I'm wrong, but I believe the sealed laser tube is under vacuum. If there is a leak, it would be air leaking into the tube and poisoning the gas mixture. Last year I swapped from a dead tube to a spare that had been sitting in my shop for over a year. The spare runs fine at the rated power. No leaks I guess.

[r691175002]

I picked that piece of info up off of sams laser faq although it seems to apply to helium-neon lasers. No idea if its actually true though.

http://www.repairfaq.org/sam/laserchn.htm

When you have confirmed good alignment and the windows are clean, you may wonder if the gas mixture is right. In lasers that have been sealed a long time there is sometimes a noticeable loss of helium by diffusion through the glass and through the Epoxy of soft-seals. A good way to check the He:Ne ratio is to view the discharge spectroscopicly. I used to use a transmission grating (600 lines/mm) for this. In the yellow region you will find two lines close together. These are neon, 585.25 nm, and helium, 587.56 nm. If the mix is right these two lines should appear approximately equal in brightness.

For a sealed tube, the helium lost by diffusion can be restored by putting the laser in an atmosphere of 100% helium (at 1 atm) for a day or two. 24 hours of inward diffusion this way is about equal to the outward diffusion of a year.

I figured the process would be more similar to osmosis since only helium can travel between the atmosphere and the tube and there is so little helium in the atmosphere that there would be more in the tube even with it being under vacuum.

[lasersafe1]

Not sure if I believe this at all. I've got a 30 year old He-Ne in the lab downstairs. Still runs fine. Glass tube. I am also not sure how much helium is in the mix with CO2. Don't really want to research it either. Now that we can buy Chinese tubes for under $200, they are almost disposable.