Laser Power Drop Off

Discussions on optics for laser cutter/engravers

Laser Power Drop Off

Postby iGull » Thu Oct 18, 2012 8:29 pm

Hi All

Just looking for some practical info for reference.

Finished my blackhole laser last year about this time (according to my buildlog). It's a 60W chinese tube, the current has been limited correctly during its one year lifetime. It hasn't had a hard life - maybe in use for a short time (perhaps an hour or two say) every other day - most of that would be switched on, but not lasing. It's been keep in a fairly temperate environment - say between 10 to 25 degC.
I suppose a log book would have been a useful thing, I'm not great at diaries and logs :-) The power is starting to drop off quite considerably - I'd hazard a guess at about 15 to 25% over the last week. Is this the general failure mode in glass tube lasers ?
I've stripped and cleaned the optics, all immaculate - also the laser output mirror. Current looks correct at full power and alignment is all OK (actually, in a year, I've never had to adjust it!).
A year doesn't seem a long time for a laser that hasn't had much use (certainly not an eight hour day, five days a week). In my mind, I had thought perhaps two or three years would be the norm (that was base on hope rather than any technical dayta :D )

Anyone with some experience of tube life in these lasers ? A 60W tube isn't really that cheap either. I'm hoping my stored one year old spare tube is OK!

I've trawled the forum, but don't see anything with valid data on this.

Anyone help?

TIA

Neil
EMOs are a sign of weakness ...
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Re: Laser Power Drop Off

Postby macona » Fri Oct 19, 2012 12:52 pm

The old tubes just dont age well. I should measure the output from mine, it has been sitting over a year now too.

Consider getting a laser power meter. You can get heads from coherent and ophir that you can connect straight to a multimeter like this (You do need a power supply for the internal amp)

http://www.ebay.com/itm/Ophir-OEM-Laser ... 5addb2b881

http://www.ebay.com/itm/MOLECTRON-COHER ... 460819afbb
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Re: Laser Power Drop Off

Postby TLHarrell » Fri Oct 19, 2012 4:35 pm

Earlier this year my 40w chinese tube bit the dust. The failure mode was similar to what you are experiencing. It just seemed I was spending more time fiddling with correcting power for what used to be a simple cut. I had a longer job running during the evening doing a set of acrylic computer fan grilles. I was having a hell of a time cutting through the white acrylic. Ruined several parts and ended up basically getting it out the door with zero profit. Started up another small job on some semi-translucent acrylic after that, doing some small jewelry parts, and the falloff in power was like a cliff dive. Started at the top of the sheet cutting ok. Second row of parts didn't cut through. Third row it was barely marking the acrylic. Fourth row it wouldn't make it through a wet paper towel.
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Re: Laser Power Drop Off

Postby iGull » Sat Oct 20, 2012 7:53 pm

Hi All
Thanks for the info.
@macona - you're right, a workable power meter would be a useful thing - if only they weren't half the price of a new tube :D
I'm guessing that there has to be an open source version that we could build for a few dollars that would be sufficiently accurate (or at least consistent) - might be worth a project slot here on buildlog.
Looks like they're nothing more than a temperature/time monitoring system. You already know the power has dropped off anyway, but it would give you a warm feeling of how much :D

@tim - looks like mine has joined yours (and many others no doubt) in the great tube graveyard :D I swapped out my spare tube this morning and it's all back up and running again. My spare tube (coletech) was actually 60mm diameter as opposed to the 55mm it should have been. I thought they had shipped an 80W (I wish :D ), but it was only 1200 long - it must have come from another company. What is annoying, is that my mounts were cut for 55mm tube diameter - fortunately, I had the presence of mind to design and cut a new set last week (deja vu ???) - did them in 6mm clear blue acrylic this time - very sexy.
I did a complete realignment from the tube up and managed to get the beam down the same hole at the four corners (bear in mind that my table is 1200x600mm - 48"x24") - I'm pretty happy about that - there was some beam divergence at the faraway corners on my last setup. Beam is nicely vertical too.
What is strange is that I thought to do another focus test - best focus was about 2.5mm below where it was before ! That IS strange - it's a 63.5mm lens - I checked it a few times, but that's where I get the best line without a doubt. I can only guess that my previous setup was wrong :roll:
I'll be checking that again !

Cheers

Neil
EMOs are a sign of weakness ...
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Re: Laser Power Drop Off

Postby macona » Sun Oct 21, 2012 10:32 am

There are plans out there for building small laser power meters using solar cells. But they are only good for small lasers up to 30 to 50mw. Due to the nature they are consistent in their voltage out vs the laser power so no real calibration is necessary, though there is issues due to the response of the solar cell to different wavelengths but it can be compensated for.

High power heads are of a totally different. They measure the power via thermal. The simplest is a chunk of coated aluminum with a metal stem thermometer stuck in it. How hot it gets is directly related to the laser's power. That could be made. But the electronic ones are probably beyond most people's ability to make. The thermopile is built up via thin film coating.

Either way you still need a working laser power meter to calibrate it.

http://www.kenteklaserstore.com/categor ... goryID=208
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Re: Laser Power Drop Off

Postby lasersafe1 » Sun Oct 21, 2012 4:24 pm

I had a reply to this, but decided to make it as a new topic since it involves building your own power meter.
viewtopic.php?f=14&t=1595
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