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Argon Troubles (Mode Hopping / Temperature)

Trevor

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Last night I fired up my Lasos multiline argon for a maintenance burn. I had fired it up a few days ago, but it needed to flicker for a solid minute before it was willing to fire. I thought this may have been due to me not burning it long enough during these burns (is one hour sufficient?).

So, I got everything set up downstairs in the usual room that I work with lasers in. It's about 15m long - quite nicely sized. I fired up the fan on the top of the laser and then turned it on (at idle). It flickered for about a minute then finally came on. While I waited for it to warm up so that I could turn the current up, I got out the prisms that I had gotten from Jack at Optotronics. I snapped this picture at idle:

1_idle.JPG


Several minutes later, I began slowly turning the current up. I finally got the laser up to full power and noticed something strange across the room. I walked over and took another picture.

2_full.JPG


I was mystified. It is certainly the cleanest TEM01 that I have ever seen. But what could be causing this? I've been quite religious about always running the fan when the laser is running, sucking air out the top of the head. I noticed that when I turned the laser back down to idle current, it hopped back into TEM00 on all lines, but, as I turned the current up, the mode on the blue lines shifted into TEM01. Alignment issues and temperature came to mind. It lases quite nicely at what appears to be rated power (I haven't actually measured it, though I will tonight), so I'm not sure it's alignment. Could the fan not be moving enough air to keep the tube happy? What else could be at play here?

Am I doing something glaringly wrong? :undecided:

-Trevor
 





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Is it just me, or is there actually 8th line appearing, it's 3rd from left on second picture to right ?

Seems a different shade of teal from the neighbouring dot in order to be one big TEM01 from 2nd line...
 

Trevor

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Is it just me, or is there actually 8th line appearing, it's 3rd from left on second picture to right ?

Seems a different shade of teal from the neighbouring dot in order to be one big TEM01 from 2nd line...

Correct, the eighth line appears at full current.

-Trevor
 
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It's interesting that only the blue lines are mode shifting.. On my JDSU system at high current (8A or more) just the 488nm line becomes TEM01* (donut mode), but that mode is seen fairly frequently in well-aligned gas lasers..
 
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TEM00 isn't the highest power mode. TEM01 and other higher order modes consume more of the energy in the cavity and produce more output power. As you increase the current, the laser will hop to a higher power mode as they become available. The different colors come from different laser transitions and so will not necessarily have the same power distribution inside the cavity. You might be able to fix this by tweaking the mirror alignment, but you will probably have less overall beam power than if you left it this way.
 

LSRFAQ

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Has the unit been G shocked or ran at very high currents?

It is very rare for a sealed mirror argon to change from TEM00 to TEM 1,0 BAR unless something happened to it, or it was set up for multimode in the first place.

You really have to change the cavity length some how to see a mode structure change, unless its internal dirt or other obscuration mechanisms.

Slight detuning does not normally change mode structure on these, but other damage mechanisms can. Slight detuning tends to push things TEM00 due to losses.

What is the rated power of this tube?

How many hours on it?

Part number?

What worries me is it may be overdriven. I suggest you find a means of measuring tube current other then the internal sensor and see where Itube is really at.

514 has a different gain and pumping mechanism then the blue lines. Mode changing to multimode in 488 and the violets with out loss of gain in 514 suggests a gas pressure issue, a optics issue, overcurrent, or impurity. A film forming on the optics will impact the shorter wavelenths at first. The easiest way to create the symptoms your reporting is to overheat the tube for a few minutes.

Steve
 
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Trevor

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Has the unit been G shocked or ran at very high currents?

This was purchased from Dave - he said it is not being overdriven. As for being shocked... other than a friend setting the head down on a table harder than I would have, no. And I doubt that counts as a "G shock" - definitely no worse than what it would experience in shipping.

You really have to change the cavity length some how to see a mode structure change, unless its internal dirt or other obscuration mechanisms.

The OC looks slightly dirty and scatters a small amount.

What is the rated power of this tube?

How many hours on it?

Part number?

I'm not sure of the first two, but I'll check on the last.

What worries me is it may be overdriven. I suggest you find a means of measuring tube current other then the internal sensor and see where Itube is really at.

That was something I talked to Dave exhaustively about - so I'm prettymuch certain it's not that.

The easiest way to create the symptoms your reporting is to overheat the tube for a few minutes.

...would it be permanent if that was the cause? I know these usually like at least 250cfm fans. The fan I have is 20W. I just looked up a 250cfm fan and it draws 38W. I'll see about trying a new fan.

-Trevor
 

LSRFAQ

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This was purchased from Dave - he said it is not being overdriven. As for being shocked... other than a friend setting the head down on a table harder than I would have, no. And I doubt that counts as a "G shock" - definitely no worse than what it would experience in shipping.

------------------------------------------------------------------------

Under no circumstances am I saying Dave did anything wrong.

...Things happen in shipping, power line spikes happen, parts get old, vibration moves adjustments, etc. It can be just fine when it left Dave's and get messed up two weeks later. Degradation is sometimes a thing that you have to do a lot of work with lasers to understand.

... I'm missing a few details that I would normally be able to see if I have the laser in front of me, such as its mode behavior in time as I turn the current knob up and down. So I can only ask you for your observations. . Its somewhat paradoxical. Things sometimes partially fail.


....I thus must rule out a few things before I tell you to make a insulated pipe and a easy to make little collar jig, then tell you to twist on the slightly flexible mirror mount portion of the tube and try to improve the mode.
That is fine if it was a mechanical or thermal shock to the tube. The shock induced misalignment problem can some times be fixed by inducing a opposite stress into the mount for a few seconds with some simple tools and patience.
Not difficult at all. However it is time consuming.

...So if I have a PSU model number and a head model number, I can go look at the data sheet and tell you where to probe a bit with just a voltmeter. Its not rocket science, it just needs some patience.

I can come up with 5 things that can do this off the top of my head, and the first one to look at is the PSU. High current causes the first two things on my list. Sometimes its best to just let things be with argons, but over driving the tube is not one of them. Lets check that out first.

... If its not the mechanical or current issues, its a dust or film issue on the optics inside. The first thing to do is get a clean cotton swab and some methanol and gently clean the OC and light sensor. The light sensor is just held on by a few screws, and its not alignment critical. So its easy to clean the optics and try to get rid of the smear in the beam. If the smear stays, odds are the tube has ran for thousands of hours and there is a film on the optics. At that point you just enjoy the laser as the beautiful thing it is.

...If there is still a major smear after cleaning, and the smear shape is not a "half moon", you have film issue. If the smear is half moon shaped, you may have a alignment issue. However any dirt on the OC or splitter makes a identical smear, so you do have to clean first. You can often see if there is a film or dust with a red hene beam or a flashlight when the tube is off and after stripping off the casing a bit. The optic is tailored to reflect blue and green, so red is used to probe and align the cavity. Dust shows up if you flash a hene down the OC and watch the reflection carefully. You can see right through the OC and down into the tube in the red in most cases.

If it is a piece of dust causing the diffraction, some times these clear up on their own, and the trick is to use gravity to get the little dustball to go some place else in the tube using gentle shock and tilting. I have one tube that has a little ball of melted copper in it. Sometimes I need to play "pinball" with that head to get a beam. I can't sell it, so its test head. This case is rare, but it does happen.
It can be caused by over current or a dirty event on the production line.

But lets start with the PSU, and work from there, OK?

You have to make measurements, you cannot assume.

Steve
 
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Trevor

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...So if I have a PSU model number and a head model number, I can go look at the data sheet and tell you where to probe a bit with just a voltmeter. Its not rocket science, it just needs some patience.

The head is an LGK 7872 ML02, made by Lasos. The power supply bears the tag "p/n: 34410609 // Rev: A" - Dave said it's a Spectra Physics 26A.

... If its not the mechanical or current issues, its a dust or film issue on the optics inside. The first thing to do is get a clean cotton swab and some methanol and gently clean the OC and light sensor. The light sensor is just held on by a few screws, and its not alignment critical. So its easy to clean the optics and try to get rid of the smear in the beam. If the smear stays, odds are the tube has ran for thousands of hours and there is a film on the optics. At that point you just enjoy the laser as the beautiful thing it is.

...If there is still a major smear after cleaning, and the smear shape is not a "half moon", you have film issue. If the smear is half moon shaped, you may have a alignment issue. However any dirt on the OC or splitter makes a identical smear, so you do have to clean first. You can often see if there is a film or dust with a red hene beam or a flashlight when the tube is off and after stripping off the casing a bit. The optic is tailored to reflect blue and green, so red is used to probe and align the cavity. Dust shows up if you flash a hene down the OC and watch the reflection carefully. You can see right through the OC and down into the tube in the red in most cases.

If it is a piece of dust causing the diffraction, some times these clear up on their own, and the trick is to use gravity to get the little dustball to go some place else in the tube using gentle shock and tilting. I have one tube that has a little ball of melted copper in it. Sometimes I need to play "pinball" with that head to get a beam. I can't sell it, so its test head. This case is rare, but it does happen.
It can be caused by over current or a dirty event on the production line.

Okay. I'll start trying to figure out how to unbutton the head, and I'll also try and pursue a fan that does 250cfm+ so that I can eliminate heat as the problem.

Also, I just read this thread regarding cooling: http://laserpointerforums.com/f43/lasos-start-up-problem-44380.html

Very interesting.

Thanks! :)

-Trevor
 
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daguin

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Sorry I missed this earlier.

You already have a better man than I helping you. I'm going to pretty much sit here and watch (and learn) as well.

Unfortunately, that was my last Lasos head. I cannot just swap it out for you. If you cannot get things "right" we'll work it out.

I do have the Lasos PSU here, but it is 220V only

In the mean time, don't turn it up ;)

peace,
dave
 

Trevor

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http://www.markettechinc.net/pdf/ArModules2.pdf

You might wish to email them and find out what the ML02 modification is.
Also ask what Imax is for that tube.

Steve.

Based on their stated max power, it seems we're running it within spec. However, I'll go ahead and drop them an email.

Also of note, I did what Hemlock_Mike had said in that other thread - I backed off the fan some by lifting it ~8mm off the top of the housing. Started in about 5 clicks, as opposed to tens of clicks. I wonder if it will hold TEM00 with slightly less cooling. That beggars experimentation. Any suggestions? Or is the faster start not a sign of increased happiness?

-Trevor
 
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My experience with air cooled argons is that you can never have too much cooling. Tube life is a factor of the temperature it's run at. Running one with inadequate cooling is the same as running it at high currents as far as tube life is concerned.

Also, I agree you should manually test what the actual tube current is. Granted i've only had 3 ML air cooled argons in my life, but none.. even the supposedly low hour one, lased on 7 lines at idle currents.
 

Trevor

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I've run the laser a couple times since I started this thread. I tried building a shroud for a large box fan to pull air out of the vent on the top of the laser, but the fan didn't have the oats to pull air through such a small opening.

I measured the air exhausted at idle and at higher current, and it started at ~130F and approached 140F until I turned the current back down (these may be a bit low - I was measuring the air temperature with my DMM temperature probe). Are these temperatures inordinately high?

-Trevor
 

LSRFAQ

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Lift the fan about a inch from the head on box heads. We usually use 6" or 4" PVC thin sewer pipe to make a riser, giving the fan the extra plenum space usually makes it much easier to start the fan, and gives a more laminar flow to the heat sinks, which is a good thing.

steve
 




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