Broken Laser Blues
June 19, 2007 11:03 PM   Subscribe

Calling all spectroscopists! My chemist friend needs your help! He writes: Our tunable dye laser is broken, and we need it for our research to continue, but we don’t have the time to send it away for repairs. What should we do?

We’re using an ancient Lambda Physik Scanmate tunable dye laser and a second harmonic generation (SHG) crystal to produce light at approximately 230 nm. The laser has a small control box inside, which is connected to a PC that only holds the Scanmate software. This PC is triggered remotely by a master computer, which also controls the pump laser (an XeCl excimer) and our molecular beam valve.

The dye circulator works fine, and we aren’t having any problems actually generating light at the desired wavelength.

The problem occurs once we start scanning. The PC is instructed to scan a region of the spectrum at a rate of about 1 nm every 24 minutes. It usually runs for about 0.1 nm, then a red box appears in the Scanmate program that reads “Data Conflict 64: Reboot System.” Sometimes this same error occurs immediately at startup, without any scanning at all.

This error is “supposed” to occur when data is not being sent correctly in some form or another within the laser. We’ve checked the fiber optic jumpers, and they seem fine.

Has anyone worked with this kind of laser before, and if so, has anyone experienced this problem? What could it be? Weak connections? Low voltages? The manual is unhelpful, and discussions with tech support haven’t resolved the issue.

Help us hive mind. Spectroscopy needs you.
posted by benign to Science & Nature (7 answers total)
heh laser "blues" indeed!

dye lasers are usually tuned by rotating a grating that serves as one side (the non-output end) of the optical cavity. also to get a very narrow linewidth there is often a high-finesse etalon inserted into the cavity; when you tune the laser you then have to synchronously tilt the etalon as well.

i have don't know for sure what "data conflict 64" might mean (hooray, human-readable error messages), but my guess is that either the drive or readback for one of the grating or the etalon is broken; the bit of software in charge of tuning them synchronously then barfs. where the broken thing is is anyone's guess; you need someone standing in front of it with experience with this particular control system and test equipment.

but, if you are willing to sacrifice some linewidth in the short term you might find out if it's possible to disable the intracavity etalon in the control software, open up the housing and remove it from the light path. (this is presuming that the grating is still what works, which sounds reasonable if you can still tune the thing around on a coarse scale). it really depends on how brave you are and i guess on how much spectral resolution you need.

if it was me though, i've spent enough time trying to fuck around with other people's control systems to realize that you have a snowball's chance in hell of figuring out what's wrong with it just by pot-shotting it. i would see if you can get coherent to send a tech out to your lab.
posted by sergeant sandwich at 3:03 AM on June 20, 2007

In my experience with electronics of all sorts, I have found nearly all problems (90-95%) to come from connectors. They are subject to vibration, oxidation, general looseness.

I cannot count the times I have solved a problem by simply removing / reconnecting, and in some cases, cleaning connections. Any cables that may be in motion, regardless of how small the motion may be, are also highly suspect for fatigue failures.

Since you use the word 'ancient', I presume the remote computer that operates the laser is likely an AT-type or an industrial PC. Both have backplanes/motherboards customarily, and just looking inside for looseness, wiggling and reseating circuit boards can resolve a problem.

You are not likely to make things any worse by giving this a shot.

The good news is that most of it sounds like it is working. It also fails reliably, which makes determining a fix much faster. Pay close attention to what you do in case the problem goes away and you may be armed for the next failure.

Make sure that you unplug things and observe appropriate safety habits, particularly where high voltage is found.
posted by FauxScot at 3:38 AM on June 20, 2007

I agree with Faux Scot. It's very likely a 'Physical Layer' problem. When troubleshooting networks and computers - work your way up the OSI model [Layer 1 -> Layer 7].

Good Luck!
posted by LakesideOrion at 6:01 AM on June 20, 2007 [1 favorite]

FauxScot has a pretty good answer.

His point about it failing reliably is an excellent one...intermittent failures are a bitch to troubleshoot.
posted by SlyBevel at 6:37 AM on June 20, 2007

FauxScot: Since you use the word 'ancient', I presume the remote computer that operates the laser is likely an AT-type or an industrial PC. Both have backplanes/motherboards customarily, and just looking inside for looseness, wiggling and reseating circuit boards can resolve a problem.

Agree with this, big time. Also, have a look at the logic boards for blown circuit components/connections (fried resistors, decayed capacitors, burnt-out fuses). When we tried to fire up a backup machine last year (that had been working perfectly when last we used it - SIX years previously), our electronics guy and his magic oscilliscope found three different problems of this type. Like yours, the interface box was a stripped-down PC (a 386!) that only ran the control software (and we used a 486 as the user control machine).
posted by hangashore at 7:30 AM on June 20, 2007

Try FauxScot's answer first; that's always the first thing to try with balky lab equipment. Beg borrow or steal some contact cleaner and a can of compressed air too. Reseat and clean all connectors. Spade and card-edge connectors are the worst, so pay close attention to them. Blow out all the dust. I used to have to do this monthly on a finicky old PDP-11-based system.

If that doesn't work, check your cables. Cables are easy and cheap to fix and often fail on older systems. Replacing a cable often fixes problems.

If that doesn't work then you have a computer hardware failure (can you try another computer?) or worst, an instrument failure. If it's the detector or instrument contoller, you may be able to fix that yourself. If it's the instrument, the grating, the optics or the laser (which don't sound likely given your description), you may need to make the compromise that sargent sandwich outlines, or suck it up and make that phone call to Coherent ($$$$$).
posted by bonehead at 9:14 AM on June 20, 2007

If your really desperate: search Google for labs that are using this laser and shoot off some e-mails. For example: Here is a newsgroup posting from a ScanMate user. Try hitting him/her up.

On other hand, it's hard to believe that Coherent aren't the experts on this topic since they service the sucker. Are they just being difficult because they want your money or do they not have a clue? If they're being difficult, threaten to drop the system and switch over to a Spectra-Physics system. If their service just amounts to swapping out electronics, just ask them to FedEx you the serviceable parts and do the swapping yourself. You still pay, but you don't have to crate up the laser.

This might be your best option because if the laser was running fine and nobody was doing anything special to it, it sounds like to me that something has died inside, especially since the error doesn't seem to be dependable on a trigger.

Can you swap out the control PC? (Just stick the HD in a another computer)
posted by toftflin at 6:00 PM on June 21, 2007

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