True Lab Stories: Death of a Laser

So, what’s the deal with last night’s silly obituary? Basically, the main laser in my experiment died because I’m a jackass.

More specifically, the laser in question is a diode laser, similar to the kind found in CD and DVD players. These are broadly tunable, available in a wide range of powers and wavelengths, and relatively cheap. They’re also extremely sensitive to static shock, to the point where I have to be careful to always touch something metal before working on the laser or anything close to it.

To check that we have successfully tuned the laser to the right wavelength, we need to do some spectroscopy– that is, we shine the laser through a cell containing krypton atoms, and confirm that they absorb the light. Unfortunately, we’re working with metastable atoms here, which means the cell needs to contain a plasma discharge. Which needs a Tesla coil to get it started.

You can see where this is headed, of course…

I wasn’t stupid enough to directly zap the laser with the Tesla coil, but it was only a couple of feet away, on the same laser table. The plasma discharge uses radio-frequency fields to sustain the discharge, which tends to put 150 MHz noise on, well, everything in the lab, and I was trying to find a way to shield out that noise without killing the discharge. The technique here consists of wrapping the cell in aluminum foil, and seeing whether the noise improves. One of the configurations I tried was a little too effective at cutting down the field, and I cranked the Tesla coil way up to try to get the cell to light. Big spark, no more laser.

The True Lab Story worthy part of this is that I knew this was a danger, and until fairly recently, the cell had been located on a different laser table than the laser itself, for precisely this reason. It made for a very long beam line between the laser and the spectroscopy set-up, though, and I was constantly having to re-align the damn thing. I moved the cell over to the main laser table, to shorten the path.

“It’ll be fine,” I said to myself. “I won’t let the students use the Tesla coil over here, but I’ll be careful…” The ancient Greeks had a word for this: hubris, which translates to “being an arrogant jackass.”

The laser went down around 2:00, and I was in the lab until 7:30 or so last night (skipping dinner) installing one of the spares (as diodes are notoriously fragile, I bought four when I started out…) and re-aligning the system. I also moved the spectroscopy cell back over to the other laser table.

On the one hand, it is sort of nice to be reminded that I do have some lab skills when I need them– that was a lot of fiddly alignment done in five hours. It’d be nicer if I didn’t have to be an idiot first before demonstrating my Mad Lab Skillz…

Comments

  1. #1 cisko
    February 7, 2007

    (Bows head.) RIP, little guy.

    How costly are these? Would it make a credible entry on Derek’s ‘Ruining Stuff’ post? (Not that hubris matches up to the kinds of willful ignorance described there.)

  2. #2 TJ
    February 7, 2007

    Yeah- it always seems unfortunate that Mad Lab Skillz are mostly displayed after a virtuoso display of dumbkopfery. Mine always are.

  3. #3 Uncle Al
    February 7, 2007

    Lighting the tiny hydrogen flame of a GC flame ionization detector can contaminate the probe causinging noise in the output. One therefore takes some care in doing it, preferably with gas lighter rather than a match. A grad student confrere of mine had a better idea. He turned off all the electronics and used a Tesla coil spark. The flame lit. The whole bank of op amps for all four detectors was trashed. It was a painful visit with the prof.

    One might place a spec of platinum catalyst at the burner rim to spontaneously ignite hydrogen in air. That then only requires disassembling the burner, cleaning it, reassembly and testing, and then cleaning the cleaning to punctiliously remove residuals. Call it much of a day… speaking from personal experience.

    “The first rule of intelligent tinkering is not to lose the parts.”

  4. #4 jeffk
    February 7, 2007

    This is very relevent as I just had some delivered and I havn’t opened them yet.

    How much do they cost? Most can be bought for a few dollars, which is no surprise since they’re probably the most expensive component in a $20 cd-rom. However, I found that some specific frequencies can be quite expensive, several hundreds.

  5. #5 Carl Brannen
    February 7, 2007

    A company I worked at reverse engineered a fairly complex integrated circuit. We got a netlist with several thousand SSI chips that were supposedly equivalent to the original. But in reverse engineering there are errors, so it had to be prototyped.

    The prototype was three wire wrap boards, each about a meter square. They were on pin boards so you couldn’t see where the chips were on the wiring side. One of our engineers had the bright idea of putting heat shrink tubing on the pin-1 posts of each chip. When he shrunk the tubing onto the pins with a heat-gun, it melted the insulation on thousands of wire wrap wires and they all shorted together.

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