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…