True Lab Stories: Evil Professor Tricks

A week or two ago, one of my students measured the power output of a grating-locked diode laser, and came into my office saying “I think I may have killed the laser.” The power output was much too low for a laser of that type, which is a bad sign.

So, we went down to the lab, and looked at the system, and after a minute, I said “Rotate the laser ninety degrees in its mount, and measure it again.”

And, lo, the power was back up at the level we expected originally.

As I explained to my student, this wasn’t actually black magic, just physics that I knew and he didn’t. The light coming out of the laser is polarized, and in the Littrow configuration that we use for the laser, the output beam comes off a diffraction grating at an angle of 50-ish degrees. At that angle, the intensity of that reflection is highly dependent on the polarization, and rotating the laser will take you from a nice, bright beam to a really dim one.

Of course, from the perspective of the student, it looks like yet another Evil Professor Trick, one of those situations in which a faculty member sets you to some task that seems impossible, and you slave away for hours getting nowehere. Then the professor comes in, and makes it work inside of five minutes.

The very worst Evil Professor Trick I experienced was when I was an undergrad. We did a lab in my junior year where we were supposed to make a really simple dye laser, and use it to do some spectroscopy. The set-up was really basic– a couple of mirrors, a lens, a dye cell, and a pump laser, but try as we might, we couldn’t get the damn thing to lase. We spent most of the three-hour lab period beating on it, adjusting every screw on the mirrors, adjusting the pump laser, moving the dye cell around. The TA for the class came in and helped, and he couldn’t get anywhere, either. Every once in a while, we’d get a tiny flicker of green light (the dye laser was supposed to be green), but it would disappear as quickly as it appeared.

Finally, after hours of this, the professor teaching the class came in and asked “What’s going on, here?” We told him our tale of woe, and he said, “Hunh. Well, sometimes turning the lens helps…” and rotated the lens by about ten degrees in its mount…

…and I swear, the beam that came out of the thing looked like the killer laser at the end of Real Genius. “There you go,” he said, and walked out of the room before we could kill him.

(The lens in question was a cylindrical lens, it turns out, and the angle matters. We were thinking in terms of spherical lenses, so rotating it in the holder never even occurred to us.)

So what’s your favorite Evil Professor Trick?