Sophomoric MIT student laser work

As undergrads in the late fall of 1969, John and I were both taking an optics course, six point something or other. It required a lab project. Perhaps primed by Star Trek and Mission Impossible, we ambitiously decided to build a long-distance laser communicator.

There was clear line-of-sight from the top of MIT Building 24, looking over the shoulder of the Great Dome, to my 5th-floor apartment on the other side of the Charles, on Mass Ave near Beacon St. At just over a kilometer, that seemed a long enough distance to prove the point. A safely low-milliwatt helium-neon laser (the red kind) would carry our signals. A high-voltage transistor would modulate the laser’s couple-hundred volt input power. (I think we asked first, before we tore into the laser to re-engineer our transistor into the circuit.)

For initial experiments with beam propagation, we needed a long yet convenient indoor space. Obviously we couldn’t set ourselves up in the Infinite Corridor, but the equally long corridor outside our third floor lab proved infinite enough. We had to use it late at night, though, because ordinary foot traffic is incompatible with beam propagation!

We couldn’t really drag a heavy optical bench out into the hallway, and rolling carts were unstable, so we just set up our mirrors and stuff on the venerable concrete floor. The laser beam was invisible except where it scattered in the clouds of dust swirling over the late-night janitor’s broom. There was some, ah, consternation when we sophomoric kids warned him to stay clear of the beam or his leg might be cut off. (Belatedly, we apologize, sir!)

Even a pinpoint laser beam spreads with distance (even more than our calculations suggested). Our brand new optics knowledge said we needed a beam spreader. Well, if John’s high school telescope could condense the cosmos to fit into an eye, we would simply reverse the process. Point the laser into the telescope eyepiece, and out the objective lens comes a nice, broad beam (really).

To receive the signal, we planned to use Rick’s high school Fresnel lens. It looked like a big, transparent LP record, and it was going to focus that nice broad laser beam down to a point on a photodetector.

At some point we had to move the experiment outdoors. One winter night found John perched by an open wintry window in a penthouse lab on top of Building 24, with the telescope pointing out the window. Voyeur-like, he aimed it at my bedroom window. Then he set up the laser to point through the eyepiece. Laser being narrower than cosmos, that aiming required fine tuning with a micrometer — but of course he couldn’t see where he was aiming. Phone lodged between shoulder and ear, he had to wait for my instructions from across the river.

Meanwhile, I was in my apartment. Perched by my own open wintry window, phone lodged between shoulder and ear and Fresnel lens at the ready, I kept watch for the laser beam as he carefully adjusted the aim. Our theory was that near-forward scattering would let me see where the beam was, even when it wasn’t aimed right at me, and I would tell John how to home in on the target. Well, the slightest micrometer tweak would swing the beam ten or twenty feet, and it was hard to see the beam to give instructions.

An interesting winter optics phenomenon is that when warm lab air meets cold outdoor air, the air currents randomly refract the beam. It’s the same principle that makes stars twinkle. In our case it made the beam seem to wander on its own. When we could see it actually land on something, it looked like a seething, swirling, organic blob. It helped to defocus a little, expanding the two-foot blob to a fainter 20-foot blob.

Snow flurries started, added an extra frisson of wintry to the proceedings. It turns out that snow flurries radically enhance near-forward beam scattering. As John scanned the beam up and down Mass Ave, from the Boston end it looked like an enormous red death ray shooting out of the MIT Great Dome, casting about for startled pedestrians to incinerate.

Due to weather, atmospheric physics, and, well, the calendar, we ultimately weren’t able to demonstrate long-distance laser communication. But we still had that lab project to do. So we brought our stuff back into the optics lab. On the bench, we didn’t need the telescope or Fresnel lens, a significant simplification.

A transistor radio, speaker disabled, served as signal source. That signal drove the high-voltage transistor which modulated the laser power. The laser beam carried the signal just two meters, sigh, to the other end of the bench. There the detector drove, I forget, probably someone’s stereo. That’s the setup, but somehow we had never actually tested it end-to-end until the long night before demo day.

It worked fine in demo. The lab filled with music from Jefferson Airplane or someone else appropriate to the era, although the sound quality was inferior to even a telephone.

There was a heart-stop moment, though. Lee Smullin, head of the EE department (and later on John’s thesis committee) had heard about a really cool demo–ours–and came over to see it. At some point he put his hand in front of the laser, interrupting the beam. But it didn’t interrupt the music! Well, the volume did decrease some, and in a real hurry we developed a sufficiently persuasive alternate theory of parasitic radio-frequency coupling to the unshielded receiver, that bypassed our beam entirely. Whew.

Somewhere along the way we used some non-laser technology to burn a hole in a piece of wood or something, making it look like the laser had done it, and oh-so-innocently left it on the laser bench to play a trick on the instructor. He might not have been completely fooled, but he was generous enough to play along.

Epilogue: Six months later, long after the end of the course, there was an inquiry from the institute Radiation Safety board. See here, boys, we have reports about a Death Ray…do you know anything about this?

Postscript: A few years later, I was recounting the experience to colleague Bob Hunter, a laser physicist. When I mentioned the poor sound quality, he wanted me to estimate the high frequency cutoff of the laser modulation. Oh, I dunno, 800 Hz, 1 kHz, something like that. Off the top of his head he produced estimates for things like He-Ne collision rates, electron mean free path, ionization relaxation times and cavity gain, not to mention actual equations, and literally used the back of an envelope to compute pretty much exactly the number I quoted. I still don’t know whether it was just a tour-de-force performance in technical BS, but either way I’m still impressed.

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