Enticing Order Out of a Chaotic Laser

Chaos may sound like anarchy, but the chaos that scientists know can be tamed--and now, it seems, even put to work. In a Report in this week's issue of Science, Gordon Chin of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and his colleagues describe how they have used chaos control to stabilize the frequency of a tunable laser, making it into an even more powerful laboratory tool for studying the planets.

Astronomers like Chin use tunable lasers to study how cold gases resembling the atmospheres of the outer planets absorb infrared light. These measurements provide a template for interpreting observations from space probes. If the frequency of the laser light were less fickle, it could be even more useful. "A real application hasn't come out [of chaos control] as yet, and this is probably a good one," says Ira Schwartz of the Naval Research Laboratory in Washington, D.C.

The device that Chin and his colleagues at Goddard and at the University of Tennessee, Knoxville, focused on is the lead-salt laser--a semiconductor device whose light can be tuned, unlike most lasers, simply by adjusting its temperature or the current that energizes it. But the laser doesn't maintain a steady frequency. Instead it hops among many closely spaced frequencies so fast that the net output is just some smeared average of many sharply defined signals.

The group tracked these fluctuations by passing the light through ethylene gas and tuning the laser so that its frequency was just at the edge of one of the gas's absorption peaks--a frequency at which ethylene absorbs light. That way, even a slight change in the laser's frequency, by shifting it either closer to the absorption peak or farther away, would result in a huge change in the intensity of the transmitted light. The frequency changes showed the mathematical hallmarks of chaos--a hair-trigger sensitivity that makes a system effectively unpredictable in the long run.

In the very short run, however, a chaotic system is predictable. Using the frequency-to-amplitude trick, Chin's setup could spot when the laser frequency was drifting chaotically and apply a correcting pulse to the laser current approximately every microsecond. Making it all work "is just staggeringly simple," says Chin. "We got success in the first try." The strategy reduced the frequency variation by a factor of 15, he says, and he and his colleagues are now planning to put the newly stabilized laser to work.