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Astronomers Rise Above the Chaos

Photonics Spectra
Feb 1997
Kathleen G. Tatterson

GREENBELT, Md. -- Astronomers at NASA's Goddard Space Flight Center are using chaos-control technology to make an infrared semiconductor laser system more effective for spectroscopy.
The term chaos can be misleading, according to Gordon Chin, head of the project team. It's actually characterized by patterns of frequency variations that occur so quickly that they are difficult to detect. The team, which includes Chin and John J. Hillman from Goddard, and Larry R. Senesac and William E. Blass from the University of Tennesee at Knoxville, found patterns in the seemingly random fluctuations of a lead-salt laser, and using an established chaos-control technique, they reduced the frequency variation by a factor of 10 to 15.
Astronomers use the lead-salt infrared diode laser for atmospheric spectroscopy because of its tunability over a 30-µm range. However, the laser line was not as sharp as it could be. Chin and his colleagues discovered that chaotic variations in frequency were occurring at a microsecond rate, making them difficult to track and control.

Controlling chaos
Using the absorption line of ethylene gas as a frequency-to-amplitude converter, the scientists could measure even the slightest change in frequency as a change in amplitude. Once they could identify and measure chaos, they could also control it.
In what Chin calls a very simple technique, dubbed occasional proportional feedback, the team applied a correcting pulse to the laser current, decreasing the frequency excursions and making the laser line sharper and more useful.
"A whole range of applications is opening up because of chaos control," he said. "Because we are already using these lasers for spectroscopic work, and the application seems to be simple, we can use this technique in applications immediately."
The team also plans to use the newly tamed laser as a local oscillator, which coherent optical systems employ to generate a composite wave that results in greater receiver sensitivity.
Further testing of this chaos-control method will take place in April at a similar facility at the University of Cologne in Germany. The researchers also believe that optically pumped lasers would be ideal candidates for this technique because the frequency time scale on such lasers is slower, making chaos easier to identify and control.

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