Achieving control of chaotic laser output.

Irregular fluctuations in intensity have long plagued the operation of a wide variety of solid-state lasers. Present-day versions of these lasers owe their markedly improved stability and performance to a succession of engineering advances aimed at modifying designs or materials to circumvent such random behavior.

But that isn't the only way to proceed. Researchers are starting to explore the possibility of exploiting rather than avoiding a laser's chaotic output. As an important step in that direction, a group at the Georgia Institute of Technology in Atlanta has applied a novel control technique that automatically locks a solidstate laser's erratically fluctuating light into a regularly repeating pattern of intensities.

"It was very exciting to see we could control [the laser's output intensity] in this way," says physicist Rajarshi Roy, who led the Georgia Tech group. "We didn't know whether we could do it until we tried the experiment." Roy and his colleagues describe their results in a paper accepted for publication in PHYSICAL REVIEW LETTERS.

The researchers demonstrated their approach with a widely used type of solid-state laser made from a crystal of yttrium aluminum oxide lacd with neodymium. The so-called Nd:YAG laser generates infrared radiation at a wavelength of 1,064 nanometers. Passing this radiation through a potassium titanyl phosphate crystal inside the laser cavity produces green light at half the original wavelength or double the frequency.

However, the use of a frequency-doubling crystal often induces random fluctuations in the laser's output of green light. Two years ago, Roy and his co-workers found they could eliminate this chaotic behavior and achieve stability simply by rotating the doubling crystals into certain positions.

"This was the unknown parameter that had varied from laser to laser when they were built in the past," Roy says. The discovery explained why some of these lasers showed chaotic behavior and others did not.

Reports last year of the development of a laboratory technique for dynamically stabilizing the chaotic vibrations of a magnetoelastic, metallic ribbon (SN: 1/26/91, p.60) prompted Roy and his group to consider a similar strategy for lasers. The trouble was that a laser's fluctuations typically occur far too rapidly for the king of computer-mediated controls used in the ribbon experiments.

Roy found the solution in the work of Earle R. Hunt of Ohio University in Athens, who had developed a simple, high-speed, electronic method of converting rapid, chaotic voltage fluctuations in an electronic circuit into a periodic signal. "I realized this [electronic] hardware was exactly what we needed for our laser," Roy says.

To achieve control, Roy and his co-workers use a modified version of Hunt's circuitry to monitor the laser's ouput periodically and, in response, slightly alter the power output of the diode laser supplying energy to the Nd:YAG system. By applying minute, brief jolts at just the right rate, the researchers can turn a Nd:YAG laser's chaotic output into a periodic signal.

"Without the feedback, it's chaotic," Roy says. With feedback, the laser stays locked to a certain pattern for tens of minutes.

In addition, the researchers can readily shift the laser's output from one periodic pattern (or waveform) of intensity fluctuations to another. "With this control technique, you may eventually have the ability to generate different, complex waveforms in a laser system and to switch between these waveforms," Roy says.

"Right now, however, we're kind of at the trial-and-error stage," he adds. The Georgia Tech researchers would like to extend their technique to faster lasers and to develop electronic means of deliberately selecting and controlling the various waveforms emerging from a laser.