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A more perfect ‘back to the future’

The perpetual motion of molecules, says Yujie Ding, makes air a turbulent medium that undermines efforts to achieve clear long-distance imaging.

“When you send a beam of light through a turbulent medium,” says Ding, professor of electrical and computer engineering, “the returning beam is no longer coherent. The phase of the light wave becomes random and this causes the image you see to be blurred.”

Improving the quality of images is critical for astronomers scanning the skies and for engineers and scientists taking advantage of remote sensing.

To overcome the negative effects of turbulent media, scientists have tried to exploit a third-order nonlinear medium. The goal is to achieve a time-reversed returning beam, or phase conjugate beam, with its phase variation and propagation direction exactly opposite to those of the incoming beam. But wave mixing in a third-order nonlinear medium, says Ding, is an inefficient process.

“We have experimental results that demonstrate a phase conjugation using second-order nonlinearities. This has not been done before.”—Yujie ding

Scientists have also tried to take advantage of the photorefractive effect. However, phase conjugation based on such an effect, says Ding, is too slow to effectively recover or correct blurred images.

Ding is experimenting with a second- order nonlinear medium that he believes much more efficiently reverses the phase variation of the incoming light wave through phase conjugation.

“A phase-conjugated returning wave is a time reversal of the incoming wave,” says Ding. “It’s like going back to the future. If the returning wave recovers the phase of the incoming wave before it enters the turbulent medium, the present meets the past in an almost instantaneous process. The recovery of the original wave results in no distortion in the image that you see.

“Using second-order nonlinearities to achieve phase conjugation in the absence of the photorefractive effect has not been done before. We have experimental results that demonstrate phase conjugation using second-order nonlinearities. We’re trying to improve the efficiency of the process.”

Ding will present his findings at CLEO 2011, the Conference on Lasers and Electro-Optics in Baltimore, in May. His work is supported by the Air Force Office of Scientific Research.