The method relies on a principle similar to that of a strobe light, which can make a moving object appear to stand still by illuminating it with very short flashes of light. The Purdue researchers have done just the opposite -- they have used an electronic "strobe" to make light appear to stand still. By "capturing" light in this way, the researchers can use laser beams to watch a moving object. The special properties of the strobe result in a cleaner signal coming back from the moving object, resulting in a more accurate measurement of its speed.
The effect is accomplished using a semiconductor device called a photorefractive quantum well. The device was developed at Purdue by David D. Nolte, professor of physics, and his graduate student, Indrajit Lahiri. Nolte says possible applications might be found in manufacturing, remote sensing and laser radar.
"Our device is unique in that it measures velocities by constantly adapting to and compensating for unwanted light signals caused by environmental factors, such as vibrations and atmospheric fluctuations," Nolte says.
The results of experiments with the device appear in the Jan. 1 issue of the journal Optics Letters. In addition to Nolte and Lahiri, authors of the article are Michael R. Melloch, professor of electrical and computer engineering at Purdue, and Marvin Klein of Lasson Technologies.
The device determines velocity by measuring the shift in frequency, or Doppler shift, of laser light as it is reflected off a moving object. When laser light hits an object moving toward you, the light waves that are reflected back are compressed, shifting them to a higher frequency. When the object moves away from you, these light waves are stretched out, lowering the frequency. This Doppler shift in light is the same thing that happens when sound waves from a train whistle pass by, going from a high pitch while it is moving toward you to a lower pitch as it moves away.
Getting a Doppler shift off a moving object is not new, Nolte says, noting that astronomers commonly use Doppler shifts to measure velocities. "When lasers came around, people started using them to determine Doppler shifts," he says. "But the big problem is that when you shine a laser on a moving object, the light that is reflected back has horrible properties. You get a hodgepodge pattern of bright and dark speckles, instead of a nice, uniform intensity pattern. This makes it difficult to get a reliable measurement of the Doppler shift."
Other factors also degrade the quality of the laser light, such as vibrations, changes in temperature and atmospheric effects. Together with the speckling problem, all these effects fall into a category that Nolte calls "nuisance" effects because they make Doppler shift measurements difficult.
"Our device eliminates these nuisance effects by using dynamic holography, where the semiconductor device acts as a holographic film," Nolte says. "This method is about the only way to completely eliminate them." A hologram is like a three-dimensional image on film.
When an electrical pulse, or "strobe," is applied across the device, it takes a holographic snapshot of the light hitting it. Each electronic strobe lasts only one millionth of a second, recording a new hologram for each pulse -- and making the hologram stand still, if only for a millisecond. The strobe frequency, on the order of a kilohertz or tens of kilohertz, filters out any changes in the light that occur below those frequencies. All the nuisance frequencies fall within this range and are therefore removed by the device, Nolte says.
On the other hand, the Doppler-shifted light coming from a moving object has a frequency in the megahertz range, one thousand times faster than the frequency of the electronic strobe. So, this light travels unimpeded through the device to a detector.
Nolte says his group is not the first to use dynamic holograms: A research group in France has used them inside bulk crystals and bulk semiconductors to measure vibrations.
"Our device is unique in that we're using adaptive dynamic holography for the first time to measure velocity instead of vibration," Nolte says. "Also, we're the first to use an electronic strobe to create temporary static holograms."
The Purdue research is funded by the National Science Foundation through its Division for Electronic and Communications Systems and Purdue's Materials Research Science and Engineering Center for Technology-Enabling Heterostructures.
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Oscillatory mode coupling and electrically strobed gratings in photorefractive quantum-well
I. Lahiri, D.D. Nolte, Department of Physics;, Purdue University; M.R. Melloch, School of Electrical and Computer Engineering, Purdue University; M.B. Klein, Lasson Technologies
Oscillatory mode coupling between two coherent laser beams is produced by movement of an interface pattern against a quasi-static electrically strobed grating in a photorefractive AlGaAs/GaAs multiple-quantum-well diode operated in the quantum-confined Stark geometry. The oscillation frequency is equal to the frequency difference between the two laser beams and provides a method to measure high-frequency Doppler shifts or large surface displacements for laser-based ultrasound. Combined photorefractive gains (normally forbidden by symmetry in the Stark geometry) and absorptive gains approach 1200 cm-1 during two-wave mixing when moving gratings are used.
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