Holography Accurately Measures Velocity

Aaron Hand

WEST LAFAYETTE, Ind. -- Scientists at Purdue University have developed a convenient method for measuring the velocity of an object. It could prove particularly useful for remote sensing in manufacturing.
Just as a strobe light makes a moving object appear to stand still, so does the researchers' electronic strobe. They can then use the "captured" light from a split laser beam to "watch" a moving object.
David D. Nolte, a professor of physics; Michael Melloch, professor of electrical engineering; and graduate student Indrajit Lahiri developed the method, which involves applying a strobe across a photorefractive quantum well. Nolte and colleagues at Bell Labs developed the detector in 1989.
The device can determine velocity by measuring the Doppler shift of laser light as it reflects off a moving object. The Purdue process is unique because the quantum-well device constantly adapts to environmental factors, Nolte said.
Reflected laser light "has horrible properties," with speckle and a non-uniform intensity pattern, he said. This makes it hard to get a reliable measurement of the Doppler shift. To eliminate the adverse effects of speckle and problems such as vibration and changes in atmosphere, the device uses dynamic holography, he said.
The electronic strobe, an electric pulse 1 ms long, activates the device, capturing a holographic snapshot of the light that hits it. The quantum well -- a dynamic holographic semiconductor thin film -- stores each hologram for about a millisecond before decaying; the hologram, in turn, removes the distortion from signals coming from the moving object.
The device acts something like a semiconductor optical modulator, absorbing the photons and filtering out undesired signals. It filters out changes that occur in frequencies below that of the strobe, which is in the kilohertz range; the frequencies of the adverse effects fall within this range. Meanwhile, the Doppler-shifted light coming from the moving object typically has a frequency in the megahertz range, so it travels unimpeded through the device to a separate detector.
The new method works at much lower intensities and at higher frequencies than previous methods; thus, it is more effective when measuring velocities in adverse conditions.
The device could be useful in quality control or process monitoring when noncontact sensing is difficult, such as in high-temperature or corrosive environments.