Purdue Researchers Make Light 'Stand Still' To Measure Motion

January 01, 1998

WEST LAFAYETTE, Ind. -- Purdue University researchers have demonstrated a new method for using lasers and semiconductors to more accurately measure the velocity of a moving object.

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.
-end-


Purdue University

Related Laser Articles from Brightsurf:

Laser technology: New trick for infrared laser pulses
For a long time, scientists have been looking for simple methods to produce infrared laser pulses.

Sensors get a laser shape up
Laser writing breathes life into high-performance sensing platforms.

Laser-powered nanomotors chart their own course
The University of Tokyo introduced a system of gold nanorods that acts like a tiny light-driven motor, with its direction of motion is determined by the orientation of the motors.

What laser color do you like?
Researchers at the National Institute of Standards and Technology (NIST) and the University of Maryland have developed a microchip technology that can convert invisible near-infrared laser light into any one of a panoply of visible laser colors, including red, orange, yellow and green.

Laser technology: The Turbulence and the Comb
While the light of an ordinary laser only has one single, well-defined wavelength, a so-called ''frequency comb'' consists of different light frequencies, which are precisely arranged at regular distances, much like the teeth of a comb.

A laser for penetrating waves
The 'Landau-level laser' is an exciting concept for an unusual radiation source.

Laser light detects tumors
A team of researchers from Jena presents a groundbreaking new method for the rapid, gentle and reliable detection of tumors with laser light.

The first laser radio transmitter
For the first time, researchers at Harvard School of Engineering have used a laser as a radio transmitter and receiver, paving the way for towards ultra-high-speed Wi-Fi and new types of hybrid electronic-photonic devices.

The random anti-laser
Scientists at TU Wien have found a way to build the 'opposite' of a laser -- a device that absorbs a specific light wave perfectly.

Laser 'drill' sets a new world record in laser-driven electron acceleration
Combining a first laser pulse to heat up and 'drill' through a plasma, and another to accelerate electrons to incredibly high energies in just tens of centimeters, scientists have nearly doubled the previous record for laser-driven particle acceleration at Berkeley Lab's BELLA Center.

Read More: Laser News and Laser Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.