Lasers find distant hidden explosives

Ashley N. Paddock, ashley.paddock@photonics.com

A new method of Raman spectroscopy uses laser light to detect chemicals inside a container from a distance of more than 100 m.

Laser light is scattered in a very specific way by various substances. This is the basis of Raman spectroscopy and can be used to analyze the contents of a nontransparent container without opening it.

Scientists at Vienna University of Technology (TU Vienna) used the technique to see what was in certain containers, irradiating samples with a laser beam. When the light was scattered by the molecules of the sample, it changed its energy; e.g., the photons transferred energy to the molecules by exciting molecular vibrations, changing the wavelength of the light and, thus, its color. By analyzing the color spectrum of the scattered light, the researchers determined what kind of molecules scattered it.

Bernhard Zachhuber mounts optical elements of the spectrometer. Courtesy of TU Vienna.

“Until now, the sample had to be placed very close to the laser and the light detector for this kind of Raman spectroscopy,” said Bernard Zachhuber of TU Vienna.

His technological advancements enabled measurements to be made over long distances.

“Among hundreds of millions of photons, only a few trigger a Raman-scattering process in the sample,” he said.

These light particles scatter uniformly in all directions. Only a tiny fraction of them travel to the light detector, and as much information as possible must be extracted from this very weak signal. This can be done using a highly efficient telescope and extremely sensitive light detectors.

The scientists collaborated with private companies and with partners in public safety, including the Spanish Guardia Civil, to apply their method to the extreme. With the help of the Austrian military, they tested frequently used explosives such as TNT, ANFO and RDX on their testing grounds.

The results proved successful. Even at a distance of more than 100 m, the substances could be detected accurately and reliably, said researcher Engelene H. Chrysostom of TU Vienna.

And when the scientists hid a sample in a nontransparent container, the laser beam was scattered by the container wall, but a small portion of the beam still penetrated the box, exciting Raman scattering processes inside the sample.

The Raman spectroscope at TU Vienna.

“The challenge is to distinguish the container’s light signal from the sample signal,” said scientist Bernhard Lendl.

This can be done using a simple geometric trick: The laser beam hits the container on a small, well-defined spot. The light signal emitted by the container stems from a very small region, while the light that enters the container is scattered into a much larger region. If the detector telescope is not aimed exactly at the point at which the laser hits the container, but rather just a few centimeters away, the characteristic light signal of the contents can be measured, instead of the signal coming from the container.

The researchers’ new method could make security checks at airports a lot easier, but they believe that applications are broader than that. It could be used wherever it is hard to get close to the subject of investigation – for studying icebergs or for geological analysis on a Mars mission, for example, or for a host of chemical industry applications.