Air laser will sniff bombs, pollutants from great distance

do that. If there’s a bomb buried on the road ahead of you, you’d like to detect it by sampling the surrounding air, much like bomb-sniffing dogs can do, except from far away. That way you’re out of the blast zone if it explodes. It’s the same thing with hazardous gases — you don’t want to be there yourself. Greenhouse gases and pollutants are up in the atmosphere, so sampling is difficult.”

Any chemical explosive emits various gases depending on its ingredients, but for many explosives the amount of gas is miniscule.

The most commonly used remote laser-sensing method, LIDAR — short for light detection and ranging — measures the scattering of a beam of light as it reflects off a distant object and returns back to a sensor. It is commonly used for measuring the density of clouds and pollution in the air, but can not determine the actual identity of the particles or gases. Variants of this approach can identify contaminants, but are not sensitive enough to detect trace amounts and cannot determine the location of the gases with much accuracy.

The returning beam is thousands of times stronger in the method developed by the Princeton researchers, which should allow them to determine not just how many contaminants are in the air but also the identity and location of those contaminants.

The stronger signal should also allow for detection of much smaller concentrations of airborne contaminants, a particular concern when trying to detect trace amounts of explosive vapors.

While the researchers are developing the underlying methods rather than deployable detectors, they envision a device that is small enough to be mounted on, for example, a tank and used to scan a roadway for bombs.

So far, the researchers have demonstrated the process in the laboratory over a distance of about a foot and a half. In the future they plan to increase the distance over which the beams travel, which they note is a straightforward matter of focusing the beam farther away. They also plan to fine-tune the sensitivity of the technique to identify small amounts of airborne contaminants.

In addition, the research group is developing other approaches to remote detection involving a combination of lasers and radar. “We’d like to be able to detect contaminants that are below a few parts per billion of the air molecules,” Miles said. “That’s an incredibly small number of molecules to find among the huge number of benign air molecules.”

—Read more in Arthur Dogariu et al., “High-Gain Backward Lasing in Air,” Science 331, no. 6016 (28 January 2011): 442-5 (DOI: 10.1126/science.1199492)