'Air laser' tech could sniff bombs, probe atmos from afar

Cunning analysis pulse remotely generated from thin air

Military- and security-funded boffins have been trying to develop better bomb-detecting technology for many years now. Most of what they come up with has obvious defects: typically sensors either don't actually detect explosives but rather associated substances or shapes (metal detectors, X-ray systems) or they need to be very close to explosives or other chemicals of interest – as with sniffer dogs or handheld chem detectors.

All that may finally be about to change, however, as engineers at Princeton uni say they have come up with a way of making a very sensitive chemical analysis of a piece of air that is a long distance from the sensing equipment. Thus the very faint traces of explosive which are emitted from concealed bombs could be picked up – just as sniffer dogs can sometimes trace it – but from outside the bomb's lethal radius.

The system works by remotely generating a pulse of laser light at the air volume to be sampled, which then shines back at the detector instrument. This strong infrared illumination right by the volume to be analysed means that potentially very low concentrations of chemicals can be detected, the more so as it is coherent light – a laser – which naturally carries information across the distance to the detecting instrument more efficiently and with less scatter than ordinary light would.

The infrared laser behind the target volume is generated out of thin air by shining a cunningly focused ultraviolet laser out from the detector. This energises a cylinder of air just a millimetre long, situated at the place to be scanned, and turns it into a gas laser aimed back at the scanner.

"We are able to send a laser pulse out and get another pulse back from the air itself," says Richard Miles, Princeton engineering prof. "The returning beam interacts with the molecules in the air and carries their fingerprints."

Miles and his colleagues say that their method sends back a beam "thousands of times" stronger than can be obtained by existing methods such as reflecting lasers off objects beyond the area of interest. They think that this will allow much lower trace levels of airborne substances to be picked up.

Thus far the "air laser" effect has only been demonstrated in the lab at distances a bit over a foot, but Miles and his colleagues say that focusing the ultraviolet energising laser at points further off should be straightforward. The engineers consider that it ought to be possible to build a scanner which could be mounted on a vehicle and be able to pick up the extremely faint plumes of explosives emitted from buried mines, hidden roadside bombs etc, from well down the road.

If it works, the "air laser" scanner could be a boon for hard-pressed Western troops fighting in Afghanistan and Iraq: improvised bombs are their number one cause of deaths and injuries, and efforts to detect the hidden devices are tremendously time-consuming, unreliable and typically only work at dangerously close range.

Professor Miles and the team think that combo sensors employing both air lasers and specialised radar could offer the genuine remote bomb detector that ISAF soldiers in Afghanistan and others before them have been waiting for so long.

"We'd like to be able to detect contaminants that are below a few parts per billion of the air molecules," he says.

Naturally the "air laser" method, if it proves out, could find many other uses apart from bomb sniffing. It would greatly improve scientists' knowledge of what's actually happening in the Earth's atmosphere, for instance.

"Greenhouse gases and pollutants are up in the atmosphere, so sampling is difficult," notes Miles.

The team's publication, High-Gain Backward Lasing in Air, was recently published in top-level boffinry mag Science (subscription link). ®