Researchers at Princeton University developed a technique
for generating a laser beam out of nothing but air.
Princeton University engineers have developed a new laser
sensing technology that is expected to enable the remote distant
detection of explosives, airborne pollutants and greenhouse
gasses. The technique differs from previous remote laser-sensing
methods in that the returning beam is not just a reflection or
scattering of the outgoing beam but an entirely new laser beam
generated by oxygen atoms whose electrons have been "excited" to
high energy levels. This "air laser" is a much more powerful
tool than previously existed for remote measurements of trace
amounts of chemicals in the air.
"We are able to send a laser pulse out and get another pulse
back from the air itself," said Richard Miles, a professor of
mechanical and aerospace engineering at Princeton, the research
group leader and co-author on the paper. "The returning beam
interacts with the molecules in the air and carries their finger
prints."
The researchers, whose work is funded by the Office of Naval
Research's basic research program on Sciences Addressing
Asymmetric Explosive Threats, published their new method Jan. 28
in the journal Science.
Miles collaborated with three other researchers: Arthur
Dogariu, the lead author on the paper, and James Michael of
Princeton, and Marlan Scully, a professor with joint
appointments at Princeton and Texas A&M University. The new
laser sensing method uses an ultraviolet laser pulse that is
focused on a tiny patch of air, similar to the way a magnifying
glass focuses sunlight into a hot spot. Within this hot spot – a
cylinder-shaped region just 1 millimeter long – oxygen atoms
become "excited" as their electrons get pumped up to high energy
levels.
When the pulse ends, the electrons fall back down and emit
infrared light. Some of this light travels along the length of
the excited cylinder region and, as it does so, it stimulates
more electrons to fall, amplifying and organizing the light into
a coherent laser beam aimed right back at the original laser.
Researchers plan to use a sensor to receive the returning
beam and determine what contaminants it encountered on the way
back.
"In general, when you want to determine if there are
contaminants in the air you need to collect a sample of that air
and test it," Miles said. "But with remote sensing you don't
need to 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."
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't 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.
Any chemical explosive emits various gases depending on its
ingredients, but for many explosives the amount of gas is
miniscule.
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 way. 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."
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