IEEE Spectrum July, 2009 - 22

watt laser. To achieve a 1-MW beam with
10 percent efficiency would require a whopping 10 MW of input energy and produce
a hefty 9 MW of waste heat. Nevertheless,
later this year a beast with such power,
called the Airborne Laser (ABL), will be
put to the test of blasting dummy nuclear
missiles from the sky.
Here's how. ABL is the latest example in
a class of high-energy lasers called flowinggas lasers. They are powered by burning chemical fuels like those that drive
rocket engines. Hot molecules in the gas
emit a cascade of light emissions, producing a powerful laser beam. Rocket-engine
lasers have generated infrared beams that
can reach a couple of megawatts for a few
seconds at a time. The technology used in
ABL can turn more than 20 percent of the
combustion energy into laser light in the
laboratory, but ABL's efficiency is undisclosed. In such a laser, the exhaust gas carries away the energy left behind as heat.
But so far the US $5 billion ABL can
barely squeeze into a Boeing 747. The laser
is completely unsuited to the battlefield.
It's being designed to destroy long-range
missiles rising through the atmosphere a
couple of hundred kilometers away, but
it's vastly overpowered for the comparatively easy job of hitting slow-moving mortar shells only a kilometer or two away. It
would be like shooting deer with a cannon.
So in 1996 the U.S. Army and the Israeli
Ministry of Defense teamed up to test
smaller lasers against mortars and rockets.
For that task, they tapped Redondo Beach,
Calif.-based aerospace contractor TRW
(acquired by Northrop Grumman Corp. in
2002) to build a 100-kilowatt-class flowinggas laser, a compact version of ABL.
The result, called the Tactical HighEnerg y Laser (THEL), made laser
defense look promising. In 2000, it shot
down a short-range Katyusha rocket over
the White Sands Missile Range in New
Mexico. But by 2004 the United States
and Israel agreed THEL wasn't up to the
job, ending any further tests.
One problem with THEL was its large
footprint. It required several trailer-size
containers; outdoor tanks filled with
exotic chemical fuels and the "exhausted"
chemicals that had fed the reaction (which
would need to be protected from enemy
fire or sabotage); and a platform-mounted
apparatus called a beam director, similar
in size and shape to a big sky-scanning
searchlight. But the real killer for field
commanders was the logistical nightmare they envisioned. Aside from its
bulk, characteristic of flowing-gas lasers,
30

NA * iEEE SpEctrum * july 2009

LASER TAG: Earlier this
year, Northrop Grumman
researchers far overshot
the 25-kilowatt goal of the
Joint High Powered
Solid State Laser
program by
sustaining a 105-kW
beam for more
than five minutes.
Photo: NorthroP
GrummaN

THEL would be useless without its special
fuels, and it also produced toxic, corrosive hydrogen fluoride gas, which would
require special handling. "A chemical
laser on a battlefield is more of a hazard
than the threat it is trying to mitigate,"
says John Boness, chief technologist at
Textron Systems, a laser-weapons contractor in Wilmington, Mass.
The U.S. Army and Israel had their
sights set on something more nimble that
could fit on an armored vehicle the size
of a motor home and that would be ready
to roll when it arrived on the battlefield.
John Wachs, chief of the directed-energy
division at the Army Space and Missile
Defense Command, in Huntsville, Ala.,
explains the ultimate result of that conclusion: "The Army decided they would
prefer solid-state, all-electric devices."

olid-state lasers are far simpler
than their flowing-gas cousins. Small
versions have been in the U.S. arsenal

since the Vietnam War, but they are just
glorified laser pointers: The low-power
beams mark targets with invisible infrared spots, which allow smart bombs to
home in. Such lasers could probably kill
a fly, but nothing bigger.
The first laser ever built was also solid
state. It used bright pulses from a flashlamp (which creates an intense white
light) to illuminate a stubby ruby rod.
The flashes excited chromium atoms in
the ruby, which prompted the atoms to
release their energy as red photons in a
laser beam.
Modern solid-state lasers are powered
in the same general way-light from an
external source transfers energy to the
atoms in a crystal, and these emit laser
light. But instead of ruby rods, engineers
have developed lasers that use more
sophisticated crystals doped with a rareearth element called neodymium. These
simple, durable lasers are used widely in
industry and the military.
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Table of Contents for the Digital Edition of IEEE Spectrum July, 2009