IEEE Spectrum July, 2009 - 23

beam
me up

tHin-zag tExtRon fibER LaSER

illustrations: steVe stanKieWiCZ; photo: laWrenCe liVermore national lab

Competing solid-state laser
weapon designs reflect
different philosophies

textron uses a single oscillator
in which the light follows
a zigzagging path through
six water-cooled slabs,
collecting more light
than it would if
it traveled in a
straight line.

noRtHRop gRUMMan
fiREStRikE

RaytHEon LaSER aREa
dEfEnSE SyStEM

Each of Northrop
Grumman's
Firestrike modules
generates a 15-kW
beam, which can
be combined into a 105-kW
beam with seven amplifiers.

raytheon's lADS is an
updated version of its
phalanx Gatling gun
and uses a brute force
broadband laser to
achieve a 50-kW
beam.

Despite the advances with materials,
at most just a few percent of the electrical energy from a flashlamp makes it into
a laser beam. Solid-state laser materials
don't dissipate heat well, so trying to
crank up the laser output too much will
warm the crystal rod. Exceeding the
strict heat limits of this material even
by a negligible amount causes internal stress and degrades the beam quality, which means it won't focus tightly
on the target. Add even more heat and
the rod can crack or shatter. For decades,
this combination of low efficiency and
poor heat dissipation made attempts to
develop high-energy solid-state lasers
seem like a waste of time.
But by the 1980s, the industry began
replacing the old flashlamps with semiconductor diode lasers. Both tools for
powering solid-state lasers use two-stage
processes, with electrical input first converted into light and the light powering
the laser. But the diode laser is far more
efficient. Like light-emitting diodes, diode
lasers generate light when mobile electrons become attached to atoms at the
border of two different semiconductor
materials. Thanks to strategically placed
mirrors, diode lasers convert much more
input electricity into light-light that's limited to a narrow range of desirable wavelengths. Contrast that with flashlamps,
which give off energy across the entire
visible spectrum, the main reason why
so much of their light is never absorbed
and converted into laser energy.
A diode laser can transform roughly
half the input electricity into light, and a

solid-state laser can, in turn, convert about
half of the energy in that diode laser's output into a high-quality laser beam. So less
than a quarter of the input energy emerges
in the beam. That's 25 times as much as
what you get with a flashlamp. And the
improved efficiency lessens the heatdissipation problem, which should make
it easier to construct a battlefield laser.

www.spectrum.ieee.org

hel had established that destroying a moving target at a distance of a
kilometer or two requires around 100 kW
of laser power. That oomph is needed
mostly because of the spreading of the
laser beam.
Although people often think of laser
beams as being pencil thin, beyond a certain distance from the source the beam
spreads out conically, like a searchlight.
If the beam starts with a diameter of one
centimeter, it might well expand to something like 10 centimeters at 200 meters'
distance. By the time it hits a target 2 km
away, the beam could be a meter across.
Extending the target distance from
200 meters for stationary objectives to
2 km for rockets and mortars reduces the
laser power per unit area-the critical factor in igniting the explosives in a bomb or
rocket-by a factor of 100. And to counter that reduction, you need to boost the
power, in turn, by a factor of 100.
Although THEL had gone nowhere,
the Army had been hedging its bets all
along. In 1997, not long after work started
on THEL, the Army tapped Sparta, a
defense contractor headquartered in
Lake Forest, Calif., to build a Humvee

LawREnCE LivERMoRE
HEat-CapaCity LaSER
lawrence livermore's
ingenious but elaborate cooling
mechanism solved the problem
of laser overheating.

with a turret-mounted solid-state laser to
destroy unexploded ordnance exposed
on the ground. They named the test system Zeus, after the thunderbolt-wielding
king of the Greek gods.
A soldier operating Zeus would use
the turret to train a green laser pointer on
the target. With the unarmored Humvee
parked between 25 and 250 meters away-
far enough to keep out of danger-the
soldier would then switch on the highenergy laser beam, invisible to the human
eye because of its 1-micrometer infrared
wavelength. The laser Sparta installed at
its Huntsville location, before shipping
Zeus to Afghanistan in 2003 for field tests,
emitted only a kilowatt-small potatoes
by laser-weapon standards. Although the
beam Zeus uses can burn exposed skin, it
is nowhere near as deadly as an ordinary
bullet. But the targets were easy ones,
stationary and clearly in the line of sight.
Other than the sparkles of dust particles
ignited by the heat, the only trace of the
beam was the zone it heated on the target,
as if it were sunlight focused onto paper.
If you were viewing the target with an
infrared camera, you'd see a small spot
begin to glow as Zeus heated the casing. The steadily brightening spot would
grow in size as the laser's heat penetrated
deeper, through the case and finally into
the explosive payload. The show would
end with, well, a bang.
Zeus went on to field trials. At Bagram
Air Base in Afghanistan, Zeus destroyed
more than 200 rounds of ordnance in
six months of field trials, including
51 rounds in one particularly successjuly 2009 * iEEE SpEctrum * NA

http://www.spectrum.ieee.org

Table of Contents for the Digital Edition of IEEE Spectrum July, 2009