IEEE Spectrum July, 2009 - 24

o tackle the problem of scaling up the
power of a solid-state laser to compensate for distance, in December 2002 the
U.S. military launched a grand challenge
called the Joint High Power Solid State Laser
(JHPSSL). The Army's goal was a 100-kW
electrically powered laser that it could use
for distant battlefield targets. The Air Force
wanted a similar laser for its F-35 Joint Strike
Fighter, and the Navy hoped such a laser
could fend off small-boat attacks like the
2000 suicide bombing that killed 17 sailors
and seriously damaged the USS Cole. Field
commanders insisted that the laser not
require any special power systems, so it had
to run on electricity from the diesel generators that run other front-line equipment.
The first step was a competition to
reach 25 kW by late 2005 between four
entrants: Northrop Grumman, Textron,
Raytheon, and Lawrence Livermore
National Laboratory. The 25-kW output
beam needed to remain tightly focused for
300 seconds while ensuring that the laser
didn't self-destruct. The basic approach
was clear-use diode lasers (here acting
as what laser physicists call pump diodes)
to inject light into thin slabs of laser material containing neodymium atoms, which
in turn emit light at 1.06 µm in the near
infrared. The pump beam passes through
the slab and excites the atoms to produce
high-output energy, and the heat can dissipate through the wide top and bottom.
Cooling is a critical issue, because
solid-state lasers convert only about
20 percent of their electrical input energy
into light output. Again, that may be good
by laser standards, but it still means that
80 percent of the input energy winds up
as heat-four watts of heat for every watt
in the output beam. Much of that heat
goes into the laser slab. The results of
sending a laser beam through a hot slab
are similar to what happens when you
park your car in an open lot on a summer
day. Just as the rising air currents bend
the light over the hood, making it ripple
and waver, uneven heating in a slab will
break up or spread a laser beam. The farther the beam has to travel in these conditions, the more it will spread and the
less damage it can do. And while rocketengine lasers can blow away their waste
32

NA * iEEE SpEctrum * july 2009

laseR
weapons
the path to a deployable
laser weapon is littered with
canceled projects

tHEL
the tactical HighEnergy laser, a joint
project between
israel and the
united States, was
designed to shoot down
short-range rockets.

zEUS

Sparta's Zeus
system was a
turret-mounted
solid-state laser
on a Humvee
that destroyed
unexploded ordnance from a safe
distance. it was the first solid-state
weapon success story.

heat as exhaust, solid-state lasers need
another way to keep from frying.
Lawrence Livermore had already
developed a clever but elaborate way to
deal with the problem, resulting in what's
known as a solid-state heat-capacity laser.
Several ceramic laser slabs, 10 cm square
and 2 cm thick, are mounted on four
rotating wheels. For 10-second intervals,
arrays of pump diodes fire 200 pulses per
second into the edges of the slabs. During
those 10 seconds, the slabs heat from room
temperature to 130 ˚C, at which point the
pumping stops for a quarter of a second as
fresh slabs are rotated into place and the
hot ones are pressed between heat sinks
to cool. Then the cycle repeats.
This design was an ingenious solution
to the cooling problem, and the Lawrence
Livermore laser reached 25 kW. But the
lab didn't make the cut for the 100-kW
round of JHPSSL. Instead, the Army
chose two other designs, from Northrop
Grumman and from Textron Systems.
Rather than moving the slabs, both
designs used liquid coolant and heat
exchangers to keep the slab temperatures
uniform. The big difference between the
Northrop Grumman and Textron systems was in the optical layout.
To appreciate how different these
designs were, you need to understand
that a laser is an oscillator with photons
bouncing back and forth between a pair
of mirrors. Those photons stimulate the
laser crystal to emit more photons of the
same wavelength and phase, which is to
say that their electric and magnetic fields
oscillate in lockstep. There are two ways
to boost laser power: The first is just to
build a bigger oscillator with more laser

abL

the Airborne laser, which is uS $4 billion
over budget and can barely fit into a
retrofitted 747, is designed to destroy
long-range missiles a few hundred
kilometers away.

material. The second is to amplify the
beam by passing it through a moreexcited laser material without mirrors.
Northrop Grumman chose the latter
method-amplification. The output of a
single oscillator was split between two
amplifiers to generate a total of 27 kW for
350 seconds, longer than JHPSSL's stated
300-second goal. For the 100-kW demonstration, Northrop redesigned the amplifiers so that each generated 15 kW and
fit onto a table 1 meter square. Reaching
100 kW required splitting the oscillator
output among seven amplifiers, then carefully matching the phases of the waves
coming out of each in the final beam.
In March of this year Northrop
Grumman crossed the finish line, generating a single 105-kW beam for more than
5 minutes and turning 19.3 percent of the
input electrical power into output photons.
The company's JHPSSL test bed weighed
in at 7 metric tons and measured 2 by 2 by
2.7 meters, but Northrop Grumman engineers have also developed a compact version called Firestrike. Its 15-kW amplifier
modules fit into field-ready boxes measuring 30.5 by 58.4 by 101.6 cm, and seven of
them stack to make a 100-kW weapon that
will weigh only a quarter as much as the
test bed. That includes power supply and
cooling equipment but not the electronics
that adjust the output phases and direct
the combined beam toward the target.
Textron avoids the problem of combining beams by building one big oscillator. "We have a single beam running
through six slab modules in series," says
Textron's John Boness, who oversees the
company's laser program. The light follows a zigzag path through the waterwww.spectrum.ieee.org

illustrations: steVe stanKieWiCZ

ful 100-minute span. The Army was
impressed: Zeus wasn't powerful enough
to destroy buried mines or munitions, but
it was small enough to be mobile, and it
could take on improvised explosive
devices-Iraq's famously deadly IEDs-
if they were not buried under sand.

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