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Sharon Weinberger
Aviation Week & Space Technology
22 May 2006
"Can you bring a gunship to Kirtland?"
That's how Rudy Martinez got involved in
laser weapons. Martinez, then an operations officer, got permission
to fly an AC-130 to Kirtland AFB, N.M., where the man who
called showed him a classified weapon that would, he said,
"revolutionize the gunship": a chemical laser. The
laser was as big as the plane, Martinez recalls.
The caller was one of the inventors of the chemical oxygen-iodine
laser, and the year was 1977. Today, Martinez is a deputy
branch chief at the Air Force Research Laboratory's Directed
Energy Directorate, and runs a simulation center that demonstrates
directed-energy and laser weapons on tactical aircraft. While
the chemical laser is still not deployed, Martinez remains
a proponent of directed energy.
Laser weapons have always seemed tantalizingly close to deployment,
but logistics, size and cost have led to cancellations, delays
and other disappointments. While chemical lasers opened the
door to megawatt-class directed-energy weapons, their size
and logistical complexity have prevented them from replacing
or supplementing tactical weapons on bombers, strike fighters
and ground vehicles.
The latest developments in solid-state lasers may finally
put directed energy on tactical platforms. Building a 100-kw.
solid-state laser that is compact enough to fit on a tactical
vehicle and simple enough to be deployed has the potential
to tip the scales toward directed-energy weapons.
Last summer, the High-Energy Laser Joint Technology Office
(JTO) at Kirtland sent a team from MIT's Lincoln Laboratory
to assess four solid-state lasers developed by Northrop Grumman,
Lawrence Livermore Laboratory, Textron and Raytheon. In December,
the JTO selected two companies for the next stage of the Joint
High-Powered Solid-State Laser (JHPSSL) program. It awarded
contracts, worth about $30 million to Textron and almost $60
million to Northrop Grumman, for a three-year effort to build
a 100-kw. solid-state laser. Though left out, Livermore and
Raytheon continue to work on solid-state lasers.
Northrop Grumman, working at its Space Technology Sector in
Redondo Beach, Calif., combined multiple low-power beams to
form one powerful laser. The design uses a yttrium-aluminum
garnet as the lasing material and combines it with a master
oscillating power amplifier, which takes a low-power beam
and amplifies it in stages. Beam combining allows Northrop
Grumman to scale up power. For the second phase of the program,
Northrop Grumman assembled two laser chains consisting of
four gain modules, each on 5 X 12-ft. optical benches.
The apparatus--benches covered with optics and an active cooling
system--is far from deployable. "This rig is huge,"
says Jeffrey Sollee, JHPSSL project manager. "But it
wasn't meant to be small and compact. It was meant to be experimental."
Even as Northrop Grumman moves to phase three of JHPSSL and
attempts to make its setup more compact, size remains a challenge.
"The one area that will probably represent the biggest
challenge is getting eight of the 12.5-kw. beams tiled together
in a small footprint side by side with a lot of mirrors close
to one another, and not have problems with stray light and
heating of components," Sollee says.
But the tradeoff is a path to scaling power. Now that the
hard part of beam-combining is proven, more power simply requires
more laser chains, and there doesn't appear to be a theoretical
upper limit other than the increase in size. "[T]here's
nothing in the physics that says I can't go beyond 100 kw.,"
says Northrop Grumman's Dan Wildt, director of directed-energy
systems.
Efficiency remains a challenge. Northrop Grumman achieved
"wall plug" efficiency (power in vs. power out)
of 10% in the second phase of JHPSSL. The final phase, which
is based on electro-optical efficiency (a light-to-light comparison),
has a requirement of 17% and a goal of 19%.
Beam quality (power directed to a specific area) is another
challenge. The goal of the next phase of JHPSSL is less than
two times the diffraction limited (this measures how tight
a beam the laser achieves). Some competitors decline to provide
exact numbers, but Northrop Grumman says it has good beam
quality--1.75 times the diffraction limited (at power up to
19 kw.). It must maintain quality at higher power levels.
Textron was an unlikely entrant in the program, because its
laser was not part of the earlier competition. It was developed
with company funds and some government support. Textron says
its investment in lasers is part of a strategy to position
itself in a growing market. "Textron is in the business
of strike weapons and munitions. It's very clear that if high-power
laser technology is successful, it's likely to play a major
role in those markets," says John Boness, vice president
of business development.
Textron's "ThinZag" approach uses very small and
thin laser-gain material, which lends itself to efficient
cooling. According to Dan Trainor, who heads solid-state laser
work at the company, the ThinZag configuration uses gain material
placed between pieces of quartz. The beam "zigzags"
through the quartz, the cooling material and the gain material,
then back through the cooling material. It repeats the process,
then turns around and goes in the other direction.
There's another advantage to this approach, notes Trainor.
Most solid-state lasers produce a tall, skinny beam, but the
ThinZag beam has a more useful dimension. "[I]t's essentially
square and rectangular," he explains, which helps maintain
the beam. "The hardest thing for anyone to do is the
beam quality."
Whereas Northrop Grumman scales up by adding lasers, Textron
adds power by increasing the size of the slabs. Textron employs
ceramic gain material rather than crystal. Trainor touts this
approach as superior: "If you restrict yourselves to
crystal, then how big you can grow crystal restricts how big
of a laser you get; you can only make crystal so big."
While declining to specify recent power-output levels, Trainor
notes the company has achieved its projected output for 1-,
5- and 15-kw. lasers. Getting to 100 kw. should not be a big
challenge, he maintains.
When it comes to power, scientists at Lawrence Livermore Laboratory
in California are heads and shoulders above the competition.
(Officially, Boeing competed for the JHPSSL using the Livermore
laser. As a government lab, Livermore can't compete with private
companies for government work.) Livermore's laser, which grew
out of the earlier Solid-State Heat-Capacity Laser program,
reached an impressive 45 kw. during JHPSSL testing, far beyond
Northrop's 27 kw.
The Livermore laser uses four ceramic slabs pumped by nine
rows of 80-diode bars, for a total of 720 diodes that emit
light angled into 10-cm.-sq. ceramic ytterbium yag slabs.
Those slabs, about 2 in. thick, power the laser. Power is
raised by adding slabs. Each slab increases output by a factor
of two.
Bob Yamamoto, program manager for Livermore's solid-state
heat-capacity laser, says this technique is better than Northrop's
beam combining, which must ensure the beams combine homogeneously
and distortions are corrected. But Yamamoto faces a challenge
in laser size. The Livermore setup is relatively large at
8 ft. long, 5 ft. tall and 4 ft. wide. But the payoff is power.
"We get oodles of power in a small package."
Another issue is thermal management: The Livermore laser has
a separate cooling system that limits runtime. The laser works
in pulsed bursts, powering up for as long as it can without
burning out diodes, then turning off to cool. The cooling
equipment occupies a room behind the laser, with pipes running
water from tanks 24 hr. a day. While the tanks could be smaller,
overall system reduction is a challenge to getting the laser
out of the lab.
Energy efficiency, runtime, cooling and beam quality remain
significant hurdles for Livermore. "We're in the 2 to
3 range [diffraction limited]," Yamamoto says of beam
quality. Moreover, the Livermore laser is about 10% efficient--within
the range of competitors, but less than the military wants.
"That's not very good, but it's state of the art."
Despite problems, high power output is an attractive feature--and
Livermore is the only one among the competitors that has a
blueprint for what so far has been the domain of chemical
lasers: megawatt-class strategic lasers. Yamamoto says the
lab has run computer simulations demonstrating the idea's
feasibility. The design involves doubling the size and increasing
the number of slabs to 16. It would require more diodes and
more cooling, but would still yield a compact, megawatt-class,
solid-state laser. "It's peanuts in the amount of real
estate required," Yamamoto says.
The question for Livermore, according to Mark Neice, deputy
director of the High-Energy Laser JTO, is if there is a place
for a heat-capacity laser. The answer is "certainly,"
but "there has to be more work done on thermal properties
of ceramic materials at higher power," he remarks.
Raytheon asserts it took the most scientific approach to solid-state
lasers. The company's work relies on a phased conjugate master
oscillator power amplifier, designed to scale up power without
adding lasers or optics. The architecture employs a phased
conjugate mirror that reverses the wave front of the beam,
correcting distortions as it bounces back in a phased conjugate
loop. "The phased conjugate mirror . . . smoothes out
degradation," notes Barry Alexia, Raytheon's director
of strategy and business development for space and airborne
systems in El Segundo, Calif.
But the laser never broke the 2-kw. threshold during the JHPSSL
program, concedes Alexia. Problems cropped up with the amplifiers.
"You're looking at some high precision in the alignment,"
he says. "There are various slabs, and slabs are unique
to amplifiers." Raytheon declines to release numbers
on efficiency or beam quality.
Nonetheless, the company remains optimistic. "We're able
to identify that this is a path forward as far as size,"
Alexia says. "We're also able to identify a path forward
as reducing the thermal radiant generated on this laser. And
we're able to identify the amount of power necessary. We feel
that the other ideas, even though they may have higher power,
don't lend themselves to [a compact] size, or to a lot of
power issues. . . ."
The design, Alexia says, is "a path forward to make this
technology viable for its intended platform." As to why
Raytheon didn't get beyond 2 kw., his response is simple:
"The technology is pretty far-reaching."
The question is whether any laser will make it to the battlefield.
Assuming they break 100 kw., the lasers still need to be integrated
into a weapons system and made compact enough to fit in a
ground or air vehicle. "The Army has said they'd like
to have the solid-state laser for counter-RAM (rockets, artillery,
mortar) in the field by 2010," Northrop's Wildt says.
"We think that's doable."
Yet the history of laser development is littered with efforts
that never got off the ground. Earlier this year, Northrop
Grumman moved the prototype of its space-based Alpha chemical
laser to a display case, a sign that chemical lasers, at least
in space, are a thing of the past. While Northrop Grumman
still sees chemical lasers as the only strategic laser in
the near future, there is a heightened focus on solid-state
lasers.
"I think you'll see a solid-state laser on some version
of a fighter aircraft," remarks Martinez at the Air Force
Research Laboratory. But he adds that solid-state lasers may
be just a "stepping-stone" to a tactical laser capable
of being outfitted on the F-35 Joint Strike Fighter. "Fiber
lasers, I believe, are the future," he says, noting their
potential to reduce size significantly over current lasers
(see sidebar).
Martinez, however, tempers his optimism with caution. He notes
that the chemical laser has also come down in size, pointing
to the Advanced Tactical Laser, the 100-kw. chemical laser
under development for the AC-130 gunship. "It does take
all the volume in the gunship, but it's a hell of a lot better
than the [version] I saw in 1977."
That said, tactical lasers have yet to appear on the battlefield.
"I'm still waiting, 30 years later," Martinez notes.
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