ONCE again, science fiction has predicted science fact. Remember
those movies where the hero (or villain) uses a beam from a compact
laser to blow a rocket out of the sky? Last December, that generic bit
of sci-fi drama took a step closer to reality. In a demonstration at the
White Sands Missile Range in New Mexico, the solid-state heat-capacity
laser (SSHCL) burned a 1-centimeter-diameter hole straight through a
2-centimeter-thick stack of steel samples in 6 seconds. The electrical
current to do so came from a wall outlet and cost no more than 30 cents.
While large chemical lasers have successfully shot down tactical
rockets, the SSHCL design supports the weight and size requirements for
a future mobile deployment.
The SSHCL, designed and developed at Lawrence Livermore, is the
prototype of a laser tactical weapon, which shows promise as the first
high-energy laser compact enough in size and weight to be considered
part of the Army’s future combat system (FCS) for short-range air
defense. The FCS is a component of the Army’s vision of sensors,
platforms, and weapons with a networked command and control system. The
more advanced version of the laser weapon system, now under development,
will be battery-powered and—at 2 meters long and less than a meter
across—small enough to be mounted on a hybrid-electric high-mobility
multipurpose wheeled vehicle (Humvee). In this configuration, the
Humvee’s generator and batteries could power both the vehicle and the
laser, requiring only diesel fuel to support full operation.
The SSHCL offers speed-of-light precision engagement and destruction
of a variety of targets, including short-range artillery, rockets, and
mortars. There is a current need for effective protection against these
weapons on the battlefield. The project is sponsored by the U.S. Army
Space and Missile Defense Command and has a number of commercial
partners, including General Atomics, Raytheon Co., PEI Electronics Inc.,
Northrop Grumman Corp., Goodrich Corp., Armstrong Laser Technology Inc.,
and Saft America.
Meeting the Challenges
The SSHCL delivered to White Sands for testing last September has an
amplifier composed of nine disks of neodymium-doped glass (Nd:glass). In
this prototype, an electrical source powers flashlamps, which in turn
pump the disks, which then release the energy in pulses of laser light.
The average output power of the SSHCL is 10 kilowatts, and it can
deliver 500-joule pulses at 20 hertz in 10-second bursts—essentially
vaporizing metal. The prototype requires 1 megawatt of input power to
produce a 13-kilowatt laser beam. Project manager Brent Dane, of
Livermore’s Laser Science and Technology program, notes that the
ultimate objective of the project is to build a next-generation system
with enough electrical efficiency to produce a 100-kilowatt laser beam
from the same 1 megawatt of input power. The final version will be
capable of firing 200 pulses per second.
The Livermore team is focusing on the technological challenges that
remain to building the 100-kilowatt system. Dane enumerated the three
areas of concentration: growing large crystals of neodymium-doped
gadolinium–gallium– garnet (Nd:GGG) for amplifier disks; developing the
technology needed to make diode arrays large, powerful, and
cost-effective; and defining the laser architecture and technology that
will allow high-quality beams to propagate precisely over long
distances.
Although the prototype uses Nd:glass for its laser amplifier disks,
the final version will use Nd:GGG. “There are many reasons for choosing
Nd:GGG,” explains Mark Rotter, an electrical engineer who is leading the
diode-pumped Nd:GGG effort. “Compared with Nd:glass, Nd:GGG boasts a
higher mechanical strength and higher thermal conductivity, which, in
combination, will allow us to rapidly cool the disks between runs and
reduce the turnaround time between laser firings. The Nd:GGG is also
twice as efficient in converting pump energy to output beam energy.” The
challenge—to grow the crystals large enough to manufacture the nine
13-square-centimeter slabs needed for the 100-kilowatt laser—is well on
its way to being met. Northrup/Grumman Poly-Scientific, the commercial
partner responsible for growing the crystals, is now producing
high-optical-quality Nd:GGG crystals up to 15 centimeters in diameter.
The ultimate goal is to grow crystals approximately 20 centimeters in
diameter.
To pump these Nd:GGG amplifier disks, the SSHCL will use arrays of
laser diodes instead of flashlamps because diode arrays are more compact
and efficient than flashlamps and, more importantly, diode radiation
generates less heat in the Nd:GGG laser crystals. The challenge is to
make the diode arrays large, powerful, and cost-effective and to come up
with a cooling scheme that will work in the field.
Lawrence Livermore’s Ray Beach, who leads the diode array portion of
the project, explains, “Cooling high-average-power laser diode arrays is
a unique and challenging problem in the field of thermal engineering.
Although laser diodes are extremely efficient devices by ordinary laser
standards—they typically convert 50 percent of their electric input
power into light output—the remaining 50 percent of the input power
shows up as high-intensity heat from a very compact source. Because the
arrays operate near room temperature, there isn’t much opportunity to
radiate away heat or use standard electronic cooling techniques such as
forced air.”
Livermore engineer Barry Freitas came up with a revolutionary
packaging technology that solves the problem of creating high-density
diode arrays. In this approach, small laser diodes are soldered to
low-cost silicon substrates that are etched with thousands of tiny
(30-micrometer-wide) microchannels. Cooling water flows through these
microchannels, which act as high-performance heat sinks. The team used
this packaging design to create the world’s highest average-power diode
array—41 kilowatts of peak power from a 5- by 18-centimeter package.
Arrays that produce 100 kilowatts of power are in production. Work is
under way with Armstrong Laser Technology to commercialize the
silicon-based diode laser package to support the production needs of the
100-kilowatt laser development.
The team is also working on an optical system that will make a beam
of high enough quality—that is, sufficiently narrow, intense, and
well-shaped—to propagate 10 kilometers and still hit and disable its
target. “In the final system, the laser pulse will travel through nine
slabs of crystal, and no matter how good the optics are, the beam will
pick up distortions along the way. It’s those distortions in the
wavefront that we are addressing, because they decrease the power that
can be extracted in the laser beam and cause that beam to diverge more
on the way to the target,” explains Dane.
A team led by Jim Brase in the Physics and Advanced Technology
Directorate is developing an adaptive resonator system that will sense
distortions in the wavefront and correct them in the system. The
resonator—which is based on adaptive optics technology developed at
Livermore—includes a deformable mirror, control electronics, and sensors
to detect the shape of the laser pulse’s wavefront. A deformable mirror
will be placed inside the laser resonator, and a wavefront sensor will
be used to measure the output beam during operation. The sensor measures
the difference between the actual shape and a perfect, flat wavefront.
Computer-controlled actuators on the mirror then raise or lower small
sections of the mirror’s surface to correct distortions in the incoming
light so that a high-quality beam is maintained from the laser
resonator.
Future Looks Bright
The solutions to these challenges are being incorporated into an
SSHCL testbed—a module made up of a three-slab Nd:GGG amplifier pumped
by laser diode arrays. This testbed will be configured as a laser system
to demonstrate the pulse energy at a high repetition rate in 2003. The
final version of the SSHCL, which would have an output power of 100
kilowatts under burst mode for several seconds, is expected to be ready
to demonstrate to the Army by 2007.
Meanwhile, at the White Sands Missile Test Range, the Army, with
Laboratory support, is putting the prototype through its paces, testing
it on aluminum and steel to determine what types of power and pulse
format will optimize the final weapon system. The Army will also use the
prototype to address issues such as lethality, beam degradation due to
atmospheric effects, and precision optical pointing and tracking.
The future for the solid-state laser looks promising, notes Dane.
“The system we delivered to White Sands is just the starting point. The
goal is to have a laser weapon system that is small, cost-effective, and
mobile, which protects against tactical threats while meeting the
sponsor’s other military requirements. We’re confident we’ll meet these
goals.”
