Defense Technology International: Solid-State Lasers; 100 Kilowatts or Bust; Rivals Vie To Develop Next Generation of Lasers

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.