ARTICLE

December 2002

Tactical Laser Weapons Still Many Years Away

Air Force solid-state laser program focuses on increasing power levels and beam quality

by Sandra I. Erwin

The prospect of highly precise, directed-energy guns has tantalized military planners for decades. But only recently has the Defense Department begun to put real money into laser technology that ultimately could deliver speed-of-light tactical weapons.

Specifically, the Pentagon will spend up to $49 million during the next two years to develop a solid-state high-power laser that could be morphed into a weapon for use on tactical jet fighters, Navy ships or Army trucks.

Lasers are light radiating devices that generally are differentiated by the lasing media, which can be gas, liquid or solid state.

So far, only chemical lasers have achieved the level of power needed for the device to be militarily useful. Examples are the Air Force Airborne Laser—a megawatt-class system installed on a Boeing jet—and the Army’s ground-based Tactical High Energy Laser. The ABL was designed to shoot down ballistic missiles, while THEL can defeat rockets, mortars and artillery shells.

But chemical lasers—where the energy is created by a chemical reaction—are unpractical as battlefield weapons. They are too big and logistically complex.

Solid-state lasers use crystal or glass as lasing media. They are electrically powered, which makes them attractive for compact weapon systems.

The technology, however, has lagged. Even though solid-state lasers have been around for 30 years, they have not reached the power levels required for a weapon-grade laser. Solid-state lasers are prevalent in low-power applications, such as laser marking and machining.

Generally, a weapon-grade laser would require, at a minimum, power levels starting at tens of kilowatts, up to megawatts.

An industry competition is now underway to develop a 25 kw solid-state laser. The project, managed by the Air Force Research Laboratory, will establish whether the technology is mature enough to warrant further development.

Even if AFRL scientists successfully prove that the 25 kw solid-state laser works in the lab, experts caution, much more development and engineering work would be needed to turn the basic technology into an operational weapon.

Defense contractors such as TRW and Raytheon now claim that a 25 kw solid-state laser is achievable and that, within five to 10 years, they could jack up the power to 100 kw.

The Air Force program was prompted, to an extent, by such claims.

“Contractors have come to us and said they feel they are ready to demonstrate this technology,” said Capt. Kalliroi Lagonik, laser physicist at the Air Force Research Lab. “Based on what our scientists know and what the contractors have said, we decided to go ahead and really push that technology,” she told National Defense.

Companies will have two years to complete their demonstrations. Lagonik said she could not disclose how many companies are participating in the competition or how many will receive a share of the $49 million program. The proposals were due in September and the winners will be selected in 2003, Lagonik said.

The 25 kw laser demonstration, she said, is “only a stepping stone.” Ultimately, “Our goal is to develop a 100 kw solid-state laser.”

Lagonik recognized, however, that several obstacles stand in the way of a 25 kw solid-state laser.

Among the technical concerns are the ability to reduce the excess heat generated in the laser, to maintain a high-quality beam over a long distance for an extended period of time and making the optics rugged enough to withstand high power levels.

Elihu Zimet, a senior research fellow at the National Defense University, said that solid-state lasers have “the most potential” for a compact engineered weapon. Nevertheless, he cautioned, there is a lot of “marketing and hype” surrounding this technology. The 25 kw effort, he said, is “high risk but not physically impossible.”

Solid-state lasers, he said, “have the largest challenge to scale up to megawatt power levels, because of waste heat removal.” Waste heat from a chemical laser is carried off in the flowing gas medium, Zimet explained. “With a solid-state laser, the heat remains in the laser medium, increasing the temperature until lasing with acceptable beam quality is impossible.”

Heat removal usually is done by physically cooling the slabs of lasing material (with water or air) or by operating the laser in short pulses followed by a cool-down. It is preferable to physically cool the laser, he said, because it does not require interruptions in the lasing.

Cooling a laser is a “very difficult engineering challenge,” said Zimet, especially when the goal is to generate high levels of power. In a 100 kw solid-state laser, for example, there could conceivably be 900 kw of wasted power that has to be eliminated. That is because, by nature, solid-state lasers are not efficient. Ten percent is considered high efficiency, compared to most solid-state lasers that average 1 percent efficiency.

Further, high-power solid-state lasers will never be affordable unless the laser diodes used to pump the laser come down in price, said Zimet. For example, pumping a 100 kw laser that is 10 percent efficient would require a million diodes. Diodes cost about $100 per watt.

“That would be quite an expensive laser,” said Zimet.

Only a handful of companies make these high-powered diodes. Low-power subwatt level diodes are used today for many commercial applications. But only high-

brightness diodes can produce a weapon-grade solid-state laser. Most solid-state lasers today are powered by flashlamps, not diodes.

Because conventional lamps consume lots of power, they require expensive cooling systems. Experts said that diode-pumped lasers are between three and five times more efficient and have a much longer lifetime, typically up to 10,000 hours compared with a few hundred hours for lamps.

Joint Strike Fighter
If the 100 kw solid-state laser ever comes to fruition, it would be a candidate weapon for the F-35 Joint Strike Fighter, now in deve-lopment at Lockheed Martin Corp.

JSF pilots would shoot the laser to blind the seeker sensors of enemy missiles fired from the ground or from aircraft.

The design of the F-35 makes it a relatively easy fit for a solid-state laser, said Neil Kacena, Lockheed Martin’s deputy for advanced development programs. As he sees it, “the challenge is to be able to focus the beam and [to develop] the fire-control system.”

For a JSF weapon, Kacena said, “We believe a 100 kw is the target power level.”

He noted that the vertical takeoff version of the F-35 has a shaft-driven lift fan, which, if removed, would open up an ideal spot for the laser. “Not only do you have the volume but also the shaft, which brings in 27,000 horsepower, so you have an enormous amount of power,” said Kacena.

As far as the range goes, he said, lasers are quite limited. “You want to get relatively close.” Conceivably, a 100 kw laser beam could reach a few hundred feet.

But the technology has yet to overcome many obstacles, Kacena cautioned. The atmospheric disturbance surrounding a fighter jet is likely to distort the laser beam and impede its accuracy.

Lockheed Martin has been a “cheerleader” for the laser manufacturers, said Kacena, because the company would like to see a laser weapon in the JSF.

At least two companies—TRW and Raytheon—are working on high-power solid-state lasers and vying for the Air Force Research Lab award.

Stephen J. Toner, deputy for missile defense at TRW Space & Electronics, said he is optimistic about the potential market for a 25 kw solid-state laser.

“We’ve seen a broad increase in interest from a number of customers,” he said in an interview.

The 25 kw laser prototype that TRW designed for the Air Force competition can be fitted in a pod, so it could easily be installed on tactical aircraft, both manned and unmanned, Toner explained. The Boeing Co. asked TRW to design a laser pod for the Air Force unmanned combat aircraft, the X-45.

The beam would be powerful enough to disable enemy cruise missiles or ground vehicles, he said. “You won’t destroy the vehicle, but you may be able to disable it by firing at the tires and burning the tires or a hole in the hood.”

The key to success in this technology, he said, is “beam quality.” The beam quality is what separates a laser from a flashlight. Toner expects that if the beam quality is high enough, a 25 kw solid-state laser could be projected out to 3 or 4 km and “burn a hole in metal.”

TRW’s prototype is an actively cooled laser. Water cools the device inside and air cools the outside.

Even though solid-state lasers cannot achieve the megawatt power found in chemical lasers, they do have a big advantage: they can penetrate the atmosphere much more easily. The chemically powered Airborne Laser can’t shoot into the atmosphere, Toner pointed out.

Looking ahead, he said, “We are probably five-10 years away from a 100 kw level.” Once the power is achieved, it will take several more years to engineer and package the weapon for specific platforms.

Toner conceded that, for a long time, he did not believe that solid-state lasers could be made into weapon-class devices. “There’s been a lot of skepticism,” he said. “I think we are finally starting to turn the corner.”

In recent years, the Lawrence Livermore National Lab and the Army successfully developed a 10 kw solid-state laser. That laser, however, is not diode-pumped nor is it actively cooled. It must be shut down after each use, so it can cool down.

The Army’s system—also co-developed by Raytheon—is called a heat-capacity laser. It is only 1 percent efficient—to get 10 kw of power, it requires a power source of 1 megawatt.

Raytheon officials said they hoped the heat-capacity laser will give the Army an opportunity to experiment with the technology and figure out potential applications.

“The Army is considering changing the heat-capacity laser from flashlamps to diodes,” said Michael Booen, Raytheon vice president for directed energy weapons.

The company is trying to convince the Army to fund a 15 kw mobile laser that could be “put in the hands of majors and colonels out there and have them get a feel for it,” Booen told reporters. Getting feedback from troops would help the contractors, “so when we start producing these weapons, we know what military utility they may have.”

Chauncy F. McKearn, manager of Raytheon’s high-energy laser program, said the company is not only competing for the Air Force 25 kw laser award but also is “actively” working with Lockheed Martin on concepts for a 100 kw weapon for JSF.

“JSF has all the desirable features that you’d like to have for a laser weapon,” McKearn said.

Other laser pod designs are in the works for the Navy’s Super Hornet aircraft.

Raytheon officials say they believe that they have cracked the toughest nut in the development of these lasers: the beam quality. The company has a patented technology based on special mirrors and optics—called phase conjugation—that McKearn claims can fix the beam distortion and poor focus.

Another breakthrough has been the advent of new lasing materials, specifically ytterbium, said McKearn.

In Raytheon’s flashlamp-pumped lasers, the lasing material is a combination of YAG and neodymium. As the company began to move to diode-pumped lasers, researchers found that the diodes can pump ytterbium, a metal that generates 75 percent less heat than neodymium. The new materials greatly diminish the cooling problem, said McKearn.

The availability of high-power diodes, however, remains a concern. McKearn predicts the cost will go down from current levels of $70 to $100 per watt down to $5 per watt during the next several years.

Five years ago, he said, “a single 100 kw laser would have used three times the world’s yearly production of diodes.”

Zimet, the NDU researcher, said the industry must address this problem so it can deliver on its promises of “affordable” weapon-class solid-state lasers. “If a laser device of 100 kw costs $30 million or $40 million, the military probably won’t be interested.”

Back To Top