Whenever aerospace and defense can tap into a commercial technology, it hooks up to a pace of development that can speed the fielding of new capabilities. That could be coming true for long-sought-after high-energy laser weapons.
Lockheed Martin has demonstrated that electric fiber lasers can be spectrally combined to produce a high-power, weapon-grade beam. Fiber lasers are used in communications and manufacturing, and component technology is being driven by commercial markets. What the aerospace industry has to do is find a way to take them to higher, lethal power levels.
With the most power yet achieved with high electrical efficiency and beam quality, Lockheed says its 30-kw fiber laser is a key step toward tactical high-energy weapons. The laser was tested under the company-funded Accelerated Laser Demonstration Initiative (Aladin), but Lockheed is under contract to supply a 60-kw fiber laser for field-testing in the U.S. Army's truck-mounted High-Energy Laser Mobile Demonstrator in 2017. A 100-kw version is to be tested in 2022.
“We think fiber lasers are the future for directed-energy lasers,” says Rob Afzal, Lockheed Martin senior fellow and principal investigator for Aladin. “They offer the highest efficiency at high power, routinely over 30 percent; fantastic beam quality, which puts more intensity on target at longer range; and are the most affordable, because component technology is being advanced by the industrial laser market.”
Individual fiber lasers produce only kilowatt-class beams, which must be combined to produce weapon-grade power levels. Instead of merging beams by manipulating the phase of each beam, which requires complex feedback loops, Lockheed uses a technique called spectral beam combining, which is “simple and robust,” Afzal says.
“It is like a prism in reverse. We take a number of beams, each with a slightly different wavelength, and bounce them off a grating. Off comes a single beam with all the beams sitting on top of each other,” he says. “We can maintain the diffraction-limited performance of each individual fiber laser as we power-scale the beam to get the maximum range or minimum dwell time.”
A Northrop Grumman solid-state laser demonstrated power levels beyond 100 kw in 2009. This technology, using diode-pumped slab lasers that are optically combined, is being readied for fielding on warships. But fiber lasers are more than twice as efficient electrically, reducing the size of the supporting power and cooling systems and allowing a high-energy weapon to be packaged into smaller platforms such as aircraft and trucks.
Slab lasers are harder to scale, Afzal says, because it is difficult to cool the lasing material at higher powers and, as temperatures increase, the beam distorts. The Aladin fiber-laser architecture is “fundamentally scalable,” he says. “Scaling is done by adding lasers, and we are no longer worried about getting the heat out. The beams combine in free space.”