With inherently higher electrical efficiency and beam quality, fiber lasers are in a race to reach maturity before the military makes decisions on the development and deployment of high-energy electric laser weapon systems for defensive and offensive missions.
A key step is the U.S. Army’s planned demonstration in 2017 of a 60-kW fiber-laser system developed by Lockheed Martin. But rival solid-state lasers have already exceeded 100 kW in demonstrations and are at a higher technology readiness level (TRL) as the services eye the potential for early fielding of directed-energy weapons.
Lockheed has begun production of the fiber-laser modules for the 60-kW system. The company was awarded a $25 million contract in April to build and test the modular laser for integration into the Army’s Boeing-developed High-Energy Laser Mobile Demonstrator (HEL MD). “We will deliver the laser to the customer at the end of 2016,” says Lockheed senior fellow Rob Afzal.
Previously, Lockheed built a 30-kW system using internal funds to demonstrate the feasibility of combining the beams from multiple fiber lasers while maintaining beam quality and electrical efficiency. The modular technology allows the laser to scale up to power levels beyond 100 kW, Afzal says. After the 2017 demo, the Army plans to upgrade the HEL MD to 100 kW and could do this simply by adding modules, he adds.
The Aladin 30-kW system demonstrated the ability to scale power by combining multiple fiber lasers. Credit: Lockheed Martin |
Generating the laser beam by diode-pumping a long optical fiber results in higher beam quality and electrical efficiency but less power than solid-state devices using slabs of laser crystal as the gain medium. This requires the beams from multiple fibers to be combined efficiently to form a single high-power beam. Lockheed says its laser system can achieve 40% efficiency, reducing the power-generation and cooling requirements for the overall weapon system.
Afzal says the beam-combined fiber laser’s higher power and beam quality puts more irradiance on the target at greater range. This can increase engagement range or reduce defeat time, allowing a laser weapon system to “shoot-look-shoot” against multiple targets. Lockheed uses spectral beam combining. The output from each fiber-laser module is at a slightly different wavelength. A diffraction grating combines the beams by laying one on top of the other to form a single high-power beam—like a prism in reverse, he explains.
Compared with coherent beam combining used in other high-power lasers, spectral beam combining provides the highest “power-in-the-bucket” efficiency, a measure of beam quality that is a function of the power delivered to the target area. “The issue with a phased array is the sidelobes. The power in the lobes does not provide effect on the target,” says Afzal. “Coherent is efficient, but there is a lot of added complexity we feel isn’t necessary for the types of power and tactical applications we are trying to achieve. We went for the simplest, most elegant approach.”
The 30-kW Aladin demo system has around 100 fiber-laser modules. The 60-kW prototype for the Army has fewer, higher-power, kilowatt-class fiber lasers. “It’s almost 1 for 1 [lasers vs. kilowatts]. You can tack on 5-10%. That’s one of the big advantages of spectral beam combining,” says Afzal. On the end of each laser module is a delivery fiber that terminates in the beam-combiner box. This outputs a single high-power beam to the weapon system’s laser-beam director turret.
One aim of the demo system was to understand how to manufacture the lasers and what life-limited elements would wear out. The production modules are “more rugged, more traceable to a tactical vehicle and to beyond 100 kW,” he says. The truck-mounted HEL MD has been tested against mortars and unmanned aircraft with a 10-kW industrial fiber laser, but range and lethality was limited. After demonstration of the 60-kW system in 2017, plans call for tests of the 100-kW version by 2022.
Lockheed makes its own fiber lasers because of the need for high beam quality, but it uses component technologies such as optical fibers and pump diodes from the commercial market. “There have been two revolutions in lasers: telecommunications, and industrial cutting and welding. We bring them together to create a new class of laser,” Afzal says.
Industrial fiber lasers are available with higher power, up to 10 kW per fiber, but not with the quality required for beam combining. Most live-fire tests of laser weapons so far have used industrial lasers but scaled the power by aiming multiple beams at a common point so they overlap. This is done with the U.S. Navy’s 30-kW Laser Weapon System prototype, which has been deployed operationally for evaluation in the Persian Gulf on the forward-staging ship USS Ponce.
The Athena prototype weapon using the Aladin fiber laser disabled the engine of a truck in tests. Credit: Lockheed Martin |
Advantages of a modular fiber laser include scaling, cooling and packaging. “With a modular design, you can scale to higher power by loading more modules into the rack, like blade servers in a server farm,” he says. Each module is independently cooled. “As we add more modules, we increase the size of the cooling system but not its complexity. It’s parallel, not serial. Previously, you ran into a scaling problem where, as the laser got more powerful and the slabs got bigger, you couldn’t get the heat out.”
Flexibility in packaging the modules is another benefit. “You can stack them vertically or horizontally, or in two cabinets. They are all independent, and the fiber delivers the power,” Afzal says. The Air Force Research Laboratory (AFRL) is looking at systems for sixth-generation fighters where the laser modules would be distributed throughout the aircraft and the beams routed by fibers through the tight confines of the airframe to a conformal array on the fuselage surface.
As it begins building the Army system, Lockheed is studying how the fiber-laser technology can be applied to other requirements. “We are looking at how we could package the system into a weapons module for the Littoral Combat Ship or into a pod for an aircraft, as well as Army tactical vehicles,” he says.
One potential application is AFRL’s planned Self-Protected High-Energy Laser Demonstration (Shield), for which a solicitation is expected shortly. Shield aims to demo an anti-missile self-defense pod for fighters by 2020 and a longer-range, 100-kW system by 2022. The Air Force wants the laser technology for a self-defense pod to be scalable to an offensive weapon that can be carried by larger aircraft, beginning with special-operations gunships.
“The Shield technology level we can do now,” says Afzal. “We would look at modifications to make it more relevant to the Air Force, but it is not a next-generation system.” But the key issue could be maturity of the fiber-laser technology versus other solid-state electric lasers. Army trials of the 60-kW system will take Lockheed’s technology to TRL 6, “arguably TRL 7 depending on how they use the system and if they do tactical engagements,” he says. The race is on.
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