Military HighTech, High Energy Laser Mobile Demonstrator
On the desert floor, on top of a big, sand-coloured truck, a cubic mechanism pivots and fires an invisible infrared beam to zap one target after another. This High Energy Laser Mobile Demonstrator (HEL MD) is a prototype laser weapon developed for the US Army by aerospace giant Boeing of Chicago, Illinois. Inside the truck, Boeing electrophysics engineer Stephanie Blount stares at the targets on her laptop's screen and directs the laser using a handheld game controller. “It has a very game-like feel,” she says.
That seems only natural: laser weapons are a staple of modern video games, and ray-guns of various sorts were common in science fiction for decades before the first real-life laser was demonstrated in 1960. But they are not a fantasy anymore. The Boeing prototype is just one of several such weapons developed in recent years in both the United States and Europe, largely thanks to the advent of relatively cheap, portable and robust lasers that generate their beams using optical fibres.
The output of these fibre weapons is measured in kilowatts (kW), orders of magnitude less than the megawatt-class devices once envisioned for the US Strategic Defense Initiative — an ultimately unsuccessful cold-war plan that sought to use lasers to disable ballistic missiles carrying nuclear warheads.
But the modern, less ambitious, weapons are on the brink of real-world deployment. Tests such as those of the Boeing system show that the lasers have enough power to overcome threats from terror groups — at a fraction of the price of conventional defences. “It's a very cost-effective solution to taking out cheaply made weapons like small mortars or rockets made out of sewer pipe,” says Blount.
In late 2014, for example, the US Navy showed that a ship-mounted laser-weapon system called LaWS could target small boats, such as those used by terrorists and pirates. That experimental weapon is currently installed on the USS Ponce, an amphibious support ship in the Gulf.
Many challenges to full-scale deployment remain, warn developers, from the need to boost the weapons' power to the difficulty of operating a laser in fog and clouds. But specialists in defence and security are starting to take lasers seriously. “After a nearly half-century quest, the US military today is on the cusp of finally fielding operationally relevant directed-energy weapons,” wrote Paul Scharre, an advanced-technology specialist at the Washington DC-based Center for a New American Security (CNAS), in a report on laser weapons released in April.
Aiming and targeting may be battle-ready, but power is still a problem. A commercial laser's 10-kW output is at the low end of what is useful for laser weapons. And using fibres puts limits on the beam's power and quality — not least because at high powers, the cascade of photons surging through the fibre can heat it up faster than it can radiate the energy, and can thus cause damage. To avoid this, researchers are working to combine the output from several lasers.
The ideal way to do this would be 'coherent combining', in which the waves from each laser march together in tightly synchronized formation. This technique is widely used in radio and microwave applications, says Tso Yee Fan, a laser scientist at the Massachusetts Institute of Technology's defence-oriented Lincoln Laboratory in Lexington. But coherence is much tougher to achieve with visible and infrared light. The waves from each laser must have almost identical wavelengths, the planes of their oscillations must precisely align, and the peaks and troughs of each wave must coincide. “In radio-frequency or microwaves, the wavelength's a few centimetres,” Fan says. “In optics, the wavelength's around a micrometre, so being able to do those kinds of controls has been really difficult.”
arkus Martinstetter from MBDA's Future Systems Directorate argues that high-precision targeting minimizes the chances of accidentally hurting bystanders while trying to shoot down targets, especially compared with conventional explosives. “There is no risk from fragmenting ammunition and we only start the irradiation when the aim point is exactly on target,” he says.
Lockheed Martin is also working on laser weapons that can take on targets that are more complex or farther away than can be tackled by its low-cost ADAM system. In March, for example, the company reported that its Advanced Test High Energy Asset (ATHENA) system could disable the running engine of a small truck mounted on a test platform. ATHENA uses a similar adaptive-optics system to the Airborne Laser, coupled with Lockheed's Accelerated Laser Demonstration Initiative (ALADIN) fibre-laser system.
ALADIN combines the output of several fibre lasers, each with a slightly different wavelength, into a single 30-kW beam. This 'wavelength beam combining' approach originated at the Lincoln Laboratory and is similar to methods that channel Internet traffic into fibre-optic cables. Fan notes that this method is easier than coherent combining, but gives better-quality beams than incoherent combining, so it can more easily hit smaller targets from longer distances.