— More than 30 years after a laser first shot down a moving target in the skies above New Mexico, US troops have yet to fire a laser weapon on the battlefield.
Despite ploughing billions of dollars into developing laser weapons and carrying out numerous high-profile tests, the Pentagon has been unable to convert the technology into a suitable system. The problem is that existing systems are huge, lumbering beasts that need their own trailer just to carry the fuel. This would be no good for protecting troops in a fast-moving battle, for example, or on fighter aircraft.
So the Department of Defense is now pinning its hopes on solid-state lasers, which can be powered by electricity rather than chemical fuel. These lasers have also been years in development, but last month prototype system designs from two competing US contractors, Northrop Grumman of Los Angeles and Textron Systems of Wilmington, Massachusetts, passed the Pentagon's preliminary review. The companies now have until the end of 2008 to demonstrate that their devices can fire a 100-kilowatt beam - enough to take down a high-speed rocket - for 300 seconds in a simulated battle. The teams are already well on their way: late last year Northrop Grumman set a new record when its laser fired a 27-kilowatt beam for 350 seconds.
The Pentagon wants to fit such lasers to the back of a truck or assault vehicle, which would move in concert with advancing troops and zap missiles within a few kilometres of them. The lasers could also protect ships against cruise missiles, or give fighter jets a defence against rocket strikes or a means to attack enemy aircraft.
"Lasers are highly precise and cause negligible damage to surrounding areas," says Mike McVey of Northrop Grumman. "They are also much safer to people in the vicinity of the threat." A solid-state laser of less than 100 kilowatts could hit the engine of a vehicle, disabling it and bringing the vehicle to a halt without harming the people inside, he claims. Instead of destroying radio or television masts to disable communications infrastructure, the idea is that a laser weapon could just cut power lines or cables, making reconstruction work quicker and easier.
So far, the power levels of solid-state lasers are nothing like those of chemical lasers, which have already fired beams of over 1 megawatt. They rely on the reaction of two chemicals to produce a light-emitting gas. This then passes between two mirrors that reflect this light, exciting more of the gas molecules and producing a more powerful beam.
Because of their power, chemical lasers will be used in the US air force's Airborne Laser programme (see "Learning to fly"). This is designed to carry out missions such as patrolling the borders of states that the US deems to be its enemy, zapping missiles during their vulnerable lift stage from hundreds of kilometres away.
This high power comes at the expense of portability, though. And the need to keep chemical lasers supplied with fuel during a military operation would be a logistical nightmare. Fuels would need to be stored on site, and if any laser were to run out of fuel it would become a defenceless sitting duck.
As a result of these concerns, the Pentagon has dropped development of its most successful laser weapon to date, the Tactical High Energy Laser (THEL). Between 2000 and 2004 this prototype chemical laser was used to shoot down mortar shells, rockets and other short-range missiles in a series of tests at White Sands Missile Range in New Mexico. But THEL was the size of several trailers parked side by side, and attempts to build a smaller version failed to convince the Pentagon that the device was worth the headache of coping with its fuel needs.
In comparison, solid-state lasers can be powered by electricity. The type used for military applications essentially involves applying a voltage across a semiconductor diode made of gallium, aluminium and arsenic, which converts the electrical energy into light. Such semiconductor lasers are very good at generating raw power, but their beams are not suitable for destroying a target. So the light from many diodes is then directed onto a separate sheet of transparent material, where it excites neodymium atoms within the material, producing a single beam of laser light. By passing this light through a series of sheets, the beam's power can be increased.
"As long as you've got the fuel to power vehicles, you have electrical energy available," says Mark Neice of the Air Force Research Laboratory in New Mexico. As a vehicle moves along it can generate enough electricity to power a laser, he says.
For example, Northrop Grumman is working with BAE Systems in Farnborough, UK, to install a solid-state laser in a hybrid-electric armoured vehicle, designed to defend against rocket-propelled grenades. Northrop claims the cost of firing the laser would be just a few pennies per second, compared to $1000 a shot for the advanced version of its chemical laser, THEL, and a few million dollars to fire a single Patriot missile.
Solid-state lasers may not need to be as powerful as their chemical cousins, says Neice. This is because the devices emit light at wavelengths of close to 1 micrometre, in the infrared range, compared to 4 micrometres for chemical lasers like THEL. As a result, the beams tend to propagate more easily through the atmosphere, meaning more energy will reach the target than with chemical laser beams.
Even if the two prototype lasers can be made to fire a 100-kilowatt beam in the lab, however, it will still be some time before the Pentagon can transform the lasers into battle-ready weapons, says Phil Coyle of the Center for Defense Information in Washington DC.
"It's not easy to get as much energy in a laser beam as in an M-16 rifle bullet. Then you have to aim it and hold it on the target long enough to damage it," Coyle says. "While those things have been done under controlled conditions at test ranges, it's much more difficult under the uncontrolled conditions of battle."