After more than a century of popular sci-fi fantasies that feature deadly energy weapons, including War of the Worlds, Flash Gordon, Buck Rogers, Star Trek and Star Wars, it looks like the ray gun has finally arrived in the real world.

And even if the first ray guns out of the lab can barely fit on the bed of a 30-ton off-road truck rather than in a soldier’s palm, the novel, "speed-of-light" capabilities that lasers could bring to the battlefield has drawn the keen interest of the Pentagon brass, which spends about $400 million a year on directed-energy beam weapons.

At the end of this year, which marks a half-century of amazing progress in lasers, defense contractors Northrop Grumman and Boeing plan to test-fire a prototype mobile laser weapon against examples of the lethal ordnance—rockets, artillery, mortars—that insurgents in Afghanistan and elsewhere shoot at U.S. troops every day, says Mark Neice, director of the Department of Defense's High Energy Laser Joint Technology Office in Albuquerque, N.M. As long as such an area-defense system is fed electrical power (from the grid or battery packs), its 100-kilowatt, solid-state, or electric, laser should be able to use its “unlimited magazine” of low-cost shots and ultra-precision tracking/targeting system to zap out of the air multiple inbound munitions from several kilometers away, he explains.

Weapons engineers will use the live-fire tests of the one-micron-wavelength (infrared) beam, which will take place at White Sands Missile Range in New Mexico, "to validate our notional models of beam propagation," Neice says. These results, “will allow us to determine what targets we can take on, at what power levels, what ranges and so forth.” The U.S. Army hopes that laser cannons can shield its bases from insurgent attacks while minimizing the risk of collateral damage to the civilian populations among which guerrillas often hide. A cannon’s powerful beam will be able reach out to incoming weaponry, and either detonate, disable or knock them off-course, whereas its ultra-precision aiming capability would presumably enable troops to pick off ground targets without hitting nearby non-combatants.

The U.S. Air Force has in the meantime taken the lead in a project sponsored by the Defense Advanced Research Projects Agency (DARPA) to develop even more powerful and compact solid-state lasers that could fit on combat aircraft. Such systems could provide the nation’s air arm with what Michael W. Zmuda, manager of the Air Force Research Lab’s Electric Laser on Large Aircraft (ELLA) program, calls the “game-changing capability” to carry out beyond-the-horizon, air-to-air engagements and precisely targeted, air-to-ground strikes. “It would open up a raft of new tactical and defensive roles, such as defeating targets that are close to our own troops while avoiding collateral damage to civilians and property, as well as a range [of] rapid-response missions against a whole new set of targets,” he says.

The Air Force plans to fit a B1-B bomber with a new 150-kilowatt solid-state laser that will be built by the winner of a contract competition between General Atomics Aeronautical (GAA) and Textron Defense. The original DARPA effort arose when “we realized that a laser beam propagates much more efficiently 1,000 meters off the ground, where atmospheric distortion and scattering effects are much less pronounced,” according to Michael Perry, vice president at GAA. To fit in a fighter jet, one of the chief Pentagon goals, the airborne laser weapon will need to generate around five kilowatts per kilogram which means the technology “has to be reduced in size and weight by a factor of 10 over the current ground-based system,” Perry notes.

Meanwhile, U.S. Navy researchers are learning to cope with the extra difficulties of running a finely tuned electro-optical device in the harsh maritime conditions near the sea surface, where water vapor in the air tends to scatter and attenuate directed-energy beams. Navy planners are interested in using lasers in a “counter-materiel role” to help naval vessels fend off harassing attacks by squadrons of small armed boats such as occurred in early 2008 in the Strait of Hormuz, says Dan Wildt, vice president of directed energy systems at Northrop Grumman. Though the Navy is not saying specifically, it is thought that a relatively low-power laser beam could set alight wood or glass-fiber hulls, fuel or vulnerable weapons from stand-off distances of a kilometer or more. Wildt’s company is supplying a 15-kilowatt solid-state laser for Navy tests at a Pacific range later this year.

Northrop Grumman and others are also working on switchable free-electron lasers that can fire beams of two or more different wavelengths of light.* These weapons could provide ship defenses with more flexible means to better penetrate the sea haze and protect against supersonic cruise missiles and other aerial threats. Free-electron lasers employ an array of electromagnets called a wiggler or undulator to force a beam of electrons to travel in a sinusoidal path that makes them release energetic, in-phase photons that form a powerful laser beam. Changes to the electron beam or the wiggler’s magnetic field alters the wavelength of the resulting laser beam.

Much of the recent interest in military laser technology stems from recent progress in solid-state, or electric, laser technology. These sources generate powerful, coherent light beams when arrays of semiconductor laser diodes pump light into the faces of “slabs” or rods—special ceramic lasing media that amplify the light greatly. The slabs are ganged into chains that progressively boost the output beam power. Over the past few years, contractors have demonstrated solid-state lasers capable of producing over 100 kilowatts of power, which specialists consider the minimum weapons-grade power rating.

Weapons-grade electric lasers have an Achilles heel, however. Their energy conversion efficiencies are only 20 to 30 percent, which means most of the input power is lost to heat. To dissipate the waste heat that would otherwise cause thermal distortions in the internal light path and reduce optical transmission, electric lasers require bulky, power-hungry liquid-cooling systems, says Mike Rinn, vice president at Boeing. Future mobile lasers will have to operate much more efficiently, to avoid the need both for huge, energy-sapping coolers and perhaps for batteries altogether if they could run directly off of a vehicle’s engine power. Two laser technologies that could fit the requirement, Rinn says, are the fiber laser, where the lasing material is a fiber-optic material, and the so-called hybrid laser, in which laser diodes pump a gas-phase lasing media.

*Correction (5/17/10): This article originally stated that Northrop Grumman and others are working on "free-energy" lasers.