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.