Directed-energy weapons are set for key tests this year, including the attempted shoot-down of a boosting ballistic missile, but they may not dispel lingering doubts over the military utility of high-power lasers.

Instead, the real future of high-energy laser weapons could be taking shape in laboratories, where later this year competing solid-state lasers are expected to run at power levels exceeding 100 kw. Different designs for 150-kw. electric lasers will also be tested in the lab this year as a step toward ground, and later airborne, demonstration of a fieldable laser weapon early next decade.

The military wants speed-of-light weapons with pinpoint accuracy, unlimited magazines and variable effects, but while the megawatt-class Airborne Laser and kilowatt-class Advanced Tactical Laser provide high power levels, their size and logistic issues with the hazardous chemical fuels limit their potential. "Warfighters want an electric laser," says Don Seeley, deputy director of the U.S. Defense Dept.'s High-Energy Laser Joint Technology Office.

Solid-state lasers promise to be much smaller and lighter, easier to integrate on to mobile and airborne platforms, and powered by electricity generated on board. Compared with fuel-hungry chemical lasers, electric weapons offer longer run times and unlimited shots.

The Pentagon's flagship effort - the Joint High-Power Solid-State Laser (JHPSSL) program - is nearing completion with the laboratory demonstration of lethal power levels. But the High-Energy Liquid Laser Area Defense System (Hellads), a U.S. Defense Advanced Research Projects Agency program, is poised to take the lead in demonstrating a deployable weapon.

Northrop Grumman and Textron Systems are developing competing 100-kw. solid-state lasers under JHPSSL. Textron is also building a more powerful derivative of its JHPSSL laser for Hellads, while General Atomics is developing the unique "liquid laser" that gave the Darpa program its name.

Full-power firings of the JHPSSL devices were planned for the end of 2008, but are now expected in February-March for Northrop Grumman and summer 2009 for Textron. Both companies have completed 30-kw. firings as a step toward full power levels. The 100-kw. demos will complete the program, but the solid-state lasers are candidates for the U.S. Army's High-Energy Laser Technology Demonstrator program to test a truck-mounted system in 2013-15 that can counter rocket, artillery and mortar projectiles.

"JHPSSL is stoking the fire of military interest in high-energy lasers," says Dan Wildt, Northrop Grumman vice president of directed-energy systems. In addition to counter-rocket/artillery, potential uses include precision strike and aircraft self-defense for the U.S. Air Force, and anti-terrorist force protection and ship anti-missile defense for the Navy.

JHPSSL is demonstrating two different approaches to scaling solid-state lasers to high power. Northrop Grumman uses a "master oscillator power amplifier" configuration where the beams from eight lasers are combined optically to get to 100 kw. Textron uses a power oscillator configuration where a single beam goes through a chain of gain modules to produce a 100-kw. laser.

Northrop Grumman's design is based on 15-kw. building blocks, or "benches," says Wildt. Eight of these benches stacked in two columns will produce "well over 100 kw.," he says. Inside each bench are four gain modules, comprising neodymium-doped yttrium aluminium (Nd:YAG) crystals, or "slabs," pumped by light from diodes to amplify the laser beam. The beams from the benches are then tiled - laid side-by-side - and their phases controlled so they combine optically into a single beam.