Our first foray into laser-equipped combat aircraft was the Airborne Laser Testbed, a Boeing 747 with a gigantic chemically-pumped megawatt laser turret in its nose. It was pretty awesome from a conceptual standpoint, but it didn't work very well, and was scrapped last year. This doesn't mean that the idea of high-powered lasers on aircraft doesn't make a lot of sense, and DARPA is still for ways to make it work. It's working on two at the moment: the High Energy Liquid Laser Area Defense System (HELLADS), and Aero-Adaptive/Aero-Optic Beam Control (ABC).
HELLADS (pictured above) will be a 150-kilowatt system that's "ten times smaller and lighter" than current systems, suitable for use both on the ground and in the air in an air-to-ground attack role. The goal is to create a laser that weighs less that five kilos per kilowatt and fits into a total space of three cubic meters, which would make it small enough to weaponize my Volvo.
The other system, ABC, is intended as more of a defensive weapon. It'll be a small laser turret mounted on things like fighter aircraft that can shoot down incoming missiles. The reason that this is hard is because most missiles approach from behind, and the laser will have to be able to fire through the turbulence generated by the aircraft's engines without losing all of its beam energy. This is where the adaptive optics and beam control comes in, and here's a terrible illustration from DARPA showing how the concept would work:
Both of these projects are actually much farther along than these lousy illustrations would suggest: HELLADS has been largely completed in terms of hardare, and will undergo integration testing on the ground this year with airborne tests starting in 2014. Meanwhile, the ABC system has passed wind tunnel tests, and Lockheed Martin is on a 30-month contract to stick a protype turret in an aircraft and see what happens. We'll be watching.