The 150 kW HELLADS system from General Atomics appears to be the current frontrunner for the USAF contract despite the system only having just recently entered ground tests. HELLADS stands for "High Energy Liquid Laser Area Defense System". The third generation prototype measures just 1.3 x 0.4 x 0.5 meters -- small enough to fit onto a Predator C UAV, exactly what DARPA wants to do by 2018 -- and runs off of a single lithium ion battery pack.
According to DARPA's Dr. David Shaver,
The goal of the HELLADS program is to develop a 150 kilowatt (kW) laser weapon system that is ten times smaller and lighter than current lasers of similar power, enabling integration onto tactical aircraft to defend against and defeat ground threats. With a weight goal of less than five kilograms per kilowatt, and volume of three cubic meters for the laser system, HELLADS seeks to enable high-energy lasers to be integrated onto tactical aircraft, significantly increasing engagement ranges compared to ground-based systems.
But before it enters operational service, the system must first pass a grueling set of field tests against mortars, rockets, UAVs, simulated surface-to-air missiles and both ground and air vehicles at the White Sands Missile Range in New Mexico. According to Ars Technica, Air Force leaders at the presentation Beyond that, the USAF is working to develop and install Lockheed Martin's ABC laser system on its 6th generation fighters by the mid-2030's.
[Image Credit: top - Air Force Research Lab, inline - DARPA]