The Adaptive Optics Revolution: A History

9: Airborne Laser

203 N I N E Airborne Laser By the early 1990s, the Air Force, DARPA, Lincoln Laboratory, the Navy, and a number of independent contractors had been actively engaged in adaptive optics research for two decades. During that time, there were many impressive technical accomplishments in this relatively new field of science. The military suspected early on that adaptive optics, if it worked, would represent a critical enabling technology for future Air Force missions—improving images of objects in space and thus strengthening the Air Force’s ability to monitor the activities of satellites and missiles. As it turned out, military research in this area also benefited astronomers around the world. Another goal of adaptive optics research was to perfect compensation techniques in order to produce powerful coherent beams of light that would be the kill mechanism of future ground-based and airborne directed-energy weapons. The Air Force was optimistic that this could be done, because for 13 years it had developed and tested the Airborne Laser Laboratory or ALL. This modified NKC-135 research aircraft was essentially a laboratory in the sky that operated a high-power CO2 laser in flight. In 1983 the ALL shot down five AIM-9B “Sidewinder” missiles over the Navy’s China Lake Test Range in California, thereby proving that an airborne laser could function in an operational environment. By the early 1990s, the Air | Nine 204 Force had proposed a second-generation Airborne Laser or ABL to be ready for duty in the early 21st century. ABL would be a critical component of the nation’s multilayered missile defense system. Originally, it was designed to intercept only short- and medium-range theater ballistic missiles. Later, it was expanded to engage a full range of enemy ballistic missile threats— theater and intercontinental ballistic missiles—during any phase (boost, midcourse, or terminal) of their flight.1 A missile is most vulnerable during boost phase, the first few minutes after launch when it climbs slowly, moves along a predictable course, and gives off a bright, hot exhaust, making it relatively easy to track. At that stage themissileisalsoatriskbecauseitisloadedwithpressurizedoxygenandkerosene . A powerful laser beam hitting the fuel tanks could heat the metal skin of the missile enough to cause it to collapse or rupture. As an initial stress fractureinthemetalgrowslarger,itquickly“unzips”themissile,causingitto break up. Or the laser could ignite fuel vapors, causing the missile to explode. At that point, nuclear, biological, chemical, or conventional explosive payloads onboard the missile would fall on the enemy’s homeland (a condition known as fratricide), thereby not causing an immediate danger or threat to friendly ground troops positioned hundreds of miles away. Also, plans called for an airborne laser system to perform its mission in a standoff mode while flying over friendly air space to reduce the chances of casualties to air crews.2 Air Force leaders felt the time was right to begin development of an ABL that could become the country’s first operational laser weapon. Hans Mark, former secretary of the Air Force under President Carter and director of defense research and development under President Clinton, supported this goal. He pointed to two recent technical achievements that accounted for a revival of interest in the ABL. One was the development of the shortwavelength 1.3-micron chemical oxygen iodine laser (COIL), which could deliver an intense beam of continuous energy over hundreds of miles to a target such as a boosting Scud missile.3 The other was adaptive optics. Mark firmly believed that the advancement of wavefront sensors, high-speed computers, and deformable mirrors made it possible to equip an ABL with an adaptive optics system that would be capable of significantly reducing the damaging effects of atmospheric turbulence on a high-power COIL beam propagating from an aircraft flying high above the clouds to a target hundreds of kilometers away.4 Airborne Laser 205 Although many in the military bet adaptive optics would be a key ingredient of any successful directed-energy weapon, most adaptive optics research focused on theoretical studies and laboratory experiments conducted under controlled conditions. By the 1990s there was more pressure from the leadership to move adaptive optics off the drawing board and into the operational arena by developing an airborne laser weapon. Meeting that challenge was more difficult than first expected, both technically and financially. It was extremely time-consuming and difficult to integrate adaptive optics into the beam control...