CCD Sets Sights On Airborne Laser Program

Daniel C. McCarthy

The threat from nuclear missiles has diminished since the end of the Cold War, but short- and medium-range tactical missile technology has proliferated. This new danger has fueled support for the US Air Force's Airborne Laser program, which, if successful, will use high-power lasers to destroy tactical missiles in the launch phase. The platform is a modified passenger jet that can take advantage of high altitudes to minimize atmospheric effects on the laser's accuracy and destructiveness.

However, even at high altitudes, the system requires some form of imaging to perform wavefront correction. For this, it uses a smaller laser to illuminate the missile and to provide a model of atmospheric effects. Three electron-bombarded CCDs developed by Intevac Inc. help this function.

The US Air Force Airborne Laser system aims to provide a defense against tactical missiles such as Scuds. Electron-bombarded CCDs onboard the aircraft are providing the sensitivity and frame rate to track and destroy the supersonic targets. Courtesy of Boeing Co.

CCDs in Concert

One CCD serves as part of the fine-tracking system aimed at distant laser-illuminated missiles. Another measures the wavefront returning from the illuminated target and helps the system correct for atmospheric-produced aberrations. The third CCD measures the uncorrected beam of the high-energy laser.

The task of aiming, firing and destroying a supersonic target at long range obviously puts high demands on the imagers' frame rate and sensitivity. Intevac's devices delivered on both counts, according to George Gal, a senior consulting scientist at Lockheed Martin who is in charge of developing the sensor suite

The design interposes a vacuum envelope between a transferred-electron photocathode and the back-thinned CCD. Electrons from the cathode accelerate very quickly in the vacuum, striking the CCD at high speed. The energy from this impact dissipates by creating electron-hole pairs to produce about 150 electrons for every electron that strikes the device.

This very low noise gain process provides a sensitivity that is higher by a factor of two or three when compared with most image-intensified CCDs, said Verle Aebi, president and general manager of Intevac's Photonics Technology Div. The sensitivity of the CCD in the airborne laser's fine-tracking system provides good resolution despite the low signal returning from laser-illuminated missiles.

There are other benefits besides sensitivity. The unique gain process enabled Intevac to eliminate the phosphor screens typical of most image-intensified CCDs. These screens have a high time constant, which means that optical signal comes onto the screen faster than it can fade out. Without them, Intevac's CCDs can achieve 10-kHz frame rates.