In counterinsurgent, counter-terror campaigns, risks associated with conventional weapons in combat operations can severely limit their use and effectiveness, particularly in urban environments. These risks are largely associated with challenges posed by confining intended effects to adversary combatants and forces. Even in cases when combat operations are deemed necessary, the consequences of collateral damage from conventional weapons can complicate and hinder the overall mission. Laboratory and field testing of high-power laser systems indicate irradiance levels for functional and lethal effects against a variety of adversary targets and surgical precision of such lasers against certain air and ground targets. However, existing high-power chemical laser systems are too large and too inefficient for deployment on tactical airborne platforms.

The DARPA Excalibur program will develop coherent optical phased array technologies to enable scalable laser weapons that are 10 times lighter and more compact than existing high-power chemical laser systems. The optical phased array architecture provides electro-optical systems with the same mission flexibility and performance enhancements that microwave phased arrays provide for RF systems and a multifunction Excalibur array may also perform laser radar, target designation, laser communications, and airborne-platform self protection tasks.

These phased arrays will coherently combine lower-power electrically driven lasers, such as diode lasers and fiber laser amplifiers. Coherently combinable single-mode diode lasers and fiber-based systems can provide overall laser efficiencies greater than 50 percent and 30 percent, respectively, while maintaining near-diffraction-limited beam quality. To produce a weapons-grade system, however, their output power must be increased without introducing additional optical phase noise and modal instability.

Beam-steering technologies will be pursued to make these arrays conformal with the airframe, to provide rapid retargeting across a large field of regard, and to compensate for the effects of atmospheric turbulence. The Excalibur program will demonstrate a high-power phased array of kW- class fiber laser amplifiers and investigate the limits of active optical phase-locking onto uncooperative targets under realistic conditions.

Once Excalibur’s technical objectives are achieved, it is envisioned that a coherent array of tens of subapertures, each driven with a multi-kW coherently combinable fiber laser amplifier, would enable ~100 kW class laser systems for precision strikes against both ground and air targets. This technology will enable the practical use of high-power lasers on a broad spectrum of military platforms without degradation of their original missions.