DARPA's Excalibur High Energy Laser Program Successfully Tests 21-element Optical Phased Array Laser

High-energy lasers (HEL) have the potential to benefit a variety of military missions, particularly as weapons or as high-bandwidth communications devices. However, the massive size, weight and power requirements (SWaP) of legacy laser systems limit their use on many military platforms. Even if SWaP limitations can be overcome, turbulence manifested as density fluctuations in the atmosphere increase laser beam size at the target, further limiting laser target irradiance and effectiveness over long distances.

Recently, DARPA’s Excalibur program successfully developed and employed a 21-element optical phased array (OPA) with each array element driven by fiber laser amplifiers. This low power array was used to precisely hit a target 7 kilometers—more than 4 miles away. The OPA used in these experiments consisted of three identical clusters of seven tightly packed fiber lasers, with each cluster only 10 centimeters across.

“The success of this real-world test provides evidence of how far OPA lasers could surpass legacy lasers with conventional optics,” said Joseph Mangano, DARPA program manager. “It also bolsters arguments for this technology’s scalability and its suitability for high-power testing. DARPA is planning tests over the next three years to demonstrate capabilities at increasing power levels, ultimately up to 100 kilowatts—power levels otherwise difficult to achieve in such a small package.”

In addition to scalability, Excalibur demonstrated near-perfect correction of atmospheric turbulence—at levels well above that possible with conventional optics. While not typically noticeable over short distances, the atmosphere contains turbulent density fluctuations that can increase the divergence and reduce the uniformity of laser beams, leading to diffuse, shifted and splotchy laser endpoints, resulting in less power on the target. The recent Excalibur demonstration used an ultra-fast optimization algorithm to effectively “freeze” the deeply turbulent atmosphere, and then correcting the resulting static optically aberrated atmosphere in sub-milliseconds to maximize the laser irradiance delivered to the target. These experiments validated that the OPA could actively correct for even severe atmospheric distortion. The demonstration ran several tens of meters above the ground, where atmospheric effects can be most detrimental for Army, Navy and Marine Corp applications. In addition, these experiments demonstrated that OPAs might be important for correcting for the effects of boundary layer turbulence around aircraft platforms carrying laser systems.

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