DARPA successfully tested an optical phased array (OPA) combining 21 laser beams, as part of the Excalibur program. With each of the 21 array elements driven by fiber laser amplifiers the low power array was able to precisely hit a target at a distance of 7 kilometers (4.3 miles). In three years the agency expects to scale up the design, delivering 100 kW weapon-class energy levels on target.

The OPA used in these experiments consisted of three identical clusters of seven tightly packed fiber lasers, each cluster measures only 10 centimeters across. According to Joseph Mangano, DARPA program manager, the agency is planning to scale up the design over the next three years, ultimately transmitting up to 100 kilowatts of power – levels otherwise difficult to achieve in such a small package.

“Beyond laser weapons, this technology may also benefit low-power applications such as laser communications and the search for, and identification of, targets.” Joseph Mangano, DARPA PM

Future tests aim to prove the OPA’s capabilities in even more intense environmental turbulence conditions and at higher powers. Such advances may one day offer improved reliability and performance for applications such as aircraft self-defense and ballistic missile defense.

“The success of this real-world test provides evidence of how far OPA lasers could surpass legacy lasers with conventional optics,” said Mangano, “With power efficiencies of more than 35 percent and the near-perfect beam quality of fiber laser arrays, these systems can achieve the ultra-low size, weight and power requirements (SWaP) required for deployment on a broad spectrum of platforms,” said Mangano. “Beyond laser weapons, this technology may also benefit low-power applications such as laser communications and the search for, and identification of, targets.”

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.

“In addition to scalability, Excalibur demonstrated near-perfect correction of atmospheric turbulence — at levels well above that possible with conventional optics.”

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 military 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.

The successful demonstration helps advance Excalibur’s goal of a 100-kilowatt-class laser system in a scalable, ultra-low SWaP OPA configuration compatible with existing weapon system platforms. Continued development and testing of Excalibur fiber optic laser arrays may one day lead to multi-100 kilowatt-class High-energy lasers (HEL) in a package 10 times lighter and more compact than legacy high-power laser systems.

Under the Excalibur program DARPA explores the development of coherent optical phased array technologies able to reduce the size and weight of laser weapons 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. As such, future multifunction Excalibur arrays may also perform tasks including laser radar, target designation, laser communications, and airborne-platform self protection.

“Excalibur fiber optic laser arrays may one day lead to multi-100 kilowatt-class HELs in a package 10 times lighter and more compact than legacy high-power laser systems”

Unlike the chemical lasers that rely on a single high power source, 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.

High-energy lasers have the potential to benefit a variety of military missions, particularly as weapons or as high-bandwidth communications devices. However, the massive 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.