Ever since the laser first appeared in the public consciousness, popular culture has reframed it as a prospective weapon of infinite range and power. This is largely because popular culture is unshackled from the concerns of engineering and physics. But the idea of light-based directed energy has nevertheless prompted interest and investment in the laser’s potential military applications — where it is most often viewed as a defensive countermeasure. Decades of government-funded demonstrations have yielded promising results that indicate laser-based weapons are finally ready for the battlefield. High-energy lasers mounted on vehicles provide mobile protection against enemy drones. Courtesy of Boeing. Chief among the recent advancements are high-power lasers capable of producing tens of kilowatts of energy in a beam that can knock down an unmanned aerial system (UAS) or a drone, which are among today’s proliferating threats. Further out on the horizon are systems operating in the hundreds of kilowatts, designed to destroy much larger and more distant targets such as missiles. Industry and government collaborators are racing to produce the high-energy laser platforms capable of such feats. But significant challenges remain, such as maximizing beam power while maintaining beam quality, and manufacturing optics that can handle such high-energy outputs. Another significant challenge is how to rapidly dissipate the considerable heat produced by these systems. Beating the heat Thermal management is a critically important area of research, said Rob Afzal, a senior fellow at Lockheed Martin’s Rotary and Mission Systems business segment. Among the company’s projects is the High Energy Laser with Integrated Optical-dazzler and Surveillance (HELIOS), a 60-kW-plus system scheduled for deployment on U.S. Naval vessels this year. The HELIOS platform is designed to be integrated into a ship’s combat system to either disable small enemy boats and drones or to provide long-range intelligence, surveillance, and reconnaissance (ISR) capabilities. It is also intended to counter — or dazzle — ISR capabilities mounted on enemy drones. “Heat’s a big problem,” Afzal said. “If you can’t get the heat out of your system, your beam quality is going to degrade. And then if you can’t get the gain [you need] out of your system, you won’t be able to scale in power.” One way to get the heat out of a directed-energy system is to avoid putting it in. Developing the system to more efficiently convert electricity into light means generating less waste heat, as well as reducing drain on a transport vehicle’s power supply, which is particularly important if its energy comes from batteries or a similar storage source. Currently, the conversion rate for many laser-based systems is about 35% efficient. So a 100-kW directed-energy weapon, such as those that Lockheed Martin is actively working on, will consume about 300 kW of power and dissipate roughly 200 kW of heat. The quest to improve efficiency in today’s directed-energy weapons often leverages two commercial technologies, Afzal said. The first is fiber lasers operating in the few-kilowatt range. Such solid-state lasers offer high beam quality with no thermal distortions. The second is wavelength division multiplexing, wherein a directed-energy weapon combines the light from several fiber lasers of slightly different wavelengths into a single beam. Higher efficiency could therefore come from more efficient pump diodes, more efficient coupling of the diodes to the gain medium in the fiber, or more effective methods to keep the beam on target. The last option presents challenges of its own. Power and control Typically, the fiber lasers used for directed-energy systems emit wavelengths around 1050 nm. Under the best conditions, the optical energy of the combined beams must travel through open air — often for many kilometers — to reach its target. And the atmosphere is not a neutral medium. “Shorter wavelengths experience increased scattering, and longer wavelengths can experience more absorption,” Afzal said. “The band around 1050 nm provides a balance between the two competing effects.” Aside from increasing power, another way to boost a beam’s damage to a target is by extending its time on the target. This requires improved targeting control, especially because most targets are distant and on the move. Also, the beam is often subject to movement and vibration at its source due to the motion of its platform and to atmospheric turbulence. Adaptive optics, which deforms mirror surfaces to adjust the outgoing beam’s waveform and thereby counter atmospheric turbulence, is one solution for compensating for this jitter when tracking long-range targets. For shorter distances of a few kilometers, such an approach may be overkill, said Ronald Dauk, program manager of Boeing’s Laser & Electro-Optical Systems division. The company’s tracking technology makes use of IR bands or illuminates the target with a separate tracking laser to provide the information needed to correct the adaptive optics. One of the reasons to implement advanced tracking is that, compared to increasing the laser power, it adds comparatively little to system size, weight, and power (SWaP), and cost, said Iain McKinnie, technical area director for EO/IR, laser radar, and high-energy lasers at Raytheon Intelligence & Space. This is because directed-energy systems that keep the beam on target require less power to achieve comparable damage. This reliance on beam control is evident in Boeing’s Compact Laser Weapon Systems, which are designed to accommodate 2-, 5-, or 10-kW lasers. Boeing is partnering with General Atomics to also develop 100- to 250-kW class systems. As part of this collaboration, Boeing will supply beam directing and targeting technology, while General Atomics will contribute the distributed gain high-energy laser component. Dauk said the two companies are planning to unveil demonstration systems later this year and in 2022. Along with improved tracking, higher output powers are always a good option to have. Boeing intends to increase the total output of its directed-energy system by boosting output per channel, from the current 1-kW levels to as much as 5 kW, and the company is exploring combining enough fiber lasers to continue to drive system powers even higher. Boeing is looking to fiber laser suppliers to help increase its products’ power output while maintaining beam quality. Dropping the cost per kilowatt of a system helps to fulfill the important goal of reducing the system’s SWaP and cost. This reduction is important when mounting a directed-energy weapon on a mobile platform with finite energy storage. A close-up of optical fibers entering a combiner assembly. Today’s laser-based directed-energy weapons often merge the output from several fiber lasers to attain combined outputs of 50 kW or higher, and even higher-power systems are on the drawing board. Courtesy of Lockheed Martin. Optics that can withstand the full power of a directed-energy system’s beam are another potential solution that industry-side suppliers can help to achieve. The task is made easier through innovations that ensure that minimal beam energy is left behind in the optical material. “Low absorption coatings have been a key development area that has enabled all of these high-energy laser systems,” Dauk said. Raytheon is also fielding high-power directed-energy systems, including 10-kW laser systems deployed by the U.S. Air Force, as well as a 50-kW-class laser system being incorporated into Stryker combat vehicles by the U.S. Army. The systems achieve these power levels by combining as many 2.5-kW-class fiber lasers as are needed, and they can knock down drones at distances of a few kilometers — as well as incoming rockets, artillery shells, and mortar rounds. The beams track targets using Raytheon’s multispectral targeting system. An artist’s rendering of a vehicle-mounted high-energy laser knocking drones out of the sky. Courtesy of Raytheon Intelligence & Space. A rendering of a possible future battlefield, with ultrashort-pulse lasers supplying high-peak-power lasers to take out a range of adversaries. Courtesy of Applied Energetics. One of the advantages of combining beams from several fiber lasers, said McKinnie, is that no light exits the gain medium during amplification. Thus, there are minimal interfaces to reduce power, or components to vibrate out of alignment. “Fiber lasers are inherently the most robust kind of laser I’ve ever worked with,” McKinnie said. Their ruggedness and reliability are particularly important in defense applications, which demand operation over a wide temperature range and in environments full of vibration and dust. McKinnie said achieving hundreds of kilowatts of output still presents component challenges. Either the power of the individual fiber lasers must be increased or more beams must be combined, or some combination of these two approaches is required to reach the desired output. However, as output power increases, aiming the beam also becomes more difficult. This is partly because higher power levels cause more severe thermal effects in the atmosphere, and the beam can trigger nonlinear effects. While such effects may inhibit the performance of continuous-wave lasers, researchers are actually finding that nonlinear effects can be leveraged in many directed-energy applications (see sidebar on page 48). Targeting tech transfer Despite these challenges, defense contractors and their clients still have plenty of incentives and sources of funding to find ways to overcome them, said Mark Neice, executive director of the Directed Energy Professional Society. The U.S. government, for instance, has been increasing its funding of directed-energy systems over the last few years, investing more than a billion dollars toward research and development in 2020 alone. The military is not the only beneficiary of this research. Neice said many of the advancements arising from weapons system development could be useful to industry. Developments in thermal management could also prove advantageous in nondefense applications by making lasers better able to handle wide temperature swings. Finally, increases in laser output power and conversion efficiency are of interest in industrial settings. The first helps improve throughput, while the second cuts costs. “It’s a two-way street, absolutely,” Neice said of the interplay between commercial and defense sectors when it comes to high-energy lasers. “The Defense Department leverages off of the commercial capability. But the tech base sometimes is not honed in on the specific defense requirements.” So the predicted ray guns of yore aren’t here — yet. However, advancements in directed-energy laser weapons are bringing them closer, with innovations that could prove useful on the battlefield, on the plant floor, and beyond. Short Pulses Pack a Powerful Punch While most directed-energy weapons rely on continuous-wave lasers with high average power to disable targets, achieving sufficiently high peak power is another approach to the same end, said Greg Quarles, CEO of Applied Energetics. The Tucson-based company is working on developing ultrashort-pulse lasers for defense and industrial applications. The energy from high-peak-power femtosecond laser pulses presents multiple threats to small or lightweight military targets, such as drones. They can cause nonlinear effects in the atmosphere to guide electrical charges to the target, convert optical energy into radio frequency energy to jam electronics, or disable a drone’s sensor array. Courtesy of Applied Energetics. Here, ultrashort refers to pulse widths of less than 500 fs. Squeezing a few watts of average power into the femtosecond range results in pulses with terawatt peak power levels, which are high enough to produce nonlinear effects in the atmosphere, such as so-called plasma channels. Such channels can guide electrical discharges to a remote target. Or they can be used to convert optical energy into radio frequency energy to jam electronics and bring a drone down, Quarles said. An ultrashort-pulse strike on a drone’s sensor can also damage it and thereby render it useless. While Quarles is bullish on weapon systems based on ultrashort-pulse lasers, he views the technology as complementary to systems with high-average power. A laser with high-average intensity, for instance, may be more effective for taking out larger, more distant targets via thermal effects, while ultrashort-pulse lasers would be more suitable to counter against smaller, more proximate targets. “What you’re looking at is this toolkit of various ways to defeat the threat,” Quarles said. “What you want to do is to combine them to be most effective.“