Amid abundant opportunity for directed energy systems, the viability of the technology’s deployment hinges on sustained investment.
MICHAEL EISENSTEIN, CONTRIBUTING EDITOR
“Keep your eye on the sky” is a critical rule in modern warfare, due to the rapid proliferation of low-cost, speedy uncrewed aerial vehicles (UAVs). These aircraft can be fitted with surveillance equipment for tracking the enemy or loaded with weapons or explosives to take out targets. Today, drones are a mainstay in modern conflicts, including the Israel-Gaza and Russia-Ukraine wars.
Courtesy of General Atomics.
“It’s almost like it’s back to World War I with trench warfare, because you’re being hunted by a drone every single time you turn around in any of these battle spaces,” said Gregory Quarles, CEO Emeritus at Applied Energetics. Eliminating UAVs with conventional weaponry
can be challenging and costly, particularly when they travel in swarms. Quarles’ company is among those developing directed energy weapons as an alternative.
The term directed energy encompasses a range of different technologies, but much of the current effort in this space is focused on the development of high-energy laser (HEL) weapons that deliver dozens or hundreds of kilowatts of
focused energy to blind, disable, or
destroy hostile targets.
Though expensive to construct, these systems offer a cost-effective defense
solution. “Once you buy the system, you’re talking about the cost of a pot of coffee to shoot down a UAV,” said Scott Forney, president of General Atomics Electromagnetic Systems (GA-EMS).
Future directed energy systems could potentially achieve sufficient power and range to disable even more serious threats, such as hypersonic missiles.
Field tests, including several conducted in the last five years, have demonstrated the feasibility of HEL-based directed
energy weapons. However, it remains costly and technically challenging to design and assemble a system that can consistently deliver a high-quality beam while also managing issues such as heat production, precision aiming, and maintaining stable performance on a chaotic battlefield. Solutions are emerging for these problems, but consistent government investment will be needed to produce a practical tool suitable for broad deployment. “To really make this a viable technology, we need to have sustained volume so that we can dedicate the necessary resources to it,” says Derek Angel, aerospace and defense market manager at optical fiber manufacturer OFS.
The best and brightest
The notion of using lasers as anti-missile defense stretches back several decades, though Quarles said that the field really picked up momentum in 2000 with the U.S.’ development of the High Energy Laser Joint Technology Office within the Department of Defense. This office worked with the military to support the development of a solid-state HEL that could deliver upward of 100 kW of energy. In 2009, Northrop Grumman reported its demonstration of the first such system.
Since then, anti-missile solid-state lasers operating in the range of hundreds of kilowatts have remained a focus of the defense sector. As of mid-2024, the Department of Defense had established a road map to support the development of a 500-kW beam by the end of fiscal year 2025.
Sources such as these, operational in the range of hundreds of kilowatts, offer necessary capabilities. “When you see people working in the hundreds of kilowatts, they’re usually talking about things like counter-cruise missiles … targets moving much faster that you need to engage [with] at much longer distances,” said Andrew Stentz, vice president of emerging products and technology, aerospace and defense at Coherent. During the past four years, companies including Lockheed Martin, nLight, and GA-EMS have demonstrated 300-kW HEL systems that could eventually be deployed for such purposes.
But these are still prototypes, and their real-world practicality remains an open question; some in the military have expressed skepticism about the defensive capabilities of current-generation HEL systems. And in recent years, defense contractors have shifted toward lower-energy systems operating in the 10- to 50-kW regime, which is well suited for debilitating or destroying UAVs within the kilometer range. Last October, for example, Leonardo DRS and BlueHalo demonstrated successful destruction of multiple drones with their vehicle-mounted 26-kW LOCUST HEL system.
Source material
Most HEL directed energy systems are based on fiber lasers, which are compact and highly energy-efficient compared with other solid-state laser formats. The lasers use hair-thin strands of transparent gain material, such as silica glass, doped with a rare-earth element, such as ytterbium, which, according to Angel, offers a certain advantage in an aerospace and defense context. “[The] wavelengths that ytterbium emits happen to be absorbed quite well by things that are flying and involved in weapons systems,” he said.
Raman fiber laser modules are optimized for high brightness fiber laser pumping, including for directed energy and aerospace and defense applications. Courtesy of OFS.
Today’s fiber lasers generally deliver output power in the range of 3 kW. To achieve HEL capabilities, multiple fibers such as these must be coupled to a beam-combining module that integrates the beams to produce higher-kilowatt-range energies. Some modules use coherent beam combining, in which an array of parallel beamlets is precisely matched in terms of wavelength, polarization, and phase. Alternatively, in spectral beam-combining modules, a diffraction grating is used to combine multiple beams with distinct wavelengths into a single overlapping beam.
But fiber lasers are not the only game in town. GA-EMS, for example, is developing systems based on distributed-gain lasers, in which light from emitter diodes passes through a series of thin “slices” of crystal gain material, with a refractive index-matched liquid coolant in between each segment. This design eliminates the need for a beam-combining component while also mitigating the heat that can undermine the performance of other nonfiber solid-state laser designs. “We have had no problem cooling the crystal, because we have plenty of safety margin in our design,” Forney said.
Beating the heat
Energy efficiency and thermal management are two of the greatest challenges in developing HEL weapons. At the time when Northrop Grumman was developing its first HEL directed energy system, according to Quarles, the inefficiency of existing laser materials and architectures was so high that up to 1500 kW of input energy might be required to achieve an output of 100 kW.
This situation has improved with contemporary laser sources — but not dramatically. “On a good day, those
systems are 30% efficient, so you’re throwing away 70% of the power from them,” said John Ballato, a materials scientist at Clemson University, who
specializes in fiber laser technology.
“The diodes, the pumps — they generate a staggering amount of heat,” Ballato said. This requires robust, reliable cooling solutions that enable the laser to withstand repeated firing over a relatively short period of time, including heat sinks and liquid coolants.
The optical fibers used in HEL systems are highly energy-efficient, with greater opportunities for heat dispersion than other solid-state laser formats. “They are hundreds of microns thick and meters long, so you can drive all the heat out in the transverse direction and that’s a huge advantage,” Stentz said.
But fiber lasers operating in the multi-kilowatt regime still generate enough heat to potentially undermine beam quality. According to Ballato, laser performance can be impeded by various nonlinear effects as the beam travels through the fiber, such as stimulated Brillouin scattering, wherein interactions between the light and the gain material result in power loss due to the conversion of laser light to other wavelengths. These effects are barely noticeable at low energies or in short fibers, but routinely manifest in HELs. “When you take the power that you generate divided by the cross-sectional area, you have energy densities that exceed the surface of the sun by many tens of thousands of times,” Ballato said. “With that type of power confined to that small of a diameter over 5, 10, 15 meters of length, even nominal ‘nothings’ become important ‘somethings.’”
Solutions to mitigate this effect, such as widening the fibers used in the system, create the potential for another problem: transverse mode instability (TMI). “As you get to bigger fibers, you have more ‘paths’ for the light to go down the fiber,” Ballato said. TMI is also exacerbated by thermal effects, and results in a less coherent beam.
Several companies, including OFS, have introduced fiber designs to precisely address these detrimental effects. “We developed chemistry and geometries that suppress TMI and stimulated Brillouin scattering, and which allow us to reach a narrow line with 3-kW output,” Angel said. Ballato’s group, meanwhile, is heavily focused on further improving materials to bring the magnitude of these effects as close to zero as possible. The group has previously developed silica-based fiber formulations that can potentially deliver stable output energies of up to 13 kW.
Rugged optical coatings are also essential for preserving laser performance at higher energies — and heat is not the only enemy present on a smoky, dusty battlefield. Any exposed surface is potentially susceptible to foreign-object damage in such an environment. “If debris falls onto the lens or the mirror and the laser is
firing, it can become a point of damage, and the mirror can fail catastrophically — so catastrophically that you get a beam-size hole through the optic,” said Mike Hyman, chief technology officer at Optimax. The company is prioritizing the use of foreign-object damage-resistant coatings in optical components intended for use in directed energy systems. Optimax has also developed a rigorous testing regimen that includes systematically dirtying those components to evaluate how well they retain performance and stability.
Ready for battle
System design and optimization aside, the challenge of packaging HELs into an effective and practical weapon is itself a major undertaking. SWAP—short for size, weight, and power—is a key metric in the directed energy community that determines the footprint of the laser system, and where it can be deployed as a result.
Among other things, SWAP is informed by the cooling requirements of the laser, and HEL systems operating at hundreds of kilowatts can occupy considerable space. For example, the 300-kW system, developed by GA-EMS for a
demonstration in 2021, occupies a 21.5-ft-long cargo container. “That container
includes the laser, the thermal management system, the beam director, and the control system,” Forney said. “And we basically were attaching a diesel generator and a diesel tank.”
A system of this size can readily be transported overland. But it also presents a potentially vulnerable target for adversaries, particularly if situated near the front as an early line of defense against aerial attack.
Soldiers may be adept at repairing conventional weapons. Handling and replacing laser
optics requires more care and protection against environmental damage.
The field is also contending with challenges related to targeting. It might take up to 10 s of continuous laser exposure to destroy a target, depending on the power of the beam, and even the simpler objective of disabling UAV electronics still requires seconds of precise, stable aim. Turbulence and other atmospheric irregularities can dramatically undermine this precision. “The hard part about getting lasers to operate at any range on the ocean or on the ground is that you really have a lot of dirty air to get through,” Forney said.
GA-EMS and other companies are working to overcome these effects by implementing strategies based on adaptive optics. These approaches measure distortions created by atmospheric effects and then precisely adjust the laser optics to compensate. Forney is also enthusiastic about airborne deployment of HELs. This could greatly simplify the aiming challenge by using uncrewed planes to position lasers above much of the smoke and turbulence that would confound ground-based armaments. At the same time, it would also bring these weapons much closer to their targets.
Finally, there is the challenge of battlefield maintenance. Soldiers may be adept at repairing conventional weapons. Handling and replacing of laser optics requires considerably more care and
protection against environmental damage.
Pump combiners as well as signal combiners are designed for efficient coupling into cladding pumped fibers. These components are essential for certain directed energy laser weapons, which commonly use fiber laser technology. Courtesy of OFS.
“Right now, the armed forces are
entertaining building deployable cleanrooms,” Hyman said. To simplify things, many HEL developers are creating laser components that can be easily replaced without disassembling the weapon system. “We provide as clean a space as we can and we have modular units that can be easily swapped out,” Forney said. “You can pull out a chunk of the laser and reinstall one that’s either refurbished or new for that application.”
Energizing aerial defense
These and other innovations could potentially deliver combat-ready directed energy systems in the near future. Unfortunately, manufacturers have struggled to achieve the steady funding that is required to make this happen. “How do you afford the R&D expense and the continued research element of this if the volume’s not there? There is no consistent program of record — there’s a lot of starting and stopping,” Angel said.
A system of this size can readily be transported overland. But it also presents a potentially vulnerable target for adversaries, particularly if situated near the front as an early line of defense against aerial attack.
The high cost of directed energy
weapons development remains a deterrent, even for the well-funded U.S. Department of Defense. In a talk from 2023, David Kiel, director of the Directed Energy Warfare Office at the Office of Naval Research, said that senior Navy
officials have balked at the estimated $1 billion price tag for directed energy weapon development. There is interest from the Navy, Kiel said in the discussion, but the directed energy community and system developers will need to overcome doubts from those who believe that the weapons do not work. Until investment arrives, it will be challenging to develop systems that work as intended. “We have got to get past this prototyping phase, and closer to first production runs of systems to really understand how they can be utilized,” Hyman said.
But an inflection point may be near. Daniel Creeden, general manager of advanced defense solutions at Coherent, believes recent demonstrations show that the current generation of counter-UAV HEL systems are almost ready for prime time. “They seem to be very, very effective,” Creeden said. “It is not meant to be an end-all-be-all for everything, but it addresses a unique set of threats that’s a real problem.”
Quarles concurs, citing ongoing
conflicts worldwide and escalating
tensions in areas such as the Taiwan Strait and Korea, as well as abundant opportunities for domestic deployment of UAVs
for nefarious purposes, including terrorism or espionage. “I really foresee that
in the next 24 months, there will be much wider acceptance of the technology in
the Department of Defense,” Quarles
said.