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The Laser Directed Energy Weapons (DEWs) offer a transformational ‘game changer’ to counter asymmetric and disruptive threats, while facing increasingly sophisticated traditional challenges. Laser technology provides major advantages for military applications over kinetic weapons due to High precision and rapid on-target effect, precise and scalable effects, avoidance of collateral damage caused by fragmenting ammunition, Low logistics overhead and minimum costs per firing.
The key functional components of any laser system are the energy supply and processing blocks that generate power for the pump source that generates an active laser beam by using a laser medium and then feed it to the beam control system. The beam control system consists of beam coupler, beam conditioner, and components that align the beam. Afterwards, the beam is directed towards the target of interest. The impact of laser beam over the desired target(s) is normally evaluated through a sensing and control system. Such a system detects turbulences and inaccuracies induced by atmospheric conditions and relative motion or state transitions of the target(s) and implements appropriate control techniques to apply corrections. These energy lasers have some special requirements for their effective operation, i.e. laser fuel/power requirements, cooling/thermal requirements, tracking and pointing requirements, personal and environmental safety requirements.
The development of laser weapons requires many critical technologies, first is development of lasers capable of generating powers in kilowatts to megawatts range to be able to produce useful damage effects on the target. Laser require a power of the order of 100 kW, to be employed as directed energy weapons, in varieties of missions such as wide-area, ground-based defense against rockets, artillery and mortars; precision strike missions for airborne platforms; and shipboard defense against cruise missiles. To destroy anti-ship cruise missiles would require a beam of 500 kilowatts and demand megawatts of power.
Solid state lasers are electrically powered, and they are separated into three types: Fiber solid-state lasers like LaWS, slab solid-state lasers, and free electron lasers. While they avoid the complicated logistics associated with chemical lasers, SSLs are generally not very efficient. The fiber laser is a variation on the standard solid-state laser, with the medium being a clad fiber rather than a rod, a slab, or a disk and Laser light is emitted by a dopant in the central core of the fiber.
The push to optimize the size, weight and power (called SWaP in military parlance) of field deployed laser weapons has driven a progression in the technology of the gain material used from chemical (e.g. deuterium fluoride), to solid state, and, most recently, to fiber. Fiber lasers have emerged most promising technology, for directed energy weapons due to their many advantages like: high electrical to-optical efficiency (40%), high reliability for operation in harsh military environments, and high beam quality near diffraction-limited light output.
Fiber lasers can satisfy extreme power requirements. The U.S. Navy’s Laser Weapon System (LaWS), tested by the Naval Sea System Command, has six fiber lasers, each emitting 5.5 kW, incoherently combined into one beam and fired through a beam director . The 33 kW system was used to shoot down an unmanned aerial vehicle (UAV). Although the beam was not single-transverse-mode, the system is of interest because it can be constructed of standard, easily-available components.
High Power and efficient Fiber Laser technology
Fiber laser is a device in which “the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium and holmium.” The idea of confining the photons within the rare-earth doped fiber is what gives the fiber laser its principal advantage over rivals: stability. Because a fiber laser generates its beam inside the core, delivery of the beam doesn’t require complex or sensitive optical equipment.
A normal laser, on the other hand, either uses an optical fiber to move the laser beam or uses mirrors to bounce it around. Either approach works, but both require extremely precise alignment. That makes normal lasers sensitive to movement and impact. And once things go out of alignment, a specialist has to set things right. A fiber laser has no such sensitivity. It’s stable. A fiber laser can handle the bumps, knocks, vibrations and general discord of any assembly line.
The most common geometry of the fiber used in fiber lasers is a dual-core structure. An undoped outer core (sometimes called an inner cladding) collects the pump light and guides it along the fiber. Stimulated emission generated in the fiber passes through the inner core, which often is singlemode. The inner core contains the dopant (ytterbium or erbium) that is stimulated to emit radiation by the pump light. The doped fiber has a cavity mirror on each end; in practice, these are fiber Bragg gratings, which can be fabricated within the fiber. Numerous noncircular variations exist on the shape of the outer core; these shapes, which include hexagonal, D-shaped, and rectangular, decrease the chances of the pump light missing the central core.
However, the fiber host is usually silica glass with a rare earth dopant in the core. The primary dopants are ytterbium and erbium. Ytterbium has center wavelengths ranging from about 1030 to 1080 nm and can emit in a broader range of wavelengths if pushed. Using pump diodes emitting in the 940 nm range can make the photon deficit very small. Ytterbium has none of the self-quenching effects that occur in neodymium at high densities, which is why neodymium is used in bulk lasers and ytterbium is used in fiber lasers (they both provide roughly the same wavelength).
Erbium fiber lasers emit at 1530 to 1620 nm, which is an eye-safe wavelength range. This can be frequency-doubled to generate light at 780 nm—a wavelength that’s not available from fiber lasers in other ways. And finally, ytterbium can be added to erbium so that the ytterbium absorbs pump light and transfers that energy to erbium. Thulium is another dopant that emits even deeper into the near-infrared (NIR; 1750 to 2100 nm), and is thus another eye-safe material
A fiber laser can be end- or side-pumped. In end-pumping, the light from one or many pump lasers is fired into the end of the fiber. In side-pumping, pump light is coupled into the side of the fiber; actually, it is fed into a coupler that couples it into the outer core.
Power limitations can arise, particularly from working within a singlemode fiber. Such a fiber core has a very small cross-sectional area, and as a result, very high-intensity light going through it. Nonlinear Brillouin scattering becomes increasingly important at these high intensities, and can limit output at multikilowatt levels. If the output is high enough, the fiber end can be optically damaged.
Another advantage is that fiber lasers are power efficient. A fiber laser can convert nearly 100 percent of the input it receives into the beam, thus limiting the amount of power that is converted into heat energy, That means the fiber tends to stay safe from heat damage or fracture. All of this adds up to a robust laser that requires next-to-no maintenance.
Beam combining
Developers got around the limits on single-mode fiber lasers by finding ways to combine the beams from multiple fiber lasers to generate high-power, high-intensity beams. The simplest approach is incoherent beam combination, which Phillip Sprangle and colleagues at the U.S. Naval Research Laboratory tested by combining beams from four fiber lasers onto a target 3.2 km away. They delivered a total of 5 kW to the target, and concluded that at such distances there “is little difference in the energy on target between coherently and incoherently combined laser beams for multi-kilometer propagation ranges and moderate to high levels of turbulence.”That inspired the U.S. Office of Naval Research to buy half a dozen 5.5-kW industrial fiber lasers from IPG, and combine their beams incoherently by aiming them with different mirrors through a single telescope toward the same target. Called Navy LaWS, for Laser Weapon System, it was mounted on the USS Ponce when it was deployed to the Persian Gulf.
Laser Weapon System Demonstrator tested by U.S. Navy
Amphibious transport dock ship USS Portland (LPD 27) disabled an unmanned aerial vehicle (UAV) with a Solid State Laser – Technology Maturation Laser Weapon System Demonstrator (LWSD) MK 2 MOD in May 2020 in a U.S. Navy test. LWSD is a high-energy laser weapon system demonstrator developed by the Office of Naval Research and installed on Portland for an at-sea demonstration. LWSD’s operational employment on a Pacific Fleet ship is the first system-level implementation of a high-energy class solid-state laser, according to officials. The laser system was developed by Northrup Grumman, with full System and Ship Integration and Testing led by NSWC Dahlgren and Port Hueneme.
HADES directed energy weapon demonstrator
The U.S. Air Force Research Laboratory (AFRL)’s unique laser weapon demonstrator called the High-power Adaptive Directed Energy System, or HADES, completed field testing in 2019. The HADES technology was conceived by Nutronics, Inc., a small business in Longmont, Colo. that won Phase I, II and III contracts with the U.S. Air Force (USAF) through the Small Business Innovative Research (SBIR) program including special contracts through the Commercialization Readiness Program. Between 2011 and 2019, the Nutronics team invented, developed, tested, and delivered the HADES directed energy weapon demonstrator to the Air Force.
“HADES technology was designed with the warfighter in mind,” said the AFRL program manager Dan Marker. “The system works by combining a large number of high power fiber lasers in a fashion that corrects for distortions caused by the atmosphere. This gives the warfighter a long-range high power laser weapon.” “The HADES concept is generic,” said Marker. “The technology can apply to any type of mission or military platform. The system can be adapted to different power levels for use in a severely turbulent atmosphere or on missions that require high speed compensation such as the effects of air flowing over an aircraft turret.” “The field tested concept produced super resolution distortion free images, and provided atmospheric data needed to pre-compensate a projected laser beam,” Marker said. “This type of technology is critical to the performance of a laser weapon system.”
In 2017, Lockheed Martin in Bothell developed 60 KW laser weapon for U.S. Army
Lockheed Martin’s Bothell office has unveiled a world record-setting laser weapon for the U.S. Army. Lockheed successfully developed and tested the 58 kW laser beam setting a world record for this type of laser.
“The inherent scalability of this beam combined laser system has allowed us to build the first 60kW-class fiber laser for the U.S. Army,” said Robert Afzal, senior fellow for Lockheed’s Laser and Sensor Systems in Bothell. “We have shown that a powerful directed energy laser is now sufficiently light-weight, low volume and reliable enough to be deployed on tactical vehicles for defensive applications on land, at sea and in the air.”
The system’s modular laser design will allow the laser’s power to be varied across a wide range — from 60 kW to 120 kW — depending on the specific mission and threat, Lockheed Martin said.
“A robust laser system with minimal operational down-time results from the integration of modular fiber-based lasers,” said Iain Mckinnie, business development lead for Laser Sensors and Systems, Lockheed Martin Mission Systems and Training. “With modular lasers, the possibility of a complete system failure due to a single-point disruption is dramatically lessened.”
Dynetics to build 100 kW laser for US Army
The US Army’s Space and Missile Defense Command has selected a Dynetics-led team to develop an experimental 100 kW laser weapon system for its Family of Medium Tactical Vehicles (FMTV).
Under the new contract, the Dynetics team is selected to prepare for the critical design review, which will determine the final laser design, before building the system and integrating it onto a 6×6 FMTV for field testing at White Sands Missile Range in New Mexico. The capability is also a potential technology insertion to address the army’s Indirect Fire Protection Capability Increment 2 – Intercept Block 2 objective requirements.
This effort is part of the service’s latest venture into directed-energy weapons and is designed to help the service counter rocket, artillery, and mortar (RAM) attacks, as well as unmanned aircraft systems (UASs).
“The project researches advanced technologies for HEL weapon systems to enable more efficient laser systems with greater power output, which in-turn enables future laser weapons on smaller vehicles for additional missions,” the army wrote in fiscal year 2020 (FY 2020) budget request documents. “This includes technologies to support development of alternate laser sources, precision optical pointing and tracking components, adaptive optics to overcome laser degradation due to atmospheric effects, more compact and lighter weight energy generation and storage devices, and more efficient thermal management systems to remove excess heat.”
For FY 2020 and FY 2021, the service plans to request just over USD22.4 million to continue HEL TVD research and development. For example, the army said it plans to complete development of the gimbal, telescope, and main optics bench for the HEL TVD beam control system in 2020, according to budget documents.
General Atomics’s compact 150 kW High Energy Laser (HEL)
General Atomics revealed in April 2015 that its Gen 3 High Energy Laser (HEL) completed beam quality and power measurements tests. The Gen 3 laser has a number of upgrades that provide improved beam quality, increased electrical to optical efficiency, and reduced size and weight; the assembly is small at only 1.3 by 0.4 by 0.5 metres (4.3 × 1.3 × 1.6 ft), and is powered by a compact Lithium-ion battery to demonstrate deployability on tactical platforms. Beam quality remained constant through the 30-second demonstration, proving that the beam quality of electrically-pumped lasers can be maintained above 50 kilowatts.
General Atomics’s Tactical Laser Weapon Module includes high-power-density lithium-ion batteries, liquid cooling, one or more laser unit cells, and optics to clean up and stabilize the beam before it enters the beam-director telescope. A unit cell produces a 75 kw beam, and modules can be combined to create beams of 150-300 kw in power.
General Atomics is undertaking a privately funded study to integrate a 150-kilowatt solid-state laser onto its Avenger (née Predator-C) drone. GA-ASI has designed a power system for drone lasers that works almost like a hybrid car, the non-plugin kind. “You use the aircraft power to charge an intermediate storage system, and then that runs the laser when it’s doing laser shots,” said Michael Perry, Vice President for Mission Systems at GA-ASI.
DoD is finalizing contracts for three competing demonstrators, aiming for a 300-kilowatt weapon by 2022 and 500 kW by 2024
“We are in the process of negotiating contracts with three different performers for three different electrically powered laser concepts,” Thomas Karr, who works for Pentagon R&D chief Mike Griffin as assistant director for directed energy, said. (DE includes both lasers and high-powered microwaves). These will be demonstration models for testing, not prototypes of operational weapons, he emphasized in an interview with Breaking Defense. Industry has proposed several designs that “have all been demonstrated at lower power levels, 50 to 150 kilowatts,” Karr said. Those power levels are enough to burn through drones and rockets, but not larger, faster and tougher targets like cruise missiles.
“We want to have a 300-kilowatt laser by 2022. We’d like to get up to 500 kilowatts by 2024,” he said, “and then, if we still haven’t hit the limit of anything, it’s on to the megawatt class. “Those are aggressive objectives,” Karr acknowledged, “[but] we have high confidence that one or more of these different fiber or slab approaches will scale up to 300 or beyond. I don’t think we’ve seen the limit yet.”
General Atomics and Boeing team up on scalable 100 kW to 250 kW- high-energy laser weapon in Oct 2020
General Atomics Electromagnetic Systems (GA-EMS) and Boeing have entered into a partnership to develop scalable 100 kW to 250 kW-class High Energy Laser (HEL) weapon system for air and missile defenses.
Of the problems that have hampered laser weapon development over the past six decades, one of the biggest is how to properly cool a laser generator. This is important because weapon-grade lasers have an efficiency between 50 and 70 percent, with the leftover percentages being lost as heat that could shut down or damage the device.
For the HEL weapon system, Boeing and GA-EMS plan on pooling their technologies to create a combat-ready system, with GA-EMS to provide its scalable distributed gain laser technology, HELLi-ion battery systems, and integrated thermal management. The last of these is particularly important because it combines the high energy density of a solid-state laser with the cooling system of a liquid laser.
Meanwhile, Boeing will provide its beam director and precision Acquisition, Tracking and Pointing (ATP) software. When combined, the hoped for result will be an HEL laser weapon system with the high-output power, range, adaptability, precision ATP requirements, and reduced logistical footprint that will allow it to be fitted to land, sea, and air-based platforms.
“GA-EMS has made significant advancements in developing and demonstrating highly scalable laser technologies to facilitate high output power in smaller, lighter weight packages,” says Scott Forney, president of GA-EMS. “We look forward to working with Boeing to deliver a laser weapon system with capabilities designed to meet current operational requirements, while providing the flexibility and adaptability to suit emerging platform requirements supporting missions across a multi-domain battle space.”
Military eyes prototype megawatt-class laser weapon for ballistic missile defense
The Ballistic Missile Defense System (BMDS) Laser Scaling project of the U.S. Missile Defense Agency (MDA) will develop a prototype laser weapon system that will weigh no more than about four tons, including the laser, electric power, and thermal management subsystems. The project’s focus is on reducing size and weight, and increasing power, electrical-to-optical efficiency, beam quality, and lasing runtime.
MDA officials issued a request for information last week (HQ0277-19-RFI-0001) for the BMDS Laser Scaling project in efforts to understand industry’s ability to demonstrate a 1,000-kilowatt electrically pumped laser sometime between 2025 and 2026.
Researchers also are interested in electrical power and thermal management subsystems for the prototype, and are not yet providing a specific platform or strategic mission. It is to be a ground demonstrator laser with technology maturation and light-weight engineering paths to potential future applications.
MDA officials envision a laser weapon able to shoot down incoming ballistic missiles with near diffraction-limited beam quality at 1 megawatt of laser power with a vertical beam quality of 1.1 at 0.25 lambda/D. It should have a laser wavelength shorter than one micron to offer high intensity on the target at long ranges. The system should have a mass efficiency of two to four kilograms per kilowatt, including electric power and thermal management. Early prototypes may have a lower mass efficiency as long as they have clearly defined paths to increase mass efficiency.
The electrical-to-optical efficiency goal is at least 48 percent, and continuous laser shot durations must be from 2 to 60 seconds. The prototype must have an energy storage system able to supply power for two minutes at full power without recharging.
Chinese laser advancements: Raycus fiber lasers exemplify nation’s Made in China plan
In 2013, Raycus succeeded in developing China’s first 10 kW fiber laser－making China the second country to master the technology in the world. Raycus earlier unveiled a 20 kW fiber laser produced by at the laser technology and industry development forum held in Wuhan, Central China’s Hubei province. That product has entered mass production according to the company.
Fiber laser, which releases laser energy through a fiber as thin as a human hair, has been widely applied in aerospace, shipbuilding, airplane and auto manufacturing, as well as 3D printing. It is an indispensable component of precision machining. Compared with carbon dioxide laser, fiber lasers feature three times faster emitting speed, 20 percent higher energy conversion efficiency, four times less power consumption, with no noise or pollution emitted. Yan said fiber laser industry is of strategic significance and an important industry in military-civilian integration.
According to Yan, the successful development of the 10 kW fiber laser has lowered the price of imported fiber laser from five million yuan ($760,000) to a little more than three million yuan. As the country realizes production of 20 kW fiber laser, the price of imported ones will decrease by 40 percent.
The United States still embargoes the export of high-power fiber lasers of more than 1,000 W to China. However, since Raycus has successfully produced fiber lasers with that much or more power, the embargo thus makes no sense, he said. However, China still lags behind other countries in fiber laser technology, said Wang Pu, a professor at the Institute of Laser Engineering of Beijing University of Technology.
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