High-power Military Lasers: The Pentagon’s laser weapon plans expand
Advances in high-energy solid-state lasers and encouraging results from field trials show expanded capabilities of recent U.S> military laser prototypes.
The Pentagon’s laser weapon programs are multiplying in the wake of continued advances in high-energy solid-state lasers, encouraging results from field trials, and the appointment early this year of former NASA administrator Mike Griffin as undersecretary of defense for research and engineering. Ongoing field trials and new construction of systems focus on high-energy lasers in the 50–150 kW range to target rockets, artillery, drones, small boats, and other tactical targets at ranges on a kilometer scale. But Griffin has hopes for developing a future generation of long-range laser-armed drones that could fill the role once envisioned for the Air Force’s Airborne Laser—boost-phase defense against ballistic missiles.
The Army Space and Missile Defense Command (Huntsville, AL) is now testing an upgraded version of its High Energy Mobile Laser Test Truck (HELMTT) at the White Sands Missile Range in New Mexico. In earlier tests with a modified 10 kW industrial fiber laser, the system, which included a beam-control system and 50 cm retractable telescope in an 8x8 military truck with a 16-ton payload, shot down small-caliber mortars and hobby-sized drones. Now upgraded with a 50 kW fiber laser from Lockheed Martin (Bothell, WA), HELMTT is going through lethality tests against harder targets. Tests of a 100 kW version are planned to start in 2022.1 This year, the Army also launched another program, the Multi-Mission High Energy Laser (MMHEL), which put a 50 kW laser on a smaller and more agile Stryker combat vehicle for short-range air defense against rockets, artillery, and drones (see figure).2
A MEHEL-equipped Stryker shot small fixed- and rotary-wing UAS out of the sky using a 5 kW fiber laser in April 2018 during MFIX-17 at Fort Sill, a first for the Army ([a]; photo credit: U.S. Army photo by C. Todd Lopez, Army News Service). Shown is one of the drones shot down by a MEHEL-equipped Stryker in April at Fort Sill during MFIX-17 (b). Lessons learned during MFIX-17 will make the MEHEL easier for Soldiers to operate. (Photo credit: U.S. Army)
In spring 2018, the Navy awarded Lockheed a $150 million contract to build two copies of a new laser weapon in the 60–150 kW range called the High Energy Laser and Integrated Optical-dazzler with Surveillance (HELIOS). It’s a follow-on to the Navy LaWS tests on the USS Ponce. One HELIOS will be delivered by 2020, then integrated into the cooling, power, and battle-management systems of a modern guided-missile destroyer for field trials. The other will go to White Sands for extensive testing. In addition to zapping drones and rockets, HELIOS is designed to use its optical system monitor and gather intelligence data on an extended area around the ship, and will include a laser dazzler able to confuse sensors and cameras on drones and small boats.
Spectral beam combination
The technology behind these lasers is spectral beam combination, a high-energy version of wavelength-division multiplexing in the ytterbium-fiber laser band that stacks lasers at closely spaced wavelengths to avoid interference. At the 2017 Conference on Lasers and Electro-Optics (CLEO; San Jose, CA), Lockheed reported that spectral beam combination of 96 fiber lasers each emitting 300 W could generate 30 kW, but could not talk about the configuration used to reach 50 kW for HELMTT.4
Single-mode emission is essential for the high beam quality needed for laser weapons, but the continuous-wave power is limited by a thermal effect called transverse mode instability that occurs above a threshold value that can range from several hundred watts to a few watts, depending on the fiber design. Once a threshold level is reached, thermal effects arise that shift much of the power from the fundamental mode to the LP11x and LP11y modes, with interference between modes writing a time-varying thermal gradient in refractive index that causes power in the lowest-order mode to vary chaotically on a millisecond scale.5
So far, spectral beam combination has reached the 100 kW class that Griffin told Congress in spring 2018 that he wanted to install on Army theater vehicles.6 However, transverse mode instability and the limited number of wavelengths that can be combined may restrict spectral beam combination to powers in the 300 kW class. That was the power Griffin said he wanted available for weapons on Air Force tankers, presumably for self-defense. Griffin also wants megawatt-class lasers in space for uses including boost-phase missile defense, but those could require different technology.
One option for megawatt-class lasers is HELLADS, the liquid-flow laser system developed by General Atomics (San Diego, CA) that already has reached 150 kW. It can be scaled readily to much higher powers by adding more modules with 75 kW output, says Mike Perry of General Atomics. Although details are classified, that technology is said to depend on flowing an index-matched coolant very smoothly through a solid slab to produce a very high-quality beam.
A second option is diode-pumped alkali lasers (DPALs), which promise very high optical-to-optical conversion because quantum defects are as low as 1%. In 2017, Greg Pitz of the Air Force Research Laboratory (Kirtland AFB, NM) reported 48% optical-to-optical conversion efficiency in a potassium-vapor laser emitting 1.5 kW, but output power remains far short of megawatt class.7 Other groups also are working on DPALs and some work is said to be classified. Their challenge is to reach megawatt output within the decade time scale that Griffin hopes to see.—Jeff Hecht
1. See https://goo.gl/YZDMC6.
2. See https://goo.gl/Ypu3wq.
3. See https://goo.gl/wMpkbc.
4. See https://goo.gl/sxstQ8.
5. F. Beier et al., Opt. Express (2017); https://doi.org/10.1364/oe.25.014892.
6. See https://goo.gl/LhvjmD.
7. G. A. Pitz and M. D. Anderson, Appl. Phys. Rev., 4, 4 (Oct. 12, 2017); https://doi.org/10.1063/1.5006913.