Longtime skeptics of high-energy lasers often say, ‘the technology is five years out… and will always be’. The DoD and defense industry have invested in laser weapons development for 40 years, in anticipation of a transition to troops that has yet to happen.
Then, between 2017 and 2018, we demonstrated our high-energy laser weapon system, or HELWS, for the U.S. Air Force and the Army, shooting down multiple drones.
The demonstration marked a milestone in the development of functional laser weapon systems and resulted in a contract with the Air Force to develop three prototype high-energy laser systems that will be deployed to troops overseas.
So what changed? What is different than in the previous 40 years of laser weapons development?
First, the advent of fiber laser technology. The systems have shrunk down to a level where they can fit on ground vehicles, helicopters and ships – platforms that the military actually uses. And the beam quality of fiber lasers – a measure of how much of the laser power actually reaches the target – is excellent.
Second is the proliferation of low-cost threats, like drones. Drones in large numbers can do a lot of damage and can simply exhaust the magazines of any current defensive weapon. Lasers can complement kinetic weapons – never running out of “bullets”, so long as electrical power is available.
Laser weapon systems have some key advantages for dealing with drone swarms. They provide speed of light “flyout” time and have deep magazines – as long as you have a power source, you can fire the laser and hit a target immediately. Lasers also have a very low cost per shot, so it makes sense to use them against low-cost threats like drones.
In the future, high-energy lasers may even be used to deal with more challenging threats, such as classes of missiles. Other defensive weapons include interceptor missiles and rapid-fire guns such as the Phalanx. But because the number of defensive missiles and ammunition magazines are finite, in the future high-energy lasers can help support the overall defense when dealing with multiple threats.
Chemical vs Fiber Lasers
Lasers have come a long way since the first type of laser weapon systems that were based on chemical lasers. Those lasers were scalable to high power, but had several downsides that ultimately led to their downfall. First of all, there were no significant commercial market opportunities for chemical lasers, so DoD had to provide the vast majority of funding for components, subsystems and the laser. Second, the chemicals themselves were toxic and large volumes were needed, driving system size and safety issues. Chemical lasers themselves fell out of favor because of these operational concerns, but from a technological standpoint, the HEL community continues to leverage the many successes and lessons learned in that era.
Fiber lasers are much smaller and cleaner than chemical lasers, and the demand for fiber lasers has exploded in the last 10 years, thanks in great part to two industries – material processing and communications. The material processing industry uses fiber lasers to accurately cut, drill and weld anything from car parts to computers. Meanwhile, most voice and data communications travel through fiber optic cables. This has created an over $2-billion-dollar commercial fiber laser industry that can be leveraged by the defense industry.
If the Air Force’s deployment of Raytheon Intelligence and Space’s HELWS is successful, lower power systems that can kill drones could proliferate quickly in the coming years. Feedback from the Air Force will help improve the performance of the system so it can one day deal with threats other than just drones. We also plan to drive down costs, as our manufacturing process and supply base are improved.
How Do High-Energy Lasers Work?
How do high-energy lasers work, anyway? At the simplest level, electrical power is used to generate a laser beam, getting rid of the waste heat from that process. You also need a system that knows where to point the laser beam and can hold it precisely on the target for long enough to kill it. Of course, this description oversimplifies what is a sophisticated series of steps.
It all starts with a radar somewhere – typically on the platform itself or in an adjacent platform – that sends a message to say: ‘there is something out there that might be a threat.’ The laser system slews and points in the direction of the threat. A camera looks at the threat, often providing a better, higher resolution picture than the radar could provide. The decision-maker then determines whether the object is a threat that must be engaged.
Once that decision is made, the beam control system engages sensors to ensure that the target is precisely tracked despite motion of both the platform and the target. Based on prior knowledge of the identified target, the most vulnerable point is selected – either manually or via automation. The beam control system ensures that the high energy laser continues to hit the same point on the target with high precision until the target is neutralized.
To further understand how the system works, it helps to look at the four subsystems that make up the HEL.
First, there is the power subsystem, which reconditions electrical power to whatever voltage is needed to drive the laser. The power can come from the platform that the laser is mounted on, such as a destroyer ship, or from lithium-ion batteries, like those in the Polaris MRZR ATV that was adapted.
Then there is the thermal subsystem. It removes the large amount of waste heat generated by the laser system and disposes of it in a way that doesn’t degrade the performance of the laser.
The third subsystem is the laser beam itself, one of the most complicated parts of the whole system. In the first step, arrays of thousands of low power semiconductor diode lasers, each similar to a laser pointer, convert the electrical power into divergent beams of laser light. In the next step, each fiber laser acts as a brightness converter, efficiently converting the divergent diode light beams into highly directional fiber laser beams. A range of different techniques are used to efficiently combine the multiple beams from multiple fiber lasers into a single, high-power, low-divergence beam.
Two of the main beam-combining techniques are spectral and coherent beam combining. With coherent beam combining, sensors measure a distorted probe laser beam at, or near, the target, then use algorithms to provide phase corrections and compensate for the distortions. Matching phase corrections can then be applied to individual fiber laser beams comprising the high-energy laser beam, correcting for the distortions in that high-power laser beam.
Spectral beam combining provides a simpler technique for generating low to moderate power level laser beams (up to ~50 kW-class). But for higher power lasers in the presence of atmospheric distortion, a separate adaptive optics system is required, including components that are unproven for long term operation in field environments. Both techniques are still being developed and refined across the industry. RI&S believes that each solution has a mission set that is better-suited to address.
The fourth subsystem, beam control, serves a critical role: pointing the beam precisely at the chosen aim point on the target with sufficient intensity to neutralize it. Any jitter in the position of impact on the target is equivalent to a lower laser power that will take longer to kill the target.
Targeting and Optics
RI&S’s high-energy laser weapon system uses a modified version of our Multi-Spectral Targeting System, or MTS, to hold the laser beam on a target with ultra-high precision. The MTS, an electro-optical and infrared sensor commonly seen on manned and unmanned aircraft, makes for a near-ideal effective beam control system. A highly integrated design philosophy makes our HEL system more robust, providing lower jitter than other beam control systems.
In addition to the four subsystems, future laser systems may also need adaptive optics. To understand adaptive optics, imagine being outside on a hot day. As you look at the horizon, the image becomes blurry or distorted. The same distortion effect happens to a laser beam as it moves towards its target – it becomes distorted and can start to break up. If you are close to the ground, dealing with threats at a similar altitude, adaptive optics becomes important. To solve this challenge, there are different adaptive optical techniques that can be used with the two, previously mentioned beam combining techniques.
There will be more challenges to overcome as lasers move to the field and towards volume manufacturing. Since the timing of a strong demand signal from the government has been uncertain, the defense industry hasn’t yet invested in largescale manufacturing infrastructure for laser weapons. At RI&S, we are drawing on existing manufactured components as much as possible – leveraging our MTS production facility, as well as readily-available fiber lasers from established commercial manufacturers. In this way, we can lower the additional investment needed to reach volume production.
The next few years will help determine whether high-energy lasers become a staple of the battlefield. The upcoming field deployment, as well as further development of the technology, will provide important milestones in the future of laser weapon systems.
This article was written by Iain McKinnie, Principal Engineering Fellow and Technical Area Director for EO/IR and Lasers at Raytheon Intelligence & Space (McKinney, TX). For more information, visit here .