The U.S. Navy's recent demonstration of its new laser weapon, designed to blast enemy drones out of the sky, proves that these systems no longer solely exist in the world of science fiction. But how do these so-called directed-energy weapons work?
The idea for laser weapons has been around for at least a century; the writer H.G. Wells even imagined "heat rays" in his 1897 novel "War of the Worlds." Lasers, though, are a demonstration of several technologies and even physics that didn't exist or wasn't known until the 1960s — and in some cases, later than that.
In part, the initial drive to build laser weapons wasn't to make ray guns — it was to help people make phone calls. It wasn't until fiber optics and cheap laser diodes became available that this technology could be used to build weapons, according to experts. [7 Technologies That Transformed Warfare]
"We could build powerful lasers in the past, but they weren't small enough or powerful enough to be tactically deployed," said Robert Afzal, a senior fellow in laser and sensor systems at Lockheed Martin, one of several companies that has been developing laser weapons for the military. "With high-powered, fiber-optic laser technology, we can now build a laser powerful and small enough for a tactical vehicle."
The laser system being developed at Lockheed isn't the same one that was demonstrated last month by the U.S. Navy, but the physics and engineering are similar, Afzal told Live Science.
Making laser light
The word "laser" is actually an abbreviation for "light amplification by stimulated emission of radiation." To make a laser, you need a lasing medium — some material that emits light when it is stimulated by energy. Further, that light needs to be a single wavelength, and all the light waves need to be in step — a state called coherence.
A neon light bulb generates light of specific wavelengths, but those waves aren't all in step; they're jumbled together, with the crests and troughs at different places. This makes it harder to focus the light into a beam that doesn't disperse over long distances. It also means less energy gets delivered to anything illuminated by that light.
Coherent light waves can be more focused. In other words, the light waves in a laser beam spread out much less than those in a flashlight beam do, directing more of its energy into a small spot.
The first laser beams in the 1960s were generated with ruby crystals that were pumped with light from a powerful type of flash lamp. The crystal was called the gain medium.
The intense light excited the atoms in the crystal, which then generated the photons, or packets of light, for the laser. A mirror was at each end of the crystal, and one of the mirrors was transparent. The light would be reflected from one side and come out the transparent side.
More modern lasers use gases as the gain medium, such as carbon dioxide, helium or neon. They all produce lasers of different wavelengths for different applications. Carbon-dioxide lasers emit infrared light, and they are often used as cutting tools. [Science Fact or Fiction? The Plausibility of 10 Sci-Fi Concepts]
Later the chemical laser was invented, but that wasn't going to work for shipboard weapons. "The old chemical lasers took up a lot of volume," said Mark Skinner, vice president of directed energy at Northrop Grumman Aerospace Systems. "They also sometimes used toxic chemicals." For example, a hydrogen fluoride lasers, first demonstrated in 1969, can deliver high-powered beams but the hydrogen fluoride is dangerous and difficult to handle.
The laser diode was a big innovation; though they were first demonstrated in the 1960s, it wasn't until the 1970s that semiconductor lasers were built that could operate continuously at room temperature. Earlier, in 1966, Charles K. Kao (who would go on to win a Nobel Prize in Physics in 2009) discovered how to transmit light over optical fibers, which meant that lasers could be used as a means of communication. Then, the development of cheap diode lasers enabled the building of devices such as CD players and laser communication arrays.
"Really, we put together two revolutions: fiber-optic telecommunications and wave-division multiplexing," Afzal said. Wave-division multiplexing (WDM) is a technique that combines lasers of different wavelengths onto a single fiber, which enables more power to be pumped through a fiber-optic strand. Originally applied to communications, it became a go-to technology for laser weapons as well, he said.
Building a ray gun
But laser weapons require more than simply making laser light — they need to transmit the light to a target and do so with enough energy to inflict damage. Laser power is usually measured in watts. The power of a laser pointer can be measured in milliwatts, but that's still enough to injure a person's eyes. The power of industrial laser cutters is in the kilowatt range. The military needs lasers that have a much more powerful range than that — in the tens of kilowatts, at least. [Flying Saucers to Mind Control: 7 Declassified Military & CIA Secrets]
The U.S. Navy's new laser weapon, which is currently deployed on the USS Ponce — an amphibious transport ship — is reportedly a 33-kilowatt laser, and it can fire several beams that add up to 100 kilowatts. The Navy said in January that it plans to test a 150-kilowatt version within a year. (A Navy spokesman said he couldn't reveal how powerful the laser actually is.)
The reason for the high power is that even though lasers are focused to a narrow point, their beams still spread out over long distances, and that cuts down the energy that gets delivered to the target. A laser damages its target because the energy from the light heats up the material it hits. As such, the beam has to stay on a target for a certain period of time (more power means less time and thus a more effective weapon). A video released to CNN shows the Navy's Laser Weapons System (LaWS) trained on a target for about 1 or 2 seconds, but none of these specifications have been publicly released yet.
The LaWS aboard the USS Ponce is a fiber-optic laser, and it combines beams to increase the power. While fans of "Star Wars" may recall the image of several separate beams joining together after they're emitted from the Death Star, real combined-beam lasers don't work like that. Instead, they use fiber optics to generate the beams, and then those beams are combined using a prism-like setup of lenses.
"Think of that cover of [the Pink Floyd album] 'Dark Side of the Moon,'" Afzal said. "You have a prism that combines several beams into one."
Another advantage of fiber optics, Afzal said, is that the beams are more "perfect." This means there is less diffraction, or spreading out of the light, than there is with a traditional lens (early lasers had beams focused by lenses, and laser pointers still do this).
Pros and cons
One of the biggest issues with developing laser guns was figuring out how to power them. Thirty kilowatts over 1 second is enough to light up a neighborhood (the average home in the U.S. uses about 10 kilowatt-hours in a year). This means that any boat using a laser weapon has to have a power plant that's robust enough to handle it. The USS Ponce demonstration showed that it could handle the power load.
The advantage of lasers, and the reason the military is interested in them, is speed. A laser beam travels at the speed of light. Practically speaking, when a laser weapon is aimed at something, it will hit instantly. There's no need to point the weapon slightly ahead of where the target is moving, as would need to be done if the military were trying to shoot down a projectile. And contrary to what's depicted in movies, there's no way to see a laser beam unless there's something scattering the light. If the beam is visible, it would simply appear to be instantly "on," just like a searchlight.
Lasers are also cheap to use, according to the Navy, because the only cost is power. This means that once the weapon is built, the price per shot goes down — a laser never runs out of ammunition. Missiles, on the other hand, can cost thousands of dollars each, Skinner noted.
Still, there are some disadvantages to using lasers as weapons. Subrata Ghoshroy, a research affiliate at MIT who worked on early laser weapons in the 1980s, noted that weather can be a problem. Laser beams are made of light, which means fog and other inclement weather will scatter that light. Range would be reduced as a result, along with the energy directed on the target.
Heat is also a factor. "Thermal management is a horrendous problem," Ghoshroy said. The reason is that all those kilowatts through a diode heat it up, and eventually, the beam quality degrades. It was not clear, he said, how often the USS Ponce's laser could fire or how long it would last before it runs into problems.
Afzal said the weather issue is common to many weapons systems, so lasers aren't unique in that sense. Fog, for example, would stop many kinds of missile launchers or guns. "If you can see it, you can engage it," he said.
Originally published on Live Science.
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