Improving fiber optic lasers means the advent of radiation weapons

Original author: Jeff Hecht
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The cunning configuration of industrial lasers will finally make laser weapons practical

The most advanced laser weapon of the US morphlot looks like an expensive telescope for beginners. It rises on the landing gear of the USS Ponce and looks at the sky above the Persian Gulf, while its operator sits in a dark room somewhere on the ship, and holds in his hands something like a game controller. In front of him on the screen you can see a small boat, located near Ponce, carrying a dark object. An infrared ray aimed directly at this object is not visible, but one of the points suddenly becomes brighter, and then the object suddenly explodes, and metal fragments fly away from it, falling into the water.

This weapon, whipped up from several industrial lasers designed for cutting and welding, should produce only about 30 kW - and this is far from the megawatt monster that military scientists have been dreaming of for many decades, capable of shooting down ICBMs . But this, as his supporters say, is a serious milestone on the path to the future, in which weapons with directed energy will be deployed in real combat conditions. They add that this future will result from changes in mission and technology. Changes in the mission have been going on for years - from global protection from "unreliable states" possessing nuclear weapons to local protection from rebels. Changes in technology happen more dramatically and lead to new solid - state fiber optic lasers. They form the backbone of the $ 2 billion fast-growing industry in the United States, re-creating existing technologies for cutting and welding metals, and scaling them to even greater power with a devastating effect.

Pentagon officials believe that high-energy laser technology, like the one tested on the already decommissioned Ponce, can play various roles both on land and at sea: destroy cheap rockets, artillery, drones, small boats loaded with weapons that the rebels introduce in operation in places like Iraq and Afghanistan. Today, the destruction of a rebel rocket worth several thousand dollars may require the use of the Patriot complex at a cost of $$ 2-3 million. For comparison, a shot from a fiber laser can cost only $ 1 for diesel fuel, according to the military.

The military of other countries also aims to create fiber lasers with a power level of about 100 kW, which is necessary for the reliable destruction of targets at a distance of several kilometers. Chinese companyPoly Technologies , Israeli Rafael and German defense company Rheinmetall have already developed lasers that are not inferior in power to the American prototype. Britain will spend £ 30 million to build a 50 kW Dragonfire laser , and Japan is exploring the possibility of using fiber lasers to block attacks with short-range missiles and ballistic missiles from North Korea. The United States goes ahead of everyone by launching a program to develop high-energy electrically pumped high-energy lasers 15 years ago, and benefits from the rapid development of industrial fiber lasers running IPG Photonics. The company, located in Oxford, Massachusetts, was founded in Russia in the 1990s, but then its headquarters moved to the United States in 1998. Its facilities are located around the world and it dominates the international fiber laser market.

“The Department of Defense has wanted to get laser weapons since the invention of the laser,” said Robert Afzal, senior laser and sensor fellow at the Lockheed Martin defense company in Bozel, Washington. “The key element was the creation of a high-energy electric laser, small enough and powerful enough to be placed on trucks, planes and ships, and not to remove all other equipment from there,” to make room for him. And although other technologies are being developed now, it seems that fiber lasers are the first to be able to satisfy these requirements.

The Pentagon has been blinded by laser weapon dreams ever since physicist Gordon GouldI went to the Agency for the Study of Promising Projects, which was then only a few months old, in 1959, and suggested they create a laser. Gould, one of three people, with whose names the creation of the laser is associated, he took advantage of the idea of ​​generating coherent light, expressed by a 37-year-old graduate student at Columbia University at the end of 1957. Within a few weeks, he sketched his version with a pair of mirrors located on opposite ends of a thin long cylinder. Smoking one cigarette after another over a pile of scientific papers and his notebook, located on the kitchen table, he realized that the laser can concentrate light into a powerful beam. Having developed the idea of ​​a laser, Gould abandoned his research for a doctorate, and patented the invention, eventually securing the support of TRG, where he began working in early 1958.

In the first place in the list of tasks of the new unit of the US Department of Defense was the task of protection against nuclear weapons. Therefore, the new agency was very interested in the possibility of building weapons operating at the speed of light. ARPA has allocated $ 1 million (at today's prices $ 8) to develop the proposal Gould. Unfortunately, when the Pentagon decided to keep this work secret, his involvement in the Communist Party at a young age prevented him from gaining access permission, which was necessary to work on his own project.

To create a powerful beam, it took more than a pair of mirrors. It was required to place a light source between the mirrors and come up with a way to pump energy into this material. The first working laser was built on the basis of a solid synthetic ruby ​​containing chromium atoms emitting light, which could be pumped by bright pulses of light from a lamp. Soon, other types of systems appeared: it turned out that a discharge of current passing through a tube filled with helium and neon can generate coherent light. The pulses of current passing through gallium arsenide diodes, the edges of which were polished to a mirror shine, could give the same thing.

But the Pentagon needed higher power, and from these sources it was impossible to achieve it. In an initially secret project by ARPA Seaside, researchers built solid-state lasers using rods a few centimeters thick, but most of the incoming light energy went into warming up the radiating solid, and little came out in the form of a beam. Therefore, this approach was abandoned. Similar problems forced the abandonment of early gas and semiconductor lasers.

The military was about to abandon laser weapons in the mid-1960s when researchers at the Avco-Everett laboratory near Boston invented an amazing new approach. They burned hydrocarbon fuels and forced hot gas to pass through a series of rocket-like nozzles so that gas flowed between pairs of mirrors and generated tens of kilowatts of infrared laser light.

Fiber laser gun: The energy of diode lasers is pumped into a long section of a special optical fiber with the addition of light-emitting ytterbium atoms. Laser light travels along the length of the fiber, and is reflected from mirrors embedded in the fiber, and then exits from one of the ends.

Inside fiber lasers: The laser consists of a three-layer fiber. The fiber sheath has the lowest refractive index. Inside the shell is the outer core core with a slightly higher refractive index. This difference causes light in its path through the fiber to bounce off the outer core. The light crossing the inner core stimulates the emission of laser light by ytterbium atoms located in the inner core, which has an even higher refractive index.

Gas flow technology supported the study of laser weapons in the U.S. Army during the Cold War. The development of new fuels capable of delivering megawatts of energy has led to dreams of creating military laser stations in orbit. US President Ronald Reagan spent billions of dollars onA strategic defense initiative that explored the possibility of creating space lasers that shoot down Soviet ICBMs. At the end of the Cold War, the United States Air Force spent another billion cramming a huge anti-ballistic missile laser into the Boeing 747 to combat launches from "unreliable countries" like North Korea. In 2010, this megawatt monster really managed to bring down a ballistic missile during tests - this was his first success - but its use did not come close to the practical level. After that, Secretary of Defense Robert Gates hacked this program, announcing: "I don’t know a single person in a uniform who believes that this concept is working."

The less powerful chemical lasers being developed for the Pentagon’s new anti-insurgency mission ended up in the same fate. In 1996, the United States and Israel launched a joint program to test 100 kW gas-fueled gas lasers created by TRW (now part of Northrop Grumman). A high-energy tactical laser shot down rockets and artillery shells in 2000 and 2001. A terrorist attack on the 2000 USS Cole at about the same time underscored the dangers of small boats and the need to seek protection from them.

But this TRW laser revealed two big problems. Firstly, it was impractically large, and consisted of four trailers the size of a truck each, and a separate beam guide, the size of a huge spotlight. Secondly, which was more important for Pentagon experts in logistics, he needed special chemical fuels. Such fuel always causes problems for logistics - any supply disruption can make weapons useless. Worse, these chemicals themselves also represented weapons. Hydrogen fluoride was generated in their reaction, a gas capable of destroying corneas, burning out the lungs and seriously damaging the skin - which created enormous risks for the soldiers serving him.

Meanwhile, another laser technology was rapidly developing. In the 1960s, Zhores Ivanovich Alferovfrom Russia [Soviet and Russian physicist, the only one of the Nobel Prize winners in physics currently living in Russia, vice president of the Russian Academy of Sciences from 1991 to 2017 / approx. trans.] and holder of the IEEE Medal of HonorHerbert Kroemer from the United States invented structures that dramatically improved the operation of semiconductor devices, including diode lasers, by restricting the flow of light and electric current. This achievement in 2000 brought them the Nobel Prize in physics. For 17 years, until 1977, Bell's laboratories extended the life of diode lasers from seconds to 100 years, making it possible to use them in fiber optic communications. After that, other laboratories dramatically increased power and efficiency, as a result of which diode lasers could convert about half of the electrical energy they received into laser light.

Diode lasers did not generate the narrow beams needed for weapons, but they opened up a new opportunity: to replace lamps as an energy source of solid-state lasers. These lasers, which emitted their light into sheets of glass or crystals containing light emitting additives, had a huge advantage over lamps because they generated much more efficient light, which also consisted of waves of the same length. If you play with the composition of the laser semiconductor, then you can choose the wavelength of light, which will be almost completely absorbed by the crystal, which will lead to an extremely efficient conversion of its energy into laser.

High efficiency not only saves the Pentagon a couple of bucks in electricity bills. The incoming energy, which has not turned into light, turns out to be spurious heat, and limits the efficiency of the laser, since it must be removed. From a practical point of view, diode lasers made it possible to hope for power reaching units or even tens of kilowatts, although they did not reach the megawatts necessary for defense against long-range missiles.

Laboratory High Energy Laser Joint Technology Office (HEL-JTO) was launched to develop weapons based on solid-state lasers. Similar systems using glass or crystal sheets peaked in March 2009 when a demonstration from Northrop Grumman took place. Their devicegave out a stable beam with a power of 105 kW for 5 minutes. The laser did not require special fuel, it did not produce toxic gases, but it weighed 7 tons and occupied 10.8 cubic meters - which is comparable to the size of a concrete mixer truck.

The military, of course, wanted something smaller than a concrete mixer. Another HEL-JTO program, Robust Electric Laser Initiative, did the job. Having been instructed to develop a solid-state laser that is better suited for use in combat, HEL-JTO set itself the task of constructing a 100 kW laser occupying 1.2 m 3 and capable of delivering more than 150 kW per kilogram, working with an efficiency of at least 30%. Two of the four projects launched by the laboratory were considering new options for fashionable laser technology: fiber lasers.

The experimental fiber lasers will appear to be capable of delivering the energy and efficiency needed to hit drones, small boats and other targets.

Fiber laser, in fact, is an optical fiber with important modifications. It has a central core with a slightly higher refractive index than the glass shell surrounding it. A telecommunication fiber uses a similar structure to transmit optical signals from laser transmitters to a central core consisting of extremely pure quartz, which practically does not cause loss. But in a fiber laser, this central core contains light-emitting atoms, usually ytterbium.

Fiber lasers have another extra layer between the color-emitting central core and the outer sheath. This intermediate layer, the outer core or inner shell, has a refractive index in the gap between the core and the outer shell. It consists of high-purity glass, and its task is to conduct light from external diode pump lasers, sent to the outer shell through separate fiber optic cables. From there, light passes through the outer core, reflected from the walls of its shell, constantly crossing the inner core, in which ytterbium atoms capture photons and emit laser light. The outer shell specially has an irregular shape - D, an ellipse, or even a square - so that as much light as possible passes through the central core.

Like a signal inside a fiber optic telecommunication cable, the light emitted by the ytterbium atoms remains inside the central core. But instead of traveling tens of kilometers in one direction to the next optical amplifier or receiver, this light moves in one direction or the other, reflected from a pair of reflectors located at both ends of the fiber. And with each pass, more and more ytterbium atoms emit light, increasing the laser power.

The tight fit of the inner and outer cores ensures that ytterbium atoms absorb most of the pumped light. In 2016, IPG Photonics announced that it was able to achieve the conversion of more than 50% of the electric energy into laser energy in the laboratory - this is much more than can be obtained with a solid crystal or glass, or an older solid-state laser circuit. Creating light in a long and thin fiber also allows you to get a beam very focused at long distances - and this is what is required to transfer lethal energy to a target several kilometers away. Since fiber lasers are obtained thin - the fiber diameter is in the range of 125-400 microns - they have a very large surface area to volume ratio, which allows them to dissipate heat much faster,

Fiber lasers started small, they were an offshoot of developing fiber amplifiers for long-distance telecommunications in the 1990s. Attempts to increase their energy began to do IPG. Starting with a 1 W fiber laser in 1995, the company added every order of magnitude to this power every three years, until 2012. The company itself also grew along with the power of their lasers. In 2017, its sales reached $ 1.4 billion, which is about a third of the revenue of the entire industrial laser market.

Industrial fiber lasers are very powerful. IPG recently sold a 100 kW laser to the NADEX Laser Research Center in Japan. He is able to weld metal parts up to 30 cm thick. But for the sake of such power, one has to sacrifice the ability to focus the beam at distances. Tools for cutting and welding need to work with objects located just a few centimeters from them. And the highest power that was achieved from a fiber laser with a beam suitable for focusing on objects located hundreds of meters from them is 10 kW. But even this is enough for stationary purposes like unexploded ordnance left on the battlefield, since the laser can be focused on the explosives for quite some time until it detonates.

Of course, 10 kW will not be able to stop a boat carrying a bomb. In the demonstration for the Navy, the USS Ponce used six industrial fiber lasers from IPG, each of which had a power of 5.5 kW, firing through the same telescope to form a beam with a power of 30 kW. But it will not work to get a beam with a power of 100 kW, capable of maintaining the focus necessary to destroy fast-moving distant targets, simply adding light from additional industrial lasers and increasing the size of the telescope. For this, the Pentagon needed a single system capable of delivering 100 kW. The laser had to track the movement of the target, focusing on a weak spot like an engine or explosive until the beam destroyed it.

Unfortunately, with the current approach this is not possible. “If I could create a 100 kW laser based on a single fiber cable, that would be great, but I can't,” says Lockheed Afzal. “Scaling a single fiber to high energy does not work.” Such power requires new technology, he adds. The leading candidate is combining the rays of many individual fiber lasers in some more controlled way than simply directing all the rays through one telescope. And in this area two approaches look promising.

One idea is to precisely equalize the phases of the light waves emanating from several fiber lasers so that they fold and form a single, more powerful beam. The light waves in each laser are coherent, that is, they move identically with each other - for all waves, the peaks and troughs coincide. In principle, the coherent alignment of the rays of several different lasers should create a powerful beam that could be focused on targets located several kilometers away. Phased Array Antennascan combine the coherent output of several radio transmitters, but with light it is much more difficult to do. The wavelengths of light are orders of magnitude shorter — of the order of a micrometer, in contrast to centimeters in the case of a radar — which makes it extremely difficult to precisely combine the waves so that they are structurally folded and not interfere.

Another approach involves ignoring the phases and combining the rays from many fiber lasers equipped with optics that limit the light emitted by them in one short section of the spectrum. As a result, each beam has its own distinct wavelength. As a result of their combination, a beam with a large spread of wavelengths is obtained, and its components do not interfere with each other. This technique is called "spectral beam combining," and was adopted from technologySpectral channel multiplexing , which has proven extremely successful in pushing more data into existing fiber-optic communication channels.

To implement this technology, Lockheed developed special optics that deflect the rays of individual fiber lasers at angles that depend on the wavelength - just like a prism separates the colors of the spectrum. After that, the rays are combined and form a single beam. In 2014, the company “created and tested for its money a 30 kW laser to deal with physics and engineering fundamentals,” says Afzal. The system combined 96 beams with different wavelengths of 300 watts each into a single beam with a total power of 30 kW. At relatively low energies, lasers emit high-quality beams, so it is easier to combine them to produce a high-energy beam than to build a single laser with high energy and the same beam quality, as Afzal claims.

Last year, Lockheed managed to scale this technology to 60 kW when it introduced a model for mounting on a military truck prepared for battle. This laser "set the world record for the efficiency of military solid-state lasers, exceeding the 40% bar," says Adam Aberle, head of the development and demonstration of high-energy laser technology. With such efficiency, a laser system with a 100 kW beam produces less than 150 kW of stray heat. Compare this to the 400 kW of stray heat generated by a laser made by a different technology in 2009 by Northrop Grumman. On March 1, Lockheed announced that by 2020 the United States Navy will deliver two copies of similar lasers called HELIOS, which will have to produce no less power. The Navy will install one of them on a destroyer,

“We regard the development of a high-energy fiber laser with a combination of beams as the last piece of the puzzle,” says Afzal. Perhaps, but the search for the perfect laser weapon is far from over. Now that the technology of high-energy lasers already looks viable, the military will have to think about how to deploy lasers in combat conditions and against which to use. And for this you will have to develop, create, test, improve the equipment necessary to turn a powerful laser into a mobile weapon: this includes trucks, ships and aircraft carrying the laser; sensors and computer systems that recognize and track targets; power management systems delivering electricity to the laser; cooling systems that prevent overheating;

And the equipment itself is not yet ready for such tasks. Research and development teams will transfer their lasers to military materiel workgroups, such as the US Naval Sea Systems Command, to integrate new technology into military plans and develop procedures to support its work. The working groups will also have to conduct critical fatality tests of lasers using potential targets and develop a strategy and tactics for their use in battle.

Simply put, the ability to shoot missiles at a test site is not enough for laser weapons to earn their place in the military arsenal. It took the military nearly 60 years to bring the lasers to a state that would make them potentially useful for combat. But in the Pentagon, many top officials grew up with "kinetic weapons" - guns and missiles - and it will be very difficult to convince them in the onset of the era of Buck Rogers . Without broad support, there will be no money to finance the complex and expensive work of deploying a radically new type of weapon. As one wit said (for another reason): "There will be no bucks, there will be no Buck Rogers."

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