One of the earliest uses of directed energy weapons, according to legend, was Archimedes' use of a set of mirrors to focus sunlight on the Roman fleet as they invaded Syracuse, setting fire to the textile sails.
Today we have lasers, and now as then some of the chief pursuits of mankind being military matters, these lasers have been put to use for missile defense, amongst other purposes.
US ADAM vs. Qassam - Lockheed Martin
Since the days of Ronald Reagan's $30 billion Star Wars program, a number of state militaries and defense contractors have started (and sometimes shuttered) a variety of laser weapon systems. In the following we look at the history of lasers, laser defense, and some of the patents involved. As an entree, some of the reasons a military might seriously consider such devices:
They have pinpoint accuracy
They offer a low cost per use (~1$ per shot as opposed to ~$50k per shot for the Iron Dome)
They have a virtually unlimited magazine capacity.
They are less lethal if tuned properly (no stray interceptors)
They can operate in all weather conditions.
They can engage multiple targets in rapid succession.
Limited collateral damage.
Laser energy travels at the speed of light.
Laser energy can pass through walls at distances of hundreds of meters or even miles.
The First Laser War
The story of the laser begins with a war - a thirty year patent war, in fact.
Gordon Gould was apparently the first to conceive of the laser, while Theodore Maiman was the first to build one.
Gould conceived a series of ideas concerning the laser (a word he coined from the acronym 'light amplification from stimulated emission of radiation') while in graduate school in 1956-1957. He had these ideas witnessed and notarized at a local candy store on the advice of the Maser's inventor Charles Townes, who was a professor at Columbia and later won the Nobel prize for his work on the maser and the laser. Townes agreed to act as a witness.
This notarized page describes a single concentrated beam of light created by two mirrors in a gas-filled chamber. But it wasn't until 1959 that Gould actually filed an application for a patent - after Townes and another physicist, Arthur Schawlow, had already filed their own patent applications.
Gould quit Columbia without his PhD and joined forces with a small research company known as TRG, which based on his ideas obtained a major research contract from the Defense Department's Advanced Research Project Agency (ARPA, aka DARPA). But on the basis of Gould’s previous involvement with communism, he was denied security clearance and effectively barred from working on his own invention.
Gould, backed by TRG, battled the courts for years until October 1977, when the U.S. Patent and Trademark Office finally awarded him a patent on the optical pumping of lasers, eventually granting all four of the patent applications Gould originally filed. The market for lasers had meantime ballooned to more than $500 million per year - after 20 years of fighting, suddenly Gould was a multimillionaire.
That initial patent war marked the start of R&D into lasers, which have found their way to multiple uses - in communications, surgery, welding and cutting metals and other materials, fundamental physics research including laser cooling and inertial confinement fusion, and more. 'Directed energy weapons' are a class of weapon including high-power lasers being developed, for use in 'dazzling and destroying'. Dazzling refers to blinding sensors and cameras (a 1995 UN resolution forbids use of laser weapons for actually blinding human beings), and destroying in this case means destroying missiles and possibly other military targets.
The laser is in a sense an energy-storage device - energy is stored by 'pumping' electrons into high-energy states (left picture above), where they are liable to be 'pushed' back into the ground state by a passing photon of the right sort, thereby releasing another photon (right picture above). If conditions are set up correctly, energy can be pumped into the electrons of a laser’s atoms over a long period of time, and then can be released much more quickly than this, resulting in the production of a pulse of light with high peak power - this is the 'amplification' in the LASER acronym.
Theodore Maiman developed the first working laser in 1960. It consisted of a ruby rod, with its end faces coated with reflective material, that was surrounded by a helical flashlamp. The bright light from the lamp pumped energy into the electrons of the ruby’s chromium ions. The resulting beam had a wavelength of 694 nanometers, a deep red color, and a power of about 100 Watts.
Since those early days , laser power has grown over 13 orders of magnitude, to 2 petawatts. The power in question however is instantaneous power, generally for a pulse that is as short as it is intense - so the total energy involved may be a modest few hundred or thousand Joules (about the energy required to lift 1kg to a height of 10 or 100 meters).
This kind of extreme pulse is good for things like inertial confinement fusion experiments (below) but less so for disabling missiles.
To take down a missile one needs to maintain high power for relatively long periods (e.g. several seconds) , with a total energy closer to a car at highway speeds (1MJ or one million Joules) than a rock at 10 meters height (100Joules).
Some hurdles
Power
In a 1987 paper [1] that served as a nail in the coffin of star-wars, the same Townes from the initial laser/maser development and others came to the conclusion that known methods for producing power lasers were all lacking in power by at least a factor of 100. Today, solid state and fiber lasers have increased in power to the point that they may do the job; 10-100kW fiber lasers are possible, and are far more portable than the star-wars chemical lasers.
Beam Spread and Atmospheric Turbulence
There is a fundamental limit on how 'straight' a laser beam can be - even the most perfect laser optics produce a beam that spreads to some degree. The fundamental limit is due to diffraction, with the minimum divergence angle given by
θ=2.44λ/D
where λ is the wavelength of the laser beam and D is the diameter of the exit pupil of the laser. The spread in spotsize due to this divergence is
s=rθ
If you are shooting your laser horizontally through the atmosphere, turbulence will spread the beam to at least 10 microradians divergence. This is still small enough that given excellent optics, large exit diameter, and perfect aim, a spot can still be put entirely onto a 30-cm diamter missile at 3 km distance.
That being said, smoke, sand, dust, fog, rain, may cause a laser to fail due to increased turbulence and absorption of the laser power.
Optics
Contamination is a killer of laser optics. Somehow one has to maintain clean-room level cleanliness for the laser optics in the field, a demanding requirement in any situation situations with dust particles, salt spray, etc. A speck of dirt on the output lens can cause it to shatter.
Tracking
Laser rangefinders were used initially to track targets. However, cloud cover, rain, or smoke can also prevent signal detection, which is why laser rangefinders fell out of favor in the 1980s.
Moreover, many basic countermeasures can exploit the fragility of these kinds of sensors, for example, possibly using dust or shrapnel to disrupt the electrooptical sensors on a system like AN/DAS-4, the most advanced laser tracker used in prototype weapons. Thus ideally a variety of systems are employed in parallel - radar, optical, and laser - for tracking.
Beam Control
Another major problem with laser technology is jitter of the laser spot. The laser has to hit a single spot for at least several seconds (at currently practical power levels, that is) until the target is disabled. Since the target is generally a a few tens of cm wide few at a distance of a few km away , the angular precision required is of the order of 10cm/10km ~ 10^-5 radians.
Position Sensing Devices (PSD), Fiber Optic Gyros (FOG), Fast Steering Mirrors (FSM), and various filters can apparently significantly reduce jitter.
Since weather and intervening terrain can disrupt the line-of-sight beam in ways that wouldn’t affect a physical, maneuvering missile, laser defense will probably be complementary to Iron-Dome type interceptors.
Where lasers shine (so to speak) are against large numbers of lower, slower, more fragile targets: small drones, mortar shells, and unguided artillery rockets — weapons that Hamas and Hezbollah have in their thousands and tens of thousands. Hezbollah’s arsenal is estimated at 150,000 rockets, while Hamas' stockpile was at least the ~4,000 rockets launched in May 2021. Even if the Iron Dome was 100% effective, the economics (given a ~$50K USD pricetag per interceptor) make defending against an all-out barrage difficult to impossible.
Current Technology
Chemical Lasers
The highest average laser power to date is 1 MW, or megawatt, first achieved by the Mid-Infrared Advanced Chemical Laser (MIRACL) at White Sands Missile Range, New Mexico - part of the aforementioned Reagan-era star-wars program. This device, which involved a good amount of highly-reactive chemicals (e.g. fluorine, deuterium, basic hydrogen-peroxide, or iodine), produced several MJ of laser energy in a seconds-long burst (1MJ is the energy of a 2-ton car at 120kph). Due to the great expense, the dangerous chemicals involved, and dubious efficacy, this program was finally scrapped in 1993, and most countries have moved to fiber and solid-state systems.
Fiber Lasers
The fiber laser uses an optical fiber as the gain medium (instead of e.g. the ruby rod shown above)
An advantage of fiber lasers over other types of lasers is that the laser light is both generated and delivered in a flexible fiber, and has high output power (kilowatts) compared to other types of laser due to a. the fiber's high surface area to volume ratio, which allows efficient cooling, and b. active regions that can be several kilometers long (with the fiber in a coiled loop taking little space).
Diode (Solid State) Lasers
The diode laser dispenses with the pumping light, instead producing stimulated emission directly from an optically conductive layer (yellow, below) sandwiched between n- and p-type regions (red,blue) as in a traditional LED.
In most cases these devices use electrical rather than optical pumping to provide a population of electrons 'ready to lase' . Sets of individual laser diodes can be stacked, forming powerful beams in several dimensions (below).
US Systems
The current leader of the pack appears to be the US Navy's LAWS system, which is currently in use and employs six solid-state welding lasers in parallel.
US Navy lays down the LAWS
A number of other programs are underway in the US. The Missile Defense Agency’s (MDA) goal is to field a laser weapon on a high-altitude, long-endurance (HALE) drone by 2023. The laser-armed drone would circle potential launch sites and shoot down ballistic missiles while they are in their most vulnerable “boost phase.” Toward this goal, the Defense Department plans to spend $563.5 million on directed energy research across five research programs.
MDA is pursuing two technologies: the diode-pumped alkali laser system (DPALS), which focuses on building a more powerful singular laser, and Fiber Combining Lasers (FCL), which combine the beam outputs of smaller lasers. Both technologies require approximately 35-40 kilograms of weight per kilowatt of energy emitted, while MDA Director James Syring avers "where we need to be is below the 5 kilograms-per-kilowatt window."
Israel
Nautilus
The Nautilus Tactical High Energy Laser (THEL) , a US-IL collaboration, was started in 1995 and cancelled by 2006. The system shot down 28 Katyusha rockets and several artillery shells in tests, but was a chemical laser suffering from high costs, sensitivity to atmospheric conditions, and questionable portability.
The late Nautilus (THEL) in action
Keren Barzel
Iron Beam ( קֶרֶן בַּרְזֶל, keren barzel) was unveiled at an airshow in 2014 and deployed in August 2020. It is based on a fiber laser packing 'tens of kilowatts' of power, and is designed to destroy short-range rockets, artillery, drones, and mortars, with a range of up to 7 km (which is too close for the Iron Dome system to intercept).
The system is based on five years of research and development in solid-state lasers and is developed by Rafael, funded by the MoD, and extensively underwritten by the United States. An Iron Beam battery is mobile and composed of an air defense radar, a command and control (C2) unit, and two HEL (High Energy Laser) systems.
Iron Beam (Keren Barzel) in action
Drone Dome
The Drone Dome is a Rafael anti-drone system. A 360-degree radar, camera and small laser are all integrated into a jeep-mounted system adapted to protect field soldiers from drone attacks.
Light Saber
Israel's "Lahav Or" (Light Saber) is a laser system designed to intercept airborne incendiary threats (e.g. balloons and drones) launched from the Gaza Strip, and is deployed operationally by the Border Police.
According to available details, the system has an effective range of 2 kilometers (1.2 miles), day or night. The system uses a relatively low-power 'eye-safe' laser capable of incinerating a balloon or a kite.
Light Saber in action
Some more exotic alternatives
The final frontier - space lasers
The idea of space-based lasers has been extended to use of ground-based lasers reflected off space-based mirrors. With three mirrors located on geostationary satellites, a ground-based laser (which can be much larger than anything one can get into space) would be able to strike anywhere on the surface of the planet.
Free Electon Laser
Intense, high-energy x-ray lasers can be devised using technology developed for particle accelerators - wigglers, undulators, and their kin.
These devices take a beam of high-speed electrons, and send them wiggling or undulating back and forth by use of an array of magnets (figure below). The accelerations involved in the wiggling (or undulation) causes the electrons to emit radiation in the directions perpendicular to the acceleration. The physical spacing of the magnets and the speed of the electrons determine the radiation that's produced, which can reach the extremes of x-ray in this case.
The US is (or was) developing a free-electron laser weapon, with Boeing having started on a $23M military contract in 2010 and reaching a power of 14kW in 2011.
Plasma Bolt
In this variant, a laser-induced plasma channel is created using an intense laser beam to plasmify the air, rendering the entire laser beam path conductive. Once a channel of air is conductive, it can carry electric current along its length - something like a directed lightning bolt. This apparently has been tested in 2012 by the Army's Picatinny Arsenal in New Jersey
"If a laser beam is intense enough, its electromagnetic field is strong enough to rip electrons off of air molecules, creating plasma," George Fischer, lead scientist for the project, said. Although a flying object will generally be electrically 'floating' and thus won't serve as a path for current, by raising a missile to extreme voltage, or by applying a directed EMP or high-frequency alternating voltage, its electronics (if it has any) can be fried, and possibly any electrically-sensitive explosives can be detonated.
Roundup
A few current and past programs for high-energy laser development:
LAWS (US NAVY)
ATHENA (US)
LIGHT BLADE (IL)
Peresvet (RU)
Boeing yal-1 (US)
Cuas - Raytheon (US)
Dragonfire (UK)
Lockheed Martin SHIELD 2021-2023 - airborne, fiber laser
Talos (EU)
[1] Report to The American Physical Society of the study group on science and technology of directed energy weapons , Rev Mod. Phys 59, July 1987
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