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 a key driver of technological development being war, lasers have been put to use for missile defense, amongst other purposes.
ADAM system vs. Qassam-type rocket, 2013 – 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 engage multiple targets in rapid succession.
• Limited collateral damage.
• Laser energy travels at the speed of light.
The First Laser War
The story of the laser begins with a war – a thirty-year patent war, in fact.
Gould 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. 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 he was denied security clearance and effectively barred from working on his own invention.
Gould 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. The market for lasers had meantime ballooned to more than $500 million per year – and 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.
Since those first lasers, the momentary power they can produce has reached the point where a single pulse can rival the entire world’s energy use – for a vanishingly small amount of time. This is useful for projects like inertial confinement fusion, but less so for missile defense which needs to impart a good amount of energy onto a target, and not just immense power for a correspondingly short time period.
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).
In a 1987 paper  that served as a nail in the coffin of star-wars, the same Townes who served as Gould’s advisor in laser/maser development came to the conclusion that all known lasers were 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, but these devices are right at the edge of what’s possible.
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. If you are shooting your laser horizontally through the atmosphere, turbulence will spread the beam to at least 10 microradians divergence, or 10mm spread for every kilometer of beam. Smoke, sand, dust, fog, rain, may cause a laser to fail due to increased turbulence and absorption of the laser power.
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.
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.
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.
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.
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. SkyGuard, a Northrop Grumman chemical laser project developed for use in Israel, met a similar fate.
The fiber laser uses an optical fiber as the gain medium (instead of e.g. Maiman’s 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).
Israel’s Laser Defense Programs
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.
Nautilus in action – Northrop Grumman
Iron Beam (Keren Barzel) animations – Israel MOD
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 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.
Lahav Or – Israel MOD
The “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.
Some more exotic alternatives
The final frontier – space mirrors
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
Free electron laser video – US Navy
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.
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.
For more info, there’s a fuller version here.