Lasers are common in science fiction as futuristic weapons. Lasers project light at damaging intensities onto a target. To be effective, this light must be focused through optics (laser beams do not travel as perfect columns! Focusing makes the beam converge to a tight spot on the target). The ability to focus is limited by diffraction and, in an atmosphere, thermal blooming; and energy can be lost from the beam due to atmospheric absorption, scattering, and other phenomena. The color (wavelength, frequency, or per-photon energy) of the light you choose can be very important for your ability to get the light to the target.
Lasers can be expected to be exceptionally accurate. Depending on the power, energy, and intensity you can deliver to the target, they could be limited to relatively shallow surface burns or could drill deeply enough to reach vital equipment or organs deep inside the target. You can defend against lasers with armor or smoke, but mirroring will not work against higher intensity lasers.
Certain colors of lasers can cause unintended blindness to bystanders or friendlies, including all visible colors of light. Use with caution.
Lasers are remarkable tools, with a wide range of applications across all realms of technology - communications, medicine, remote sensing, computing, data storage - but of course you don’t care about any of that. No, what draws us to places like this is using lasers to blow stuff up! So what is a laser, anyway? What makes it good at blowing stuff up, and why can’t it blow stuff up even better?
When we think about some new technology, we often default to thinking about it the same way as some analogous technology we are already familiar with. Early writers imagined airplanes as giant cruising air battleships rather than jet fighters. Early robots were very often metal humans that could talk and sense and interact with their environment rather than mechanical arms carrying out rote motions in factories or disk-shaped vacuum cleaners. In the same way, we often default to thinking about laser weapons as just like bullet-shooting guns. But they are not. Laser weapons, even laser guns, will be shaped by their own physics and design constraints into machines that optimize their potential and very often this leads to appearance and behavior that will be very different from the bullet guns that we expect.
If you look at the usual definitions of a laser, they will say that a laser is a device that uses stimulated emission to produce monochromatic, unidirectional, coherent light. All of this is mostly correct, but there are exceptions to each of these cases. Real life gets complicated!
- Stimulated emission is a process where an atom, molecule, or other quantum system in an excited (energized) state can be made to transition to a lower energy state by being hit by a quantum unit (or particle) of light (a photon), and in the process produce a second photon that is identical in every way to the first. Stimulated emission is a useful method to make directed beams of light, but there are machines called lasers that produce laser-like light without stimulated emission.
- Monochromatic means the light the laser produces is only one color (or wavelength, frequency, or photon energy … any one of these terms suffices to specify the color). In fact, monochromatic means that the laser is exactly one color - but this is unphysical. Every real phenomena has some finite distribution of colors, even if this distribution is so narrow that it can be hard to detect and can be neglected for many purposes. Most lasers are very close to only one color. But a number of lasers, especially those made for extremely high powered pulses, produce beams with a wide spectral range of colors.
- Unidirectional means all the light is going in one direction. As we will see later, this is ultimately not possible. The wave nature of light makes light that seems to originally be going in the same direction eventually diverge. In fact, it is often helpful to have the light coming out of a laser be slightly diverging or converging, and adjust that later with lenses or mirrors.
- Coherent (or, for our purposes, transversely coherent - there’s also longitudinal coherence but we don’t need to worry about that here) means that if you look across the beam at any given distance along the beam, all the light will be pointing or wiggling the same way. It is this property that allows lasers to be focused and directed so well. Loss of coherence means your beam can’t focus as well as it potentially could. Even incoherent light can be focused to some extent by forming an image - witness using a magnifying lens to focus sunlight - but coherent light can be focused much better.
Fundamentally, a laser is a tool that amplifies light. But even if we could, we wouldn’t want to amplify just any old light. No, we want to amplify light that has only those properties we want. For laser combat, that usually means light that will be focused on the target after going through the appropriate optics.
Technically, what happens is that to make it efficient to amplify light, we put the light we are trying to amplify into a cavity, a structure that allows light of a certain kind to resonate (or bounce back and forth a very large number of times with little loss). The material, substance, or device that amplifies the light is called the gain medium. This gain medium is placed so that the resonating cavity light goes through it, getting amplified with each pass.
The ways that the light can bounce around inside of a cavity are called its modes. A given mode will have only one color, and will have a specific relative intensity and direction profile that depends on the geometry of the cavity. The light in a given mode might be converging (focused) or diverging (defocused), but that can be altered later with optics. Any single mode will be fully coherent. The gain medium will usually only allow certain colors to be amplified, and thus only certain cavity modes can be excited. What we want to do is arrange the cavity so that only the modes that are most useful to us are amplified. When we do this, we can make beams with a minimum divergence that we can focus to destructive power on our enemies.
The Nature of Light
Lasers emit light. So to understand lasers we need to understand light. Lasers and the electromagnetic spectrum goes over some fundamental properties of light, and how the different colors of light impact what your laser can do.
So, we have a beam of really bright light that’s mostly one color and going mostly in one direction. That’s great. Now what? If you are fighting someone, you probably want to shine that light on them. And you probably want the light to be as intense as possible when it gets to them. However, if the intensity of the laser is too high while it is in your laser generating machinery, it will melt all that expensive machinery that is making the laser. You need the laser to be spread out enough in your laser gun that your laser gun can survive, but intense enough at your target that you can do bad things to it.
The trick is to use a lens or mirror to focus the laser to a tight spot on your target. This lets you use a large, spread out beam in your laser gun, but still blast your target with concentrated energy!
To do this, you need to be able to focus the laser at different distances. Also, you need to know the distance at which the beam needs to be focused. If you get either of these wrong, your target will just get heated up a bit, or get a painful but mostly harmless surface burn. The first issue can be solved by using motors and moveable or deformable lenses or mirrors. This is similar to the auto-focus technology of modern cameras. The second issue also uses the same technology that cameras use, either by bouncing a ranging laser pulse off whatever the laser is aimed at and seeing how long it takes to return, or by adjusting the focus until whatever is aimed at displays crisp lines. A consequence of all this is that laser weapons will see their beams directed to some large focal assembly that looks like a large camera lens or telescope mount, rather than just taking the beam that comes directly out of the laser generator.
But you want a big focusing aperture for more than just preventing damage to your beam optics. A wide aperture also lets you better overcome diffraction to focus better at longer ranges. In addition, in the air your ability to focus will be limited by thermal blooming, twinkle, and jitter. If your beam is too high powered and you focus too much, you will run into problems with thermal blooming, two-photon absorption, stimulated scattering, and cascade breakdown.
Diffraction gives the ultimate limit to how well you can focus your laser light. This page covers the effect in more detail, but the end result is that the larger your focal aperture, the shorter the wavelength of the light you are using, or the shorter the distance to your target the tighter your focus will be.
Getting light through air can be tricky. Sure, we can see through air just fine, so maybe you’d think it wouldn’t be too much of an issue. And sometimes it isn’t! But when the intensity of the light gets really high all sorts of crazy non-linear things can happen. We’ll try to go through the more important of these things so you can get some idea of what the limitations are for your laser and how you might get around them.
If your only concern is blowing up enemy spacecraft with your own spacecraft, you don’t need to worry about atmosphere because there isn’t any. Have fun floating around up there in orbit, though, wistfully gazing down on all those nice juicy targets that you can’t blast with your vacuum-only rated lasers.
- Attenuation will remove energy from your laser beam, giving less juice to blow up your target. There are two main forms of attenuation:
- Twinkle can make your beam wander around randomly and lose focus if you don't correct for it.
- Thermal blooming will make your beam lose focus. It is a non-linear phenomenon so you need to be careful that the steps you take to correct it don't just end up making it worse.
- Two-photon absorption can absorb high power pulses of ultraviolet light when they are tightly focused.
- Stimulated scattering turns your laser into a laser with the result that energy is removed from your beam. It is mostly important for high power pulses.
- Cascade breakdown can occur when a beam of longer wavelengths is focused too tightly. It turns the air into a plasma, which absorbs the beam and prevents it from getting to your target.
- Atmospheric hole burning might allow the really high frequency beams that are rapidly absorbed by air a way to get through the air anyway.
- Filamentation is a strange phenomenon where the light in your beam self-focuses. Laser filaments have unexpected behavior – they might cause problems but they might end up being useful.
Before your beam can do anything, you need to shine it on your target. Fortunately for you (and unfortunately for your target) lasers can be exceptionally accurate.
Imagine shooting out a ray of light. It goes through an optical obstacle course, consisting of any variety of lenses, prisms, and mirrors until it finally lands somewhere on a screen and makes a bright spot. So here’s a neat trick - if you take the light at the spot and reverse its direction (or perhaps more practically, put a light source at the spot that shoots out a ray backwards along the direction that the original ray came in at), the resulting ray moves back exactly along the path it originally took, retracing the same route through the mirrors and prisms and lenses, so that it ends up at the original source.
Now let’s see what that means for lasers. Suppose you have a laser gun. You see your target in your scope and you line up your scope reticle with the image of the target, then slowly squeeze your trigger. Zap! You send a beam of collimated photonic death at your enemy. But wait! It is a hot day, full of mirages, and the heat distortion is bending the light from your target so that it takes a round-about path to get to you. Your laser was never actually pointing at your target. Surely you must miss, right? Well, no. Because the laser beam takes the same path back to the target as the light from the target took to get to you. As long as you aim at the target’s image, no matter how it was displaced by optical trickery, the beam will arrive at the target.
In this way a laser can be much more accurate at long range than bullet guns, which have to contend with drop due to gravity and deflection by the wind.
Leading the target
Laser beams also travel much faster than bullets. Any waterfowl hunter knows they need to lead their target by a bit if they want duck dinner, because it takes the shot a slight amount of time to get there. Laser beams travel so fast that over planetary scales you don’t need to worry about leading your target.
Over vast distances in space, though, even the incredible speed of light gives a slight lag between when the information you are using for targeting is given off by your target, and when the beam reaches your target. If your target is moving on a known trajectory, you can correct for this and most spacecraft gunnery control systems should have this functionality. But this lag does give the target a small amount of time to dodge your beam.
This light lag might never be a problem, even in space, for some kinds of lasers. A visible or near visible light wavelength would need to be focused through an impractically large mirror to cause damage out to even a light second with any reasonably achievable beam power, due to the focusing limits of diffraction. So for lasers like this, lag and dodging will not be a problem. But to get very long range lasers by going farther and farther into the ultraviolet or x-rays, or by making enormous focusing mirrors, may give you beams that can reach many light seconds or even light minutes. At these ranges dodging becomes an issue. Very long range lasers might end up being tools that force an enemy to expend propellant to avoid being hit, eventually resulting in insufficient propellant left to complete their mission.
The scope on top of a rifle is set maybe 6 cm above the barrel. So if the scope is perfectly aligned with the barrel (it isn’t) and the bullet goes exactly straight (it doesn’t) , when you shoot a rifle you will hit about 6 cm below where you shot at. This is called parallax. Usually, this being within 6 cm is adequate - it’s still close enough for a hunter to put a bullet through a deer’s vitals, or a sniper to drop a terrorist with a shot to the center torso. (As an aside, scopes are usually set up to point somewhat down compared to the barrel, so that the initial rise of the bullet is pulled back down by gravity, and the bullet first rises a bit above the line of aim before coming back down - thus nicely countering one effect by the other and extending the distance over which you don’t have to adjust for bullet drop considerably.)
But what if we could do better? A laser uses optics to focus the beam. The shooter uses optics to aim the beam. So let’s use the same optics for both purposes. This idea is common in cameras, where it is called a single lens reflex camera. A flippable mirror directs light from the lens to an eyepiece, showing exactly what the camera will record when you click the shutter and flip the mirror around so the light goes to the sensor (or film) instead. You can do the same thing with your laser gun, except that now instead of letting the light in to a sensor you let the light out from the laser beam generator. Either way, the laser beam goes directly to where the reticle was pointing, without any parallax at all.
Sometimes it's not so much about how much the beam itself moves around, but how steady you can hold the beam. This page covers much of the issues with jitter on fixed mounts. Hand-held laser weapons will mostly be limited by the jitter imposed by the person aiming the gun – their breathing, pulse, tiny muscle tremors – all these can throw off a gunner's aim. Fortunately, modern camera makers have come up with ways to correct for that. Electronic auto-stabilization that works on consumer camera optics will also work on laser optics, making it easier to hold your aim rock-steady.
A gun that shoots bullets will kick back as the bullet is launched. This might be from the pressure of the gas inside the barrel pushing back on the gun. For a science-fictional gauss gun, it might be from the interaction of the induced currents in the bullet with the magnetic field and the currents in the gun’s barrel. But no matter how you do it, Newton’s laws of motion and the conservation of momentum mean that when you shoot the bullet forward, something else has to go backward and almost always that thing that goes backward is the gun.
Recoil has a number of drawbacks. If you don’t have experience shooting and you are holding the gun wrong, it can knock you off balance. If you are shooting really big bullets going really fast, the gun recoiling can hurt or even leave bruises, especially if you are a small framed person. Many shooters start to subconsciously anticipate the kick, and flinch as they pull the trigger which will throw off aim. Most seriously, perhaps, is that when you are shooting rapid fire the recoil makes it hard to control the gun. On fully automatic fire, after the first two or three bullets the rapid recoil usually torques the barrel of the gun up into the air so all you are shooting at is the sky. And for really big guns, like cannons, you need to engineer in shock absorbers so the recoil doesn’t damage the cannon’s mount or throw off the aim of subsequent shots.
Lasers don’t have this problem. Or rather, if they do have this problem you are dealing with such massive overkill that your battlefield will look like nothing we have ever experienced. For any practical laser power, the recoil will be so minuscule that you can’t notice it. Lasers do produce a tiny amount of recoil. The force they give is the power divided by the speed of light; the total momentum given to the gun is the energy divided by the speed of light. Because light is so incredibly fast unless the power and energy is extraordinarily high the recoil will never be felt. If your laser kicks with the recoil typical of a normal gun, you are throwing energies typical of the detonation of tons of TNT downrange at your enemies with each shot.
Effect on target
So now you are shining an intense beam of light on your target. What does it do? Does it just smoulder? Does it burst into flames? Is it drilled through in a shower of sparks? Does it explode messily? This page will help you find out.
So we’ve been having fun pretending to blow up bad guys with lasers. But what happens when it’s the bad guy that has the laser? How are we going to protect ourselves from becoming the extra crispy variety of barbeque?
The first step in not getting shot is not being seen. While stealth methods have long been known and used to prevent the enemy from shooting at you, for lasers not being seen has an additional meaning. Lasers operate by emitting light. If light can’t get between you and your attacker, his beam can’t reach you. It doesn’t matter if the evil emperor's goons have a precise lock on you with their radar. If you are behind a kilometer thick fog bank, their laser guns are not going to reach you.
Now clearly, mere darkness is not going to stop a laser. Even if your foe can’t see you because the lights are out, his beam can still get to you. But if you can put something in the way that will absorb or scatter the light out of the beam, then his beam can’t reach you and you will be safe. This might involve deploying a smoke screen. Or flying above the clouds - or below the clouds if the enemy is in orbit and you are not.
A rough rule of thumb for visible and near-visible colors is that if they can see you through clouds or smoke, they can shoot you with the laser. This is not a strict rule. Human eyes are good at picking out low-contrast or faint patterns, so the smoke might let someone see faintly through it but still attenuate laser beams to near uselessness. But there are factors that also swing the other way, too. A lot of the reason that we have trouble seeing in fog is not just because light is being scattered out of the path between us and the thing we are looking at, making it dimmer; but also because other light is getting scattered into the light path from the sides, which reduces the signal to noise (or contrast, if you prefer). A laser beam will be affected by the first of these, but won’t care one whit about the second (although the second effect will make the whole fog bank light up like a colored flash bomb when the beam turns on). Taken together, this usually means that if the bad guy can’t see you (which, due to the reversibility of light paths means you can’t see the bad guy) he can’t shoot you. If he can barely see you, he can shoot you but with reduced effectiveness. And if he can see you clearly you’ve got no protection from his laser beam.
Very powerful beams might be able to burn through smoke or fog. This is more difficult when the beam is wide (near the shooter) than when it is focused and narrow (near the target) because it will be of lower intensity and will also have to burn more junk out of the way. The upshot is that if you can burn aerosols out of the air, it is harder to shoot out of your own smokescreen than it is to shoot into it.
On the other hand, smoke that absorbs light will make thermal blooming much worse. You might be able to get your beam to your target, but the beam may be so smeared out by thermal distortion that it doesn’t do much.
Of course, you don’t need a cloud of aerosols to block a light beam. A solid barrier can do so as well. A sheet of paper could, in principle, stop the beam. If the beam is still spread out and not focused, it might not have enough intensity to burn through. Even if it can burn through, it might spend enough energy doing so that the effect of the beam is reduced. If the beam is focused on the paper sheet, it will blast through no problem. But then the beam is focused on the paper - not you. When a beam focuses to a point, it then starts to spread out again once it gets past that point. In photography, this is called depth of field. When you focus on something, both the stuff in front of and behind the thing you are focusing on can be blurry. The same effect in lasers is depth of focus, where if a beam focuses on something and shoots through it, it will spread out and become less intense once it passes through its focus point. So if you are hiding in bushes, the laser that your enemy shoots at you might put holes in a few leaves and sticks but if you are far enough into the bushes the beam might not have much of an effect on you by the time it reaches you.
One complication of all this “not being seen” stuff is that it is assuming visible or near-visible light beams. A glass window might let you see through it and even shoot a visible light beam through it, but glass is good at blocking ultraviolet light. If you tried to shoot through the glass with an ultraviolet-B beam, the glass would act like an opaque barrier. Likewise, fog will affect infrared light differently from visible or ultraviolet light. To make things even more complicated, “infrared” is a very broad spectral range, and light at one color of infrared will go through fog quite differently than light at another color of infrared. So when you start getting to off-visible colors, a more useful comparison might be “if your enemy can see you with a sensor that detects the same color of light as the beam he is using, he can shoot you.”
So we can defend against laser beams by absorbing or scattering the beam out of the way. What about reflecting it? It seems like everybody’s favorite laser defense is covering themselves with mirrors. But does it work?
The answer is “it depends, but mostly no.” The problem is that mirrors are not perfect. And even if you could somehow make a perfect mirror in the clean room of your multi-million dollar laboratory facility, it will not long remain perfect once it is out in the environment collecting dust and corrosion and micro-cracks and who knows what. The mirrors used for your beam guides and beam pointer telescopes can be sealed away. Mirrors used for armor - not so much.
That can be okay for heat ray type lasers. If you have an outer surface that reflects away 80% of the incident light, that can be enough to keep you from getting a hole melted through your innards, or catching on fire, or something equally bad.
But once the intensity at the point of focus of the laser starts getting more intense than heat ray scales, bad things start happening to the mirror. The small amount of light the mirror absorbs starts to damage the mirror. And if the mirror gets damaged, it doesn’t work as well so it absorbs more light. So it gets even more degraded. And then you don’t really have a mirror any more. This will happen very quickly, once the mirror starts to degrade it is all over. Some very intense pulses can just bypass the reflectivity of a mirror altogether, with electric field strengths high enough to simply rip electrons right off the atoms so that mere linear optical properties like reflectivity mean nothing. The mirrors used to direct and focus the beam work with the beam dispersed over a larger area and are thus protected. If the beam is focused onto a tiny spot, it could damage even the same mirrors used to direct it.
But there is one way that mirrors could potentially help against lasers. Remember that the laser needs to be focused on its target. If the laser beam pointer can’t get the right range to the thing it's pointing at, the laser will focus someplace else. A mirrored surface can potentially confuse most methods of range finding. Send out a ranging laser pulse and see how long it takes to come back? Oops, the beam pointer measures the range to the image reflected in the mirror, not the mirror. Use passive methods of adjusting focus until the lines are sharp? Same story. So if you wear a really shiny suit, your enemy’s lasers just might focus on whatever they see reflected in your mirrored suit instead of you. And then the laser is not tightly focused when it hits you and your mirror works just fine to bounce the beam away to whatever it thinks it should be focusing on.
The time-honored way of protecting yourself from something bad is to put a bunch of stuff between you and it. Usually something that is strong and tough. With the hopes that the bad thing will not be able to get through the strong and tough stuff, give up, and go someplace else.
This works for lasers, too. Armoring is a perfectly reasonable way to keep a laser from blasting your innards into outards.
Against a laser of heat ray intensity, mirroring has already been mentioned. In addition, it will help to use materials with a high melting temperature, high thermal conductivity (to wick heat away from the laser-heated spot), high specific heat capacity (so it can absorb a lot of heat without raising its temperature too much), and preferably something that won’t thermally decompose or catch fire. Active cooling can also help - send a flow of water or other coolant through capillary channels to absorb the heat and transport it somewhere else.
When the laser gets intense enough to start melt ejection, you probably don’t have much time to engage in active cooling. But a high specific heat of fusion (the amount of energy it takes to turn something into a vapor) will help.
For lasers that are so intense that they are blasting out craters or channels in things through the sheer force of their vapor pressure, thermal properties no longer really matter. All the damage is mechanical, so you want the same things that will protect you against bullets and explosive blasts. A high strength-to-weight ratio is good for making it difficult for the pulses to blast very far into the armor. And a low brittleness will help you take multiple hits without your armor shattering.
As a laser gets closer to its target, its spot size gets more tightly focused. And a very small spot size makes the laser more effective at penetrating armor. More so than bullets, a laser’s ability to penetrate armor will fall off with range, and will also increase faster as range decreases. A laser gun shooting a train of blast-pulses with the same overall energy of a bullet will punch a significantly deeper hole through armor at close enough range where the focus can be kept tight.
The main limitation on how much you can increase your armor penetration just by getting closer so you have a smaller spot size and more concentrated beam is that lasers have trouble drilling out holes with too high of an aspect ratio. Plausibly, a laser could drill a 1:20 or even 1:30 aspect ratio hole in a target’s armor, but is likely to stall out around that depth. If you get even closer, you won’t drill deeper. So get to within the range where your laser is giving you its maximum achievable aspect ratio hole, and shoot from there.
Deflector shield force screens
Science fiction likes to imagine fantastical technology that projects invisible barriers that stop incoming attacks. Note the "invisible" part. These technological force screens are almost invariably portrayed as letting light go right through them. Logically, this means they won't do much to a laser beam; at least not a visible light laser beam. If the force screen is a completely reflective perfect mirrored surface or totally black sphere of inky onyx midnight or something, protecting from laser beams becomes a lot easier to justify. But then of course the people inside can't see out. So choose your poison.
Logistics and Energy Supply
You've supplied all your troops with laser guns. Your tanks have laser cannons, and your orbital spacecraft have giant laser mirrors. But how do you make sure your lasers have enough ammo to keep working?
This is one area where lasers are really expected to stand out from the crowd of other sci-fi weapons, because most of them run on electricity. You're going to be using electricity anyway, to run your computers and lights and communications gear and maybe even your hover tanks and VTOL jets and space-jeeps or whatever else you're using to get around. So you won't need an entirely different logistics leg to keep your lasers in ammo. In addition, electricity is cheap. Even if you have to generate it out in the field using Diesel fuel, it is still much cheaper than bullets. And if you can plug into the grid, it gets cheaper still.
There are sometimes ideas for laser ammo that doesn't just involve plugging their batteries in to recharge. Chemical lasers are one of the biggest culprits. They would require shipments of exotic, reactive, flammable, toxic, and corrosive chemicals with special handling needs. So needless to say, they were abandoned pretty quickly when electrical lasers came along.
Another idea occasionally floated is using an explosively pumped flux compression generator. These generators are a way to make a very high power pulse of electricity, presumably to energize a very high power laser pulse. The way they do this is by detonating a length of high explosive cord inside of an electromagnet. Needless to say, neither the explosive nor the electromagnet survive the process. So you end up with laser "bullets", where one cartridge supplies one shot. Probably accompanied by a very loud bang. Flux compression generators don't have a great specific energy, but they are one way to get very high specific powers. They'll also end up with expensive ammo and extra logistical headaches, so militaries will avoid them if they can get away with rechargeable energy storage.
So what is that electrical energy storage? A number of examples are covered here.
The cornea, lens, and vitreous humor of the eye are transparent to wavelengths between roughly 0.350 μm and 1.4 μm (the entire visible range, as well as near infrared and part of the ultraviolet-A band). Light that scatters off of your target can be focused by your very own eyes into an intense spot on your retina that can cause burns and permanent injury. Even the light that remains after multiple reflections can be hazardous. High power lasers in this wavelength range can cause instant total and permanent blindness. Anyone working with laser weapons in this range will need to take precautions to protect their eyesight. Not every eye injury will be permanent. Bright lasers can also dazzle, leave you seeing spots, or temporarily blind you. And some injuries may be permanent but only cause partial loss of vision. But this is a major concern anywhere that lasers are used.
Note that part of the wavelength range of eye-hazardous light is invisible. You might never see the beam that blinds you. But every visible light laser weapon represents a threat to your eyesight.
The incandescent plasma produced by an incident laser beam can also be bright enough to cause eye damage. This is less hazardous than the directly scattered light from the beam, however - similar to the danger involved with directly viewing an electric arc.
Eye damage from indirect exposure to the beam can be avoided by wearing protective glasses that filter out the wavelength of the laser being used. If you want to have a laser gun that can be used without worrying about blinding people without eye protection, you can use short-wave infrared or longer wavelengths (wavelength > 1.4 μm) or the higher registers of the UV-A band or shorter wavelengths (wavelength < 0.35 μm).
Some lasers emit beams of ionizing radiation. Ionizing radiation is radiation whose energy-per-photon is high enough to knock electrons off of atoms, creating ions. Free ions in tissue can attack cells, causing cell death, mutations, or cancer. This is the same stuff people worry about when exposed to radioactive elements or nuclear technology. The main wavelengths to worry about are hard x-rays and gamma rays. Light with these wavelengths can penetrate deeply through matter and deposit its energy inside the body. At high enough doses this can lead to increased cancer risk, sickness, or death.
Technically, extreme ultraviolet and soft x-rays are also ionizing radiation. But since they can’t get through air, clothes, or environment suits they are not much to worry about. Even if you do reach you, they will primarily cause radiation burns similar to sunburn on your skin, which is definitely unpleasant and will increase your risk of skin cancer, but not nearly as bad as having all your bone marrow killed so you die slowly of anemia and infection like the deeply penetrating forms of radiation.
But what if you don’t have a laser that emits ionizing radiation? Do you have to miss all the fun? Not necessarily. Absurdly high intensities of non-ionizing radiation can do crazy physics shenanigans that result in the production of some ionizing radiation. These kinds of laser intensities will probably be confined to laboratories, because you don’t actually need situations that extreme to blow a hole in somebody and that kind of intensity in air can actually impede the propagation of your beam in many cases. But who knows? Maybe future research will show that the best way to blast bug eyed aliens is to make uber-powered beams that kinda sorta make some ionizing radiation as a minor side effect. Nothing to worry about, citizen. Move along. Our experts have it all under control.
There are certain basic precautions that should be used with any ranged weapon, including laser weapons.
- Treat every laser weapon as if it were charged and ready to fire. Always keep the aperture pointed in a safe direction.
- Keep your finger off the trigger until you are ready to fire. Use your safety, but remember that safeties sometimes fail.
- Be sure of your target and what lies behind it before firing.
- Be certain nobody around you is at risk of being blinded or irradiated by the laser.
- Never use a laser weapon unless you are familiar with how it works.
- Be sure the aperture is clear of obstructions.
- Never point a laser weapon at anything you do not want to shoot.
- Remove the power source of a laser weapon when not in use. Store power supplies and laser weapons separately.
- Never use alcohol or drugs before or during shooting.
If you are designing a hand-held laser weapon for your science fiction setting, there are a few common-sense safety designs to include in your weapon. Trigger guards, for example. You don't want some stray branch or obstruction to catch on the trigger and shoot a hole in your foot. Safety switches are also a Really Good Idea, to help prevent negligent discharges.
What does it look like?
So you're making science fiction, and you want to know what a person at the scene would experience when lasers are around or in use. We'll go over that a bit here. In this discussion, we assume that the lasers under discussion are laser weapons, and intense enough to cause some damaging effect to the target.
The beam in vacuum
In vacuum, there is nothing to scatter the light out of the beam and into your eye. So unless the laser is pointed into your eye, you won't see anything. If the laser is pointed into your eye, then in a very short time you won't be seeing anything either.
This might seem disappointing to people who want flashy beams of light flickering between their spacecraft during cinematic space battles, but sadly that's the way it is.
The beam in air
If your laser weapon is operating at visible light wavelengths in an atmosphere, it will produce a visually obvious beam due to scatter. Even lasers too weak to be effective weapons (in the few watt range) produce quite visible beams in daylight for Earth-typical conditions. This also holds true for lasers going through other transparent media, like water.
An invisible light beam will, usually, be invisible. If there are a lot of particulates in the air like dust, smoke, or pollen, you might get a glow as the particulates heat up and combust or shine from their heat. Of course, if you are looking at the beam with optics that let you see light of that wavelength, it will be as if the beam was using visible light.
The same atmospheric heating that produces thermal blooming will also produce pressure waves that you can hear. Any pulsed beam will make a snap, pop, or crack sound from this heated air. A continuous beam that turns on suddenly will also make a sound like this.
Long wavelength beams that trigger cascade breakdown will make a glowing ball of plasma where the beam is being absorbed – but if this is happening you are doing it wrong and your laser will not be very effective.
If your beam is powerful enough to cause filamentation, it will appear as a white glow once filametation has been initiated.
A beam of ionizing radiation – x-rays or gamma rays – will produce a blue glow from the ionized air molecules (and Cherenkov radiation in water or other transparent condensed media). If the beam is being used for hole burning, the air plasma in the beam will be blazingly bright like a straight lightning stroke.
And remember – any light that you can see is energy that is not being deposited in your target. To make a beam that efficiently blows up your enemy you incidentally want to make your beam as invisible as possible even disregarding considerations like not giving away your position.
The point of incidence
The place where the beam strikes its target (or, if it misses, the point where the beam hits something) is called the point of incidence.
For heat ray type lasers, the point of incidence is likely to get heated to glowing incandescence. If it combusts, you will get flames and smoke. If the target is merely being cooked, you will still probably get steam and vapors and some smoke coming off. You will hear the crackling of the flame, the sizzling of the flesh, and smell the smoke or odor of cooking meat.
At higher powers, you will see a brilliant flare of plasma and incandescent vapors. Melt ejection will produce a spray of glowing sparks. You will get a roar for a continuous beam cutting through an object. A high powered pulse beam will produce a brilliant plasma flash and explosion; as with any explosion there will be a very loud bang.
Most obvious of all, however, will be visible light beams. A reasonable fraction of the light will be scattered away, this makes a dazzling flare the color of the beam. The intense glare from the point of incidence will make it hard to look at anything in that direction.
The laser itself
The actual device that produces the beam will require a large aperture for its focal array (unless you are trying to go for the atmospheric hole burning route). For hand-held models, this will result in something that looks like a cross between a firearm and a camera. There will not be any barrel. Instead, you'll have a big lens. Because it will probably use single lens reflex sights, you won't have a separate scope (although you might include the notches of iron sights for when you don't want to use the in-built scope for some reason). But the ergonomics of a pistol grip or rifle-style stock mean that you will still get these design elements on your weapon (for weapons designed for human use, anyway. Aliens might want other ways to grab on to their lasers).
|Various possible designs of portable laser weapons.|
Larger lasers are likely to have a beam generator that's in a box, probably below decks or deep under the armor of your AFV or buried underground or something. It will send its laser light up through beam pipes using mirror arrays or fiber optics to a beam pointer. The beam pointer will be a turreted telescope with the afore-mentioned large aperture.
|This spider tank robot shows a laser beam pointer, as the rotating spotlight-like turret on top of the main turret. The actual laser generator is in the middle of the robot's body.|
People have invented many different kinds of lasers. This page summarizes some of the more important ones from a weapons perspective.
Vehicular heat ray
This is an example of a potential near term laser weapon. It uses a fiber laser to put out 100 kW of 1 μm near infrared light through a 30 cm aperture beam pointer telescope-turret. The entire device has a mass of 400 kg, and is mounted in an all-terrain hybrid Diesel-electric truck. It gets its power from the same generator and battery pack that powers the truck.
The truck and heat ray are meant to provide zone defense for the army units they are deployed with. The laser is used for shooting down incoming missiles, mortar shells, artillery shells, drones, and thin-skinned aircraft, although it can certainly be used to fry personnel, light ground vehicles and sensitive points on light armored vehicles as well - or anything the gunner can see, really. The truck has a really big radar array for detecting airborne threats. Let’s consider this weapon being used to burn down some incoming mortar shells. The shells are flying toward us at 200 m/s from 1 km away. The shell’s transverse speed is 50 m/s at this moment. The windspeed is 3 m/s, but with the weather, dust kicked up by the vehicles, and battlefield smoke the attenuation and absorption length in the air is about 3 km.
Looking just at the diffraction limit, we might naively expect the laser can focus all 100 kW of its beam down to a mere 4 mm in diameter, delivering intense photonic drilling death in mere moments. But it has all this air to go through. The range is still far less than the attenuation length, so that won’t cause us to lose too much power from our beam. We’re at far too low of a frequency for two photon absorption to happen, our adaptive optics are correcting the twinkle, cascade breakdown and stimulated scattering are not a worry, and our jitter is less than our diffraction limit. But thermal blooming is going to be an issue. This early model heat ray doesn’t have fancy algorithms for predicting and correcting the non-linear effects of thermal blooming. In order to keep runaway blooming from happening, the spot size at the target can only be focused down to 15 cm at best. At this intensity, the beam is burning through the steel casing at about a quarter millimeter per second. It takes four to five seconds for the beam to burn through the case and deflagrate the explosives inside. Because it only takes five seconds for the mortar shells to reach their target, this seems to be a problem. Fortunately, as the shells get closer the beam can be focused tighter. At 800 m, the beam spot focuses down to 10 cm, and now it can burn down a shell in just over a second. With the damage done from the first second of burning, the first of the mortar shells is incinerated after two second. Now at 600 m, the beam switches to the second shell with a 6 cm wide beam. The blistering radiant heat flashes through the second, and third shell in about half a second each. At 400 m, the beam spot is down to a mere 3 cm. The laser is blasting down the shells nearly as soon as the beam touches them. It is limited more by its rate of slew between shells than its dwell time on each shell.
Man-portable heat ray pack
Now let’s step a few decades into the future. Laser technology has advanced enough to allow a 10 kW laser to be fit into a 10 kg backpack with a fiber optic cable to a 4 kg beam pointer “rifle” with a 20 cm aperture. The pack is fueled by another 10 kg of high-capacity, high discharge rate lithium-sulfur batteries. With 50% electricity-to-laser-light efficiency, the batteries supply enough juice for 80 seconds of continuous lasing. This laser pack is designed to emit light at 1.5 μm, to reduce the blinding hazard to bystanders.
The heat ray pack is a weapon of the Space Marines, for use in vacuum or when boarding enemy vessels. The low recoil is considered to outweigh the limited range in atmosphere due to thermal blooming - you can keep a 1 mm spot size focus at 70 m in air even with poor air quality, and very few spacecraft corridors are anywhere near that long. In vacuum or clearer air, the heat ray can maintain its 1 mm focus up to 110 m away.
Within that range, this weapon is murderous. In 1/10th of a second, it can cut an unarmored person in half with a slash across their torso, blood and viscera splattering from the wound as explosive vaporization of tissue blasts out a 25 cm deep, 1.3 cm wide swath of destruction. Against hardened steel, sparks of molten metal fly out in an incandescent jet as the narrow beam cuts a hole up to 2 cm deep at up to 2 cm/s. Thinner steel can be cut proportionally faster. SpectraTM woven armor, effective against bullets, withers and burns under the beam as it vaporizes a 2 cm deep hole at up to 13 cm/s. Anywhere the beam penetrates leaves grievous wounds. Only hard armor provides much protection - it takes 1/10th of a second to burn a 2 cm deep hole into a rifle plate, and the beam can only cut to 2 cm depth at a rate of 1 cm/s - again, thinner plates can be cut proportionally faster. It is hard to keep a beam focused on a maneuvering target to within 1 cm for a second, so heat pack users usually aim for less armored parts.
While the Space Marines are storming contested space habitat units, what is the Space Force doing when facing enemy spacecraft? They use a deep ultraviolet free electron laser. This beast of a weapon emits a time average of 100 MW of 50 nm wavelength light. Because FELs naturally operate in pulse mode, this beam is made up of a rapid-fire train of 10 J, 100 fs pulses emitted at a rate of 10 MHz. With efficient superconducting linacs, energy recovery of the linac pulses, and good resonators and amplifiers, the laser gets 66% efficiency - always useful when heat rejection is such a problem in the vacuum of space. The linacs and laser light generators and other associated equipment have a total mass of 40 metric tons. The 2.5 meter diameter beam pointer telescope and turret mount have a mass of another 10 tons, although the Vanguard-class attack beamstar spacecraft generally mount four beam pointers to give full coverage of all firing arcs with overlapping fields of fire.
As a bonus, FELs are frequency-agile. This laser can tune the wavelength of its beam down into the near ultraviolet, visible or near infrared for through-atmosphere bombardment.
As an example of this laser in action, we’ll look at the Excelsior, a Vanguard-class beamstar in medium orbit around the colony world of Clementine. She’s patrolling with the Wyvern and Desperado, both Excalibur-class missile slingers. The task force has been tracking several sets of inbound craft, presumably alien hostiles first noted by their drive plumes weeks ago.
The aliens are coming in on a hyperbolic trajectory, plotted to pass within a few tens of kilometers of the planet’s surface. Anticipating an Oberth burn at periapsis, the two Excalibur-class vessels spend precious propellant to adjust their orbits to planet-skimming retrograde to the incoming craft. The Excelsior puts herself in a higher overwatch position. As the hostiles approach to within the orbits of Clementine’s two small moons, Wyvern and Desperado loose their first flight of missiles, boosted by the Oberth effect for extra closing speed. At around the same time, multiple high thrust burns are detected coming from the invaders as they release their own swarms of kinetics.
Coming up over the limb of the planet, the enemy missiles come into view of the Excelsior, at a range of 25,000 km or almost two planetary radii. She opens up, sending searing beams of ultraviolet at the incoming swarms. At this distance, the beam has expanded from diffraction to a 60 cm spot. The pulses flash a thin layer of the armored graphite nosecones to plasma, ten million times per second. Gradually, the alien missiles exposed to this hellish radiation erode away, their fronts being ground off at a rate of a few millimeters per second. But seconds are what the Excelsior has - at a closing speed of 20 km/s, it will take ⅔ of an hour for the missiles to cross the gap. Initially, it takes over a minute and a half for the thickly armored nosecones to be ablated away. One by one the missiles lose their armor, and take a flood of hard UV radiation through their innards. The Excelsior’s own beam pointer scopes act as telescopes to assess the damage, watching for the change in spectral lines to indicate a change from carbon plasma to iron and silicon and aluminum and oxygen to tell her she got through the outer shell of the missile to the chewy center. As the missiles get closer, the beam gets more intense. At 12,000 km, the missiles are falling at a rate of one every half minute and at 6,000 km the last missile is burned through in less than eight seconds.
But by this time the enemy spacecraft are behind the limb of the planet. They had waged their own battle with the missiles from the two missile craft, using some kind of radiation beam to fry the missiles’ electronics. Now it was time to wait. The Excelsior pivoted to bring her beam pointers to bear on where she expected the craft to re-appear. The crew wait, fidgeting, watching the telemetry from the Wyvern and Desperado. They release the last of their missiles to skim around the top of the planet, then burn hard for a higher orbit. And sure enough, the missiles meet the enemy coming from the opposite direction. With little time to engage with beams, the enemy runs straight into the incoming munitions. Flashes of plasma and the sparkle of tumbling debris light up the planet’s limb. Only three of the targets made it through the barrage. They turn their radiation beams on the two missile carriers. At this range, the sleeting radiation burns holes in the armor while delivering a lethal dose to the crew.
But as the enemy climb into view, the Excelsior is waiting. At a range of only 300 km, her beams focus to less than a centimeter across. With a beam pointer dedicated to each enemy vessel, beams flicker from one to the next nearly instantly. Her targets flare brightly, the beams first coring their particle emitters, then piercing the reactor coolant lines, and finally cutting gashes in the hull at over 12 m/s. The alien spacecraft come apart, their pieces tumbling away into the void. Over the next several weeks, the people on the colony world below occasionally see meteor trails of infalling debris, a reminder of the heroic Space Force men and women who risked and gave their lives to defend them.
Infantry pulse laser
As the Galactic Federation expands ever further into the galaxy, their laser tech increases. Instead of cumbersome heat packs, the glorious Space Marines are now equipped with powerful new pulse lasers. A pulse laser emits a 2.5 kJ pulsed beam with a 1 millisecond duration, for a peak power of 2.5 MW. The beam is tuneable from 1.5 μm short-wave infrared across the entire visible spectrum up to 0.3 μm UV-B. For eye safety, 1.5 μm is the default setting unless longer range is needed.
This laser has a 7 cm aperture. At up to 50 m, it can put its pulse into a 1.25 mm diffraction-limited spot - any tighter and the laser just wastes its energy trying to drill a hole with too high of an aspect ratio. This range can be extended by decreasing the wavelength, in the usual fashion for diffraction, at up to 180 m for a violet beam. Without compensation, the beam would be moderately impacted by thermal blooming in dirty air, but improvements in non-linear control systems implemented in the laser’s adaptive optics allow it to overcome this level of beam disruption. At longer ranges, the beam loses focus, but can still cause lethal damage to an unarmored target at up to 400 m in the short-wave infrared, or 1.5 km in violet. Two photon absorption pretty much kills any ultraviolet beam in air, but in space ultraviolet frequencies can extend the range of maximum effect to 250 m and potentially lethal range to 2 km.
At close focus, the beam can instantly punch a 2.5 cm deep hole through the hardest armor steel. It can flash-drill through 1.2 cm of the nano-diamondoid armor the Space Marine’s foes can be expected to wear. And it can literally explode out a cavity that can punch through up to three people in a row (if they’re all close enough to be within the depth of focus of about 6 m for short-wave infrared or 25 m for violet). At the maximum effective kill range, it will blast out an even wider and messier hole with enough penetration to reach vital organs.
A pulse laser is equipped with a 1 kg ring shaped superconductive magnetic energy storage (SMES) magazine, roughly doughnut shaped if you like doughnuts large enough that you can fit your hand through the hole in the middle. This SMES stores up to 20 MJ of circulating electrical energy, enough for 4000 shots at the 50% efficiency of the laser generator. Space Marines usually don’t need to carry extra magazines. The entire laser gun, magazine included, has a mass of 5 kg.
The venerable pulse laser has seen the Federation Marines through numerous wars. It saw the downfall of the K’mrugh Alliance, the Ghrell Hegemony, and other foes of the Feds. But now a new weapon has come along to replace it.
The Mk. 28 Infantry Micropulse Laser, or blaster as the gunts affectionately call it, emits a train of 100 pulses within a millisecond duration. Spaced 10 μs apart, each pulse has an energy of 35 J and lasts for only 10 ns. At close focus, it has a spot size of 1.5 mm. The pulses, falling one on top of the other, can blast out a sequence of stacked craters up to 7.5 cm deep in armored steel, 3 cm deep into the toughest and strongest nano-carbon armor, and blows right through any reasonable number of people that can fit in its depth of focus. With a similar 7 cm focal aperture to the pulse laser, a blaster has only about a 20% longer range for its close focus. But its effective lethal range extends out to 4 km for short-wave infrared and 15 km for violet light and 20 km for ultraviolet.
In order to keep the beams from dispersing due to stimulated Raman scattering, each shot contains a number of weak “seed” beams tuned specifically to stimulate Raman scattering themselves. But these beams stimulate the scattering in a direction that is still focused, preemptively removing available photons that would otherwise be scattered at random and allowing the beam to keep tight focus much further. The number of additional parallel seed wavelengths ranges from just one for short-wave infrared up to 25 for violet light.
Each shot has a combined energy for the full pulse train of 3.5 kJ. But despite the increased energy, improved efficiency and better SMES energy rings allow the blaster to get up to 6000 shots from a single fully energized magazine. An infantry blaster has a mass of 4.5 kg fully loaded.
- Philip E. Nielsen, “Effects of Directed Energy Weapons”, (2012)
- Bob Preston, Dana Johnson, Sean J.A. Edwards, Michael Miller, Calvin Shipbaugh, “Space Weapons Earth Wars”, RAND 2002.
- N. Bloembergen et al., “Report to The American Phsyical Society of the study group on science and technology of directed energy weapons”, Reviews of Modern Physics, Vol. 59, No. 3, Part ll, July 1987
Author: Luke Campbell
Helpful comments by Sevoris Doe and Gerrit Bruhaug
- Halliday, Resnick, and Walker, “Fundamentals of Physics”, John Wiley & Sons (2005)
- Proceedings of the U. S. Army Natick Laboratories Flash Blindness Symposium, 8-9 November 1967, John M. Davies and David T. Randolph, Ed.