Laser Weapons

Lasers at the Starfire Optical Range.

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.

Introduction

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.

Focus

Boeing YAL-1 Airborne laser beam pointer turret.

Tactical High Energy Laser (THEL) beam pointer.

The laser beam pointer of the AN/SEQ-3 LaWS (Laser Weapons System) aboard the USS Ponce.

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

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.

Beam-Atmosphere Interactions

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:
  • Absorption.
  • Scatter.
• 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 ith the result that energy is removed from your beam. It is mostly important for high power pulses and longer wavelengths.
• 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.

Accuracy

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.

Deflection

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.

Parallax

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.

Jitter

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.

Recoil

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.

Credit

Author: Luke Campbell