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Laser Weapons

Green Laser
Image from Luke Campbell

Categories of Laser Weapon

Heat Rays

These are lasers that cause damage primarily through thermal mechanisms. The energy of the beam directly heats the target, causing charring, melting, ignition, and vaporization. All lasers that emit continuous beams fall into this category, while short pulsed beams can still have a heat ray effect when their intensity falls off enough they can no longer cause mechanical effects at the target. Early lasers were almost always heat rays.

Blasters

It is far more efficient to damage matter through mechanical means than thermal mechanisms. Lasers can cause primarily mechanical damage by emitting ultra-short pulses of extreme intensity, causing matter to flash suddenly into plasma. The resulting explosion creates a shockwave which can gouge craters and drive cracks into the target. In this way, a blaster can cause damage comparable to a heat ray with one tenth to one hundredth the amount of energy.

In order to achieve deep penetration, blasters typically emit a very rapid train of pulses. Each pulse falls into the crater created by the previous pulse, allowing the beam to drill a deep hole.

Visible and Near Visible Wavelengths

The best performance in an atmosphere is found with beams in the near infrared and visible wavelengths. These colors allow beams to be focused at sufficiently long ranges to engage distant enemies, yet avoid excessive losses due to scattering, absorption, multi-photon ionization, and catastrophic beam absorption due to cascade plasma formation. As the blue end of the visible range is approached, scattering losses can become significant for distant targets. In the near ultraviolet scattering can become excessive and multi-photon ionization often makes pulsed beams impractical. Any farther into the ultraviolet and any gases become opaque, immediately absorbing the laser beam. Many mid infrared wavelengths are absorbed by the atmosphere; even for those wavelengths where the air is transparent the difficulty of focusing these long wavelengths means these frequencies are typically neglected in favor of shorter wave light. These problems are exacerbated for far infrared, which also suffers when too-intense beams cause cascade ionization which absorbs the rest of the beam.

The only wavelengths of laser light suitable for underwater weapons are blue and green. Light of these colors can penetrate several tens of meters before being absorbed. Other wavelengths of visible light are absorbed within a few meters, while infrared light is absorbed almost immediately.

Visible and near visible light lasers are easily directed by mirrors and lenses. As a consequence, the laser itself is often buried under thick armor or in the center of a vehicle, and the beam directed using a series of mirrors and a turret that can rapidly swivel to engage targets. Portable lasers used for infantry weapons, self defense, or sport hunting are compact, lacking a barrel but sporting a large aperture lens for focusing the beam.

X-ray Wavelengths

X-rays are ideal for space combat, due to their extremely short wavelengths that allow them to be focused at long distances. However, air is opaque to x-rays, so lasers of these wavelengths are not used in atmospheres or for bombardment of planets with atmospheres. X-rays are tricky to handle, since they cannot be directly reflected and are absorbed by any lens material. Soft x-rays can be focused using complex grazing incidence reflection mirrors, but even these fail for hard x-rays. Many x-ray lasers rely on a process known as self amplified spontaneous emission, which requires very long lasing chambers and results in unstable beams with very small diameters. Alternately a seed-beam from a shaped crystalline diffractive cavity generator can be ramped up by an x-ray laser amplifier to destructive levels, allowing powerful beams to be emitted which often are used without secondary focusing. The high powered death beam need never touch matter until it reaches its target, thus eliminating the difficult issues of dealing with extreme power levels impinging on delicate focusing optics.

Operation

Cooling

Early laser weapons are often very inefficient, converting as little as 10% of the input energy into the output light beam. The remainder of the energy creates heat, which will damage the laser if not disposed of. As technology increases, so does laser efficiency, until advanced laser weapons can can approach, or even exceed, 90% efficiency. This greatly reduces the heat load on the weapon to the point where often only passive cooling is needed.

Where heat management is a problem, active cooling systems are needed. In an atmosphere or hydrosphere, this can be as simple as using a fan to blow cool air or water over the hot parts of the laser or as complicated as pumping a closed cycle working fluid past the heat sensitive components which then is shunted through a series of cooling pipes or fins that allow the heat to escape to the ambient surroundings through thermal diffusion. In the vacuum of space, a closed cycle systems must pass through large radiator fins or droplet radiators, increasing the size of the weapon system greatly. These radiators are vulnerable targets and can be damaged in combat, often rendering the laser inoperable. Open cycle cooling in space can be accomplished by venting the hot coolant directly into vacuum. This obviously can only continue until the supply of coolant is exhausted. In cases where the lasing medium is itself expendable (as in primitive chemical lasers), the spent hot lasing fluid can be directly vented to reduce the heat load.

Focus and Diffraction

A laser beam must be focused by a suitable lens or shaped mirror, in order to concentrate the beam onto a small enough spot to cause damage. In order to focus the beam at the right spot, the range to the target must be known. Laser weapons incorporate an automatic range finder for this purpose.

A laser's range is limited by its ability to focus at long distances. Even a perfectly focused beam spreads, because of diffraction. This spread depends on the ratio of the initial width of the beam to the wavelength of the beam. The smallest possible spot size to which a beam can be focused can be calculated; if the initial beam width is D, the wavelength of the light is L, and the distance to the target is R, the smallest spot size (S) is given by

S = 1.2 R L / D.

This means that a green light laser (with a wavelength of 0.5 microns) emitted through a lens ten metres in diameter can be focused into a spot 6 millimetres in diameter 100 kilometers away. As a consequence, long range laser weapons will have large apertures for focusing, use short wavelengths, or both.

Scattering and Visibility

Visible light lasers will be visible their light is scattered out of the beam. Even pure gases will scatter light through an effect known as Rayleigh scattering. Rayleigh scattering is more effective for shorter wavelength light, so blue light will scatter the most, followed by green, yellow, and orange, with red light scattering the least. Any particulates suspended in the air will greatly increase the amount of light scattered; light scattered from particulates does not depend on wavelength. In an Earth-like atmosphere, the beam from an antipersonnel laser will be obvious even in clear air at night, indoors, and in cloudy weather, although it could take green and blue light to be clearly visible in full sunlight. Anti-armor and anti-vehicular lasers emitting visible beams will always be obvious in broad daylight. Any amount of dust, smog, mist, pollen, lint, smoke, or other suspended particles will make a visible light beam shine like a beacon.

Short, high power laser pulses of any wavelength to which air is transparent may undergo self focusing in an atmosphere to produce glowing filaments of plasma. These are clearly visible as white-hot streaks delineating the beam. Because creating the plasma channel saps energy from the laser beam, weapons beams are generally designed to only initiate self focusing just in front of the target, if at all.

Any laser will be visible where it is incident on its target. Those operating at invisible wavelengths will produce a flare of plasma, sparks, and flying debris. Visible light lasers will produce all this, plus a dazzling flare the same color as the beam at the point of incidence.

Fresnel lenses and Zone plates

One useful arrangement is to separate the final focusing element and the laser emitter, so that a large lens or mirror is placed many kilometres from the emitter at a point where the beam has spread out by diffraction to be very large. In this way a wide beam can be focused many light seconds away. A flat Fresnel lens is commonly used for this application. The greater the distance between the emitter and the lens, the larger the lens needs to be; but the larger the lens, the greater the effective range of the beam.


Zone Plate
Image from Steve Bowers
Note: illustration not to scale. The gunship may be more than 300 metres long, the zone plate may be tens of thousands of kilometers away, and the target light minutes away.

X-Ray laser beams can be focused by diffraction through a solid, dense zone plate. Unfortunately the diffraction pattern on the zone plate absorbs or otherwise dissipates 50% or more of the energy of the beam, but what is left can be focused onto a very small spot on a very distant target (due to the very small wavelength of the beam and the large size of the plate).

Defensive measures

A target can protect itself against laser fire by using armor. Bulk carbon, such as diamond, diamondoid, graphite, fullerite, or woven nanotubes, offers both excellent mechanical protection coupled with extraordinary resistance to melting and vaporization, and is thus commonly used for protection against lasers (not to mention projectiles and any other thermal or mechanical threat). Thick, rotating outer shells of armor are often seen where lasers are the primary threat. Unless the beam is sufficiently intense to drill a hole through all the armor on a time scale much shorter than the rotation period, the differing rotation of the concentric shells prevents a single hole from penetrating all the shells and reaching vital components.

Reflective materials offer little protection at close ranges. For pulsed beams, the intensity gets so high that reflectivity doesn't matter much - the beam just rips electrons off any surface it encounters. Even at lower intensities, the beam can rapidly heat a reflective surface, pitting it or scorching it (and thus reducing its reflectivity) or flashing it to plasma (at which point the beam would heat the plasma, and the plasma would heat the surface, bypassing the reflectivity). At the limit of laser engagement range in space, however, the beam has spread out enough that reflective materials may be useful.

However, the best defense is not to be hit at all. Since an x-ray laser can be a very long-distance weapon, considerations of light speed delay are important- if an object is several light seconds or even light minutes away, it can thrust away from its apparent position and any x-ray beam directed towards it would miss. This is not without cost, however, as it forces the dodging craft to expend valuable propellant. If the laser armed spacecraft can force his adversary to use up enough of his propellant, that adversary may no longer be able to complete his mission, resulting in victory for the laser craft.

Types of lasers

Gas lasers, Chemical Lasers and Solid state Lasers

These types of laser are generally of historical interest only. Gas lasers were generally used in medical or industrial contexts, but were
little use in warfare.

Chemical lasers are powered by chemical reactions in a fluid, some lasers of this type have been used as weapons. The Chemical Oxygen Iodine laser, (COIL), for instance was used as an antimissile weapon, and could be mounted in vehicles. However, their drawback of requiring bulky, expensive chemicals that are consumed upon use meant they were dumped in favor of solid state lasers as soon as technically feasible.

Solid state lasers use a glass or crystal material with an added metal ion dopant as the lasing medium. In fact the most ancient form of lasers, in the Early Information age, used ruby crystals (according to surviving historical records). Some lo-tech and lower middletech societies still use these kinds of lasers, occasionally using them as weapons, but lasers of this kind have been almost completely superseded by diode/quantum dot/nanoemitter lasers and Free Electron Lasers, and by optical phased arrays.

Free Electron Lasers

Free electron lasers can be tuned to produce laser beams of a very wide range of frequencies, up to and including x-rays. They have no lasing medium per se, instead a beam of ultra-relativistic electrons is shot through an alternating magnetic field (called a wiggler) to produce an intense beam of coherent light. Most commonly, an Energy Recovery Linear accelerator is used to accelerate femtosecond electron pulses; after passing through the wiggler the electron pulses are then recirculated 180 degrees out of phase with the accelerating field allowing their energy to be recovered and used to accelerate the next batch. However these linear accelerators are very large, so need to be housed inside large structures or warships. A typical free electron x-ray laser gunship might be ten kilometers long, and used in conjunction with a large and distant metal zone plate to attack targets many astronomical units away. Longer wavelength light requires less energetic electrons to create, and thus visible and near visible free electron lasers are significantly more compact even though they are still tens of meters long and must be mounted in fixed installations or very large vehicles such as sea going vessels, jet transport aircraft, dirigibles, or orbital bombardment spacecraft.

Diode Lasers

These can be very small, and use a diode to stimulate light emission from a semiconducting substrate One common use of semiconductor lasers is for smaller weapons for use in an atmosphere or underwater. These use manufactured optical resonators to generate a beam -as distinct from "natural" resonators such as the fluorescing atoms used in modern solid state lasers phase-locked semiconductor (or diode) lasers.. By phase locking the beam by means of a phase-conjugate "mirror", very high quality beams can be generated. More advanced systems use quantum wells or quantum dots, or more advanced nanoscale smart matter emitters to replace the semiconductor junction lasing medium. Lasers of this type often operate in the near IR or visible part of the spectrum and have high efficiencies — the most advanced forms can be more than 90% efficient at converting energy input into light.

More on handheld lasers here.

Optical Phased Arrays

The most flexible form of laser weapon is that produced by optical phased arrays. An Optical Phased Array consists of a layer of light emitting material, sensors and associated processors. Such a configuration allows a wide range of optical effects to be broadcast, including lightshows and moving images, and three-dimensional projections of real or imaginary objects and scenes. A phased array system can allow an object to apparently disappear by projecting an image of the scene behind it (in all directions).

This technology also allows an entire object to emit light as if it were a single aperture laser. Any object- a vehicle, a satellite, a building or a spacesuit- could be used to concentrate emitted light onto a target. Such an array can even be used as an interplanetary, or even interstellar weapon. Entire warships, habitats or even planet-sized objects are used as phased array lasers. Using a visible wavelength phased array laser emitted by a Dyson sphere, the entire energy output of a star can be focused onto a planet in a nearby solar system, boiling it entirely away into space in a matter of days. Such systems are known as Nicoll-Dyson beams; such beams have rarely been used in warfare, although a number were used recently during the Oracle War.

However it is a very difficult task to accurately target objects separated by distances of light years; because of the chaotic nature orbital mechanics involving more than two bodies, the precise location of a planet or any other object is somewhat unpredictable on a timescale of more than several years, the time it takes for the beam to reach its target across interstellar space.

More on Optical Phased Arrays here.

Nuclear-pumped x-ray laser

These inefficient weapons are sometimes used by medium-tech societies which are culturally isolated and do not have access to more advanced designs. A fission weapon is detonated inside a cluster of metal rods, which have been aimed at a target; the rods become a lasing medium for a brief pulse of x-rays shortly before they are destroyed. An antimatter-pumped version is used by the MASS weapons system, versions of which are still used in modern times.

Stellar Lasers

Certain natural stellar phenomena result in coherent emissions; these are often quite transient and associated with various energetic objects such as highly luminous Wolf-Rayet stars or rapidly evolving protostars. Using stellar engineering techniques, it is possible to induce such high energy events even in quiescent main sequence stars, and powerful coherent beams can be produced for offensive or defensive purposes.

 
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Development Notes
Text by Luke Campbell; additions by Steve Bowers

Initially published on 15 November 2007.

 
 
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