Lasers

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Laser application

When the laser was invented in 1960, it was amazingly, a solution looking for a problem. While the laser's weapons potential was clear, most of the uses of lasers that have changed the World were not foreseen even by the so-called experts of the time. We consider those areas where lasers are now indispensable, or at least have the potential to be in the future.

Scientific

Spectroscopy

Most types of laser are an inherently pure source of light; they emit near-monochromatic light with a very well defined range of wavelengths. By careful design of the laser components, it is possible to improve the purity of the laser light (measured as the linewidth) beyond that of any other light source. This makes the laser a very useful source for spectroscopy. The high intensity of light that can be achieved in a small, well collimated beam can also be used to induce a nonlinear optical effect in a sample, which makes techniques such as Raman spectroscopy possible. Other spectroscopic techniques based on lasers can be used to make extremely sensitive detectors of various molecules, able to measure molecular concentrations in the parts-per-trillion (ppt) level. Due to the high power densities achievable by lasers, beam induced atomic emission is possible, this technique is termed Laser Induced Breakdown Spectroscopy (LIBS).

Lunar laser ranging

When the Apollo astronauts visited the moon, they plantedretroreflector arrays to make possible the Lunar Laser Ranging Experiment. Laser beams are focused through large telescopes on Earth aimed toward the arrays, and the time taken for the beam to be reflected back to Earth measured to determine the distance between the Earth and Moon with high precision.

Photochemistry

Some laser systems, through the process of modelocking, can produce extremely brief pulses of light - as short as picoseconds or femtoseconds (10^-12 - 10^-15 seconds). Such pulses can be used to initiate and analyse chemical reactions, a technique known as photochemistry. The short pulses can be used to probe the process of the reaction at a very high temporal resolution, allowing the detection of short-lived intermediate molecules. This method is particularly useful in biochemistry, where it is used to analyse details of protein folding and function.

Laser cooling

A technique that has had recent success is laser cooling. This involves ion trapping, a method where a number of ions are confined in a specially shaped arrangement of electric and magnetic fields. By shining particular wavelengths of laser light at the ions, it is possible to transfer momentum from the ions to the light photons, causing the ions to lose energy and to slow down, thus cooling the ions. If this process is continued, eventually all the ions in the trap are slowed and have the same energy level, forming an unusual arrangement of matter known as a Bose-Einstein condensate.

Nuclear fusion

The most extravagant use of lasers in science is in the field of fusion research. Some of the world's most powerful and complex arrangements of multiple lasers and optical amplifiers are used to produce extremely high intensity pulses of light of extremely short duration. These pulses are arranged such that they impact pellets of tritium-deuterium simultaneously from all directions, hoping that the squeezing effect of the impacts will induce atomic fusion in the pellets. This technique, known as inertial confinement fusion so-far has not been able to achieve breakeven, that is, less power is generated by the fusion reaction than is used to power the lasers, but research continues.

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Military

Death ray

The first role envisioned for the laser in military applications was as a "death ray": a hand-held device that might replace the gun as a weapon for infantry, or a vehicle-mounted "laser cannon" able to destroy tanks, ships and aircraft. However, practical considerations have severely constrained these ideas; any laser capable of seriously wounding a human would (with its requisite power supply) be inevitably too heavy for a single soldier to lift, and a high-power laser capable of burning through tank armour would be extremely complex and very sensitive to misalignment from any knocks or vibration it might suffer, making it unsuitable for field deployment.

There remains the possibility of using lasers to blind, since this requires much lower power levels, and is easily achievable in a man portable unit. However, most nations regard the deliberate blinding of the enemy as forbidden by the rules of war. Russia, China, and Jordan possess such weapons, which were banned in most western countries in 1980.

Defensive applications

Recently, some progress has been made in the use of the laser as a directed energy weapon, mostly in defensive applications. By using a chemical laser, one in which the laser operation is powered by an energetic chemical reaction, the requirement for generating and storing a large amount of electical energy (which directly or indirectly is used to power most high-power lasers) is removed. This makes the laser system much more compact, and easier to transport. One example is a laser system which is designed to destroy missiles in flight. It is mounted in a converted commercial airliner, and could be used, for example, to protect assets such as AWACS aircraft or to destroy ballistic missiles. However, the practical problems of reliably generating and aiming the laser beam remain formidable.

The Mobile Tactical High-Energy Laser (MTHEL) is another defensive laser system under development; this is envisioned as a field-deployable weapon system able to track incoming artillery projectiles and cruise missiles by radar and destroy them with a powerful deuterium fluoride chemical laser.

Strategic Defense Initiative

Another example of direct use of a laser as a defensive weapon was researched for the Strategic Defense Initiative (SDI, nicknamed "Star Wars"), and its successor programs. This project would use ground-based or space-based laser systems to destroy incoming intercontinental ballistic missiles (ICBMs). Again, the practical problems of using and aiming these systems would be many; particularly the problem of destroying ICBMs at the most opportune moment, the boost phase just after launch. This would involve directing a laser through a large distance in the atmosphere, which, due to optical scattering and refraction, would bend and distort the laser beam, complicating the aiming of the laser and reducing its efficiency.

Another idea to come from the SDI project was the nuclear-pumped X-ray laser. This was essentially an orbiting atomic bomb, surrounded by laser media in the form of glass rods; when the bomb exploded, the rods would be bombarded with highly-energetic gamma-ray photons, causing spontaneous and stimulated emission of X-ray photons in the atoms making up the rods. This would lead to optical amplification of the X-ray photons, producing an X-ray laser beam which would be minimally affected by atmospheric distortion and capable of destroying ICBMs in flight. The X-ray laser would be a strictly one-shot device, destroying itself on activation. Some inital tests of this concept were performed with underground nuclear testing, however, the results were not encouraging. Reseach into this approach to missile defense was discontinued after the cancellation of the SDI program.

In recent years, the United States Air Force has begun experimenting with using lasers combined with high-altitude airships as a potential means for a missile defense shield but also as a means to destroy enemy spacecraft or satellites in low-earth orbit. For more information, see Evolutionary Air and Space Global Laser Engagement.

Laser sight

Instead, the laser has in most military applications been used as a tool to enhance the targeting of other weapon systems. For example, a laser sight is a small, usually visible-light laser placed on a handgun or rifle aligned to emit a beam parallel to the barrel. Since a laser beam typically has low divergence, the laser light appears as a small spot even at long distances; the user simply places the spot on the desired target and the barrel of the gun is aligned. Recent studies (2001) have shown that laser sight has become an effective deterring tool for law enforcement. Criminals are more likely to surrender without resistance when they find a red laser dot on their chest. Modern laser sights are so small that they can fit below the barrel as part of gun instead of a separate attachment.

Illuminator

Saber 203 Laser Illuminator (U.S. Air Force)

This menacing but allegedly "non-lethal" laser weapon, shown in the accompanying photo attached to an M-16 rifle, was developed by the U.S. Air Force to temporarily impair an adversary’s ability to fire a weapon or to otherwise threaten friendly forces. The Saber 203 briefly illuminates an opponent with harmless, low-power laser light. Realizing he has been targeted, the aggressor (according to the Air Force) hides or flees rather than risk death by aiming his weapon and attracting defensive fire.

Rangefinder

Main article: Laser range-finder

A laser range-finder is a device consisting of a pulsed laser and a light detector. By measuring the time taken for light to reflect off a far object, and knowing the speed of light, the range to the object can be found. A laser rangefinder is thus a simple form of LIDAR. The distance to the target can then be used to aim a weapon such as a tank's main gun.

Target designator

Another military use of lasers is as a laser target designator. This is a low-power laser used to indicate a target for a laser guided munition such as a smart bomb or missile, typically launched from an aircraft. The guided munition adjusts its flight-path to home in to the laser light reflected by the target, enabling a great precision in aiming. The laser designator can be shone onto the target by an aircraft or nearby infantry. Lasers used for this purpose are usually infrared lasers, to prevent easy detection of the guiding laser light by the enemy.

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