Soon after the invention of the laser in 1960, it was described as "a solution in search of a problem". However, since that time, the laser has found a place as a useful tool in many scientific, military, medical and industrial applications.
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 achived in a small, well collimated beam can also be used to induce a nonlinear optical effects 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.
Some laser systems 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.
A technique that has had recent sucess 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.
The most extravagent 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 achive breakeven, that is, less power is generated by the fusion reaction than is used to power the lasers, but research continues.
The first rôle 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 solider 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 posibility of using lasers to blind, since this requires much lower power levels, and is easily achivable in a man portable unit. However, most nations regard the deliberate blinding of the enemy as forbidden by the rules of war.
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.
A laser rangefinder is a device consisting of a pulsed laser and a light detector. By measuring the time taken for light to reflect of 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.
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.
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 incoming air-to-air missiles. It is mounted in a converted commercial airliner, and could be used, e.g., to protect assets such as AWACS aircraft. However, the practical problems of reliably generating and aiming the laser beam remain fearsome.
Another example of direct use of a laser as a defensive weapon was researched for the Star Wars strategic defense initiative (SDI), 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.
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