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Originally an acronym for "Light Amplification by Stimulated Emission of Radiation", a laser uses a quantum mechanical effect, stimulated emission, to generate a very collimated, monochromatic and coherent beam of light.

Common light sources, such as the electic light bulb emit photons in all directions, usually over a wide spectrum of wavelengths. The light is also incoherent, i.e., there is no fixed phase relationship between the photons emitted by the light source. By contrast, a laser emits photons in a narrow, well-defined beam of light. The light is often near-monochromatic, consisting of a single wavelength or color, and is highly coherent, and is often polarised.

A laser can also function as an optical amplifer when seeded with light from another source. The amplfied signal can be very similar to the input signal in terms wavelength, phase and polarisation; this is particularly important in optical communications. As a source, lasers are able to produce a very "pure" light in that the output can have a close approximation to a single wavelength, polarisation, and direction. Some types of laser, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for the generation of extremely short pulses of light, on the order of a femtosecond (10-15 seconds).

Laser light can be highly intense, able to cut steel and other metals. The beam emitted by a laser often has a very small divergence (i.e. it is highly collimated), but will eventually spread under the effect of diffraction, though much less so than a beam of light generated by other means. A beam generated by a small laboratory laser such as a helium-neon (HeNe) laser spreads to approximately 1 mile in diameter if shone from the Earth's surface to the Moon. The common image of a bright beam of light emitted from a laser is a consequence of light being scattered from dust and air particles in the beam, rather than the beam itself. In a vacuum, a laser beam is invisible unless shone directly into the eyes.

Even low power lasers can be hazardous to a person's eyesight. The coherence and low divergence of laser light means that it can be focused by the eye into an extremely small spot on the retina, resulting in localised burning and perminant damage in seconds. Certain wavelength of laser light can cause cataracts, or even boiling of the vitreous humor, the fluid in the eyeball. Infrared and ultraviolet lasers are particularly dangerous, since the body's "blink reflex", which can protect an eye from excesively bright light, only works if the light is visible. Lasers are classified by wavelength and maximum output power into safety classes, from class I (inherently safe; no possibility of eye damage even from hours of direct exposure) to class IV (highly dangerous; even non-direct scattering of light from the beam can blind). Users of class III lasers and above must usually wear approprate eye protection when operating the laser.

The basic physics of lasers centres around the idea of producing a population inversion in a laser medium. The medium may then amplify light by the process of spontaneous emission, which if the light fed back into the medium by means of an optical resonator, will continue to be amplified into a high-intensity beam. A great deal of Quantum Mechanics and thermodynamics theory can be applied to laser action (see laser science), though in fact most of the more useful laser types were discovered by trial and error.

Population inversion is also the concept behind the maser, which is similar in principle to a laser but works with microwaves. The first maser was built by Charles H. Townes in 1953, who later worked with Arthur L. Schawlow to describe the principle of the laser. The first working laser was made by Theodore H. Maiman in 1960 at Hughes Research Laboratories. Maiman used a flashlamp-pumped ruby crystal to produce red laser light at 694 nanometeres wavelength. (See laser construction).

Even earlier was a similar device working at radio frequencies. These were first developed in the 1930s, and further developed in the early 1950s. They don't seem to have a name of their own, possibly because they don't have any obvious practical uses. (Ordinary electronic radio amplifiers and oscillators perform better for less effort.) Some of the basic theory of lasers was in fact developed to explain these earlier devices.

Types of lasers include:

  • Ruby lasers, producing red (694 nm) light, used for medical purposes including the removal of tattooes and birthmarks.
  • Semiconductor lasers, used in laser pointers, laser printers, and CD/DVD players;
  • Excimer lasers, producing ultraviolet light, used in semiconductor manufacturing;
  • Neodymium-doped YAG lasers (Nd:YAG), a high-power laser operating in the infrared, used for cutting, welding and marking of metals and other materials;
  • Titanium-doped sapphire (Ti:sapphire) lasers, a highly tunable infrared laser, used for spectroscopy;
  • Erbium-doped fiber lasers, a type of laser formed from a specially made optical fiber, which is used as an amplifier for optical communications.

See also Laser applications, Laser construction, Laser science.