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

Common light sources, such as the electric 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 amplifier when seeded with light from another source. The amplified 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 permanent 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 excessively bright light, works only 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 output of a laser may be a continuous, constant-amplitude output (known as c.w. or continuous wave), or pulsed, by using the techniques of Q-switching or modelocking.

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 stimulated emission, which if the light is fed back through 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 many 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. Townes later worked with Arthur L. Schawlow to describe the theory of the laser, or optical maser as it was then known.

Independently from their American colleagues, a team of Soviet scientists lead by Nikolai Basov[?] and Aleksandr Prokhorov has also been working on producing the quantum oscillator. The first maser, developed by Townes, was incapable of continuous output. In order to achieve continuous output, new systems with more than two energy levels had to be designed. These systems could release stimulated emission without falling to the ground state, thus maintaining a population inversion. Nikolai Basov and Alexander Prokhorov of the USSR first developed this idea. In 1964, Charles Townes, Nikolai Basov and Alexandr Prokhorov shared a Nobel Prize in Physics "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle."

The first working laser was made by Theodore H. Maiman in 1960 at Hughes Research Laboratories, beating several research teams including those of Townes at Columbia University, and Schawlow at Bell laboratories. Maiman used a solid-state flashlamp[?]-pumped ruby crystal to produce red laser light at 694 nanometeres wavelength. (See laser construction).

Types of lasers include:

  • Ruby lasers, producing red (694 nm) light. The first type invented, often used for medical purposes including the removal of tattoos 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;
  • Holmium-doped glass, a high-power laser operating in the infrared, it is explosively absorbed by water-bearing tissues in sections less than a millimeter thick. It is usually operated in a pulsed mode, and passed through optic fiber surgical devices to resurface joints, remove rot from teeth, vaporize cancers, and to pulverize kidney and gall stones.
  • 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, LADAR, active laser medium, laser diode

The verb "to lase" means to give off coherent light or possibly to cut or otherwise treat with coherent light, and is a a back-formation of the term laser.

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