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Laser
A laser (Light Amplification by Stimulated
Emission of Radiation) is a device which uses a quantum mechanical
effect, stimulated emission, to generate a coherent beam of light from a lasing
medium of controlled purity, size, and shape. The output of a laser may be a
continuous, constant-amplitude output (known as CW or continuous wave),
or pulsed, by using the techniques of Q-switching, modelocking, or
gain-switching. In pulsed operation, much higher peak powers can be achieved.
A laser medium 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 of wavelength, phase, and polarisation; this is particularly
important in optical communications. The verb "to lase" means "to produce
coherent light" or possibly "to cut or otherwise treat with coherent light",
and is a back-formation of the term laser.
Common light sources, such as the incandescent light bulb, emit photons in almost all directions, usually over a wide spectrum of wavelengths. Most light sources are also incoherent; i.e., there is no fixed phase relationship between the photons emitted by the light source. By contrast, a laser generally emits photons in a narrow, well-defined, polarised, coherent beam of near-monochromatic light, consisting of a single wavelength or hue.
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