LASER
A LASER (Light Amplification by Stimulated Emission of Radiation)
is an optical device which uses a quantum mechanical effect called
stimulated emission (discovered by Einstein while researching the
photoelectric effect) in order 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, polarized, coherent beam of near-monochromatic
light, consisting of a single wavelength or hue.
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). 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.
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