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Characteristics of laser light
In addition to "directionality", laser rays are characterized by
other properties that distinguish them from all other sources of
electromagnetic radiation existing in nature. Laser rays are coherent,
monochromatic and extremely bright.
Monochromaticity
The light beam is composed of a ray of electromagnetic waves of a single
frequency. Theoretically, this concept can be easily understood, in
practice, it cannot be achieved, because the beam is made up of various
wavelengths, even though they are very close to each other, and for this
reason they are situated in an extremely restricted band of the spectrum.
However, it is legitimate to speak of monochromaticity especially with
respect to radiation obtainable with any other conventional light source
including monochromatic lights (spectral lights).
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Brightness
This is the parameter that most profoundly distinguishes the optical
qualities of a laser and explains its applications. By brightness, we mean
the power (in Watts or Joules per second) emitted per unit of surface area
and per unit of solid angle. The enormous brightness of the laser beam
allows it to reach a great energy density, such as can sublimate or melt the
hardest of metals or heat-resistant materials. We recall that the ratio of
focalized laser brightness to the brightness of the sun at its Zenith is
approximately 60,000.
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Coherence
Coherence is the property directly related to stimulated emission, by which
the individual emissions occur in phase with each other. We distinguish
spatial and temporal coherence:
Spatial coherence because the wave fronts maintain a constant form;
Temporal coherence because the phase difference of the wave remains constant
whatever the time under consideration, a concept linked directly to having a
sole frequency or spectral line, and, therefore, to monochromaticity.
Unidirectionality
Unidirectionality of the radiation is intimately related to spatial
coherence, and results from the structure of the laser resonance cavity
which, consisting of parallel mirrors, allows only those beams which are
exactly perpendicular to the surface of the mirror to propagate and escape.
This also explains why the laser light beam has a very limited angular
divergence.
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