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Operating principles
There are three essential parts to the laser structure:
The resonant cavity, the active medium and the pumping system. The pumping
system delivers energy to the active medium. In a particular medium and in a
manner specific to that medium, this energy raises a large number of the
medium's particles (atoms, molecules, ions) to one or more excited states.
Because of the general principles of physics, the individual particles tend
to return to the non-excited state emitting a quantum of energy known as a
photon. If a photon interacts with or is absorbed by another similar excited
particle, this excited particle emits a photon in synchrony or phase (with
coherence) with the one absorbed and which is identical to it in terms of
energy or wavelength and direction of propagation. In the right conditions,
a chain reaction phenomenon is obtained, that is, an amplification of the
original wave or of the photons generated by the decay of the excited
particles, which are spontaneously de-energized. In the active material, the
stimulated emission can occur only at the wavelength or wavelengths
characteristic of the absorption of the de-energization of the atoms,
molecules or ions of which it is constituted.
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The resonant cavity where the active medium
is located is composed of precisely parallel mirrors, one of which is
totally reflecting and the other, partially reflecting. Only light that
propagates at a right angle, or near right angle to the mirrors can undergo
amplification. Consequently, the electromagnetic wave that is unsaturated
within the cavity and that which escapes through the partially reflecting
mirror possesses a high directionality.
The directionality of a laser beam, quantified by means of the
characteristic angle of divergence is high and is limited by the phenomenon
of diffraction, which is associated with the phenomena of wave propagation.
For example, a green ray from an argon laser with a cross section
1-centimeter in diameter, widens up to a cross section of three centimeters
after traveling a distance of 500 meters. The high directionality of the
laser beam permits laser radiation to be sent to the moon and the reflected
portion to be received on earth.
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In addition, the resonant cavity, as such,
forces the electromagnetic oscillation to occur only at frequencies
associated with the physical distance of the mirrors.
The selectivity of the resonant cavity along with the foregoing fact that
stimulated emission occurs at the wavelengths of the active material,
account for the property of monochromaticity of laser radiation.
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For a light source, we define brightness as
the power emitted per unit of surface area (power density) and per solid
angle unit. The very high brightness of laser radiation is due to the low
divergence value of the beam. Observing the light from a conventional, even
nearly monochromatic light source (spectral lamp), of a 50 Watt power at 100
meters distance, we would measure a light intensity on a sensor very much
lower than that coming from a laser source of the same 50 Watt power
measured along the axis of the beam. In the first case the light is
distributed on a surface 'that increases greatly with distance, while for
the laser source, the light is distributed on a surface that increases
almost imperceptibly with distance.
In pumping the active medium, or rather, in supplying the energy by means of
a feeder to the particles which must be raised to an excited state and in
the gain within the cavity which, if greater than a threshold value, will
permit laser emission, the transfer of energy is not complete, and,
therefore, there is an output in the laser system that can be measured,
according to the type of laser, in values that range from a fraction of a
percent to 15-20 percent.
Although this negative element accounts for considerable loss of energy, it
is acceptable in all laser applications, in that the characteristics of
laser light can produce results which otherwise cannot be achieved.
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