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| http://www.dmphotonics.com/ |
LASER means a technique of “Light Amplification
by Stimulated Emission of Radiation,” was invented in 1960. In fact consist on
active medium, that when stimulated by some specific energy source (photon
energy) emits LASER light, which consists on the first photon plus the emitted
photon, as showed on figure 1.
[http://u20science.org/akshivb/2013/04/17/laser/
- 17/04/2013]
LASERs are a coherent, monochromatic and
directionality light source. The coherence is a property that indicates the
ability of the waves to interfere with each other. Two waves that are coherent
can be combined to produce an unmoving distribution of constructive and
destructive interference (a visible interference pattern) depending on the
relative phase of the waves at their meeting point. Waves that are incoherent,
when combined, produce rapidly moving areas of constructive and destructive
interference and therefore do not produce a visible interference pattern [1 - http://www.worldoflasers.com/laserproperties.htm].
LASERs are said as monochromatic light source because they generate light on a
very narrow band spectrum of wavelength, and in this case the peak of intensity
is on one specific wavelength that makes them as a monochromatic light source.
Directionality is related with the multiple
reflections inside of optical cavity that also produce a well-collimated beam,
because only photons traveling parallel to the cavity walls will be reflected
from both mirrors. If the light is the slightest bit off axis, it will be lost
from the beam [1].
Active medium to LASER can be gas, liquid or
solid. This medium is localized on the cavity center and is placed on the first
stage of the laser system the oscillator, this is the place where the active
medium receives energy to make a population inversion and is knew as a
amplification medium. When the active medium has a good population inversion it
emits a LASER light, which the wavelength depends of medium properties and of
wavelength seed (utilized as an energy source to make a population inversion on
active medium).
Here I will show as work a LASER oscillator to
a state solid LASER, which means that we have as active medium a crystal. In
fact LASER cavity consist on a crystal on the center of cavity, a set of mirrors
and an energy source, as show the figure 2.
As we can see on figure 2, oscillator has a set
of two mirrors, where one mirror has high reflection, and the other has a
partial transmit, which means that some part of the amplified light can be
emitted by the oscillator when it gets a specific energy, and before get this
energy the beam is trapped on the cavity and will be how many tours inside of
oscillator as necessary to get the energy to could be emitted. This specific
energy is correlated with the longitudinal mode of the cavity.
To a cavity with l length some a specific set of
frequencies could satisfy boundary conditions on mirrors, on the other hand, it
means that just some frequencies we will have a constructive interference each
time that a beam did a complete turn inside of the cavity. These specific frequencies are cavity
longitudinal modes:
C is light velocity, neff is
effective refractive index inside of the cavity and l is cavity
length.
Furthermore longitudinal modes, the frequencies
that will be emitted by optical cavity need to be inside of the gain curve that
has, in general, a big bandwidth. By the equation [1] is possible to find the
frequencies permitted spacing:
It is possible to estimate longitudinal modes
number N of a LASER by the relationship between the gain curve bandwidth and frequencies
permitted spacing:
In this way the frequencies that will be
emitted need satisfy the gain curve and the equation [1]. On the next post about lasers I will explain more about lasers system.
Kelly Cristine Camargo, PhD
