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-The following is a discussion of a
solid state ruby laser only.- Understanding how a Laser works requires a brief
discussion of quantum mechanics. An atom is composed of a nucleus, made up
of
protons and neutrons, and an orbiting cloud of electrons, as shown in the figure
to the right.
Protons have a positive charge of +1, neutrons are electrically neutral, and
electrons have a charge of -1. In order for an atom to be electrically
neutral, e.g. not an ion, the number of protons must be equal to the number of
electrons. Electrons orbit the nucleus of the atom in specific orbitals
surrounding the nucleus, shown as a simple diagram in the figure below (the Bohr model of
the atom). Electrons do not always stay in the same orbit, but are able to
transition between orbits, provided that a particular orbit is not already at
maximum occupancy, by absorbing or releasing energy in the form of photons
(light). The amount of energy required to transition between orbits is
dependant on how far the electron is moving. A longer jump requires a
higher energy photon to be absorbed
A typical solid state ruby Laser has a power source, an active medium, and a
resonant chamber. As described above, the energy source excites electrons
from the active medium, producing photons at a specific wavelength. The
resonant chamber is composed of two mirrors, one at each end of the chamber, and
contains the active medium. One mirror is 100% reflective while the other
mirror is 95-99% reflective. This allows some of the light to escape,
creating the Laser beam. The main function of the resonant chamber is to
allow photons to bounce back and forth between the mirrors to intensify the
light before it escapes through the partially reflective mirror. this
intensification of light is made possible by the fact that photons are bosons.
A boson is a type of particle that is able to occupy the same space at the same
time as another boson. The resonant chamber provides an environment where
the photons can synchronize into the same resonance, forming an intensified beam
of light that is .
Normal light is emitted in as spontaneous radiation, producing various wavelengths of visible and non-visible light. In contrast, Laser light has three unique properties: 1) Monochromaticity: Laser light is monochromatic, meaning that all the photons of a particular laser beam have same wavelength. 2) Coherence: All the photons of the laser beam a synchronized in time and space. 3) Collimation: all the elements of the laser beam are parallel, that is they do not diverge; as a result they may be focused into a very small space. So, what makes a cold laser different? |
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electron.
The current quantum mechanic view of electron orbitals is shown in the figure to
the right. Knowing that electrons emit photons as they move down from
a higher energy orbit to a lower one is the basic principle of how to generate
the light for a Laser. In order to create a lot of light at a specific
wavelength, a Laser uses an energy source to excite electrons to a specific energetic state, say from n=2 to n=3 in fig.2.
As
the electrons return to their previous lower energy state, they emit a photon at
a specific wavelength. If many electrons are excited to the same level and
return to the same level, all emit the same wavelength photon. As more and
more electrons are excited and then decay, more and more photons are emitted. 