The laser light produces an intense, concentrated, and
highly parallel beam of coherent light. Laser is the outgrowth of maser, a similar
device using radio microwaves instead of visible light waves.
Some basic
principles involved in the operation of laser. Meta stable states, optical
pumping, fluorescence, population inversion, resonance, stimulated emission,
coherence, polarization, fabry-perot interferometry, cavity oscillation.
Most
of the principles are already used in some other applications but coherence and
stimulated emission are the keys to the realization of maser and laser operation.
Consider a gas enclosed in a vessel containing free atoms
having several energy levels, at least one of which is meta-stable. White
light into these gas atoms can be raised from the ground state to the excited state.
If the light intensity is high, we can obtain more electrons in excited states which
means population inversion. When an electron in one of these excited states
spontaneously jumps to the ground state, as it eventually will, it emits a
photon of energy hv. This is called fluorescent or phosphorescent radiation.
As the photon passes by another nearby atom in the same Meta stable state, it can,
by the principle of resonance, immediately stimulate that atom to radiate a
photo of the exact same frequency and return it to its ground state. That
stimulated photon has exactly the same frequency, direction, and polarization
as the primary photon and exactly the same phase and speed.
Both of
these photons may now be considered primary waves, and upon passing close to
other atoms in their meta-stable states, they stimulate them to emission in the
same direction with the same phase.
However, the transition from the ground state
to the excited state can also be stimulated, thereby absorbing the primary
wave. An excess of stimulated emission therefore requires a population inversion,
more atoms in the Meta stable state than the ground state. Thus if the
conditions in the gas are right, a chain reaction can be developed, resulting
in high-intensity coherent radiation.
The Ruby laser
The ruby is primarily a transparent crystal of corundum doped
with trivalent chromium ions. The chromium ions are the active ingredients. The
aluminium and oxygen atoms are inert. The ruby crystal is cylindrical in shape.
Generally ruby laser one end is highly reflective and the other end is close to
half silvered.
When white light enters into a crystal, absorption by the
chromium ions in the blue-green part of the spectrum occurs. The electrons
immediately rise excited state and randomly jump back to the ground level,
emitting visible red light.
Helium-neon gas laser
The helium-neon gas laser was the first gas laser operation.
Many scientists have tried to make gas lasers because it is inexpensive,
unusually stable, and emit continuously, this helium-neon laser is widely used
in optics laboratories.
There are ten times as many helium atoms present in a
mixture as there are neon atoms, the orange colour of the gaseous discharge is
characteristic of neon atoms. The visible spectrum of helium contains lines in red, yellow, green and blue, so the discharge appears as white light.
The carbon dioxide
laser
A high-power molecular gas laser operates on
carbon dioxide gas molecules. This optical device produces a continuous laser
beam with a power output of several kilowatts and at the same time maintains a
relatively high degree of purity and coherence.
At present hundreds of different kinds of lasers using many
different materials have been made, emitting radiation over a wide range of
wavelengths from the ultraviolet at one end of the spectrum to microwave at the
other.
Modulated laser beams have been used for communication. The modern
medical industry depends upon lasers and in future all technology will have some
kind of laser. Laser communication is used to communicate small and large
distances. Lasers have been used like radar, and determine distances. Lasers are also used in all micro-level cutting machinery in industries.
Students can do many
projects about wireless communication with the help of laser. We can communicate using laser beam transmission to the hills area in the case of mobile communication.
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