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LASER
an acronym
for

Light Amplification by Stimulated Emission of Radiation
It is an intense concentrated and highly parallel beam
of (quasi)monochromatic light Obtained by a process
involving energy states within the atoms of the material
...

 based on solid, liquid, or gaseous media
...

high or low power,
pulsed or continuous beam of light
at wavelengths anywhere from the far IR to the X- ray region
...
Monochromaticity: The light is very nearly monochromatic
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Coherence: The light is coherent, with the waves all exactly in phase with one
another
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Directionality: The light is highly directional
...

4
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LASER
Absorption and Emission of Electromagnetic Radiation:

 An atom consists of a centrally placed nucleus around which electrons revolve in
Some Permitted orbits
...
e
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The state of lowest energy of atom is called as its
Ground state
...

A no of excited states are possible for any atom or configuration of atoms
...
It can be excited by supplying energy to
the material in the form of heat/light
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 An excited state is an unstable state
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The
Wavelength of a photon depends on its energy which, in turn depends upon the energy
of transition involved
...

 Since various excited energy states are possible, hence a large no of transition are
also possible
...
Also the time interval between the gaining of thermal energy and the
emission of this energy (as light) differs for various energy states
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Such a process is
termed as random emission and the emitted light is incoherent
...
The beam is also
well collimated and so spreads out very little, even over long distances
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There are three mechanisms
Involved in the process
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Absorption
2
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Stimulated Emission

Absorption:

E2

When a quantum of radiation (photon) strikes an atom, there is a
h  E2  E1
possibility that, the atom will absorb energy and be excited from the
ground state (with energy Ei) to a Higher state (with energy Ef)
...
Here
E1
the photon stimulated the atom to make the upward
Stimulated absorption
Transition

LASER

E2

Spontaneous Emission:
E  h

E1

Spontaneous
emission

An atom in an excited state has a tendency to jump back to the
Ground state which is a stable state
...
This process is called as spontaneous
emission as it does not require any outside stimulus
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It remains in the
Meta stable
excited state for a short interval of time
state
h
(called as its life time Corresponding To
that ex

LASER
E2

h '

h ' 'h ' '
h ' '
h ' '

E2

E1

E2

E1

E2

E1

E1

h '
h '

h

h '
E0

Atoms in ground
state(E0 ) are
pumped to state E2
By photons of
frequency
 '

E 2  E0
h

E0

E0

Metas table state
Rapid transition or
Spontaneous emission Occupied many
To mata stable state E1 atoms
By emission of photons
Of frequency
E  E1
 '' 2
h

Or in some other way

h
h
h
h
h

E0

Induced emission occurs
When photons of energy
h  E1  E0 are incident with
the secondary photons
themselves inducing further
Transitions to produce an
Avalanche of coherent and
Resulting amplification
...
Once this is arranged with the help of pumping, then shinning light of frequency
ν on the assembly, there will be more induced emissions from atoms in the meta stable
State than induced absorption by atoms in the ground state
...
This is the concept that underlies in the operation of
Ruby Laser
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More photons of frequency ν we pump into the
assembly of atoms, there will be more upward transition from the ground state to
the meta stable state
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When half of
the atoms are in each state, the rate of induced emission will be equal to the rate of
induced absorptions, so that the assembly cannot have more than half its atoms in
the metastable state
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A population
inversion is only possible only when the induced absorptions are to a higher energy
level than the meta stable one from which the induced emission take place, which
prevents pumping from depopulating the meta stable state
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Active Medium
2
...

3
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In its
simplest version, it is a pair of mirrors set on an optical axis which defines the
direction of Laser beam
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After the active medium is energized
By the pulses of light from the energy
Source(Normally a flash tube),
Emission starts
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During the reflections in their way
They interact with other excited atoms
Components of Laser system
Of the active material and thus produce
more photons by stimulated emission
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LASER
If one of the reflector is partially transparent (say 10%), then some of the coherent light
(stimulated photon beam which is parallel to the axis of cylinder) would emerge
through this reflector and serves as the output laser beam
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What do you mean by a meta stable state?
2
...

3
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What are the basic components of a laser?

LASER
Ruby Laser
Ruby laser is a three level solid state laser
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Construction:
It consists of a ruby
crystal rod (Al2O3)
doped with
approximately 0
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The Cr3+ ions
replaces the Al3+ ions in
the lattice of Al2O3
Cooling
Crystal
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Cr3+ ion has a meta stable
Level whose life time is about 0
...
Hen
Laser is used in investigating structure of molecules
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