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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
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high or low power,
pulsed or continuous beam of light
at wavelengths anywhere from the far IR to the X- ray region
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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
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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
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A no of excited states are possible for any atom or configuration of atoms
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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
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 Since various excited energy states are possible, hence a large no of transition are
also possible
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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
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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)
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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
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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 excited state)
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If this life time for an excited state An atom can exist in a metastable energy level
for a longer time before radiating than it can in
in an atom is much longer than the
an ordinary energy level
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In this case, the
time interval until spontaneous emission occurs is relatively long
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There is a possibility that the interaction

LASER

E

2
between the incoming photon and the atom will cause
h
h  E2  E1
the atom to return to the ground state and thereby
h
emit a second photon of energy h  E2  E1
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These two photons are in
Stimulated emission
Phase and they travel in the same direction
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The quantum state of the stimulated photon is identical to that of the incident
photon
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Photons produced in this fashion are the source
Of the intense coherent light in a laser]
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Questions:
1
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What do you mean by meta stable states?
3
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LASER
Pumping
It is energizing process of a medium so that the population of atoms in higher energy
Level is increased largely in comparison of population of the lower level
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1
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2
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This method of pumping is preferred in gaseous laser
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Inelastic collisions Between atoms: provides the pumping energy to excite the
atoms
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Direct Conversion: In semiconductor laser, the electrons recombine with the holes
across the p-n junction and provide the pumping energy for population inversion
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Chemical Reaction: Radiations come out of a chemical reactions provide the
pumping energy for population inversion
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Write a brief note on population inversion
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What is pumping? Name five methods of pumping to result population inversion
in lasers

LASER

Active Medium:
The key to the LASER is the presence of many atoms of one or more excited energy
levels whose lifetime is large ~ 103 sec in comparison to normal life times ~ 108 sec
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Laser emission is possible only if the medium (through which the external radiation is
passing) contains large no of atoms having meta stable states
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Such a medium is called as active medium
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What is an active medium in Laser? Give examples
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 2l  2n  l  n


2

What is an optical resonator or resonant cavity?
Three Level Laser:
A three level laser, the simplest kind, uses an assembly of atoms (or molecules)that
have a meta stable state h in energy above the ground state and a still higher
excited state that decays to the meta stable state
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LASER
What we want is more number of atoms in the meta stable state than in the ground
State
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The result will be an
Amplification of original light
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Why there are three levels needed?
Suppose there are only two levels, one a ground state and the other being a meta
Stable state above the ground state
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However, at the same time, the pumping will induce
downward Transitions from the meta stable state to the ground state
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In this situation, laser amplification cannot occur
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LASER
The Basic Components of a Laser
The basic components for operation of a laser are:
1
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Energy source for energizing the active medium (Pumping): This energy source
energizes the active medium and raises its atoms/molecules/ions to excited states
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Optical Cavity (or Resonator cavity): It is the principal component of Laser
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One of the mirrors is fully reflecting and the other is
partially transparent with respect to the emitted radiation
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Photons that do
not travel parallel to the axis of the
Cylinder suffers several reflections
Between the two end reflectors
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Intensity of the photon beam parallel to the
axis of the cylinder increases gradually
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1
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What is a three level Laser? Explain its working with a neat diagram
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Why a two level system cannot result laser emission?
4
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It is a high powered pulsed laser having
Power of hundreds of mega watt with very short duration (10-6 sec)
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05%
Of trivalent Chromium
Ions (Cr3+ ) in the form
Of Cr2O3
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The Aluminum
(Liquid Nitrogen)
And oxygen atoms are
Inert

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