Transcript LIGHT APLIFICATION by STIMULATED EMISSION of RADITIONS

LASER
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LASER stands for LIGHT APLIFICATION
by STIMULATED EMISSION of
RADITIONS
First laser was constructed by Maiman
Laser action has been obtained with
atoms, ions & molecules in gases, liquids,
solids, semiconductors at wavelength( λ )
ranging from ultraviolet to radio frequency
regions and with power ranging from a
few milli watts to mega watts.
Basic principle involved in laser
Stimulated
absorption
Spontaneous
emission
Stimulated
emission
Stimulated Absorption
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E1 & E0 are two energy
levels such that E1 is
excited energy level & E0
is lower energy level.
If hƒ =E1- E0 ,where ƒ is
frequency of incident
radiation, then radiation
photon is absorbed &
electron jumps from E0 to
E1.
Spontaneous Emission
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The excited atom does
not remain in higher
energy state for
long.It gives out a
photon of energy
hƒ= E1- E0 & returns
to ground state. As
this process does not
require stimulus from
outside,so it is termed
as spontaneous
emission.
Stimulated Emission
The excited atom in energy
state E1 may not come to
ground state at given instant.
So it requires external
stimulation. When a photon
of energy hƒ= E1- E0 strikes
the atom in excited state , it
comes to ground state. The
photon emitted goes parallel
to incident photon & both of
these photons are in phase.
For laser to have Stimulated
Emission, the following two
conditions must be satisfied
1)The higher energy state
should have longer life time
2) The number of atoms in
higher energy state E1 must
be more than that in E0
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Population Inversion
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The situation in which
number of atoms in higher
energy state is greater
than in lower energy state
is called population
inversion. It can be
obtained even at room
temperature by laser
pumping.various pumping
methods are:optical
pumping,chemical pumping,
electrical pumping. when
population inversion is
obtained, emitted photons
have same frequency and
phase. They travel in same
direction. Thus the number
of photons goes on
multiplying by stimulated
emission. Hence we get a
highly intense,
monochromatic, coherence
and unidirectional beam.
RESONANCE CAVITY
Fully
Reflecting
mirror
Active medium
Partially
reflecting
mirror
A medium with population inversion is capable of amplification but in order
to get a continous supply of light energy it is required to couple a part of
output back into the medium. It can be made by placing an active medium
between two mirrors such that one is fully reflecting and other is partially
reflecting.
Active medium: A medium which on excitation produces population
inversion and promotes stimulated emission
Characteristics of Laser
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1)Directionality
2)Intensity
3)Monochromaticity
4)Coherence
1)Directionality
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The conventional source
emits radiation in all
direction. But Laser
emits light in particular
direction. Directionality
of laser can be
expressed in terms of
full angle beam
divergence, which is
twice the angle that
outer edge of beam
makes with the axis of
beam.
The angular spread Δθ
=λ/d where d is
diameter of aperture.
2) Intensity
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Laser gives out light in form of narrow beam with
energy concentrated in a small region.
Even 1 watt laser appears thousands times more
intense than 100 watt ordinary lamp.
The power output from a small gas laser is 10-3
watt whereas that from a pulsed solid state laser
may be as high as 109 watt.
3) Monochromaticity
Laser emits light which is more monochromatic
than any other monochromatic conventional
source.
Light emitted from a monochromatic
conventional source is not perfectly sharp.It
spreads whereas laser light spreads very little
as shown in figure.
4) Coherence
It is measure of degree of phase
correlation that exists in radiation field of
light source at different locations and
different times.
 Cohernce is of two types
a) Spatial Coherence
b) Temporal Coherence
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Temporal Coherence
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If the phase difference between any
two points at an instant of time lying
in direction of propagation of the
electromagnetic wave is independent
of time, the coherence is temporal
coherence.
Spatial Coherence
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If the phase difference between any
two points at an instant of time lying
in the plane perpendicular to
direction of propagation is
independent of time, the coherence
is spatial coherence.