LASER

1. LASER
Light Amplification by Stimulated
Emission of Radiation

2.  Objectives…
Introduction and understand the principle of
LASER
• Light Amplification by Stimulated Emission of
Radiation
• Absorption
• Spontaneous Emission
• Stimulated Emission
• Population Inversion
• Optical Pumping

3.  Objectives…
 Characteristics or Properties of Laser Light
• Coherence
• High Intensity
• High directionality
• High monochromaticity
Laser light is highly powerful and it is capable
of propagating over long distances and it is not
easily absorbed by water.

4. Introduction
• LASER
“Light Amplification by Stimulated Emission
of Radiation”
• MASER (1939 Towner)
“Microwave Amplification by Stimulated
Emission of Radiation”
• Stimulated Emission - Einstein in 1917.
• Ruby Crystal LASER - Maiman, California in 1960.
• He-Ne LASER - Ali Javan in 1961.
• Diode LASER- Hall in 1962.

5.  Light having following Properties
 Wavelength
 Frequency
 Amplitude
 Phase
 Coherence/Incoherence
 Velocity
 Direction

6. Absorption
• E1 = Ground state
• E2 = Excited State
• E = hν (Photon Energy)

7. • According to Bohr’s law atomic system is
characterized by discrete energy level.
• When atoms absorb or release energy it
transit upward or downward.
• Lower level E1 & Excited level E2
• So, h ƒ = E2 – E1
• The rate of absorption depends on no. of
atoms N1 present in E1 & spectral energy
density u(ƒ) of radiation
• So, P12 α N1 u(ƒ)
• P12= B12N1 u(ƒ)

8. Spontaneous Emission
• E1 = Ground State
• E2 = Excited State
• E = E2 – E1
= ΔE
= hν

9. • System having atoms in excited state.
• Goes to downward transition with emitting
photons, hƒ = E1 – E2.
• Emission is random, so if not in same phase
becomes incoherent.
• The transition depends on atoms in excited state
N2.
P12(spont) α N2 = A21 N2
• Where,
A21 = Einstein coefficient for spontaneous
Emission. we get Incoherent radiation forms heat
by light amplification of radiation by spontaneous
emission.

10.  Stimulated Emission

11. • System having atoms in excited state.
• Goes to downward transition with emitting
photons.
• 2hƒ = E1 – E2. After applying photon energy hƒ.
• Emission is depends on energy density u(ƒ) & No. of
atoms in excited state N2
• P12(stimul) α u(ƒ) N2 = B21 N2 u(ƒ)
• Where, B21 = Einstein coefficient for Stimulated
Emission.
• Thus one photon of energy hƒ stimulates two
photons of energy hƒ in same phase & directions.
So, we get coherent light amplification of radiation
by stimulated emission.

12.  Population Inversion
• It is the process of increasing exited electrons in
higher energy levels.
• Due to this process the production of laser is
possible.
• The energy level between the ground state E1 (1st
level) and exited state E3 (3rd level) is known as
metastable state E2 (2nd level).
• By optical pumping electrons from ground state
jumps to exited state by absorbing photons.

13. • The electrons remain only for 10-8 sec in exited
state E3, so most of them jumps back to the
ground state E1 by emitting photons. But some of
them jumps to the metastable state E2.
• They (electron) stay in metastable state for more
then 10-3 sec.
• So electron density increases in metastable state.
• Thus the transitions are possible it takes more
no. of electrons together and ν – (knew)
12 photon
beam is produced which constitute laser beam.

14.  Optical Pumping
There are no of techniques for pumping a
collection of atoms to an inverted state.
• Optical pumping
• Electrical discharge
• Direct conversion
When photon of blue green light incident on
Ruby crystal, electrons from ground state absorbs
and exited and jumps on higher energy state levels
and comes back to metastable state. They increase
population of electrons in metastable state.
This process is called optical pumping which is
done by flash tube.

15.  Relation between Einstein’s ‘A’ and ‘B’ coefficients
• Einstein obtained a mathematical expression for
the existence of two different kinds of processes,
(1) Spontaneous emission
(2) Stimulated emission
• Consider all atoms r in thermal equilibrium at T.
• Radiation of freq. ƒ & energy density u(ƒ).
• N1 & N2 r atoms in E1 & E2 respectively.
• In equilibrium absorption rates & emission rates
must be same.
• i.e. B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)
A21 N2= u(ƒ) [B12N1 – B21N2]
So, u(f) = [A21 N2 / (B12 N1 – B21 N2)] ---------(1)

16. ------------(2)
• Boltzmann distribution law,
------------(3)
• So, -----------(4)
• But, E2 – E1 = hf -----------(5)
• So, -----------(6)
21
21
12 1
21 2
( )
[ ]
ƒ
1
A
B
u
B N
B N


1
2
/
1 0
/
2 0
E kT
E kT
N N e
N N e




2 1( )/1
2
E E kTN
e
N


h /1
2
ƒ kTN
e
N


17. ---------- (7)
• According to plank’s radiation formula,
----------- (8)
• Where, B12 = B21 & A21 / B21 = ------------ (9)
• So, Ratio of spontaneous to stimulated emission:
--------- (10)
21
21
ƒ12
21
h /
ƒ
1
( )
[ ]kT
e
A
B
u
B
B


3
3 ƒh /
8 1
( ) ( )
[ ]
ƒ
ƒ
1kT
u
c
h
e



3
3
8 ƒh
c

2 21 21
2 21 21
3
3
8
( ) ( ) ( )
ƒ
ƒ ƒ ƒ
N A A h
R
B u B u ucN

  

18. • So,
--------- (11)
--------- (12)
• So, R = ---------- (13)
If hƒ << kT, in thermal equilibrium,
then R = << 1
• hƒ<<kT – Stimulated emission
–Valid in microwave region (MASER)
• hƒ>>kT – Spontaneous emission
–Valid in visible region, incoherent
3
3 /
3
3
ƒh
8
( )
8
ƒ
ƒ
&
ƒ
ƒ
1
1
( ) ( )
[ ]kT
h
u
c
u
R
h
e
c





ƒh /
1[ ]kT
e 
ƒh /
1[ ]kT
e 

19.  Types of LASER
There are three types of lasers
1. Solid Laser (Ruby Laser)
2. Liquid Laser
3. Gas Laser ( He – Ne Laser, CO2 Laser)

20.  Ruby Laser…
To produce laser from solid, Ruby crystal is used.
Ruby is an aluminum oxide crystal (Al2O3) in which
some of the aluminum atoms have been replaced
with Cr+3 chromium atoms (0.05% by weight).
It was the first type of laser invented, and was first
operated by Maiman in Research Laboratories on
1960.
Chromium gives ruby its characteristic pink or red
color by absorbing green and blue light.
For a ruby laser, a crystal of ruby is formed into a
cylinder. The ruby laser is used as a pulsed laser,
producing red light at 6943 Å.

22. Ruby crystal is surrounded by xenon tube. Ruby
crystal is fully silvered at one side and partially
silvered at the other end.
A strong beam of blue green light is made to fall up
on crystal from xenon tube and this light is
absorbed by the crystal.
Because of this, many electrons from ground state
or normal state are raised to the excited state or
higher state and electron falls to metastable state.
During this transition photon is not emitted but
excess energy of the electrons absorbed in crystal
lattice.

24. As electron drops to metastable state they remain
there for certain time ~ 10-6 sec.
 Thus the incident blue green light from tube
increases the number of electron in metastable
state and then the population inversion can be
achieved.
 If a light of different frequency is allowed to fall
on this material, the electrons move back and
forth between silvered ends of the crystal.
While moving through they get stimulated and
exiced electrons radiate energy.

25. Thus readia photon has the same frequency as
that of incident photon and is also in exactly same
phase.
When the intensity of light beam is increased the
same process is repeated.
Finally extremely intensified beam of light energies
from the semi silvered side of the crystal.
This way it is possible to get extremely intensified
and coherent beam of light from the crystal. This
beam is nothing but higher energetic beam – ie.
LASER beam.

26.  Applications of Ruby Laser…
Ruby lasers have declined in use with the
discovery of better lasing media. They are still used
in a number of applications where short pulses of
red light are required. Holography's around the
world produce holographic portraits with ruby
lasers, in sizes up to a meter squared.
Many non-destructive testing labs use ruby lasers
to create holograms of large objects such as
aircraft tires to look for weaknesses in the lining.
Ruby lasers were used extensively in tattoo and
hair removal.

27. Drawbacks of Ruby Laser…
• The laser requires high pumping power because
the laser transition terminates at the ground state
and more than half of ground state atoms must be
pumped to higher state to achieve population
inversion.
• The efficiency of ruby laser is very low because
only green component of the pumping light is used
while the rest of components are left unused.
• The laser output is not continues but occurs in the
form of pulses of microseconds duration.
• The defects due to crystalline imperfections are
also present in this laser.

28.  Gaseous Laser (He – Ne Laser)
A helium - neon laser, usually called a He-Ne laser,
is a type of small gas laser. He-Ne lasers have many
industrial and scientific uses, and are often used in
laboratory demonstrations of optics.
He-Ne laser is an atomic laser which employs a
four-level pumping scheme.
 The active medium is a mixture of 10 parts of
helium to 1 part of neon.
Neon atoms are centers and have energy levels
suitable for laser transitions while helium atoms
help efficient excitation of neon atoms.

29. The most common wavelength is 6328 Å. These
lasers produced powers in the range 0.5 to 50 mW
in the red portion of the visible spectrum.
They have long operating life of the order of
50,000 hrs.

30.  Construction…
It consists of a glass discharge tube of about
typically 30 cm long and 1.5 cm diameter.
The tube is filled with a mixture of helium and
neon gases in the 10:1.
Electrodes are provided in the tube to produce a
discharge in the gas.
They are connected to a high voltage power
supply. The tube is hermetically sealed with glass
windows oriented at Brewster angle to the tube.
The cavity mirrors are arranged externally.

31.  Working…
When the power is switched on , a high voltage of
about 10 kV is applied across the gas.
 It is sufficient to ionize the gas.
The electrons and ions are produced in the process of
discharge are accelerated toward the anode and
cathode respectively.
The electron have a smaller mass, they acquire a
higher velocity. They transfer their kinetic energy to
helium atoms through inelastic collisions.
 The initial excitation effects only the helium atoms.
They are in metastable state and cannot return in
ground state by the spontaneous emission.

32. The excited helium atoms can return to the ground state
by transforming their energy to neon atoms through
collision. This transformation take place when two
colliding atoms have initial energy state. It is called
resonant transfer of energy.
So, the pumping mechanism of He-Ne Laser is when the
helium atom in the metastable state collides with neon
atom in the ground state the neon atom is excited and
the helium atom drops back to the ground state.
The role of helium atom is thus to excite neon atom and
cause, population inversion. The probability of energy
transfer from helium atoms to neon atoms is more as
there are 10 atoms of helium per 1 neon atom in gas
mixture.

33. Without the Brewster windows, the light output is
unpolarized, because of it laser output to be
linearly polarized.


34.  When the excited Ne atom passes from metastable
state (3s) to lower level (2p), it emits photon of
wavelength 632 nm.
 This photon travels through the gas mixture
parallel to the axis of tube, it is reflected back and
forth by the mirror ends until it stimulates an
excited Ne atom and causes it to emit a photon of
632nm with the stimulating photon.
 The stimulated transition from (3s) level to (2p)
level is laser transition.

35. Although 6328 Å is standard wavelength of He-Ne
Laser, other visible wavelengths 5430 Å (Green)
5940 Å (yellow-orange), 6120 Å (red-orange) can
also produced.
Overall gain is very low and is typically about 0.010
% to 0.1 %.
The laser is simple practical and less expensive.
The Laser beam is highly collimated, coherent and
monochromatic.

36. Applications of He-Ne Laser…
The Narrow red beam of He-Ne laser is used in
supermarkets to read bar codes.
The He-Ne Laser is used in Holography in
producing the 3D images of objects.
He-Ne lasers have many industrial and scientific
uses, and are often used in laboratory
demonstrations of optics.

37.  Semiconductor Laser (Diode Laser)
• A semiconductor laser is a laser in which a
semiconductor serves as a photon source.
• The most common semiconductor material that
has been used in lasers is gallium arsenide.
• Einstein’s Photoelectric theory states that light
should be understood as discrete lumps of energy
(photons) and it takes only a single photon with
high enough energy to knock an electron loose
from the atom it's bound to.
• Stimulated, organized photon emission occurs
when two electrons with the same energy and
phase meet. The two photons leave with the
same frequency and direction.

38. P type Semiconductors
• In the compound GaAs, each Ga atom has three
electrons in its outermost shell of electrons and
each As atom has five.
• When a trace of an impurity element with two
outer electrons, such as Zn (zinc), is added to the
crystal.
• The result is the shortage of one electron from one
of the pairs, causing an imbalance in which there is
a “hole” for an electron but there is no electron
available.
• This forms a p-type semiconductor.

39. N type Semiconductors
• When a trace of an impurity element with six
outer electrons, such as Se (selenium), is added
to a crystal of GaAs, it provides on additional
electron which is not needed for the bonding.
• This electron can be free to move through the
crystal.
• Thus, it provides a mechanism for electrical
conductivity.
• This type is called an n-type semiconductor.

40. • Under forward bias (the p-type side is made
positive) the majority carriers, electrons in the n-
side, holes in the p-side, are injected across the
depletion region in both directions to create a
population inversion in a narrow active region. The
light produced by radioactive recombination across
the band gap is confined in this active region.

41.  Application of Lasers…
 Laser beam is used to measure distances of sun,
moon, stars and satellites very accurately.
 It can be used for measuring velocity of light, to
study spectrum of matters, to study Raman
effect.
 It can be is used for increasing speed and
efficiency of computer.
 It is used for welding.
 It is used in biomedical science.
 It is used in 3D photography.

42.  Application of Lasers…
 It is used for communication, T. V. transmission,
to search the objects under sea.
 It can be used to predict earthquake.
 Laser tools are used in surgery.
 It is used for detection and treatment of cancer.
 It is used to aline straight line for construction of
dam, tunnels etc.
 It is used in holography.
 It is used in fiber optic communication.
 It is also used in military, like LIDAR.
 It is used to accelerate some chemical reactions.