Laser and its Application strictly based on AKTU syllabus

1. Engineering Physics I Unit IV
Presentation By
Dr.A.K.Mishra
Professor
Jahangirabad Institute of Technology, Barabanki
Email: akmishra.phy@gmail.com
Arun.Kumar@jit.edu.in
6/24/2017
Dr A K Mishra, Academic Coordinator,
JIT Jahangirabad
1

2. Engineering Physics I LASER
• At the end of this module successful students
will be able to:
• Construct energy level diagrams
• Derive the relationship between the Einstein coefficients
• Determine a general formula for laser gain in a generalized four-level laser
• Describe the operation of Helium-Neon and carbon dioxide lasers
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JIT Jahangirabad
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3. LASER
• When an atom is in its excited state and a photon of energy hν
is incident over it, atom comes to its ground state. But now
instead of one, two photon of energy hν each are released.
When these two photons of energy hν each are incident on
another two excited atoms, four photons of energy V
released. This process goes on continuously and a result, a
monochromatic unidirectional, beam of photon released. This
is known as stimulated emission of Radiation.
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JIT Jahangirabad
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E

4. • Absorption of Radiation:
The probability of stimulated emission of radiation is given by
Spontaneous emission of radiation: Probability of
Occurring the spontaneous emission of radiation is
defined as the following
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JIT Jahangirabad
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density.energytheis)U(andtCoefficienEinstien
theisB12photon,theoccuringofyprobabilitheisP12where
(1)..............................)U(B1212

P
)2.......(....................A2121 P

5. • Stimulated emission of radiation: When an atom is in its excited
state and a photon of energy hν is incident over it, atom come to its
ground state. But now instead of one, two photon of energy hν each are
released. When these two photons of energy hν each are incident on
another two excited atoms, four photons of energy V released. This
process goes on continuously and a result, a monochromatic
unidirectional, beam of photon released. This is known as stimulated
emission of Radiation.
• The probability of stimulated emission of radiation is given by
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JIT Jahangirabad
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density.energyis)(Uandemission
stimulatedoftCoefficienEinsteinisBwhere
)U(BA
21
212121

P

6. Relation between Einstein Coefficients
• Let be the no. of atom in the state 1 &2
respectively.
The probability of transition for no. of atom from state 1 to 2
per unit time is given by
N1 P12 = N1 B12 u(ν)……………………………………………………(4)
And total probability of Transition for state 2 to 1 is given by
N2 P21 = N2 A21 + N2 B21 u(ν)……………………………………….(5)
At thermal equilibrium Absorption and Emission probability
are equal
N1 P12 = N2 P21………………………………………………………….(6)
N1 B12 u(ν)= N2 A21 + N2 B21 u(ν)
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21 NandN

7. Continue……………
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bygivenisT
at temp.mequilibriuat thermalEandEstateenergythe
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8. • At thermal equilibrium
N1 = N0 e-E1/KT
N2 = N0 e-E2/KT
= e(E2-E1)/ KT …………………………………………………………(8)
Where K is Boltzmann constant. Now equation (7) becomes
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JIT Jahangirabad
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B.&AtscoefficienEinsteinbetweenrelationtheis
)11.......(..............................
h8A
(9)&(8)eqnCompairing
.(10)....................
)1(
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is)(Uradiationof
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9. Population Inversion
• We know that According to Boltzmann's equation.
• Where is the energy difference b/w ground and excited state, K is
Boltzman Constant and T is absolute Temperature.
• But for atomic radiation is larger is than KT and at thermal equilibrium the
population of excited state is smaller than ground state i.e.
• N1 > N2, As a result stimulated emission are very little as compared to
absorption. Therefore LASER action will not take place. If we increase the
number of atom’s in excited state than ground state i.e. N2 > N1, means the
process of stimulated emission dominate and the LASER Action be achieved.
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JIT Jahangirabad
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KT
2
1
KT
)E-(E
kT
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10. • The state in which the population of atoms is Larger than
the ground state is known as population inversion i.e.
N2 > N1
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11. Pumping
• The technique of creating population
inversion in a medium is called pumping and
external source of energy is called pumping
source. There are following method of
pumping –
• Electrical Pumping
• X- ray Pumping
• Chemical Pumping
• Optical Pumping
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12. Pumping
• Electrical Pumping- it is used in gas and semiconductor Laser. In this
process pumping is achieved by allowing a current of suitable value
to pass through gas. Therefore by creating electric field ion’s and
free electron are produced. Since they are accelerated by electric
field. They acquired additional kinetic energy and are able to excite
a neutral atom by collision.
• X – Ray Pumping: Some time atom’s of solid material can energized
by an X-Ray beam called X- Ray pumping.
• Chemical Pumping: It is occurs when the required population
inversion is produced directly from an exothermic chemical reaction
called as chemical pumping.
• Optical Pumping: When population inversion is achieved by optical
energy is known as optical pumping. When light from a powerful
source is absorbed by active material and atom pumped into
excited state. This is used in solid (Ruby) laser.
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13. Ruby LASER
• Working:
• In 1960, T.H. Maima Constructed the first LASER using Ruby
Crystal (Al2O3). In Ruby Laser there is a cylindrical vessel
whose one side is partially silvered polished and other side
is fully silvered. This tube is known as resonator. In this
tube (Al2O3) is filled or a cylindrical rod of (Al2O3). On this
rod 0.05 % Cr+++ polished. When light is incident on Cr
atoms. They give rise to pink colour. Hence the name of this
laser is Ruby.
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14. Ruby LASER
• Cr. Atoms are excited due to pumping and raise to
excited state E2 the excited atoms return to
ground state E1 from E2 in two steps. First they
return to metastable state. This transmission is
non-radiative and energy of this transition is
passed to the crystal lattice as a heat. The Cr
atoms that return to E3 level that remains in this
state for 10-8 Sec.
• Thus the accumulation of coming excited atoms
at E3 level increase its population and then
transition occur from E3 to E1 level by emitting out
photons.
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JIT Jahangirabad
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15. Ruby LASER
• Energy Level Diagram:
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16. Advantages of Ruby Lasers:
• From cost point of view, the ruby lasers are
economical.
• Beam diameter of the ruby laser is comparatively
less than CO2 gas lasers.
• Output power of Ruby laser is not as less as in
He-Ne gas lasers.
• Since the ruby is in solid form therefore there is
no chance of wasting material of active medium.
• Construction and function of ruby laser is self
explanatory.
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JIT Jahangirabad
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17. Disadvantages of Ruby Laser
• In ruby lasers no significant stimulated
emission occurs, until at least half of the
ground state electrons have been excited to
the Meta stable state.
• Efficiency of ruby laser is comparatively low.
• Optical cavity of ruby laser is short as
compared to other lasers, which may be
considered a disadvantage.
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18. Applications of ruby Laser
• Due to low output power they are class-I
lasers and so may used as toys for children’s.
• It can be used in schools, colleges, universities
for science programs.
• It can be used as decoration piece & artistic
display.
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19. He-Ne Laser
• He-Ne stands for Helium-Neon. The He-Ne
laser active medium consists of two gases
which do not interact form a molecule.
Therefore He-Ne laser is one type of atomic
gas lasers.
• Construction of He-Ne Lasers
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20. • The construction of typical He-Ne laser plasma tube can be
shown as:
• The tube where the lasing action takes place consists of a
glass envelop with a narrow capillary tube through the
center. The capillary tube is designed to direct the
electrical discharge through its small bore to produce
very high current densities in the gas.
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21. • The output coupler and the HR (high reflective) mirror
are located at the opposite ends of the plasma tube. To
make laser tubes more economical and durable
manufacturers often attach the mirror s directly to the
ends of the capillary tube as shown above. This is very
common with small low power lasers. With high power
tubes or when optically polarized output is desired, the
capillary tubes ends are cut at an angle and sealed with
glass planes called Brewster windows. When this is
done then the mirrors mush be mounted in
mechanically stable but adjustable mounts. This allows
the operator to align the mirror surfaces parallel to
each other but perpendicular to the axis of the
capillary tube.
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22. • The plasma tube has a large cylindrical metallic cathode
and a smaller metallic anode. The current is directed from
cathode to anode.
• In figure shown, the gas reservoir provides a supply of extra
gas. This reservoir helps to maintain a uniform pressure
over long period of time and provides extra gas to replace
any gas that may escape through the tube or through the
seals where the loads pass through the glass envelop.
Usually all He-Ne plasma tubes have a gas reservoir.
• Note that! In He-Ne lasers active medium is low pressure
gas mixture of Helium & Neon gas, which is contained in
the plasma tube. The ratio of He to Ne within the tube vary
from 5:1 to 20:1. Usually this ration average 8:1 can be
considered.
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23. Function of He-Ne Laser
• In the He-Ne laser the light is produced by atomic
transitions within the Neon atom. The Helium
does not directly produce laser light but it acts as
a buffer gas, the purpose of which is to
assist/help the atoms of the other gas to produce
lasing in as manner.
• When energy from the pumping source is applied
He-Ne gas mixture then some of the energy is
absorbed by the Helium atoms. In other words
we can say that helium atoms achieve an excited
state.
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24. • Now when the Helium atoms move within the
laser tube, they collide with the Neon atoms.
At each collision some of the energy within
the helium atom is transferred to the Neon
atom and so raising it to an excited meta-
stable state. When a sufficient number of
Neon atoms reach to this state then
population inversion occurs and hence the
lasing can take place.
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25. • This can be shown by simplified energy level
diagram as:
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26. • Here upward transition shows the absorption of energy
from the pumping source by Helium atom. While down
ward transition shows the emission of energy / light or
lasing present in the Neon atom only.
• In diagram above there are 3 down word energy transitions
for Neon that produce lasing. If transition occurs at the
relatively small energy step from E5 to E4 then low energy
infrared photon is released with a wavelength of 3.391
microns. If transition occurs at E5 to E2 which is much
larger energy step then it produces short wavelength more
energetic photon at 632.8nm. This gives the red light which
is most desirable for He-Ne laser applications.
• E3 to E2 then it produces a laser output at 1.152 microns in
infrared portion of the spectrum.
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27. Characteristics of He-Ne Laser
• The He-Ne laser is a relatively low power device
with an output in the visible red portion of the
spectrum. The most common wavelength
produced by He-Ne lasers is 632.8 nm, although
two lower power (1.152 µm and 3.391 µm)
infrared wavelengths can be produced if desired.
Majority of He-Ne lasers generate less than 10m
watt of power, but some can be obtained
commercially with up to 50m watts of power. For
He-Ne lasers the typical laser tube is from 10 to
100 cm in length and the life time of such a tube
can be as high as 20,000 hours.
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28. Applications / Uses of He-Ne Laser
The Helium-Neon gas laser is one of the most commonly
used laser today because of the following applications.
• He-Ne lasers are produced in large quantities from many
years.
• Many schools / colleges / universities use this type of laser
in their science programs and experiments.
• He-Ne lasers also used in super market checkout counters
to read bar codes and QR codes.
• The He-Ne lasers also used by newspapers for reproducing
transmitted photographs.
• He-Ne lasers can be use as an alignment tool.
• It is also used in Guns for targeting.
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JIT Jahangirabad
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29. Advantages of He-Ne Laser
• He-Ne laser has very good coherence property
• He-Ne laser can produce three wavelengths that are
1.152 µm, 3.391 µm and 632.8 nm, in which the 632.8
nm is most common because it is visible usually in red
color.
• He-Ne laser tube has very small length approximately
from 10 to 100 cm and best life time of 20.000 hours.
• Cost of He-Ne laser is less from most of other lasers.
• Construction of He-Ne laser is also not very complex.
• He-Ne laser provide inherent safety due to low power
output.
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30. Disadvantages of He-Ne Laser
• The weak points of He-Ne laser are
• It is relatively low power device means its output
power is low.
• He-Ne laser is low gain system/ device.
• To obtain single wavelength laser light, the other two
wavelengths of laser need suppression, which is done
by many techniques and devices. So it requires extra
technical skill and increases the cast also.
• High voltage requirement can be considered its
disadvantage.
• Escaping of gas from laser plasma tube is also its
disadvantage.
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