3. LASER
• A laser is an amplifier of light. When the
laser is suitably excited by optical or
electrical energy, the light of the proper
frequency entering the laser cavity is
amplified in such a manner that laser
output wave is in phase with input.
Practical utility of a laser is as an
OSCILLATOR –-- a generator of light. Thus
laser is also known as GENERATOR of
light.
3
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4. 4
LASER ACTION
• Laser action is based on amplification of EM waves by means
of forced or induced atoms or molecules.
• A laser radiation uses three fundamental phenomena when EM
waves interacts with the matter namely
Laser interaction
Spontaneous emission Stimulated emission Spontaneous absorption
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5. Excited atoms emit photons
spontaneously.
When an atom in an excited state falls to a lower energy level, it emits
a photon of light.
Molecules typically remain excited for no longer than a few
nanoseconds. This is often also called fluorescence or, when it takes
longer, phosphorescence.
Energy
Ground level
Excited level
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6. Atoms and molecules can also absorb photons,
making a transition from a lower level to a
more excited one.
This is, of
course,
absorption.
Energy
Ground level
Excited level
Absorption lines in an
otherwise continuous
light spectrum due to a
cold atomic gas in front
of a hot source.
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7. 7
Spontaneous absorption
• Let us consider two energy
level having energy E1 &
E2 resp.
• The atom will remain in
ground state unless some
external stimulant is applied
to it.
• When an EM wave i.e
photon of particular freq fall
on it , there is finite
probability that atom will
jump form energy state E1
to E2.
photon
E1
E2
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8. 8
Spontaneous emission
• Consider an atom in higher
state (E2).
• It can decay to lower energy
level by emitting photon.
• Emitted photon have energy
hv=E2-E1.
• Life time of excited state is
10-9sec.
Photon
hv=E2-E1
E2
E1
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9. 9
Stimulated emission
• There are metastable state
i.e. transition from this state
is not allowed acc to
selection rule.
• There life time is 10-3 sec.
• Atom in this state can’t
jump to lower state at there
own.
• When an photon of suitable
freq arrive it make the atom
in metastable unstable.
• The emitted photon is in
coherence with incident
photon.
Incident photon
Emitted
Photon
coherent
Metastable state(10-3sec)
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10. 10
Stimulated Emission
The stimulated photons have unique
properties:
– In phase with the incident photon
– Same wavelength as the incident photon
– Travel in same direction as incident
photon
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11. Stimulated vs Spontaneous Emission
Stimulated emission requires the presence of a photon. An
“incoming” photon stimulates a molecule in an excited state to
decay to the ground state by emitting a photon. The
stimulated photons travel in the same direction as the
incoming photon.
Spontaneous emission does not require the presence of a
photon.
Instead a molecule in the excited state can relax to the
ground state by spontaneously emitting a photon.
Spontaneously emitted photons are emitted in all directions.
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12. another process, stimulated
emission, can occur.
Before
After
Absorption
Stimulated
emission
Spontaneous
emission
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13. 13
The processes that account for absorption and emission of
radiation and the attainment of thermal equilibrium. The
excited state can return to the lower state spontaneously as
well as by a process stimulated by radiation already present
at the transition frequency.
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16. 16
EINSTEIN’S THEORY OF
RADIATIONS
• Ra=rate of absorption per unit volume
• It depends upon:
1.N1: no. of atom in ground state.
2.ρ(v): energy density per unit freq of
incident wave.
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17. 17
EINSTEIN’S THEORY OF
RADIATIONS
• Rsp=rate of emission per unit
volume.
• It depends upon:
1.N2: no. of atom in exicited state.
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18. 18
EINSTEIN’S THEORY OF
RADIATIONS
• Rst= rate of stimulated emission per unit
volume
• It depends upon:
1.N2: no. of atom in exicited state.
2.ρ(v): energy density per unit freq of
incident wave.
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19. 19
Properties of Laser
• Monochromatic
The light emitted from a laser is
monochromatic, that is, it is of one wavelength
(color). In contrast, ordinary white light is a
combination of many different wavelengths
(colors).
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20. Properties of Laser
• Directional:
• Lasers emit light that is
highly directional. Laser
light is emitted as a
relatively narrow beam in a
specific direction. Ordinary
light, such as coming from
the sun, a light bulb, or a
candle, is emitted in many
directions away from the
source.
20
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21. Properties of Laser
• Coherent
• The light from a
laser is said to be
coherent, which
means the
wavelengths of the
laser light are in
phase in space and
time.
21
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22. 22
Population Inversion
• A state in which a substance has been energized, or
excited to specific energy levels.
• More atoms or molecules are in a higher excited state.
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23. Population Inversion
• The process of producing a population
inversion is called pumping.
• Examples:
→by lamps of appropriate intensity
→by electrical discharge
23
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24. Achieving inversion:
Pumping the laser medium
Now let I be the intensity of (flash lamp) light used to pump energy
into the laser medium:
R = 100% R < 100%
I0 I1
I2
I3 Laser medium
I
Will this intensity be sufficient to achieve inversion, N2 > N1?
It’ll depend on the laser medium’s energy level system.
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25. In what energy levels do molecules reside?
Boltzmann population factors
Ni is the
number
density of
molecules in
state i (i.e.,
the number
of molecules
per cm3).
T is the
temperature,
and kB is
Boltzmann’s
constant.
exp /
i i B
N E k T
Energy
Population density
N1
N3
N2
E3
E1
E2
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26. Boltzmann Population Factors
In equilibrium, the ratio of the populations of
two states is:
N2 / N1 = exp(–DE/kBT ), where DE =
E2 – E1 = hn
In the absence of collisions,
molecules tend to remain
in the lowest energy state
available.
Collisions can knock a mole-
cule into a higher-energy state.
The higher the temperature,
the more this happens.
2
2
1 1
exp /
exp /
B
B
E k T
N
N E k T
Low T High T
Energy
Molecules
Energy
Molecules
3
2
1
2
1
3
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36. 36
Pump Source
• A pump is basic energy source for a laser. It gives
energy to various atoms of laser medium & excites
them . So that population inversion can take place &
it is maintained with time. The excitation of atom
occur directly or through atom or atom collision.
• There is various type of pump depending upon
nature of medium .Examples: electric discharges,
flashlamps, arc lamps and chemical reactions.
• The type of pump source used depends on the gain
medium.
→A helium-neon (HeNe) laser uses an
electrical discharge in the helium-neon gas
mixture.
→Excimer lasers use a chemical reaction.
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37. 37
Gain Medium
• When energy is given to laser medium a
small fraction of medium shows lasing action.
This part of laser medium is called Active
centers. For examples in ruby laser Cr+++ is
active center, in He-Ne laser Ne are active
centers.
• It is the Major determining factor of the
wavelength of operation of the laser.
• Excited by the pump source to produce a
population inversion.
• Where spontaneous and stimulated emission
of photons takes place.
• Example:
solid, liquid, gas and semiconductor.
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38. Optical Resonator
• It is an set up used to obtain amplification of
stimulated photons, by oscillating them back &
forth between two extreme limits. Consist of:
1.Two plane or concave mirrors placed co-axially.
2.One mirror is reflecting & other is partially
reflecting.
38
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39. 39
Optical Resonator
• Two parallel mirrors placed around the gain
medium.
• Light is reflected by the mirrors back into the
medium and is amplified .
• The design and alignment of the mirrors with
respect to the medium is crucial.
• Spinning mirrors, modulators, filters and
absorbers may be added to produce a variety of
effects on the laser output.
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40. to a chain reaction and laser
emission.
Excited medium
If a medium has many excited molecules, one photon can become many.
This is the essence of the laser. The factor by which an input beam is
amplified by a medium is called the gain and is represented by G.
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43. 43
Four-level Laser System
• Laser transition takes
place between the
third and second
excited states.
• Rapid depopulation of
the lower laser level.
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44. 44
FOUR LEVEL LASER:
• STEP 1- PUMPING: atoms are excited to
higher energy level by providing energy
from ext. source.
• STEP 2- POPULATION INVERSION:
atom via radiation less decay, decays to
metastable state and hence population
inversion take place.
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45. 45
FOUR LEVEL LASER:
• STEP 3- LASER ACTION: atom from
metastable state decays to lower state by
stimulated emission and hence laser
action take place.
• STEP 4- BACK TO GROUND STATE:
atom from excited state decays to lower
state by spontaneous emission.
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47. 47
Three-level Laser System
• Initially excited to a
short-lived high-energy
state .
• Then quickly decay to
the intermediate
metastable level.
• Population inversion is
created between lower
ground state and a
higher-energy
metastable state.
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49. 49
Two-level Laser System
• Unimaginable
as absorption and stimulated processes
neutralize one another.
• The material becomes transparent.
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50. Two-Level System
En, Nn
Em, Nm
En, Nn
Em, Nm
Even with very a intense pump source, the best one can achieve with a two-
level system is
excited state population = ground state population
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51. Usually, additional losses in intensity occur, such as absorption, scat-tering,
and reflections. In general, the laser will lase if, in a round trip:
Gain > Loss This called achieving Threshold.
The Laser
A laser is a medium that stores energy, surrounded by two mirrors.
A partially reflecting output mirror lets some light out.
A laser will lase if the beam increases in intensity during a round trip:
that is, if
3 0
I I
R = 100% R < 100%
I0 I1
I2
I3 Laser medium
with gain, G
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52. 52
Laser Types
• According to the active material:
solid-state, liquid, gas, excimer or
semiconductor lasers.
• According to the wavelength:
infra-red, visible, ultra-violet (UV) or x-ray
lasers.
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53. 53
Laser Types
• According to the nature of pumping.
flash type, chemical pumping & electric
discharge lasers
• According to the nature of output:
pulsed & continuous wave lasers.
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55. Continuous vs Pulsed Lasers
Pump Source : Excitation of the lasing atoms or molecules by
an external source of light (such as a lamp) or another laser
The output of the laser light can be a continuous wave (cw) if
the pumping is continuous or pulsed if the pumping is pulsed.
Pulsed lasers have very high peak intensities because the
laser intensity is concentrated in a very short time duration.
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56. 56
Solid-state Laser
• Example: Ruby Laser
• Operation wavelength: 694.3 nm (IR)
• 3 level system: absorbs green/blue
•Gain Medium: crystal of aluminum oxide (Al2O3)
with small part of atoms of aluminum is replaced
with Cr3+ ions.
•Pump source: flash lamp
•The ends of ruby rod serve as laser mirrors.
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57. 57
How a laser works?
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59. 59
1. High-voltage electricity causes the
quartz flash tube to emit an intense
burst of light, exciting some of Cr3+
in the ruby crystal to higher energy
levels.
2. At a specific energy level, some
Cr3+ emit photons. At first the photons
are emitted in all directions. Photons
from one Cr3+ stimulate emission
of photons from other Cr3+ and the
light intensity is rapidly amplified.
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60. 60
3. Mirrors at each end reflect the
photons back and forth, continuing
this process of stimulated emission
and amplification.
4. The photons leave through the
partially silvered mirror at one
end. This is laser light.
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62. 62
• As the flash lamp stop operting, the
population of the upper level decreases
very rapidally & lasing action stops till the
further operation of next flash. As the
production of laser beam depends upon
the operation of flash lamp the ruby laser
is pulsed type laser.
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63. 63
• During the period of operation of two
flash laser output is oscillating & output is
highly irregular function of time, shows
random fluctuations in the amplitude. This
type of output is called as laser SPIKING.
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64. He-NE LASER
• A helium-neon laser, usually called a HeNe
laser, is a type of small gas laser. HeNe lasers
have many industrial and scientific uses, and are
often used in laboratory demonstrations of
optics. Its usual operation wavelength is 632.8
nm, in the red portion of the visible spectrum
64
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65. He-Ne laser
He-Ne lasers are
normally small, with cavity
lengths of around 15 cm up
to 0.5 m.
The optical cavity of the
laser typically consists of a
plane, high-reflecting
mirror at one end of the
laser tube, and a concave
output coupler mirror of
approximately 1%
transmission at the other
end.
Electric discharge
pumping is used.
Optical output powers
ranging from 1 mW to 100
mW. 65
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67. CARBON
DIOXIDE
Carbon dioxide lasers are
the highest-power
continuous wave lasers that
are currently available.
They are also quite
efficient: the ratio of output
power to pump power can
be as large as 20%.
The CO2 laser produces a
beam of infrared light with
the principal wavelength
bands centering around 9.4
and 10.6 micrometers.
67
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69. Semiconductor
laser
Lasers which use semiconductor as
active medium. The majority of
semiconductor materials are based
on a combination of elements in
the third group of the Periodic
Table (such as Al, Ga, In) and the
fifth group (such as N, P, As, Sb)
hence referred to as the III-V
compounds.
69
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70. P- and N-type Semiconductors
• In the compound GaAs, each gallium atom has three electrons in its
outermost shell of electrons and each arsenic atom has five. When a trace
of an impurity element with two outer electrons, such as 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.
• When a trace of an impurity element with six outer electrons, such as
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.
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71. 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
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73. 73
Components of LASER
1. PUMP.
2. ACTIVE MEDIUM.
3. OPTICAL RESONATOR.
A pump is basic energy source for a laser. It gives
energy to various atoms of laser medium & excites
them . So that population inversion can take place & it
is maintained with time. The excitation of atomoccur
directly or through atom or atom collision.
There is various type of pump depending upon nature
of medium
When energy is given to laser medium a small
fraction of medium shows lasing action. This
part of laser medium is called Active centers.
For examples in ruby laser Cr+++ is active center,
in He-Ne laser Ne are active centers.
It is an set up used to obtain amplification of stimulated photons,
by oscillating them back & forth between two extreme limits.
Consist of:
1. Two plane or concave mirrors placed co-axially.
2. One mirror is reflecting & other is partially reflecting.
11jan 2009
Made by Mrs MANDEEP KAUR,
NWIET, DHUDIKE
