The document discusses the history and theory of lasers. It begins by explaining that a laser is an optical amplifier based on stimulated emission of radiation, as proposed by Einstein in 1917. The first laser was built in 1960 by Maiman using a ruby crystal. Key aspects discussed include: - Laser operation requires a population inversion between energy levels. - Common laser types include ruby, He-Ne, and semiconductor lasers. - Semiconductor lasers use the stimulated emission from a p-n junction. - Holograms were first made possible by the invention of the laser as a coherent light source. Applications of holography include credit cards, medical imaging, and art.

1. LASER

2. 1 IGHT
MPLIFICATION BY
TIIMULATED
MISSION OF
ADIATION

3. LASER AND ITS APPLICATIONS
1) LASER
2) APLLICATION OF LASER
3) EINSTEIN COEFFICIENT
4) COMPONENTS OF LASER
5) RUBY LASER
6) HE-NE LASER
7) SEMICONDUCTOR LASER

4. 1.Introduction (Brief history of laser)
The laser is perhaps the most important optical
device to be developed in the past 50 years. Since its
arrival in the 1960s, rather quiet and unheralded
outside the scientific community, it has provided the
stimulus to make optics one of the most rapidly
growing fields in science and technology today.

5. The laser is essentially an optical amplifier. The word
laser is an acronym that stands for “light amplification by
the stimulated emission of radiation”. The theoretical
background of laser action as the basis for an optical
amplifier was made possible by Albert Einstein, as early
as 1917, when he first predicted the existence of a new
irradiative process called “stimulated emission”. His
theoretical work, however, remained largely unexploited
until 1954, when C.H. Townes and Co-workers developed
a microwave amplifier based on stimulated emission
radiation. It was called a maser.

6. Following the birth of the ruby and He-Ne lasers, others devices followed
in rapid succession, each with a different laser medium and a different
wavelength emission. For the greater part of the 1960s, the laser was
viewed by the world of industry and technology as scientific curiosity.
In 1960, T.H.Maiman built the first laser device (ruby
laser). Within months of the arrival of Maiman’s ruby laser,
which emitted deep red light at a wavelength of 694.3 nm,
A. Javan and associates developed the first gas laser (He-
Ne laser), which emitted light in both the infrared (at
1.15mm) and visible (at 632.8 nm) spectral regions..

7. THE OPERATION OF LASER
In 1958, Charles Townes and Arthur Schawlow theorized
about a visible laser, an invention that would use infrared
and/or visible spectrum light.
Light Amplification by Stimulated Emission of Radiation-
(LASER).

8. THE OPERATION OF THE LASER
1E
2E
3E
4E

9. ABSORPTION
1E
2E
3E
E4

10. SPONTANEOUS EMISSION
1E
2E
3E
E4

11. iE
fE
MirrorMirror
Stimulated emission

12. EINSTEIN’S COEFFICIENT
In 1916, according to Einstein, the interaction of
radiation with matter could be explained in terms of
three basic processes: spontaneous emission,
absorption and stimulated emission. The three
processes are illustrated and discussed in the
following:

13. BEFORE AFTER
(i) Stimulated absorption
ii) Spontaneous emission)
(iii) Stimulated emission

14. )II) SPONTANEOUS EMISSION
Consider an atom (or molecule) of the material is existed
initially in an excited state E2 No external radiation is
required to initiate the emission. Since E2>E1, the atom will
tend to spontaneously decay to the ground state E1, a
photon of energy h =E2-E1 is released in a random
direction as shown in (Fig. 1-ii). This process is called
“spontaneous emission ”
Note that; when the release energy difference (E2-E1) is
delivered in the form of an e.m wave, the process called
"radiative emission" which is one of the two possible ways
“non-radiative” decay is occurred when the energy
difference (E2-E1) is delivered in some form other than e.m
radiation (e.g. it may transfer to kinetic energy of the
surrounding)

15. (III) STIMULATED EMISSION
Quite by contrast “stimulated emission” (Fig. 1-iii)
requires the presence of external radiation when an incident
photon of energy h =E2-E1 passes by an atom in an excited
state E2, it stimulates the atom to drop or decay to the lower
state E1. In this process, the atom releases a photon of the
same energy, direction, phase and polarization as that of the
photon passing by, the net effect is two identical photons
(2h) in the place of one, or an increase in the intensity of
the incident beam. It is precisely this processes of stimulated
emission that makes possible the amplification of light in
lasers.

16. GROWTH OF LASER BEAM
Atoms exist most of the time in one of a number of
certain characteristic energy levels. The energy level or
energy state of an atom is a result of the energy level of the
individual electrons of that particular atom. In any group
of atoms, thermal motion or agitation causes a constant
motion of the atoms between low and high energy levels. In
the absence of any applied electromagnetic radiation the
distribution of the atoms in their various allowed states is
governed by Boltzman’s law which states that:
The theory of lasing

17. if an assemblage of atoms is in state of thermal
equilibrium at an absolute temp. T, the number of
atoms N2 in one energy level E2 is related to the
number N1 in another energy level E1 by the
equation.
Where E2>E1 clearly N2<N1
K Boltzmann’s constant = 1.38x10-16 erg / degree
= 1.38x10-23 j/K
T the absolute temp. in degrees Kelvin
KTEE
eNN /)(
12
12 


18. At absolute zero all atoms will be in the ground state.
There is such a lack of thermal motion among the electrons
that there are no atoms in higher energy levels. As the
temperature increases atoms change randomly from low to
the height energy states and back again. The atoms are
raised to high energy states by chance electron collision
and they return to the low energy state by their natural
tendency to seek the lowest energy level. When they return
to the lower energy state electromagnetic radiation is
emitted. This is spontaneous emission of radiation and
because of its random nature, it is incoherent

19. As indicated by the equation, the number of atoms decreases
as the energy level increases. As the temp increases, more atoms
will attain higher energy levels. However, the lower energy levels
will be still more populated.
Einstein in 1917 first introduced the concept of stimulated
or induced emission of radiation by atomic systems. He showed
that in order to describe completely the interaction of matter
and radiative, it is necessary to include that process in which an
excited atom may be induced by the presence of radiation emit a
photon and decay to lower energy state.

20. An atom in level E2 can decay to level E1 by emission of photon.
Let us call A21 the transition probability per unit time for
spontaneous emission from level E2 to level E1. Then the number
of spontaneous decays per second is N2A21, i.e. the number of
spontaneous decays per second=N2A21.
In addition to these spontaneous transitions, there will induced
or stimulated transitions. The total rate to these induced
transitions between level 2 and level 1 is proportional to the
density (U) of radiation of frequency , where
 = ( E2-E1 )/h , h Planck's const.

21. Let B21 and B12 denote the proportionality constants for
stimulated emission and absorption. Then number of
stimulated downward transition in stimulated emission per
second = N2 B21 U
similarly , the number of stimulated upward transitions per
second = N1 B12 U
The proportionality constants A and B are known as the
Einstein A and B coefficients. Under equilibrium conditions
we have

22. by solving for U (density of the radiation) we
obtain
U [N1 B12- N2 B21 ] = A21 N2
212121
212
)(
BNBN
AN
U

 
N2 A21 + N2 B21 U =N1
B12 U
SP ST
A b

23. 







1
)(
2
1
21
12
21
21
N
N
B
B
B
A
U 
KThKTEE
ee
N
N //)(
1
2 12 









1
)(
/
21
12
21
21
KTh
e
B
B
B
A
U


According to Planck’s formula of
radiation
 1
18
)( /3
3

 KTh
ec
h
U 


)2)
)1)

24. from equations 1 and 2 we have
B12=B21 (3)
213
3
21
8
B
c
h
A


equation 3 and 4 are Einstein’s
relations. Thus for atoms in
equilibrium with thermal radiation.
)4 (
21
21
212
212 )()(
tan A
UB
AN
UBN
emissioneousspon
emissionstimulate 

from equation 2 and 4

25.  1
18
8
)(
8.
.
/3
3
3
3
3
3



KTh
ec
h
h
c
U
h
c
emissionspon
emissionstim





 1
1
.
.
/

 KTh
eemissionspon
emissionstim

Accordingly, the rate of induced emission is extremely
small in the visible region of the spectrum with ordinary
optical sources ( T10 3 K (.

26. If n1 > n2
radiation is mostly absorbed
spontaneous radiation dominates.
• most atoms occupy level E2, weak absorption
• stimulated emission prevails
• light is amplified
if n2 >> n1 - population inversion
Necessary condition:
population inversion
E1
E2
Condition for the laser operation

27. Three level laser
The laser operation
E1
E3
E2
Fast transition
Laser action
• 13 pumping
• spontaneous emission 3 2.
• state 2 is a metastable state
• population inversion between states 2 and 1.
• stimulated emission between 2 i 1.

28. AMPLIFICATION IN A MEDIUM
Consider an optical medium through which
radiation is passing. Suppose that the medium contains atoms in
various energy levels E1, E2, E3,….let us fit our attention to two
levels E1& E2 where E2>E1 we have already seen that the rate of
stimulated emission and absorption involving these two levels are
proportional to N2B21&N1B12 respectively. Since B21=B12, the rate
of stimulated downward transitions will exceed that of the
upward transitions when N2>N1,.i.e the population of the upper
state is greater than that of the lower state such a condition is
contrary to the thermal equilibrium distribution given by
Boltzmann’s low. It is termed a population inversion. If a
population inversion exist, then a light beam will increase in
intensity i.e. it will be amplified as it passes through the medium.
This is because the gain due to the induced emission exceeds the
loss due to absorption.

29. x
o eII 
 ,
gives the rate of growth of the beam intensity in the direction
of propagation,an is the gain constant at frequency

30. Two-, three-, and four-level systems
Two-level
system
Laser
Transitio
n
Pump
Transitio
n
At best, you get
equal populations.
No lasing.
It took laser physicists a while to realize that four-level systems are
best.
Four-level
system
Lasing is easy!
Laser
Transition
Pump
Transitio
n
Fast decay
Fast
decay
Three-level
system
If you hit it hard,
you get lasing.
Laser
Transitio
n
Pump
Transition
Fast decay

31. THE HELIUM-
NEON LASER
Energetic electrons in a
glow discharge collide with
and excite He atoms, which
then collide with and
transfer the excitation to
Ne atoms, an ideal 4-level
system.

32. Ruby laser
• discovered in 60-ies of the XX century.
• ruby (Al2O3) monocrystal, Cr doped.

33. • Lasing from the Cr3+.
• three level laser
Energy
4A2
4T
2
4T
1
2T
2
2E
LASING
• optical pumping: 510-600nm
and 360-450nm.
• fast transition on 2E.
• lasing: 2E on 4A2,
•694nm
rapid decay
Ruby laser
Al2O3
Cr+

34. RUBY LASER
First laser: Ted Maiman
Hughes Research Labs
1960

35. SEMICONDUCTOR LASERS
Laser diode is similar in principle to an LED.
What added geometry does a Laser diode require?
An optical cavity that will facilitate feedback in order to
generate stimulated emission.
Fundamental Laser diode:
•Edge emitting LED. Edge emission is suitable for adaptation
to feedback waveguide.
•Polish the sides of the structure that is radiating.
•Introduce a reflecting mechanism in order to return radiation
to the active region.
•

36. LASER DIODES
Polishing of the emitting sides of the cavity. A considerable
percentage of the radiation is reflected back alone from the
difference in reflective indexes of the air-AlGaAs interface.
Therefore mirror coating not necessary.
.

37. L
Electrode
Current
GaAs
GaAsn+
p+
Cleaved surface mirror
Electrode
Active region
(stimulated emission region)
A schematic illustration of a GaAs homojunction laser
diode. The cleaved surfaces act as reflecting mirrors.
L
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

38. p+ n+
EFn
(a)
Eg
Ev
Ec
Ev
Holes in V B
Electrons in C B
Junction
Electrons
Ec
p+
Eg
V
n+
(b)
EFn
eV
EFp
The energy band diagram of a degenerately doped p-n with no bias. (b) Band
diagram with a sufficiently large forward bias to cause population inversion and
hence stimulated emission.
Inversion
region
EFp
Ec
Ec
eVo
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

39. HOLOGRAM
Hologram is from the Greek word holos, meaning whole and
gramma meaning message.

40. HOLOGRAM HISTORY
Theory Developed in 1947 by British/Hungarian scientist
Dennis Gabor
Developed because he was trying to improve the resolution of
electron microscope
Development in this field was stifled during the 1950’s
because light sources were not coherent

41. HISTORY
In 1962 Emmett Leith and Juris Upatnieks realized that
holography could be used as a 3-D visual medium
From their work, they used a laser to create the first hologram
in history, that of a toy train and bird
This type of hologram required laser light to be viewed,
though.

43. RECORDING OF TYPES OF HOLOGRAMS

44. APPLICATIONS OF HOLOGRAPHY
Design of containers to hold
nuclear materials
Credit cards carry monetary
value
Supermarket scanners
Optical Computers
Improve design of aircraft
wings and turbine blades
Used in aircraft “heads-up
display”
Art
Archival Recording of fragile
museum artifacts

45. HOLOGRAPHY IN THE FUTURE
Medical Purposes
Gaming Systems
Personal Defense
Computers
Artwork
Amusement Park Rides