Light amplification by stimulated emission of radiation (LASER).

1. Laser

3. LASER
• LL ight
• AA mplification by
• SS timulated
• EE mission of
• RR adiation

4. Properties of laser

5. Monochromatic

6. Directionality

7. Coherence

8. Spatial and
Temporal
Coherence
Beams can be
coherent or
only partially
coherent (indeed,
even incoherent)
in both space and
time.
Spatial and
Temporal
Coherence:
Temporal
Coherence;
Spatial
Incoherence
Spatial
Coherence;
Temporal
Incoherence
Spatial and
Temporal
Incoherence

9. The temporal coherence time is the time the wave-fronts remain
equally spaced. That is, the field remains sinusoidal with one
wavelength:
Temporal
Coherence
Time, τc
Coherent time

10. Coherent length
coherence length is the propagation distance from
a coherent source to a point where a wave (e.g.
an electromagnetic wave) maintains a specified degree of
coherence. Within this distance, the wave is most similar to a
perfect sinusoidal wave. The significance is that wave
interference will be strong within a coherence length of the
source
Spatial
Coherence
Length
cL τ= λ
λ
∆
=
2
L

11. Incandescent vs. Laser Light
1. Many wavelengths
2. Multidirectional
3. Incoherent
1. Monochromatic
2. Directional
3. Coherent

12. BASIC PRINCIPLE NEEDED
FOR LASER

13. Stimulated Absorption
• Energy is absorbed by an atom, the electrons are
excited into vacant energy shells.

14. Absorption
E1
E2

15. Spontaneous Emission
• The atom decays from level 2 to level 1 through the
emission of a photon with the energy hv. It is a
completely random process.

16. Spontaneous Emission

17. Stimulated Emission
atoms in an upper energy level can be triggered or
stimulated in phase by an incoming photon of a specific
energy.

18. Stimulated Emission

19. 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

20. Electron/Photon Interactions

21. WHY WE NEED META STABLE
STATE?
ANSWER IS
With having the metastable state above the
ground level. Atom reaches the meta stable
state (after first stimulated emission) can
remain there for longer time period. So the
number of atom increases in the meta stable
state. And when these atoms come back to the
original ground level it emits laser beam.

22. • A state of a medium where a higher-lying electronic level has
a higher population than a lower-lying level
POPULATION INVERSION

23. • The method particle of raising a particle from lower
energy state to higher energy state is called pumping.
• TYPES OF PUMPING :
1. Optical pumping
2. Electrical pumping
3. X-ray pumping
4. Chemical pumping
PUMPING

24. All lasers have 3 essential components:
• A lasing or "gain" medium
• A source of energy to excite electrons in the gain medium
to high energy states, referred to as "pump" energy
• An optical path which allows emitted photons to oscillate
and interfere constructively as energy is added or
"pumped" into the system, ie, a resonator
LASER COMPONENTS

27. LASER ACTION

28. Types of Laser
a.According to their sources:
1.Gas Lasers
2.Crystal Lasers
3.Semiconductors Lasers
4.Liquid Lasers
a.According to the nature of emission:
1.Continuous Wave
2.Pulsed Laser
a.According to their wavelength:
1.Visible Region
2.Infrared Region
3.Ultraviolet Region
4.Microwave Region
•X-Ray Region
d. According to different levels
1. 2-level laser
2. 3-level laser
3. 4-level laser
e. According to mode of pumping
1. optical
2. chemical
3. electric discharge
4. electrical

29. 2- Level Laser
νh
E1
E2
Absorption
E1
E2
νh
Spontaneous
Emission
E1
E2νhνh νh
Stimulated
Emission

30. THREE STEP LASER
• Stimulated absorption
• Spontaneous emission to the meta stable
state
• Stimulated emission from meta stable state
to ground state. E2
E1
E0
E2 – E1
E1 – E0
META STABLE STATE

31. 4-Level LASER

32. PRACTICAL LASERS

33. RUBY LASER
Construction

34. ENERGY LEVEL IN RUBY
LASER
Energy
4
A2
4T
2
4T
1
2T
2
2
E
LASING
rapid decay
Cr+
Al2O3

35. LASER ACTION IN RUBY
LASER

36. HE-NE LASER
Construction

37. .
Energy Level Diagram of He-Ne

38. Energy Level Diagram of He-Ne cont…

39. Combined Laser Action

40. Emission wavelengths of common
lasers

41. APPLICATION S OF LASER

42. 04/09/15
Not to be Taken Lightly
The Weighty Implications of Laser Technology
Applications of
Laser
Technology
Medical
Entertainment
Telecommunications
Military
• Optical Surgery
• General Surgery
• Tattoo removal
• CD Players
• DVD Players
• Video Game Systems
• Information tech.
• Holograms
• Weapons
• Satellites
• Radar
Industry

43. 04/09/15
Can You See the Light?
Dentists use
laser drills
Bad eyesight can be
corrected by optical
surgery using lasers
CD-Audio is
read by a laser
Tattoo removal is
done using lasers
Cd-Rom discs
are read by lasers
Laser pointers can
enhance
presentations Bar codes in
grocery stores are
scanned by lasers
Video game systems such as
PlayStation 2 utilize lasers
DVD players read
DVD’s using lasers
Airplanes are
equipped with
laser radar
Military and Space
aircraft are equipped
with laser guns
Laser tech. is used in printers,
copiers, and scanners

44. Einstein’s Coefficients
Einstein gives a probability that stimulated
emission is same as absorption.
Means that if a stimulated absorption can
occur then there is same probability that
stimulated emission can occur.

45. Equilibrium condition
In case of energy states the number of electron
absorbed and emitted should be equal or the
rate of change of numbers of atoms in two
states should be equal.
0=
dt
dN

46. Planck’s Radiation Law
( )
1
18
3
3
−
=
kT
h
e
c
h
E ν
νπ
ν
Plank’s gives the formula that how
a gas radiate energy.

48. Spontaneous emission
A21 :- correspond to spontaneous emission
probability per unit time
This particular emission can occur
without the presence of external field
E(v)

49. Stimulated Absorption
B12 :- correspond to stimulated absorption
probability per unit time
This type of absorption can occur in
presence of external field E(v) only

50. Stimulated Emission
B21 :- correspond to stimulated emission
probability per unit time
This type of emission can occur in presence of
external field E(v) only

51. Total Emission Probability
Spontaneous Emission + Stimulated Emission
A21 + B21 E(v)
Number of atoms that can jump from level E2 to E1 is
( )[ ] 22121 NEBA ν+

52. Total Absorption Probability
( )[ ] 112 NEB ν

53. The rate of change of atoms in E2
It can be given by differentiation
(probability)
or
emissionAbsorption
dt
dN
−=2
( )[ ] ( )[ ] 22121112
2
NEBANEB
dt
dN
νν +−=

54. At Equilibrium
Then
02
=
dt
dN
( ) ( )[ ] 22121112 NEBANEB νν +=

55. Emission and absorption are same
( )








−





=
1
1
21
12
2
121
21
B
B
N
NB
A
E ν

56. Maxwell Bolzman Distribution
KT
E
eNN
1
01
−
=
KT
E
eNN
2
02
−
=
In thermal equilibrium

57. So the equations become
kT
h
e
N
N
ν
=
2
1
So equation becomes
( )
1
1
21
1221
21
−





=
B
B
e
B
A
E
kT
hν
ν

58. After comparing with Planks
Radiation Law
3
3
8
21
21
c
h
B
A νπ
=
And
1
21
12
=
B
B
Units of Einstein
coefficient: Joules-
Sec/m3

59. Conclusions
• Stimulated emission have same probability as
stimulated absorption
• Ratio between spontaneous and stimulated
emission varies with v3
• All we need is to calculate one of the
probability to find others.