The document provides an overview of lasers, including: 1) It defines what a laser is and describes the three main processes that occur in lasers: absorption, spontaneous emission, and stimulated emission. 2) It explains the key components of lasers - the active medium, pumping mechanism, and optical resonator. 3) It provides examples of different types of lasers, including ruby, He-Ne, and semiconductor lasers, and describes their workings. 4) It discusses applications of lasers in various fields such as industry, medicine, communications, defense, and more.

1. LASER
G. S. Gawande College, Umarkhed
Dist. Yavatmal
By
Dr. Praful D. Shirbhate
Assistant Professor
DEPARTMENT OF PHYSICS

2. SYLLABUS
Introduction & Quantum Transitions: Absorption,
Spontaneous Emission & Stimulated Emission,
Metastable State, Population Inversion
Pumping Schemes & LASER Properties,
Spatial and temporal coherence of a light wave
Principle & working of He-Ne LASER
Ruby Laser
Semiconductor laser & applications

3. LASER
L:LIGHT
A:AMPLIFICATION by
S:STIMULATED
E:EMISSION of
R:RADIATION

4. • Coherent
• Less Divergence
• High Intensity
• Monochromatic
Difference between Ordinary
& laser light
Ordinary light Laser light
• Incoherent
• High Divergence
• Low Intensity
• Polychromatic

5. Characteristics of Laser
Monochromaticity
Coherence
 Temporal coherence
 Spatial coherence
Intensity
Unidirectionaly
Divergence

6. Coherence length (lcoh ) : The length of the
wavetrain upto which it is perfectly sinusoidal .
lcoh = c.tcoh = c 
 Coherence time(tcoh) : The time for which the
wave train is perfectly sinusoidal.
 Since  = Δt = 1/ Δ‫טּ‬
lcoh = c  = c. ∆t = c / ∆ν,
 We know , ν = c / λ
∴ ∆ν = - c . ∆ λ / λ2 , ignoring minus sign
lcoh = λ2 / ∆ λ , Where Δ λ Bandwidth

7. Three distinct processes can take place.
i) Absorption
ii) Spontaneous emission
iii) Stimulated emission
Quantum processes in Laser:

8. Absorption and Emission
Atom

9. 1. Absorption
 Energy of photon h=E2-E1 ‘ absorb by atoms in
the lower energy states and excites to higher
energy states.
Einstein Equation for absorption
Nab = B12N1ρ(υ)Δt
 A + h  A*

10. Energy
Absorption
Ground State
LASER
Excited State

11. The process of emission of photons by an excited
atoms by its own , without the influence of external
agent is called spontaneous emission.
A* A + h  Spontaneous Emission
Nsp = A21N2Δt , A* A + h (photon)
1. Spontaneous Emission

12. Energy
photon
Spontaneous Emission
Ground State
LASER
Excited State

13. Energy
photon
photon
photon
photon
photon photon
Spontaneous Emission
Ground State
LASER

14. The process of emission of photons by an excited
atom through a forced or triggered transition
A* + h = A + 2 h
Nst = B21 N2 ρ(υ) Δt
3. Stimulated Emission

15. Energy
photon
Stimulated Emission
Ground
State
Metastable
State
photonphoton photon photon
photon
LASER

16. Comparison between Spontaneous and
Stimulated Emission
Spontaneous Emission Stimulated Emission
1. Spontaneous emission is a random
and probabilistic process.
1. Not a random process.
2. The process is not controllable from
outside
2. Controllable from outside
3. The resultant light is not
monochromatic
3. Highly monochromatic
4. Light emitted through this process is
incoherent
4. Highly coherent
The net intensity is proportional to the
number of radiating atoms, thus
I total = NI
Where N – no. of atoms
I - Intensity of light emitted by one
photon.
5. The net intensity is proportional to
the square of number of radiating
atoms, thus
Itotal = N2I
Where N – no. of atoms, I -
Intensity of light emitted by one
photon

17. 1) Condition for Stimulated emission to dominate
Spontaneous Emission
2) Condition for stimulated emission to dominate
absorption transitions
Condition for Light Amplification
This condition indicates that Stimulated
transition will overwhelm the absorption process
if N2 is greater than N1. The system must
achieve the state of population inversion.

18. Population Inversion
 In thermal equilibrium state, N 1 >> N2 which is
governed by Boltzman’s equation
N2 / N1= e- ΔE/kT
 Population inversion is a condition in which
population of upper energy level N2 far exceeds
the population of lower energy level N1
i.e. N2 >> N1.
N1
N 2
N1
N2
Normal State N 2 << N1
Thermal Equilibrium State
Inverted State N 2 >> N1
Population inversion State

19. Metastable State
Metastable state can be defined as a state where
excited atom can remain for longer time than the
normal excited state.
Atoms stay in metastable states for about 10-6 to
10-3s. This is 103 to 106 times longer than the
time of stay of atom at excited levels.
If the metastable states do not exist, there could
be no population inversion, no stimulated
emission and hence no laser operation.

20. Components of Laser
The essential components of Laser are
 An active medium
 A pumping agent
 Optical Resonator

21. An active medium
A medium in which light gets amplified is called
an active medium.
The medium may be solid , liquid or gas.
Active centres are those atoms which are
responsible for stimulated emission.

22. Pumping
 The process of supplying energy to the medium with
a view to transfer it into the state of population
inversion is known as pumping.
 Techniques to achieve the state of population
inversion:
 Optical pumping (used in Ruby Laser)
 Electric discharge (used in He-Ne Laser)
 Direct Conversion (used in Semiconductor
Laser)

23. Fabry - Perot optical resonator
(Resonant cavity)
Optical resonator consist of two opposing plane
parallel mirrors, with an active material placed in
between them.
One of the mirror is semitransparent while the
other is 100 % reflecting.
The mirrors are set normal to optic axis of the
material.
100 %
reflecting
mirror
Semi-
transparent
mirrorActive
medium
Optic axis

24. Action of Optical Resonator

25. Condition for Steady State Oscillation
 For waves making a complete round trip inside
the resonator, phase delay must be some
multiple of 2.
2L = m  ; ( m = 1,2,3,…)


2
m
L 
 Length L of the optical resonator should
accommodate an integral number of standing half
waves.

26. Pumping Schemes
Pumping Schemes are Classified as:
 Two-level
 Three-level and
 Four –level schemes.
• Two-level scheme will not lead to laser action.
• Three-level and four-level schemes are
important and widely employed.

27. Pumping radiation excites the ground state atoms.
Induces transitions from the upper level to the
lower level.
Hence, population inversion cannot be attained in
a two-level pumping scheme.
E2
E1
Energy
Two Level Pumping Scheme

28. E
E
Laser transition
Absorption band
Non-radiative
transition
Metastable
state
Ground
state
E3
E2
E1
Major disadvantage of a three level scheme 
Efficiency is less
Output  Pulsating beam
Three Level Pumping Scheme

29. Stimulated
Emission of
Radiation
E4
E3
E2
E1
Ground State
Non radiative
transition
Natural
depletion
Pumping
Four level lasers are more efficient.
Four level lasers can operate in a Continuous Wave
mode.
Four Level Pumping Scheme

30. Types of LASER
 Solid-state lasers – Ruby laser, “Nd:YAG“,
Nd:Glass, lasers etc
 Gas lasers - He-Ne, He-Cd, CO2, N2 lasers
etc
 Semiconductor lasers ( diode lasers)-These
electronic devices are generally very small and
use low power. GaAs, GaAsP

31. H. V.
Power
Supply
Coolant
Partially
silvered
Mirror
Fully
Silvered
Mirror
Inlet Outlet
4 cm
0.5
cm
A helical
Photographic flash
lamp filled with
Xenon
RUBY LASER
 Ruby Laser is a first solid state laser developed
in 1960 by T.H. Maiman
 Ruby laser rod; a synthetic Ruby crystal of
Al2O3 doped with 0.05% Cr3+ ions.

32. Stimulated
emission
Metastable
state
Energy level Diagram (Ruby Laser)
Cr3+ atom absorb
green and blue bands
of wave length from
xenon flash lamp &
excited to E3 & E3’
respectively
Radiative transitions
from E2 to E1 emits 
Red photon with peak
near 6943 A0 .
Non radiative
transition
Green
Blue
E3’
E2
E1
Pumping
Energy(ev)
Ground state
E3

33. • First gas laser was developed in 1961 by Ali
Javan and his coworkers
He-Ne Laser

34. +
Electrodes
He-Ne Mixture
-
100%
Reflecting
Mirror
Partially
Reflecting
Mirror
Structure of He Ne Laser
LASER

35.  Discharge tube of about 50 cm long, 1 cm in
diameter, filled with a mixture of He & Ne gases in
the ratio of 10:1 which is active medium.
 Ne-atoms are active centers- have energy levels
suitable for laser transitions
 He-atoms is efficient to excite the Ne-atoms.
 Energy transfer between He and Ne-atom takes
place through collision and the Ne atoms get
excited.
Construction:

36. Energy Transfer Through Atomic
Collisions :
Energy levels of Helium and Neon atoms and transitions between
the levels.
Helium Neon
F1
F2
F3
Excitation by
collision with
electrons
De-excitation by
collision with
walls
E1
E2
E3
E5
E4
E6
Spontaneous
emission (6000Å)
3.39 µm
1.15 µm
6328Å
Laser
transition
20.61eV 20.66eV

37. He atoms are excited to levels F2 & F3 – metastable
levels.
E4 & E6 levels in Ne are metastable states 
accumulation of atoms takes place in level E6 and E4
Population inversion can be achieved between:E6 a nd
E5, E6 and E3 levels E4 and E3 levels
 E6 E3 transitions; laser beam of red colour at 632.8
nm (6328 A)
 E4 E3 transitions; laser beam at wavelength of 1150
nm(11500 A )
E6 E5 transitions; laser beam in IR region at
3390nm(33900 A)
Working:

38. Semiconductor Laser
R.N. Hall and his coworkers made the first
semiconductor laser in 1962.
A semiconductor diode laser is a specially fabricated
p-n junction device that emits coherent light when it is
in forward biased.
Laser output
Current flow
P type
N type
Active region
Roughened
surface
Optically flat and
parallel faces
Optically flat and
parallel faces

39. Working:
.
 Semiconductor laser is heavily doped PN junction
diode
 When diode is forward biased electron from C.B.
recombine with holes in V. B.
 During recombination it emits energy in the form of
light and junction acts as laser which emites
coherence beam of laser light
 At low FB. Junction acts as a LED which emits
incoherent light
 A GaAsP laser emits light of wavelength 9000 Ao in
IR region (red)

40. Energy band structure of a semiconductor
diode
When the junction is F.B, electrons and holes are
injected across the junction to cause population
inversion.
 Population inversion is created in a very narrow
zone called the active region

41. Applications of Laser
 Industrial applications
 Applications in the field of medical science
 Astronomical and geophysical applications
 Metrology applications
 Applications in communication
 Defence application
 Environmental monitoring and Scientific Research

42. Laser show
ENTERTAINMENT APPLICATION

43. DEFENCE APPLICATION
Finger print detection
Detection of submarine
and mines
Laser at war time

44. OPTICAL COMMUNICATION
Frequency in the visible region ~ 1014 cycle/sec
Frequency in the microwave region ~ 109 cycle/sec
i.e. communication capacity: light wave 105 > microwave

45. SCIENCE AND TECHNOLOGY
Laser fusion
Compact Disk (CD)
Laser Eraser

46. HOLOGRAPHY
DISPLAY HOLOGRAM: EXHIBIT

47. SECURITY HOLOGRAM
An Embedded Hologram™ cannot be removed,
erased, duplicated or simulated by
photocopying, photography or scanning.

48. MEDICAL APPLICATION
Brain tumor surgery
Eye surgery

49. THANK YOU