Fundamentals of Lasers for Engineering Students

1. Laser
by:
Dr. K. MADHUSUDHANA
Mahaveer Instirute of Science and Technology
Bandlaguda, Hyderabad – 5

3. LASER
• L ight
• A mplification by
• S timulated
• E mission of
• R adiation
C.H. Towns, 1954
T.H. Maiman, 1960
Noble prize, 1964

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

5. Coherence

6. Coherence
• Temporal coherence
Coherent length l0 = λ2/Δλ = cT0

7. Spatial Coherence

8. Monochromaticity
• Degree of non monochromaticity
τ = Δν/ν = c/l0ν=1/νT0
• Quality factor = λ/Δλ = l0/λ
Ideally coherent: same energy, momentum,
polarization

9. Absorption and Emission
• Excitation potential and critical potential

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

11. Absorption
E1
E2

12. 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.

13. Spontaneous Emission

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

15. Stimulated Emission

16. 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

17. Electron/Photon Interactions

18. 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.

19. Einstein’s Coefficients

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

21. 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

22. 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

23. Absorption
E2  E1  h
• The probability of this absorption from state 1 to state 2
is proportional to the energy density u(v) of the radiation
P12 α N1
P12α ρ(v)
P12  B12N1ρ(v)
Where B12 is known as the Einstein’s coefficient of absorption of radiation.

24. Spontaneous Emission
s
The probability of occurrence of this spontaneous emission transition
from state 2 to state 1 depends only on the 2 and 1 and is given
by
P21 α N1
(P21 )sp A21 N2
Where A21 is known as the Einstein’s coefficient of spontanious Emission of
radiation.

25. Stimulated Emission
The probability of occurrence of stimulated emission transition from
the upper level 2 to the lower level 1 is proportional to the energy
density u(v) of the radiation and is given by
Thus the total probability of emission transition from the upper level
2 to the lower level 1 is
(P21 )st
P21 (P21)sp
P21 N2[ A21  B21 ρ()]
P21 α N2
P21α ρ(v)
(P21 )st  B21N2ρ(v)
Where B21 is known as the Einstein’s coefficient of stimulated Emission of
radiation.

26. Relation between Einstein’s Coefficients
Let N1 and N2 be the number of atoms at any instant in the state 1
and 2, respectively. The probability of absorption transition for
atoms from state 1 to 2 per unit time is
P12  N1B12 ρ(v)
The probability of transition of atoms from state 2 to
1,either by spontaneously or by stimulated emission per unit time
is
P21  N2[ A21  B21 ρ( )]
at temperature t, the emission and absorption probabilities are
equal and thus
P12  P21

27. N1B12 ρ( )  N2[ A21 B21 ρ( )]
N1B12
N2 A21
N2 B21
ρ( ) 
N1B21  N2 B21
N2 A21
ρ( ) 
But Einstein proved that probability of (stimulated)
absorption is equal to the probability of stimulated
emission, So
B12  B21
B21 (N1 / N2 )1
1
ρ( ) 
A21

28. ann’s
hermal equilibri
According to Boltzm law, the distribution of atoms among the
energy states E1 and E2 at the t um at temperature T
is given by N0 =
e E0 / kT
eE1 / kT
N e
N
2
E2 /kT
1
 2 1( E E ) / kT
 e
where k is the Boltzmann constant
 eh /kT
N2
N1
1
1eh / kT
21B
ρ( ) 
A21 (1)

29. 1
ρ( ) 
e3
eh / kT
8h 3
1
Planck’s radiation formula gives the energy density of radiation u(v)
as
(2)
from equation (1) and (2)
e3
B
A 8h 3
21

21
This equation gives the relation between the probabilities of
spontaneous and stimulated emission.

30. Condition for the laser operation
If N1 > N2
• radiation is mostly absorbed
if N2 >> N1 - population inversion
•most atoms occupy level E2, weak absorption
• stimulated emission prevails
• light is amplified
Necessary condition:
population inversion

31. BASIC PRINCIPLE NEEDED
FOR LASER

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

33. • 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

34. 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

36. LASER ACTION

37. Types of Laser
a.According to their sources:
1.Gas Lasers
2.Crystal Lasers
3.Semiconductors Lasers
4.Liquid Lasers
b.According to the nature of emission:
1.Continuous Wave
2.Pulsed Laser
c.According to their wavelength:
1.Visible Region
2.Infrared Region
3.Ultraviolet Region
4.Microwave Region
5.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

38. 2- Level Laser
h
E1
E2
Absorption
E1
E2
h
Spontaneous
Emission
E1
E2hh h
Stimulated
Emission

39. 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

40. 4-Level LASER

41. PRACTICAL LASERS

42. Ruby Laser
Ruby Laser is the first type of laser,
demonstrated in the year 1960 by
T.H.Maiman.
Ruby Laser is a solid state laser.
It is a pulsed three level pumping scheme.
• Active medium: The active medium in
Ruby rod (Al2O3+Cr2O3) is Cr3+ions.
Some of the Aluminum atoms are replaced
by 0.05% of Chromium atoms.
Lasing action takes place in Chromium
energy levels.
• Energy Source: The pumping of ions is
through optical pumping, using Xenon flash
lamp.

43. Construction:
Ruby Laser consists of a cylindrical shaped Ruby crystal rod. One of the end faces is
highly silvered and the other face is partially silvered so that it transmits 10-25% of the
incident light and reflects the rest.
The ruby crystal is placed along the axis of a helical Xenon or Krypton flash lamp of
high intensity. This is surrounded by a reflector.
The ruby rod is protected from heat by enclosing it in a hollow tube, through which
cold water is circulated.
The ends of the flash lamp are connected to a pulsed high voltage source, so that the
lamp gives flashes of an intense light.

44. Working:
The Chromium ions are responsible for the stimulated emission of radiation, whereas
Aluminum and Oxygen ions are passive, sustaining the lasing action.
The Chromium ions absorb the radiations of wavelength around 5500Ao (Green) and
4000Ao ( Blue),emitted by the flash lamp and get excited to 4F2 and 4F1 energy levels
respectively, from ground state.
After the life time, the ions make non- radiative transition to the metastable state 2E,
consisting of a pair of energy levels (doublet).
Population inversion takes place between metastable and ground state. As a result,
stimulated emission takes place giving rise to the emission of light of wavelengths
6929Ao and 6943Ao , of which 6943Ao is the laser radiation of high intensity.

46. Optical
Pumping
Short-live state
Radiation-less Transition
Metastable state
Spontaneous
Emission
Ground State
E2
E1
E3
10-8sec
10-3sec
5500 Å Stimulated
6943 Å Emission
6943 Å
6943 Å

47. HE-NE LASER
Construction
Helium: Neon= 10:1

48. .
Energy Level Diagram of He-Ne
Power output: mW

49. He: Ne LASER

50. He-Ne Laser
Electron
Impact
Radiation-less
Transition
Metastable state
Spontaneous
Emission
Ground
StateHe
20.61 eV 20.66 eV
c
Ne
c
6328 Å 63
63
28 Å
28 Å
18.70 eV
Energy
Transfer

51. In a molecular gas laser, laser action is achieved by transitions between vibrational
and rotational levels of molecules. Its construction is simple and the output of this
laser is continuous.In the CO2 molecular gas laser, transition takes place between
the vibrational states of Carbon dioxide molecules.
CO2 Molecular gas laser
CO2 laser was the first molecular gas laser developed by Indian born American
scientist Prof.C.K.N.Pillai.It is a four level laser and it operates at 10.6 μm in the far
IR region. It is a very efficient laser.
CO2 LASER

52. Working Principle Of CO2 Laser:
The active medium is a gas mixture of CO2, N2 and He. The laser transition
takes place between the vibrational states of CO2molecules.
It consists of a quartz tube 5 m long and 2.5 cm in the diameter. This discharge tube
is filled with gaseous mixture of CO2(active medium), helium and nitrogen with
suitable partial pressures.The terminals of the discharge tubes are connected to a
D.C power supply. The ends of the discharge tube are fitted with NaCl Brewster
windows so that the laser light generated will be polarized.
Two concave mirrors one fully reflecting and the other partially form an optical
resonator.

53. Working Of CO2 Laser-
When an electric discharge occurs in the gas, the electrons collide with
nitrogen molecules and they are raised to excited states. This process is
represented by the equation
N2 + e* = N2* + e
N2 = Nitrogen molecule in ground state e* = electron with kinetic energy
N2* = nitrogen molecule in excited state e= same electron with lesser energy
Now N2 molecules in the excited state collide with CO2 atoms in ground state and
excite to higher electronic, vibrational and rotational levels.
This process is represented by the equation N2* + CO2 = CO2* + N2

54. N2* = Nitrogen molecule in excited state. CO2 = Carbon dioxide atoms in ground state
CO2* = Carbon dioxide atoms in excited state N2 = Nitrogen molecule in ground state.
Since the excited level of nitrogen is very close to the E5 level of CO2 atom, population
in E5 level increases.
As soon as population inversion is reached, any of the spontaneously emitted photon
will trigger laser action in the tube. There are two types of laser transition possible.
1.Transition E5 to E4 :
This will produce a laser beam of wavelength 10.6μm
2.Transition E5 to E3
This transition will produce a laser beam of wavelength 9.6μm. Normally 10.6μm
transition is more intense than 9.6μm transition. The power output from this laser is
10kW.
The carbon dioxide laser usually produces a beam that is invisible to the naked eye,
but can go through the atmosphere so that it can be used as a cutting or welding
device in industry. CO2 laser-engraved walnut plaques were a popular item in the
1970s. In those days, CO2 lasers were developed for defense, but it was soon
realized that enough of the wavelength is absorbed in the atmosphere to cause
severe problems in pointing and focusing the beam at long distances. CO2 lasers are
no longer considered in this role. However, they are still considered for target
designation and to induce fluorescence in trace materials for detection, where the
beam only has to propagate short distances through the atmosphere.

55. Generation diagram of Laser
SEMICONDUCTOR LASER

56. Classification Of
Semiconductor
Laser
Semiconductor Laser
Homojunction
Semiconductor
Laser
Heterojunction
Semiconductor Laser
Double
Heterojunction
Semiconductor
Laser
Single
Heterojunction
Semiconductor
Laser

57. Homojunction SemiconductorLaser
Homojunction diode lasers are those in which P end and N end of the
diode are made of the same semiconductor material.
Example : Ga As laser
They use Direct Band Gap
Semi- conductor material.
P-N Junction act as the active
medium.
The crystal is cut at a thickness of
0.5 mm
Applied voltage is given through
metal contacts on both surfaces of
the diode.
 Pulse beam of laser of 8400 Å is produced

58. Energy Level Diagram :Homojunction

59. Heterojunction Semiconductor
LaserHeterojunction Semiconductor lasers are those in which P end is made of one type
of semiconductor material and the N end is made of another type of semiconductor
material
Example : GaAlAs diode laser
 Use Direct Band gap Semiconductor
Consist of five layers namely
GaAs – p type
GaAlAs – p type
GaAs – p type (Active Medium)
GaAIAs – n type
GaAs – n type
Diagram of Heterojunction
Semiconductor Laser

60. Energy Level Diagram :Heterojunction
Energy level Diagram of Heterojunction Semiconductor Laser

61. APPLICATIONS OF LASER

62. 2020/4/13
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
Nuclear fusion
Long distance measurement
Holography

63. 2020/4/13
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

64. THANKS SEE YOU IN NEXT
LECTURE