4. 1917
• Albert Einstein lays the foundations for laser.
• he predicts the phenomenon of “Stimulated Emission,”
which is fundamental to the operation of all lasers
History of LASER
1917
• Albert Einstein lays the foundations for laser technology
• he predicts the phenomenon of “Stimulated Emission,” which is
fundamental to the operation of all lasers.
1939
• Valentin Fabrikant theorizes the use of stimulated emission to amplify
radiation.
1950
• Charles Townes, Nikolay Basov, and Alexander Prokhorov develop the
quantum theory of stimulated emission
5. 1917
• Albert Einstein lays the foundations for laser.
• he predicts the phenomenon of “Stimulated Emission,”
which is fundamental to the operation of all lasers
History of LASER
1954
• C.H. Townes and colleagues created microwave amplification by
stimulated emission of radiation (maser)
1959
• Columbia University graduate student Gordon Gould proposes that
stimulated emission can be used to amplify light
• He describes an optical resonator that can create a narrow beam of
coherent light, and calls it a LASER
1960
• The first laser device was developed by T.H. Maiman in 1960. The name
laser is an acronym for Light Amplification by Stimulated Emission of
Radiation.
10. Interaction of matter with radiation: Spontaneous Emission
• The probability of occurrence of this spontaneous emission transition
from state 2 to state 1 depends only on the properties of states 2 and 1
and is given by
• P’12 = A21 …………(i)
• Where A21 is known as the Einstein’s coefficient of spontaneous
emission of radiation.
12. Spontaneous Emission VS. Stimulated Emission
NO: Spontaneous emission Stimulated emission
1. The spontaneous emission was
postulated by Bohr
The stimulated emission was postulated by
Einstein
2. Additional photons are not required in
spontaneous emission
Additional photons are required in stimulated
emission
3. One photon is emitted in spontaneous
emission
Two photons are emitted in stimulated
emission
4. The emitted radiation is
polymonochromatic
The emitted radiation is monochromatic
5. The emitted radiation is monochromatic The emitted radiation is Coherent
6. The emitted radiation is less intense. The emitted radiation is high intense
7. The emitted radiation have less
directionality Example: light from sodium
or mercury lamp
The emitted radiation have high directionality
Example: light from laser source.
14. Characteristics of Laser
2. Intensity:
• If all the rays can parallelly travel up to certain distance that light should be
highly focused.
• So, the intensity of the laser beam is very high. If a person is allowed to
observe an ordinary light, emitted by a 100 W bulb at a distance. One foot
from the source, he can perceive only one thousand Watt of light.
• While, if the person is allowed to observe the laser beam from the same
distance, the entire laser beam penetrates through his eye. It will damage the
eye of the observer. This shows the high intensity of the laser beam.
15. Characteristics of Laser
3. Monochromaticity:
• The simple meaning of this word is that the source is pure in colour or
wavelength which means single wavelength and hence it has single
frequency.
• The laser beam is strictly monochromatic than any other conventional
monochromatic source.
• The light from a laser typically comes from one atomic transition with a single
precise wavelength.
16. Characteristics of Laser
4. Coherence:
• A traveling light ray propagates with some phase value or phase difference.
• In terms of light the meaning of coherence is having single value of phase
or having constant phase difference.
• This coherence is described in terms of temporal coherence (coherence in
time) and spatial coherence (coherence in space) which are required to
produce high quality interference.
17. Components of Laser
Laser requires three Components: 1) Active Medium
2) Pumping scheme
3) Optical Cavity
18. Components of Laser: Active Medium
1) Active Medium:
• The fundamental component of laser is material medium which is known as an
Active Medium.
• This active medium can be solid, liquid or gaseous. After the invention of ruby
laser, other active media such as glasses, plastics, liquids, gases and even plasma
were used.
• What should be the characteristics of an active medium?
• A medium which provides upper and lower energy level.
19. Population Inversion:
• Normal State (under equilibrium) - Lower energy level populated and higher
energy level almost empty
• Population of an energy level E, at temperature T is given by Ni = N0 e(-E
i/KT)
N1 = N0 e(-E
1/KT)
N2 = N0 e(-E
2/KT)
• At ordinary conditions N1 > N2, i.e., the population in the ground or lower state is
always greater than the population in the excited or higher states.
20. Population Inversion:
• Population inversion : Higher energy level more populated and lower
energy level is less populated
• Population of an energy level E, at temperature T is given by Ni = N0 e(-E
i/KT)
N1 = N0 e(-E
1/KT)
N2 = N0 e(-E
2/KT)
• Population of higher energy level is greater than the population of lower energy
level is called population inversion i.e., N2 > N1
21. Components of Laser : Pumping techniques
Pumping techniques:
• The method of raising the particles from lower energy state to higher energy
state is called pumping
• The most commonly used pumping methods are:
I. Optical pumping
II. Electrical discharge pumping
III. Chemical pumping
IV. Injection current pumping
22. Components of Laser : Pumping techniques
I. Optical pumping:
• Majorly used in solid laser.
• Xenon flash tubes are used for optical pumping.
• Examples of optically pumped lasers are ruby, Nd: YAG Laser
23. Components of Laser : Pumping techniques
II. Electrical discharge pumping
• Electrical discharge pumping is used in gas lasers.
• Electrical discharge pumped lasers are He-Ne laser, CO2 laser, argon-ion laser,
etc
24. Components of Laser : Pumping techniques
Chemical pumping
• Chemical reaction may also result in excitation and hence creation of population
inversion in few systems.
• Examples of such systems are HF and DF lasers.
25. Components of Laser: Pumping scheme
I. Two-level System:
• Two energy level E1 and E2
• Einstein coefficients (or constants) for the upward (B12) and downward (B21)
transitions can be found easily and are equal, i.e.B12 = B21.
• Population inversion cannot be achieved in two level system.
• Solution: Metastable state
• Metastable state : where electrons can stay longer.
26. Components of Laser: Pumping scheme
II. Three-Level System:
• Atoms are pumped to an excited state E3.
• In addition to this excited state (e.g. E3) the system has a
metastable state (e.g. E2).
• E3 has short life time and the atoms come out the upper level E3
spontaneously decays into this metastable state.
• This transition is usually weakly radiative or non-radiative (energy
is released to the lattice which give rise to phonons).
27. Components of Laser: Pumping scheme
II. Three-Level System:
• Atoms decay from the higher
level to the metastable level state,
which results in a population
inversion between the metastable
level and ground state.
• Population inversion can only be
achieved by pumping a higher
level, followed by rapid radiative
or non-radiative transfer to the
upper laser level.
28. Components of Laser: Pumping scheme
III. Four-Level System:
• Four energy levels, E4 > E3 > E2 > E1 with corresponding populations
of N1, N2, N3 and N4.
• The atoms are excited by optical pumping of the E1 ground state in
the E4 band.
• By rapid decay (radiative transition) they come to the metastable
energy level E3.
• The population inversion of level E3 with level E2 takes place during
the lifetime of the transition of E3 to E2 is long compared to E4 to E3.
29. Components of Laser: Pumping scheme
III. Four-Level System:
• Atoms in the metastable state E3
start spontaneous and stimulated
emission in the E2 energy level.
• The transition from the E2 energy
level E1 is just as fast as the E4
level.
• This rapidly de-energized atom
leads to a negligible population in
the E2 state and maintains the
population inversion..
30. Components of Laser: Optical Cavity
• In a laser, the active medium or the gain medium is kept in an optical cavity (or
resonant cavity) usually made up of two parallel surfaces. Among them one is
perfectly reflecting reflector and the other surface is partially reflecting
reflector. In this resonant cavity, the intensity of photons is raised tremendously
through stimulated emission process
33. Nd:YAG Laser: Working Principle
• Flash lamp is used to excite the atom of
active medium.
• neodymium ions are excited to energy
levels E3 and E4.
• This transition is due to absorption of
energy with wavelengths of respectively
0.73 mm and 0.80 mm
• non-radiative transition from E3 and E4
states to a metastable state E2
34. Nd:YAG Laser: Working Principle
• The metastable state is not a stable state. Therefore, Nd3+ ions will stay in this
state until the population inversion is achieved.
• When the population inversion is achieved between E2 and E1, a stimulated
emission takes place from the energy levels E2 to E1.
• The emitted energy is amplified between the resonators and then radiates a
pulsed laser beam at a wavelength of 1.064 mm in the infrared region.
• Further, Nd3+ ions take a very rapid non-radiative transition from E1 to E0.
• Maximum output power of 70 W either in continuous or pulsed mode of
operation can be produced using the Nd-YAG laser.
35. Carbon Dioxide Laser: Construction
• Quartz tube of length of 2.6 cm &
area of cross section 1.5 mm2
• Tube is filled with carbon dioxide,
helium & nitrogen gases in the ratio
of 1:4:5
• An ac supply of frequency 50
cycles or dc supply is given in
order to produce electric discharge
in the gaseous mixture.
• Water vapour or air is circulated in
tube to maintain cooler.
• The gas mixture can be pumped
either longitudinally or transversely
into the gas discharge tube
36. Carbon Dioxide Laser: Working Principle
• Molecules excites to the
higher energy states E5 since
it is metastable state the
population inversion could be
achieved between two sets of
energy levels say E5 & E3 and
E4 & E3
• Transition E5 → E3 generates
the laser beam of wavelength
of 9.6µ lies in IR region
• Transition E4 → E3 generates
the laser beam of wavelength
of 10.6µ lies in IR region.
37. Semiconductor Laser: Construction
• The active medium :p-n junction
diode
• Single crystal of gallium arsenide is
cut in the form of a platter having
thickness of 0.5μmm.
• The photon emission is stimulated in
a very thin layer of PN junction
• The end faces of the junction diode
are well polished and parallel to each
other. They act as an optical
resonator through which the emitted
light comes out.
38. Semiconductor Laser: Working Principle
• forward biased with large applied
voltage is applied
• forward biased voltage is
increased-more and more light
photons are emitted and the light
production instantly becomes
stronger
• photons will trigger a chain of
stimulated recombination resulting
in the release of photons in phase
39. Applications of Lasers: Communication
In case of optical communication semiconductors laser diodes are used as
optical sources and its band width is (1014Hz) is very high compared to the
radio and microwave communications.
_ More channels can be sent simultaneously
_ Signal cannot be tapped
_ As the band width is large, more data can be sent.
_ A laser is highly directional and less divergence, hence it has greater potential
use in space crafts and submarines
40. Applications of Lasers: Computers
Computers
_ In LAN (local area network), data can be transferred from memory storage of
one
computer to other computer using laser for short time.
_ Lasers are used in CD-ROMS during recording and reading the data.
41. Applications of Lasers: Chemistry and Photography
Chemistry
_ Lasers are used in molecular structure identification
_ Lasers are also used to accelerate some chemical reactions.
_ Using lasers, new chemical compounds can be created by breaking bonds
between atoms are molecules.
Photography
_ Lasers can be used to get 3-D lens less photography.
_ Lasers are also used in the construction of holograms.
42. Applications of Lasers: Industry
Industry
_ Lasers can e used to blast holes in diamonds and hard steel
_ Lasers are also used as a source of intense heat
_ Carbon dioxide laser is used for cutting drilling of metals and nonmetals, such
as ceramics plastics glass etc.
_ High power lasers are used to weld or melt any material.
_ Lasers are also used to cut teeth in saws and test the quality of fabric.
43. Applications of Lasers: Medicine
. Medicine
_ Pulsed neodymium laser is employed in the treatment of liver cancer.
_ Argon and carbon dioxide lasers are used in the treat men of liver and lungs.
Lasers used in endoscopy to scan the inner parts of the stomach.
_ Lasers used in the elimination of moles and tumours which are developing in
the skin
tissue.
44. Applications of Lasers: Military
Military
_ Lasers can be used as a war weapon.
_ High energy lasers are used to destroy the enemy air-crofts and missiles.
_ Lasers can be used in the detection and ranging likes RADAR.
45. Applications of Lasers: Scientific Research
. Scientific research
_ Lasers are used in the field of 3D-photography
_ Lasers used in Recording and reconstruction of hologram.
_ Lasers are employed to create plasma.
_ Lasers used to produce certain chemical reactions.
_ Lasers are used in Raman spectroscopy to identify the structure of the
molecule.
_ Lasers are used in the Michelson- Morley experiment.
_ A laser beam is used to conform Doppler shifts in frequency for moving
objects.
