A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term "laser" originated as an acronym for "light amplification by stimulated emission of radiation

1. LASER
Dr. Madhavrao K. Deore
M.Sc., Ph. D.
Department of Physics,
M.V.P.Samaja’s, Arts, Science and
Commerce College, Ozar(Mig),
Nashik, -422206, India
deoremadhav63@gmail.com

2. A laser is a device that emits light through a process of optical
amplification based on the stimulated emission of electromagnetic
radiation. The term "laser" originated as an acronym for "light
amplification by stimulated emission of radiation
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.
The development of laser has been proved to be turning point in the
history of science and engineering. It has produced completely new
type of system with potential applications in wide verity of fields.
LASER

3. 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.
The first laser was built in 1960 by Theodore H. Maiman at
Hughes Laboratories, based on theoretical work by Charles
Hard Townes and Arthur Leonard Schawlow.

4. Laser light and ordinary light difference
Laser light Ordinary light
It is highly coherent- single
frequency
It is incoherent in nature- discrete
freqn.
There coherence length as of the
order of few km (Kilometer).
There coherence length as of the
order of few mm (Millimeter).
It is travel in only one direction,
which is parallel to the optic axis.
Intensity of ordinary light decreases
with distance as it travels in form of
short pulses of small length and
short duration.
It is highly monochromatic as it
travels in form of large length and
long duration.
Only one wave lenght
It is not strictly monochromatic as it
travels in form of short pulses of
small length and short duration.
Wavelenght- 400-700 nm
Spread of plane wave front is less
hence they spread least i.e. their
divergence is less.
Spread of spherical wave font is
more hence they spread heavily i.e.
their divergence is more
polarized Mostly unpolarized

6. Brief history of laser
1) 1900 –Max Planks- light is electromaganetic radiation
2) 1917- principle of laser discovered- Albert Einstein- describe the theory of
stimulated emmision- Stimulated emission is the process by which an incoming
photon of a specific frequency can interact with an excited atomic electron (or other
excited molecular state), causing it to drop to a lower energy level. The liberated
energy transfers to the electromagnetic field, creating a new photon with
identical phase, frequency, polarization, and direction of travel as the photons of the
incident wave

7. 1951: Charles H Townes, Alexander Prokhorov, Nikolai G Basov, Joseph
Weber - The invention of the MASER (Microwave Amplification of Stimulated
Emission of Radiation) at Columbia University, Lebedev Laboratories, Moscow and
University of Maryland.
1958: Schawlow, A.L. and Townes, C.H. - Proposed the realization of masers for
light and infrared at Columbia University .
1960-Townes, C.H- Patented and Nobel(1964)
1960: Maiman, T.H. - Realization of first working LASER based on Ruby at
Hughes Research Laboratories.
1961: Javan, A., Bennet, W.R. and Herriot, D.R. - First gas laser : Helium- Neon
(He-Ne laser) at Bell Laboratories.
1961: Fox, A.G., Li, T. - Theory of optical resonators at Bell Laboratories.
1962: Hall,R. - First Semiconductor laser (Gallium-Arsenide laser) at General
Electric Labs.
1962: McClung,F.J and Hellwarth, R.W. - Giant pulse generation / Q-Switching.
1962: Johnson, L.F., Boyd, G.D., Nassau, K and Sodden, R.R. - Continuous
wave solid-state laser.
1964: Geusic, J.E., Markos, H.M., Van Uiteit, L.G. - Development of first working
Nd:YAG LASER at Bell Labs.
1964: Patel, C.K.N. - Development of CO2 LASER at Bell Labs.
1964: Bridges, W. - Development of Argon Ion LASER a Hughes Labs.

8. 1965: Pimentel, G. and Kasper, J. V. V. - First chemical LASER at University of
California, Berkley.
1965: Bloembergen, N. - Wave propagation in nonlinear media.
1966: Silfvast, W., Fowles, G. and Hopkins - First metal vapor LASER - Zn/Cd - at
University of Utah.
1966: Walter, W.T., Solomon, N., Piltch, M and Gould, G. - Metal vapor laser.
1966: Sorokin, P. and Lankard, J. - Demonstration of first Dye Laser action at IBM
Labs.
1966: AVCO Research Laboratory, USA. - First Gas Dynamic Laser based on CO2
1970: Nikolai Basov's Group - First Excimer LASER at Lebedev Labs, Moscow
based on Xenon (Xe) only.
1974: Ewing, J.J. and Brau, C. - First rare gas halide excimer at Avco Everet Labs.
1977: John M J Madey's Group - First free electron laser at Stanford University.
1977: McDermott, W.E., Pehelkin, N.R,. Benard, D.J and Bousek, R.R. - Chemical
Oxygen Iodine Laser (COIL).
1980: Geoffrey Pert's Group - First report of X-ray lasing action, Hull University, UK.
1984: Dennis Matthew's Group - First reported demonstration of a "laboratory" X-
ray laser from Lawrence Livermore Labs.
1999: Herbelin,J.M., Henshaw, T.L., Rafferty, B.D., Anderson, B.T., Tate, R.F.,
Madden, T.J., Mankey II, G.C and Hager, G.D. - All Gas-Phase Chemical Iodine
Laser (AGIL).
2001: Lawrence Livermore National Laboratory - Solid State Heat Capacity
Laser (SSHCL).

9. Interaction of radiation with matter
Electromagnetic radiations are the radiations consisting of waves of energy
associated with electric field and magnetic field which act perpendicular to
each other and also perpendicular to the direction of propagation of waves.
Characteristics of electromagnetic radiations:-
a) These radiations transmit energy through space.
b) These radiations travel with speed of light in vacuum.
c) These radiations show dual nature – particle and wave nature.
d) These radiations are associated with electrical component and magnetic
component.

10. Interaction of E.R. with matter produces- Absorption & spontaneous emission.
Absorption & spontaneous emission- natural process.
In absorption, the energy of photon is taken up by matter. There is reduction in intensity of
light wave.
The velocity of light c= vλ
When the beam of light passes through absorbing medium , then attenuation of light in
medium describes the decrease in intensity of light (dI) ( absorption) which is propotional to
intensity of light and thickness
dI α I * dx
dI = - œ I * dx
-Ve sign indictaes decrease intensity w. r t. t. thickness of medium.
dI/ I = - œ dx
By integration varies from Io to I and 0 to x I = I e - œ *x
Thus intensity decrease with intensity exponentially

11. Energy levels
The stationary state of a quantum mechanical system called energy state.
A quantum mechanical system or particle that is bound—that is, confined spatially—can
only take on certain discrete values of energy. These discrete values are called energy
levels. The term is commonly used for the energy levels of electrons in atoms, ions,
or molecules, which are bound by the electric field of the nucleus.
After absorbing energy, an electron may jump from the ground state to a higher energy
excited state.
A decrease in energy level fromE2 to E1 resulting in emission of a photon represented by
the red squiggly arrow, and whose energy is h ν
An increase in energy level fromE1 to E2 resulting from absorption of a photon
represented by the red squiggly arrow, and whose energy is h ν
The atom absorb or emits light in discrete packets called photons. A photon is
an elementary particle, the quantum of light and all other forms of electromagnetic
radiation.

12. As you may remember from chemistry, an atom consists of electrons orbiting around a nucleus.
However, the electrons cannot choose any orbit they wish. They are restricted to orbits with only
certain energies. Electrons can jump from one energy level to another, but they can never have
orbits with energies other than the allowed energy levels.
Electrons in a hydrogen atom must be in one of the allowed energy levels. If an electron is in the
first energy level, it must have exactly -13.6 eV of energy. If it is in the second energy level, it
must have -3.4 eV of energy. An electron in a hydrogen atom cannot have -9 eV, -8 eV or any other
value in between.
Let's say the electron wants to jump from the first energy level, n = 1, to the second energy level
n = 2. The second energy level has higher energy than the first, so to move from n = 1 to n = 2, the
electron needs to gain energy. It needs to gain (-3.4) - (-13.6) = 10.2 eV of energy to make it up to
the second energy level.
The electron can gain the energy it needs by absorbing light. If the electron jumps from the
second energy level down to the first energy level, it must give off some energy by emitting light.
The atom absorbs or emits light in discrete packets called photons, and each photon has a
definite energy. Only a photon with an energy of exactly 10.2 eV can be absorbed or emitted when
the electron jumps between the n = 1 and n = 2 energy levels.

13. The energy that a photon carries depends on its wavelength. Since the
photons absorbed or emitted by electrons jumping between the n = 1 and n
= 2 energy levels must have exactly 10.2 eV of energy, the light absorbed or
emitted must have a definite wavelength. This wavelength can be found
from the equation
E = hc/λ,
where E is the energy of the photon (in eV), h is Planck's constant (4.14 x
10-15 eV s) and c is the speed of light (3 x 108 m/s). Rearranging this
equation to find the wavelength gives
λ = hc/E.
A photon with an energy of 10.2 eV has a wavelength of 1.21 x 10-7 m, in the
ultraviolet part of the spectrum. So when an electron wants to jump from n
= 1 to n = 2, it must absorb a photon of ultraviolet light. When an electron
drops from n = 2 to n = 1, it emits a photon of ultraviolet light.

15. Population Density and Population inversion
No.of atoms per unit volume present in an energy level is called population
density
In science, specifically statistical mechanics, a population inversion occurs
while a system (such as a group of atoms or molecules) exists in a state with
more members in an excited state than in lower energy states. It is called an
"inversion" because in many familiar and commonly encountered physical
systems, this is not possible. The concept is of fundamental importance
in laser science because the production of a population inversion is a
necessary step in the workings of a standard laser

16. Boltzmann distributions and thermal equilibrium
To understand the concept of a population inversion, it is necessary to
understand some thermodynamics and the way that light interacts
with matter.
To do so, it is useful to consider a very simple assembly of atoms forming
a laser medium.
Assume there are a group of N atoms, each of which is capable of being
in one of two energy states, either
The ground state, with energy E1; or
The excited state, with energy E2, with E2 > E1.
The number of these atoms which are in the ground state is given by N1,
and the number in the excited state N2.
Since there are N atoms in total, N= N1 + N2
The energy difference between the two states, given by
∆E12 = E2 – E1

17. The characteristic frequency of light which will interact with the atoms; This is given by the relation
E2-E1 =∆E = hv12, h being Planck's constant.
If the group of atoms is in thermal equilibrium, it can be shown from Maxwell-Boltzmann distribution that
the ratio of the number of atoms in each state is given by the Boltzmann factor:
where T is the thermodynamic temperature of the group of atoms, and k is Boltzmann's
constant.
We may calculate the ratio of the populations of the two states at room temperature
(T ≈ 300 K) for an energy difference ΔE that corresponds to light of a frequency
corresponding to visible light (ν ≈ 5×1014 Hz). In this case ΔE = E2 - E1 ≈ 2.07 eV, and kT ≈
0.026 eV. Since E2 - E1 ≫ kT,
it follows that the argument of the exponential in the equation above is a large negative
number, and as such N2/N1 is vanishingly small; i.e., there are almost no atoms in the
excited state.
When in thermal equilibrium, then, it is seen that the lower energy state is more populated
than the higher energy state, and this is the normal state of the system.
As T increases, the number of electrons in the high-energy state (N2) increases,
but N2 never exceeds N1 for a system at thermal equilibrium; rather, at infinite temperature,
the populations N2 and N1 become equal. In other words, a population inversion (N2/N1 >
1) can never exist for a system at thermal equilibrium. To achieve population inversion
therefore requires pushing the system into a non-equilibrated state.

18. Absorption and Emission
Absorption:If light (photons) of frequency ν12 pass through the group of
atoms, there is a possibility of the light being absorbed by atoms which
are in the ground state, which will cause them to be excited to the
higher energy state. The rate of absorption is proportional to the
radiation intensity of the light, and also to the number of atoms currently
in the ground state,N1.

19. Spontaneous emission
Spontaneous emission is the process by which a quantum system such as
an atom, molecule, nanocrystal or nucleus in an excited state undergoes a transition to a state with a
lower energy (e.g., the ground state) and emits quanta of energy. Light or luminescence from an atom
is a fundamental process that plays an essential role in many phenomena in nature and forms the
basis of many applications, such as fluorescent tubes, older television screens (cathode ray tubes),
plasma display panels, lasers, and light emitting diodes. Lasers start by spontaneous emission, and
then normal continuous operation works by stimulated emission.

20. Stimulated emission
Stimulated emission is the process by which an incoming photon of a specific frequency can
interact with an excited atomic electron (or other excited molecular state), causing it to drop
to a lower energy level. The liberated energy transfers to the electromagnetic field, creating a
new photon with identical phase, frequency, polarization, and direction of travel as the
photons of the incident wave. This is in contrast to spontaneous emission which occurs at
random intervals without regard to the ambient electromagnetic field. Further calculation can
be obtained by statistical mechanics.
If an atom is already in the excited state, it may be perturbed by the passage of a photon that
has a frequency ν21 corresponding to the energy gap ΔE of the excited state to ground state
transition. In this case, the excited atom relaxes to the ground state, and is induced to
produce a second photon of frequency ν21. The original photon is not absorbed by the atom,
and so the result is two photons of the same frequency. This process is known as stimulated
emission.

22. Specifically, an excited atom will act like a small electric dipole which will
oscillate with the external field provided. One of the consequences of this
oscillation is that it encourages electrons to decay to the lowest energy state.
When this happens due to the presence of the electromagnetic field from a
photon, a photon is released in the same phase and direction as the
"stimulating" photon, and is called stimulated emission.
The critical detail of stimulated emission is that the induced photon has the
same frequency and phase as the incident photon. In other words, the two photons
are coherent. It is this property that allows optical amplification, and the production of
a laser system. During the operation of a laser, all three light-matter interactions
described above are taking place. Initially, atoms are energized from the ground state
to the excited state by a process called pumping, described below. Some of these
atoms decay via spontaneous emission, releasing incoherent light as photons of
frequency, ν. These photons are fed back into the laser medium, usually by an optical
resonator. Some of these photons are absorbed by the atoms in the ground state, and
the photons are lost to the laser process. However, some photons cause stimulated
emission in excited-state atoms, releasing another coherent photon. In effect, this
results in optical amplification.

23. Einstein coefficients
In 1917 Einstein postulated on thermodynamic grounds that probability for spontaneous
emission) is related to the probability of stimulated emission(B) and relation between them is
calculated from quantum mechanics.
Einstein coefficients are mathematical quantities which are a measure of the probability of
absorption or emission of light by an atom or molecule.[The Einstein A coefficient is related to the
rate of spontaneous emission of light and the Einstein B coefficients are related to
the absorption and stimulated emission of light.
Consider the two level energy E1 & E2.
let N1 and N2 be the no. of atoms in the ground excited state.
The distribution of atoms in the two energy levels will change by absorption or emission of
radiation. Einstein introduced three empirical coefficients to quantify the change of population of
the two levels.

24. the upper level increases.
The rate is clearly proportional to the population of atoms in the lower level and to the energy
density of radiation in the system.
Thus the rate of increase of population of the excited state is given by
( dN2/dt) = B12 ρ(ν) N1 dN1/dt = - B12 ρ(ν) N1
where B12 is a constant of proportionality with dimensions m /s -J.
Spontaneous Emission - The population of the upper level will decrease due to spontaneous
transition to the lower level with emission of radiation.
The rate of emission will depend on the population of the upper level.
the rate at which N2 decays is:
dN2/dt = - A21 N2 → dN1/dt = A21 N2 ,
If A21 is the probability that an atom in the excited state will spontaneously decay to the ground
state,
where A21 is the rate of spontaneous emission. In the rate-equation is a proportionality constant for this
particular transition in this particular light source. The constant is referred to as the Einstein A coefficient,
Stimulated Emission - Stimulated or induced emission depends on the number of atoms in the
excited level as well as on the energy density of the incident radiation.
If B21 be the transition probability per unit time per unit energy density of radiation,
the rate of decrease of the population of the excited state is .
dN2/dt = -B21 ρ(ν) N2 → dN1/dt = B21 ρ(ν) N2
B21 is known as the Einstein B coefficient for that particular transition,

25. At thermodynamic equilibrium, the net change in the number of any excited atoms is
zero, being balancing, by loss and gain due to all process
0 = - B12 ρ(ν) N1 + B21 ρ(ν) N2 + A21 N2
B12 ρ(ν) N1 - B21 ρ(ν N2 = A21 N2
ρ(ν) = A21 N2 / ( B12 N1 - B21 N2)
= (A21/B12) / [ ( N1/N2) –(B21/B12)]
= (A21/B12) / [ ( exp((E2-E1/KT)) –(B21/B12)]
According to Plank’s radiation law for any value of T
A21/B12 = (8πһν3μ3)/c3
ρ(ν) = (8πһν3μ3)/c3 / [ ( exp((E2-E1/KT)) –(B21/B12)]
When an atom with two energy levels is placed in the radiation field then
B12= B21
ρ(ν) = (8πһν3μ3)/c3 / [ ( exp((E2-E1/KT)) –1]
B12, B21 and A12 are known as Einstein coefficient.

26. A laser consists of a gain medium, a mechanism to energize it, and something to
provide optical feedback.[7] The gain medium is a material with properties that allow it
to amplify light by way of stimulated emission. Light of a specific wavelength that
passes through the gain medium is amplified (increases in power).
For the gain medium to amplify light, it needs to be supplied with energy in a process
called pumping. The energy is typically supplied as an electric current or as light at a
different wavelength. Pump light may be provided by a flash lamp or by another laser.
The most common type of laser uses feedback from an optical cavity—a pair of
mirrors on either end of the gain medium. Light bounces back and forth between the
mirrors, passing through the gain medium and being amplified each time. Typically one
of the two mirrors, theoutput coupler, is partially transparent. Some of the light escapes
through this mirror. Depending on the design of the cavity (whether the mirrors are flat
or curved), the light coming out of the laser may spread out or form a narrow beam. In
analogy to electronic oscillators, this device is sometimes called a laser oscillator.
Most practical lasers contain additional elements that affect properties of the emitted
light, such as the polarization, wavelength, and shape of the beam.

27. Components of a typical laser:
1. Gain medium
2. Laser pumping energy
3. High reflector
4. Output coupler
5. Laser beam