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" Lasers have many important applications. They are used in common consumer devices such as DVD players, laser printers, and barcode scanners. They are used in medicine for laser surgery and various skin treatments, and in industry for cutting and welding materials. They are used in military and law enforcement devices for marking targets and measuring range and speed. Laser lighting displays use laser light as an entertainment medium. Lasers also have many important applications in scientific research

1. 1ASN
Laser is an acronym for light amplification by
stimulated emission of radiation. They produce a
highly directional and intense beam with a narrow
frequency range.
COMPPONENTS OF A LASER
A laser device consist of three components
1. The pump: A pump is basically the energy
source for a laser. It gives energy to
various atoms of laser medium and
excites them so heavily that population
inversion is achieved and maintained.
Different methods of pumping are optical
pumping, direct electron excitation, in
elastic atom-atom excitation etc.
2. Laser medium: It is the medium in which
the laser action is made to take place. It
may be solid, liquid or gas. The main
requirement of the laser medium is that
we should be able to obtain population
inversion in it. The atoms or molecules of
the laser medium are called active centers.
3. The resonator:
An optical resonator is a setup which is used to
obtain amplification of stimulated photons, by
oscillating them back and forth between two
extreme points. It consist of two plane mirrors
placed coaxially. One of the mirror is partially
reflecting while the other is totally reflecting. It
enables a part of the internally reflected beam to
escape out as a laser beam. The space between
two mirrors is called cavity.
Working:
Photons emitted in any direction other than
parallel to optical axis will pass through the sides
and are lost. Photons emitted parallel to the axis
are augmented. On reaching the semi-transparent
mirror, some of the photons are emitted out and a
part of them will be reflected back. While
propagating in the opposite direction they de-
excite more and more atoms and build up their
strength. The photons again gets reflected from
the 100 % reflecting mirror.
As the oscillation build up to high intensity it
emerges through the front mirror as a highly
collimated intense beam.
CHARACTERISTICS OF A LASER
Main characteristics of a laser are
1. Directionality: Laser light emit light only in
one direction as the photons travelling
along the optical axis of the system are
selected by the optical resonator. The
energy carried by a laser beam can be
controlled easily and focused into a small
area.
2. Intensity: The laser beam is highly intense
because energy is concentrated in a very
narrow region. Hence it finds application
in welding, drilling etc.
3. Mono chromaticity: A laser beam is highly
mono chromatic. The light spread is of the
order of a few angstroms.

2. 2ASN
4. Coherence: A laser light is coherent
because the waves emitted by a laser will
be in phase and are of same frequency.
Coherence is classified as
A) Spatial coherence: If two waves have
identical propagation characteristics
at two different points in space, then
the waves are said to be spatially
coherent. Coherence length Lc is the
distance travelled by a wave without
losing its coherence. 𝐿𝑐 = 𝜆(
𝜆2
Δ𝜆
)=
𝜆2
Δ𝜆
where λ is the wave length of laser
beam and Δ λ is the wave length
spread (line width) of a laser beam.
B) Temporal coherence: If two waves
have identical propagation
characteristics at different instants of
time then they are said to be
temporally coherent.
Coherence time 𝜏 𝑐 = (
1
Δ𝜈
)
Coherence time is related to coherent
length as
𝑙 𝑐 = 𝑐 𝜏 𝑐
PRINCIPLE OF LASER
The laser works on the principle of stimulated
emission of radiation. Atoms or molecules are
characterized by a set of discrete allowed energy
states. An atom can move from one energy state
to another when it receives or releases an amount
of energy equal to the energy difference between
the two states.
Let E1 be the lower energy state and E2 be the
excited state. Let a monochromatic radiation of
frequency ν and energy hn be incident on the
medium. If hν= E2-E1, three distinct process may
occur in the medium
1. Stimulated absorption: This is the
transition in which an atom in the lower
energy state E1 absorbs the incident
photon and gets excited to the higher
energy state E2. The number of transition
occurring at any instant is directly
proportional to the number of atoms in
the lower energy state and the photon
density of the incident beam.
The rate of absorption can be written as
R12=B12ργN1. Where B12 is the Einstein’s
coefficient, N1 number of atoms in lower
state, ργ is the photon density of incident
radiation.
2. Spontaneous emission: Atoms residing in
the excited state will be unstable and has
a natural tendency to seek out to the
lowest energy state. They give up the
excess energy hn = E2-E1, in the form of
photons. The process does not requires
any external impart of energy and is called
spontaneous emission. The number of
transition taking place does not depends
on the photon present in the incident
radiation and depends only on the density
of N2 in the excited state E2. Also the
light emitted by the medium will be
incoherent.
The rate of spontaneous emission can be written
as R21 = A21N2 where A21 is Einstein’s coefficient
and N2 is the number of atoms in the excited
state.

3. 3ASN
3. Stimulated emission: Here a photon is
incident on the atom residing in the
excited state thereby inducing a
downward transition of the atom to the
lowest level. The transition generates a
second photon which would be identical
to the triggering photon in respect of
frequency, phase, direction and
polarization. If the number of atoms in the
excited state is large, an avalanche of
secondary photons can be generated
which results in intensified beam of
coherent light.
The rate is R21 = B21N2ρn where B21 is Einstein’s
coefficient.
Active medium: The medium in which light gets
amplified is called an active medium. It can be a
solid, liquid or gas containing atoms or molecules.
The main requirement of an active medium is that
we should be able to obtain population inversion
in it.
Active centers: Out of the large number of
different atoms in a species, only a fraction of
them are responsible for stimulated emission.
Such atoms are called active centers.
Metastable states: A higher energy state where the
life time is very large is called a metastable state.
Normally excited atoms have short life time.
Hence population inversion does not happens
even though pumping process is done. Therefore
it is necessary for the excited states to have a
longer life time.
Population inversion: A non-equilibrium condition
where the excited state is more populated than
the ground state.
The number of atoms occupying an energy state
is called population of that state. Let N1 and N2
be the populations of energy levels E1 and E2. In
thermal equilibrium, the population of energy
levels are fixed b Boltzmann factor as
N1 α 𝑒
−𝐸1
𝑘𝑡 and N2 α 𝑒
−𝐸2
𝑘𝑡
𝑁1
𝑁2
= 𝑒
−(𝐸1−𝐸2)
𝑘𝑡
-Ve sign shows that N1 > N2 at equilibrium.
Therefore more atoms are at the ground state.
Hence for the stimulated emission to occur, the
population distribution between E1 and E2 are
inverted.
Before After
Pumping: For maintaining the condition of
population inversion, atoms must be excited
continuously. The process of supplying energy to
the medium in order to achieve population
inversion is called pumping.
Methods of pumping:
1. Optical pumping- Widely used in solid state
lasers. A light source is used for pumping.
Photons are used to excite atoms. A+hν=𝐴∗
,
A is the atom in the lower state and 𝐴∗
is the
atom in the excited state. E g: ruby lase, Nd-
YAG laser
2. Direct electron excitation- Used in gaseous
lasers. An electric field is applied which
causes the flow of electrons from cathode to

4. 4ASN
anode. While travelling these electrons
collides with atoms of the active medium and
hence excites the atoms. A+𝑒−
= 𝐴∗
E g:
Argon laser.
3. In elastic atom-atom collision: Used in
gaseous laser where a mixture of gases are
used as the active medium. Due to electron
bombardment atoms of one gases gets
excited into higher level. These excited atoms
collides with atoms of other gas and excites
them. A+𝑒−
= 𝐴∗
+𝑒−
𝐴∗
+ 𝐵 = 𝐴 + 𝐵∗
e g:
He-Ne laser
4. Direct conversion: Here energy is supplied
directly in the form of electrical energy. E g:
semiconductor laser (electron –Hole
recombination leads to emission of radiation).
5. Chemical reaction: Used in chemical laser.
Due to the chemical reaction produced in the
excited state. E g: H2+F2=2HF
Pumping schemes:
1. Three-level pumping: In three level pumping,
between the ground state and excited state,
there lies a metastable state having a longer
life time. The active centers are excited from
ground level E1 to higher level E3 by some
pumping process. After pumping more than
half of the active centers, they gets decayed
to level 2 by non-radiated transition. It is a
fast decay process (10−8
𝑠𝑒𝑐). They reside in
level 2 for(10−4
− 10−3
𝑠𝑒𝑐). Thus population
in E2 > E1. This leads to population inversion
and finally results in stimulated emission.
Disadvantage: The output is pulsated and
very high pumping source is required for
pumping more than 50% of active centers.
2. Four level pumping: In four level pumping,
between the ground state and excited state,
there lies a metastable state and an ordinary
excited state with a very wide energy gap. By
the process of pumping, atoms gets excited
to level 4 and gets readily de excited to level
3 by radiative transition. This results in a
condition of population inversion between
level 3 and level 2.
Advantage: Output is continuous and power
requirement for pumping is low.
Spontaneous emission Stimulated emission
Atoms return to lower
energy state without the
help of external energy.
Atoms return to lower
energy state with the help
of external energy.
Uncontrolled random
process
Controlled regular process
Polychromatic radiation Monochromatic radiation
Incoherent radiation Coherent radiation
Emission cannot be
multiplied with chain
reaction
Emission can be multiplied
with chain reaction
Intensity of output radiation
depends on the atoms
present in the excited state.
Intensity of output radiation
depends on the atoms
present in the excited state
and also on the number of
incident photons.