1) Laser light is produced through stimulated emission of radiation using a process called optical amplification. This involves exciting atoms in an active medium to a higher energy state and using stimulated emission to generate coherent light. 2) The key requirements for laser action are population inversion, where more atoms are in an excited state than a lower energy state, stimulated emission of radiation, and optical feedback from an optical cavity formed by mirrors. 3) Lasers produce highly directional, coherent light that is monochromatic, while ordinary light is polychromatic, spreads in many directions, and is incoherent. The essential elements of a laser are an active medium, a power supply to excite the atoms, and an optical resonator

1. L ight
A mplification by
S timulated
E mission of
R adiation
Laser
• Light Amplification by Stimulated Emission of
Radiation

2. The contents
• Bohr model of atom
• Einstein Quantum theory of radiation
• Basic theory (necessary conditions of
laser)
• Essential elements of laser
• Properties of laser
• Comparison between laser and
ordinary light

3. Light Absorption and Emission from the atom

5. Boltzmann’s principle,
a fundamental law of thermodynamics, states that,
when a collection of atoms is at thermal equilibrium,
the relative population of any two energy levels is
given by
𝑁2
𝑁 1
= 𝑒
−
𝐸2−𝐸1
𝑘𝑇
N2, N1 are the population of the upper and lower energy
states respectively, T is equilibrium temperature, k
Boltzmann, s constant.

6. 2- Einstein's Quantum Theory of Radiation
Einstein proposed the following assumptions:
• The atom population densities N1 and N2 at energy levels E1 and
E2, respectively, are distributed according to the Boltzmann
distribution at that temperature.
• Population densities N1 and N2 are constant in time.
According to Einstein, the interaction of radiation with matter
could be explained in terms of three basic processes:
a- Absorption
b-Spontaneous Emission,
c Stimulated Emission

7. a-photons of energy h = E2 – E1 is incident on matter with
ground-state energy E1 , Excited state energy E2.
The resonant photon energy h raises the atom from energy
state to E2 .
In the process, the photon is absorbed.
a-Absorption,

8. The rate at which N1 atoms are raised from energy level
E1 to E2 is given by
Where the coefficient B12 is a constant characteristic of the
atom.
ρ (υ) is a factor represented the photon density as
a function of frequency.
 


1
12
.
1
N
B
dt
dN
abs









9. b-Spontaneous Emission,
• In this process, when an atom in an excited state E2
spontaneously gives up its energy and falls to E1, a photon
of energy h = E2 - E1 is emitted.
• The photon is emitted in a random direction.

10. The rate at which N2 atoms are decreased from energy
level E2 to E1 is given by
𝒅𝑵𝟐
𝒅𝒕 𝒔𝒑𝒐𝒏𝒕
= −𝑨𝟐𝟏𝑵𝟐
• The N2 population decreases with a time constant  =
1/A21,
• The constant  is referred to as the spontaneous
radiative lifetime of level E2;
• the coefficient A21 is referred to as the radiative rate.
The coefficient A21 is a constant, characteristic of the
atom.

11. Stimulated Emission
When an incident photon of resonant energy h = E2 – E1
passes by an atom in excited state E2,
it stimulates the electron to drop to the lower state, E1 and two
photons in the same direction are emitted

12. The rate at which the N2 atoms are stimulated by the photons is
proportional both to the number of atoms present and the
density of the radiation field
𝒅𝑵𝟐
𝒅𝒕 𝒔𝒆
= −𝑩𝟏𝟐𝑵𝟏𝝆 𝝂
for the Boltzmann distribution of atoms between the two energy
levels
𝑵𝟐
𝑵𝟏
= 𝒆
−
𝑬𝟐−𝑬𝟏
𝒌𝑻 = 𝒆−
𝒉𝝂
𝒌𝑻
that the rates
dN2/dt = N2B21p ()
and
dN1/dt = N1 B12p ()

13. The Basic Theory or (necessary conditions) of
laser
a -Population Inversion
There are two ways to obtain population
inversion,
• One way is reducing the atoms in the
lower level and this is difficult.
• The other is increasing the number of
atoms in the upper level via electrical
excitation, collisionnal energy transfer and
chemical reaction.

14. 3 -level system e .g Ruby laser

15. 4 -level system e g He-Ne laser

16. • When an incident photon of resonant energy h = E1 - E0
passes by an atom in excited state E1,
• it stimulates the atom to drop to the lower state, E0.
• In the process, the atom releases a photon of the same
energy, direction, phase,
• The net effect, then, is two identical photons in the place of
one, or an increase in the intensity of the incident beam
b- Stimulated emission

17. C- Light amplification
• When a source of energy such as light or electrical
energy is supplied to system of atom in excited state
may be some atoms spontaneously emitted photon
or stimulate other atoms to emit a photon to a lower
energy level
• By repeating the same method the umber of photons
increases and the laser source emits light

19. Properties of laser
a - Monochromaticity
b- Coherence
c-Directionality

20. a - Monochromaticity
The monochromaticity is that property of the laser
radiation which describes its spectral distribution correlated
with its intensity.
Fig. shows in the laser beam case the maximum intensity is
obtained at a given wavelength (o) around which the most part
of the radiation is emitted. The laser beam intensity drops to
zero very fast at wavelengths other than the wavelength of the
maximum. This property is often described as the high spectral
purity of the laser beam.

23. Properties of ordinary light
Polychromatic:
Ordinary light consists of many
wavelengths of light such as
mercury vapour lamp
Directionality :
Ordinary light spreads in all
directions
Incoherence:
In ordinary light the wave
trains of same frequency are in
out of phase .

24.  laser is monochromatic light while In contrast, ordinary
white light is a polychromatic light.
 Lasers emit light that is highly directional, In contract
ordinary light is spreads in all directions
 The light from a laser is said to be coherent, In contract
ordinary light is incoherent
Comparison between laser and ordinary light

25. Essential elements of LASER
ACTIVE MEDIUM
Solid (Crystal)
Gas
Semiconductor (Diode)
Liquid (Dye)
SOURCE OF ENERGY
Optical
Electrical
Chemical
OPTICAL
RESONATOR
HR Mirror and
Output Coupler
The Active Medium contains atoms which can emit light
by stimulated emission.
The source of energy is a source of energy to excite the
atoms to the proper energy state.
The Optical Resonator reflects the laser beam through
the active medium for amplification.
High Reflectance
Mirror (HR)
Output Coupler
Mirror (OC)
Active
Medium
Output
Beam
Excitation
Mechanism
Optical Resonator
Laser-Professionals.com