Lasers, which stand for Light Amplification by Stimulated Emission of Radiation, are devices that generate light through stimulated emission, and possess unique properties such as high directionality, monochromaticity, intensity, and coherence. The document outlines the principles of laser operation including population inversion and various pumping mechanisms, as well as applications in medicine, communication, industry, science, and military. Additionally, it describes the working principle of optical fibers based on total internal reflection and their role in communication systems.

1.  The acronym LASER stands for Light Amplification by
Stimulated Emission of Radiation.
 A laser is a device that generates light by a process called
STIMULATED EMISSION.
 The theoretical explanation for laser oscillation was given by
A.L Schawlow and C.H. Townes in the year 1958. The 1st laser,
namely, Ruby Laser, was demonstrated by T.H. Maiman in the
year 1960.
Introduction
Unit-4 –LASER And Fibre Optics

2.  The laser light exhibits some peculiar properties than compare with the convectional light.
Those are
• Highly directionality
• Highly monochromatic
• Highly intense
• Highly coherence
 Highly directionality The light ray
coming ordinary light source
travels in all directions, but laser
light travels in single direction.
Characteristics of LASER
Highly directionality

3.  The light from a normal monochromatic source spreads over a
range of wavelength of the order of 100 nm. But the spread is
of the order of 1 nm for laser
 Since an ordinary light spreads in all directions, the
intensity reaching the target is very less. But in the case
of laser, due to high directionality, the intensity of laser
beam reaching the target is of high intense beam. For
example, 1 mill watt power of He-Ne laser appears to
be brighter than the sunlight.
Characteristics of LASER . . .
Highly intense
Highly monochromatic
 Hence laser is highly monochromatic, that is, it can emit light
of single wavelength

4.  The wave trains which are identical in phase and direction are
called coherent waves.
Characteristics of LASER . . .
Highly coherent
 Since all the constituent photons of laser beam possess the
same energy, momentum and propagate in same direction,
the laser beam is said to be highly coherent.
Differences between ordinary light and Laser beam

5.  If a photon of energy hv is incident on the atom. In the lower
state, the atom absorbs the incident photon and gets excited,
to jump the higher energy state. This process is called
absorption.
where B12 is the proportionality const. known as prob. of absorption of radiation per unit time.
Spontaneous and Stimulated emission
 The rate of absorption R12 is proportional to the population of lower energy level N1 and to the
density of incident radiation ρ. Hence
Absorption
Atom + photon = Atom*
E1 + hv = E2
hv = E2 - E1
R12 α N1ρ
or R12 = B12N1ρ

6. Spontaneous and Stimulated emission . . .
Spontaneous emission
It is a process in which there is an emission of a photon
whenever an atom transits from a higher to lower energy state
without the aid of any external agency.
 For this process to take place, the atom has to be in the excited state.
 Since the higher energy level is an unstable one, the excited atom in the higher level E2
spontaneously returns to the lower energy level E1with the emission of photon of energy
hν = E2 - E1
Atom* = Atom + photon
where A21 is the proportionality constant known as prob. of spontaneous emission per unit time.
 The rate of spontaneous emission of radiation R21(sp) is proportional to the population N2 at
the higher energy level E2. Hence
R21(sp) α N2
or R21(sp) = A21N2

7. Spontaneous and Stimulated emission . . .
Stimulated emission
It is a emission of photon whenever an atom transits from a
higher to lower energy state under the influence of an external
agency i.e., an external photon.
 For this process to take place, the atom has to be in the excited state.
 Let a photon having an energy hν interact with the atom in the excited state. This incident
photon triggers the excited atom in the higher level E2 to transit to lower level E1, resulting in
the emission of another photon of energy hν.
 Both the inducing (incident) and the emitted photon have the same phase, energy and
direction of movement.
where B21 is the proportionality constant known as prob. of stimulated emission per unit time.
 This kind of emission is responsible for laser action.
R21(st) α N2 ρ
or R21(st) = B21N2 ρ
 The rate of stimulated emission of radiation R21(st) is proportional to the population N2 at the
higher energy level E2 and to the density ρ of the inducing photon.

8. Spontaneous and Stimulated emission . . .
Differences between Spontaneous and Stimulated emission

9. Working of LASER
Population Inversion
 It is a state of achieving more
number of atoms in excited state
compared to ground state. It is an
essential condition for producing
laser beam.
 Population inversion can be
achieved by a process called
pumping.
Pumping Mechanism
 It is the mechanism of exciting atoms from the lower energy state to
a higher energy state by supplying energy from an external source.
 The most commonly used pumping mechanism are: optical,
electrical and direct.

10. Working of LASER . . . Pumping Mechanism . . .
Optical Pumping
In this type a direct conversion of electric energy into light takes place. This technique is
adopted in semiconductor laser.
In addition to above three, the other types of pumping are inelastic collision between atoms and
chemical methods which are respectively adopted in He-Ne gas laser and in dye and chemical
lasers.
In this type of pumping, atoms are excited by means of an
external optical source. This type is adopted in solid state lasers
such as ruby and Nd: YAG laser.
Electrical Pumping
In this type of pumping the electrons are accelerated to a high velocity by a strong electric
field. These moving electrons collide with the neutral gas atoms and ionize the medium. Thus,
due to ionization they get raised to a higher energy level. This technique is adopted in gas
lasers such as CO2 laser.
Direct Conversion

11. Working of LASER . . .
Life time
The limited time for which an atom remains in the excited is known as life time.
Metastable state
It is an energy level in an atomic system where
the life time of atoms is very large (of the order
10-3 to 10-2 seconds).It helps in achieving the
population inversion.

12. Working of LASER . . .
Components of LASER

13. Working of LASER . . .
Components of LASER . . .
It is a pair of reflecting surfaces (mirrors) of which one is a
perfect reflector and the other is a partial reflector. It is used
for amplification of photons thereby producing an intense
and highly coherent output.
OR

14. Applications of LASER
The most significant applications of lasers include:
1. Lasers in medicine
2. Lasers in communications
3. Lasers in industries
4. Lasers in science and technology
5. Lasers in military
1. Lasers in Medicine
 Lasers are used for bloodless surgery.
 Lasers are used to destroy kidney stones.
 Lasers are used in cancer diagnosis and therapy.
 Lasers are used for eye lens curvature corrections.
 Lasers are used in fiber-optic endoscope to detect ulcers in the intestines.
 The liver and lung diseases could be treated by using lasers.
 Lasers are used to study the internal structure of microorganisms and cells.
 Lasers are used to create plasma.

15. Applications of LASER . . .
1. Lasers in Medicine . . .
 Lasers are used to remove tumors successfully.
 Lasers are used to remove the caries or decayed portion of the teeth.
 Lasers are used in cosmetic treatments such as acne treatment, cellulite and hair removal.
2. Lasers in Communications
 Laser light is used in optical fiber communications to send information over large distances
with low loss.
 Laser light is used in underwater communication networks.
 Lasers are used in space communication, radars and satellites.
3. Lasers in Industries
 For cutting, welding, melting and drilling.
 To test the quality of the material.
 For the heat treatment.

16. Applications of LASER . . .
4. Lasers in Science and Technology
 A laser helps in studying the Brownian motion of particles.
 With the help of a helium-neon laser, it was proved that the velocity of light is same in all
directions.
 With the help of a laser, it is possible to count the number of atoms in a substance.
 Lasers are used in computers to retrieve stored information from a Compact Disc (CD).
 Lasers are used to store large amount of information or data in CD-ROM.
 Lasers helps in determining the rate of rotation of the earth accurately.
 Lasers are used in computer printers.
 Lasers are used for producing three-dimensional pictures in space without the use of lens.
 Lasers are used for detecting earthquakes and underwater nuclear blasts.
5. Lasers in Military
 Laser range finders are used to determine the distance to an object.
 The ring laser gyroscope is used for sensing and measuring very small angle of rotation of the
moving objects.

17. Introduction
FIBER OPTICS

18. Optical Fibre Construction
Refractive index 𝜂 =
𝑐
𝑣
The ratio of the speed of light in a vacuum to its
speed in a specific medium.
The refractive index of glass ng is 1.52 and refractive index of water nw is 1.33. Since the
refractive index of glass is higher than the water, the speed of light in water is faster than the
speed of light through glass.

19. Optical Fibre Construction . . .
Refractive
index

20. Working Principle of Optical Fibre
 Let us consider a point source O in optically denser
medium (Water or medium with high refractive index).
 As the angle of incidence increases, the angle of refraction also increases.
Total Internal Reflection
 Let XY be the boundary separating the optically denser
medium.
 At a particular angle of incidence ic, called as Critical angle, the angle of refraction is 90°
and hence the refracted ray moves along the surface of water i.e. along XY.
 If the angle of incidence is more than ic, there is no
refracted ray, the incident ray is completely reflected
back in the water. This phenomenon is known as total
internal reflection.
 Optical fiber works on the principle of total internal
reflection.

21. Working Principle of Optical Fibre . . .
Explanation

22. Working Principle of Optical Fibre . . .
Explanation . . .

23. Acceptance Angle:
Numerical Aperture:

24. Types of Optical Fibers:

25. Attenuation (Power loss) in Optical Fibers:

26. Optical Fiber in Communication System:
Information signal source:
•The information signal to be transmitted
may be voice, video or computer data
(analog signals).
• In order to communicate through
optical fiber, the analog signals are
converted into electrical signals.( by
Analog to Digital converter)
•The converted electrical signals are
passed through the transmitter.

27. Optical Fiber in Communication System:
Transmitter:
•The transmitter is a modulator device used
to receive electrical input signal, and then
modulate it into digital pulses for
propagation into an optical fiber.
•The modulator consists of a driver and a
light source as shown in fig. The driver
receives the electrical signals and then
converts into the digital pulses. These digital
pulses are converted into optical signals after
passing through a light source, generally
either light emitting diodes (LED’s) or a
semi conductor laser is used as light source.
•The optical signals are then focused into the
optical fiber as shown in fig.

28. Optical Fiber in Communication System:
Optical Fiber (or) Transmission
medium:
•The optical fiber is used as transmission
medium between the transmitter and the
receiver.
•The optical signals are then fed into an
optical fiber cable where they are
transmitted over long distances using the
principle of total internal reflection.

29. Optical Fiber in Communication System:
Receiver:
•The receiver is a demodulator device used to receive the
optical signals from the optical fiber and then convert
into electrical signals.
•The demodulator consists of a photodetector, an
amplifier and a signal restorer.
•The optical signals which are emerging from the optical
fiber are received by photo detector.
•The photodetector converts the optical signals into
electrical signals.
•The electrical signals are then amplified by the amplifier
and the amplified electrical signals are converted into
digital form.
•The amplified electrical signals are fed to a signal
restorer where the original voice is recovered.

30. Advantages of Optical Fiber:

31. Advantages of Optical Fiber:

32. Applications of Optical Fibre: