1. 1. LASER
2.OPTICAL FIBER

2. 1. LASER

3. SUB TOPICS:-
• Definition of lasers
• Characteristics of laser
• Emission and absorption of radiation
• Population Inversion
• ND YAG laser
• Application of laser

4.  A laser is a device which is built on the
principles of quantum mechanics to create a
beam of light where all of the photons are in a
coherent state - usually with the same frequency
and phase. (Most light sources emit incoherent
light, where the phase varies randomly.) Among
the other effects, this means that the light from a
laser is often tightly focused and does not
diverge much, resulting in the traditional laser
beam.

5. Characteristics of laser:-
 Laser Light is Highly Coherent:-
Wave trains for laser light may be several hundred
kilometers long. Interference fringes can be set up by combining
two beams that have followed separate paths whose lengths
differ by as much as this amount. The corresponding coherence
length for light from a tungsten filament lamp or a gas discharge
tube is typically considerably less than 1 m.
 Laser Light is Highly Monochromatic :-
Tungsten light, spread over a continuous
spectrum, gives us no basis for comparison. The light from
selected lines in a gas discharge tube, however, can have
wavelengths in the visible region that are precise to about 1 part
in 106. The sharpness of definition of laser light can easily be a
thousand times greater, or 1 part in 109.

6.  Laser Light can be Sharply
Focused:-
This property is related to the parallelism of
the laser beam. As for star light, the size of the
focused spot for a laser beam is limited only by
diffraction effects and not by the size of the source.
Flux densities for focused laser light of 1015 W cm-
2are readily achieved. An oxyacetylene flame, by
contrast, has a flux density of only 103Wcm-2.
 Brightness:-
The primary characteristic of laser radiation is
that lasers have a higher brightness than any oilier
light source. We define brightness as the power
emitted per unit area per unit solid angle.

7. Emission and absorption of
radiation:-
1. Absorption
2. Spontaneous Emission
3. Stimulated Emission
Therefore 3 process
of light emission:

8. Absorption:-
E2
E2
E1

9. Spontaneous Emission
E2
E1
„‟ Consider an atom (or molecule) of the material is existed
initially in an excited state E2 No external radiation is required to
initiate the emission. Since E2>E1, the atom will tend to
spontaneously decay to the ground state E1, a photon of energy h
=E2-E1 is released in a random direction as shown in (Fig. 1-ii). This
process is called “spontaneous emission‟‟

10. Stimulated Emission
E2
E1
“stimulated emission” requires the presence of external
radiation when an incident photon of energy h =E2-E1 passes by
an atom in an excited state E2, it stimulates the atom to drop or
decay to the lower state E1.

11. Population Inversion:-
 Therefore we must have a mechanism where N2 > N1
 This is called POPULATION INVERSION
 Population inversion can be created by introducing a so call met stable centre
where electrons can piled up to achieve a situation where more N2 than N1
 The process of attaining a population inversion is called pumping and the
objective is to obtain a non-thermal equilibrium.
 It is not possible to achieve population inversion with a 2-state system.
 If the radiation flux is made very large the probability of stimulated emission
and absorption can be made far exceed the rate of spontaneous emission.
 But in 2-state system, the best we can get is N1 = N2.
 To create population inversion, a 3-state system is required.
 The system is pumped with radiation of energy E31 then atoms in state 3 relax
to state 2 non radioactively.
 The electrons from E2 will now jump to E1 to give out radiation.

12. 3 states system

13. Population Inversion

14. ND YAG Laser:-

15.  Nd YAG is a solid state laser four level laser. Nd stands for
neodymium and YAG for Yttrium aluminium garnet (Y3Al5O12). It is
developed by J.E. Geusic, H.M. Marcos and L.G. Van Vitert in 1964.
The rod of Y3Al5O12- is doped 1% with triply ionized neodymium.
Nd3+ ions will replace the Y3+ ions in the crystal. Maximum length of
the rod is about 10 cm and diameter is 6-9 cm.
 Active medium: Nd3+ ions act as active medium or active centers.
YAG is just the host.
 Pumping source: The pumping of Nd3+ ions to upper levels is done by
krypton arc lamp. Xenon lamp can also be used as pumping source.
Thus, the optical pumping is used to achieve population inversion.
 Optical resonator system: The ends of the ND YAG rod are polished
and silvered so as to act as the optical resonator system.

16. Application of laser:-
• Medicine : Bloodless surgery, laser healing, surgical treatment,
kidney stone treatment, eye treatment, dentistry, etc.
• Industry : Cutting, welding, material heat treatment, marking parts.
• Defense : Marking targets, guiding munitions, missile defense,
electro-optical countermeasures (EOCM), alternative to radar.
• Research : Spectroscopy, laser ablation, Laser annealing, laser
scattering, laser interferometer, LIDAR (Light Detection And
Ranging).
• Product development : Laser printers, CDs, barcode scanners,
thermometers, laser pointers, holograms, bubble grams.
• Laser lighting displays : Laser light shows.
• Laser skin procedures : such as acne treatment, cellulite reduction,
and hair removal.

17. 2. FIBER OPTICS

18. Advantages of Optical Fiber
 Thinner
 Less Expensive
 Higher Carrying
Capacity
 Less Signal
Degradation& Digital
Signals
 Light Signals
 Non-Flammable
 LightWeight

19. Advantages of fiber optics
 Much Higher Bandwidth (Gbps) - Thousands of
channels can be multiplexed together over one
strand of fiber
 Immunity to Noise - Immune to electromagnetic
interference (EMI).
 Safety - Doesn’t transmit electrical
signals, making it safe in environments like a gas
pipeline.
 High Security - Impossible to “tap into.”

20. Advantages of fiber optics
 Less Loss - Repeaters can be spaced 75 miles
apart (fibers can be made to have only 0.2 dB/km
of attenuation)
 Reliability - More resilient than copper in extreme
environmental conditions.
 Size - Lighter and more compact than copper.
 Flexibility - Unlike impure, brittle glass, fiber is
physically very flexible.

21. Total Internal Reflection in Fiber

22. Critical angle, θc
 The minimum angle of incidence at which a light
ray ay strike the interface of two media and result
in an angle of refraction of 90° or greater.

23. Acceptance angle:-
 The maximum angle in which external light rays
may strike the air/glass interface and still
propagate down the fiber.

24. Acceptance angle:-
 θin (max) = sin-1
 Where,
 θin (max) – acceptance angle (degrees)
 n1 – refractive index of glass fiber core
 n2 – refractive index of quartz fiber cladding

25.  Consider an optical fiber having a core of refractive
index n1 and cladding of refractive index n2. let the
incident light makes an angle i with the core axis as
shown in figure. Then the light gets refracted at an
angle θ and fall on the core-cladding interface at an
angle where,
.............(1)
By Snell’s law at the point of entrance of light in
to the optical fiber we get,

26.  When light travels from core to cladding it moves from
denser to rarer medium and so it may be totally
reflected back to the core medium if θ‘ exceeds the
critical angle θ'c. The critical angle is that angle of
incidence in denser medium (n1) for which angle of
refraction become 90°. Using Snell’s laws at core
cladding interface,
 or

27. Therefore, for light to be propagated within the core
of optical fiber as guided wave, the angle of incidence
at core-cladding interface should be greater than θ'c.
As i increases, θ increases and so θ' decreases.
Therefore, there is maximum value of angle of
incidence beyond which, it does not propagate rather
it is refracted in to cladding medium . This maximum
value of i say im is called maximum angle of
acceptance and n0 sin im is termed as the numerical
aperture (NA)

28. The significance of NA is that light entering in the cone of semi
vertical angle im only propagate through the fiber. The higher the value of
im or NA more is the light collected for propagation in the fiber. Numerical
aperture is thus considered as a light gathering capacity of an optical fiber.
Numerical Aperture is defined as the Sine of half of the angle of fiber's
light acceptance cone. i.e. NA= Sin θa where θa, is called acceptance
cone angle.

29. Numerical Aperture (NA)
 Used to describe the light-gathering or light-
collecting ability of an optical fiber.
 In optics, the numerical aperture (NA) of an
optical system is a dimensionless number that
characterizes the range of angles over which the
system can accept or emit light

30. The numerical aperture in
respect to a point P depends on
the half-angle θ of the
maximum cone of light that can
enter or exit the lens.

31.  Two main categories of
optical fiber used in
fiber optic
communications are
multi-mode optical
fiber and single-mode
optical fiber.

32.  Single-mode fibers – used to transmit one signal per
fiber (used in telephone and cable TV). They have
small cores(9 microns in diameter) and transmit
infra-red light from laser.

33.  Multi-mode fibers – used to transmit many
signals per fiber (used in computer networks).
They have larger cores(62.5 microns in
diameter) and transmit infra-red light from
LED.

34.  Multimode fiber has a
larger core (≥ 50
micrometres), allowing
less precise, cheaper
transmitters and
receivers to connect to it
as well as cheaper
connectors.

35.  However, multi-mode fiber introduces
multimode distortion which often limits the
bandwidth and length of the link.
Furthermore, because of its higher dopant
content, multimode fiber is usually more
expensive and exhibits higher attenuation.

36.  multimode step-index fiber
 the reflective walls of the fiber move the light pulses to
the receiver
 multimode graded-index fiber
 acts to refract the light toward the center of the fiber
by variations in the density
 single mode fiber
 the light is guided down the center of an extremely
narrow core
36

37.  The index profile of an optical fiber is a
graphical representation of the magnitude of
the refractive index across the fiber.
 The refractive index is plotted on the
horizontal axis, and the radial distance from
the core axis is plotted on the vertical axis.

38.  A step-index fiber has a central core with a
uniform refractive index. An outside cladding
that also has a uniform refractive index
surrounds the core;
 however, the refractive index of the cladding
is less than that of the central core.

39.  In graded-index fiber, the index of refraction
in the core decreases continuously between
the axis and the cladding. This causes light
rays to bend smoothly as they approach the
cladding, rather than reflecting abruptly from
the core-cladding boundary.

41.  Communication -
 Telephone transmission method uses fibre-optic cables. Optical fibres
transmit energy in the form of light pulses.The technology is similar to that of
the coaxial cable, except that the optical fibres can handle tens of thousands of
conversations simultaneously.
 Medical uses -
 Optical fibres are well suited for medical use.They can be made in
extremely thin, flexible strands for insertion into the blood vessels, lungs, and
other hollow parts of the body. Optical fibres are used in a number of instruments
that enable doctors to view internal body parts without having to perform
surgery.
 Simple uses -
 The simplest application of optical fibres is the transmission of light to
locations otherwise hard to reach. Also, bundles of several thousand very thin
fibres assembled precisely side by side and optically polished at their ends, can be
used to transmit images.

42. THANK YOU!!!