Presentation on Optical Fiber for UG Physics students by Dr. P D Shirbhate assistant Professor, Department of Physics G S Gawande college, Umarkhed Dist Yavatmal.

1. OPTICAL FIBER
G. S. Gawande College, Umarkhed
Dist. Yavatmal
By
Dr. Praful D. Shirbhate
Assistant Professor
DEPARTMENT OF PHYSICS

2. SYLLABUS
Introduction: Optical fibers
Total internal Reflection
Propagation of light through a Optical Fiber
Acceptance angle, Numerical aperture
Mode of Propagation & classification
 Fiber Losses: Attenuation and dispersion
 Applications of the Optical Fiber

3. Fibre Optics
3

4. Fibre optics
 Optical fiber are made up of glass or plastic as thin
as human hair, designed to guide light waves
along its length.
 Optical fiber works on the principle of Total internal
reflection.
4

5.  Core – Innermost
cylindrical region is light
guiding region know as
core.
 Cladding – coaxial
middle region having RI
less than core( n2< n1)
 Protective Buffer
coating(Sheath) -
Plastic coating that
glass or plastic
cladding
fiber core
Plastic jacket
5
Structure of Optical Fiber

6. Core
Cladding
Sheath
≈ 8-60µm
125 µm
250 -900µm
Structure of optical fibre
6

7. 7
Total Internal Reflection
The phenomenon in which light is totally reflected from a denser-
to-rarer medium boundary is known as total internal reflection
Snell’s law for this case maybe written as
Where θ1 is the angle of incidence of light ray in the denser medium and θ2 is the
angle of refraction in the rarer medium. Thus,
•If θ1< θc, the ray refracts into the rarer medium
•If θ1 = θc, the ray just grazes the interface of rarer-to-denser media
•If θ1 > θc, the ray is reflected back into the denser medium.
•Where, θc is the critical angle.
……………..(1)

8. 8
Normal
2
3
4
5
µ2
µ1
1
θ2
θ1
The angle of incidence for which the angle of
refraction is 900 is known as critical angle θc.
Acc. Snell’s law,
Critical Angle
When θ2 = 90o,
When the rarer medium is air, µ2 = 1 and
writing µ2 = µ, we obtain
…………………… (3)

9. Propagation of light through a Optical fiber /
Acceptance Angle
9
 Let us consider a optical fiber into which light is launched at
one end, as shown in Figure .
 Let the refractive index of the core be n1 and the refractive
index of the cladding be n2, (n2<n1).
 Let n0 be the refractive index of the medium from which
light is launched into the fiber.

10. Derivation
 Applying Snell’s law to the launching face of the
fibre, ------------ (1)
 Applying Snell’s law,
------------------ (2)
 Using equation (1) & (2)
 For air medium, n0= 1.The angle θ0 is called the
acceptance angle of the fibre.
1
2
90sin
sin
n
nc


10

11. Propagation of light through a cladded
fibre:
Acceptance Angle: The maximum angle that a
light ray can have relative to the axis of the fibre
and propagate down the fibre.
θ0 = sin-1[ n1
2 - n2
2 ]1/2
It depends on core diameter and the material.
Acceptance cone:- It is twice of the acceptance
angle i.e. 2θ0
.
11

12. Numerical Aperture
 Numerical Aperture is defined as the sine of
the acceptance angle.
12
Fractional Refractive Index Change
Fractional refractive index change is the fractional
difference between the refractive indices of the
core and the cladding.

13. Modes of propagation
 Modes are the possible number of paths of light that
propogates through optical fibre.
 The waves travel in number of directions and not all
waves are trapped within the optical fibre.
 The light rays traveling through a fibre are classified
as 1. Axial rays 2. Zigzag rays.
13

14. Classification of Optical Fibre
Optical Fibre
On the basis of
refractive
index profile
Step index
fibre
Graded
index fibre
On the
basis of
materials
On the basis
of light
propagation
Plastic
fibre
Multimo
de fibre
(MMF)
Single
mode fibre
(SMF)
Glass
fibre
PCS
(polymer
clad
silica)
14

15. Single Mode Step Index Fiber
 A single mode step index fiber has a very fine thin core of diameter of 8 µm
to 12 µm as shown in fig.
 The core is surrounded by a thick cladding of lower refractive index.
 The cladding is composed of silica lightly doped with phosphorous oxide.
 The external diameter of the cladding is of the order of 125 µm
 Since the refractive index profile of Single mode fiber is step-type, the fiber is called
a single Mode step index fiber.
Only one mode can propagate through a single mode step index fiber. This mode is
known as the zero order mode.
Both ∆ and NA are very small for single mode fibers. ∆ is of the order of 0.002.

16. Multimode Step Index Fiber
 A multimode step index fiber is very much similar to the single mode step
index fiber except that its core is of larger diameter.
 The core diameter is of the order of 50 to 100 µm, which is very large
compared to the wavelength of light.
 The external diameter of cladding is about 150 to 250 µm
 Since the refractive index profile of Multimode fiber is step-type, the fiber is called
as Multimode step index fiber.
 Multimode step index fibers allow finite number of guided modes.
 The direction of polarization, alignment of electric and magnetic fields will be
different in rays of different modes.
 In other words, many zigzag paths of propagation are permitted in a MMF

17. Graded Index (GRIN) Fiber
 The size of the graded index fiber is about the same as the step index fiber
 A graded index fiber is a multimode fiber with a core consisting of
concentric layers of different refractive indices.
 The refractive index of the core has a high value at the centre and falls of
with increasing radial distance from the axis. shown in Fig.
 The index profile is parabolic and is preferred for different applications
 A GRIN fiber supports a finite number of guided modes. As a light ray goes from a
region of higher refractive index to a region of lower refractive index, it is bent away
from the normal.
In the graded index fiber, rays making larger angles with the axis traverse longer path
but they travel in a region of lower refractive index and hence at a higher speed of
propagation. Thus, there occurs a self focusing effect.

18. Advantages of optical Fibre
1) Cheaper
2) Smaller in size, lighter in weight, flexible yet strong
3) Not hazardous
4) Immune to EMI(electromagnetic interference) and
RFI(radiofrequency interference)
5) No cross talk
6) Wider bandwidth
7) Low loss per unit length
18

19. Losses
19
ATTENUATION:
DISPERSION:
1) Absorption by material
2) Rayleigh Scattering
3) Geometric effects
1) Intramodal Dispersion
2) Intermodal Dispersion
3) Material Dispersion
change in shape
of signal .

20. ATTENUATION
The attenuation of optical signal is defined as
the ratio of the optical input power from a fibre of
length L to the optical output power.
It is expressed in decibel per kilometer
(dB/Km).
Pi is the power of optical signal launched at one
end of the fibre
P0 is the power of the optical signal emerging from
the other end of the fibre.
20

21. Different Mechanisms of Attenuation:
Intrinsic
Losses
.
Absorption
Losses
Scattering
Losses
Extrinsic
Losses
Microbend
Losses
Macrobend
Losses
Waveguide
Losses
Mode
Coupling
Losses

22. 1) Intrinsic losses:-Intrinsic losses are influenced by the material composition and
purification level. Impurities and inhomogeneities
in material cause signal absorption and scattering.
a) Absorption Losses: Highly pure glass absorbs light in specific wavelength region.
Absorption loss is caused by the presence of impurities such as traces of metal ions
(e.g., Cu2+, Fe3+) and hydroxyl (OH–) ions. . This phenomenon causes a light signal
to be absorbed by natural impurities in the glass, and converted to vibration energy
or some other form of energy. Absorption losses can be limited by controlling the
amount of impurities during manufacturing process.
b) Scattering Losses : Despite the careful manufacturing techniques, most fibers are
inhomogeneous that have disordered, amorphous structures Inhomogeneities can be
either structural or compositional in nature. In structural inhomogeneities, the basic
molecular structure has random components, whereas, in compositional
inhomogeneities, the chemical composition of the material varies. The local
microscopic density variations in glass cause local variations in refractive index.
These variations which are inherent in manufacturing process and cannot be
eliminated and act as obstructions and scatter light in all directions. This is known as
Rayleigh scattering. This type of loss can be reduce by using improved
manufacturing methods

23. 2) Extrinsic losses:- They are cause by geometric effects. Irregularities of geometric nature cause light energy losses.
•Any bends in the optical fiber produce radiative losses.
a) Microbend losses:- Microbend is a small-scale distortion. It is generally form
due to pressure on the fiber. The bend may not be clearly visible on inspection.
Microbends may be introduced during manufacturing or installation processes.
Microbending may occur, for example, due to winding of optical fiber cable over
spools. Light rays get scattered at the small bends and escape into the
cladding. Such losses are known as microbend losses.
b) Macrobend losses:- A macrobend is a large-scale bend that is visible. When a
fiber is bent through a large angle, strain is placed on the fiber along the region that is
bent. The bending strain will affect the refractive index and the critical angle of the light
ray in that specific area. As a result, light traveling in the core can refract out, and loss
occurs
Microbend losses Macrobend losses

24. c) Waveguide losses:- Due to irregularities in the optical fiber geometry, the
incident angle becomes less than the critical angle for higher order modes. As a
result, part of the light ray will be refracted into the cladding. They are known as
waveguide losses.
d) Mode coupling losses:- The power launched into a propagating mode may
get coupled into a leaky mode at some points of the fiber. The coupling occurs
due to the small imperfections present in the core and imperfectly aligned
connectors.

25. Dispersion:
A light pulse launched into a fiber decreases in amplitude, as it travels along the
fiber, due to losses in the fiber. It also spreads during its travel. The pulse received at
the output is wider than input pulse, as shown in Fig. It means that the pulse becomes
distorted as it propagates through the fiber. Such distortion arises due to dispersion
effects.
Dispersion is typically measured in nanoseconds per kilometer (ns/km.)
There are three mechanisms in the distortion of the light pulse in a fiber.
(i) Material dispersion,
(ii) Waveguide dispersion
(iii) Intermodal dispersion.

26. Fiber optic Communication:
Fiber optic communication system consist of three major part:
Transmitter
Optical fiber
Receivers

27. Applications of the Optical Fiber:
1) They are used for illumination and short distance transmission of images.
2) They are used as fiber optics sensors.
3) They are used as waveguides in telecommunications.
4) They are used in medical diagnostics and military applications

28. Thank You