Optical fibers use the principle of total internal reflection to carry light along their length. Light is kept in the core of the fiber by total internal reflection at the core-cladding interface. This occurs when the angle of incidence is greater than the critical angle, which depends on the refractive index difference between the core and cladding materials. Dispersion and attenuation are factors that limit the performance of optical fibers by causing signal degradation over long distances. Optical fibers have advantages over metal wires including higher bandwidth capacity, lighter weight, and immunity to electromagnetic interference.

1. OPTICAL FIBER
Course: B.Tech
Subject: Engineering Physics
Unit: I
Chapter: 1
1

2. Basic principle Total Internal Reflection in Fiber
An optical fiber (or fibre) is a glass or plastic fiber that
carries light along its length.
Light is kept in the "core" of the optical fiber by total
internal reflection.
2

3. 3
What Makes The Light Stay in Fiber
Refraction
The light waves spread out along its beam.
Speed of light depend on the material used called
refractive index.
Speed of light in the material = speed of light in the free
space/refractive index
Lower refractive index  higher speed

4. 4
The Light is Refracted
This end travels
further than the
other hand
Lower Refractive index Region
Higher Refractive index Region

5. 5
Refraction
When a light ray encounters a boundary separating two
different media, part of the ray is reflected back into the first
medium and the remainder is bent (or refracted) as it enters
the second material. (Light entering an optical fiber bends in
towards the center of the fiber – refraction)
Refraction
LED or
LASER
Source

6. 6
Reflection
Light inside an optical fiber bounces off the
cladding - reflection
Reflection
LED or
LASER
Source

7. 3

8. 8
Critical Angle
If light inside an optical fiber strikes the cladding too
steeply, the light refracts into the cladding - determined by
the critical angle. (There will come a time when, eventually,
the angle of refraction reaches 90o and the light is refracted
along the boundary between the two materials. The angle of
incidence which results in this effect is called the critical
angle).
Critical Angle
n1Sin X=n2Sin90o

9. 9
Angle of Incidence
Also incident angle
Measured from perpendicular
Exercise: Mark two more incident angles
Incident Angles

10. 10
Angle of Reflection
Also reflection angle
Measured from perpendicular
Exercise: Mark the other reflection angle
Reflection Angle

11. 11
Reflection
Thus light is perfectly reflected at an interface between
two materials of different refractive index if:
 The light is incident on the interface from the
side of higher refractive index.
 The angle θ is greater than a specific value called
the “critical angle”.

12. 12
Angle of Refraction
Also refraction angle
Measured from perpendicular
Exercise: Mark the other refraction angle
Refraction Angle

13. 13
Angle Summary
Refraction Angle
Three important angles
The reflection angle always equals the incident
angle
Reflection Angle
Incident Angles

14. 14
Refractive Index
n = c/v
c = velocity of light in a vacuum
v = velocity of light in a specific
medium
light bends as it passes from one
medium to another with a different
index of refraction
air, n is about 1
glass, n is about 1.4
Light bends in towards normal -
lower n to higher n
Light bends
away from
normal - higher
n to lower n

15. 15
Snell’s Law
The amount light is bent by refraction is given by Snell’s
Law:
n1sinq1 = n2sinq2
Light is always refracted into a fiber (although there will be
a certain amount of Fresnel reflection)
Light can either bounce off the cladding (TIR) or refract
into the cladding

16. 16
Snell’s Law
Normal
Incidence
Angle(1)
Refraction
Angle(2)
Lower Refractive index(n2)
Higher Refractive index(n1)Ray of light

17. 17
Critical Angle Calculation
The angle of incidence that produces an angle of
refraction of 90° is the critical angle
 n1sin(qc) = n2sin(90°)
 n1sin(qc) = n2
 qc = sin-1(n2 /n1)
Light at incident angles
greater than the critical
angle will reflect back
into the core Critical Angle, qc
n1 = Refractive index of the core
n2 = Refractive index of the cladding

18. OPTICAL FIBER CONSTRUCTION
Core – thin glass center of the fiber where light travels.
Cladding – outer optical material surrounding the core
Buffer Coating – plastic coating that protect the fiber.
4

19. OPTICAL FIBER
The core, and the lower-refractive-index cladding, are
typically made of high-quality silica glass, though they
can both be made of plastic as well.
5

20. 20
NA & ACCEPTANCE ANGLE DERIVATION
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.”
optical fiber will only propagate light that enters the
fiber within a certain cone, known as the acceptance
cone of the fiber. The half-angle of this cone is called
the acceptance angle, θmax.
2 2
max 1 2nsin n nq  

21. 21
When a light ray is incident from a medium of refractive
index n to the core of index n1, Snell's law at medium-core
interface gives
i 1 r
r c c
1 2
c
1
nsin n sin
sin sin(90 ) cos
n
sin ( )
n

q  q
q  q  q
q 
6

22. Substituting for sin θr in Snell's law we get:
By squaring both sides
Thus,
22
i c
1
n
sin cos
n
q  q
22
2 2 1
i c2 2
1 2
nn
sin cos 1
n n
q  q  
2
1
i 2
2
n
sin 1
n
q  

23. from where the formula given above follows.
NUMERICAL APERATURE IS
ACCEPTANCE ANGLE
23
2
1
2
2
n
NA 1
n
 
2
1 1
max 2
2
n
sin 1
n

q  

24. Definition:-
Acceptance angle:-
Acceptance angle is defined as the maximum angle of
incidence at the interface of air medium and core medium
for which the light ray enters into the core and travels along
the interface of core and cladding.
Acceptance Cone:-
There is an imaginary cone of acceptance with an angle
.The light that enters the fiber at angles within the
acceptance cone are guided down the fiber core
Numerical aperture:-
Numerical aperture is defined as the light gathering capacity
of an optical fiber and it is directly proportional to the
acceptance angle.
24

25. 25
Classification of Optical Fiber

26. 26
Three common type of fiber in terms of the
material used:
• Glass core with glass cladding –all glass or
silica fiber
• Glass core with plastic cladding –plastic
cladded/coated silica (PCS)
• Plastic core with plastic cladding – all plastic or
polymer fiber

27. Plastic and Silica Fibers
7

28. BASED ON MODE OF PROPAGATION
Two main categories of optical fiber used in fiber
optic communications are
 multi-mode optical fiber
single-mode optical fiber.
28

29.  Single-mode fiber
 Carries light pulses along single path
 Multimode fiber
 Many pulses of light generated by LED travel at different
angles 29
8

30. Based on the index profile
30
The boundary between
the core and cladding
may either be abrupt,
in step-index fiber, or
gradual, in graded-
index fiber
9

31. 31
Step Index Fibers
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.
The refractive index profile may be defined as
n(r) = n1 r < a (core)
n2 r ≥ a (cladding)

32. GRADED-INDEX
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.
32

33. 33
Figure.2.6
(a)
(b)
10

34. 34
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

35. Figure 2.10 Two types of fiber: (Top) step index fiber; (Bottom)
Graded index fiber
11

36. Attenuation
 Definition: a loss of signal strength in a lightwave,
electrical or radio signal usually related to the distance the
signal must travel.
Attenuation is caused by:
Absorption
Scattering
Radiative loss
36

37. Losses
Losses in optical fiber result from attenuation in the
material itself and from scattering, which causes some
light to strike the cladding at less than the critical angle
Bending the optical fiber too sharply can also cause
losses by causing some of the light to meet the cladding
at less than the critical angle
Losses vary greatly depending upon the type of fiber
Plastic fiber may have losses of several hundred dB
per kilometer
Graded-index multimode glass fiber has a loss of
about 2–4 dB
per kilometer
Single-mode fiber has a loss of 0.4 dB/km or less
37

38. Macrobending Loss:
The curvature of the bend is much larger than fiber
diameter. Lightwave suffers sever loss due to radiation of
the evanescent field in the cladding region. As the radius of
the curvature decreases, the loss increases exponentially
until it reaches at a certain critical radius. For any radius a
bit smaller than this point, the losses suddenly becomes
extremely large. Higher order modes radiate away faster
than lower order modes.
38
12

39. Microbending Loss
Microbending Loss:
microscopic bends of the
fiber axis that can arise
when the fibers are
incorporated into cables.
The power is dissipated
through the microbended
fiber, because of the
repetitive coupling of
energy between guided
modes & the leaky or
radiation modes in the
fiber. 39
13

40. Dispersion
The phenomenon in an optical fibre whereby light photons
arrive at a distant point in different phase than they entered
the fibre.
Dispersion causes receive signal distortion that ultimately
limits the bandwidth and usable length of the fiBer cable
The two main causes of dispersion are:
Material (Chromatic) dispersion
Waveguide dispersion
Intermodal delay (in multimode fibres)
40

41. Dispersion in fiber optics results from the fact that in
multimode propagation, the signal travels faster in some
modes than it would in others
Single-mode fibers are relatively free from dispersion
except for intramodal dispersion
Graded-index fibers reduce dispersion by taking advantage
of higher-order modes
One form of intramodal dispersion is called material
dispersion because it depends upon the material of the core
Another form of dispersion is called waveguide dispersion
Dispersion increases with the bandwidth of the light source
41

42. Advantages of Optical Fibre
Thinner
Less Expensive
Higher Carrying
Capacity
Less Signal
Degradation& Digital
Signals
Light Signals
Non-Flammable
Light Weight
14

43. 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.”

44. 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.

45. Fiber Optic Advantagesgreater capacity (bandwidth up
to 2 Gbps, or more)
smaller size and lighter weight
lower attenuation
immunity to environmental
interference
highly secure due to tap
difficulty and lack of signal
radiation
15

46. Disadvantages include
the cost of interfacing
equipment necessary to
convert electrical
signals to optical
signals. (optical
transmitters, receivers)
Splicing fiber optic
cable is also more
difficult.
Disadvantages of fiber optics
16

47. Areas of Application
Telecommunications
Local Area Networks
Cable TV
CCTV
Optical Fiber Sensors

48. 48
Formula Summary
 Index of Refraction
Snell’s Law
Critical Angle
Acceptance Angle
Numerical Aperture
v
c
n 
2211 sinsin qq nn 






 
1
21
sin
n
n
cq
 2
2
2
1
1
sin nn  

2
2
2
1sin nnNA  

49. STUDENTS CAN ALSO REFER IT……
49
http://hank.uoregon.edu/experiments/Dispersion-in-
Optical-Fiber/Unit_1.6%20(2).pdf
http://www1.ceit.es/asignaturas/comuopticas/pdf/chapter4.
pdf
http://course.ee.ust.hk/elec342/notes/Lecture%206_attenua
tion%20and%20dispersion.pdf
1 Engineering Physics by H Aruldhas, PHI India
2 Engineering Physics by B K Pandey , S. Chaturvedi,
Cengage Learning
3 Resnick, Halliday and Krane, Physics part I and II, 5th
Edition John Wiely
4 Engineering Physics by S.CHAND
5 Engineering Physics by G VIJIYAKUMARI

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