OPTICAL FIBERCourse: B.Tech

Subject: Engineering Physics

Unit: I

Chapter: 1

1

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

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

The Light is Refracted

This end travels

further than the

other hand

Lower Refractive index Region

Higher Refractive index Region

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

Reflection

Light inside an optical fiber bounces off the

cladding - reflection

Reflection

LED or

LASER

Source

3

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

Angle of Incidence

Also incident angle

Measured from perpendicular

Exercise: Mark two more incident angles

Incident Angles

10

Angle of Reflection

Also reflection angle

Measured from perpendicular

Exercise: Mark the other reflection angle

Reflection Angle

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

Angle of Refraction

Also refraction angle

Measured from perpendicular

Exercise: Mark the other refraction angle

Refraction Angle

13

Angle Summary

Refraction Angle

Three important angles

The reflection angle always equals the incident

angle

Reflection Angle

Incident Angles

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 1.4Light bends in towards normal -

lower n to higher n

Light bends

away from

normal - higher

n to lower n

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

Snell’s Law

Normal

Incidence

Angle(1)

Refraction

Angle(2)

Lower Refractive index(n2)

Higher Refractive index(n1)Ray of light

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 coreCritical Angle, qc

n1 = Refractive index of the core

n2 = Refractive index of the cladding

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

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

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 2n sin n nq

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 2c

1

n sin n sin

sin sin(90 ) cos

nsin ( )

n

q q

q q q

q

6

Substituting for sin θr in Snell's law we get:

By squaring both sides

Thus,

22

i c

1

nsin cos

nq q

222 2 1

i c2 2

1 2

nnsin cos 1

n nq q

2

1i 2

2

nsin 1

nq

from where the formula given above follows.

NUMERICAL APERATURE IS

ACCEPTANCE ANGLE

23

2

1

2

2

nNA 1

n

21 1

max 2

2

nsin 1

n

q

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

Classification of Optical Fiber

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

Plastic and Silica Fibers

7

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

Single-mode fiber

Carries light pulses along single path

Multimode fiber

Many pulses of light generated by LED travel at different

angles 29

8

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

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)

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

33Figure.2.6

(a)

(b)

10

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

Figure 2.10 Two types of fiber: (Top) step index fiber; (Bottom)

Graded index fiber

11

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

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 less37

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

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

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

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

Advantages of Optical Fibre

Thinner

Less Expensive

Higher Carrying Capacity

Less Signal Degradation& Digital Signals

Light Signals

Non-Flammable

Light Weight14

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.”

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.

Fiber Optic Advantagesgreater 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

radiation15

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

Areas of Application

Telecommunications

Local Area Networks

Cable TV

CCTV

Optical Fiber Sensors

48

Formula Summary

Index of Refraction

Snell’s Law

Critical Angle

Acceptance Angle

Numerical Aperture

v

cn

2211 sinsin qq nn

1

21sinn

ncq

2

2

2

1

1sin nn

2

2

2

1sin nnNA

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.

pdfhttp://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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