FIBER OPTICS

Dr D. Arun Kumar

Assistant Professor

Department of Physical Sciences

Bannari Amman Institute of Technology

Sathyamangalam

General Objective

To understand the propagation of light through optical

fibers and obtain expressions for the acceptance angle

and numerical aperture

Specific Objectives

1. Illustrate the propagation of light through optical fiber

(S, E)

2. Define acceptance angle and numerical aperture (S)

3. Derive the expressions for the acceptance angle and

numerical aperture (S, M)

4. Compute the relation between fractional index and

numerical aperture (M)

Optical fiber

The development of lasers and optical fiber – revolution

in field of communication systems.

carrying light waves through an open atmosphere - rain,

fog etc affected the efficiency

guiding medium

The optical fiber is a wave guide

It is made up of transparent dielectrics (SiO2), (glass or

plastics).

transmit voice, video and digital data signals using light

waves

Fiber Construction

Core - inner cylinder made of glass or plastic - high

refractive index n1.

Cladding - core is surrounded by cylindrical shell of

glass or plastic – less refractive index n2

Jacket - cladding is covered – polyurethane - protects

from moisture and abrasion.

Total Internal reflection

The light is transmitted through this fiber by total

internal reflection.

Core diameters - from 5 to 600μm

cladding diameters - from 125 to 750μm

Core transmits the light waves

The cladding keeps the light waves within the core by

total internal reflection.

Principle of propagation of

light in an optical fiber

Total internal reflection at the fiber wall can occur only if

two conditions are Satisfied.

The refractive index of the core material n1 must be

higher than that of the cladding n2 surrounding it.

At the core – cladding interface, the angle of incidence (

between the ray and normal to the interface) must be

greater than the critical angle.

critical angle

Propagation of light through

fiber Let the refractive indices of the core and cladding be n1

and n2 respectively; n1> n2.

Propagation of light through

fiber

Propagation of light through

fiber

This angle im is called the acceptance angle of the fiber.

the maximum angle at or below which the light can

suffer Total Internal Reflection is called acceptance

angle.

Propagation of light through

fiber

Acceptance cone

An optical fiber accepts only those rays which are

incident within a cone having a semi angle im.

Numerical Aperture

the sine of the acceptance angle

NA = sin im

determines the light gathering ability of the fiber.

measure of amount of light that can be accepted by a

fiber

Refractive indices of the core and cladding materials.

Fractional Index change

TYPES OF OPTICAL FIBERS

Optical fibers are classified into three major categories

Based on the type of the

material

Glass fiber

Example:

Core: SiO2 Cladding: SiO2

Core: GeO2- SiO2 Cladding: SiO2

Plastic fiber

Example:

Core: polymethyl methacrylate : Cladding: Co- Polymer

Core: Polystyrene : Cladding: Methyl methacrylate

Based on the number of

modes

Single mode fiber

allow only one mode of propagation

very small core diameter

Multimode fiber

allow many modes to propagate

core diameter is very large

STEP INDEX FIBER

core of uniform refractive index surrounded by cladding

of refractive index lower than that of the core

refractive index abruptly changes at the core cladding

boundary.

Based on the refractive

index profile

GRADED INDEX FIBER

refractive index varies radially decreasing continuously

in a parabolic manner from the maximum value of n1.

Based on the refractive

index profile

THE FIBER OPTIC

COMMUNICATION SYSTEM

The major components of an optical fiber communication

system are

The optical transmitter

The optical fiber

The optical receiver

THE FIBER OPTIC

COMMUNICATION SYSTEM

INFORMATION SIGNAL

SOURCE

The input signal can be voice, music or video

It is in the form of analog signal

The analog signals are converted into electrical

signals

Then the signals are passed through transmitter

TRANSMITTER

It receives from the information source

It modulates the electrical signal into a digital pulse

The modulator consists of a driver and light source

The digital pulses are converted into optical pulses.

The light source can be LED or semiconductor

LASER

TRANSMISSION MEDIUM

The medium is the optical fiber

The principle involved is total internal reflection

Due to total internal reflection, there is no loss of light

on the core cladding junction

RECEIVER

It consists of demodulator device

It consists of photodetector, an amplifier and a

signal receiver

The photodetector converts the optical pulses

and electrical pulses

The signals are amplified by the amplifier

A single optical fiber is used to transmit several

optical signals using a device known as

multiplexer

ADVANTAGES

15,000 Signals can be transmitted

No cross talk and signal leakage is nil due to total

internal reflection

It is ideal means for communication

It gives foolproof communication during wartime

The cost of the cable is very low

Optical fibers have immunity to adverse

temperature, moisture and chemical reactions

Mind Map

Questions

1. Can we use fiber optics for lighting in houses?

2. Assume that an optical fiber is bent at right angles. Can

we get output?

Thank you……….