L.D. ENGINEERING COLLEGE AHMEDABAD

-By:

Bhautik Vakhariya

[B.E.(C.E.5TH Sem)]

In 1854, British physicist John Tyndall discovered the principle of optical fiber by watching a stream of water flowing out of a barrel."Water is carrying light!"Indeed, light was moving forward in a repeated process of "total internal reflection" at the border of the water and air."Any transparent object can convey light even if it's bent," thought Tyndall.In other words, water and air are like the core and cladding in optical fiber.

However, the light was gradually weakened and lost its strength in the course of being carried several hundred kilometers.

In 1936, two Japanese scientists conducted an experiment to carry light from Tokyo to Osaka through a string of quartz crystals. Light was to be passed through the quartz, which is a transparent mineral like glass.

Basic phenomenon

Consider an ordinary glass of water.

Following observations are made:-

•Image appear distorted

•Light slows down as it enters the water

•Speeds up again when moves back into the air.

•Inference

•Any substance that light can travel through will exhibit REFRACTION phenomenon.

•Looking glass from below the surface of water we can find that:-

•If angle of view is steeper, light will see entering the surface at shallower angle.

•If you look at the underside of the top surface from shallower angle,you will find a point at which light can’t enter the top surface at a yet shallower angle.

•At this point ,the top surface of water looks like a perfect mirror though it isn’t.

First analog signal is converted to its digital form(usually 0 or 1 )

It is then fed into electric to optical converter (LEDs or Laser) which generates the flashes of light or pulse.

This pulse of light is then incident in optical fiber keeping in mind its critical angle.

Light is then reached to its receiving end by total internal reflection.

At the receiving end sensors like photodiode is used to convert light flash back into digital signal.

An optical fiber contains three layers:-

1.Core of highly pure glass with high refractive index for light to travel.

2.Middle layer of glass with lower refractive index known as cladding.

3.Outer polymer jacket to protect the fiber from damage.

For glass optical fiber the diameters are:-

1. Core :-10-600 microns

2. cladding:-125-625 microns

3. Jacket:-300-1000 microns

For plastic optical fiber (POF)all diameters range between 750-2000 microns

Because refractive index of the core glass is greater then that of the cladding , light travelling in the core will remain in it due to total internal reflection as long as the light strikes the core-cladding interface at an angle greater than the critical angle.

STEP INDEX MULTI MODE FIBERBANDWIDTH-20 MHZ*KMCORE DIAMETER –100-300 MICRONS

GRADED INDEX MULTIMODE FIBERBANDWIDTH-800 MHZ*KMCORE DIAMETER-50-100 MICRONS

STEP INDEX SINGLEMODE FIBERBANDWIDTH-5 GHZ*KMCORE DIAMETER –5-7 MICRONS

MULTIMODE

STEPPED INDEX

CROSS

SECTION

MULTIMODE

GRADED INDEX

SINGLE - MODE

STEPPED INDEX

INDEX

PROFILE

INPUT

PULSE

LIGHT PATH OUTPUT

PULSECLADING

DESIGNING OF FIBER CABLES

First fiber is surrounded by buffer tube(layer of silicone or epoxy).It keeps the fiber from “microbends”.The whole is then covered with fiber glass and polyurethane.

To have tensile strength and electrical insulation kevlar(arranged in form of filaments)is placed.

Finally PVC is coated as external protective jacket.It make the fiber to sustain in adverse condition.

Buffer jacket is constituted by two sheaths:-Silicone and Hytrel

Moisture barrier(plastic or aluminum) is inserted between two layers of kevlar

It is an underground fiber cable .Since it requires a further protection against humidity, two sheaths have been inserted between a Kevlar layer:A moisture barrier and a metallic shield.

This phenomenon is called Pulse dispersion or Pulse spreading.It has three basic causes:-

1. Model dispersion:-The larger the diameter of optical’s fiber core,the more different propagation modes it can support .So more pronounce the model dispersion will be.

2. Material dispersion:-It causes due to different wavelength of light and its velocity.The effect is more pronounced with white light.

3. Wave-guide Delay distortion:- It is because each propagating mode within fiber experiences a slight dispersion effect due to confined wave guide.

Any dispersion limits the rate of data transmission and in some case data may be lost.

Ability to carry much information .

Support higher data rates,and at greater distances.

Immune to all kind of interference including lightning.

Unaffected by most chemicals.

Unaffected by outdoor atmospheric condition.

Smaller and lighter.

Ideal for secure communication systems because it is difficult to tap.

Not susceptible to noise from electronic instruments.

Fiber-Optic

Copper

Distance between repeater stations, km

200 1-2

Cable size for equivalent transmission, inch

3/8 3

Cable weight per kilometer, lb 132 16,000

Electrostatic or electromagnetic interference

None Present

Error rate Lower Higher

1. Long-haul Telephone and computer networks.

2. Television picture tube

3. Cathode ray tube.

4. Can be used in illumination

5. For sensing changes in temperature or information

6. Welding and cutting

7. Used in secret information interchange

8. Bio-Medical :

8.1.Illuminating different parts of scientific Instruments.

8.2.Used in Gastroscope for viewing different parts of stomach

• Cost of initialization and installation is high.

• Require specialized skills and knowledge much different from those required for installation of electrical cables.

•There is possibility of hazard when working with optical fiber:

like glass shards and optical radiation.