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Physics Project Report on Optical Fibre

















This is to certify that the investigatory Project Report entitled "Optical Fibre" submitted by student of Class XII is original and has been completed by him under my supervision.



As a student of Class XII. I did this project on a a part of my studies entitled to "Optical Fibre". I owe a deep sense of gratitude to my Physics Teacher whose valuable advice, guidance and helped me in doing this project from conception to completion.

At the same time, I can not forget to express my thankfulness to our school Principal for extending his generous, patronage and constant encouragement.

Finally I thankful my parents for helping me economically and my friends for giving me a helping hand at every step of the project.


Signature of the Student


In the field of communication various communication modes are present like satellite communication, line communication etc. In line communication various transmission medium are present parallel wire (Ribbon), Twisted wire line, coaxial cables, optical fibre etc. In these medium optical fibre is a latest and widely used process in communication for transmit signals from one place to another without any practical loss in intensity of light signals.

Optical Fibre

Optical fibre are used to transmit light signals from one place to another without any practical loss in the intensity of the light signal. Optical fibre in thin (2 to 125 mm), Flexible and capable of conducting an optical ray. Various glasses and plastics can be used to make optical fibres. The lowest losses have been obtained using fibres of ultra pure fused silica. Ultra pure fibre is difficult to manufacture, higher-loss multi component glass fibres are more economical and still provide good performance. Plastic fibre is even less costly and can be used for short-haul links, for which moderately high losses are acceptable.

Parts of Optical Fibre

An optical fibre consists of a very thin glasses core (5 mm to 50 mm in diameter) surrounded by a glass coating called cladding. The glass core and cladding are enclosed in a protective jacket made of plastic. The refractive index of the glass used for making core (m) is a little more than the respective index of the glass used for making the cladding (m2) i.e.m1> m2.


In optical fibre, the value of refractive index of core is 1.52 and the value of refractive index of cladding is 1.48 respectively.

Graded-index multimode fibre :- A multimode optical fibre whose refractive index decreases smoothly from its center to the outer surface of the fibre. There is no noticeable boundary between core and cladding.

The different paths followed by two wavelength l1 and l2 inside a graded index multimode fibre, at some layer in the fibre, different fore each, the rays are totally reflected. The speed of l1 is greater than l2. l1 covers longer path than l2 but at a greater speed. So that time of travel is same for both the wavelengths.

Optical Communication


The use of optical carrier waves for transmission of information from one place to another is called optical communication.

The information sources provides on electrical signal to an electrical transmitter. Which drives an optical source to give modulation of the light waves. The optical source which provides the electrical optical convession, may be either a semiconductor laser or light emitting diode. At receiver's end, the optical signal is detected by optical detector, which converts the optical signal into an electrical signal. The optical detector may be photodiode or a phototransistor. The electrical signal is further demodulated by electrical receiver so as to reproduce the original information signal which is then received at the destination.

Advantages of Optical Fibre Communication

1. Small size and weight :- Optical fibre have very small diameter, which are often no greater than the diameter of human hair. It is much lighter also than coaxial cable.

2. Immunity of electrical disturbance :- The optical fibres are free from spark hazards and short circuits.

3. Immunity to cross talk :- There is no optical interference between fibres. Even when may fibres are cabled together, unlike communication using electrical wires, crosstalk is negligible.

4. Signal Security :- The light from optical fibres does not radiate significantly, hence optical fibre communication provides very high degree of signal security.

5. Low Transmission loss :- The recent development in optical communication systems have resulted in fabricating optical fibres, which exhibit very low attenuation .

6. Enormous transmission bandwidth :- The optical carrier frequency lies in the frequency range 1013 to 1016 Hz.

7. Ruggedness and flexibility :- Optical fibre can be twisted without any damage. This property is very helpful in storage transportation, handling etc.

8. Ideal for multiplexing :- Optical fibre can carry large number of signals at the same time much more efficiently than that can be done in ratio communication.


1. The optical fibre technology can be handled only by the experts in this field.

2. If there are any scratches on the optical fibre cable, the light signal can escape at such points. It is because a ray of light can meet the surface of the scratch at an angle of incidence less than the critical angle.

Light Source Used in Optical Fibre System

Two different types of light source are used in fibre optic systems. The light emitting diode (LED) and the injection laser diode (ILD). Both are semiconductor devices that emit a beam of light when a voltage is applied. The LED is less costly, operates over a greater temperature range, and has a longer operational life. The ILD, which operates on the laser principle, is more efficient and can sustain greater data rates.

Process of Transmission

Consider that a ray of light is incident along AO on the glass core from air. It will get refracted along the path OP inside the core and then after suffering refraction at the core cladding interface, it may follow the path PQ inside the cladding.

If the angle of incidence is decreased to a value, so that the ray of light inside the core meets the core cladding interface at an angle equal to or grater that the critical angle for it, then the ray of light will undergo total internal, when it comes to incident on the core cladding interface at critical angle. We know the value of Critical angle at the interface of a media having refractive indices m1 and m2.

SinC = m2/m1 m1>m2

The Optical fibre, for which m1=1.52 and m2=1.48. By calculation

C=770 (approx).

It can be easily deduces that the maximum angle of incidence (i) in air for which all the incident light is totally reflected at the interface of a pair of media having refractive indices m1 and m2 is given by :

SinI= m12 -m22

This maximum value of maximum angle of incidence is known as acceptance angle.

The optical fibre for which m = 1.52 and m2 = 1.48, by calculation, the value of angle of incidence is 200.


1. Modernís abc of Physics.

2. Pradeepís Fundamental Physics.

3. Dinesh a to z Physics.

4. Internet Sources :,

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