FIBER OPTICS. Dr D. Arun Kumar Assistant Professor Department of Physical Sciences Bannari Amman Institute of Technology Sathyamangalam

Transcription

1 FIBER OPTICS Dr D. Arun Kumar Assistant Professor Department of Physical Sciences Bannari Amman Institute of Technology Sathyamangalam

2 General Objective To understand the propagation of light through optical fibers and obtain expressions for the acceptance angle and numerical aperture

3 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)

4 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 (SiO 2 ), (glass or plastics). transmit voice, video and digital data signals using light waves

5 Fiber Construction Core - inner cylinder made of glass or plastic - high refractive index n 1. Cladding - core is surrounded by cylindrical shell of glass or plastic less refractive index n 2 Jacket - cladding is covered polyurethane - protects from moisture and abrasion.

6 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.

7 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 n 1 must be higher than that of the cladding n 2 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.

8 critical angle

9 Propagation of light through fiber Let the refractive indices of the core and cladding be n 1 and n 2 respectively; n 1 > n 2.

10 Propagation of light through fiber

11 Propagation of light through fiber

12 Propagation of light through fiber This angle i m 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.

13 Acceptance cone An optical fiber accepts only those rays which are incident within a cone having a semi angle i m.

14 Numerical Aperture the sine of the acceptance angle NA = sin i m 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.

15 Fractional Index change

16 TYPES OF OPTICAL FIBERS Optical fibers are classified into three major categories

17 Based on the type of the material Glass fiber Example: Core: SiO 2 Cladding: SiO 2 Core: GeO 2 - SiO 2 Cladding: SiO 2 Plastic fiber Example: Core: polymethyl methacrylate : Cladding: Co- Polymer Core: Polystyrene : Cladding: Methyl methacrylate

18 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

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20 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

21 Based on the refractive index profile GRADED INDEX FIBER refractive index varies radially decreasing continuously in a parabolic manner from the maximum value of n 1.

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23 THE FIBER OPTIC COMMUNICATION SYSTEM The major components of an optical fiber communication system are The optical transmitter The optical fiber The optical receiver

24 THE FIBER OPTIC COMMUNICATION SYSTEM

25 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

26 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

27 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

28 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

29 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

30 Mind Map

31 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?

32 Thank you.