Optical Fiber Communication System

OPTICAL FIBER COMMUNICATION SYSTEM :

Like all other communication system, the primary objective of optical fiber communication system also is to transfer the signal containing information (voice, data, video) from the source to the destination. The general block diagram of optical fiber communication system is shown in the figure9.

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The source provides information in the form of electrical signal to the transmitter. The electrical stage of the transmitter drives an optical source to produce modulated light wave carrier. Semiconductor LASERs or LEDs are usually used as optical source here. The information carrying light wave then passes through the transmission medium i.e. optical fiber cables in this system. Now it reaches to the receiver stage where the optical detector demodulates the optical carrier and gives an electrical output signal to the electrical stage. The common types of optical detectors used are photodiodes (p-i-n, avalanche), phototransistors, photoconductors etc. Finally the electrical stage gets the real information back and gives it to the concerned destination.

                 It is notable that the optical carrier may be modulated by either analog or digital information signal. In digital optical fiber communication system the information is suitably encoded prior to the drive circuit stage of optical source. Similarly at the receiver end a decoder is used after amplifier and equalizer stage.

4. PRIMARY ELEMENTS OF OPTICAL FIBER COMMUNICATION SYSTEM :

                 Figure10 shows the major elements used in an optical fiber communication system. As we can see the transmitter stage consists of a light source and associated drive circuitry. Again, the receiver section includes photodetector, signal amplifier and signal restorer.

                   Additional components like optical amplifier, connectors, splices and couplers are also there. The regenerator section is  a key part of the system as it amplifies and reshapes the distorted signals for long distance links.

Figure 10.

4.1  Transmitter section :

The main parts of the transmitter section are a source (either a LED or a LASER), efficient coupling means to couple the output power to the fiber, a modulation circuit and a level controller for LASERs. In present days, for longer repeater spacing, the use of single mode fibers and LASERs are seeming to be essential whereas the earlier transmitters operated within 0.8µm to 0.9µm wavelength range, used double hetero structure LASER or LED as optical sources. High coupling losses result from direct coupling of the source to optical fibers. For LASERs, there are two types of lenses being used for this purpose namely discrete lenses and integral lenses.

Theory of Relativity:

Space, time and motion, frame of reference, Galileo's principle of relativity. Michelson-Morely experiment, Special theory of Relativity, transformation of space and time, Time dilation, Doppler effect, length contraction, twin paradox, Relativistic mass, Variation of mass with velocity, kinetic energy, equivalence of mass and energy. Relation between energy and momentum.

[No. of Periods: 8+2]

Unit – II

Geometrical Optics and Acoustics

(i) Geometrical Optics: Combination of thin lenses, Cardinal points of coaxial system of thin lenses, location and properties of Cardinal points, Newton's formula.

(ii) Acoustics: Magnetostriction oscillator and Piezo-electric oscillator for production of ultrasonic waves, determination of wavelength of Ultrasonic waves and its engineering applications, Basic requirements for an acoustically good hall. Reverberation and Sabine's formula for reverberation time, Absorption coefficient and its measurement, Factors affecting architectural acoustics and their remedy.

[No. of Periods: 10+2]

Unit – III

Wave Optics

Wedge shaped films, Interference by division of amplitude: Newton's rings, Interference by division of wave front: Fresnel's biprism, fringe width, Diffraction at single slit, diffraction grating. Resolving power of grating, Polarisation by reflection (Brewster's law), refraction, double refraction (only introduction).

[No. of Periods: 6+2]

Unit – IV

Digital Electronics

Number system used in digital electronics: decimal, binary, octal, hexadecimal. Conversion of decimal, binary, octal & hexadecimal to one another and vice versa. Addition, subtraction, multiplication, division, 1's, 2's compliment and use in subtraction, AND, OR, NOT, NAND, NOR, EX-OR gates & their representation. Logic symbols, Equivalent simplified switching circuits & truth table. Law of Boolean algebra. De Morgan's theorems & De Morganization, implementations of Boolean expressions using gates, NAND Gate and NOR Gate as universal gates, AND, OR, NOT, EX-NOR logic operation using NAND gates or NOR gates.

[No. of Periods: 6+2]

Unit – V

Quantum Physics

De Broglie's wave length, Davisson and Germer's experiment, Compton Effect, Quantisation of atomic energy, Electron spin, concept of wave packet & their properties, wave function & probability interpretation, Heisenberg's Uncertainty Principle, its elementary proof and applications. Duane-hunt limit for continuous X-rays, Moseley law of characteristic X-rays.

Bragg's law of X-ray diffraction. Bragg's X-ray spectrometer. Electron optics:Bethe's law, Electric and magnetic focusing Devices: CRT

[No. of Periods: 10+2]

Name of the Text Books:

1. Gaur and Gupta "Engineering Physics"

2. Beiser, "Modern Physics", McGraw-Hill Inc., New Delhi.

3. Avadhanulu and Kshirsagar "Engineering Physics".

Name of the Reference Books:

1. Jenkins and White: "Optics", McGraw-Hill Book Company.

2. Singh R. B.: "Physics of Oscillations and Waves"

3. Ghatak A.K.: "Optics"

4. Mani and Mehta: "Modern Physics", Affiliated East-West Press Pvt. Ltd., 1998.

5. Sanjeev Puri: Modern Physics, Narosa Pub.Co. 2004.

6. Azroff: Solid State Physics, Tata McGraw-Hill, 2004.

7. Kaplan: Nuclear Physics, Narosa Publishing, 1987.

8. Theraja: B. L., Basic Electronics, S. Chand, 2002.

9. Puri: Digital Electronics, Tata McGraw-Hill, 2002.

10. Millman, J and Halkias: Integrated Electronics, Tata MeGraw-Hill, 2004.

11. Tyagrajan and Ghatak: Lasers, Macmillan, 2001.

12. Keiser: G Optical fiber Communication, McGraw-Hill, 2000.