1. Optical Fiber Communications
Week 1
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2. Course Schedule Lectures ( Hand outs) Quizzes Assignments
Research paper/ Case Study
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3. Marks Distribution Quizzes 10 marks
Assignments/ Research paper marks
Mid Term Exam marks
Final Examination marks
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4. Course Contents Introduction to optical fiber communications
Optical fiber waveguides
Transmission characteristics of optical fibers
Fiber Fabrication
Optical Fiber Connections, joints and couplers
Light sources for optical fibers
Optical Detectors
Modulation
Optical Networking
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5. Recommended Text Books
John M. Senior, “ Optical fiber communications” Second edition.
Subir Kumar, “ Optical fibers and fiber optic communication systems”.
Gerd Keiser, “Optical Fiber Communication 3rd Edition”, McGraw-Hill
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6. Introduction
Communication may be broadly defined as the transfer of information from one point to
another.
The information transfer is frequently achieved by superimposing or modulating the information onto an electromagnetic wave which acts as a carrier for the information signal.
Communication may also be achieved using
an electromagnetic carrier which is selected from the optical range of frequencies
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7. Historical Development
In, 1880 Alexander Graham Bell reported the transmission of speech using a light beam, Photophone proposed by Bell just four years after the invention of the telephone modulated sunlight with a diaphragm giving speech transmission over a distance of 200 m.
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8. Conti….
But using electromagnetic waves as a carrier lies in Optical region have a lot of limitations and problems regarding light transmission in the atmosphere is restricted to line of sight and is severely affected by disturbances such as rain, snow, fog, dust and atmospheric turbulence.
Nevertheless lower frequency and hence longer wavelength electromagnetic waves (i.e. radio and microwave) proved suitable carriers for information transfer in the atmosphere, being far less affected by these atmospheric conditions.
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9. Invention of LASER
A renewed interest in optical communication was stimulated in the early 1960s with the invention of the laser.
Although the use of the laser for free space optical communication proved somewhat
limited, the invention of the laser instigated a tremendous research effort into the study of optical components to achieve reliable information transfer using a lightwave carrier.
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10. Invention of Optical fibers
The proposals for optical communication via dielectric waveguides or optical fibers fabricated from glass to avoid degradation of the optical signal by the atmosphere were made almost simultaneously in 1966 by Kao and Hockham.
Initially the optical fibers exhibited very high attenuation (i.e dB/ km) and were therefore not comparable with the coaxial cables they were to replace (i.e. 5 to 10 dB/ km), but within the space of 10 years optical fiber losses were reduced to below 5 dB/ km.
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11. General Communication System
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12. Digital Optical Fiber link
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13. Advantages of optical fiber communication
Enormous potential bandwidth:
The optical carrier frequency range yields the far greater potential transmission bandwidth than metallic cable system.
Small size and less weight:
Optical fibers have very small diameters which are often no greater than the diameter of a human hair. Hence, even when such fibers are covered with protective coatings they are far smaller and much lighter than corresponding copper cables.
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14. Conti…. Electrical Isolation:
Optical fibers which are fabricated from glass, or sometimes a plastic polymer, are electrical insulators and therefore, unlike their metallic counterparts, they do not exhibit earth loop and interface problems.
Signal security:
Signal sniffing cannot be carried out until physical insertion of detecting instrument, which bit difficult.
Widely applicable in military, banking and other secure networking.
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15. Conti… Low transmission loss:
Efforts are made in the field of fiber development……… 0.2dB/KM,
GLASS FIBER CABLE is efficient then plastic ( plastic polymers).
Ruggedness and flexibility:
Flexibility in uses, manufactured for high tensile strength.
Can be bent till certain dia w/o prominent loss.
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16. Optical fiber Waveguides
Topics in this chapter
Introduction
Ray theory transmission
Electromagnetic mode theory for optical propagation
Cylindrical Fibers
Single mode fibers
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17. Introduction
A transparent dielectric rod, typically of silica glass with a refractive index of around 1.5, surrounded by air, proved to be an impractical waveguide due to its unsupported structure (especially when very thin waveguides were considered in order to limit the number of optical modes propagated) and the excessive losses at any discontinuities of the glass–air interface.
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18. Refractive Index n= speed of light in air/speed of light in substance
Naturally the refractive index of the air is 1 and refractive index of the water is approximately 1.3 where as that of glass is 1.5.
To get better understanding, consider a piece of glass with refractive index 1.5 that means if light will travel through 1.5 ft of air but during the same time light will travel 1 ft through the glass, the glass slows down the speed of the light wave.
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19. Clad- Core Fiber
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20. Conti…
The invention of the clad waveguide structure led to the first serious proposals by Kao and Hockham in 1966, to utilize optical fibers as a communications medium, even though they had losses in excess of 1000 dB/ km.
These proposals stimulated tremendous efforts to reduce the attenuation by purification of the materials.
This has resulted in improved conventional glass refining techniques giving fibers with losses of around 4.2 dB/ km
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21. Ray Theory transmission
Total Internal Reflection:
A ray of light travels more slowly in an optically dense medium than in one that is less dense, and the refractive index gives a measure of this effect.
When a ray is incident on the interface between two dielectrics of differing refractive indices (e.g. glass–air), refraction occurs.
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22. Conti…
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23. Conti….
It may be observed that the ray approaching the interface is propagating in a dielectric of refractive index n1 and is at an angle φ1 to the normal at the surface of the interface.
If the dielectric on the other side of the interface has a refractive index n2 which is less than n1, then the refraction is such that the ray path in this lower index medium is at an angle φ2 to the normal, where φ2 is greater than φ1.
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24. Conti….
The angles of incidence φ1 and refraction φ2 are related to each other and to the refractive indices of the dielectrics by Snell’s law of refraction which states that:
n1 sin φ1 = n2 sin φ2 Or
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25. Critical Angle
As n1 is greater than n2,the angle of refraction is always greater than the angle of incidence.
Thus when the angle of refraction is 90° and the refracted ray emerges parallel to the interface between the dielectrics, the angle of incidence must be less than 90°. This is the limiting case of refraction and the angle of incidence is now known as the critical angle φc,
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26. Propagation of light wave through Optical fiber
Any light wave which travels along the core and meets the cladding at the critical angle of incidence will be totally internally reflected. Therefore light wave is propagated along the fiber core by a series of total internal reflections.
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27. Acceptance angel
Any light wave which is meeting the core cladding interface at or above the critical value will also be reflected and hence will propagate along the core.
So, any light wave impinging on the core within its maximum external incident angel is coupled into the fiber and will propagate, this angel is called acceptance angel.
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28. Conti….
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29. NA( Numerical Aperture) and Half acceptance angle Calculation
On White Board
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30. Conti…
It has been observed that numerical apertures for the fibers used in short distance communications are in the range of 0.4 to 0.5 where as for the long distance communications numerical aperture are in the range of 0.1 to 0.3.
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31. Example
A silica optical fiber with a core diameter large enough to be considered by ray theory analysis has a core refractive index of 1.50 and a cladding refractive index of 1.47.
Determine:
(a) the critical angle at the core–cladding interface;
(b) the NA for the fiber;
(c) the acceptance angle in air for the fiber.
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