Definition, types of fibers, losses in fibers, advantages of fibers, applications of fibers

1. Unit-5
Fiber Optics
PROF. ARVIND D. KANWATE
S.V.C. D. RAJA

2. What is Optical Fiber ?
 An optical fiber is a hair thin cylindrical fiber of glass or any transparent dielectric medium.
 The fiber which are used for optical communication are wave guides
made of transparent dielectrics.
 Its function is to guide visible and infrared light over long distance.

3. Structure of optical fiber

4. Components of optical Fiber
 Core- central tube of very thin size made up of optically transparent
dielectric medium and carries the light form transmitter to receiver. The core
diameter can vary from about 5um-100 um.
 Cladding- outer optical material surrounding the core having reflecting
index lower than core. It helps to keep the light within the core throughout the
phenomena of total internal reflection.
 Buffer Coating- plastic coating that protects the fiber made of silicon
rubber. The typical diameter of fiber after coating is 250-300um.

5. Refraction Indices
 Vacuum…….1.00000 (exactly)
 Air ……1.00029
 Alcohol ......1.329
 Diamond ...... 2.417
 Glass ........ 1.5
 Ice ....... 1.309
 Sodium Chloride (Salt) .... 1.544
 Sugar Solution (80%) ........ 1.49
 Water (20 C) ................ 1.333

6. Total Internal Reflection
 When an ray of light travels from a denser to a rarer medium such that the angle
of incidence is greater than the critical angle, the ray reflects back into the same
medium this phenomena is called TIR.
 In the optical fiber the rays undergo repeated total number of reflections until it
emerges out of the other end of the fiber, even if fiber is bend.

7. Critical Angle (𝜃𝑐)
 If the incident ray exceeds the critical angle, the refraction would be turned in to
reflection called total internal reflection. The critical angle is used for the mathematical
expression to the occurrence of total internal reflection.
 Critical Angle is the angle of incidence beyond which rays of light passing through a
denser medium to the surface of a less dense medium are no longer refracted but
totally reflected.

9. Numerical Aperture
 The numerical aperture of the fiber is closely related to the critical angle and is
often used in the specification for optical fiber and the components that work
with it
 The numerical aperture is given by the formula:
 The angle of acceptance is twice that given by the numerical aperture
2
2
2
1.. nnAN 

10. Classification of optical fiber
 Optical fiber is classified into two categories based on:-
1) The number of modes-
 Single mode fiber(SMF) and
 Multi-mode fiber(MMF)
2) The reflective index-
 Step index optical fiber
 Graded- index optical fiber

11.  Single mode
 only one signal can be transmitted
 use of single frequency
 Multi mode
 Several signals can be transmitted
 Several frequencies used to modulate the signal

12. a) Single mode fiber
 It is having only one path for light to pass.
 Very small diameter of core.(7 to 10 μm)
 It have bandwidth up to 40Ghz.
 Mostly use in long distance and low cost circuit like T.V.
cable.

13. b) Multimode fiber
 Light takes more than one path to travel .
 Core is having diameter of 20 to 100μm.
 Usually use for medium distance and high bandwidth.

14. A) Step index fiber
 Step index have uniform reflective index of core.
 Core have bigger refractive index than cladding.
 Graph of radial distance vs. refractive index is seems like a step-
index fiber.

15. b) Gradedindex fibre
 In this type of fiber core has a non uniform refractive index that gradually
decrease from the center towards the core cladding interface.
 The cladding has a uniform refractive index.
 The light rays propagate through it in the form of helical rays. They never cross
the fiber axis.

17. Losses in optical fibers
 Attenuation loss
 Dispersion loss
 Waveguide loss

18. Attenuation
 Attenuation is the loss of the optical power.
 Attenuation in optical fiber take place due to elements
like coupler, splices, connector and fiber itself.

19. Dispersion
 Dispersion in fiber optics results from the fact that in multimode
propagation, the signal travels faster in some modes than it would in others
 Single-mode fibers are relatively free from dispersion except for intramodal
dispersion
 Graded-index fibers reduce dispersion by taking advantage of higher-order
modes
 One form of intramodal dispersion is called material dispersion because it
depends upon the material of the core
 Another form of dispersion is called waveguide dispersion
 Dispersion increases with the bandwidth of the light source

20. Optical Fiber Communication System
Information source- It provide an electrical signal to a transmitter
comprising an electrical stage.
Electrical transmitter- It drives an optical source to give an modulation of
the light wave carrier.
Optical source- It provides the electrical-optical conversion. It may be a
semiconductor laser or an LED.

21. OPTICAL SOURCES
LEDS(GaAlAs)
 850 nm, 1310 nm
 Low cost easy to use
 Used for multimode fibers
 Special “edge-emitting “ LEDs for SMFs
Laser Diodes(InGaAsP, InGaAsSb)
 850nm, 1310nm, 1550nm
 Very high power output
 Very high speed operation
 Very expensive
 Need specialized power supply and circuitry

22. OPTICAL DETECTORS
PIN Diodes(Si, Ge, InGaAs)
 850nm, 1310nm, 1550 nm
 Low cost
APDs(Avalanche Photodiodes, GaAlAs)
 850nm, 1310nm, 1550 nm
 High sensitivity-can operate at very low power levels
 Expensive

23.  Optical cable- It serve as transmission medium.
 Optical detector- It is responsible for optical to electrical conversion
of data. It may be a photodiodes, phototransistors or photoconductors.
 Electrical receiver: It is used for electrical interfacing at the receiver
end of the optical link and to perform the signal processing electrically.
 Destination: It is the final point at which we receive the information in
the form of electrical signal.

24. Advantages of optical fibers
 Can carry much more information
 Much higher data rates
 Much longer distances than co-axial cables
 Immune to electromagnetic noise
 Light in weight
 Unaffected by atmospheric agents
Metallic cable

25. 1.ExtremelyWide (Large) Bandwidth
 The bandwidth available with a single glass fiber is more than 100 GHz.
 With such a large bandwidth, it is possible to transmit thousands of video
conversations or dozens of video signals over the same fiber simultaneously.
 Irrespective of weather the information is voice ,data or video or a
combination of these, it can be transmitted easily over the optical fibers.
 Whereas ,only a very less number(40-50) of independent signals alone can be
sent through metallic cables.

26. 2.Immunityto electrostatic Interference
 As optical fibers are being made of either glass or plastic external
electrical noise and lightning do not affect the energy in a fiber
cable.
 The result is noise free transmission.
 However ,this is not true for metallic cables made of metals ,as
they are good conductors of electricity.

27. 3. Elimination of cross talk
 Fiber system are immune to cross talk between cables caused by
magnetic induction .
 Whereas ,in a metallic cable cross talk results from the
electromagnetic coupling between two adjacent wires.

28. 4. Lighter weight andsmaller size
 Optic fiber are very small in the size.
 The size reduction make fiber the ideal transmission medium for
ships, aircraft and high rise buildings where bulky copper cables
occupy too much space.
 Reduction of size is reduction of weight also.

29. 5. Lower cost
 The material used in fiber is silica or silicondioxide which is one of the most abundant
material on earth ,resulting in lower cost.
 Optical fiber costs are continuing to decline.
SILICA SILICON DIOXIDE

30. 6. Security
 Fiber cable are more secure than metallic cable.
 Due to it’s immunity to electromagnetic coupling and radiation,
optical fiber can be used in most secure environments.
 Although it can be intercepted ,it is very difficult to do so because
at the receiving user’s end an alarm would be sounded.

31. 7. Greater safety
In many wired system, the potential hazard of
short circuits requires precautionary designs.
 Whereas, the dielectric nature of optical fibers
eliminates the spark hazard.

32. 8. Corrosion
 Fiber cables are more resistive to environmental extremes.
 They operate over large temperature variation than their metallic
counter parts ,and are less affected by corrosive liquids and gases.

33. 9. Longer life and easy to maintenance
A longer life span of 20 to 30 years is predicated for the
fiber optic cable as compared to 12 to 15 years for the
conventional cables.
Fiber-optic Fusion Splicer

34. Disadvantage
System installation is very costly.
 Only point-to-point communication is possible.
 Precise and costly instruments would be required.
 Splicing is time consuming.
 It accept only unipolar codes.

35. Application of optical fiber
(1) Communication
 Optical fiber is mostly use in communication.
 It is use in Wi-Fi router, Landline phone and
server connector.
 A single optical fiber can carry over
3,000,000 full-duplex voice calls or 90,000
TV channels. So it is use in Broad bandwidth.

36. (2) Military
 Optical fiber is use to make military equipment, and weapons.
 It is also use to make antenna to communicate in far areas.

37. (3) Sensor
 Most of sensors are made from optical fiber.
 Optical is also use to make detectors i.e. Metal detector

38. (4)Monitoring in Structural Engineering
 Buildings and Bridges: concrete monitoring during setting, crack (length, propagation
speed) monitoring, prestressing monitoring, spatial displacement measurement, neutral
axis evolution, long-term deformation (creep and shrinkage) monitoring, concrete-steel
interaction, and post-seismic damage evaluation
 Tunnels: multipoint optical extensometers, convergence monitoring, shotcrete /
prefabricated vaults evaluation, and joints monitoring Damage detection
 Dams: foundation monitoring, joint expansion monitoring, spatial displacement
measurement, leakage monitoring, and distributed temperature monitoring
 Heritage structures: displacement monitoring, crack opening analysis, post-seismic
damage evaluation, restoration monitoring, and old-new interaction

39. (5) Other application
 Optical fiber is use to make lamps, decorative application, art, toys, micro scope and outer
body of devices.
 Many medical devices are made from optical fiber.

40. (6) In medical field
 There are two optical fibers in endoscope:
 1) One for light, to illuminate the inside of patient.
 2) Another for a camera to send the images back to doctor.

41. Endoscope

42. Data Center using Fiber Optics

44. Dr. N. S. Kapany