Optical Fibre

1. N. Jayaprakash

2. Course Contents
1. Fundamentals of Communication
2. Fundamentals of Optical Communication
3. Construction and Type of Cables
4. O.F Components and Connectors
5. Railtel
6. Light Sources ,Detectors and Receivers
7. O.F Measurements Instruments
8. Optical System Design
9. Fibre Splicing
10. PCM Principles & Line Codes
11. SDH
12. DWDM & FTTH

3. 1. Fundamentals of Communication
Encoder
Light
Source
Light
Detector
Decoder
inf
inf
OF Cable
E -> O O ->E

4. Transmission Impairment using
Copper Cable
1. Attenuation :Repeaters and Signal to Noise Ratio( SNR)
2. Noise
3. Distortion

5. Digital Transmission- Performance Criteria
 1 in 106 : Better
 1 in 105 : Good
 1 in 104 : Reasonably Good
 1 in 103 : Just Acceptable
 < 1 in 103 : Unacceptable

6. 2.Fundamentals of Optical Fibre Communication
 Introduction(Content Material)
1. What is Fiber Optics?
2. How are Fiber Optics Used today?
3. What are the advantages of Fiber Optics over Copper wire?
4. Where will Fiber Optics take me in the next 20 years?

7. Need for Fibre Optic Communication??
 In long haul transmission system there is a need of low
loss transmission medium.
 There is need of compact and least weight transmitters and
receivers
 There is need of increased span of transmission
 There is need of increased bit rate-distance product

8. Block Diagram of OFC systems
Channel Coupler
Processing
Modulator
Carrier Source Amplifier
Optical
Detector
Repeater
Message Origin Message Output
Optical Fiber
Transmitter Receiver

9.  Modulator :
1) Converts the electrical message into the proper format.
2) It Impresses the signal onto the wave generated by
carrier source.
 Carrier Source :
It generates the wave on which the information is
transmitted. This wave is called Carrier.
 Channel Coupler:
It feeds the power into the information channel. The
Channel Coupler design is an important part of fiber
system because of possibility of high losses.

10. Transmission Sequence
1. Information is Encoded into Electrical Signals
2. Electrical Signals are Converted into Light Signals
3. Light Travels Down the Fiber.
4. A Detector Changes the Light Signals into Electrical
Signals
5. Electrical Signals are Decoded into Information.

11. Advantages of Fibre Optics
1. Optical Fibres are non Conductive (Dielectrics)
2. Electromagnetic Immunity
3. Large Bandwidth (>5 GHz for 1 Km length)
4. Low loss
5. Security
6. Small, Light weight Cables etc….

12. Application of Fibre Optics
1. Military applications
2. Medical applications
3. Undersea cables
4. Control Systems
5. Factory Communications
6. Railways
7. Communication networks…. Etc…,

13. Principle of Fibre Optics
 Total Internal Reflection (TIR)
 Single Mode Fibre
 Multimode Fibre
Core (um) Cladding(um)
8 125
50 125
62.5 125
100 140

14. Total Internal Reflection (TIR)
• Refractive index of core(n1) is higher than the
cladding(n2) n1>n2
• When a ray of light strikes the boundary at an angle
greater than critical angle it gets reflected and no light
passes through Fiber Optic Cables | Group

15. Single Mode Fibre
• Carries light pulses along single path. Only the lowest
order mode (fundamental mode) can propagate in the
fiber and all higher order modes are under cut-off
condition (non-propagating) .
• Uses Laser Light source

16. Cont.…
Advantages
 Less dispersion
 Less degradation
 Large information capacity
 Core diameter is about 10 μm
 Difference between the RI of core and cladding is small
Drawbacks
 Expensive to produce
 Joining two fibers is difficult
 Launching of light into single mode is difficult

17. Multi-Mode Optical Fiber
 Multi-mode optical fiber is a type of optical fiber
mostly used for communication over short distances,
such as within a building or on a campus.
 Typical multimode links have data rates of 10 Mbit/s
to 10 Gbit/s over link lengths of up to 600 meters.

18. About MMF
 Multi-mode fibers are described by their core and
cladding diameters. example: 62.5/125 μm multi-
mode fiber.
 The two types of multi-mode optical fibers are:
 Step index multi-mode optical fibers
 Graded index multi-mode optical fibers
 The transition between the core and cladding can be
sharp, which is called a step-index profile, or a
gradual transition, which is called a graded-index
profile.

19. Cont.…

20. Types of MMF

21. Step Index Fiber
 Step-index multimode fiber has a large core, up to 100
microns in diameter.
 As a result, some of the light rays that make up the digital
pulse may travel a direct route, whereas others zigzag as
they bounce off the cladding.
 These alternative pathways cause the different groupings
of light rays, referred to as modes, to arrive separately at
the receiver.

22. Cont.
 The pulse begins to spread out, thus losing its well-
defined shape.
 The need to leave spacing between pulses to prevent
overlapping limits bandwidth that is, the amount of
information that can be sent.
 Consequently, this type of fiber is best suited for
transmission over short distances, in an endoscope, for
instance.

23. Light Propagation in Step Index Fiber

24. Modal Dispersion
 The arrival of different modes of the light at different
times is called Modal Dispersion.
 Modal dispersion causes pulses to spread out as they
travel along the fiber, the more modes the fiber transmits,
the more pulses spread out.
 This significantly limits the bandwidth of step-index
multimode fibers.
 For example, a typical step-index multimode fiber with a
50 μm core would be limited to approximately 20 MHz
for a one kilometer length, in other words, a bandwidth of
20 MHz·km.

25. Graded-Index Multimode Fiber
 Graded-index multimode fibers solves the problem
of modal dispersion to a considerable extent.
 Graded-index multimode fiber contains a core in
which the refractive index diminishes gradually from
the center axis out toward the cladding.
 The higher refractive index at the center makes the
light rays moving down the axis advance more slowly
than those near the cladding.

26. Light Propagation in MMF

27. Multi-Mode v/s Single-Mode

28. Advantages of Multi-Mode
 Easily supports most distances required for premises
and enterprise networks
 Can support 10 Gb/s transmission upto 550 meters
 Easier to install and terminate in the field
 Connections can be easily performed in the field,
offering installation flexibility and cost savings
 Have larger cores that guide many modes
simultaneously

29. Applications
 Step-index multimode fibers are mostly used for imaging
and illumination.
 Graded-index multimode fibers are used for data
communications and networks carrying signals for
typically no more than a couple of kilometers.

30. Optical Fiber Components
 Fiber Connector: an optical fiber connector terminates the end
of an optical fiber, and enables quicker connection and
disconnection
 Broadband light source (BBS):a light source that emit lights
over a large wavelength range . Example: ASE source,
EELED,SLED
 Fiber coupler: an optical device that combines or splits power
from optical fibre's
 Circulator: a passive three-port device that couple light from
Port 1 to 2 and Port 2 to 3 and have high isolation in other
directions

31. Cont.…
 Mode scrambler: an optical device that mixes optical
power in fiber to achieve equal power distribution in
all modes
 Index matching fluid: A liquid with refractive index
similar to glass that is used to match the materials at
the ends of two fibers to reduce loss and back
reflection
 Wavelength division multiplexer: a device that
combines and split lights with different wavelengths

32. Optical Fibre Parameters
 Wavelength
 Frequency
 Window
 Attenuation
 Dispersion
 Bandwidth

33.  Wavelength
It is a Characteristic of light that is emitted from the light
source and is measured in nanometers(nm).
In the Visible spectrum , wavelength can be described as
the colour of the light.
 Frequency
It is a number of pulse per second emitted from a light
source.
Frequency is measured in units of hertz (Hz)
1Hz= 1 pulse/sec

34.  Window
A narrow window is defined as the range of wavelengths
at which a fiber best operates .
Window Operational Wavelength
800nm-900nm 850nm
1250nm-1350nm 1300nm
1500nm-1600nm 1550nm

35.  Attenuation
It is defined as a loss of optical power over a set distance,
a fibre with lower attenuation will allow more power to
reach a receiver
 Intrinsic Attenuation
 Absorption
 Scattering
 Extrinsic Attenuation
 Micro bending
 Macro bending

36.  Bandwidth
It is defined as the amount of information that a system
can carry such that each pulse of light is distinguishable
by receiver.
Numerical Aperture :
Numerical aperture(NA) is the “light- gathering ability” of
a fibre.
Light injected into the fibre at angles greater than the
critical angle will be propagated .
NA= n1
2 - n2
2
where n1 and n2 are refractive indices of Core and
Cladding resp..

37. Dispersion
 Modal Dispersion
 Material Dispersion
 Waveguide Dispersion

38. Optical Fibre Construction

39. Parts O.F Cables
Component Function Material
Buffer Protect fibre from Outside Nylon, Plastic
Central Member Facilitate Stranding
Temperature Stability
Anti-Bulking
Steel, Fibre Glass
Cable Jacket Contain and Protect cable
Core
PVC, Teflon
Cable Filling Compound Prevent Moisture intrusion Water Blocking Compound
Strength Member Tensile Strength Aramid Yam, Steel

40. Buffer
 Loose Buffer – For Outdoor applications
 Tight Buffer - for indoor applications

41. Colour Codes
 6 F OF Cable
Fiber No.
1. Blue
2. Orange
3. Green
4. Brown
5. Slate
6. White
 12 F OF Cable
1. Blue
2. Orange
3. Green
4. Brown
5. Slate
6. White
7. Red
8. Black
9. Yellow
10. Violet
11. Pink
12. Glass/Natural

42. O.F Components
 Components like pig tail, patch cord , FDF & Connectors
 Connector Requirement
1. The attenuation in optical fibre connectors should be less
than 1 dB.
2. The Connector must provide consistent performance on
each remating .
3. The connector must provide protection to the fibre so that
it does not break while being handled
4. Connector must be cost effective.

43. O.F Measuring Instruments
 Main Tests on OFC
 Cable loss
 Splice loss
 Connector loss
 Fibre Length
 Continuity of Fiber
 Fault Localization/ Break Fault

44. Main Instruments Required
 Calibrated Light Source
 Optical Power Meter
 Optical Attenuator
 Optical Time Domain Reflectometer (OTDR)

45.  Calibrated Light Source
 Generates Light Signals of Known Power and Wavelength
 Calibrated Power Source
 Measures Optical Power over wide range
 It is never measured directly, but Measured through
Electrical Conversion using Photo Electric conversion. It is
known as Optical Sensor of known Wavelength.
 Accuracy depends upon the stability of the Detector’s power
to current conversion.

46. Power Attenuator
 Types
 Fixed Attenuators
 Variable Attenuators
 Applications
 To Simulate the Regenerator Loss at the FDf
 To provide Local Loop Back for Testing
 To measure the Bit Error Rate by varying the Optical Signal
at the Receiver Input

47. OTDR is used for measuring
1. Fiber Loss
2. Splice Loss
3. Connector Loss
4. Fiber Length
5. Continuity of Fiber
6. Fault Localization

48. PCM-Principles
 PCM system use TDM technique to provide a number of
circuits on the same transmission medium.
 Basic Requirements for PCM System
 Filtering
 Sampling
 Quantization
 Encoding
 Line Coding

49. Enclosure

50. Termination box

51. PDH

53. PDH

54. FDF

55. PDH

56. Cooler

57. OTDR

58. Indicators in SDH,PDH

59. OTDR