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Physics (Second Semester) > Optical Fibres > Flashcards

Optical Fibres Flashcards

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1
Q

What is the shape of an optical fiber ?

A

Cylindrical

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2
Q

How many parts does it basically consist of ?

A

Two

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3
Q

What are they ?

A

Core and Cladding

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4
Q

Where is the core ?

A

It is the inner cylindrical part

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5
Q

What is the core generally made up of ?

A

Glass or plastic

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6
Q

What is the cladding ?

A

It is a concentric cylinder, surrounding the core

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7
Q

Which material has a higher refractive index ?

A

Core has a higher refractive index

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8
Q

What is used to prepare a jacket for the cladding ?

A

A polyurethane jacket

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9
Q

Why is it protected with a polyurethane jacket ?

A

It safeguards the fiber from chemical reaction,

And Crushing

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10
Q

What physical principle are optical fibers built on ?

A

Total internal reflection

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11
Q

Why is total internal reflection a special case of reflection ?

A

The ENTIRE incident energy is reflected back. There is ZERO absorption of light energy

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12
Q

Would fiber optics communication be possible if there was a very minute loss ?

A

No, it would be impossible

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13
Q

What is a waveguide ?

A

A tubular structure, which guides energy in the form of waves

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14
Q

Which property of light is generally used for analyzing signal propogation in fibers ?

A

Wave property

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15
Q

What is fractional index change ?

A

It is the ratio of the refractive index difference of the core and cladding to the refractive index of the core

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16
Q

Do all the light rays that enter the core at an angle less than the angle of acceptance travel in the core, theoretically ?

A

No. Maxwell’s laws tells us that only light waves of certain number of modes are sustained

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17
Q

Which parameter determines the number of modes supported for propagation ?

A

V - number

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18
Q

What is refractive index profile ?

A

The curve which represents the variation of refractive index with respect to the radial distance from the fiber axis

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19
Q

What are the three categories optical fibers are classified into ?

A

Single mode fiber
Step index multi mode fiber
Graded index multi mode fiber

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20
Q

What is a mode ?

A

Modes are the possible solutions of Helmholtz equations, combined with Maxwell’s equations and the boundary conditions

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21
Q

What does a mode tell us about a wave ?

A

How a wave is distributed throughout space

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22
Q

Can two waves have the same mode ?

A

Yes, but with different frequencies

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23
Q

What is the diameter of the core in single mode optical fiber ?

A

8-10 micro meter

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24
Q

Which is the most commonly used optical fiber in the world ?

A

Single Mode Optical Fiber

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25
Q

Where do Single mode Optical Fibers find application, particularly ?

A

Submarine cable system

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26
Q

What is splicing ?

A

Joining the strands of one optical fiber to another

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27
Q

Can it be done with twisting and soldering ?

A

No

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28
Q

How is it done ?

A

The ends of respective strands are fused using mechanical splices

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29
Q

What is the special property of the core of Graded-Index Multimode Fiber ?

A

The refractive index decreases radially outward

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30
Q

Which is the most expensive type of optical fiber ?

A

Graded Index Multimode Fiber (GRIN) is the most expensive optical fiber

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31
Q

Where does GRIN generally find application ?

A

Telephone trunk between central offices

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32
Q

What happens to the refractive index of the core at the interface with the cladding in GRIN ?

A

At the interface, they are equal

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33
Q

What is attenuation ?

A

The loss of power suffered by the optical signal as it propagates through the fiber

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34
Q

What are the three mechanisms through which attenuation takes place ?

A
  1. Absorption
  2. Scattering
  3. Radiation Losses
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35
Q

What are the two types of absorption losses ?

A
  1. Absorption in impurities

2. Intrinisic Absorption in the glass itself

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36
Q

Is the amount absorbed intrinsically large ?

A

No, it sets the lowest limit for absorption. However, it is not zero

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37
Q

What are the two types of scattering ?

A
  1. Rayleigh scattering

2. Others

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38
Q

What is Rayleigh Scattering ?

A

It is the elastic scattering of light and other electromagnetic radiation by particles much smaller than the wavelength of radiation

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39
Q

What causes the Rayleigh scattering ?

A

Localized structural inhomogenity

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40
Q

Why does this happen ?

A

Structural inhomogenity sets in during solidification of th molten state

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41
Q

How does Rayleigh scattering depend on wavelength ?

A

Inversely proportional to the fourth power of wavelength

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42
Q

Can Rayleigh scattering be stopped via a processing technique ?

A

No, there is no processing technique at present to stop it

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43
Q

What causes the other forms of scattering ?

A

Fiber defects

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44
Q

What kind of defects ?

A

Trapped gas bubbles

Un-reacted starting materials

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45
Q

Can it be minimise using processing techniques ?

A

Yes, they can be made negligible compared to Rayleigh scattering using the right techniques

46
Q

What causes the radiation losses in optical fibers ?

A

Macroscopic bends

Microscopic bends

47
Q

What is a macroscopic bend ?

A

It is a bend with a radii larger than the fiber diameter

48
Q

When is it caused ?

A

While turning it around a corner

49
Q

What is the magnitude of radiation losses caused by macroscopic bends ?

A

It is negligible, till a critical radius of curvature, after which it increases suddenly

50
Q

What are microscopic bends ?

A

Repetitive small-scale fluctuations in linearity of wire

51
Q

Why do they occur ?

A
  1. Non uniformities in fiber manufacturing

2. Non uniformities in lateral pressure created during cabling

52
Q

What is caused by microscopic bends ?

A

Irregular reflections and leaking

53
Q

How can microbending loss be minimized ?

A

Extruding a compressible jacket over the fiber

54
Q

How does the jacket help ?

A

It withstands the stresses while keeping fiber relatively straight

55
Q

State Lambert’s Law

A

The rate of decrease of intensity of light with distance travelled in proportional to initial intensity

56
Q

What kind of mediums does Lambert’s Law apply to ?

A

Homogeneous mediums

57
Q

What happens to the voice over telephone in a point to point communication system ?

A

It is converted to analog electrical signals

58
Q

Where do these signals come out of ?

A

Transmitter

59
Q

What does the coder do ?

A

Convert the electrical signal to binary data

60
Q

What is the output of a coder ?

A

Stream of electrical pulses

61
Q

What does an optical source (like LED or laser diode) do with these electrical pulses ?

A

Give out pulses of optical power

62
Q

What is this unit called ?

A

Optical transmitter

63
Q

How much of incident light funneled into core is sustained for propagation ?

A

Only within half angle of acceptance

Certain number of modes

64
Q

How does the signal propagate ?

A

Total internal reflection

65
Q

What are the problems signals face ?

A

Attenuation and delay distortion

66
Q

What is delay distortion ?

A

The reduction in quality of signal because of spreading of pulses

67
Q

Why do the pulses spread ?

A

Mainly due to variation in velocity of various spectral components of pulse during propagation

68
Q

Do pulses overlap ?

A

Yes, they do because of spreading

69
Q

What does a repeater consist of ?

A

A receiver and transmitter arranged adjacently

70
Q

Where are repeaters placed ?

A

In the transmission path, at points where signals may reach a limiting stage

71
Q

What is the limiting stage ?

A

A stage beyond which information cannot be extracted from the signal

72
Q

What does the receiver of the repeater do ?

A

It converts the optical signal into corresponding electrical signal

73
Q

What happens to this electrical signal ?

A

It is amplified and recast to original form

74
Q

What does the amplification and recasting ?

A

Electrical generator, which is a part of the receiver section

75
Q

What happens to the reshaped electrical signal ?

A

It is sent to optical transmitter in the form of binary data

76
Q

What happens at the receiving end ?

A

Optical signal is fed to a photodetector

77
Q

What does the photodetector do ?

A

Transforms it into pulses of electrical current

78
Q

What does the decoder do ?

A

Converts the binary sequence into an analog signal

79
Q

Where can optical fibers be used in sensing devices ?

A

It can be used to sense parameters like pressure, voltage or current

80
Q

What is a data link ?

A

Communication over a distance much shorter than telecommunication, but greater required reliability

81
Q

What are problems encountered by using metal cables ?

A

Cross talk, impedance mismatching (Reliability reduces)

82
Q

Can they be overcome by using optical fibers ?

A

Yes

83
Q

How do optical fibers benefit in local area networks ?

A

Provide more efficient and faster communication

84
Q

Which cable can carry greater information (signals over a larger bandwidth) : Optical fibers or metallic cables ?

A

Optically fibers can carry much greater amounts of information

85
Q

What are the advantages of using optical fibers, material-wise ?

A

They are made with easily available materials (Silicon dioxide and plastic)

86
Q

What is the average life of an optical fiber ?

A

40 years

87
Q

What is a massive materialistic advantage of optical fibers over metallic cables ?

A

Optical fibers do not corrode

88
Q

Why are optical fibers easy to transport ?

A

They are compact and light-weight

89
Q

Why is the cost/unit/channel of an optical fiber very less ?

A

It carries a lot more information

90
Q

What are the common disturbances encountered by metallic cables that optical fibers are immune to ?

A

Radio and telecommunication signals

Lightning

91
Q

Why is it difficult to tap optical fibers ?

A

There is no leaked energy radiation to detect

92
Q

What happens when information is tapped by directly hooking to the information carrying fiber ?

A

The power at the receiver end drops, warning of an intrusion

93
Q

Why does cross-talk not take place in optical fibers ?

A

There is a higher purity of signal

94
Q

Why is there a greater purity of signal ?

A

Light cannot enter a fiber from it’s sides and cause interference

95
Q

Why are optical fibers superior for long distances ?

A

Superior Attenuation characteristics (80 km before amplification vs 5 km)

96
Q

How are optical fibers protected from flammable and corrosive environments ?

A

There is no spark

97
Q

Why ?

A

The signal is optical

98
Q

How do optical fibers reduce complexity in transmission of information ?

A

There are far fewer components than metallic cables

99
Q

How do the lesser number of components reduce the cost ?

A
  1. Eliminates costly electronic modules

2. Fewer spare parts in inventory

100
Q

How do the lesser number of components affect reliability ?

A

It improves the reliability

101
Q

Why ?

A

There are a fewer number of parts which can fail ?

102
Q

What is a difficulty in optical fibers, manufacturing wise ?

A

Splicing is difficult , skillful and therefore expensive

103
Q

What happens when splicing is not done precisely ?

A

It causes a virtual break in communication

104
Q

Why are maintenance costs high for optical fibers, in case of accidents ?

A

Repairing connection is highly skillful and time consuming

105
Q

What causes upsets in the critical alignments which lead to loss of signal power (from the weather) ?

A

They undergo expansion and contraction with temperature

106
Q

Can they be bent to circles ?

A

It is risky since the loss becomes considerable even for small curvature bends

107
Q

What is diameter of the core of Step Index Multi mode Fiber ?

A

50-200 micro meters.

They are very similar to Single Mode Fibers, otherwise.

108
Q

What is the difference in between the refractive index profiles of Single Mode and Step Index Multi Mode Fibers ?

A

They are similar but step-index multi mode fibers have a larger plane region for the core

109
Q

What can be a light source for Step Index Multi Mode Fibers ?

A

Can accept lasers and LEDs

110
Q

Which is the least expensive optical fiber ?

A

Step Index Multi Mode Fibers are the least expensive optical fibers

111
Q

Where do Step Index Multi Mode fibers find application ?

A

Data links with lower bandwidth requirements

Physics (Second Semester) (6 decks)

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