Optical Fibres Flashcards
1
Q
What is the shape of an optical fiber ?
A
Cylindrical
2
Q
How many parts does it basically consist of ?
A
Two
3
Q
What are they ?
A
Core and Cladding
4
Q
Where is the core ?
A
It is the inner cylindrical part
5
Q
What is the core generally made up of ?
A
Glass or plastic
6
Q
What is the cladding ?
A
It is a concentric cylinder, surrounding the core
7
Q
Which material has a higher refractive index ?
A
Core has a higher refractive index
8
Q
What is used to prepare a jacket for the cladding ?
A
A polyurethane jacket
9
Q
Why is it protected with a polyurethane jacket ?
A
It safeguards the fiber from chemical reaction,
And Crushing
10
Q
What physical principle are optical fibers built on ?
A
Total internal reflection
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
12
Q
Would fiber optics communication be possible if there was a very minute loss ?
A
No, it would be impossible
13
Q
What is a waveguide ?
A
A tubular structure, which guides energy in the form of waves
14
Q
Which property of light is generally used for analyzing signal propogation in fibers ?
A
Wave property
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
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
17
Q
Which parameter determines the number of modes supported for propagation ?
A
V - number
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
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
20
Q
What is a mode ?
A
Modes are the possible solutions of Helmholtz equations, combined with Maxwell’s equations and the boundary conditions
21
Q
What does a mode tell us about a wave ?
A
How a wave is distributed throughout space
22
Q
Can two waves have the same mode ?
A
Yes, but with different frequencies
23
Q
What is the diameter of the core in single mode optical fiber ?
A
8-10 micro meter
24
Q
Which is the most commonly used optical fiber in the world ?
A
Single Mode Optical Fiber
25
Where do Single mode Optical Fibers find application, particularly ?
Submarine cable system
26
What is splicing ?
Joining the strands of one optical fiber to another
27
Can it be done with twisting and soldering ?
No
28
How is it done ?
The ends of respective strands are fused using mechanical splices
29
What is the special property of the core of Graded-Index Multimode Fiber ?
The refractive index decreases radially outward
30
Which is the most expensive type of optical fiber ?
Graded Index Multimode Fiber (GRIN) is the most expensive optical fiber
31
Where does GRIN generally find application ?
Telephone trunk between central offices
32
What happens to the refractive index of the core at the interface with the cladding in GRIN ?
At the interface, they are equal
33
What is attenuation ?
The loss of power suffered by the optical signal as it propagates through the fiber
34
What are the three mechanisms through which attenuation takes place ?
1. Absorption
2. Scattering
3. Radiation Losses
35
What are the two types of absorption losses ?
1. Absorption in impurities
| 2. Intrinisic Absorption in the glass itself
36
Is the amount absorbed intrinsically large ?
No, it sets the lowest limit for absorption. However, it is not zero
37
What are the two types of scattering ?
1. Rayleigh scattering
| 2. Others
38
What is Rayleigh Scattering ?
It is the elastic scattering of light and other electromagnetic radiation by particles much smaller than the wavelength of radiation
39
What causes the Rayleigh scattering ?
Localized structural inhomogenity
40
Why does this happen ?
Structural inhomogenity sets in during solidification of th molten state
41
How does Rayleigh scattering depend on wavelength ?
Inversely proportional to the fourth power of wavelength
42
Can Rayleigh scattering be stopped via a processing technique ?
No, there is no processing technique at present to stop it
43
What causes the other forms of scattering ?
Fiber defects
44
What kind of defects ?
Trapped gas bubbles
| Un-reacted starting materials
45
Can it be minimise using processing techniques ?
Yes, they can be made negligible compared to Rayleigh scattering using the right techniques
46
What causes the radiation losses in optical fibers ?
Macroscopic bends
| Microscopic bends
47
What is a macroscopic bend ?
It is a bend with a radii larger than the fiber diameter
48
When is it caused ?
While turning it around a corner
49
What is the magnitude of radiation losses caused by macroscopic bends ?
It is negligible, till a critical radius of curvature, after which it increases suddenly
50
What are microscopic bends ?
Repetitive small-scale fluctuations in linearity of wire
51
Why do they occur ?
1. Non uniformities in fiber manufacturing
| 2. Non uniformities in lateral pressure created during cabling
52
What is caused by microscopic bends ?
Irregular reflections and leaking
53
How can microbending loss be minimized ?
Extruding a compressible jacket over the fiber
54
How does the jacket help ?
It withstands the stresses while keeping fiber relatively straight
55
State Lambert's Law
The rate of decrease of intensity of light with distance travelled in proportional to initial intensity
56
What kind of mediums does Lambert's Law apply to ?
Homogeneous mediums
57
What happens to the voice over telephone in a point to point communication system ?
It is converted to analog electrical signals
58
Where do these signals come out of ?
Transmitter
59
What does the coder do ?
Convert the electrical signal to binary data
60
What is the output of a coder ?
Stream of electrical pulses
61
What does an optical source (like LED or laser diode) do with these electrical pulses ?
Give out pulses of optical power
62
What is this unit called ?
Optical transmitter
63
How much of incident light funneled into core is sustained for propagation ?
Only within half angle of acceptance
| Certain number of modes
64
How does the signal propagate ?
Total internal reflection
65
What are the problems signals face ?
Attenuation and delay distortion
66
What is delay distortion ?
The reduction in quality of signal because of spreading of pulses
67
Why do the pulses spread ?
Mainly due to variation in velocity of various spectral components of pulse during propagation
68
Do pulses overlap ?
Yes, they do because of spreading
69
What does a repeater consist of ?
A receiver and transmitter arranged adjacently
70
Where are repeaters placed ?
In the transmission path, at points where signals may reach a limiting stage
71
What is the limiting stage ?
A stage beyond which information cannot be extracted from the signal
72
What does the receiver of the repeater do ?
It converts the optical signal into corresponding electrical signal
73
What happens to this electrical signal ?
It is amplified and recast to original form
74
What does the amplification and recasting ?
Electrical generator, which is a part of the receiver section
75
What happens to the reshaped electrical signal ?
It is sent to optical transmitter in the form of binary data
76
What happens at the receiving end ?
Optical signal is fed to a photodetector
77
What does the photodetector do ?
Transforms it into pulses of electrical current
78
What does the decoder do ?
Converts the binary sequence into an analog signal
79
Where can optical fibers be used in sensing devices ?
It can be used to sense parameters like pressure, voltage or current
80
What is a data link ?
Communication over a distance much shorter than telecommunication, but greater required reliability
81
What are problems encountered by using metal cables ?
Cross talk, impedance mismatching (Reliability reduces)
82
Can they be overcome by using optical fibers ?
Yes
83
How do optical fibers benefit in local area networks ?
Provide more efficient and faster communication
84
Which cable can carry greater information (signals over a larger bandwidth) : Optical fibers or metallic cables ?
Optically fibers can carry much greater amounts of information
85
What are the advantages of using optical fibers, material-wise ?
They are made with easily available materials (Silicon dioxide and plastic)
86
What is the average life of an optical fiber ?
40 years
87
What is a massive materialistic advantage of optical fibers over metallic cables ?
Optical fibers do not corrode
88
Why are optical fibers easy to transport ?
They are compact and light-weight
89
Why is the cost/unit/channel of an optical fiber very less ?
It carries a lot more information
90
What are the common disturbances encountered by metallic cables that optical fibers are immune to ?
Radio and telecommunication signals
| Lightning
91
Why is it difficult to tap optical fibers ?
There is no leaked energy radiation to detect
92
What happens when information is tapped by directly hooking to the information carrying fiber ?
The power at the receiver end drops, warning of an intrusion
93
Why does cross-talk not take place in optical fibers ?
There is a higher purity of signal
94
Why is there a greater purity of signal ?
Light cannot enter a fiber from it's sides and cause interference
95
Why are optical fibers superior for long distances ?
Superior Attenuation characteristics (80 km before amplification vs 5 km)
96
How are optical fibers protected from flammable and corrosive environments ?
There is no spark
97
Why ?
The signal is optical
98
How do optical fibers reduce complexity in transmission of information ?
There are far fewer components than metallic cables
99
How do the lesser number of components reduce the cost ?
1. Eliminates costly electronic modules
| 2. Fewer spare parts in inventory
100
How do the lesser number of components affect reliability ?
It improves the reliability
101
Why ?
There are a fewer number of parts which can fail ?
102
What is a difficulty in optical fibers, manufacturing wise ?
Splicing is difficult , skillful and therefore expensive
103
What happens when splicing is not done precisely ?
It causes a virtual break in communication
104
Why are maintenance costs high for optical fibers, in case of accidents ?
Repairing connection is highly skillful and time consuming
105
What causes upsets in the critical alignments which lead to loss of signal power (from the weather) ?
They undergo expansion and contraction with temperature
106
Can they be bent to circles ?
It is risky since the loss becomes considerable even for small curvature bends
107
What is diameter of the core of Step Index Multi mode Fiber ?
50-200 micro meters.
| They are very similar to Single Mode Fibers, otherwise.
108
What is the difference in between the refractive index profiles of Single Mode and Step Index Multi Mode Fibers ?
They are similar but step-index multi mode fibers have a larger plane region for the core
109
What can be a light source for Step Index Multi Mode Fibers ?
Can accept lasers and LEDs
110
Which is the least expensive optical fiber ?
Step Index Multi Mode Fibers are the least expensive optical fibers
111
Where do Step Index Multi Mode fibers find application ?
Data links with lower bandwidth requirements
