fiber optics
DESCRIPTION
FIBER OPTIC COMMUNICATIONTRANSCRIPT
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UNIT V OPTICAL FIBER COMMUNICATION
SUDHEESH.S
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7.2
Figure 7.1 Transmission medium and physical layer
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7.3
Figure 7.2 Classes of transmission media
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7.4
Figure 7.3 Twisted-pair cable
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7.5
Figure 7.7 Coaxial cable
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Fiber-optic CableMany extremely thin strands of glass or plastic bound
together in a sheathing which transmits signals with light
beams
Can be used for voice, data, and video
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Introduction to Optical Fibers. Fibers of glass
Usually 120 micrometers in diameter
Used to carry signals in the form of light over distances up to
50 km.
No repeaters needed.
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Fiber v. Copper
Optical fiber transmits light pulses
Can be used for analog or digital transmission
Voice, computer data, video, etc.
Copper wires (or other metals) can carry the same types of
signals with electrical pulses
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Optical Fiber & Communications System
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FREQUENCIES
Frequency refers to the modulating message signal
Frequency.
The rapid exchange of energy from the beam to the dot
excites the phosphor into the radiating photon of energy
which agitate at 4.2857×10^64 times/sec.
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Fiber Optics are cables that are made of optical fibers that
can transmit large amounts of information at the speed of
light.
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Glass Fibers
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Characteristics
Glass Core
Glass Cladding
Ultra Pure Ultra Transparent Glass
Made Of Silicon Dioxide
Low Attenuation
Popular among industries
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Plastic Fibers
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Optical Fiber
Core
Glass or plastic with a higher index of refraction than the
cladding
Carries the signal
Cladding
Glass or plastic with a lower index of refraction than the core
Buffer
Protects the fiber from damage and moisture
Jacket
Holds one or more fibers in a cable
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Total Internal Reflection
Optical fibers work on the principle of total internal
reflection
With light, the refractive index is listed
The angle of refraction at the interface between two
media is governed by Snell’s law:
n1 sin1 n2 sin2
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Reflection
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Refraction
When a ray of light crosses from
one material to another, the amount
it bends depends on the difference
in index of refraction between the
two materials
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17.1 Index of refraction
The ability of a material to bend rays of light is described by the
index of refraction (n).
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Refraction & Total Internal Reflection
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Total Internal Reflection
Optical fibers work on the principle of total internal
reflection
The angle of refraction at the interface between two
media is governed by Snell’s law:
n1 sin1 n2 sin2
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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
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7.23
Figure 7.12 Propagation modes
Figure 7.13 Modes
1. Single-mode fiber
Carries light pulses
along single path.
2. Multimode fiber
Many pulses of light
travel at different
angles
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Multi-Mode vs. Single-mode
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Singlemode Fiber Singlemode fiber has a core diameter of 8 to 9 microns, which
only allows one light path or mode
Images from arcelect.com (Link Ch 2a)
Index of
refraction
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Singlemode FIber
Best for high speeds and long distances
Used by telephone companies and CATV
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Multimode Step-Index Fiber Multimode fiber has a core diameter of 50 or 62.5 microns
(sometimes even larger)
Allows several light paths or modes
This causes modal dispersion – some modes take longer to pass
through the fiber than others because they travel a longer distance
See animation at link Ch 2f
Index of
refraction
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Step-index Multimode
Large core size, so source power can be efficiently coupled to
the fiber
High attenuation (4-6 dB / km)
Low bandwidth (50 MHz-km)
Used in short, low-speed datalinks
Also useful in high-radiation environments, because it can be
made with pure silica core
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Multimode Graded-Index Fiber The index of refraction gradually changes across the core
Modes that travel further also move faster
This reduces modal dispersion so the bandwidth is greatly increased
Index of
refraction
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Graded-index Multimode
Useful for “premises networks” like LANs, security systems,
etc.
62.5/125 micron has been most widely used
Works well with LEDs, but cannot be used for Gigabit Ethernet
50/125 micron fiber and VSELS are used for faster networks
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7.31
Table 7.3 Fiber types
In multimode step-index fiber, the density of the core remains constant from the
center to the edges. A beam of light moves through this constant density in a straight
line until it reaches the interface of the core and the cladding. At the interface, there is
an abrupt change due to a lower density; this alters the angle of the beam's motion. The
term step index refers to the suddenness of this change, which contributes to the
distortion of the signal as it passes through the fiber.
In multimode graded-index fiber, decreases this distortion of the signal through the
cable. The word index here refers to the index of refraction. As we saw above, the index
of refraction is related to density. A graded-index fiber, therefore, is one with varying
densities. Density is highest at the center of the core and decreases gradually to its
lowest at the edge. Figure 7.13 shows the impact of this variable density on the
propagation of light beams.
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Optical Fiber Cable Construction
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How are Optical Fibre’s made??
Three Steps are Involved
-Making a Preform Glass Cylinder
-Drawing the Fibre’s from the preform
-Testing the Fibre
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Modified Chemical Vapor
Deposition (MCVD)
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Fiber and Acrylate Coating
Optical fiber is covered by an acrylate
coating during manufacture
Coating protects the fiber from moisture and
mechanical damage
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Advantages of Optical Fibre
Thinner
Less Expensive
Higher Carrying Capacity
Less Signal Degradation& Digital Signals
Light Signals
Non-Flammable
Light Weight
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Areas of Application
Telecommunications
Local Area Networks
Cable TV
CCTV
Optical Fiber Sensors
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Type of Fibers
Optical fibers come in two types:
Single-mode fibers – used to transmit one signal per fiber
(used in telephone and cable TV). They have small cores(9
microns in diameter) and transmit infra-red light from laser.
Multi-mode fibers – used to transmit many signals per fiber
(used in computer networks). They have larger cores(62.5
microns in diameter) and transmit infra-red light from LED.
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Splices and Connectors In fiber-optic systems, the losses from splices and connections can be
more than in the cable itself
Losses result from: Axial or angular misalignment
Air gaps between the fibers
Rough surfaces at the ends of the fibers
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How are Optical Fibre’s made??
Three Steps are Involved
-Making a Preform Glass Cylinder
-Drawing the Fibre’s from the preform
-Testing the Fibre
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Testing of Optical Fiber
Tensile Strength
Refractive Index Profile
Fiber Geometry
Information Carrying Capacity
Operating temperature/humidity range
Ability to conduct light under water
Attenuation
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Optical Fiber Laying
Mechanical Linking Includes coupling of two connectors end to end
Optical distribution frames allow cross connect fibers from by means of connection leads and optical connectors
Soldering: This operation is done with automatic soldering machine that ensures:
Alignment of fiber’s core along the 3 axis
Visual display in real-time of the fibers soldering
Traction test after soldering (50 g to 500 g)
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Optical Fiber Laying (Cont…)
Blowing
Used in laying optical cables in roadways.
Cables can be blown in a tube high density Poly Ethylene
Optical fiber is then blown in the tube using an air compressor
which can propel it up to 2 kilometers away.
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Tools of Trade Cleaning fluid and rags
Buffer tube cutter
Reagent-grade isopropyl alcohol
Canned air
Tape (masking or scotch)
Coating strip
Microscope or cleaver checker
Splicer
Connector supplies
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Fiber Optics Test Kit
Features
Includes Smart FO Power Meter and Mini LED or laser source
FO test lite software for data logging
Tests all networks and cable plants
New versions of Gigabit Ethernet
Low Cost
Applications
Measure optical power or loss
Trouble shooting networks
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Protecting Fibers
Tougher than copper wires
Designed in three concentric layers
Core – Cladding – Buffer
Two basic buffer types
Tight buffer
Loose tubes
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Implementation of Different LANs
IEEE 802.3
FOIRL
Fiber optic inter repeater link
Defines remote repeaters using fiber optics
Maximum length – 1000 meters between any two repeaters.
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IEEE 802.3 (Cont…) 10BASEF Star topology with hub in the center
Passive hub:
Short cables
No cascading
Reliable Active hum:
Synchronous
May be cascaded
Do not count as one repeater
Any 10BASEF active hub must have at least two FOIRL ports
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Token Ring
Advantages
Long range
Immunity to EMI/RFI
Reliability
Security
Suitability to outdoor applications
Small size
Compatible with future bandwidth requirements and future LAN standards
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Token Ring (Cont…)
Disadvantages
Relatively expensive cable cost and installation cost
Requires specialist knowledge and test equipment
No IEEE 802.5 standard published yet
Relatively small installed base.
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Fiber Distributed Data Interface
Stations are connected in a dual ring
Transmission rate is 100 mbps
Total ring length up to 100s of kms.
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Conclusion
This concludes our study of Fiber Optics. We have
looked at how they work and how they are made. We have
examined the properties of fibers, and how fibers are
joined together. Although this presentation does not
cover all the aspects of optical fiber work it will have
equipped you knowledge and skills essential to the fiber
optic industry.