optical fibre cable an overview
TRANSCRIPT
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OPTICAL FIBRE CABLE
An overview
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INTRODUCTION
Optical Fibre is a new medium in whichinformation (Voice, Data or Video) is
transmitted through a Glass or Plastic Fibre, inthe form of light.
The transmission sequence is
-Information is encoded into Electrical Signal-Electrical Signals are converted into lightsignals.
-Light travels down the fibre
-A detector changes the Light Signals intoElectrical Signals
-Electrical Signals are decoded intoinformation
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ADVANTAGES OF FIBRE OPTICS
Optical Fibres are non-conductive(dielectric)
Electromagnetic Immunity
Large Bandwidth ( > 50 Ghz for 1 Km ) Low Loss ( 5 dB/km to < 0.25 dB/km)
Small and light weight cables
Available in long lengths ( > 12 kms)
Security
Universal Medium
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APPLICATIONS OF FIBRE OPTICS
Common carrier Nationwide Networks
Telephone Inter-Office Trunk Lines Customer premise Communication Networks
Under-Sea Cables
High Electro-Magnetic Interference areas Factory Communication/Automation
Control Systems
Expensive environments High lightning areas
Military applications
Classified (secured) Communications
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PRINCIPLE OF FIBRE OPTICS
Light traveling from one material to anotherchanges speed, which results in light
changing its direction of travel. This deflection
of light is called refraction.
A ray of light passing from a material of lower
refractive index to a material of higher
refractive index is bent towards the normal.
But light going from a material of higherrefractive index to a material of lower
refractive index refract away from the normal.
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PRINCIPLE OF FIBRE OPTICS ( contd. )
As the angle of incidence increases, the angle
of refraction approaches 90o to the normal. The
angle of incidence that yields an angle of
refraction of 90o is the critical angle. If the angle
of incidence increases more than the criticalangle, the light is totally reflected back into the
first material. The angle of incidence and
reflection are equal. This phenomenon is called
Total Internal Reflection which forms the basis ofpropagation of light through a optical fibre.
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INCIDENT RAYS
REFLECTED RAYS
REFRACTED RAYS
1
1
3
2
2
3
n2r
i
(principle of total internal reflection)
n1 = 1.48
n2 = 1.46
n1
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OPTICAL FIBRE
The Optical Fibre has two concentric layers-
-The Core which forms inner part.
-The Cladding which is the outer part.
The inner Core is the Light carrying part.
The index of the Cladding is 1% less than thatof the Core.
The typical values for r.i. of the Core is 1.47while that for the Cladding is 1.46.
Most of the Fibres have an additional coatingaround the Cladding. This buffer coating is ashock absorber and has no optical properties.
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The Optical Fibre
Cladding
125 mCore8-10 m
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LIGHT PROPAGATION THROUGH FIBRE
Light injected into the fibre and striking Core toCladding interface at an angle greater than the
Critical angle is reflected back into the Core
according to the principle of Total Internal
Reflection. The reflected Light ray strikes otherside of the Core to Cladding interface and again
it is reflected. This process continues and Light
travels in a zigzag way along the length of the
fibre. Light striking the interface at less than the
critical angle passes into the cladding where it is
lost over distance.
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Light propagation in fibre
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Light propagation in fibre
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Light propagation in fibre
http://localhost/var/www/apps/conversion/tmp/scratch_2/FIBER00.swf -
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FIBRE GEOMETRY The Optical Fibre consists of a core usually of silica
or borosilicate glass surrounded by a cladding of
the same material but of slightly lower refractiveindex.
Fibres have exceedingly small diameters. Thediameters of the core and cladding are as follows.
Core (m) Cladding (m)
8 125
50 125
62.5 125
100 140 Fibre sizes are usually expressed by first giving the
core size followed by the cladding size. Thus50/125 means a core diameter of 50 m andcladding diameter of 125 m.
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CLASSIFICATION OF OPTICAL FIBRE
Classification based on refractive index
profile :
SI fibre (Step Index Fiber)
GRIN fibre(Graded Index Fiber)
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CLASSIFICATION OF OPTICAL FIBRE (contd.)
The step index fibre has a core with
uniform index throughout. There is a sharp
step at the junction of core and cladding.
n1= r.i. of coren2= r.i of cladding
index n(r)
n2n1
r
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CLASSIFICATION OF OPTICAL FIBRE (contd.)
The graded index fibre has a non uniformcore. The index is highest at the centre
and gradually decreases until it matches
with that of the cladding. There is no sharpbreak in indices between the core and
cladding.
Refractive
index n(r)
n2
n1
r
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Modal classification :
In case of fibre, a mode is simply apath that a light ray can follow intravelling down a fibre.
The types of fibre based on mode:
Single Mode Fibre ( SMF )Multi Mode Fibre ( MMF )
CLASSIFICATION OF OPTICAL FIBRE (contd.)
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CLASSIFICATION OF OPTICAL FIBRE (contd.)
A single mode fibre supports only a single
propagating mode. Single mode fibres
have small core diameters ranging from 4
to 10 ms.
A multi mode fibre supports more than
one propagating mode. Multi mode fibres
have relatively large core diameters.Typical values of core diameters are 50,
62.5,80,100 ms.
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CLASSIFICATION OF OPTICAL FIBRE (contd.)
By this classification there are three types offibres:
Multi mode step index fibre (Step Index fibre)
Multi mode graded index fibre (Graded Index
fibre)
Single mode step index fibre (Single mode
fibre)
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Input pulse Output pulse
Refractive
index n(r)
n2n1
r CoreCladdingAmplitude
t
Amplitude
Multimode step index fiber
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Refractive
index n(r)
n2n1
r
t
Multimode graded index fiber
Input pulse Output pulse
Amplitude Amplitude
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n2n1
r
tN(r)
Input pulse Output pulseAmplitude Amplitude
Single-mode step index fiber
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FIBRE USED IN BSNL
BSNL uses Single Mode Fibres.
Typical values related to the sizes of the
Cables are-
-Core diameter= 5 to 10 micrometer-Cladding diameter=125 micrometer
-Coating diameter=250 micrometer
SM Fibres are used in Telecom for theirlarge information carrying capacity over
longer distances.
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OPTICAL FIBRE PARAMETERS
Wave length
Frequency
Window Attenuation
Dispersion
Bandwidth
Numerical aperture
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Wave length:
Wave length is a characteristic of light that
is emitted from the light source and is
measured in nm.
Frequency:
Frequency is number of pulses per second
emitted from a light source. Frequency is
measured in units of hertz (Hz).
In terms of optical pulse
1 Hertz = 1pulse/second.
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Window :
Window is defined as the range of wavelengths on which a fibre best operates.
Each window is centered on the typicaloperational wave lengths. Typical windows
areWindow Operational wavelength
800nm 900nm 850nm
1250nm --1350nm 1300nm1500nm1600nm 1550nm
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Attenuation :
Attenuation is defined as the loss of optical
power over a set distance. A fibre with lower
attenuation will allow more power to reach a
receiver than fibre with higher attenuation.
Attenuation is categorized as intrinsic and
extrinsic.
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Intrinsic Attenuation :
Cause of intrinsic attenuation is inherentor within the fibre. Intrinsic attenuation mayoccur due to absorption and scattering.
Natural impurities in the glass absorb light
energy.Light rays travelling in the core reflect
from small imperfections into a new pathway
that may be lost through the cladding. Thelight is scattered in all directions whichcauses loss of optical power in forwarddirection.
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Extrinsic Attenuation :
Extrinsic attenuation is loss due toexternal sources. Extrinsic attenuation occur
due to Microbending and Macrobending.
Microbending in the fibre caused by
crushing, contraction etc. Microbends cause
some of the light to couple out of the fibre.
Macrobending is actually excessive
bending of the fibre. If the fibre is sharply
bent, the light travelling down the fibre can
not make the turn and is lost in the cladding.
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Dispersion :
Dispersion is defined as the spreading oflight pulse as it travels down the fibre.
Because of the spreading effect, pulses
tend to overlap, making them unreadable
by the receiver. Dispersion is undesirableas it limits the bandwidth or information
carrying capacity of a fibre. The bit rate
must be low enough to ensure that pulsesdo not overlap.
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Types of Dispersion
Modal Dispersion
Material Dispersion
Wave guide Dispersion
Modal dispersion occurs only in multimodefibres. It arises because rays follow different
paths through the fibre and consequently
arrive at the other end of fibre at different
times. Typical modal dispersion figures for
step index fibres are 15 to 30 ns/Km.
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Modal dispersion can be reduced in three ways
Using a smaller core diameter, which
allows fewer modes.
Using a graded index fibre. Using a single mode fibre, which permits
no model dispersion.
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Material Dispersion :
Material dispersion occurs as different
wave lengths (colours) travel at different
velocities through a fibre, even in the
same mode.
Waveguide Dispersion :
Waveguide dispersion occurs in a
single mode fibre because optical energy
travels in both the core and cladding which
have slightly different refractive indices.
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Bandwidth :
Bandwidth is defined as the amount of
information that a system can carry such that
each pulse of light is distinguishable by the
receiver. System bandwidth is measured in
MHz or GHz.
Numerical Aperture :
Numerical aperture (NA) is the light
gathering ability of a fibre. A fibre with a large
NA accepts light well, a fibre with a low NA
requires highly directional light.
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Cladding
Core
NA= Sin Typical = 100
Input Surface Refraction
NUMERICAL APERTURE
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OPTICAL TRANSMITTERS
In Optical Line Systems we need light sources in the
infra-red spectum part. The wavelengths used are in one of the following
windows of Optical Fibres-
850nm, 1300 nm, and 1550 nm.
Features of an ideal source for Fibre OpticCommunication are-
-High brightness
-Small emission area
-Small emission Core angle
-Fast response to electrical modulation
-Long life
-Emission wavelength compatible with Fibre
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Optical Transmitting Devices
Commonly there are two Semi-conductor devices for
using as Optical Transmitters--LED (Light Emitting Diode)
-LASER ( Light Amplification by Stimulated Emission ofRadiation ) Diodes
LEDs are composed of a P-N junction with dopedsemiconductor layers. Injected electrons combine withholes in the P-layer where this phenomenon results in
emission of Photons. In case of LASER Diodes, the emission of Photons are
spontaneous and are stimulated by other Photons ( byamplifying light) and we get large quantities of highenergy Photons emitted.
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CATEGORIES OF LEDs
There are LEDs of two categories.
Surface Emitting LEDs
Edge Emitting LEDs
Surface Emitting LEDs radiate Photons in apattern where power diminishes away from a
direction normal to the surface.
Edge Emitting LEDs can concentrate
radiation with improved coupling efficiency.
OPERATING PRINCIPLE
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OPERATING PRINCIPLEOF LASER DIODE
The Energy levels of the Electrons in an atom/molecule
may be written as E1, E2, E3, E4 etc. If an Electron goes from E1 to E2, E3 etc. there will be
absorption of energy.
On the other hand if the Electron comes from E4 to E3 or
E2 or E1 etc , there will be emission of energy in theform of radiation. The radiation consists of Photons ofvarious energy levels. This will be spontaneous.
If the photons at the time of spontaneous emission arestimulated by other Photons ( I.e., by amplifying light ),
the situation is called Stimulated Emission and we willget very large quantities of high energy photons. This isthe principle used in LASER Diodes.
O C C O
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OPTICAL DETECTOR
The Optical Detectorconverts Optical Energy into ElectricalEnergy.
It is basically an Opto-Electronic Transducer doing the oppositefunction of an Optical Source.
There are two distinct mechanisms for Photo detection-
External Photo-Electric Effect
Internal Photo Electric Effect
In External Photo-Electric Effect , the Electrons are freed fromthe surface of a metal by the Energy absorbed from an incidentstream of Photons. The Vacuum Photodiode and the Photo
Multiplier Tubes come under this category. In Internal Photo Electric Effect , Semi-conductor Devices allow
generation of free Charge carriers , Electrons and Holes byabsorption of Incoming Photons. PIN Photo Diode and APDcome under this category.
PIN PHOTODIODE
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PIN PHOTODIODE
PIN stands for Positive, Intrinsic, Negative.
PIN Photodiode has resistive Intrinsic layer sand-witchedin between P and N layers. The width of Intrinsic layer is
sufficient and the depletion layer is spread over the
Intrinsic layer under the influence of high field due to
reverse bias.
Under reverse bias condition, when a Photon enters the
depletion region, it is absorbed and a pair of Electron
and Hole are generated. The Electron and the Hole so
generated move towards the opposite Electrodes. This
results in the flow of Current in the external field.
The PIN Photo Diodes have lower capacitance, high
Quantum efficiency and high speed of operation.
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APD ( AVALANCHE PHOTO DIODES )
The APD has a very wide Intrinsic Layer in between the
P and N Semiconductor materials. Provision of another
P type material in between the N type material and the
Intrinsic layer makes working much more efficient.
As the Photon enters the Intrinsic layer, Electron-Holepairs are formed. Movement of the Charge carriers
towards the opposite terminals results in collision inside
the Diode with neutral atoms. As a result of such
collision more numbers of Electron-Hole pairs aregenerated. Consequently, we get large flow of current in
the external Circuit.
APDs are often influenced by various Noises.
GENERAL FEATURES OF DETECTORS
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GENERAL FEATURES OF DETECTORS
The Detectors are supposed to have the following
characteristics-
High efficiency
Fast response
Low Noise
Small Size
Light Weight
Long Life
Reliability
Low Cost
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CABLE CONSTRUCTION
Optical Fibre need to be protected before it is used.
Cabling is a outer protective structure surroundingone or more Fibres. Cabling protects fibresenvironmentally and mechanically from beingdamaged.
Important consideration in cable design are--Tensile Strength (Pull)
-Ruggedness
-Durability-Flexibility
-Environment resistance
-Temperature extreme
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MAIN COMPONENTS OF A CABLE
In general, an OF Cable is divided into two types-
-Metallic
-Non-metallic
Metallic cables use metallic component for
protection. Non-metallic cables use non-metallic protection
material. Non-metallic cables are used in areas
suffering from high frequency of lightning.
BSNL has gone for non-metallic cables.
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PROTECTIVE COMPONENTS
The following components are commonlybeing used in O F Cables for protection-
-Buffer
-Strength Member-Filler and Core Wraps
-Jacket and Moisture barrier
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OF CABLE CONSTRUCTION
CORE
CLADDING
SILICON COATING
BUFFER JACKET
STRENGTH MEMBER
BLACK POLYETHANE INNER JACKET
ORANGE NYLON OUTER JACKET
PROTECTIVE COMPONENTS
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PROTECTIVE COMPONENTS
BUFFERS: The Fibres are coated with Buffer
immediately after being drawn. This Buffer isknown as primary coating. The Buffer being a
primary coating serves as mechanical protection
during the subsequent stages of cable
manufacturing. After primary coating , the Fibresare coloured by passing through the colouring
machine and then coloured Fibres are passed
through additional Buffer Tubes which are of
two types--Loose Buffer
-Tight Buffer
LOOSE BUFFER & TIGHT BUFFER
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LOOSE BUFFER & TIGHT BUFFER
LOOSE BUFFER: The fibres are placed inside the
Tubes having diameter much larger than those of the
Fibres. The Tubes are filled with jelly like compound
to provide additional cushioning and prevention from
intrusion of moistures. Such Buffers are suitable for
outdoor applications having temperature variations. TIGHT BUFFER: The Tight Buffer has a plastic
coating directly applied over the primary coating of
the Fibre. This type of buffer provide bettercrushand impact resistance. These are suitable for indoorapplications where temperature variation is minimum
and ability to make tight turns is desired.
STRENGTH MEMBER
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STRENGTH MEMBER
The function of the strength member
is to add mechanical strength to the fibre. Itprotects the fibre from tensile stresses duringand after installation. The most commonstrength members are-
1. Kevlar
2. Aramid Yarn
3. FRP (Fibreglass Reinforced Plastic)
or GRP (Glass Reinforced Plastic)
The strength members are centrallylocated or applied over the Buffer jacket.
FILLERS & CORE WRAPS
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FILLERS & CORE WRAPS
Fillers are employed to provide cushioning
to the Fibres and to give shape to thecable. Typical materials are PVC,polyethylene etc.
Cable Core is generally filled with a waterblocking or filling compound to preventmoisture intrusion.
Binder tapes are applied to hold the
assemblies of coated fibres and fillerstogether. Usual materials are polyester,cellulose etc.
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JACKET & MOISTURE BARRIER
The Jacket or sheath provides protectionfrom the effects of abrasion, oil, acids,
alkali etc.
Materials like PVC, nylon, low densitypolythene, high density polythene,
polyurethane etc. being used successfully
for this purpose.
There may more than one layer of
protection forming the jackets.
MULTI FIBRE CABLE
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MULTI FIBRE CABLE
-It contain several loose buffer tubes.
-Tubes used are made of thermo-plastic.-Each Tube may contain one or more fibres.
-These Tubes are stranded helically around
a central strength member which is made ofGRP or FRP.
-The interstices are filled with materials to
protect from temperature variation andmoisture. The cable core is wrapped with a
wrapping tape.
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MAIN COMPONENTS
SIECOR
OUTER JACKET (NYLON)
INNER SHEATH
BINDER
BUFFER TUBES
CENTRAL
STRENGTH MEMBER
FILLERS (SOLID)PVC / CELLULOSE
A
AA
A
FIBER
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THANKS