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Communication Basics
TRANSMITTER RECEIVERMEDIUM
Input to the transmitter is the information signal. Transmitter modifies thissignal into a suitable form so as to send along the medium to the distant end.
Receiver receives the signal from the medium and converts it to suitable
form for further application.
Medium can be of different types like copper cable, radio wave or Optical
Fiber Cable.
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Communication Media
Guided Unguided
e.g.Atmosphere(Wire Less)e.g.Twisted Pair Wire,Co-axial Cable (Copper),Fiber Optic Cable.
Communication Media
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Optical Fiber Link
Light is guided through fiber.
TRANSMITTER
DRIVER LASER
SOURCE
Converts elec. signal to light signal.
Driver modifies the information into a
suitable form for conversion into light
Source is LED or laser diode which does
the actual conversion.
FIBER LINK
MEDIUM FOR CARRYING LIGHT
RECIEVER
DETECTOR
Detector accepts light andconverts it back to elec.
signal.
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Light Ultraviolet (UV)
Visible
Infrared (IR)
Communication
wavelengths 850, 1310, 1550 nm
Low-loss wavelengths
UV IR
Visible
850 nm
980 nm1310 nm
1480 nm
1550 nm
1625 nm
l
Wavelength:l (nanometers)Frequency:(tera hertz)
c= xl
Optical Spectrum
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Advantages Of Optical Fiber
Very largeinformation carrying capacity (band width) ofthe order of several GHz.
Low loss:- Information can be sent over a large
distance. Unlike other medium the attenuation is flat in
optical fiber i.e. independent of information frequency. Fibers are immuneto ELECTRO MAGNETIC
interference.
Small size and light weight.
Greater safetyFiber is made of dielectric material
which do not conduct electricity .It cannot cause fire or
explosions.It is not prone to lightning.
Higher securityNo tapping possible.
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Applications of Optical Fiber
Telecommunication Trunk Network
Subscriber Loop
CATV Control Systems
Local Area Network
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Light travels with different velocities in different media. The speed of light
changes when it travels from one material to another.
Also the direction of propagation changes.
This deflection is called refraction.
Index of refraction (refractive index) of a material denoted by n is the ratio
of the velocity of light c in free space to the velocity of light in that material v.
i.e. n =c/v
(e.g.- Refractive Index Of Glass ~ 1.5 )
A small portion of light always reflect back when it passes from one material to another.
Light Propagation
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Snells Law
A1
A2
n2
n1
n1sinA1 = n2sinA2
As A1increases A2also increases. At a value
of A1=A called critical angle ,A2 becomes
900 i.e. No light enters material 2
At any angle of incidence greater than A all light will bereflected back to material 1.
1
2
n1> n2A
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Propagation Of Light In Fiber
When a ray of light is incident at an angle greater than the
critical angle, it gets completely reflected back to the same
material.
This is calledTOTAL INTERNAL REFLECTION
Communication Through Fiber Uses This Principle.
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CladdingCore
Coating
Fiber Geometry
An optical fiber is made ofthree sections:
The core carries the light signals
The cladding keeps the light in the core
The coating protects the glass
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Fiber dimensions are measured in m 1 m = 0.000001 meters (10
-6)
1 human hair ~ 50 m
Refractive Index (n) n = c/v
n ~ 1.467
n (core) > n (cladding)
Cladding
(125 m)Coating
(245250
m)
Core(862.5 m)
Fiber Dimensions
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Classification Of Fibers
A. Material Classification
B. Mode Classification
C. Refractive Index Classification
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A. Material Classification
Glass Core And Glass Cladding (Most Widely Used)
Glass Core And Plastic Cladding
Plastic Core And Plastic Cladding- (Inexpensive , ButSupport Very Low Band Widths)
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n2
n1
Cladding
Core
n2
n1
Cladding
Core
Multimode fiberCore diameter varies
50 micro-m for step index
62.5 micro-m for graded index
Primarily used for intra-officeapplications.
Notless expensive than single mode.
Single-mode fiberCore diameter is about 9 micro-m
Only one mode (ray) propagates.Bit rate - distance product>100 THz-km
B. Mode Classification
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C. Refractive Index Classification
Step Index fiber (SIFiber)
Graded Index fiber (GRINFiber)
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Single Mode Step Index Fiber
n1
n2
n
5-10 m
125 m
n1
n2
n
n2
n
n1> n2> n
n1refractive index of core
n2refractive index of cladding
In Step Index Fiber Core has uniform refractive
index. A sharp step in refractive index at core -cladding junction.
CLADDING
CORE
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Multi Mode Fiber
Multi mode fibers are of two types:
1. Multi mode Step Index 2. Multi mode GradedIndex
Refractive index profile
50-100 m
125 m
n1
n2
n
n2n
125 m
n1
n2
n
n2n
50-100 m
Core Has Uniform Refractive Index. A Sharp
Step In Core And Cladding Junction.
(n1 to n2)
Ref. Index Of Core Is Not Uniform. Rather
Gradually Decreases Radially Outwards
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Types Of Single Mode Fiber
SMF : G.652 (standard, 1310 nmoptimized, unshifted) Most widely deployed by far.
Introduced in 1986
SMF DS (dispersion shifted) : G.653 For single channel operation at 1550 nm
SMF : G.654 For WDM operation in the 1550 nm region
LEAF and True Wave (Non-Zero DispersionShifted) : G.655 Latest generation fiber developed in mid 90s
For better performance with high-capacity DWDM Systems
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Characteristics of Optical Fiber
A. Numerical Aperture
B. Dispersion
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B. Dispersion
The spreading of light pulse as they travel
through the entire length of the fiber.
Dispersion limits the bandwidth.
Dispersion increases in direct proportion tothe square root of fiber length.
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What Is Dispersion?
Dispersion is the spreading or broadening of light pulses as they propagate
through the fiber.
Too much dispersion gives rise to bit-errors at the receiver (i.e., the inability to
distinguish a 0 from a 1).
Not recognizable
1 0 1 1 ? 1
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Classes of Dispersion
A. Modal Dispersion
Dispersion caused due to different paths thelight rays taketo travel from one end to the
other. This is prominent in Multi Mode Fibers. B. Chromatic Dispersion
Dispersion caused due to the variation in
velocities of different wave lengthcomponentsof the transmitted light w.r.t therefractive indexof the material.
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Types Of Dispersion Visualized
MMF (Step Index)
l1l2
Optical Paths
Wavelengths
SMF
Difference in
arrival times
Modal
Chromatic
The difference in arrival times of the different components, would cause the
broadening of the signal at the receiving end, the result being dispersion.
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Attenuation
It is a major factor considered in the designing of any transmissionsystem.
In fiber optics, attenuation is one factor which determines thepower loss.
Note:Power Loss is calculated in dB/km (decibels/kilometer).
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Attenuation varies with the wave length of light.
6
5
4
3
2
1
Wave length
0 800 850 1000 1310 1550 1600
The fiber exhibits minimum attenuation at wavelength slots
850nm, 1310nm, and 1550nm . These are called first window,
second window and third window.
Graph of Loss in dB/Km versus Wavelength
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Wave length Attenuation range
850nm 2 to 2.5 dB/km
1310nm 0.4 to 0.5 dB/km
1550nm 0.25 to 0.3 dB/km
Wavelength and Attenuation Range
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Sources of Losses in Fibers
(1) Absorption
(2) Scattering
(3) Geometric Effects
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(1) Absorption
Intrinsic Absorption:It is a natural property of glass - even purest glass
absorbs energy in selected wavelength regions nearto Ultra Violet region.
Absorption Due to Impurities:
Due to the presence of impurities like metal ions andhydroxyl ions light energy is absorbed.
The peak of OH_
ion absorptionoccurs atapprox.1400nm wave length range.
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(2) Scattering
Loss of optical energy due to imperfections in the fiber(localized density variations).
At imperfections light scatters in different directions and
thus energy is lost . This is known as Rayleigh Scattering. It is inversely proportional to the fourth power of wave
length.
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(3) Geometric Effects
Micro bending
Deformation of fiber axis (axial distortion)
during cabling causes light to couple out
of the fiber.
Macro bending
Loss due to excessive bending.
Fiber Bending radius = 3 mm (apprx)
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Central strengthening member (Fiber-
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g g (
Reinforced-Plastic)
Dummy tube
Fibers
Filler (Cellulose paper/
bonded polyester) Kevlar yarn
Polyethylene sheath
Polyethylene jacket
Loose tubes
Cable Construction
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COLOR CODING IS VERY IMPORTANT: See Below,
BLUE
ORANGE
GREEN
BROWN
GREY
WHITE
RED
BLACK
YELLOW
VIOLET
ROSE
AQUA
Fiber Color Coding
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Actual Cable (
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Structure of 48 FIBER cable used in NBB route G.655
BLUE
ORANGE
GREEN
BROWN
GREY
WHITE
RED
BLACK
YELLOWVIOLET
ROSE
AQUA
DUMMY
Actual Cable (
NBB)
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Optical fiber Manufacturing is a 3 Step Process:
(I) Pre-form Manufacture
(II) Fiber Drawing (III) Cabling
Finally, Fiber & Cable Characterization
Manufacturing of the OFC
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Glass Purity Breakthrough
For Ordinary Glass propagation distance will reduce the
transmitted Light Power by 50% (i.e. 3 dB)
Window Glass 1 inch (~3 cm)
Optical Quality Glass 10 feet (~3 m)
Fiber Optics 9 miles (~14 km)
Fiber Optics Requires Very High Purity Glass
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Low Attenuation : To give wider Repeater pacing.
Low Dispersion : To achieve High Data transmission.
High Strength : To use Fiber in demanding environments.
Low attenuation is achieved by
use of extremely high purity materialin the deposition process.
Meticulous control of Process to prevent contamination.
Low Dispersion is achieved by
Accurate Control of Deposition Process.
Precise control of Dopants Flow Rate & Temperature.
High Strength is achieved by
Use of high quality pure material.
Precise control of Lathe traverse & Deposition Process.
Control of Pulling Process(Fiber Drawing).
Essential Fiber Parameters
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Reasons for Fiber Joints
Fibers / Cables are not endless.
At both Transmitter and Receiver points, fiber
must be joined to that equipment.
Cable cuts and their subsequent restoration.
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Connecting Fiber Optic Cables
Two general methods of joining fiber optic cables
Connectors
A disconnectable junction device where
removal and re-connections is needed.
Fusion Splicing
Precision splicing equipment used to fuse fibers
together for non-removable permanent cable
splices.
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Components of Fiber Optic Connector
Dust Cap
Ceramic Ferrule
Crimp Sleeve
Strain Relief Boot
Connector Body
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Types of Connectors
FC Connector
Used widely for Telecom and Datacom.
ST Connector
Limited data use. Control and Opto -electronics.
SC ConnectorUsed mainly for Datacom and CATV.
From 70+ designs only few dominate real-world applications:
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The Connector Ferrule End face
(Not to scale)
Ferrule
(2.5mm)
Glass Cladding
(125 micron)
Glass Core
Ferrule Materials:
Ceramic
Polymer / Plastic
Stainless Steel
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Physical Contact
PC Connectorferrule are formed with aconvex end face of 15mm 5mm radius ofcurvature to ensure the fiber cores are inpositive contact with each other.
The ferrules are pressed securely togetherby a spring in each connector to maintainthis contact.
Fiber
Ferrule
End face
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Connector End Face
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Connector End Face
Radius of Curvature
Connectors have convex ferrule end face. Proper physical contact
requires convex mating ferrule end faces. A convex end face insures accurate contact between fiber ends
and eliminates a glass-to-air gap between mating fibers. As theradius of curvature is made smaller, the losses are reduced.
Physical Contact Super Physical Contact Ultra Physical
ContactPC SPC UPC
Smaller Radius
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Insertion Loss
Determined by measuring how much transmitted light is lost as it passes through theconnector junction.
Expressed in dB.
Note : dB = 10log10 (Pout/ Pin)
(example: 3 dB loss is 50 % loss of signal, because 10log10(0.5) ~3)
Typical Insertion loss is 0.2 dB (This represents 5% of signal loss)
Better the polishing, better is the insertion loss.
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Loss Factors
End Gap
Finish and Dirt
Co-axiality
End Angle
Axial Run-Out
Core Mismatch
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Magnified Connector End Face
Excellent Condition Scratched Core
Chipped Connector Cleaning Residue
Unclean, Lint or Dirt Scratched Face
Multimode Singlemode
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Protrusion & Undercut
These are the defects in the ferrule polishing process.
Either are caused by failing to match the spherical surfaces
of the ferrule and fiber.
Protrusion: Undercut:Result of insufficient Result of excess
polishing. polishing.
Fiber
Ferrule
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Return Loss / Back Reflection
Return Loss / Back Reflection is expressed in dB (Decibels)
The typical return losses for various ferrule end face types:
PC Connector - 40 dB 1/10,000 reflected back
SPC Connector - 50 dB 1/100,000 reflected back
UPC Connector - 60 dB 1/1,000,000 reflected back
APC Connector - 70 dB 1/10,000,000 reflected back
We see that APC is the best since the loss is minimum.
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Selection Criteria
1. Connector Performance
Insertion Loss: 0.1 to 1.0 dB per connection.
Return Loss: -20 dB to -70 dB ( for APC )
Repeatability of connection(specified at per 1000 mating)
2. Strength of ConnectorReliability / Strength of connection ( Rough handling)
Effect of environmental changes on losses.
3. Ease of Termination
4. Cost
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Cleaning FO Connectors
With Fiber Optics, tolerance to dirt is near Zero.
Dust particles may scratch the ferrule/fiber end face if not cleaned properly, and
remedy will be changing the connector!
Use lint-free pads and Iso-propyl Alcohol for cleaning connectors.This is effective and inexpensive.
Always keep dust caps on connectors, bulkhead splices, patch panels etc.
A system is only as good as its weakest link. Do not allow the connector to
become the point of failure because of poor attention. Choose the best connectorpossible, frequently measure the losses of the connectors to check the
degradation, and clean every connector, every time.