OFC Technology

8/10/2019 OFC Technology

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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.

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