Principle of Light Transmission and Fiber Geometry

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Principle of Light Transmission 1TOFCPLT024

T818

ITEC - SCAAP Training Programme

Mod Id : TOFCPLT024

Principle of Light Transmission

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Light Propagation through O.F

� Rarer to Denser medium

- Refracted rays move towards the normal

� Denser to Rarer medium

- Refracted rays move away from normal

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�Critical angle:- The angle of incidence in the

denser medium for which the angle of

refraction is 90°

�When the angle of incidence is greater than

the critical angle, Total Internal Reflection

occurs.

�Refractive Index of core is 1.48

�Refractive Index of cladding is 1.46

Contd..

Total Internal Reflection

n1

n2

n1

n2

n1

n2

Angle ofincidence

Angle of

Refraction

Light is bent away

from the Normal

Light does not

enter second

material

Light is reflected in

the same material

Angle ofincidence

Angle ofreflection

Critical

angle

(Note:- n1 is greater than n2)

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Refractive Indices of Various Materials

Material Index (n) Light velocity (Km/s)

Vacuum 1.0 300,000Air 1.0003 (1) 300,000

Water 1.33 225,000Fused Quartz 1.46 205,000

Glass 1.5 200,000Diamond 2.0 150,000Silicon 3.4 88,000

Gallium Arsenide 3.6 83,000

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Total Internal Reflection

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Fiber Geometry

�Consists of Core and Cladding

�Core and Cladding are made up of samematerial

�The material used is optically transparent

�Silica or borosilicate glass will be the usualmaterial

�The R.I of the core will be slightly higher thanthe R.I of the cladding

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Fiber Geometry contd…

The core and cladding dia in mm

140100

12562.5

12550

1258

Cladding (µm)Core (µm)

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Construction of O.F.Cables

�Optical fiber cable needs protection before itis used.

�Cabling means process to package one ormore fibers in an outer protective structure.

�Cabling improves the mechanicalcharacteristics of a fiber without causing adeterioration of its optical properties.

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Important parameters, which needstrengthening and protection, are :

� Tensile strength (pull)

� Crushing resistance

� Protection from excess bending

� Abrasion protection

� Twist and

� Chemical protection.

O.F cables are light, small, flexible, flameretardant and temperature insensitive besidesbeing mechanically strong and rugged.

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Main Components of Optical Fiber Cable

�OF cable is mainly divided into two parts :

- Metallic

- Non-metallic

�Metallic frame cables are those which usemetallic component for protection and alsomay be metallic conductors. Non metallicfiber cables use non-metallic protectionmaterial.There is no metal componentinvolved in the cable.

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Parts of A Fiber Optic Cable

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Optic Fiber Cable

The following components are commonly used inmost of the cables.

� Buffer

� Strength member

� Filler and core wraps

� Jacket and moisture barrier

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�Optical Fibers used are either

1. Multi-mode (graded index or GRIN fibers)

2. Single-mode (Step index or SI fibers)

�DOT uses Single mode fibers.� Contain core and clad (core diameter is 8 to 10

micrometers. Cladding diameter is approximately 125micrometers.

� Difference is in their Refractive Index.

� This is because to achieve Total Internal Reflection.

� Single Mode is preferred for Telecommunicationapplications due to its larger information capacity, for

larger distances.

Fiber

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Aramid yarn,

steelTensile strength

Primary strength

Member

Steel, Fiberglass

Facilitates stranding

Temperature stabilityAnti-Buckling

Central member

Nylon, Mylarplastic

Protect fiber fromoutside stress

Buffer

Material FunctionComponent

Cable Components

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Steel tapeRodent protection

Crush Resistance

Armouring

Water blockingcompound

Prevent moistureIntrusion and

Migration

Cable fillingCompound

PE, PUR. PVC,

Teflon

Contain and protect

cable core. AbrasionResistance

Cable jacket

MaterialFunctionComponent

Cable Components

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Buffers

�Fibers are coated with a buffer immediatelyafter being drawn. This buffer known asprimer.

�Primary coating is made of silicon rubber,Acrylate, or lacquer.

�Applied to the cladding by the fibermanufacturer. This coating serves asmechanical protection during the subsequentstages of the cable manufacturing.

�Typical diameter of the fiber after primarycoating is 250-350 micrometer.

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Buffers

�After primary coating fibers are normallycoloured by passing through colouringmachines.

�Coloured fibers are passed through theadditional buffers.

�Additional buffers are called Secondarycoating.

1. LOOSE BUFFER

2. TIGHT BUFFER

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Loose Buffer

�To isolate the optical fiber from external forcesit is placed in a “ buffer ” tube.

�The cable fills these tubes with Jelly likecompound which provides additionalcushioning and prevents the intrusion ofmoisture.

�The inside diameter of buffer is several timesthat of the fiber. One or more fibers lie withinthe buffer tube.

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Loose Buffer Contd..

�Protects the fiber from any mechanical forcesacting on the cable.

�The fiber can adjust itself within the tube whenthe cable is distorted. As the cable expandsand shrinks with change in temperature, it doesnot affect the fiber much.

�The fiber in the tube is slightly longer than thetube itself. Thus, the cable itself can easilyexpand and contract without stressing the fiber.

�The diameter of this tube is 1.2 to 1.9 mm. Theloose buffer is preferred for almost all outdoorapplications

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Tight Buffer

� The tight buffer has a plastic coating directly appliedover the primary coating of the fiber.

� This type of construction provides better crush andimpact resistance.

� Tight buffer is more flexible and allows tighter turnratio.

� Tight tube buffers have general application in indoorcables where temperature variations are mixed andthe ability to make tight turns inside walls is desired.

� They do not protect the fiber from stressing ortemperature variations.

� Single fiber pigtail cables use tight buffer. They areused for termination in equipment room.

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Strength Member

� Function is to add mechanical strength to thefiber.

� Strength Member protects the fiber from thetensions and strains generated during pulling,shearing and bending of the cable.

� Most common strength members are:

– Kevlar

– Aramid Yarn

– Fiber Glass Reinforced Plastic (FRP)

– Glass Reinforced Plastic GRP).

� FRP & GRP are used in Multi Mode fibers.

� Kevlar is used for individual fibers.

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Fillers and Core Wraps

� Fillers are employed to provide cushioning to thefibers and to give shape to the cable.

� Typical materials are PVC, Polyethylene, low densitycellulose paper, spun bonded polyester.

� Cable core is generally filled with a water blocking orfilling compound for preventing moisture intrusion.

� Binder tapes are applied to hold the assemblies ofcoated fibers and fillers, together.

� It also provides a heat barrier to the fiber during theextrusion process of outer sheath. Usual materialsare polyester, Mylar, cellulose paper, etc.

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Jacket & Moisture Barrier

� The jacket or sheath provides protection from theeffects of cut and abrasion, oil, ozone, acids, alkali,solvents etc.

� Materials such as low density polyethylene, highdensity polythene, poly urethene, polyvinyl chloride(PVC), nylon, etc. have been successfully employedin commercial cables.

� The choice of jacket material depends on applicationand cost.

� In cable containing several layers of jacketing andprotective material, the outer layers are often calledthe SHEATH. The inner layer which directly protectsthe fibers becomes the jacket.

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� Types of cables

- Loose tube structure

- Slotted core structure

� Length of the cable drum is 2 Kms.

Optical Fiber Cable Configurations

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Loose tube structure� Uses protective loose tube which is made up of

thermo plastic material.

� Tubes are stranded helically in continuous oralternate paths around a central strengthmember.

� Central strength member is FRP or GRP.

� Tubes are filled with materials which have stablephysical characteristics over a wide range oftemperature.

� Cable interstices are filled with moisture resistantfilling compound to retard ingress and axialmigration of water.

� Cable core is wrapped with a wrapping tape.

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Colour code scheme in Seicor cable

Orange/Black12White6

Blue/Black11Slate5

Violet10Brown4

Yellow9Green3

Black8Orange2

Red7Blue1

Colour codeFiber NoColour codeFiber No

�Colour code simplifies fiber identification

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Optical Fiber Cable Configurations

896

648

424

212

26

Fibers in each tubeNo of fibers

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Slotted core structure

�Consists of a plastic rod abstracted over a

central strength member assuring good

mechanical and thermal performance of the

structure.

�V-grooves or slots are cut in the surface of the

plastic rod.

�These grooves or slots may contain one or

more fibers protected only by primary coating.

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Slotted core structure contd..

�After insertion of the fiber, the slotted core is

closed by applying plastic or synthetic

covering.

�Wherever necessary, slots are filled with a

filling compound having stable physical

characteristics over a wide range of

temperature.

�Each slotted core may be used alone with

protective outer jacket, or assembled with

similar cable units before providing outer

protection.

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Optical Fiber Construction

9 µm125µm

400 µm

Secondary Coating

Primary Coating

Cladding

Core

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Optical Fiber Construction

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�Cabling is packing one or more fibres in a

outer protective cover

�Cabling protects environmentally and

mechanically

O.F Cable Construction

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Components of Cable

�Fibers

�Buffers

�Fillers

�Strength Members

�Core wraps

� Jacket/Moisture Barrier

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Outer Sheath

�Also known as External Sheath

�Nylon sheath of not less than 0.7 mm thick

�Orange coloured -- used by communication

units

�Green / Black coloured -- used by Railways

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Inner Sheath

�Also known as inner jacket

�Black coloured sheath

�Not less than 2.0 mm thick

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Central Strengthening Member

�To add mechanical strength to cable so that

pulling tension and tension at bends can be

with stood.

�Solid GRP (Glass Reinforced Plastic) non-

metalic

�Outer dia 2.1±0.15 mm

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Slotted Core Structure

�Plastic rod extruded over a strength member

with good mechanical and thermal

performance.

�V- Grooves or slots cut on the surface of rod.

�Each group is containing one/more primary

coated fiber.

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Fillers & Core Wraps

�Provides cushioning and give shape.

�Materials- PVC, Polyester, Polyethylene,

Cellulose Paper.

�Filled with PJ like compound for Moisture

Barrier.

�Binder tapes are wrapped around fillers.

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Loose Tube Structure

�Fibers (normally coloured) placed loosely

in tubes of thermoplastic

�The tubes are helically stranded over a

FRP/GRP central member

�Tubes are filled with PJ compound

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Tight Buffered Cable

�Used for indoor purposes

�Pigtail cable - single tight buffered fiber

whose one end is free and one end is

connectorised

�Free end is for splicing

�Patch Cords - Both the ends are

connectorised.

� It is equivalent for Jumper

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FDF

Cable Fibres Pigtail Connector

Pigtail cable

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Colour Identification Scheme

----White6

Filler (Natural)5Slate5

Filler (Natural)4Brown4

Filler (Natural)3Green3

Orange2Orange2

Blue1Blue1

ColourBuffer Tube

No.Colour

Fiber

No.

Note : Colour codes will be different from Manufacturer to Manufacturer

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Thank You

Principle of Light Transmission and Fiber Geometry

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