07-Optical Fibre Manufacturing

8/13/2019 07-Optical Fibre Manufacturing

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Optical fibers and cables


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Optical fiberis a dielectric medium which is transparent tovisible and some part of IR spectrum, allows propagation of

designated wavelengths at low loss, minimum dispersion,

scattering and attenuation .In general, it comprises of two

different refractive indices materials, a Core with higherrefractive index and a Cladding with lower refractive index

and enclosed into a protective sheath to form a cable which

may contain several fibers


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Common Fiber Materials Materials selected for fiber manufacture must meet the

following requirements:-Transparent at the selected optical wave lengths

Possible to make thin, long, flexible fibers from thematerial

ys ca y compat e mater a s , av ng s g t y erentrefractive indices must be available

Fiber must possess stable transmission characteristics atminimum cost and minimum attenuation.

Such materials are generally glasses made from Silica orsilicates.

Plastic glasses are less widely used, these have higherresistance and used at short range, rugged applications(tensile strength is higher)


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Glass A state of matter characterise by its disorder

Glass is a state of matter that appears as a solid but actually its

structure is like a liquid with long range disorder. High

viscosity makes it rigid.

It is a non-crystalline solid that continuously converted to

liquid upon heating

Any organic or inorganic material or metal may be made asglass but generally we deal with the glasses made from

melting different inorganic oxides and silica as a major

component


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Glass transformation Glass is characterised by its transformation behaviour


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Silica glass Major component of glass is SiO2which occurs naturally as

silica sand Silica alone is excellent glass former but making glass from

just silica is very expensive and is not commercially economicon the large scale

Melting temperature of silica is 1723

o

C and to produce a glasswith considerable viscosity to handle, we need the temperatureas high as 2000oC or even more

To overcome this difficulty an alternative vapour-deposition

process is employed where silicon tetrachloride is heatedabove 1500 oC in the presence of oxygen forming purevitreous silica on a substrate at a temperature above 1800oC


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Pure SiO2glass has wonderful qualities like:

corrosion resistance

low electrical conductivity

very low coefficient of thermal expansion

ood ultra violet trans arenc and

can withstand temperature as high as 1200oC.

This type of glass is used for applications like:

optical fibre

refractory crucibles for silica and outer window pans of space shuttle


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In pure silica glass SiO4 tetrahedras (3 sided pyramids) are

formed by covalent bonding between Si and O These tetrahedras are then joined to each other at corners in

different orientation with oxygen bridges between

For this reason the oxygen in this arrangement is called as

bridging oxygen (BO). In pure silica glass without any

impurities or defects all the oxygen atoms are BO.


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Multicomponent silicate fibers Multicomponent silicate glass can be used for to make low-

loss transmission fiber but requires extensive purification ofbatch chemicals and control of melting conditions

For details see the paper, K. Sawamura, 'The development of

Toshiba', Glass Technol.,45(2004) 21-32


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Multicomponent silicate glasses Commercial production of silicate glasses (multicomponent )

requires addition of additional compounds, called as fluxes toreduce the melting temperature of silica

As the glass forming involves the inability to rearrange the

,

bond strength. Glass network formers have single bond strength exceeding about

334.944 kJ/mol (80 kcal/mol)

Glass network modifiers have single bond strength of about 251.208

kJ/mol (60 kcal/mol)

Intermediates single bond strength lie in between the network modifiers

and network formers.


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Glass structures


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Network formers:

Theses are the glass forming oxides.These glass formers are the bulk of any glass.

Silica (SiO2) is the most common glass former and thus thelass roduced this wa is called silicate lass.

Other glass formers include B2O3, GeO2, P2O5, Al2O3,Bi2O3, As2O3, Sb2O3, TeO2, Ga2O3and V2O5.

Some of these glass formers, like As2O3 and Sb2O3 mayform glass if cooled significantly rapidly.

Except SiO2, B2O3, P2O5 and GeO2all other glass formersrequire other oxides as well to form glass


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Network Modifiers:

Property modifiers or network modifiers includes alkalineearth and alkali metal oxides

These oxides used to reduce the melting temperature thusreduce the melting cost.

Mostly Na2O (derived from soda ash) and CaO (derivedfrom limestone) are used. Sometimes MgO (magnesia),K2O or BaO may also be used. These are commonly addedin the form of naturally occurring carbonates

Extensive use of network modifiers especially Na2Oreduces water durability and degradation of manyproperties.

CaO can be used alone but mostly it is used to counter the

degradation effects of Na2O, which is a good flux. TiO2andZnO is also used to maintain durability that is reduced byadding Na2O


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The addition of alkali oxide breaks the oxygen bridge and attaches

oxygen from the alkali oxide to the bridge.

In this way nonbridging oxygen (NBO) atoms are produced, that are

covalently bonded to just one silicon atom and having negative

charge. Oxygen from alkali oxide also attaches itself to this broken

bridge.

The negative charge is balanced by the charged alkali metal cations

in the neighbourhood that forms ionic bond to oxygen.

ese re uces t e s ca networ connect v ty y re uc ng .

Alkali also increases electrical conductivity

Na2CO3(12-18%) and CaCO3(5-12%) are used with SiO2(60-75%)

to make soda lime silica glass

It is the most common and least expensive glass but does not havemuch resistance to temperature variations and corrosive

environment


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Borosilicate glass has B2O3in addition and less alkali. It is

expensive for common use because of the additional cost of

B2O3 but has high resistance to temperature changes andchemical corrosion.

B2O3aids melting and reduce forming temperature

CaO and B2O3 also reduce viscosity and increases durability

B2O3is highly volatile and vaporised in the form of HBO2in

the presence of water vapours from the glass melt

Evaporation of B in the atmosphere causing environmental

issues thus current environmental regulation restricting its use


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Intermediates

These are the compounds that are intermediate betweenglass formers and modifiers.

They cannot make glass on their own but can form part ofthe network made by silica

Two of the most important intermediates are Al2O3(alumina) and PbO

Al2O3helps to make the glass durable and temperature &

PbO is also an excellent flux. PbO makes the glass denser,increases its index of refraction and absorbs radiations

Lead glasses contain lead and potassium oxide. These

glasses are commonly called as crystal glass just because oftheir colourless and high optical brilliance. It has highrefractive index, ~ 1.545


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Fining Agents

In the glass melt gases arise from reaction gases orthe air trapped between melting particles.

Fining agents are added to have a final glass void

of any gas bubbles by removal or dissolution of thebubbles and homogenise the melt.

These fining agents work in two ways:

initially they produce gases to make the bubbles biggerso that these bigger bubbles can easily come to thesurface and

later they dissolve the gas trapped in small bubbles as

the micro bubbles produced in the glass do not come outat the surface even at higher temperatures (lowviscosity).


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In the glass melt the rising rate of bubble to the glass surface isgiven by Stokes law

Where v is velocity of rise, g gravitational acceleration, r isbubble radius, is glass density and is glass viscosity

We see that large bubbles in a low viscosity glass can rise to thesurface much easily. Lower viscosity is achieved by increasingthe melt temperature and fining agent produces large bubbles

Sulphur containing compounds (ferrous sulphate (FeSO4.7H2O),

22

9v gr =

so um su p a e a2 4

, gypsum a4

.2

, ar umsulphate (BaSO4) and ammonium sulphate (NH4SO4))areefficient fining agents. Sulphates are commonly used butchlorides and fluorides may also be added in a small quantity

Arsenic trioxide (As2O3) and Antimony oxide (Sb2O3) are alsoused as fining agents

Nitrates (KNO3, NaNO3) evolve oxygen and oxides of nitrogen atglass melting temperatures thus used with arsenic and/orantimony for improved effect


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Melting techniques


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Fiber drawing


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Fiber Types

Silica Glass fiber Principal raw material is sand; glass made of pure silica is

called Silica Glass/ Fused silica / vitreous silica

Addition of GeO2 or P2O5inc ref index

Addition of B2O3or Fluorine decrease refractive index

Features of fused silica

Highly resistant to any deformity up to 1000oC

Highly resistant to thermal shock (for breakage)

Good chemical durability.

Highly transparent in visible as well as IR region.High melting point which is also a disadvantage when

fiber is prepared from molten state . It can be avoided

partially by using Silicon Vapour Deposition

Technique.


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Halide Glass Fibers Research at university of Derennes (1975) resulted into

development of Fluoride glass

low resistance at mid IR (0.2 8m) Addition of rare earth metals (Atomic # 57-71) modifies the

optical and EM properties of fiber.

ommon y use ear me a s are er um an eo ym um.

Loss min (0.001dB -0.01 dB Km-1)

Formation of micro crystallites if not controlled, may resultinto enhanced scattering losses.

Fabricating long lengths is difficult Concentration of these dopants is kept low and can be

employed both for silica as well as halide glasses


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Plastic Fibers

Plastic Clad Silica (PCS) Fibers with glass core and glass cladding are suited for

long dist applications & expensive

For short range applications that can tolerate higherlosses , less expensive, plastic clad fibers can be used

Plastic clad is employed on multimode fibers, with stepindex fiber being more common. They are sometimes

use or gra e - n ex ers as we . Larger NA, Core diameter (150-600 m), std 50 m in

graded index fibers

Large relative ref index difference allows low cost

optical source, inexpensive, quality also lower May be used for low grade / low quality circuits

Have limited performance as compared to glass fibers.


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All Plastic Fibers All plastic fibers are only used for multi mode step index fibers

They are used for short range low cost links(100m)

They give greater Optical signal attenuation More rugged, durable ,do not require careful handling

Higher ref index difference allows larger NA(0.6), acceptance angle700

Lager core (110-1400m), low cost optical source, inexpensive ruggedsolution

Attenuation leves range between 50-1000 dB/Km, but recent work hasenabled to bring this down to 20dB/Km.

polystyrene n1

= 1.6 , methyl methacrylate n2

= 1.49 NA= 0.6

Polymethyle methacrylate n1= 1.49, with methyl copolymer n2= 1.40 ,NA = 0.5


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Common Fibers & their Characteristics Lastly performance characteristics discussed pertained to lab tests ,

Characteristics of fibers commonly available are discussed in the

sequel.

High quality Silica fibers are available in the wave lengths of 0.8

0.9, 1.3 and 1.5m

Polarization keeping and modified multi mode dispersion fibers

ave a so een eve ope .

Cylindrical optical fibers will be discussed

2nd area of fiber manufacture is mid Infra Red range 2-5m, this

employs heavy metal fluoride technology these are generally multi

mode, losses are higher, expected to improve during ensuing period. Commonly available fibers are discussed in the next part of the

lesson.


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Multimode Step Index Fibers Manufactured either from multicomponent glass or

doped silica Performance of doped silica fiber is better

Structure

Core dia 50 400m

Cladding dia 125-500m

Buffer Jacket dia 250-1000m

NA 0.16- 0.5


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Performance Characteristics

Loss 40 dBKm-1 , 5dB Km-1 , = 0.85m

Reduces to 0.4 dB Km-1 , = 1.13m

- -

Large Core dia, NA, facilitate efficient incoherent

light coupling (LEDs)

Applications, suited for short haul ,limited

Bandwidth & low cost applications


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Multimode Graded Index Fibers Have similar two types as for mm step index , however

materials used are more pure , less loss ,consequent better

performance, core size is smaller, cable size almost the same

StructureCore dia 30 100m

-

Buffer Jacket dia 250-1000m

NA 0.2- 0.3

Fol types of fibers are commonly available50 m / 125 m , NA 0.20 0.24 , at0.85-1.3 m ,

used for data link & LAN applications


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85m/125m , NA 0.260.29, at0.85-1.3m , dev forlonger dist but being used for LAN

100m/125 m , NA 0.29 , provide highly efficientcoupling to LEDs at 0.85m even at 1.3m ,suited for

short dist and LAN application Performance Characteristics

-1 -1, , .

Reduces to 0.4 dBKm-1 at 1.3 m & to 0.25dB Km-1

at 1.55m

Bandwidth 300 MHz-km at 850 nm to 3 GHZ - km at

1.55mApplications, suited for medium haul , medium to high

Bandwidth applications using LEDs, injection lasers


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Performance Characteristics

Loss 2-5 dB Km-1 , with scattering limit of 1dB

at =0.85m

Reduces to 0.35 dB Km-1 at =1.3 m &to-1. .

Bandwidth 500 MHZ-km at = 850 nm,

10 GHZ-km at=1300 nm

Applications, suited for very long haul higherBandwidth applications using injection lasers .


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Plastic Clad Fibers These are multimode and have graded or step index profile. Cladding

is made of silicone polymer (Plastic clad silica ,pcs)

StructureCore dia Step Index 100 -500m , Graded Index 50 - 100m

Clad dia Step Index 300 -800m , Graded Index 125 - 150m

Buffer Jacket dia Step Index 500 -1000m , Graded Index 250

1000m

NA Step Index 0.2-0.5 , Graded Index 0.2 0.3

Fol types are commonly available

50 m/ 125 m NA 0.20 0.24at ,0.85-1.3 m , used for data

link & LAN applications


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Performance CharacteristicsLoss Step Index 5-50 dB Km-1

Graded Index 4-15 dB Km-1 ,

Though cheaper but have very limited performancecharacteristics, more rugged, low level, short

distance applications.


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All Plastic Fibres Exclusively multimode step index

Core & Cladding dia are large , no requirementof buffer jacket

as er to pro uce an e or nsta at on Structure

Core (methyl mehta crylate) 200 600m

Cladding (fluorinated methyl mehta crylate)

450 1000 m


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Transmission characteristics

Attenuation 50 100dB / Km

NA: 0.5 0.6

Band width : not measured due to short distance

coverageLosses : lower in polystyrene core fibres

Loss 20 dB /Km @0.68m

Improvement being sought for highertemperature tolerance & other characteristics


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Optical Fibre CablesNeed for cable

Optical Fibre are alternative to electricaltransmission lines

It is im erative that the should be ca able of

installation & maintenance in all thoseenvironments of transmission lines

Underground ducts

Overhead Over ground

Partially OverHead & partially ground

just like

metallicconductors


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Mechanical properties of Optical Fibres assume

greater importance for the above purpose.However, the Optical Fibres are in pure form are

ver weak structures & are characterized as under

Brittle Small cross sectional area

Low tensile strength

Low durability

Prone to chemical contamination / abrasion


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There is a need to cover these fibres to improve

their tensile strength & protect them fromsurrounding environments. Thus a series ofprotective layers surround the Optical Fibre

Initial plastic coating is applied to the claddingwith high elastic modulus

Plastic coated & buffered fibre is then placed intoan optical cable to increase its resistance tomechanical strain as well as adversecircumstances


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Summary of functions of Optical Cable Provide fibre protectionagainst fibre damage &

breakage.

Provide stability in transmissioncharacteristics of the fibre comparable to the

.

must be reduced during manufacture of cable. Provide cable strength: optical cable must

possess similar mechanical properties as those ofconductor cables. For this purpose a strengthmember thick outer sheath is incorporated in thedesign.


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Facilitate identification & jointing of the

fibresis very important for multifibre cables.If the fibres are arranged in a suitable

,

jointing kits / techniques rather than jointingeach fibre individually.


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Cable design Must take into account that strain on the fibre

in the cable must not be more than 0.2%usually < 1%. Cable design must incorporate

Fibre buffering

Cable structural & strength members

Cable sheath & water barrier


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Fibre buffering Fibre is given a protective primary coating to prevent abrasion

of the glass surface & subsequent flaws in the material. Then a

secondary buffer coating or jacket to provide protectionagainst external mechanical & environmental effects. It also

protects fibre against micro bending.

Buffer Jacket may take following threeforms:

Tight Buffer jacket

Consisting of hard plastic in direct with

the fibre (0.25 1mm thick) provides

stiffening for the fibre against-bends


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Loose buffer jacket

Fibre placed in an over sized cavity,

mechanically isolating he fibre from

external forces / effects diameter (1-

2mm) Loose filled buffering

Similar to above, cavity filled with a

moisture resistant compound. The fillingmaterial must be soft , self healing &

stable over wide range of temperatures. It

generally consists of specially blended

petroleum or silicon based compounds.


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Cable Structural & Strength Member There may be one or more structural members to serve as basic

foundation around which they are slotted

Structural member may also be strength member e.g. stranded

steel , Kevlar.

Kevlar youngs modulus up to 13 x 1010 Nm-2, four times to

When using a braided or stranded member, it be covered withsmooth cushioning surface(bedding layer) required to avoid

Micro-bending losses

Primary purpose of strength member is load bearing Extruded plastic structural member around strength member

may be required as slotted member for accommodating optical

fibres


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Cable Sheath & Water Barrier Cable covered with sheath to reduce abrasion and

crushing

Cable sheath contains cable core, cable may be in theform of extruded plastic jacket or multilayer structure

Plastic sheath material ol eth lene has little rotectionagainst penetration of water

Additional water barrier is included in the design orspace in the cable is filled with water resistantcompound i.e. specially formulated silicon or petroleum

compound. Filling compounds must not cause degradation of outer

materials & remain stable under pressure &temperature variations.

07-Optical Fibre Manufacturing

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