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Fiber Materials

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Selection of Materials

• A number of requirements must be satisfied for selecting material for optical fibres,– Must be possible to make long, thin, flexible

fibres – Must be transparent at a particular optical

wavelength for the fiber to guide light efficiently

– Physically compatible materials having slightly different refractive indices for the core and cladding must be available.

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Selection of Materials

• Majority of fibers are made of glass made of silica (SiO2) or a silicate

– Available glass fibers ranges from high loss glass fibers with large cores used for short transmission distances to very transparent (low – loss) fibers employed in long haul applications

– Plastic fibers are less widely used because of their higher attenuation than glass fibers

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• Plastic fibers are mainly used in short distance ( several hundred meters ) application and in abusive environments, where greater mechanical strength of plastic fibers offers an advantage over the use of glass fibers.

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Glass Fibers

• Glass is made by fusing mixtures of metal oxides, sulfides or selenides, resulting material is a randomly connected molecular network.

• A consequence of this random order is that glasses do not have well-defined melting points.

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Glass Fibers

• Largest category of optically transparent glasses from which optical fibers are made consists of the oxide glasses.

• Most common is silica ( SiO2 ), which has a refractive index 1.458 at 850nm.

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Glass Fibers

• To produce two similar materials that differ in indices of refraction for the core and cladding, following is added to the silica,– GeO2 or P2O5 is added to increase the

refractive index

– Fluorine or B2O3 is added to decrease the refractive index

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– Examples:• GeO2 – SiO2 Core ; SiO2 Cladding

• P2O5 - SiO2 Core ; SiO2 Cladding

• SiO2 Core ; B2O3 - SiO2 Cladding

• GeO2 – B2O3 - SiO2 Core ; B2O3 - SiO2 Cladding

• The raw material for silica is sand.

• Glass composed of pure silica is referred to as either silica glass, fused glass, or vitreous silica.

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• Desirable properties are:– Resistance to deformation at temp. as high as

1000C– High resistance to breakage from thermal

shock because of its low thermal expansion– Good chemical durability– High transparency in both visible and infrared

regions

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• Disadvantage:– High melting temperature if the glass is

prepared from a molten state

• This problem is partially avoided when using vapor deposition techniques

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Halide Glass Fibers

• Discovered in 1975

• Fluoride glasses that have extremely low transmission losses at mid-infrared wavelengths (0.2 – 8 µm with the lowest loss being around 2.55 µm )

• It is a heavy metal fluoride glass, using ZrF4 as the major component and glass network former.

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Halide Glass Fibers• Core of the glass fiber is made of,

• and is referred to as ZBLAN (after its elements)

Material Molecular percentage

ZrF4 54

BaF4 20

LaF3 4.5

AlF3 3.5

NaF 18

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Halide Glass Fibers

• To make lower refractive index glass, ZrF4 is partially replaced by HaF4 and is referred to as ZHBLAN cladding

• Advantage– Offers intrinsic minimum losses of 0.01 – 0.001 dB

/km

• Fabricating long lengths of these fibres is difficult– So ultrapure is used to reach low loss level– Fluoride glass is prone to devitrification

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Active Glass Fibers

• Incorporation of rare – earth elements into a normally passive glass gives the resulting material new optical and magnetic properties

• These new properties allow the material to perform amplification, attenuation, and phase retardation on the light passing through it

• Doping can be carried out for both silica and halide glasses

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Active Glass Fibers• Commonly used materials for fiber laser are

erbium and neodymium• Ionic concentrations of these elements are low

to avoid clustering effects• By examining the absorption and fluorescence

spectra of these materials, one can use an optical source which emits at an absorption wavelength to excite electrons to higher energy levels in the rare earth dopants

• And as these excited electrons drop to lower energy levels, they emit light in a narrow optical spectrum at the fluorescence wavelength

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Chalgenide Glass Fibers

• Non linear properties of glass fibers can be exploited for applications such as all- optical switches and fiber lasers

• Chalgenide glass is one example for this

• This has high optical nonlinearity and its long interaction length

• These glasses contain at least one chalcogen element S, Se or Te

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Chalgenide Glass Fibers

• And typically one other element such as P, I, Cl,Br, Cd, Ba, Si or Tl is added to tailor the thermal, mechanical and optical properties of the glass

• As2S3 is well known chalgenide glass

• Single mode fibers have been made using As40S58Se2 for the core and As2S3 for cladding

• Losses in these glasses typically range around 1 dB / m.

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Plastic Optical Fibers

• High bandwidth graded index polymer ( plastic ) optical fibers (POF) have been developed for delivering high speed services directly to the workstation

• Core of the fiber is– Polymethylmethacrylate (PMMA POF)– Or– Perfluorinated Polymer ( PFP POF)

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• Advantage– Tough and durable than glass fiber

– Since the modulus of these polymers is nearly two orders of magnitude than that of silica, even a 1mm diameter graded index POF is sufficiently flexible to be installed in conventional fiber cable routes

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– Compared with glass fibers core diameters of plastic fibers are 10 – 20 times larger, which allows a relaxation of connector tolerances without sacrificing optical coupling efficiencies

– Connectors, splices and transceivers can be fabricated with inexpensive technologies

• Disadvantage– Greater optical attenuations than glass fiber

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Characteristics of PMMA and PFP Optical Fibers

Characteristics PMMA POF PFP POF

Core Dia. 0.4mm 0.125 – 0.3 mm

Cladding Dia. 1.0mm 0.25 – 0.60 mm

Numerical Aperture

0.25 0.2

Attenuation 150 dB/km at 650nm

60 -80 dB/km at 650 -1300 nm

Bandwidth 2.5 Gb/s over 100m

2.5 Gb/s over 300m