Claddings and Coatings

Optical fibers for telecommunication purposes generally consist of a core with a high refractive index surrounded by a cladding with a lower refractive index which confines the propagating light waves to the core (Fig. 3). 

Most fibers have a core which consists of silica doped with oxides of germanium or phosphorus to raise the refractive index. The cladding usually consists of pure, undoped silica 

Polymers such as poly(dimethylsiloxane) are also used as cladding material but owing to the absorption losses upon reflection at the glass-polymer interface they are mainly used in short-distance applications. 

The same restriction applies to all-polymer fibers. Modern, all-glass fibers may have losses of less than 0.5 dB km at a wavelength of 1.3 pm . Typical diameters of the core and the cladded fiber are 50 and 125 pm, respectively, but there is a trend to switch over to so-called monomode fibers which have cores with a diameter of 5-10 pm. 

At present most commercial fibers possess a dual coating, a soft buffer coating surrounded by a stiff external coating (Fig. 3). Typical diameters of fibers with their soft and hard coatings are 185 and 250 pm, respectively. Both can be cured in situ by means of photopolymerization. 

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The nonlinear optical and dielectric properties of polymers find increasing use in devices, such as cladding and coatings for optical fibres, piezoelectric and optical fibre sensors, frequency doublers, and thin films for integrated optics applications. It is therefore important to understand the dielectric, optical and mechanical response of polymeric materials to optimize their usage. The parameters that are important to evaluate these properties of polymers are their dipole moment polarizability a, hyperpolarizabilities 0... 

Lead coatings are mainly applied by cladding and find principal use in the chemical industry for resistance to sulphuric acid, for cable sheathing resistant to attack by soils and in architectural applications where resistance to industrial atmospheres is particularly good. They rely for their protective action on the formation of insoluble corrosion products which stifle the corrosion reaction and lead to very long service lives, but the corrosion resistance is impaired when chlorides are present. 

The most widely used methods for the application of coatings of gold, silver and the platinum group metals (platinum, palladium, rhodium, iridium, ruthenium, osmium) to base metals are mechanical cladding and electroplating. 

Refractory metals -explosion cladding [METALLIC COATINGS - EXPLOSIVELY CLAD METALS] (Vol 16) -metal coatings for [METALLIC COATINGS - SURVEY] (Vol 16) -by sodium reduction [SODIUM AND SODIUMALLOYS] (Vol 22)... 

Corrosion Evaluation of Epoxy-coated Metallic-clad and Solid Metallic Reinforcing Bars in Concrete, US Department of Transportation, Federal Highway Administration, Publication FHWA-RD-98-153, December 1998. 

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