Critical Problems Associated With Fabrication Of Halide Glass Fibers

Critical Problems Associated with Fabrication of Halide Glass FibersJohn Lau, Alana M. Nakata and John D. MackenzieDepartment of Materials Science and EngineeringUniversity of California, Los Angeles, California 90024AbstractThe fabrication of halide glass fibers is much more difficult when compared with oxideglass fibers. This is because of the high fluidity of most molten halides, their unusualviscosity- temperature characteristics and the poor chemical durability of the glass.Protective coatings are necessary and because of the high expansion coefficients of halideglasses and their relatively low softening temperatures the choice of an ideal coating isdifficult. Some of the critical problems associated with fiber formation and the choice ofproper coatings are discussed. The effects of the penetration of water through Tefloncoatings and that of the presence of crystallites on tensile strengths of fluoride fibersare presented.IntroductionHalide glasses offer the possibility of ultra -low loss transmission and this has beenthe major motivation behind the development of halide glass fibers for long distancecommunication. In the fabrication of halide glass fibers, apart from the difficulty ofobtaining raw materials of a sufficient purity to minimize impurities absorption losses,there are other serious problems associated with halide glasses which are not normallyencountered in silicate systems. For example, halide glasses in general have a much greatertendency to crystallize during fiber drawing, their chemical durability are poor and theyhave lower intrinsic strength. These problems are often inter -related, for instance, thegrowth of crystals in the fibers creates flaws which can degrade the mechanical strength andincrease scattering losses. The introduction of alkali halides into the glass melt willimprove the glass forming ability and reduce the tendency to crystallize. However, thechemical durability becomes poorer which in turn adversely affect the optical and mechanicalproperties. Halide glass fibers must therefore be protected from the ambient atmosphere.However, their relatively high thermal expansion coefficients and low softening temperaturescan present difficulties in the selection of suitable coatings. Some of these criticalproblems associated with the fabrication of halide glass fibers are discussed in this paper.Fabrication of Halide Glass FibersOxide glass fibers can generally be fabricated by two different methods. The first oneis by the direct cooling from the melt by down -draw through a bushing. The second one isby the redraw from a pre- formed glass rod. At the liquidus temperature, most halide melts,other than those based on BeF2, have relatively low viscosities as compared to oxidesystems.1 One exception is ZnC12. A comparison between halides and other glass- formingsystems is shown in Table 1. For the continuous drawing of oxide glass fiber through abushing, the ideal viscosity is between 500 and 1000 poises.2 It is seen from Table 1 thathalide systems have much lower viscosities. Crystallization tendency usually increaseswith decreasing temperatures until some maximum crystallization from the liquidus tempera-ture is reached. At and near the liquidus, the activation energies for viscous flow arelow for halides. To reach a viscosity of 500 to 1000 poises, a halide melt would have tobe very substantially sub -cooled below the liquidus. Although it is not impossible tofabricate glass fibers under these conditions, the experimental problem to minimize crystal-lization is a serious one. Despite such difficulties, continuous fluoride glass fibers havebeen made via down -draw from a bushing.3