Vincenzo V. Rondinella

Effect of chemical stripping on the strength and surface morphology of fused silica optical fiber

Examination of the surface profile of silica optical fiber using the atomic force microscope (AFM) has proved a useful technique for understanding strength degradation of the fiber upon aging in aggressive environments in terms of the production of surface roughness. However, before AFM examination it is necessary to remove the polymer protective coating and this is usually achieved by dipping the fiber sample in methylene chloride (MeCl) or hot (approximately 200 degree(s)C) sulfuric acid. This raises the possibility that the stripping technique modifies the fiber surface. In this work it is shown that hot acid stripping does not affect the fiber strength. It does, however, remove a surface layer from the aged fiber, probably of hydrated silica, which does not contribute to the strength. Therefore, treatment with hot acid is necessary in order to reveal the strength controlling surface profile, even if there is no polymer coating requiring removal. MeCl does not remove the surface layer and does not reveal the strength controlling surface.

Influence of solubility on the reliability of optical fiber

Recent advances in the understanding of reliability of silica optical fiber indicate that the chemical durability of the fiber can control the long duration lifetime both under stress- induced fatigue and zero-stress aging conditions. In particular, dissolution of surface material produces strength degrading surface roughness. These mechanisms are discussed and strategies for improving reliability by inhibiting dissolution are examined. As an example, a modified polymer coating formulation is described that is shown to increase the lifetime of the fiber by up to a factor of thirty-fold. Strategies for improving the strength and durability of non-oxide fibers using a similar approach are also discussed.

Enhanced fatigue and aging resistance using reactive powders in the optical fiber buffer coating

We have shown in previous work that the addition of small quantities of colloidal silica to the UV- curable polymer coating of fused silica optical fiber causes a dramatic improvement in the fatigue and aging resistance both in aqueous and in constant humidity environments. The presence of silica in the coating inhibits the mechanisms responsible for the surface roughening that causes the fatigue knee and strength degradation during zero-stress aging. This work presents results which show the effect of higher concentrations of the silica additive (6 wt%) and of an adhesion promoting agent on both the rheological properties of the polymer coating and the fatigue and zero-stress aging behavior of the fiber. Viscosity measurements show thixotropic behavior which indicates that the silica particles tend to form a network structure in the prepolymer. Filtration of the prepolymer to remove large particles is hampered by this phenomenon. The fiber coated with the silica-containing polymer exhibits substantial improvement in the long term mechanical reliability compared to a reference fiber without additive in the coating.

Organically modified silicate coatings for optical fibers

Three kinds of UV-curable organically modified silicates have been prepared to be used as protective coatings for optical fibers. The synthesis involves the reaction of the thiol group of 3-mercaptopropyl-trimethoxysilane with a C equals C bond in one of the acrylic groups of three commercially available aliphatic triacrylates. The methoxysilyl groups of the synthesized diacrylate methoxysilanes were subjected to hydrolysis and condensation to form Si-O-Si units. Transparent, viscous, solvent-free resins were obtained that hardened in seconds when exposed to UV radiation. The coating derived from the reaction with glycerol propoxy triacrylate (GPTA) proved to adhere the best of the three to both plastic and glass substrates. It was then tested as a protective coating for silica fibers. Reliability tests were carried out including bending strength and fatigue tests at pH 7 and 10. The results show improved water resistance of the coated fiber in neutral conditions.

Ionic effects on silica optical fiber strength and models for fatigue

ABSTRACTThe strength and fatigue behavior of bulk fused silica is well understood in terms of the growth of microcracks under thecombined influence of stress and environmental attack. The behavior of high strength, flaw free silica optical fiber showssignificant differences from the bulk material for poorly understood reasons making long term predictions unreliable. It isknown that silica fiber strength and fatigue are sensitive to such environmental parameters as temperature, humidity andpH. However, this paper presents results which also show a sensitivity to ionic species in the environment. These resultsare interpreted in terms of possible models for the fiber behavior. 1. INFRODUCTIONIn most ceramic materials and, in particular, in oxide glasses, failure can occur after prolonged application of a constantstress which is significantly lower than the stress required to cause catastrophic failure on a short time scale. This phenomenon, called fatigue or delayed failure, can determine the long term reliability of ceramic components. A complete

Atomic Force Microscopy Studies on Optical Fibers

The strength of brittle materials is controlled by the presence of stress concentrating defects which can be produced during manufacture or subsequent use. Surface defects, which dominate for glasses, can slowly grow under the combined influence of applied stress and environmental moisture, leading to delayed failure. However, the pristine as-drawn surface of optical fibers is preserved by the application of a polymer coating immediately after drawing so that defects are of atomic dimension. For such high strength material (1GPa), the strength can be degraded in aggressive environments by surface corrosion. AFM has proved a valuable tool for examining changes in the surface morphology of optical fibers on the nanometer scale and relating them to strength degradation. The surface of fused silica fiber corrodes in aggressive (i. e., warm and wet) environments to form roughness which, while only a few nm root mean square (rms), can substantially degrade the strength leading to drastically foreshortened lifetimes. The incorporation of nanosized silica particles in the fiber coating extends the lifetime under stress by factors of up to 300 or more. AFM has verified that the particles operate by slowing the surface roughening process. Heavy metal fluoride glass fibers (based on “ZBLAN” compositions) have many potential applications but are considerably less durable than fused silica and can rapidly degrade in room temperature water. Surface analysis using AFM has shown that the fastest degradation is brought about by conditions that lead to the formation of crystals in the fiber surface.