Deposition of Hermetic Carbon Coatings on Silica Fibers
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DEPOSITION OF HERMETIC CARBON COATINGS ON SILICA FIBERS R. G. Huff, F. V. DiMarcello, A. C. Hart, Jr., K. L. Walker AT&T Bell Laboratories Murray Hill, NJ 07974
ABSTRACT
Carbon coated optical fibers have recently been shown to have excellent resistance to both static fatigue and hydrogen induced losses. The deposition technique used to form the carbon coating strongly affects the coating's ability to resist these degradation mechanisms. The system developed by AT&T utilizes an atmospheric CVD chamber in which a hydrocarbon has is pyrolyzed on the fiber surface. The heat retained in the fiber from the fiber forming process is used to drive the reaction, and high draw speeds are typically used to attain the -900"C temperature required to deposit the hermetic form of the carbon coating. Deposition rates of -lpm/sec are required to produce the -500 A coating. INTRODUCTION
Hermetic carbon coatings have been shown to be effective barriers to both water and hydrogen and can add greatly to fiber reliability 1 2' '31 . Static fatigue, which is a stress induced corrosion of the fiber surface when water is present, can lead to significant strength reductions in a standard fiber if the applied stresses are high. The polymer coatings which are applied to the fiber during the drawing process are not effective barriers to environmental humidity, and often themselves contain significant amounts of free water. Therefore, if a standard fiber is highly stressed, it will undergo stress corrosion, become weaker with time, and potentially lead to fiber failure. While stress levels on most cabled fiber are small and fatigue not a serious issue, in applications where the fiber encounters high stresses and or high concentrations of water, fatigue does constitute a reliability issue. Military, oil logging, local area networks, and underwater cables are examples of uses where stress corrosion could be a problem and a hermetic coating may be desirable. Hydrogen induced optical loss increases are another reliability issue that can seriously affect a fiber's performance. Hydrogen, if present in the core of an optical fiber, will lead to loss increases due to molecular vibrations of H2 and also due to reactions at defects in the glass structure. The magnitude of these loss increases are dependent on the amount of hydrogen present and the temperature. Small amounts of hydrogen can be generated in the polymers used in the fiber's coating and in the cables, while large amounts can be generated if galvanic corrosion from dissimilar materials in the cable occurs. Mat. Res. Soc. Symp. Proc. Vol. 172. ©1990 Materials Research Society
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In order for a coating on the glass surface to be hermetic, it must have a tight enough molecular structure to form the barrier. Pyrolytic carbon is known to exhibit very low hydrogen permeationt 41 and would appear ideal for use as a hermetic coating. Early attempts, however, were not successful[5' 6' 7'81 . Little or no change in fatigue behavior was noted for these carbon coated fibers (though Maurer et al repo
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