Diffusion of Hydrogen Through Hermetic Carbon Films on Silica Fibers
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DIFFUSION OF HYDROGEN THROUGH HERMETIC CARBON FILMS ON SILICA FIBERS P.J. Lemaire, K.L. Walker, K.S. Kranz, R.G. Huff, and F.V. DiMarcello AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974
ABSTRACT Recent work has made it possible to deposit hermetic carbon coatings on optical fibers during the drawing process. These coatings are used to protect the silica portion of the fiber from undesirable loss increases and strength reductions caused by H2 and H2 0, respectively. The hermetic properties of the carbon films have been evaluated using accelerated test conditions where the coated fibers are exposed to H2 at elevated temperatures and hydrogen pressures. In-situ spectral loss monitoring has made it possible to measure changes in the characteristic optical loss features associated with either molecular H2 or with species such as OH which form when hydrogen reacts with defects in the silica. By using long lengths of fiber it is thus possible to optically measure the extremely small amounts of hydrogen that penetrate the carbon films during accelerated tests. At temperatures in the range of 100 to 145°C the diffusion of H2 is readily modeled using classical diffusion theories for a composite cylinder, allowing calculation of the diffusion coefficient and the solubility for H2 in the carbon. At higher temperatures the diffusing 1H2is partially depleted by reaction with defects in the glass. For these conditions the inward diffusion of the H2 and its reaction at defect sites tend to balance each other, giving rise to a constant, but extremely low, concentration of H2 in the fiber.
INTRODUCTION The use of hermetic coatings on optical fibers can greatly improve fiber reliability when the fibers are used in adverse environments. Potential problems in severe environments include the mechanical failure of a fiber and/or increases in optical loss due to the presence of hydrogen. Degradation in the strength of a fiber is often attributable to corrosion of the silica surface of a fiber by trace amounts of moisture. This fatigue weakening of the fiber is especially severe when a highly stressed fiber is continuously exposed to a source of water. High stresses can arise during fiber installation and use, and can also be caused by small radius bends in the fiber. Small concentrations of water are typically present due to normal environmental humidity. Higher water concentrations can be encountered when fibers are used in underwater cables or in severe environments such as those encountered for data logging in oil well drilling. Polymer coatings will not, in general, be sufficient to totally prevent the long term penetration of water in these adverse environments. Thus, long term weakening of a fiber due to stress corrosion can be a real concern in some applications. Another reliability concern is that of hydrogen induced loss increases. Hydrogen molecules become optically absorbing when they are present in the silica structure of a fiber. This causes absorption peaks in the spectral region from 1.08 to 1.241gm [1
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