Hermetic carbon coatings for optical fibers: Evaluation of hydrogen related loss increases.

Single mode fibers with hermetic carbon coatings were evaluated for their high temperature loss stability and for their performance in hydrogen atmospheres. The presence of the carbon coating markedly reduced the diffusion rate of hydrogen into the fibers, such that the predicted hydrogen induced loss increases are negligible under anticipated operating conditions. Small loss increases can occur due to trapped hydrogen species under the carbon films, if the fibers are held at high temperatures. The sources of trapped hydrogen can be minimized by the use of chlorine containing reactants during the coating process, and/or by the use of low OH substrate tubes during the MCVD process.