Hydrogen permeation in hermetically coated optical fibers

The use of hermetic coatings on optical fibers can prevent strength degradation caused by the presence of H₂O at the fiber surface and can also prevent loss increases that might arise from H₂ in the fiber environment. The properties of hermetic coatings can vary, and are dependent on both the coating type and deposition conditions. Results are described pertaining to AT&T hermetic fibers with amorphous carbon coatings. Early experiments showed that, if improperly applied, amorphous carbon coatings can have increased permeabilities, especially with respect to H₂. The ability of a hermetic coating to prevent hydrogen induced losses depends on the coating structure being `tight´ enough to prevent not only the diffusion of H<sub>2</sub>O, but also the diffusion of the smaller H<sub>2</sub> molecule. Thus, a coating which is hermetic with respect to H<sub>2</sub> should also be hermetic with respect to H<sub>2</sub>O, although the converse will not necessarily be true. By this reasoning, the hydrogen evaluation of a hermetic coating will usually constitute a `tougher´ test than the measurement of fatigue properties. The single mode fibers were exposed to hydrogen at higher than normal pressures and temperatures. The aim of these experiments was to accelerate the permeation of H₂ using high pressures and moderate temperatures, so as to allow the prediction of permeation rates at the less extreme conditions characteristic of actual fiber operating conditions