Carbon can be applied as a thin primary coating in combination with a secondary coating of high temperature acrylate or polyimide, depending on the operating temperature required.
The carbon acts as a hermetic seal to prevent hydrogen ingress up to 150°C. The fact that carbon behaves as a barrier to hydrogen in this way means that increased attenuation associated with the presence of hydrogen in the core can be prevented. This is of particular importance in hydrogen-rich environments such as oil wells.
Beyond 150°C, the carbon layers begin to move at a microscopic level and the hermeticity of the coating is compromised. However, even >150°C, carbon significantly delays hydrogen ingress into optical fibers.