Hydrogen resistant fibers can withstand hydrogen-rich environments whilst maintaining a high level of optical performance.
There are two fundamental design optimisations that are harnessed to manufacture hydrogen resistant fibers:
1. Carbon coating
2. Pure silica core matrix (germanium-free core design)
The carbon coating facilitates the hydrogen resistance of the fiber by acting as a hermetic seal and thereby preventing hydrogen ingress up to 150°C. By behaving as a barrier to carbon, hydrogen darkening of the fiber cannot occur.
Even with a carbon coating, beyond 150°C it is possible for hydrogen to enter the fiber and so extra precautions should be taken in terms of the fiber design. By removing germanium from the core matrix, any hydrogen within a pure silica core fiber cannot bind with germanium-associated defect sites. The prevention of such irreversible bonds ensures that the transmission of the fiber is maintained to a high level.