Hydrogen darkening

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Hydrogen darkening is a physical degradation of the optical properties of glass. Free hydrogen atoms are able to bind to the SiO₂ silica glass compound forming hydroxyl (OH) - a chemical compound that interferes with the passage of light through the glass.

The problem is particularly relevant with regards to fiber optic cables - particularly in oil and gas wells where fiber optic cables are used for Distributed temperature sensing the presences of hydrogen can be common due to the cracking of hydrocarbons in the well. The darkening of the fiber can distort the DTS reading and possible render the complete DTS system inoperable due to the optical loss budget being exceeded.

To prevent this special coatings such as carbon are applied to the fiber, hydrogen capturing gels are used to buffer the fiber and other proprietary techniques may be used to prevent the passage of hydrogen atoms to the glass fiber (i.e. via the cable sheath).