Molecular hydrogen is well known to diffuse and dissolve in silica glass increasing the fibre optical losses when it reaches the Germanium doping of the core and introduces a number of OH absorption lines in the 1000÷2500nm spectrum region. This problem, commonly known as “hydrogen darkening” is typical of downhole D-FOS sensing in oil and gas applications and it can sensibly reduce the useful life of sensing fibres.

At 1550nm, that is the typical wavelength used by commercial Brillouin interrogators, hydrogen darkening can raise the optical loss up to ~1dB/km, that, considering the typical loss 0.2dB/km of single-mode fibres, means reducing the distance range and/or worsening the SNR of 5 times.

Due to the diffusivity of hydrogen in both metals and polymers, it is in general difficult to prevent the darkening issue with coatings, with the exception of carbon coatings that have been proven hermetic to hydrogen at least up to 150°C. A long-term solution of the hydrogen darkening issue is the use of pure silica core fibres, since the OH responsible of the light absorption forms readily in Germanium doped Glass but not in pure silica.