Photo darkening in ytterbium co-doped silica material

A model description of photo darkening based on 30 characterised fibres in an un-seeded amplifier setup is presented. Photo darkening of ytterbium / aluminium and/or phosphorous co-doped silica fibres is found to saturate following prolonged exposure to pump radiation. The photo darkening is associated with non-binding oxygen at surfaces of ytterbium / aluminium clusters. The dominant colour centre at near infrared wavelengths in MCVD material is a combination of 1.9 eV (FWHM of 0.62 eV) and 2.4 eV (FWHM of 0.85 eV) absorption dependent on average phonon energy of the glass material.