Lifetime quenching in Yb-doped fibres

We have discovered that in ytterbium-doped silica fibres of different compositions the excited state lifetime of a fraction of the Yb ions (between 1% and over 90%) is quenched to a very small value, leading to a strong unbleachable loss. This unexpected behaviour seems to be caused by a yet unidentified impurity or structural defect, probably a colour centre. It can seriously degrade the performance of Yb-doped lasers and amplifiers (including Er.Yb codoped fibres as used in telecommunication amplifiers), especially if the pump or signal wavelength is around 976 nm. Fibres with a low level of quenching exist, but it is not yet understood which precautions have to be taken to guarantee successful production.