Role of Ce in Yb/Al laser fibers: prevention of photodarkening and thermal effects.

We report on detailed investigations of ytterbium (Yb) and aluminum (Al) doped silica fiber and preform samples co-doped with cerium (Ce). The prevention of pump-induced photodarkening (PD) by temporary oxidation of Ce³⁺ to Ce⁴⁺ (or rather Ce³⁺⁺) was proved by observed modifications in the ultraviolet (UV) spectra of transient absorption during near-infrared (NIR) pumping of thin preform slices. Only a small part of available Ce³⁺ ions (< 4%) was found to be involved in this process despite Yb inversions of up to 0.28. The modifications in the UV absorption spectra disappeared completely when the pump power was switched-off. From these observations we conclude that the recombination to Ce³⁺ takes place very fast thereby enabling these ions to capture liberated holes h⁺ perpetually during further pumping. We found a concentration ratio of Ce/Yb ≈0.5 to be sufficient to reduce PD loss to 10% in comparison to Ce-free fibers. Thus, the thermal load caused by absorption of PD color centers at pump (and laser) wavelength is expected to be also reduced. Unfortunately, new heat sources arise with the presence of Ce which cannot be explained by the absorption of Ce ions at the pump wavelength but must be attributed to the interaction with excited Yb ions. Fiber temperature increase of more than 200 K was observed if both, Yb₂O₃ and Ce₂O₃ concentration exceed 0.4 mol%.