Photodarkening in Yb-Doped Silica Fibers - Mitigation by Co-Doping and Thermal Effects

Photodarkening in rare-earth doped fibers has been subject of intense research over the last decade as the main issue limiting high power performance and long-term stability of fiber lasers. Regarding our previous and most recent experiments on Yb-doped silica fibers, the relationship between photodarkening mitigation and fiber heat load will be discussed for the most prominent co-dopants. The main focus of attention will be paid on the mitigation of photodarkening by co-doping with aluminium and cerium. Beside this, we dedicate ourselves on the thermal properties of NIR-pumped fibers and show that, especially for high pump intensities, methods of PD mitigation can also result in a temperature enhancement of the active fiber. This can even become a critical issue for high power laser performance, e.g. by lowering the thermal mode instability threshold of LMA fibers.