Temperature effects in core-pumped Yb fibers and their correlation to photodarkening

Summary form only given. Owing to their high beam quality and enormous power scalability, Yb-doped fiber lasers and amplifiers are currently employed for many scientific and industrial applications. However, photodarkening losses and high operating temperatures can limit their performance or even destroy the laser itself. PD loss reduces the laser efficiency during run-time of fiber laser. Technically, this problem can be compensated by enhancing the pump power. But it has been shown that beyond the quantum defect caused by the Stokes shift, also PD losses contribute to the heat load of a fiber laser and thereby can cause damage at high power. Another consequence of heat load is the significant reduction of the beam quality as well as the transverse mode instability threshold [1]. The appropriate solution for overcoming this limitation is the drastic reduction of PD, which can be achieved by co-doping, e.g. with phosphorous, aluminium and cerium.