LAUSR

It has been recently shown that photodarkening can significantly reduce the mode instability threshold in high power Yb-doped fiber amplifiers, thus resulting in an even more severe limitation to the scaling of the output average power of these systems. Therefore, an efficient reduction of photodarkening in an Yb-doped active fiber will lead to very significant gains in the output average power delivered by such systems. In this context, it has been reported that photodarkening can be significantly mitigated when co-doping a fiber core with Al and P, which makes this approach potentially appealing to increase the TMI threshold. Unfortunately co-doping the fiber core with Al and P also alters the effective cross-sections of the fiber, which has repercussion in the amplification efficiency. Thus, a fiber with a higher P concentration will exhibit lower cross-sections, therefore requiring a higher Yb-ion concentration to reach a certain desired amplification efficiency. However, increasing the Yb-ion concentration leads to higher photodarkening losses, which might potentially counteract the benefits of using P co-doping. In this paper we present a comparative analysis of the expected performance of different fiber amplifiers for a given constant average heat-load and amplification efficiency as a function of the ratio of Al:P concentration in the fiber core. This study indicates which core compositions are more beneficial for increasing the mode instability threshold in Yb-doped high-power fiber amplifier systems.