Dependence of the mode instability threshold of high-power fiber laser systems on core co-dopants

Summary form only given. The phenomenon of Transverse Mode Instabilities (TMI) [I] has become one of the effects more discussed about in the field of high-power fiber laser systems. The reason is that TMI results in the sudden degradation of the quality and stability of the beam emitted by a fiber laser system once that a certain average power threshold has been reached, which seriously threatens the future applicability of this technology.In this work, we carry out a thermal optimization of fiber laser systems attending to the composition of the fiber core. For this, we have also created a heuristic model that predicts the expected PD losses of a specific material composition and this is then used to estimate the TMI threshold (modelled as being reached for a constant average heat-load). Fig.1a shows, as an example, the dependence of the TMI threshold on the PD losses (at the signal wavelength) in an alumino-silicate fiber. As can be seen, even small amounts of PD loses can have a significant impact on the TMI threshold. Additionally, Fig.1b shows the predicted TMI threshold as a function of the PD losses for different material compositions. The dashed black line indicates the evolution of the minimum PD losses with the material composition as predicted by our model. Thus, following that line allows for an estimate of the expected impact that the inclusion of Phosphorous co-dopants in the core can have on the TMI threshold. As can be deduced from Fig.1b, the optimum material composition is having an equal concentration of P2O5 and Al2O3 in the fiber core