Impact of modal interference on high-power fiber laser systems

In the last year there have been reports from various research groups around the globe about the onset of modal instabilities (or dramatic degradations of the beam quality) in high average power laser systems. Many of these reports describe how the usual output Gaussian beam of the fiber laser system transforms into a higher order mode (most typically a LP11-like mode). This effect is commonly attributed to transversal hole burning. However, this theory alone cannot really explain reports of this effect being observed in fibers in which only the central part of the fiber core has been doped (such as the Rod-Type fibers). As far as we know, up to date no theoretical work has been published in the subject to investigate the true physical origin of this effect. In this paper we present such study and the conclusions obtained from it. It has been found that conventional transversally-resolved rate equation models, able to take transversal hole burning into account, cannot explain this effect when preferential gain designs are investigated. According to our investigations the inclusion of modal interference along the fiber is crucial to explain this effect in all type of fibers. Unfortunately current BPM models (able to account for modal interference along the fiber) are not able to take into account transversal hole burning. Thus, we have developed an advanced active fiber model that combines BPM and the transversally-resolved rate equations. This model reveals, for the first time, the important role played by modal interference along the fiber