The impact of core size scaling on the transverse mode instability threshold in fiber laser amplifiers

Both transverse mode instability (TMI) and non-linear effects hinder the power scaling in fiber laser amplifiers with a diffraction-limited beam profile. In this context core size scaling is a key design parameter to counteract nonlinear effects and, therefore, it is of outmost importance to understand the impact of this parameter on the TMI threshold. In this work we present the first systematic experimental studies that investigate the impact of core size scaling on TMI characteristics. In this study, the unique characteristics of large pitch photonic crystal fibers is exploited to ensure comparability between fibers with different core sizes. This has been done by manufacturing a set of test fibers (Ytterbium-doped LPFs) with different core sizes which maintain their modal characteristics when operated under the same conditions. Furthermore, our experiments reveal a TMI power threshold of 570 W, the highest ever reported in a rod-type fiber with a very large core diameter.