CC BY 4.0Kholaif, Sobhy E.Sobhy E.KholaifJauregui, CesarCesarJaureguiNold, JohannesJohannesNoldHaarlammert, NicolettaNicolettaHaarlammertKuhn, StefanStefanKuhnSchreiber, ThomasThomasSchreiberLimpert, JensJensLimpert2026-01-212026-01-212025https://publica.fraunhofer.de/handle/publica/50386810.1364/OE.58000110.24406/H-5038682-s2.0-105026346406We present what we believe to be the first systematic experimental study investigating the impact of core size on transverse mode instability. This is a very complex measurement that requires a significant amount of attention to detail. For example, this study can only be done using fibers which modal characteristics do not change with the core size, a property offered by Ytterbium-doped large-pitch fibers. Additionally, it is mandatory to consider the influence of gain saturation and photodarkening to isolate the impact of the core size on the transverse mode instability threshold. The findings of the measurements reveal that the dependence of this threshold with the core size is weaker than predicted by some theoretical models. Additionally, during the course of these measurements, a mode instability threshold over 600 W was achieved in one of the fibers, which represents the highest, diffraction-limited, average output power reported from an Ytterbium-doped, rod-type fiber so far.entruejournal article