CC BY 4.0 UnportedLeich, MartinKalide, AndréEschrich, TinaLorenz, AdrianLorenz, MartinWondraczek, KatrinSchönfeld, DörteLangner, AndreasSchötz, GerhardJäger, Matthias2021-12-012021-12-012020https://oa.tib.eu/renate/handle/123456789/7575https://doi.org/10.34657/6622We report on the first, to the best of our knowledge, implementation of a fluorine co-doped large-mode-area REPUSIL fiber for high peak power amplification in an ultrashort-pulse master oscillator power amplifier. The core material of the investigated step-index fiber with high Yb-doping level, 52 µm core and high core-to-clad ratio of 1:4.2 was fabricated by means of the REPUSIL powder-sinter technology. The core numerical aperture was adjusted by fluorine codoping to 0.088. For achieving high beam quality and for ensuring a monolithic seed path, the LMA fiber is locally tapered. We demonstrate an Yb fiber amplifier with near-diffraction-limited beam quality of M2 = 1.3, which remains constant up to a peak power of 2 MW. This is a record for a tapered single core fiber. © 2020 Optical Society of Americaenghttps://creativecommons.org/licenses/by/4.0/530CoremakingFibersFluorinePower amplifiersSinteringCore materialDiffraction-limited beamsHigh beam qualityHigh peak powerLarge mode areaMaster oscillator power amplifiersNumerical apertureStep-index fibersFiber amplifiersArticle