Influence of pedestral diameter on mode instabilities in Yb/Ce/Al-doped fibers

in: Optics Express (2020)
Möller, Friedrich; Distler, Victor; Walbaum, Till; Haarlammert, Nicoletta; Nold, Johannes; Schreiber, T.; Unger, Sonja; Reichel, Volker; Kobelke, Jens; Bierlich, Jörg; Schwuchow, Anka; Eschrich, Tina; Leich, Martin; Wondraczek, Katrin; Tünnermann, Andreas
In this paper we present numerical and experimental results revealing that the mode instability threshold of highly Yb-doped, Ce/Al co-doped pedestal fibers is affected by the size of the index-increased pedestal structure surrounding the core. An alternative preparation technology for the realization of large mode area fibers with very large Al-doped silica pedestals is introduced. Three different pedestal fiber design iterations characterized by low photodarkening were manufactured and tested in counter-pumped amplifier setups. Up to 1.9 kW continuouswave output power of near-diffraction-limited beam quality (M2 = 1.26) was achieved with an 18/200/420 μm fiber of very low NA = 0.042, limited only by the occurrence of mode instabilities.

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