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.
We present an Ytterbium-doped, multicore fiber with 7x7 cores. The fiber is realized in a rod-type geometry with step index cores and an air-cladding for pump guiding. Using a segmented-mirror beam splitter followed by a double-pass multicore pre-amplifier and a main-amplifier of 1 m length, the stretched femtosecond input pulses emitted from the frontend system are amplified. The cores of the main amplifier have a mode-field diameter of 28 µm. Operating at 10 MHz repetition rate, a high average power in excess of 1 kW with near single-mode operation is achieved.
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