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Summary form only given. Ultrafast ps and fs laser systems using ROD fiber amplifiers delivering megawatts of peak power and several hundreds of watts in average power have attracted significant academic and industrial interest in recent years. However, the power scaling has reached a temporary limit caused by thermal effects, which has set a limit of average power scaling of these amplifiers due to so called modal instabilities (MIs) [1]. New types of fiber designs with very large cores have been suggested to increase the average power limit [2, 3]. One way of achieving this, is using resonant structures [3], however it is often speculated that these structures are sensitive to temperature causing unstable beam quality under high thermal load (high power extraction pr unit length).A 25-ps mode-locked linearly polarized seed source at 1032 nm (40MHz rep rate, 16 W average power) is used to seed a distributed mode filtering ROD fiber with 90μm core diameter. The ROD fiber shows excellent power conversion efficiency as shown in Fig. 1 with almost linear slope and the MI threshold level is observed at 314 W of average output power corresponding to ~320kW of peak power. Optical to optical conversion efficiency is 69% and the mode quality is stable below the MI threshold. No non-linear effects are observed and the ASE is suppressed by ~35dB, shown in Fig.1. The experiments are modeled using a semi-analytical approach [4, 5], where a nonlinear coupling constant that depends on the thermo-optical effect is determined. χ is combined with FEM-calculated mode distributions to predict the onset of modal instabilities for this particular fiber design.We demonstrate that a ROD fiber design having a resonant structure can deliver efficient and linear amplification to high average output power without suffering from thermal load induced beam quality degradation. In addition, we evaluate the influence of the noise of the seed source on the MI threshold level. We model the MI threshold for this fiber and show good agreement between measurements and simulations, when the MIs are seeded by system dependent noise. |
