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In single-mode fiber lasers, power scaling is typically limited by effects such as stimulated Raman-scattering and transverse mode instabilities. However, in highly multi-mode fibers, these two effects are usually negligible due to the high number of transverse modes and therefore, higher optical powers can be achieved. To further decrease the complexity, increase the robustness and reliability of multi-mode fiber resonators, we develop highly reflective multi mode Fiber Bragg Gratings (FBGs) in active doped fibers, which is challenging e.g. due to the required homogeneity of the FBG within the fiber. Here, we present a proof of concept of a fiber-integrated multi-mode resonator using a highly reflective multi-mode FBG for spectral stabilization in a fiber with a rectangular core of 20 x 60 µm² and a numerical aperture of 0.1. More than 60 modes can be guided within this fiber, which can result in a homogeneous near-field beam profile. In our experimental verification, we build up a fiber resonator and achieve an optical-optical efficiency of 50 % and stable wavelength operation at 1072 nm. 10 % of the power is emitted as ASE at a wavelength of 1030 nm, which can be removed by using longer fiber lengths to further improve the efficiency. Approximately 12 % of the signal power is propagating through the HR-FBG, which leads to an FBG-reflectivity of 70-80 % based on our simulations. As a next step, we plan to transfer the HR-FBG concept to round-core XLMA-fibers to improve our earlier demonstrated setups and realize full in-fiber multimode resonators.
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