Very large mode area, active optical fibers with a low high order mode content in the actively doped core region were designed by removing the inner cladding symmetry. The relevance of the numerical approach is demonstrated here by the investigation of a standard air-silica Large Pitch Fiber, used as a reference. A detailed study of all-solid structures is also performed. Finally, we propose new kinds of geometry for 50 μm core, all-solid microstructured fibers enabling a robust singlemode laser emission from 400 nm to 2200 nm.
The first demonstration of a 40 μm core homogeneously ytterbium-doped fully aperiodic large-pitch fiber laser, to the best of our knowledge, is reported here. In this concept, the amplification of unwanted high-order modes is prevented by means of an aperiodic inner-cladding structure, while the core and inner-cladding material has a higher refractive index than pure silica. In a laser configuration, up to 252 W of extracted power, together with an optical-to-optical efficiency of 63% with respect to the incident pump power, have been achieved. While an average M2 of 1.4 was measured, the emitted power becomes temporally unstable when exceeding 95 W, owing to the occurrence of modal instabilities.
Thanks to their capability to provide very large mode area together with effective suppression of high-order modes, while allowing strong pump absorption and efficient conversion, Yb-doped double-cladding photonic crystal fibers are one of the key enabling factors for the development of high power fiber lasers. Thermal effects are currently appointed as the main bottleneck for future power scaling since, beyond a certain average power, they allow guidance of high order modes and energy transfer to them, causing a sudden degradation of the beam quality. In this paper the effects of heat load on the modes of double cladding fibers are thoroughly analyzed with a full-vector modal solver based on the finite-element method with integrated steady-state heat equation solver. Fibers with different inner cladding designs are compared to provide a deeper understanding of the mechanisms beyond the mode reconfinement and coupling. The influence of the fiber design on the robustness of the single-mode regime with respect to fiber heating has been demonstrated, providing a clear picture of the complex interaction between modes. On the basis of simulation results it has been possible to group fiber modes into three families characterized by peculiar reaction to heating.
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