Thermal effects are currently appointed as the main bottleneck to power scaling of fiber lasers. In this letter, the single-mode properties of Yb-doped rod-type photonic crystal fibers with reduced symmetry in the inner cladding under severe heat load have been analyzed. Three fibers with different cross-sectional geometry have been compared by calculating the single-mode regime and effective area with a full-vector modal solver based on the finite-element method, properly combined with a thermal model to include the effects of fiber heating on the guiding properties. Results have demonstrated that, with an optimized cross-sectional design, it is possible to obtain single-mode propagation for a coupled pump power up to 200 W, while keeping the effective area ~2400μm2.
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