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Large-size 0.1 Yb2O3–1.0 Al2O3–98.9 SiO2 (mol%) core glass was prepared by the sol–gel method. Its optical properties were evaluated. Both large mode area double cladding fiber (LMA DCF) with core diameter of 48 µm and large mode area photonic crystal fiber (LMA PCF) with core diameter of 90 µm were prepared from this core glass. Transmission loss at 1200 nm is 0.41 dB/m. Refractive index fluctuation is less than 2 × 10−4. Pumped by 976 nm laser diode LD pigtailed with silica fiber (NA 0.22), the slope efficiency of 54% and “light-to-light” conversion efficiency of 51% were realized in large mode area double cladding fiber, and 81 W laser power with a slope efficiency of 70.8% was achieved in the corresponding large mode area photonic crystal fiber.