Flat-top mode from a 50 µm-core Yb-doped leakage channel fiber.

We demonstrate for the first time a flat-top mode from a 50 µm-core Yb-doped leakage channel fiber (LCF). The flat intensity distribution leads to an effective mode area of ~1880 µm(2) in the straight fiber, an over 50% increase comparing to that of regular LCF with the same core diameter. The flat-top mode was achieved by using a uniform Yb-doped silica glass in the core center with an index of ~2 × 10(-4) lower than that of the silica background. The fiber was also tested in a laser configuration, demonstrating an optical-to-optical efficiency of ~77% at 1026 nm with respect to the pump at 975 nm.