We report on a ytterbium-doped phosphate glass single-mode fibre laser, 4 cm long, with a maximum output power of 11.6 W and a slope efficiency of 66.6%. We developed the double-clad fibre with a 19 µm diameter step-index core and a pump waveguide with a high numerical aperture due to external air-cladding. The estimated pump absorption in the fibre exceeded 700 dB m−1.