Characterizations of 0.4 and 1 mm diameter Yb:YAG single-crystal fibers grown by the micro-pulling-down method for laser applications

70 cm long Yb3+-doped Y3Al5O12 (Yb:YAG) single-crystal fibers of 1 and 0.4 mm diameters were grown by the micro-pulling-down method. The structural and optical characterizations of those fibers showed that they exhibited a perfect single crystalline structure and have a homogeneous distribution of the Yb3+ ion in the YAG host. In diode-pumped laser operation, a continuous wave output power of 50 W at 1030 nm was demonstrated for 180 W of pump power at 940 nm, for 1 mm diameter fibers. For 0.4 mm diameter fibers, 28 W at 1030 nm for 100 W of incident pump at 940 nm power was achieved. To the best of our knowledge, these results are the highest laser power ever obtained with directly grown single-crystal fibers.