Multi-watt amplification in a birefringent Yb:LiLuF₄ single crystal fiber grown by micro-pulling-down

We present, to the best of our knowledge, the first demonstration of a single crystal fiber solid-state amplifier based on a birefringent Yb³⁺-doped thin crystal grown by the micro-pulling-down method. We measured a small signal gain >30 in a four-passes Yb:LiLuF₄ amplifier pumping with a 120-W maximum power fiber-coupled laser diode. At an absorbed pump power of 80 W, a maximum output power of 8.4 W was obtained seeding the 42-mm-long, 2%Yb³⁺-doped single crystal fiber sample with 0.8 W at 1021 nm. Even at the maximum incident pump power, the amplified laser beam polarization and spatial quality were excellent (M²=1.15×1.06).