Multi-watt multi-pass amplification in a 42-mm-long Yb:LuLiF4 single crystal fiber grown by the micro-pulling-down method

By exploiting a more favourable surface/volume ratio for heat exchange and pump absorption distribution over significantly increased length, single crystal fibers (SCF) are considered promising candidate as a replacement for bulk crystals in high-power laser systems. So far, micro-pulling down (µ-PD) technique was successfully employed mainly in growing cubic crystals, most impressive results being obtained with Yb:YAG. Birefringent SCF are also attractive, as they provide a quite straightforward means of avoiding beam quality degradation at high thermal load due to depolarization. The first Nd:YLF laser based on fiber crystals was reported earlier. However, from the perspective of high-power applications, Ytterbium-doped materials are definitely more interesting, owing to the much smaller quantum defect and absence of excited-state absorption