Multi-Watt Multi-Pass Amplification in a 42-mm-Long Yb:LuLiF4 Single Crystal Fiber Grown by the Micro-Pulling-Down Method

Summary form only given. 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 [1]. 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 in [2]. 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. In this work we present what is to the best of our knowledge the first demonstration of a SCF amplifier based on a birefringent Yb:LLF crystal grown by the μ-PD technique. The crystal growth apparatus consists of a custom μ-PD furnace with an RF heating system. Starting materials were prepared from a mixture of 5N pure powders LuF
₃
, LiF and YbF
₃
in proper quantities. The doping concentration of Yb
³⁺
in the melt was 2 at. %. To avoid the reduction of Yb
³⁺
ions to Yb
²⁺
ions, a high-purity (99.999%) argon-CF
₄
atmosphere was carried out in the growth process. An α-cut, undoped single crystal of LLF grown by Czochralski was used as the seed. The SCF obtained was about 110 mm in length and 3 mm in diameter. Since the fiber was grown with a perfectly oriented seed, the fiber shows the c -axis orthogonal to the direction of growth.