Float zone growth of thulium-doped rare-earth orthovanadate single crystals for eye-safe lasers

在激光雷达传感和医疗应用领域，对人眼安全激光器的需求一直在增加。掺铥（Tm）晶体由于易于用激光二极管泵浦，是人眼安全激光器的理想材料之一。作为Tm激光器的基质晶体，GdVO_4单晶从导热系数和吸收系数的观点来看，比YAG和萤石等其它晶体具有上级优势。本研究尝试了浮区法生长掺Tm：GdVO_4单晶并对其光谱特性进行了研究，采用纯氧流作为生长气氛，成功地生长出透明、无裂纹的Tm：GdVO_4单晶。在20 mm/h的高生长速率下，最高达20 at%的Tm掺杂是可能的。沿[110]方向生长的晶体中存在大量的小角度晶界，这会影响晶体的光学均匀性，而沿沿着[110]方向生长的晶体中存在大量的小角度晶界。颈缩工艺对提高光学质量也是必不可少的。通过优化生长速率和晶体旋转速率的组合，有效地抑制了气泡夹杂物的形成，在800 nm附近的吸收系数足够大，可以用808 nm的激光二极管泵浦，这是传统的Nd激光器。甚至在1950 nm以上也观察到强发射，其中自吸收可以忽略不计。荧光衰减时间从2.1到0.5 ms随Tm浓度的增加而减小，但对于激光振荡来说足够长。虽然激光振荡还在进行中，但用浮区法生长的Tm：GdVO_4单晶有望成为2 μm激光器的潜在材料。

The demand for eye-safe lasers has been increasing in the field of lidar sensing and medical applications. Thulium (Tm)-doped crystals are one of the promising materials for the eye-safe lasers because they can easily be pumped usuing laser-diode. As a host crystal of the Tm-lasers, GdVO_4 single crystals are superior to other crystals such as YAG and fluorite from the viewpoints of thermal conductivity and absorption coefficient. In this study, float zone growth of Tm-doped GdVO_4 single crystals were tried and their spectroscopic properties were investigated.Transparent, crack-free Tm : GdVO_4 single crystals were successfully grown by the floating zone method using an pure oxygen flow as a growth atmosphere. Tm-doping of up to 20 at% was possible at a high growth rate of 20 mm/h. Growth direction contained a number of low-angle grain boundaries, which deteriorate their optical uniformity, whereas the crystals grown along the [110] direction had to low-angle grain boundaries. The necking process was also essential to enhance the optical quality. The formation of bubble inclusions was effectively suppressed by optimizing the combination of growth rates and crystal rotation rates.The absorption coefficient around 800 nm was large enough for pumping with a laser-diode of 808 nm which is conventionally used for Nd-lasers. Intensive emission was observed even above 1950 nm where the self-absorption could be neglected. The fluorescence decay time decreased from 2.1 to 0.5 ms with increasing Tm-concentration but is long enough for laser oscillation. Although the laser oscillation is under way, the Tm : GdVO_4 single crystals grown by the floating zone method are expected to be potential materials for 2 μm lasers.