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，对人眼安全激光的需求一直在增加。掺Tu(TM)晶体可以用激光二极管泵浦，是一种很有前途的人眼安全激光器材料。作为TM激光器的主体晶体，GdVO_4单晶在导热系数、吸收系数等方面均优于其他晶体如：石榴石、石榴石等。本研究尝试采用浮区法生长掺Tm的GdVO4单晶，并对其光谱特性进行了研究，以纯氧气为生长气氛，采用浮区法成功地生长出透明无裂纹的Tm：GdVO4单晶。在20 mm/h的高生长速率下，Tm的掺杂量达到20at%是可能的。生长方向含有大量的小角度晶界，这使其光学均匀性变差，而沿[110]方向生长的晶体必须是小角度晶界。缩口过程对提高光学质量也是必不可少的。通过优化生长速度和晶体旋转速度的组合，有效地抑制了气泡包裹体的形成，800 nm附近的吸收系数足够大，可以用传统的激光二极管808 nm泵浦。即使在1950 nm以上也观察到强烈的发射，其中自吸收可以忽略不计。随着Tm-浓度的增加，荧光衰减时间从2.1ms减小到0.5ms，但对于激光振荡是足够长的。虽然激光振荡还在进行中，但浮区法生长的Tm：GdVO4单晶有望成为2μm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.