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

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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.