Abstract
Single crystal fibers (SCFs), especially ytterbium (Yb) doped crystal fibers, have great potential in the field of high-power lasers. Colorless Yb:YAG single crystal fibers were fabricated using a laser heated pedestal growth (LHPG) method with a diameter fluctuation of less than 2% and a length to diameter ratio greater than 320 : 1. An abnormal color issue exists with respect to Yb:YAG crystals. The origin of coloration was studied via density functional theory, single-crystal X-ray diffraction, XPS and Raman spectroscopy and it was confirmed that the cyan coloration of Yb:YAG crystals is due to oxygen vacancies. Yb:YAG SCFs prepared via the LHPG method could avoid this type of defect due to the large specific surface area and melt convection caused by surface tension. The fundamental properties of the cyan Yb:YAG crystal source rod and colorless Yb:YAG SCFs were systematically investigated. The colorless Yb:YAG SCFs have higher infrared transmittance and thermal conductivity. The distributions of Yb3+ along the radial and axial directions were also measured. Meanwhile we demonstrated the propagation loss and a fiber laser using the colorless Yb:YAG SCFs, obtaining a minimum loss coefficient of 0.008 dB cm−1 and a maximum continuous-wave (CW) output power of 3.62 W. The colorless Yb:YAG SCFs with good thermal conductivity, low propagation loss, wide transparency and uniform ion distribution show promise for acting as the host material in single-mode lasers.
Highlights
Nowadays, with the explosive development of solid-state lasers, high-power lasers are gradually being applied to materials processing, information and communication technology, medical treatment and many other elds
This color issue relating to Yb:Yttrium aluminium garnet (YAG) crystals should be the primary problem to be solved, especially if they are going to be used in highpower lasers
Compared with other crystal growth methods, there is no crucible in the laser heated pedestal growth (LHPG) method, which greatly improves the purity of the raw materials
Summary
Yttrium aluminium garnet (YAG) belongs to a cubic system, with the unit cell parameter a 1⁄4 12.0089 A. The close absorption (900–980 nm) and emission wavelengths (980–1100 nm) lead to a low thermal load (11% relative to 30% in Nd3+-doped laser hosts), thereby signi cantly improving the slope efficiency, which reaches almost 90% theoretically.[24,25,26] Yb:YAG crystals are considered to be the most promising ber material. It is interesting to nd that commercial Yb:YAG crystals are usually different shades of cyan, and the color always deepens with an increase in the crystal size. This color issue relating to Yb:YAG crystals should be the primary problem to be solved, especially if they are going to be used in highpower lasers. A Yb:YAG ber laser is reported in this paper
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