Laser floating zone growth of Yb, or Nd, doped (Lu0.3Gd0.7)2SiO5 oxyorthosilicate single-crystal rods with efficient laser performance

Abstract

Disordered crystals are being presently developed to enlarge the fluorescence bandwidth of trivalent lanthanides incorporated for generation of ultrashort (femtosecond) laser pulses in mode-locked oscillators and amplifiers, but crystal disorder induces a reduction of thermal conductivity which hampers the uniform crystal cooling after growth, leading to internal stresses. This is particularly remarkable when using the Laser Floating Zone (LFZ) growth technique; thus so far laser operation has been obtained only for LFZ-grown crystals with high thermal conductivity (κ ≥ 10 W m⁻¹ °C⁻¹) but without disorder, i.e. YAG, Y₂O₃ or REVO₄. To overcome this limitation we present the LFZ growth of (Lu_0.351Gd_0.630Yb_0.019)₂SiO₅ and (Lu_0.307Gd_0.612Nd_0.081)₂SiO₅ refractory (melting point ≈ 1950 °C) oxyorthosilicate single-crystal rods with dimensions suitable for high power diode laser pumping, despite these crystals having medium/low thermal conductivity, κ < 4 W m⁻¹ °C⁻¹. Rods with ≈10 mm length and ≈1.75 mm diameter were grown in air under a CO₂ laser at 10 mm h⁻¹. X-ray diffraction analyses confirm the monoclinic C2/c structure of the obtained crystals. For the chosen ≈0.3Lu/0.7Gd ratio some of the crystals are transparent and free of macro-defects. The continuous wave laser performance of Yb³⁺ and Nd³⁺ incorporated ions is demonstrated under Ti-sapphire laser pumping in an astigmatism compensated Z-shaped optical cavity. The laser performance of these LFZ oxyorthosilicates is found to be comparable to that reported in Czochralski (Cz) grown crystals. The faster pulling rate (almost one order of magnitude larger for LFZ than for Cz), the high crystal composition purity, and the absence of crucible or atmosphere control make the LFZ technique a low cost alternative for the present needs of diode laser pumped mode-locked medium/high power laser oscillators.