Beam shaping system for laser-induced crystal growth

We report on the development and setup of a beam shaping system for the Laser-Diode-Floating-Zone (LDFZ) crystal growth, a crucible-free technique used for growing high-purity crystals. The system takes a multi-kW diode laser beam with a rectangular intensity profile as an input. The intensity distribution is top-hat in the horizontal and gaussian-shaped in the vertical axis. The optical system contains a beam splitter unit, which divides the ingoing beam geometrically in 5 partial beams of equal power and size. By a two-lens imaging system, each partial beam is imaged onto the target plane by a magnification factor of 5. Adjustable mirrors of high purity fused silica allow for a radial irradiation of the sample, consequently resulting in a homogeneous heating and melting of the floating zone. After the setup and characterization, the optical system has been integrated into the existing LDFZ furnace at AIST, Tsukuba. First successful experiments on the LDFZ growth of β-Ga2O3 single crystals have been carried out, by which the crystal diameter could be scaled from 10 mm to 18 mm.