An improved cutting plan for removing laser amplifier slabs from Yb:S-FAP single crystals using residual stress measurement and finite element modeling

摘要

This paper presents a measurement of the bulk residual stress distribution in a large single crystal boule and examines the residual stress redistribution during sectioning required to produce laser amplifier slabs. Experiments are currently under way to develop high-efficiency, diode-pumped laser systems that use Sr_5(PO_4)_3F crystals doped with Yb~(3+) ions (called Yb:S-FAP) as the amplifying medium. The progress has been protracted since the cylindrical crystal boules have experienced an extremely high rate of fracture when they are cut to form rectangular amplifier slabs. It was hypothesized that fracture was caused by residual stresses acting in the presence of small chip-like defects generated by the cutting process. Attempts were made to measure the residual stress using X-ray diffraction, neutron diffraction, ultrasound, photo-elasticity, and the slitting method (also called crack compliance). The latter method turned out to be the most effective technique and it is presented here in detail. The measured residual stress field in the crystal boule closely resembled the result expected from a thermal process, with a maximum tensile stress of 50 MPa at the center of the crystal. Additionally, the experimental residual stresses were applied to a finite element model of the boule and various cutting plans were simulated to try and minimize the stresses acting along the edges of the cut. It was determined that a cut from only one side of the crystal boule and slightly off the central axis significantly reduced the stresses from the original cutting plan which used a two-sided cut through the center of the crystal. The modeling results showed that the maximum principal stress at the cut tip was reduced by 40% with the one-sided, off-center cutting plan, which would significantly reduce propensity for fracture during cutting.