Evidences of the existence of a correlation between structural stress and laser operation in monoclinic double tungstate epitaxial lasers

Significant differences have been observed in the efficiency of lasing action between KY_0.8Yb_0.2(WO₄)₂/KY(WO₄)₂ and KLu_0.8Yb_0.2(WO₄)₂/KLu(WO₄)₂ epitaxies. In KY_0.8Yb_0.2(WO₄)₂/KY(WO₄)₂, CW-laser operation could be obtained for pump wavelengths between 963 and 997 nm, using an astigmatically compensated resonator. However, when approaching the maximum absorption peak near 981 nm, thermal problems occurred and resulted in lower slope efficiency. Nevertheless, a stable laser operation was observed for output coupler transmissions (T_oc) up to 3.5%. The best performance was achieved when pumping at 984 nm with a T_oc = 2%, reaching the laser threshold at an absorbed pump power of about 275 mW. The maximum output power amounted to 40 mW, resulting in a slope efficiency of 33% with respect to the absorbed pump power. For KLu_0.8Yb_0.2(WO₄)₂/KLu(WO₄)₂ epitaxies, a much better performance was achieved using a similar cavity and the same tunable pump source, reaching a maximum measured output power of 415 mW using a T_oc = 3%. The difference in the performance of both highly doped samples is investigated by synchrotron X-ray white beam topography. This is an X-ray diffraction imaging technique capable of providing information on character and distribution of crystallographic defects and structural stress in large, nearly perfect, single-crystal materials.