Multilayered YAG-Yb:YAG ceramics: manufacture and laser performance

Thermal effects in transparent laser crystals and ceramics are generally an unwanted consequence of the pumping process: temperature gradients give rise to an unevenly distributed refractive index variation and a distortion of the optical surfaces crossed by the laser beam (thermal lens); birefringence due to thermomechanical stress can cause depolarization losses; and absorption from the ground level usually increases with temperature in quasi-three-level systems. All these effects can seriously impair laser performance, especially in high-power devices. Layered structures with a tailored modulation of the doping level can be used to reduce the peak temperature, the temperature gradients and also the thermally induced deformation of the laser material, thus mitigating the overall thermal effects. In the present work, structures comprising two and three layers of different compositions (pure YAG/10 at% Yb:YAG and pure YAG/10 at% Yb:YAG/pure YAG) were designed with a view to control deformation and stresses, and to reduce the thermal lensing effect. The multilayered samples were assembled by linear and cold isostatic pressing, and co-sintered under a high vacuum in a clean-atmosphere furnace. The microstructure of the layered samples obtained was characterized by FEG SEM, ESEM and TEM. The Yb diffusion profile across the doped/undoped interface was identified and related to the laser's output power. An internal optical transmittance up to 96% was obtained. A laser output power up to 5 W, with a slope efficiency as high as 74.3%, was also achieved.