Power scaling strategy for longitudinally diode-pumped Tm:YLF lasers

Tm:YLF is an attractive active medium as it offers very weak thermal lensing, high polarisation purity, and strong absorption bands for laser diode pumping. Despite these attributes, Tm:YLF is a fragile crystal with a low tensile strength, moderate thermal conductivity, and a quasi-three-level energy structure. Our strategy for power scaling end-pumped Tm:YLF lasers revolves around increasing the crystal's stress fracture limit. The approach is two-fold, the first is to reduce the heat deposition density and the second is to use a thin-slab geometry and high aspect-ratio pump-volume. The only significant degree of freedom available for decreasing the heat deposition density in longitudinally-pumped quasi-three-level lasers is to reduce the doping concentration and hence the absorption coefficient. Thus, for power scaling, there are conflicting requirements which imply that very careful selection of the Tm concentration is needed to maximise the fracture limit. We have compared the laser performance of single end-pumped 2at.% and 4at.% Tm:YLF rods and demonstrate that the former has greater power scaling potential, albeit with greater demand on the pump brightness.