Investigation of thermal effect on the output characteristics of end-pumped Ho:YLF laser in concave-concave and concave-convex resonators

The thermal effect on the output characteristics was analyzed in an end-pumped Ho:YLF laser by applying the Kirchhoff integral analytic solution and finite element analysis using LASCAD software. At a Tm:fiber laser power of 70 W, the maximum temperature of a Ho:YLF crystal with 8 mm diameter was found to be 297.6 K. The temperature distribution results of the two calculation methods were matched with an acceptable error. The calculated thermal lens focal length for the maximum power was -985 and -976 mm in the 3 and 8 mm diameter rods, respectively. The simulation results showed that for the larger diameter crystal, the thermal lens effect is less. Moreover, in this study, the calculation results were validated as experimental results. A focal length of the thermal lens of -1012.6 +/- 101.26 mm at double-pass pumping was measured. The experimental study showed that laser characteristics such as the far-field divergence angle were improved in the concave-convex resonator with 0.5% Ho:YLF crystal. The experimentally measured divergence angle was 0.7 mrad, which confirms the simulation results. The output power was 13.5 W, corresponding to 19.3% optical-to-optical efficiency.