High-power slab-based Tm:YLF laser for in-band pumping of Ho:YAG

Several remote sensing applications require pulsed sources in the mid-infrared spectral regime with high average powers and good beam quality. Ho:YAG lasers have a number of attractive features for high power generation at 2.1microns, either for direct applications or as a pump source for parametric conversion to longer infra-red wavelengths. Unfortunately, direct diode pumping of Ho:YAG is not practical, so a two-step process is generally employed in which one or more diode-pumped thulium-doped lasers are used to directly pump (in-band) the Ho:YAG laser. In response, we have investigated a slab-based architecture for scaling the output power of a Tm:YLF laser to the 100W power regime at 1.91microns, corresponding to a strong Ho:YAG absorption line. Multiple slab lasers with moderate beam quality in the plane of the slab can be combined to efficiently end-pump a low-doping concentration Ho:YAG rod in a pump-guided configuration. In a preliminary demonstration, two 2at.% doped Tm:YLF slab lasers with a spatially multiplexed output of 74W were employed to end-pump a 1.5mm diameter, 80mm long, 0.25at.% Ho:YAG barrel-polished rod. A two-mirror plano-concave cavity, with 11% output coupling transmission, produced a CW output of 38W with a slope efficiency of 60% with respect to the incident power. Q-switched operation at a repetition rate of 20Hz with two intra-cavity Brewster plate polarizers and a 60% transmitting output coupler produced 14mJ pulses with a pulse duration (FWHM) of 18ns. This architecture offers an attractive route for future high-power 2micron lasers.