Limiting factors in PRF scaling of barium vapour lasers

We have studied the factors limiting the performance of a barium vapour laser (BVL) as the pulse repetition frequency (PRF) is elevated. By measuring the population densities of key barium species, we have found that increasing the PRF leads to progressively larger depletion of the axial pre-pulse Ba ground-state density (up to 85% at 15 kHz PRF), due to the combined effects of ion pumping and gas heating. A direct consequence of this Ba density depletion is a proportional decrease in the peak upper laser-level density of the 1.50 μm and 1.13 μm laser transitions (i.e. Ba ¹P ₁) during the excitation pulse. The results suggest that ground-state depletion is the dominant factor limiting laser output at high PRFs, and that the effects of the pre-pulse lower laser-level and electron densities play a secondary role. The implications to the PRF scaling of BVLs and other metal vapour lasers are discussed.