Abstract

Modeling of the efficiency and output beam quality of a Rb flowing-gas diode-pumped alkali amplifier (DPAA) based on a 3D computational fluid dynamic model coupled to a wave optics model for amplified beam propagation is reported. Both end and two-way side pumping schemes were considered and compared for two cases of buffer gas composition where either a ${\rm He}/{{\rm CH}_4}$ mixture or pure He was used as a buffer gas. Dependencies of the DPAA efficiency and of the output beam quality on different flow and laser parameters were found. For the end pumping scheme and ${\rm He}/{{\rm CH}_4}$ buffer gas, the amplifier efficiency is almost independent of the gas velocity, whereas beam quality deteriorates at low flow velocities. For side pumping, both efficiency and beam quality are independent of the flow velocity. The effect of amplified spontaneous emission (ASE) on the efficiency of the DPAA is moderate and leads to a decrease in the efficiency of the amplifier by a maximum of ${\sim}{10}\%$ compared to the case when ASE is not taken into account. The ASE noise to signal ratio in the end-pumped amplifier was found to be negligible, while in the side-pumped amplifier, the ASE noise power at the output was comparable to the power of the coherent amplified output beam. The presented results show that end pumping is preferable to side pumping in terms of efficiency, output beam quality, and ASE noise to signal ratio. The maximum efficiency of the amplifier with end pumping using pure He as a buffer gas is significantly lower than that for the amplifier using ${\rm He}/{{\rm CH}_4}$ buffer gas. However, despite the low efficiency, an amplifier using pure He has higher output beam quality. The maximum efficiency of a two-way side-pumped amplifier with pure He as a buffer gas does not exceed 0.01. Thus, such an amplifier cannot be used for efficient energy extraction.

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