Study on the effect of pump beam on the purity of high-order HG mode in off-axis pumped Tm:YLF laser

Abstract

In this paper, the effect of the pump beam radius on the purity of the HG modes is investigated based on the off-axis pumped Tm:YLF laser, and LG mode vortex laser of 1.9 μm is realized. The multi-mode rate equations are established to reveal that a smaller pump beam radius leads to higher purity of the HG mode beam. At the same time, it is experimentally found that each HG mode alternated with increasing off-axis displacement at a pump beam radius of 200 μm, with a purity of 87.9 % for the HG8,0 mode. However, at the pump beam is 400 μm, the phenomenon of multiple HG modes output increases in the process of adjusting the off-axis displacement. The consistency between the experimental and theoretical results proves the effect of the size of the pump beam on the purity of the high-order HG mode laser. The 1.9 μm LG vortex beam within the 8th order is achieved with a pump beam radius of 200 μm, and the slope efficiency of the LG1,0 mode could reach 34.51 %.