Influence of wind-dominated thermal blooming on orbital angular momentum and phase singularity of dual-mode vortex beams

Abstract

The influence of thermal blooming on orbital angular momentum (OAM) and phase singularity of dual-mode vortex beams under different wind direction and wind speed has been studied in this paper. Due to the different symmetries of dual-mode vortex beams superimposed by different modes, the impact of thermal blooming on them not only depends on wind speed, but also on wind direction. Based on the scalar wave equation and the hydrodynamic equation, a 4D computer code to simulate the time-dependent propagation of dual-mode vortex beams in the atmosphere is devised by using the multiphase screen method and finite difference method. It is found that, for certain wind direction, the value of OAM increases with the decreasing wind speed because the thermal blooming becomes more serious, i.e., the thermal blooming effect promotes the OAM of dual-mode vortex beam growth. For an example, when the angle between the wind direction and the beam is 0＜θ＜50°, the OAM of the dual-mode vortex beams with a topological charge difference of 2 increases with decreasing wind speed, and there is an optimal angle (θ≈20°) to maximize OAM. Therefore, for certain wind direction and wind speed, the OAM of dual-mode vortex beam propagating in the atmosphere could be larger than that in free space, and could be larger than the OAM of single-mode vortex beam. The dual-mode vortex beam with higher modes requires smaller wind speed to make its OAM larger than the OAM in free space. In addition, the larger the topological charge difference between the two element beams of a dual-mode vortex beam is, the more stable the OAM of the dual-mode vortex beam is. On the other hand, the evolution of linear edge dislocation singularity under atmospheric thermal blooming are also investigated in this paper. When the wind direction is perpendicular to the dislocation line, the linear edge dislocation singularity disappears. If the wind direction is parallel to the dislocation line, the linear edge dislocation singularity always exists. At other angles, the linear edge dislocation singularity will evolve into optical vortex pairs. The results obtained in this paper are useful to laser propagating in the atmosphere and optical communication.