Propagation characteristics of rotating beams under combined effect of atmospheric turbulence and thermal blooming

Abstract

The rotating beam is a beam that is generated by the heterodyne interference of vortex beams with different topological charges, whose intensity and wavefront rotate rapidly with time. Under Taylor’s frozen air hypothesis, such a rotating beam, whose rotating period is much shorter than the characteristic time of atmospheric turbulence and the thermal blooming, can travel through all the inhomogeneities due to its rapid rotation during its propagation in the air. The rotation can contribute to the smooth of the wavefront distortion caused by the atmospheric turbulence and the thermal blooming, and thus improve the beam quality and reduce the centroid drift of the rotating beam in far field. The physical model of rotating beam is established by the heterodyne interference of two vortex beams. Taking the atmospheric turbulence and the thermal blooming effect into consideration, the propagation model of the rotating beam in the air is established by using the split-step Fourier method. For simplicity without loss of generality, the influences of the atmospheric turbulence and the thermal blooming effect on the propagation of rotating beam, are both treated as wavefront screen. The difference is that the wavefront screen of atmospheric turbulence is generated by the Kolmogorov power spectrum, while that of thermal blooming is generated by the fluid mechanics equation. The physical mechanism of how the rotating beam mitigates the atmospheric turbulence and the thermal blooming effect is analyzed in detail, that is, when the rotating speed of laser beam is faster than the airflow, the laser beam can pass through all the inhomogeneous and anisotropic atmosphere in the azimuthal direction within the time interval of airflow. After the laser propagates through the frozen air, the total wavefront distortion in the azimuthal direction becomes centrosymmetric with lower PV value, reducing the beam quality degradation. On the basis, the influence of rotation frequency, power ratio of the sub beams, strength of turbulence and thermal blooming on the propagation characteristics of the rotating beam in atmosphere are analyzed. Within a certain range, as the beam rotation frequency increases, the mitigation effect of the rotating beam on atmospheric turbulence and thermal blooming effects are enhanced. With the increase of turbulence intensity and thermal blooming intensity, the mitigation effect of the rotating beam is weakened but still can maintain well, which can provide reference for the engineering applications of laser beam in atmosphere.