Is the orbital angular momentum spectrum of an optical vortex beam propagating in isotropic turbulence symmetrically distributed?

The propagation of optical vortex beams through atmospheric turbulence is pivotal for orbital-angular-momentum (OAM)-based free-space optical communications and remote sensing. It has long been known that owing to the turbulence-induced phase aberration, a propagating vortex beam results in modal scattering and distortions and its power will spread uniformly onto neighboring OAM modes. However, some studies reported a contradictory result that the scattering OAM modes take precedence over the side near the OAM mode with topological charge <i>l</i> = 0, resulting in the asymmetric distribution of the OAM spectrum. Here, we present a detailed theoretical analysis to explain the physical mechanism that leads to the above contradiction. We show that the asymmetry of the OAM spectrum of a vortex beam propagating in atmospheric turbulence is a universal phenomenon arising from the interaction between beam diffraction and turbulence, while neglecting the aforementioned interaction, the OAM spectrum becomes symmetrically distributed. We further carry out proof-of-principle experiments to demonstrate the validity of our theoretical analysis and find a good agreement between the experimental results and theoretical predictions. Our findings are of great help to the in-depth understanding of interactions of atmospheric turbulence and OAM beams.