Experimental validation of structured beam properties under convectively induced optical turbulence

Structured light propagation experiments were carried out in a Rayleigh-Bénard (RB) convective water tank in order to evaluate beam characteristics and susceptibility of several topological charges to optical turbulence conditions spanning several fluid turbulence levels set by the system Rayleigh number. Structured light fields were generated using a spatial light modulator, which imparts a phase change to create light that carries optical orbital angular momentum (OAM). Beams were propagated over a 1.2 m path under weak, moderate, and strong optical turbulence conditions. The flow dynamics, relative to the OAM beam dynamics, are such that a "frozen" state is realized in the R-B tank for set flow conditions. This ensures consistent turbulence across all tested beams, enabling reliable comparisons of beam performance under identical scenarios. The study focuses on observing turbulence dynamics within the structured beam's profile using intensity fluctuation analysis in the temporal and spatial domains, correlations with dynamic masking between consecutive realizations of the beam's intensity, and power spectral densities and histograms. The scintillation index (SI) was evaluated using three methods: (1) at the point of maximum intensity within the annulus, (2) at the centroid within the vortex, and (3) averaged over the region of interest containing all non-zero beam intensities. It was found that a reduction in the SI for OAM-carrying beams with increasing topological charge was independent of the optical turbulence conditions. Since SI represents a normalized variance, this reduction is not simply a result of intensity redistribution associated with higher topological charge; rather it demonstrates that the SI systematically decreases with increasing topological charge under all experimental conditions. In addition, to gain deeper insight into the optical turbulence dynamics, histograms of the annular maximum intensity fluctuations, spectra of the correlation coefficients, and maximum intensity measurements are presented.