Generation of Vector Vortex Needle Beams and Their Propagation in Turbulent Atmosphere

Atmospheric turbulence has always posed a significant challenge to the stable propagation of lasers, especially for long-distance transmission in free-space optical communication (FSOC) systems. To mitigate the adverse effects of atmospheric turbulence, there is an urgent need to develop specialized optical beams.

Paper Information:

In this study, the propagation characteristics of Vector Vortex Needle Beams (VVPBs) in free space are theoretically analyzed and derived, with their behaviors in atmospheric turbulence numerically simulated. Simulation results show that under the same conditions, VVPBs exhibit lower scintillation indices and less beam wander than conventional Pinhole Beams (PBs) during long-distance transmission. Additionally, both VVPBs and PBs are experimentally generated, and their scintillation indices and beam wander are measured in a laboratory environment with thermally induced turbulence. The experimental data validate the theoretical findings, confirming that VVPBs have significant advantages in reducing the effects of turbulence. These findings are expected to have important implications for the development of free-space optical communication and remote sensing technologies.

The following are parts of the experimental procedures and results:

To validate the theoretical results, we first experimentally generated VVPBs and PBs, and then measured their propagation intensities in a turbulent environment. The experimental setup is shown in the figure below. A linearly polarized laser beam at 532 nm was expanded and then split into two paths by a 50:50 beam splitter (BS1) to form a Mach-Zehnder interferometer. The light in each path was reflected by a phase-only spatial light modulator (SLM) to modulate the phase and amplitude of the incident beam. The modulated beams passed through two 4f systems, which image the light onto the end plane of the 4f system with unity magnification. The end planes of each 4f system were perfectly overlapped at BS4, which can be regarded as the source plane. Two cylindrical lenses (CA1 and CA2) were placed at the back focal planes of lenses L1 and L3, respectively, to filter out unwanted diffraction orders and background noise. The light emerging from BS4 formed a single beam that passed through a heated plate to generate turbulence. After passing through the heated plate, the beam was recorded by a charge-coupled device (CCD). The holograms loaded onto the two SLMs were computer-generated. The amplitude and phase on the two SLMs were identical, except that an additional π/2 phase was loaded onto SLM2. This made the light reflected from the SLMs in paths 1 and 2 orthogonal, allowing the formation of VVPBs. Additionally, by blocking one path and changing the hologram, PBs could also be generated. Therefore, this experimental setup enabled the convenient generation of both VVPBs and PBs.

Figure 1 Experimental setup for generating VVPBs and investigating their propagation in turbulent environments, including beam expander (BE), lenses (L1-L4), charge-coupled device (CCD), beam splitters (BS), spatial light modulators (SLM1 and SLM2), and circular apertures (CA1 and CA2).

Figure 2 Comparison between simulation results (first row) and analytical results (second row) of normalized intensity distributions for VVPBs at the z = 400 mm plane. The third row shows cross-sectional comparisons between the first and second rows. The first column displays total intensity, the second column shows the x-component, and the third column shows the y-component. The white bar indicates 3 mm. Figures (a) to (f) have the same dimensions.

Figure 3 illustrates the simulation model of VVPBs propagating in turbulent atmosphere.

Figure 4 Axial intensity fluctuations of VVPBs (first row) and PBs (second row) under different turbulence intensities.  

Figure 5 Beam centroid distributions from 1000 intensity realizations of VVPBs (first row) and PBs (second row) at varying turbulence intensities.  

Figure 6 Beam spots of VVPBs recorded by a beam profile analyzer in free space at 1 m from BS4: (a) total intensity distribution, (b) x-component, and (c) y-component. Each panel measures approximately 7×5 mm².  

Figure 7 Experimental results showing the variation of (a) scintillation index and (b) beam wander with hot plate temperature for both PBs and VVPBs.

The parameters and specifications of the phase-type spatial light modulator used in this experiment are as follows:

Note: Different models vary in phase modulation range and light utilization efficiency. For specific requirements, please contact the sales manager in your respective region for details.

Final Statement:

The spatial light modulator (SLM) is a core optoelectronic device that utilizes the electro-optic effect of liquid crystal materials to dynamically control the spatial distribution of light wavefronts (including amplitude, phase, or polarization). As an extremely powerful tool in modern optics and photonics, it enables active, precise, and dynamic manipulation of light propagation in space. Currently, the main methods for generating vortex beams include the spiral phase plate method, spatial light modulator, computer-generated hologram, mode conversion method, metasurface, etc. Among them, the spatial light modulator demonstrates absolute advantages in dynamically changing topological charge numbers and other parameters due to its programmability.

Article Information:

https://doi.org/10.1063/5.0248643