## 4 Enabling Technologies for FSO Communication
### Robust Beam and Multi-Modulation Compatible System Design
#### 4.1.1 Robust Beam Design  
The propagation of laser beams through the atmosphere has been extensively studied both theoretically and experimentally. Recently, significant attention has been focused on laser beams with phase singularities moving through ideal atmospheric turbulence and non-Kolmogorov turbulence. Research indicates that partially coherent vortex beams are less impacted by atmospheric turbulence compared to partially coherent non-vortex beams, and that the topological charge of these beams can serve as an information carrier in optical communications [102]. To mitigate the disturbances caused by turbulence, extensive research has focused on the effects of non-diffracting and pseudo-non-diffracting vortex beams in resisting  
Figure 8: Techniques for mitigating atmospheric turbulence. turbulent disturbances. The findings indicate that these beams' favorable characteristics can significantly reduce the impact of turbulence on the transmission of OAM modes [103; 104; 105; 106]. Specifically, experimentalists are seeking laser beams that are either non-diffracting or possess a definite value of photon OAM along the propagation direction. These specialized laser beams have proven useful in various applications, including optical tweezers, optical trapping, laser radar, metrology, remote sensing, microlithography, image processing, astronomical imaging, medical imaging and surgery, as well as wireless, optical, and FSO communications [107]. In OAM-based underwater optical wireless communication (OWC), the OAM mode is influenced by the wavefront spatial structure of the carrier. As a result, absorption, ocean turbulence, and scattering inevitably distort the OAM mode and cause crosstalk between its energy states. To mitigate the perturbations caused by seawater turbulence, employing various beam structures has become a new trend in this field. These structures include the Gaussian Schell-model (GSM) vortex beam [102], random frozen photons (RFP) beam [108], Hermite-Gauss (HG) vortex beam [109], perfect Laguerre-Gauss (PLG) beam [110], and several quasi-non-diffractive beams such as the Bessel-Gauss (BG) vortex beam [111; 112], Lommel-Gauss (LMG) beam [113], Airy circle (SiC) vortex beams, Airy ellipse (AiE) vortex beams [114; 115; 116], and Hypergeometric-Gaussian (HyGG) vortex beam [103]. Meanwhile, most previous studies on OAM-based systems have concentrated on Laguerre-Gaussian (LG) beams in turbulent atmospheres. In contrast, non-diffraction BG beams exhibit clear advantages over traditional LG beams in reducing turbulence effects and enhancing the performance of OAM-based FSO communication links in such conditions. These benefits arise from two notable characteristics of BG beams: their ability to propagate without altering their intensity profile (non-diffraction nature), and their exceptional capacity to reconstruct after encountering an obstacle (self-healing mechanism) [111; 117]. The BG beam is essentially a Bessel beam (BB) truncated by a Gaussian profile, exhibiting nearly diffraction-free properties up to a specific distance known as the non-diffractive range (NDR). The primary advantage of BG beams over BBs is that the radial modulation tapers the aperture fields, thereby reducing the on-axis ripples caused by edge diffraction [118]. These characteristics are crucial for optical communications based on LoS operations. Consequently, for OAM-based FSO communication systems, the non-diffraction and self-recovery properties of BG beams make them highly valuable. The transmission characteristics of BG beams in free-space have been explored, and their non-obstruction properties in FSO communication systems are examined in [117; 119]. Moreover, structured light beams, particularly pin beams (PBs), exhibit reduced scintillations in free space compared to Gaussian beams. A novel class of optical beams has been introduced, characterized by a main lobe with a Bessel-like transverse profile and a width that gradually increases during propagation. These are termed inverted pin beams (IPBs).