Free-space optical (FSO) communication systems face significant challenges in maintaining image fidelity under atmospheric turbulence, particularly for high-bandwidth RGB applications. This paper introduces a novel dual-polarized (DP) FSO transmission framework enhanced by particle swarm optimization (PSO) to achieve turbulence-resilient RGB image recovery. By exploiting polarization-division multiplexing, the system doubles transmission capacity while employing a physics-informed PSO algorithm to dynamically optimize a multi-stage correction pipeline comprising Wiener deconvolution, gamma correction, histogram matching, and non-local denoising. The proposed method uniquely correlates optimized restoration parameters with propagation distance, enabling simultaneous image recovery and ranging under Gamma-Gamma turbulence modeling. Experimental validation across weak and strong turbulence conditions $\left( {C_n^2 = {{10}^{ - 17}} - {{10}^{ - 13}}{{\text{m}}^{ - 2/3}}} \right)$ demonstrates significant improvements in structure-similarity-index-measure (SSIM), particularly achieving 0.4397 SSIM recovery for Llama images at 2.9 km under strong turbulence and a 119% improvement over uncorrected results. Comparative analysis shows superior performance to deep learning baselines, with 11.2% higher SSIM at 2 km distances. By combining turbulence mitigation with accurate ranging, this framework advances high-fidelity FSO imaging for remote sensing and surveillance applications.

The need for high-bandwidth, low-attenuation communication in underwater environments is driving the development of new technologies. The Internet of Underwater Things (IoUT) addresses this need by enabling underwater devices to communicate, sense, and transmit data. Hybrid optical communication systems, combining Underwater Optical Wireless communication (UOWC), Single-Mode Fiber (SMF), and Free-Space Optics (FSO), have emerged as promising solutions to overcome these challenges and achieve high-speed, long-distance transmission. In this study, we propose a hybrid UOWC-SMF-FSO system utilizing a Photodetector, Remodulate, and Forward Relay (PRFR) for efficient wavelength conversion from 532 nm (visible spectrum) to 1550 nm (infra-red spectrum) at the water-fiber-air interface. System performance is evaluated under varying underwater distances, data rates, and water types. Key metrics such as Bit Error Rate (BER), Quality-factor (Q-factor), and eye diagrams are used to analyze the robustness and reliability of the proposed system. The results demonstrate that the hybrid system achieves at BER below threshold$(3.8\times 10^{-3})$an overall transmission range of up to 2035 m ($35\mathrm{m}$underwater range + 1000 m SMF length$+1000\mathrm{m}$FSO range) in Pure Seawater (PW) at 10 Gbps, with decreasing ranges for Clear Ocean (CL) and Coastal Ocean (CS) water types due to higher attenuation. These findings pave the way for the development of high-capacity underwater sensor networks and contribute significantly to the advancement of IoUT application.

This study presents and investigates a free space optics (FSO) transmission technique that utilizes polarization division multiplexing (PDM) and orbital angular momentum (OAM) multiplexing. The aim is to provide an effective and reliable backhauling solution. The transmission system uses four-level-pulse amplitude modulation (PAM-4) signals achieving a high capacity of 400 Gbps. Two orthogonal polarized beams of 1550 nm Laser diode are used. For each polarized beam, 4 OAM beams are further generated, and each OAM beam transports 50 Gbps data using PAM-4 modulation over the free space channel. The impact of varying FSO ranges on the system performance under clear weather sky (sunny), light rain (LR), medium rain (MR), and heavy rain (HR) is considered. The bit error rate (BER) metric is used to evaluate the proposed system's performance. Results from the simulation indicate that transmission of 400 Gbps is achieved successfully with a range of 1800 m, 900 m, 775 m, and 537 m under clear weather, LR, MR, and HF, respectively, with an acceptable Log(BER) of -2.42 for ultra-FEC limit.