Orbital angular momentum multiplexed photonic crystal multimode fiber–free-space optics link under fog-induced attenuation: design and performance analysis

In this work, we propose the design and performance analysis of an orbital angular momentum (OAM) multiplexed hybrid multimode fiber (MMF)-free space optical (FSO) link with an aggregate data rate of 80 Gbps. The guided-wave segment is realized using a specially designed photonic crystal fiber (PCF) with a P 2 O 5 -doped annular ring core. The PCF is optimized using the finite element method (FEM) in MATLAB to support the simultaneous propagation of the four Laguerre-Gaussian (LG) modes LG 0 0 , LG 2 0 , LG 4 0 , and LG 6 0 with low confinement loss, high mode purity (∼0.967), and appropriate effective refractive index separation. An OptiSystem-based link model which includes a realistic Kim-model FSO channel is used for importing the optimized PCF parameters. The performance of the system is tested for four weather conditions: clear sky, light fog, medium fog and heavy fog. For each of the four LG channels, a plot of the Q-factor and log (BER) versus the FSO range is made with thresholds of Q ≥ 4 dB (BER ≤10 −4 ). The simulated FSO link range reaches 7 km when the sky conditions are clear, while it is restricted to a range of 1,400 m, 925 m and 750 m in light fog, medium fog and heavy fog, respectively, corresponding to a reduction of about 78%, 86%, and 89% compared to clear-sky conditions. Eye diagrams support the quantitive performance metrics. Channel 1 ( LG 0 0 ) always shows the most graceful degradation, and inter-channel Q-factor spread stays between 0.3–0.4 dB in all weather conditions, demonstrating that there is no dominance of inter-modal crosstalk compared to Mie scattering in foggy weather. The results offer deployment guidelines for OAM multiplexed hybrid fiber–FSO system in foggy environment quantitatively.