Performance analysis of a 100 Gbps dual-polarization PAM-4 free-space optical link under clear weather and fog-induced attenuation

A comprehensive performance analysis of a dual-polarization pulse amplitude modulation-4 (PAM-4)-based free-space optical (FSO) link with each polarization branch carrying 50 Gbps is presented in this paper. The proposed system is tested in clear weather (0.14 dB/km) with three beam divergence angles (0.20 mrad, 0.25 mrad, and 0.30 mrad) under three different performance metrics: log(BER), error vector magnitude (EVM%), and received optical power (ROP) plotted as a function of the link range; at the maximum supported range, eye diagram analysis is provided to support the results. The link is further evaluated under light fog (9 dB/km), medium fog (16 dB/km), and heavy fog (22 dB/km) conditions, and the maximum range of the link is determined based on the forward error correction (FEC) threshold (log(BER) = −2.42). The data indicate that under clear weather, a 0.20 mrad divergence angle can maintain acceptable performance up to 9 km compared to the wider divergence configurations, which have more geometric spreading loss and lower received power. With the introduction of fog, the maximum link range decreases significantly to 1,500 m with light fog, 1,000 m with medium fog, and 800 m with heavy fog, reflecting the reduction in maximum link range as the scattering of the atmosphere increases.