Abstract: Free Space Optics (FSO) offers easy deployment and high data rates, making it ideal for terrestrial and space communications. However, FSO systems, despite their numerous advantages, are highly vulnerable to environmental factors such as atmospheric attenuation, turbulence, and adverse weather conditions. According to existing literature, the maximum range of an FSO link in terrestrial settings is typically under 5 km with a bit rate of maximum10 Gbps in hazy environments. In this paper, we propose a performance analysis of an FSO system that integrates Coherent Detection with an Intelligent Adaptive Code Modulation (ACM) technique, offering enhanced resilience, increased link length, and improved availability under varying environmental conditions. Performance analysis and simulation results demonstrate the system’s ability to maintain data transmission rates of 40 Gbps over distances up to 1 km and 10 Gbps over distances up to 6 km under adverse conditions such as fog, rain, and snow, while preserving high Q-factors, low BER, and extended link availability. The system dynamically switches between 16QAM, 4QAM, and PAM modulation schemes, depending on real-time assessments of Signal-to-Noise Ratio (SNR) and weather conditions.