Phase Noise Compensation in Long-Distance Atmospheric Free-Space Channels for High-Speed Coherent Communication

Abstract

Free-space optical communication (FSOC) has emerged as a compelling solution for high-speed, secure, and interference-free data transmission, particularly in environments where conventional fiber deployment is impractical. However, the performance of coherent FSOC systems is severely degraded by atmospheric turbulence, which induces random phase fluctuations and compromises signal integrity. This study presents a robust phase noise compensation system designed to mitigate such impairments in long-distance coherent communication links. The proposed approach integrates a frequency-stabilized narrow-linewidth laser, a phase-locked loop (PLL), and an acousto-optic modulator (AOM) to perform real-time phase correction over a 1.3 km atmospheric channel. By extracting the phase noise from the interference between a reference and a recirculated beam, the system dynamically adjusts the beam phase to counteract turbulence-induced distortions. Experimental results demonstrate a 40 dBc/Hz phase noise reduction at a 10 Hz offset and a beat linewidth of 2 Hz with a signal-to-noise ratio (SNR) of 45 dB, confirming excellent optical coherence. These results validate the proposed system as an effective solution for maintaining phase stability in turbulent environments and enabling reliable long-distance FSOC using advanced modulation formats