Phase noise compensation with highly-stabilized laser for long-distance free-space optical coherence communication

Free-Space Optical communication (FSO) is a promising technology for next-generation communication systems, offering ultra-high data rates, immunity to electromagnetic interference, and enhanced security. Coherent communication further enhances FSO by enabling advanced modulation formats that improve spectral efficiency through the simultaneous use of amplitude and phase information. However, FSO performance is significantly affected by atmospheric disturbances such as turbulence-induced phase noise, which degrades signal coherence and stability. This study proposes a phase noise compensation framework designed to mitigate the effects of atmospheric turbulence. A highly stabilized laser locked to a high-finesse cavity is used as the light source, ensuring narrow linewidth and high coherence. Phase noise introduced by a 1.3 km atmospheric channel is measured by comparing a reference beam with a recirculated beam and dynamically compensated in real time using a phase-locked loop (PLL). Experimental results demonstrate a phase noise suppression of 40 dBc/Hz at a 10 Hz offset, an RF beat signal with a narrow 2 Hz linewidth and a 45 dB signal-to-noise ratio (SNR), highlighting the effectiveness of the proposed system. These achievements validate the system's capability to maintain phase stability and mitigate atmospheric disturbances, paving the way for reliable and robust FSO networks in inter-satellite and other long-range communication scenarios.