Coherent Free-Space Optical Communications: Opportunities and Challenges

The ever-increasing data rate demand for wireless systems is pushing the physical limits of standalone radio-frequency communications, thus fostering the blooming of novel high-capacity optical wireless solutions. This imminent penetration of optical communication technologies into the wireless domain opens up a set of novel opportunities for the development of a new generation of wireless systems providing unprecedented capacity. Unlocking the full potential of free-space optics (FSO) transmission can only be achieved through a seamless convergence between the optical fiber and optical wireless domains. This will allow taking advantage of the staggering progress that has been made on fiber-based communications during the last decades, namely leveraging on the latest generation of Terabit-capable coherent optical transceivers. On the other hand, the development of these high-capacity optical wireless systems still faces a set of critical challenges, namely regarding the impact of atmospheric turbulence and pointing errors. In this work, we provide an in-depth experimental analysis of the main potentialities and criticalities associated with the development of ultra-high-capacity FSO communications, ultimately leading to the long-term (48-hours) demonstration of a coherent FSO transmission system delivering more than 800 Gbps over $\sim$ 42 m link length, in an outdoor deployment exposed to time-varying turbulence and meteorological conditions.