Neural network nonlinear mitigation and coherent combining to improve the SNR of free-space optical communication systems

Optical satellite communications offer a viable path to meet the growing needs in transmission capacity of space communication networks, as well as the saturation of conventional RF bands. Yet long propagation distances and atmospheric turbulence limit coupled power, necessitating high-power amplification. To date, high-speed transmission at such power levels has not been demonstrated in a single system. Here, we report 1 Tbit/s data rates at 100 W optical power with a signal-to-noise ratio (SNR) advantage of up to 3.9 dB over a conventional single-amplifier system. The SNR advantage of 3.0 dB is obtained by mitigating amplifier-induced nonlinearities by an artificial neural network (ANN) equalizer, thereby outperforming conventional approaches by up to 2.3 dB. An additional SNR gain has been enabled by coherent beam combining of two 50 W amplifiers. The impact of beam combining is investigated, and an advantage of up to 0.9 dB SNR is shown. A system-level investigation into the SNR penalty due to high power amplification, the influence of the polarization multiplexing, and the best-case performance of the system is given. These findings provide critical insights for future high-capacity optical satellite links.