Parallel optical amplification and multi-aperture transmission with digital coherent reception for wavelength division multiplexed high capacity FSO and its real-time evaluation

For high capacity free space optical (FSO) communication systems, expected be used to support extended coverage for the sixth generation mobile service, the digital coherent technology and wavelength division multiplexing used in optical fiber communications are promising technologies. These technologies can generate optical signals supporting Tb/s level capacity. However, to achieve the link budget required for Tb/s optical links, transmit power in the order of 100 W is required, and achieving 100 W power output with an optical fiber amplifier is challenging. In this work we propose parallel optical amplification of channel groups split out from the WDM signal, instead of amplification of the undivided WDM signal, and the transmission of the amplified signals as separate beams passed through multi-aperture optics. This configuration can reduce the required output power from the individual optical fiber amplifiers. We designed the FSO terminals for the proposed configuration with 3 transmitter apertures, such that the apertures fell within the directivity of the FSO receiver terminal. We evaluated the configuration in an outdoor experiment with a 500 m FSO link and wavelength division multiplexed real-time 100 Gb/s digital coherent QPSK signals. The experimental results show that the proposed configuration can increase the total capacity by 3 times, from 200 Gb/s to 600 Gb/s, without needing to increase the output power from the individual optical fiber amplifiers.