Demonstration of a Real-Time 14 Tb/s Multi-Aperture Transmit Single-Aperture Receive FSO System With Class 1 Eye-Safe Transmit Intensity

To achieve a link budget capable of supporting high capacity free space optical communication (FSO), a high transmit power is required. However, the transmitter can become potentially hazardous to the human eye. For a C-band transmitter to be classified as Class 1 eye-safe, the transmit intensity should be limited such that the optical power passing through a 24.5 mm diameter area is below 10 dBm. In this work we used a 9-aperture transmit, single-aperture receive FSO system to reduce the transmit intensity. Wavelength division multiplexed (WDM) signals were divided into 9 groups and transmitted through 9 apertures as separate beams. The apertures were arranged in a 3 x 3 array at a 25 mm pitch, and the transmit power from each aperture was set to 10 dBm. As a validation of the system by numerical simulation of propagation over 220 m, we showed that the additional free space loss due to pointing errors for the beams transmitted through the off-center apertures was less than 7.4 dB, and that using multiple apertures can significantly increase the transmission capacity. In the outdoor demonstration we transmitted 35 WDM 400 Gb/s dual-polarization probabilistically shaped 16-ary quadrature amplitude modulation signals over 220 m. By distributing the WDM channels across the 9 apertures to suit the transmission loss from their positions in the 3 x 3 array, we demonstrated error-free real-time transmission of a record capacity of 14 Tb/s, while satisfying the requirements of the Class 1 laser safety standard.