Design and deployment of a physical channel simulator for performance analysis of free-space optical links

The use of free-space optical (FSO) links has the potential to significantly increase the data transmission rates compared to radio links and they enable the possibility of performing quantum communication. For the development of FSO components, including transmitters and receivers, and of novel communications protocols, it is essential to have the ability to comprehensively test the performance across several different communication scenarios – e.g., aircraft-to-ground, aircraft-to-aircraft, satellite-to-ground, and ground-to-satellite. To this end, we have built and deployed two fibre-based FSO channel simulators, having identical design, for testing optical communication in the C-band. These devices are capable of emulating in-the-lab the effects that the transmission through Earth’s atmosphere would have on the received optical signal. Three parameters can be controlled and monitored in real-time: the average attenuation, the signal amplitude fluctuations and the signal phase fluctuations. The (constant) average attenuation emulates the end-to-end transmission loss, from signal source to sink, including the combined effect of geometric loss, due to long communication distances, of atmospheric absorption and fibre-coupling loss. The (time varying) amplitude fluctuations emulate the fast changes in signal intensity due to atmospheric turbulence and implementation specific disturbances like pointing errors. Finally, turbulence can also cause the phase of the signal to fluctuate in time and this effect can also be physically emulated. The amplitude and phase fluctuations time series can be loaded from previously measured data, or from synthetic data. These features allow testing the impact that the transmission through an FSO channel has on different communication schemes, including but not limited to telecommunication applications, like on-off keying and phase shift keying, and quantum communication applications, like quantum key distribution.