Increasing system tolerance to turbulence in a 100-Gbit/s QPSK free-space optical link using both mode and space diversity

In this paper, we experimentally explore using both mode and space (i.e., aperture) diversity to improve system reliability and performance of a free-space optical (FSO) link under atmospheric turbulence. When a system has a limited number of transmitter and receiver apertures, introducing mode diversity to the existing space diversity scheme might further enhance the system performance. We utilize orbital angular momentum (OAM) modes for mode diversity as an example. A 100-Gbit/s quadrature phase-shift keying FSO link is demonstrated through emulated turbulence using two transmitter and receiver aperture pairs. For each aperture pair, a Gaussian beam and an OAM beam are transmitted and received, and all four beams carry the same data stream. Our experimental results indicate that introducing mode diversity to space diversity systems could help to further reduce the variance of received signal power, thereby improving system tolerance to turbulence under a given detection threshold. Bit-error-rates (BERs) mostly below 3.8×10⁻³ are achieved under emulated turbulence with a Fried parameter of 0.4 mm. The dependence of system performance on different diversity schemes, OAM topological charge differential, and aperture spacing are also investigated.