Adaptive-optics-based simultaneous pre- and post-turbulence compensation of multiple orbital-angular-momentum beams in a bidirectional free-space optical link

As a recently explored property of light, orbital angular momentum (OAM) has potential in enabling multiplexing of multiple data-carrying beams, to increase the transmission capacity and spectral efficiency of a communication system. For the use of OAM multiplexing in free-space optical (FSO) communications, atmospheric turbulence presents a critical challenge. In this paper, we experimentally demonstrate simultaneous pre- and post-turbulence compensation of multiple OAM beams, in a bidirectional free-space optical communications link, using a single adaptive optics (AO) system. Each beam carries a 100  Gbit/s signal, and propagates through an emulated atmospheric turbulence. A specifically designed AO system, which utilizes a Gaussian beam for wavefront sensing and correction, is built at one end of the bidirectional link. We show that this AO system can be used to not only post-compensate the received OAM beams, but also pre-compensate the outgoing OAM beams emitted from the same link end. Experimental results show that this compensation technique helps reduce the crosstalk onto adjacent modes by more than 12 dB, achieving bit error rates below the forward error correction limit of 1×10⁻³, for both directions of the link. The results of work might be helpful to future implementation of OAM multiplexing, in a high-capacity FSO bidirectional link affected by atmospheric turbulence.