Multimode fiber based multi-OAM mode group receiver for turbulence-resistant free-space optical communications

Free-space optical (FSO) communication systems play an important role in modern communications for its high speed, excellent privacy, and large unlicensed bandwidth. However, in practical applications, especially for long-distance transmission, atmospheric turbulence can induce power coupling from the transmitted Gaussian mode to the neighboring higher-order modes, which will cause serious power loss and power fluctuation at the receiver side and limit the performance of the communication system. Orbital angular momentum (OAM) modes, as a set of orthogonal spatial mode basis, offer a degree of freedom for data transmission in FSO links. Here, using a self-designed and fabricated multimode fiber (MMF) based multi-OAM mode group receiver, we experimentally demonstrate a 72 Gbit/s FSO communication link under moderate to strong turbulence without adaptive optics for beam compensation. Using the MMF-based multi-OAM mode group receiver, one can significantly increase the average received optical power and reduce the power fluctuation caused by turbulence. In the proof-of-principle experiment, the distorted Gaussian beams after turbulence transmission are successfully received by converting them to three OAM mode groups (OAM 0 , OAM ± 1 , and OAM ± 2 ) with the MMF based receiver, which can reduce the required transmitted power for more than 6 dB and improve the system stability under moderate to strong atmospheric turbulence.