Demonstration of Simultaneous Mitigation of Turbulence and Misalignment in a Free-Space Mode-Division-Multiplexed Optical Link Using Beam Shaping

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We experimentally demonstrate a beam shaping technique for simultaneously mitigating crosstalk effects due to turbulence and misalignment in a 50 Gbit/s/channel quadrature phase shift keying (QPSK) modulated two-channel mode-division-multiplexing (MDM) free-space optical (FSO) communication link. Co-propagating probe beams on a different wavelength close to the data channels are used to measure the channel crosstalk caused by turbulence and misalignment. We modify the transmitted beams to be a superposition of different orbital angular momentum (OAM) modes using the inverse modal coupling of the channel, which reduces the crosstalk between the two data channels at the receiver. Our experimental results show that when both turbulence and misalignment are present: (a) crosstalk reduces by > 9 dB between OAM +1 and +2 to <-14 dB when considering both turbulence and misalignment; (b) the proposed approach induces an optical signal-to-noise ratio (OSNR) penalty of ∼2 dB for both data channels compared to the back-to-back (B2B) scenario near the 7% forward error correction (FEC) threshold.