In this paper, we experimentally demonstrate the mitigation of atmospheric turbulence effects using phase patterns that apply the inverse transmission matrix for pre-compensation in the 200-Gbit/s free-space orbital angular momentum (OAM) multiplexed optical link. In this link, two OAM channels are multiplexed, each carrying a 100-Gbit/s quadrature-phase shift keying (QPSK) signal. In the uni-directional (forward) link, a combination of two OAM modes is generated using a designed phase pattern in each of the two channels. After the two channels are multiplexed, each OAM mode will carry the combination of signals from both channels. Such beams could perform the inverse function of turbulence induced crosstalk. We explore the link performances including crosstalk and bit error rate (BER) under 6 different turbulence realizations with a Fried parameter of 1-mm. We find the following: (a) the inter-channel crosstalk is reduced by up to 21 dB; (b) the inter-channel crosstalk is below -10 dB for the 6 turbulence realizations when the compensation is applied; (c) a BER below the FEC limit could be realized for the 6 turbulence realizations with an optical signal-to-noise ratio (OSNR) of 16.8 dB for the uni-directional link with the compensation. Moreover, the post-compensation is also investigated for the two 100-Gbit/s OAM-multiplexed backward channels in the bi-directional link. For the backward channels, the phase patterns are used to mitigate turbulence induced crosstalk at the receiver for backward channels, and the inter-channel crosstalk is reduced by up to 16 dB.

We experimentally utilize a tunable, broadband pixel-array-based integrated receiver for recovering orbital-angular-momentum (OAM) multiplexed channels. The pixel-array-based OAM receiver comprises a pixel-array-based mode converter, four phase controllers, and a pixel-array-based 4-to-3 coupler. Utilizing the mode converter, a free-space data-carrying OAM beam (l=-1 or l=+1) is vertically coupled into four waveguides. The resulting waveguides modes carry an OAM-order-dependent phase delay between each other. Subsequently, a 4-to-3 coupler coherently combines four waveguide inputs with the specific phase delay into a specific output port. By further tuning the relative phase delay of the waveguides using the phase controllers, the data channel carried by a specific OAM beam (l=-1 or l=+1) could be routed to different ports. As there is no specific resonant wavelength, the OAM receiver could be operated with a relatively broad bandwidth. The experimental results show that: (a) the target OAM mode to be received by the OAM receiver could be tuned from l=-1 to l=+1 with a <-14-dB inter-modal power coupling; (b) in a wavelength range of ~6 nm, two OAM-carried 50-Gbaud quadrature-phase-shift-keying (QPSK) data channels (l=-1 and l=+1) could be recovered by the tunable OAM receiver with a bit error rate (BER) performance below the 7% forward error correction threshold; and (c) a 400-Gbit/s OAM-multiplexed and wavelength-division-multiplexed (WDM) QPSK link is demonstrated with a <2-dB optical signal-to-noise ratio (OSNR) penalty compared to that of a single-beam link.