Mode crosstalk suppression in turbulent free-space vortex beam multiplexing link by OAM-based turbulence phase...

Vortex beam carrying orbital angular momentum (OAM), which features a helical phase front, has shown its potential applications in diverse areas, especially in mode division multiplexing (MDM) based free-space optical (FSO) communications. However, in practical applications, atmospheric turbulence induced phase distortion will cause serious OAM modal degradation and mode crosstalk, fundamentally limiting the performance of vortex beam multiplexing link. To suppress such crosstalk, in this paper, we apply and demonstrate an OAM-based turbulence phase reconstruction (OAM-TPR) method, which can decompose the turbulent phase into a superposition of multiple OAM modes. The OAM mode coefficients can be successfully obtained from the intensity profiles of received distorted beam by a convolutional neural network. By using the proposed OAM-TPR method, we demonstrate efficient compensation of the turbulent phase and a statistically significant reduction in inter-modal crosstalk under weak to strong turbulence (D/r₀ = 2, 4 and 6) condition, comparing with the conventional Zernike-based method and direct-prediction method. Specifically, for D/r₀ = 4, the proposed method reduces the adjacent mode crosstalk from -8 dB to -27 dB, outperforming both the Zernike-based method (reduction to -16 dB) and the direct-prediction approach (reduction to -21 dB). Experimental results validate the effectiveness of the proposed method, as distorted intensity patterns are successfully restored to well-defined annular profiles following compensation. Moreover, the proposed OAM-TPR method can also be used for turbulence compensation under superposed OAM modes transmission conditions. The obtained results demonstrate the method's inherent modal adaptability, enabling optimal compensation with corresponding turbulent OAM coefficients.