Resisting Turbulence by Equalizing Frequency-Domain for Multiplexing Orbital Angular Momentum Modes Over a Free Space

Owing to the mutual orthogonality among different orbital angular momentums (OAM), OAM mode has been considered as a promising candidate for improving channel capacity and spectrum efficiency. However, the spiral phase-front of OAM mode is susceptible to atmosphere turbulence (AT) in free pace. The distorted spiral phase induces random noise and signal shake (fluctuation) among symbols and then degrades system performance. In order to overcome the issues, an equalization method in frequency-domain is proposed for mitigating the signal shake induced by the AT. For verifying the feasibility and validity of the proposed concept, the corresponding theoretical analysis and experiments are derived and implanted, respectively. The calculated results demonstrate that the signal shake induced by AT can be effectively mitigated when an optimized estimation response matrix is achieved by using a least squares (LS) algorithm. The bit error rate (BER) and constellation plot (CP) of quadrature amplitude modulation (QAM) signal are also investigated to evaluate the system performance, which demonstrate that the proposed concept can effectively resist the AT in an OAM-based free space optical communication system.