Effects of oceanic channels degrade the performance of underwater optical communication (UOC) systems based on orbital angular momentum (OAM) multiplexing. Considering both oceanic turbulence and water attenuation, a more comprehensive channel model is proposed. We derive the expressions of bit error rate (BER) and aggregate capacity of UOC-OAM systems. We also investigate the system performance with wavelengths of 400-700 nm . The numerical simulation results show that the blue-green wavelength is not always the best choice to provide large channel capacities for UOC-OAM systems. For large chlorophyll-a concentration channel, due to the strong attenuation the optimal wavelength is shifted toward red wavelength region. Meanwhile, with increasing of oceanic turbulence strength, the frequency bandwidth decreases and systems with larger wavelength show better performance. These results will be beneficial to the design and application of UOC-OAM systems.
Atmospheric turbulence limits the performance of orbital angular momentum-based free-space optical communication (FSO-OAM) system. In order to compensate phase distortion induced by atmospheric turbulence, wavefront sensorless adaptive optics (WSAO) has been proposed and studied in recent years. In this paper a new version of SPGD called MZ-SPGD, which combines the Z-SPGD based on the deformable mirror influence function and the M-SPGD based on the Zernike polynomials, is proposed. Numerical simulations show that the hybrid method decreases convergence times markedly but can achieve the same compensated effect compared to Z-SPGD and M-SPGD.
The fine tracking system is important to improve the performance of orbital angular momentum (OAM) space optical communication system. In order to improve the accuracy and stability of the centroid algorithm, this article proposes a modified algorithm to detect the center position of OAM light beam. The modified algorithm firstly takes a binary-conversion with the threshold based on the intensity profile of OAM beam, and then calculates the center position of the light spot using centroid algorithm. The threshold for binary-conversion of Laguerre-Gaussian (LG) beam is obtained by theoretical analysis and numerical calculation. The accuracy, stability and complexity of the modified algorithm are studied by numerical simulation. The results show that the new algorithm, compared with the centroid algorithm, has 2 orders of magnitude higher in computing accuracy and 3∼4 orders of magnitude higher in stability. However, the computing time has increased by about 1.5 times compared with the centroid algorithm.
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