In this paper, we introduce a vortex class of a partially coherent source of Schell type with an electromagnetic Gaussian Schell-model vortex (EGSMV) beam, which is the product of the partially coherent EGSM beam passing through a spiral phase modulator. The analytical expressions for the degree of polarization (DoP) and orientation angle of polarization (OAoP) of the EGSMV beam propagating through atmospheric turbulence are derived. The expressions are used to analyze the influence of topological charge, wavelength, and atmospheric turbulence on the DoP and OAoP of the EGSMV beam, and we come to some new conclusions. The larger the topological charge is, the bigger the dark spot in the center of the partially coherent EGSMV beam, the more dispersed the DoP distribution, and the wider the OAoP distribution are; therefore, there is more information received by the detector when the partially coherent EGSMV beam propagates in atmospheric turbulence. The longer the wavelength is, the smaller the near-surface refractive index structure constant and the larger the inner scale are and the more concentrated the DoP distribution is, but the influence of the outer scale is negligible. The number of petals, which is the shape of the OAoP distribution, is equal to just twice the topological charge. These results provide a theoretical basis for better control of coherent detection in optical communication when the partially coherent EGSMV beam propagating through atmospheric turbulence is used as the local oscillator in a wireless optical communication system.

Vortex beams with orbital angular momentum (OAM) significantly enhance system capacity, and high-precision recognition of OAM mode through atmospheric turbulence (AT) channels can markedly improve the information transmission capability of free-space optical communication systems. In this paper, with a cylindrical lens-assisted distinguish between positive and negative OAM, a reliable neural network combining multi-scale dilated convolution (MSDC) unit and multi-level feature fusion (MLFF) module is proposed to detect high order AT-distorted OAM modes. The network fully exploits the features in light-intensity images to achieve a highest recognition accuracy of 99.4% for mode-orders from -20 to +20 in a hybrid ATs dataset (C _n ² = 5×10⁻¹⁶, 5×10⁻¹⁴, 5×10⁻¹²m^-2/3), and almost 96% even in strong turbulence. Experimental results on accuracy, efficiency, reliability, and robustness demonstrate that the proposed method excels and provides a trustworthy solution for complex AT-distorted OAM mode recognition.

Maksutov–Cassegrain (MC) systems are widely used in long-range free-space optical (FSO) communication. In this study, analytical expressions for the diffraction field of a Laguerre–Gaussian (LG) beam passing through an MC system have been derived. The numerical results reveal that, in the long-range FSO system, the replacement of a Gaussian beam by the LG beam enhances the emission efficiency of the MC. Furthermore, the MC has a shaping effect on the LG beam, and the orbital angular momentum of the LG beam is not dispersed when the beam is diffracted by the MC.

The differential microphone array, or differential beamformer, has attracted much attention for its frequency-invariant beampattern, high directivity factor and compact size. In this work, the design of differential beamformers with small inter-element spacing planar microphone arrays is concerned. In order to exactly control the main lobe beamwidth and sidelobe level and obtain minimum main lobe beamwidth with a given sidelobe level, we design the desired beampattern by applying the Chebyshev polynomials at first, via exploiting the structure of the frequency-independent beampattern of a theoretical Nth-order differential beamformer. Next, the so-called null constrained and least square beamformers, which can obtain approximately frequency-invariant beampattern at relatively low frequencies and can be steered to any direction without beampattern distortion, are proposed based on planar microphone arrays to approximate the designed desired beampatterns. Then, for dealing with the white noise amplification at low-frequency bands and beampattern divergence problems at high-frequency bands of the null constrained and least square beamformers, the so-called minimum norm and combined solutions are deduced, which can compromise among the white noise gain, directivity factor and beampattern distortion flexibly. Preliminary simulation results illustrate the properties and advantages of the proposed differential beamformers.

In this article, we take partially coherent electromagnetic Gaussian-Schell model (EGSM) beam as a research object. Based on the unified theory of coherence and polarization and the generalized Stokes parameters of random electromagnetic beams, the expressions of degree of polarization (DoP) and orientation angle of EGSM beam that propagates in the slant atmospheric turbulence, are derived. And the expressions are used to investigate the difference in polarization property of EGSM beam between propagating along an uplink path and a downlink path in the atmospheric turbulence. The results show that under the same conditions, the DoP distribution of EGSM beam propagating along a downlink path is more concentrated than that along an uplink path. When EGSM beam propagates along a downlink path, the maximum DoP corresponding to the propagation distance is larger than along an uplink. Therefore, when EGSM beam propagates along a downlink path, the detector can receive beam information at the longer distances.