Research On Propagation Of Partially Coherent Beams And Vortex Beams In Random Media And Its Application

With the rapid progress of optical wave detection, imaging and wireless optical communications, propagation and evolution characteristics of optical wave (electromagnetic wave) in random media and optical systems are paid more and more attention by the community of engineering. In the background of optical imaging and communication applications, evolution rules of plane wave, spherical wave, partially coherent laser beam and electromagnetic beams with orbital angular momentum propagating in turbulent media satisfying non-Kolmogorov statistics and anisotropy are investigated in detail and their potential as carriers to transfer data in turbulent channel is analyzed.Based on the modified Rytov theory, irradiance fluctuations and aperture averaging of scintillation for partially coherent Gaussian laser beams propagating through non-Kolmogorov turbulence are studied in the framework of small-scale scintillation modulated by large-scale scintillation. The semi-analytic expression of intensity fluctuation under the condition of aperture receiving is derived. With the use of Gamma-Gamma fading channel model, the ergodic channel capacity of free space optical systems choosing Gaussian-Schell beams as the source is explored and the influences of spectral index of turbulence, the spatial coherence of source, and the size of receiver on signal scintillation and communication channel are analyzed. With the application of weak fluctuation theory and generalized atmospheric turbulence spectral model, approximate expression of intensity fluctuation of Gaussian-Schell model beams propagating through non-Kolmogorov turbulence is derived and analytic formulas of scintillation for plane wave and spherical wave are obtained. The effects of the inner scale, the outer scale and the spectral index of non-Kolmogorov turbulence on beam scintillation characteristics are analyzed and the performance of the above optical wireless links is studied in detail. With the use of Rytov approximation and generalized atmospheric spectral model, the time averaging of scintillation variance of plane wave is investigated under the condition of weak fluctuation. With the employment of generalized Von-Karmon spectrum and annular aperture filtering function, the annular aperture averaging factor of spherical wave scintillation in weak turbulence is studied; the influence of the spectral index and inner scale of non-Kolmogorov on its averaging effect is explored.With the use of generalized Huygens-Fresnel principle and Collins formula for partially coherent beams propagating through ABCD optical systems, the analytic expression of cross-spectral density for multi-Gaussian Schell-model beam passing through misaligned optical system and the accurate formula of its spreading in non-Kolmogorov turbulence are derived. The effects of the inner scale, outer scale and spectral index of turbulence on beam spreading are analyzed. The curve of spectral coherence evolves into Gaussian pattern. The inner scale of turbulence, the correlation width of this source and the spectral index influence its spreading in turbulence dramatically. With the application of optical beam propagation theory in turbulent media with the existence of optical systems, the single-fold integral expressions of spectral density and analytic expressions of beam spreading for the evolution of Bessel-Gaussian Schell-model beams and Laguerre-Gaussian Schell-model beams in complex systems are derived. The effects of the turbulence parameters and spatial coherence parameters of the sources on their evolution are studied carefully.With the application of orbital angular momentum (OAM) theory of electromagnetic beam, the OAM spectrum characteristics of several kinds of partially coherent beams are investigated. The results show that partially coherent beams with different partially spatial coherence own obviously different OAM spectrum properties. In order to further improve the capacity of communication systems, the communication capacity of 7x7 millimeter wave quasi-optical array systems with circular symmetry is investigated in the framework of the combination of OAM multiplexing and multi-input multi-output (MIMO) technology. The numerical analysis show that the combination of OAM technology and MIMO can enhance system capacity; however, the misalignment of the transceiver arrays has obvious influence on MIMO-OAM system capacity.The analytic formulas of phase structure function and wave structure function for spherical wave propagating through anisotropic non-kolmogorov turbulence are derived. Based on the obtained wave structure function and perturbation theory, the influence of anisotropy of non-kolmogorov turbulence on OAM spectrum of single photons is investigated; the probability of receiving initial transmitted OAM state under the condition of different turbulence is analyzed; and finally the channel capacity employing 11 different OAM transmitting states is calculated. In the regime of strong fluctuation, the analytic expression of cross-spectral density for Laguerre-Gauss (LG) beam passing through anisotropic turbulence is derived. Based on the cross-spectral density of LG beam, the spreading properties of its OAM spectrum in strong anisotropic turbulence are analyzed. Results show that it is difficult to employ the degree of OAM of the beam in free space optical communication under the condition of strong turbulence fluctuation.