| Comparing to the traditional optical communication using the polarization of light as the information carrier, the capacity of the optical communication system based on the orbital angular momentum(OAM) obviously increases. However, when OAM modes carried by vortex beams propagate in the atmosphere, the scattering and absorption of aerosols will lead the attenuation of the signal, and the interference of turbulence will cause the irradiance fluctuations, beam wander, beam spread and so on. The attenuation of the signal and interference of turbulence consequentially degrade the performance of free space optical communications. Therefore, it is an important issue to learn about the interaction between the optical wave carrying OAM and atmosphere, to control the effect of turbulence on OAM modes carried by the vortex beam and to improve the transmission performance of signal in optical communications. Main innovation points of this dissertation are as follows:1. On one hand, based on the Rytov approximation in weak fluctuation region and non-Kolmogorov power spectrum, we propose a model for the received power of OAM modes of the non-diffraction Hypergeometric-Gaussian beam, and investigate the characteristic parameter of Hypergeometric-Gaussian beam for mitigating turbulence. On the other hand, when inner-scale, outer-scale effects and anisotropy of atmospheric turbulence must be considered, modified Rytov approximation is used to deduce the moderate-to-strong anisotropic non-Kolmogorov power spectrum and the corresponding spatial coherence radius. To some extent, the property of auto-focusing of Hypergeometric-Gaussian beam may alleviate the effect of turbulence on the propagation of OAM mode.2. We invoke the factor of the Gaussian Schell model into the optical field of non-diffraction Airy beam to investigate the mitigating function of the turbulent interference by utilizing the spatial coherence and non-diffraction beam. The factor of the Gaussian Schell model describes the partial coherence of the light source. A model for the spiral spectrum of Airy-Shell beam is developed.3. Due to the multi-Gaussian-Schell mode possessing lower scintillation induced by turbulence, we model the spiral spectrum of multi-modified-Bessel-Gaussian Schell beams. We also study the influence of the source structure on the OAM communication system.4. On one hand, by invoking the modified Rytov approximation and a marine atmospheric spectrum in Kolmogorov turbulence, and considering the effect of aperture averaging, we develop the model for the received power and crosstalk power of OAM modes carried by Airy beam in moderate-to-strong marine turbulence. On the other hand, by considering the property of the non-Kolmogorov and anisotropy of the marine atmosphere, we obtain a new spatial coherence radius. Considering the fading of the spatial coherence of a beam propagation in a turbulent atmosphere and the new spatial coherence radius, we deduce the analytic expression of the received power of OAM mode carried by the partially coherent modified Bessel-Gaussian beam in anisotropic non-Kolmogorov marine atmosphere and achieve the optimal parameters of source for the mitigation of turbulence.In summary, we focus on the propagation of OAM mode carried by non-diffraction beams to minimize the disturbance of atmospheric turbulence and improve the signal quality in optical communication links. Considering the structure, spatial coherence and mode of the signal source and the types of turbulent channel, we analyze the influence of multi-Gaussian Shell source, spatial partially coherence beam and atmospheric turbulence links on the performance of a communication system. We propose the optimum parameters of the source in the communication system to mitigate the effect of atmospheric turbulence. |
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