Research On System Performance Of Free Space Optical Communications Based On Orbital Angular Momentum Multiplexing

In recent years, due to the rapid development of mobile communication and Internet technology, the pressure of the existing communication infrastructure has been increased. The speed of information transmission, the capacity of the communication system and the safety of the communication system have not been able to meet the increasing demands of the people. Therefore, the higher requirement is put forward for the spectral efficiency and channel capacity, so we need more innovation and emergence of new technologies in the field of communication. It is in this environment that all kinds of multiplexing technology are widely used in various fields of optical communication, but due to the electromagnetic communication capacity of resource constraints, the existing problems problem cannot be solved effectively, this kind of multiplexing technology including DWDM mode multiplexing, etc..In theory, orbital angular momentum is similar to the frequency, wave length and it can be regarded as a new degree of freedom, so the number of orbital angular momentum will be unlimited. It is used in the field of optical communication, there is a great application prospect to improve the capacity of optical communication in a greater extent.So, the generation method of the angular momentum beam and the transmission performance of the atmospheric channel in free optical space have attracted more and more attention of scholars both at home and abroad.considering the atmospheric turbulence in free space optical communication,atmospheric turbulence conditions, how to find higher diffraction efficiency,more stable and easier way to generate multiple OAM states, how to protect beam from the disturbance of the external environment to achieve more reliable transmission.In this paper, we focus on the study of multiple OAM states. The optimization design scheme is proposed, including amplitude-only and phase-only generator. And we fully take into account the impact of the vortex beam in the atmospheric turbulent environment, through the MATLAB numerical simulation,the results are analyzed, the specific work and results are as follows?Firstly, Based on the angular spectrum diffraction theory, we design the symmetric, covering area and phase delay of the mask to generate multi -OAM states with the amplitude-only generator.Secondly, To generate multiple orbital angular momentum modes a single phase-only element, simultaneously, We combined with the simulated annealing iterative (SAI). The method shows good optimization performance in operation,and the numerical simulation results show that the method has high diffraction efficiency, and relatively low root mean square error.Thirdly, Elaborate the transmission effect on the formation and characteristics of atmospheric turbulence. Through the MATLAB simulation, the vortex beam is transmitted in the atmospheric turbulence, and the influence of the intensity and the transmission distance of the turbulence on the transmission results is analyzed.