| Recently, the space laser communication becomes the hotspot and frontier of investigation in communication area.In the atmospheric transmission process of wireless optical communication, the following problems exist: light intensity flicker,light beam drift, high bit error rate and so on, which are the main obstacle for realizing the project of FSO.In the current investigation of the near-earth atmospheric and terrestar WOC, a lot of effort has been devoted into studying the technology that remits or restrains the influence of atmosphere. The problems mentioned above could lead to the nonuniform of optical field distribution on the reception plane of outgoing beam, light intensity variation flicker, light beam drift, variation of arrival angle and high bit error rate, so that the communication quality is affected severely. Therefore, how to solve these problems becomes the hotspot and difficult point in the study of laser propagates in atmospheric turbulence.In this paper, the field of atmospheric turbulence is studied and the solution of multiple aperture emission is proposed in order to restrain the effect of flicker, spread and light beam drift, which is caused by turbulence field. It is indicated from the theoretical study and simulation analysis that: the effect of atmospheric turbulence on communication link is restrained by adapting the technology of multiple aperture emission, so that the quality of communication is improved. The major achievements are listed as the following:1)Based on the spectrum model of Von Karman, the phase screen is achieved by simulation software, and the description of phase screen in long distance transmission is realized by using the circular accumulate algorithm. The transmission light intensity of several coherent Gaussian beams in atmospheric turbulence is obtained in simulation. The comparison of several coherent Gaussian beams and single Gaussian beam propagate through the atmospheric turbulence is carried out, and it is demonstrated that: because of the coherent overlying and complementary effect of several Gaussian beams, the number of the maximum light intensity occurrence on the reception plane increases, and the energy ratio of reception and emission in detector is raised. Meanwhile, the shift phenomenon in the transmission of single beam is effectively retrained.2)The analytical model of atmospheric light intensity flicker is established. When several Gaussian beams are overlapped coherently, the effect of vortex flow in atmospheric channel is reduced, so that the variation of light intensity is restrained. The nonlinear inverse relation between the number of aperture and light intensity is found. Within a certain range, the increase of aperture number could reduce the variation of light intensity, and the number of aperture is suggested to be 6 for some practical consideration.3)In order to investigate the situation of light turbulence, the theoretical model of the relation of light intensity flicker and bit error rate is established. The theoretical results are compared with that of simulation. It is concluded that multiple aperture emission could reduce and restrain the bit error rate in near-earth atmospheric WOC. Within the same channel condition, the more the aperture is, the lower the bit error rate is.The reduction of the error ratio is notTherefore, the number of aperture shouldn't be set as many as possible and is suggested for practical situation.Based on the investigation of several atmospheric turbulence models and the theory of phase screen, the propagation properties of partly coherent Gaussian beam in atmosphere are studied. It is indicated from the theoretical and simulation results that: the technology of multiple aperture emission has the remarkable advantage on solving the problems exist in near-earth atmospheric . |
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