Research On Transmission Performance Of Random Phase-controlled Array Beams In Turbulent Atmospheric Channels

Free-space optical communication has received extensive attention and research due to its high communication capacity,strong anti-interference and anti-interception capabilities.However,the turbulence effect in the atmospheric environment will seriously affect the transmission performance of the laser signal,reduce the tracking accuracy of the optical communication system,enhance the scintillation effect of the optical signal,and affect the communication error rate,communication rate and distance.Theoretical and preliminary experimental studies have shown that partially coherent light has the ability to suppress the turbulence effect of light transmission.Traditional partial coherent light acquisition methods are difficult to apply to optical communication systems.Therefore,the development of new partially coherent light sources and their communication performance analysis and experimental research have important academic significance and application value for ensuring and improving the performance of wireless optical communication systems in atmospheric environment.This thesis first introduces the characteristics of atmospheric turbulence and the evaluation parameters of the influence on the beam transmission characteristics.Based on the turbulent channel model,the expressions of the output signal-to-noise ratio and the average bit error rate of the communication system in turbulent atmosphere are given.Aiming at the coherent light of the random phase-regulated fiber array,the experiment of light source coherence control was carried out.By applying a random modulation voltage of different amplitude range to the Li Nb O3 phase modulator,the fiber array produced partially coherent light with an intensity correlation coefficient of 1.08~1.32.Partially coherent light transmission experiments under different turbulence intensities were carried out using a hot-air turbulence simulator: when the Fried parameter is 0.11 m,the scintillation index is the smallest when the random modulation voltage is-4 to 4 V,and the Fried parameter is 0.0658 m,the scintillation index of the random modulation voltage-5~5V is the smallest;the experimental results show that for the determined Fried parameter,the scintillation index of partially coherent light is smaller than the coherent light.However,the degree of improvement is related to the turbulence intensity and the coherence of the light source.The scintillation index has a minimum value with the modulation voltage,which indicates that the turbulence effect can be effectively reduced by adjusting the coherence of some coherent light for a given turbulence intensity.It is shown that as the turbulence intensity increases,increasing the correlation coefficient can reduce the scintillation index.The experimental results provide experimental support for the adaptive regulation of random phase to reduce the influence of turbulence.Based on the coherent optical transmission experiment of fiber array,this thesis analyzes the scintillation characteristics and communication error rate of random phase-regulated array collimated beam in turbulent atmosphere.The multi-phase screen simulation method is used to numerically calculate the intensity distribution and scintillation characteristics of the array beam in the turbulent atmosphere.The effects of random phase amplitude,duty cycle and number of beams on the coherence of the source and its influence on the scintillation index and the average bit error rate are analyzed.The results show that reducing the coherence of the beam by random phase modulation and increasing the number of beams(such as increasing from 7 to 19)can significantly reduce the light intensity scintillation index of the beam and improve the communication performance;The effect of random phase amplitude on beam scintillation index is related to turbulence intensity.When the Fried parameter is 0.136 m,the beam scintillation index with the intensity correlation coefficient of 1.1 is the smallest.With the increase of turbulence effect,increase the correlation coefficient and reduce the coherence,the flicker index can be reduced.Array collimated beam coherence controllable,high power and other characteristics provide an important basis and reference for the design and improvement of new optical sources for wireless optical communication systems.