Research On Transmission Characteristics Of Laser In Turbulent Plasma Sheath

A plasma coated flow field?also called plasma sheath?is formed around hypersonic vehicles,which causes communication interruption.The study of the communication interruption in the plasma sheath is of great significance to the development of the aerospace industry.With the continuous study of the characteristics of plasma sheath and the interaction mechanism between sheath and electromagnetic wave,it is found that the turbulence effect in sheath of sheath is also an important factor affecting the quality of communication.Therefore,the turbulence property in the sheath has begun to attract extensive attention.1.In order to study the propagation characteristics of laser in turbulent plasma sheath,first,turbulent plasma sheath around the typical flight target RAM-C was calculated based on the density-weighted Navier-Stokes?N-S?and using different chemical reaction models and turbulence models in a typical flight scenario with a flight speed of 23.9 Ma and at a speed of 61 km.The electron density on the wall with the flight test results under the same conditions.The results show that the simulation results of Gupta chemical reaction model and shear stress transport turbulence model or k-?turbulence model are similar to the flight test results.Then the statistical analysis of the turbulent plasma sheath is carried out at a flight altitude of 61 km and a flying speed of 23.9 Ma.The results show that the intensity range of the refractive index fluctuation variance is 1 0^-1110^-14 in this flight scenario,and the relationship between the refractive index variance and the x coordinates of the typical transmission path of the flying target head and the side is in accordance with the five polynomial function.The refractive index variance near the wall is 1-2 orders of magnitude larger than that of the external flow field.Finally,based on the simulation data of turbulent plasma sheath,ray tracing in the turbulent plasma sheath is studied.The deflection of light is closely related to flight conditions.2.In the study of turbulence using random phase screen,the Single Fast Fourier Transform Algorithm is used mostly in the transmission process between two random phase screens.However,this algorithm is only suitable for long distance transmission,and the thickness of plasma coated flow field around hypersonic vehicle is Fast centimeter level.For the ultra-short distance transmission problem,the algorithm suffers from undersampling.The thickness of the plasma sheath is about centimeter order of magnitude.Double Fast Fourier transform algorithm is used for the beam propagation between two phase screens.The turbulence effect of the plasma sheath surrounding the hypersonic vehicle is simulated by the multi-random phase screens.So the problem of ultra short distance transmission in the simulation of turbulence with multiple random phase screens.The spot drift and intensity scintillation index of Gauss beam in turbulent plasma sheath are numerically simulated by using multi random phase screen,and the factors that affect the quality of Gauss beam are analyzed.Studying the characteristics of spot drift,it is found that after the 2000 independent random experiments,the frequency of the drift distance of the centroid of the spot in the x axis tends to the normal distribution.It is shown that 2000 can be used as a sample number for studying the overall distribution.The above results can be used to estimate the overall distribution characteristics by using the distribution characteristics of the samples,that is,to study the distribution of the centroid drift distance of the spot under specific transmission conditions.It is found that the spot centroid moves randomly around the centroid of the original spot,and the spots fluctuation in the two directions of the x axis and the y axis are asically the same.The intensity scintillation index of the Gaussian beam in the turbulent plasma sheath has a nonlinear BoxLucas1 function relationship with the transmission distance,has a linear relationship with the refractive index variance,but the scintillation index is positively correlated with the transmission distance and the refractive index variance.Besides,it is found that the refractive index fluctuation,wavelength and transmission distance are important factors affecting the Gaussian beam quality.The larger the refractive index variance is,the more severe the spot dispersion is and the more obvious the light intensity fluctuation is.As wavelength is longer,the ability of the Gaussian beam to resist turbulence becomes stronger and the dispersion of the light spot and the intensity fluctuation are smaller.The beam distortion and the spot dispersion are more severe as the transmission distance is longer.3.Based on the simulation method of ultra-short distance multi-random phase screen,the propagation characteristics of Laguerre Gauss beams in free space and turbulent plasma sheath are studied.The results show that the main factors that affect the beam quality of the Laguerre Gauss beam in turbulent plasma sheath are transmission distance and refractive index variance.With the increase of transmission distance or refractive index variance,Laguerre Gauss beam can be obtained.The light intensity is obviously weakened,the intensity fluctuation is increased,the degree of spot dispersion is aggravated,and the damage degree of its phase distribution is also enhanced.The phase structure of Laguerre Gauss beam in the turbulent plasma sheath is opposite to the rotation reversal in free space transmission,which indicates that the Laguerre Gauss vortex beam has strong resistance to the plasma sheath turbulence.However,with the increase of transmission distance,the rotation force of phase structure in the free space transmission is stronger than that in the turbulent plasma sheath.Therefore,within a certain transmission distance,Laguerre Gauss vortex beam can maintain its phase structure without rotation.4.Based on the Gauss beam intensity scintillation index in turbulent plasma sheath and the phase shift keying subcarrier intensity modulation method,the BER of the system is studied and deduced considering the new fading caused by turbulent plasma sheath,and the BER of different transmission conditions is simulated according to different premodulation modes.The results show that the BER of the system is related to many factors such as modulation mode,refractive index fluctuation variance of plasma sheath and wavelength.The modulation mode affects frequency band utilization,power utilization and transmission capacity.The larger the refractive index fluctuation variance is,that is to say,the stronger the turbulence effect is and the greater the BER is.The longer the wavelength and the smaller the BER is.Therefore,in order to achieve efficient and reliable communication in a system in which the plasma sheath turbulence is superimposed,we need to compromise the factors such as band utilization,power utilization,transmission capacity and wavelength.