Research On Propagation Characteristics For Gaussian Laser Beam And Estimation Method Of Optical Parameters Through Turbulent Channel

Wireless laser communication(WLC)is a mode of point-to-point communication technology.It uses the laser beam propagating in the unguided medium to carry the digital signal for data transference and information exchange with high speed and long distance.Its outstanding advantages include large channel capacity,high transmission rate,anti-electromagnetic interference and so on.Undoubtedly,the atmosphere is one of the most complicated unguided medium,whose atmospheric turbulence effects(ATE)with fading characteristic can degenerate the inner coherence of the laser beam,causes a series of optical properties to no longer satisfy the effective working conditions and thus reduce the performance of the WLC links.It is the comprehensively understanding of the optical amplitude and the wavefront phase in turbulence that is the necessary prerequisites for the wide application of WLC technologies.Consequently,it is significant to both establish the improved theoretical models for various ATE and develop the advanced metrical techniques for atmospheric optical parameters(AOP).Focused on the nonlinear relationships between ATE and AOP,this paper selects the Gaussian laser beam(GLB)as the research subject.The theoretical models for ATE and the metrical techniques for AOP are investigated,respectively.On one hand,the generalized models for GLB propagating in non-Kolmogorov turbulence are established.On the other hand,a scientific solution for the consistency of the optics-based metrical technique for AOP is discussed.The completed work includes the following points:1)The modulation transfer(MT)function describes the ability of an optical system to transfer the modulation degree.The model of MT function for GLB is established.This model is applicable to the situation when the inner and outer scales of nonKolmogorov turbulence are non-negligible.By the complex function,higher algebra and other mathematical tools,this model has been reduced to simpler expressions which is convenient for practical application.Numerical simulations are performed to analyze the influence of various parameters on MT function,and these properties of MT function are generalized from the plane and spherical optical waves to the case of GLB.2)The irradiance scintillation(IS)is the fluctuation of the intensity around its average for the incident beam at the receiving antenna when the power of the outgoing beam is fixed at the transmitting antenna.The model of the aperture-averaged IS index for GLB is established.Numerical simulations are performed to analyze the influence of various parameters on the aperture-averaged IS index.It is found that the apertureaveraged IS index is less sensitive to the change of the outer scale than the inner scale.The influence of the aperture-averaged IS index on the performance of WLC systems is also discussed.3)The beam wander(BW)is the high-frequency and disordered jitter of the imaging spot barycenter in the plane of the receiving antenna after the laser beam propagating through the turbulent channel,whereas the beam spread(BS)is the expansion and widening in the plane of the receiving antenna of the imaging spot area between the turbulent channel with the vacuum transmission.Without the geometric optics approximation(GOA)method,the models of BW variance and the long-term BS radius for GLB are established,respectively.Numerical simulations are performed to analyze the influence of various parameters on both the BW variance and the long-term BS radius.The established models are also compared with the GOA-based models,which indicates the spectral power law is related to the errors induced by the GOA method.The applicability conditions of GOA method in non-Kolmogorov turbulence are also summarized.4)The angle of arrival is the angle between the normal of the distorted and undistorted envelope of wavefront phase.A heuristic model of the aperture-averaged angle-ofarrival fluctuation(AF)variance for GLB is proposed,which considers that the AF variance of GLB can be approximated by the linear combination of the AF variance of the plane and spherical optical waves.Numerical simulations are performed to verify the rationality of the heuristic model by phase screen and other means,and also to analyze the influence of various parameters on the AF variance,which indicates that the aperture-averaged AF variance is more sensitive to the change of the outer scale than the inner scale.5)The optics-based metrical techniques are generally adopted for measuring the pathaverage AOP of the entire WLC link.For one given AOP,however,there may be significant differences between the unequal measured values based on different ATE,which is detrimental to the calibration of the WLC channel.A method for synchronous measurement of multiple AOP based on multiple ATE test values is proposed.According to the established models under the uniform refractivity spectral power model,the synchronous measurement of multiple AOP is equivalent to the solution of a dual optimization problem.Combined with the improved swarm intelligence optimization algorithm with higher convergence rate,both simulated data and measured data are used to verify the feasibility and availability of the proposed metrical method,which ensures the consistency of the optics-based metrical method for multiple AOP.The forementioned results of this paper not only embody unique advantages in the application of the unguided medium channel analyzer,but also provide important basis for the development of the hybrid millimeter-wave and infrared-light wireless transceiver.