Reasearch On Random Beams And Its Atmospheric Turbulence Transmission Characteristics By Manipulating Correlation Structure

Laser beam propagation through atmospheric turbulence is an important issue that must be considered in the fields of free-space optical communication,Lidar and remote sensing,etc.,The refractive index of turbulence medium is fluctuating randomly on the transmission path,which induces negative effects on laser beam,such as decoherence effect,beam spread,beam wander,angle-of-arrival fluctuations and scintillations.These negative effects seriously restrict the application and development of laser in the field of atmospheric optics.In this paper,we designed a variety of new random beam models,based on composite modulating the spatial correlation function,the polarization and phase of random beams.To deal with the long-distance,high-date-rate free-space optical communication system,the novel models and methods have been put forward in this paper.The negative effects of atmospheric turbulence have been resisted by manipulating the correlation structure,polarization and phase of light beams.Coherence,polarization and phase are the fundamental properties of light beams.Generation of prescribed structure beams with tailored intensity,polarization and phase by means of correlation structure manipulation,and exploration of nontrivial modulation properties have seen a rapid growth of interest.In this paper,The spatial correlation function of a random beam is proposed based on the non-negative definiteness constraint on the cross-spectral density(CSD).Under the conditions of strictly satisfying the physical authenticity of the beams,the radial polarization and the twisted phase can be carried.A variety of new models,i.e.,a radially polarized cosine-gaussian schell model(CGSM)beam,a radially polarized twisted CGSM beam,and a radially polarized array-gaussian schell model beam are proposed herein.The statistical properties of these beams focused by a thin lens are detailed analyzed.The results show that by constructing the source correlation structure,the intensity distributions,such as an array-shaped,a "NUST"-shaped,and an optical cage,have been obtained.Regular non-uniform polarization states distribution is obtained in this paper,which provides a new method of the generation of various vector light field.The propagation of an electromagnetic CGSM beam through an active gradient-index(GRIN)medium is investigated.It is shown that the initial correlation structure has a significant influence on the intensity gain.Furthermore,a radially polarized CGSM beam is experimentally generated and its focusing properties are measured.The experimental results are consistent with the theoretical predictions.For the long-distance and high-date-rate free-space optical communication system,the actual air-to-air or ground-to-air line of sight propagation paths are considered.The transmission characteristics of radially polarized CGSM beams,twisted array beams,and tunable multi-Gaussian vector beams in anisotropic and Non-Kolmogorov atmospheric turbulence are studied.Analytical formulas for the cross-spectral density matrix elements of these beams in turbulence are derived.The statistical properties are analyzed in detail,such as the spectral density,the degree of coherence,the degree of polarization and the state of polarization.It is found that the second-order statistics are greatly affected by the source correlation function,polarization and phase,which means these novel beams are insensitive to the anisotropy and Non-Kolmogorov of turbulence,and the change in the turbulent statistics induces relatively small effect.Analytical formulas for the Wigner distribution functions and the second-order moments(SOMs)of a twist Gaussian Schell-model(TGSM)beam propagating through anisotropic turbulence have been derived by means of a tensor method.It is found that the propagation law for the SOMs of a TGSM beam spreading in turbulence can be described as a first-order optical systems propagation law with an additional turbulence-induced effect Based on the SOMs,second-order statistics in terms of the effective beam width,the Mi-factor and the orbital angular momentum are analyzed in detail.The method can be extended to study the propagation characteristics of various complex Gaussian beams.It is worth noting that the turbulence-induced effects can be restrained by optimizing the initial beam parameters when an optical signal transmission application is not very far.For an anticipated atmospheric channel,a comprehensive selection of initial optical signal parameters,receiver aperture size and receiver capability,etc.,is necessary.Laser beams induce scintillations when propagating in the turbulence,and scintillations saturation occurs in strong turbulence.By use of the Coherent-mode representation of the twisted random beams,it is shown that a random beam carrying twisted phase can effectively reduce turbulence scintillations compared to a coherent single mode laser.A tensor method is used to calculate the scintillations of a twisted array random beam in weak and extremely strong turbulence,and the influence of each initial optical parameter on the scintillations is detailed analyzed.The researches in this paper promote important supports in free-space optical communications,Ladar and remote sensing.