Study On The Properties Of Several Beams Propagating Through Random Media

Light propagation through random media always attracts attention,especially in the field of laser communication,laser ranging,laser remote sensing and laser radar,the influence of random turbulent medium on the transmission characteristics of various beams is prominent and inevitable.Due to the the fluctuation of the refractive index induced by turbulence,the irregular fluctuation will be produced in the amplitude and phase of the beam,and the intensity scintillation,beam wander and wavefront distortion will appear.The nature of the light beams and the statistical laws of the turbulent medium involved in the actual application are different,which directly affect the performance of the various optics systems,even determine the success or failure of the relevant technology application.Therefore,in-depth study of the propagation properties in random media and awareness of the interaction mechanism between turbulent media and beams is a necessary precondition for evaluating the negative effects of turbulence and exploring the technical means of suppressing turbulence.The transmission characteristics of several different types of beams in random media are studied in this paper,and theoretical analysis and numerical simulation methods are developed to provide theoretical basis for its practical application.Using the orbital angular momentum as an optical signal carrier is promising in next-generation optical communication.Exploring the technology of suppressing divergence of orbital angular momentum mode is a priority.Based on the Rytov theory and the Kolmogorov spectrum,the orbital angular momentum spectrum of a rotationally symmetric vortex beam is investigated,and the mathematical relationship between the amplitude modulation and the receiving probability of orbital angular momentum spectrum is discovered.Then the orbital angular momentum spectrum of the flat-top Bessel Gauss beam propagation through turbulence is analyzed,and the ability in mitigating turbulence effect is compared with the Bessel Gaussian beam.Especially,the effects of the beam order,transvere parameter,wavelength and refractive index structure constant on the mode detection probability are stressed.At last,the orbital angular momentum spectrum of the multi-wavelength vortex beam is investigated.Taking the Laguerre Gaussian beam as an example,the influences of the number of wavelengths and the beam width on the detection probability of the signal mode are discussed.The second order moment of the beam helps us comprehensively analyze the characteristics of the beam.Based on the generalized Huygens-Fresnel principle and the Wigner distribution function,the optical statistic properties,such as beam width,angular width,beam quality factor,Rayleigh length and equivalent radius of curvature of the radially polarized partially coherent light in non-Kolmogorov turbulence are studied in detail,the influences of turbulence power exponent and refractive index structure constant on beam characteristics are discussed.Recently,more and more attention is paied to the partially coherent beams with special correlation functions due to their unique optical properties.Based on the condition for devising the correlation functions,the general analytical expression for the second order moment of Schellmodel beams with any correlation functions propagation through turbulence is derived.At the same time,the second order moment for Schell-model beam under construction rule,such as the multiplication and convolution construction rule is also derived.Finally,the beam quality factor of the power-law Schell-model beam propagation through ocean turbulence is explored.The influences of ocean parameters,power exponent and coherence radius on beam quality are discussed.The elliptical vortex acts as an unconventional vortex,its evolution behavior under turbulence has not attracted much attention.Based on the generalized Huygens-Fresnel principle,the spectral density,degree of coherence and the coherent vortex of partially coherent elliptical vortex under anisotropic non-Kolmogorov turbulence are studied.The coherent vortices,an important feature of the partially coherent vortex beam,are stressed.The influences of the elliptic rate,coherence length and turbulence parameters on the conservation distance are analyzed,and the selection rules of reference points in the partially coherent elliptical vortex beam are summarized.Partially coherent beam can carry not only vortex phase but also the twisted phase.Twisted phase modulation is performed on the background of the partially coherent elliptical vortex beam,and the beam wander under the twisted phase modulation is studied.It is discovered that a saturation region exists in the twisted phase modulation and the coherent modulation.Outside the saturation region,twisted phase modulation can effectively suppress beam wander.The intensity scintillation index determines the signal-to-noise ratio and bit error rate of the optical communication system.Suppressing the intensity scintillation is one of the fundamental purposes to investigate propagation properties in random media.An important method for turbulence mitigation is the use of partially coherent beam.Based on the Split-Step beam propagation method,Monte Carlo turbulence phase screen and modified phase screen of the partially coherent beam,the simulation of the new partially coherent beam is realized.In order to reduce the operation time,the pseudo-partially coherent beam is introduced to replace the fully developed partially coherent beam.Taking the Bessel-Gaussian Schell-mode beam as an example,the intensity scintillation index in the strong fluctuation region is stressed.In addition,the simulation provides a guiding role to investigate other partially coherent beams with special correlation functions.