Propagation Properties Of Vortex Beam Under Oceanic Turbulence

With the continuous expansion and extension of human production and life,scientific exploration and military applications from atmospheric space to oceanic space,underwater communication system is facing the pressure of increasing information transmission.The vortex beam carrying orbital angular momentum(OAM)can realize the transmission of multiplex data streams in the same underwater environment,which will become an effective way to improve the capacity and bandwidth efficiency of the underwater wireless optical communication system.And it will open up a new developmental direction for the continuous expansion of underwater wireless optical communication.However,the vortex beam will inevitably be disturbed by oceanic turbulence when it is transmitted through underwater environment that will severely degrade the communication quality of the OAM-based underwater wireless optical communication system.Therefore,the research on the propagation properties of vortex beam in oceanic turbulence is of great significance for the design and application of OAM-based large-capacity,long-distance and ultra-high-speed underwater wireless optical communication links.The main work and results are as follows:(1)Propagation properties of off-axis vortex beams in oceanic turbulence.Based on the generalized Huygens Fresnel principle,the evolution of the intensity distribution and the spectral coherence of an off-axis hollow Gaussian-Schell model vortex beam with self-focusing characteristics in anisotropic oceanic turbulence is analyzed.The numerical simulation results show that the increase of position factor is beneficial to the focusing distribution of off-axis hollow Gaussian Schell model vortex beams in various anisotropic oceanic turbulence environments.When the beam propagates in oceanic turbulence,its intensity distribution expands and gradually loses the central structure of the dark space,and finally evolves into a Gaussian-like beam in the far field.The larger the hollow order,the topological charge,the transverse coherence length and the waist of the beam’s parameters are,the shorter the wavelength is,the longer the distance required for the beam intensity distribution to evolve into a Gaussian like beam is.The smaller kinetic energy dissipation rate per unit fluid mass,inner scale factor,anisotropy factor and the larger mean square temperature dissipation rate,temperature salinity contribution ratio will accelerate the expansion of beam intensity distribution and the attenuation of spectral coherence.(2)Propagation properties of flat topped vortex beams in oceanic turbulence.A new theoretical model of Hankel-Bessel flat topped vortex beam generated by multi-Gaussian Schell-model source is achieved.Based on the Rytov approximation theory,the OAM mode detection probability model of multi-Gaussian correlated Hankel-Bessel flat topped vortex beam propagating in anisotropic oceanic turbulence and the channel capacity model of multi-Gaussian correlated Hankel-Bessel flat topped vortex beam system in anisotropic oceanic turbulence are constructed.The numerical simulation results show that larger kinetic energy dissipation rate per unit fluid mass,internal scale factor,anisotropy factor,and smaller mean square temperature dissipation rate,temperature salinity contribution ratio significantly improve the detection probability and channel capacity.The beam with longer wavelength,transverse coherence length,and smaller beam order is conducive to improving the propagation performance of the beam in various anisotropic oceanic turbulence.In the same oceanic turbulence environment,the multi-Gaussian correlation Hankel-Bessel flat topped vortex beam has stronger oceanic turbulence interference resistance in comparison with Airy vortex beam,and the former is more suitable for the information carrier as underwater wireless optical communication system.(3)Propagation properties of non-diffraction vortex beams in oceanic turbulence.Based on Rytov approximation theory,the OAM mode detection probability models of Airy vortex beam,power exponent phase vortex Airy beam and Whittaker-Gaussian vortex beam propagating through oceanic turbulence are obtained.The numerical simulation results show that the beam source parameters with smaller topological charge and longer wavelength can alleviate the influence of oceanic turbulence on the beam propagation and obtain a larger detection probability.Meanwhile,increasing the main ring radius of the airy vortex beam,the power exponent of the power-exponential-phase vortex Airy beam and the waist radius of the Whittaker-Gaussian vortex beam is also helpful to improving the detection probability.When the beams propagate in the anisotropic oceanic turbulent environment with larger kinetic energy dissipation rate per unit fluid mass,inner scale factor,anisotropy factor,and smaller mean square temperature dissipation rate,temperature salinity contribution ratio,the higher signal OAM mode detection probability can be obtained.(4)Propagation properties of vortex beam in oceanic turbulence under random phase screen model.Combined with the refractive index power spectrum and power spectrum inversion method of oceanic turbulence,the random phase screen model characterizing the characteristics of oceanic turbulence is constructed,and a numerical calculation method for oceanic turbulence is established to analyze the influence of different ocean turbulence and beam parameters on the OAM mode detection probability of hypergeometric-Gaussian vortex beam through oceanic turbulence.The results show that the influence of oceanic turbulence on the Hypergeometric-Gaussian vortex beam decreases with the increase of kinetic energy dissipation rate per unit fluid mass,and increases with the increase of mean square temperature dissipation rate and temperature salinity contribution ratio.Hypergeometric-Gaussian vortex beams with smaller topological charge and longer wavelength can resist the interference of oceanic turbulence.At the same time,the theoretical derivation results are compared with the numerical calculation results.That the two methods are highly consistent with each other verifies the effectiveness of using the random phase screen model of oceanic turbulence to analyze the propagation properties of vortex beams in oceanic turbulence.It provides an effective way to study the beam propagation properties in oceanic turbulence under difficult or impossible theoretical derivation.