| With the depletion of terrestrial resources,the exploration of ocean resources and ocean military research work become more and more frequent.It is an urgent problem for whole world to construct long distance,big data,high speed,secret ocean communication systems and sensing network.Compared with ocean electromagnetic communication and ocean acoustic communication,ocean wireless optical communication has the advantages of wide frequency band,large information capacity,fast data transmission rate,low delay,good confidentiality and low energy consumption.In particular: ocean wireless optical communication system of the vortex beam with orbital angular momentum(OAM)as the signal carrier has higher channel capacity,spectrum efficiency and confidentiality.In addition,under the condition of no heat halo,ultrashort vortex pulsed beam with high power can alleviate the signal transmission loss in ocean channels.However,oceanic turbulence can cause the light intensity fluctuation,beam diameter extension,beam center jitter and other effects,which reduces the communication quality of ocean wireless optical communication system.Therefore,it is very necessary to study the propagation characteristics of continuous vortex beam or pulsed vortex beam in oceanic turbulence,which effectively optimizes the performance of ocean wireless optical communication system.For this reason,this paper has carried out the following innovative research work aiming at the above problems:1.Establishment of a new oceanic turbulent refractive index power spectrum.By introducing scale-limited outer scale factors of oceanic turbulence,we established a new oceanic turbulent refractive index power spectrum from low wavenumber to high wavenumber region under unstable stratification.The results of the new spectrum simulation are also proved to be more consistent with the experimental data.The new spectrum not only eliminates the error of the stable stratification hypothesis,but also solves the singularity problem in the null space wavenumber and the deviations problem from the experimental results in the low wavenumber region.Considering the presence of anisotropic oceanic turbulence,we further generalized the new oceanic turbulent spectrum to the asymmetric oceanic turbulent refractive index power spectrum.2.Mechanism research of the influence of oceanic turbulence on optical wave propagation.Based on the Rytov approximation principle,the average polarizability model in received aperture of Gaussian Schell-model quantized beams through anisotropic weak oceanic turbulence was established.It is concluded that the interference effect of oceanic turbulence on Gaussian Schell-model quantized beams can be controlled by setting the receiver aperture reasonably.In addition,based on the new oceanic turbulent refractive index power spectrum,the analytical expression of scintillation index of Gaussian beam was also derived.It is concluded that,under the stable stratification hypothesis,oceanic turbulencedominated by the salinity fluctuations underestimates the influence of the diffusion of salinity on the scintillation index,and oceanic turbulence dominated by the temperature fluctuations overestimates the influence of the diffusion of salinity on the scintillation index;the off-axis scintillation index of Gaussian beams can be significantly reduced by finite outer scale.Finally,the analytical expressions of the wave structure function and spatial coherence radius of Gaussian beams propagation through oceanic turbulence with outer scale were derived.The mathematical model of pointing error and the channel capacity model with pointing error in oceanic turbulence were also established.The influence of oceanic turbulence parameters and light source parameters on pointing error and channel capacity was studied.3.Establishment of the propagation model of Hermite Gaussian vortex beam in oceanic turbulence.Based on angular spectrum theory,a new type of Hermite Gaussian vortex beam in homogeneous medium was proposed and the received probability density model of Hermite Gaussian vortex beam carrying OAM states propagation in oceanic turbulence was established.It is concluded that,in the case of long-distance communication,Hermite Gaussian vortex beam than Laguerre Gaussian beam has a stronger anti-interference ability to resist oceanic turbulence;the effect of anisotropic oceanic turbulence on the received probability of signal OAM states is weaker than the effect of isotropic oceanic turbulence.4.Establishment of the received probability model of the OAM states of Gaussian pulsed X wave in anisotropic oceanic turbulence.By developing two-frequency mutual coherence function of the vortex pulsed beam,the influence of oceanic turbulence and source parameters of Gaussian pulsed X wave on the received probability of signal OAM states and the pulse broadening was studied.These results demonstrate that the large initial half-pulse width,large initial beam waist radius and small Bessel cone angle of the Gaussian pulsed X beams have a stronger resistance to oceanic turbulent interference.In addition,the propagation of the Gaussian pulsed X wave with small OAM quantum number generates the broad bandwidth and the large received probability of signal OAM states,which can realize the high capacity communication.5.Derivation of the analytical expression of special coherence function and establishment of the received probability model of OAM states for ultrashort Laguerre Gaussian pulsed beam.Based on the orthogonal characteristics of the Laguerre function and the harmonic function,the pulsed OAM eigenfunctions were developed.Then the received probability distribution model of OAM states of ultrashort Laguerre Gaussian pulsed beam was established.It is concluded that the larger transmission distance and the smaller OAM quantum number cause the smaller pulse delay.In addition,the vortex pulsed beam in shallow-sea regions dominated by temperature has a better transmission performance than that in deep-sea regions dominated by salinity.6.Establishment of the received probability model of the Lommel-Gaussian pulsed beamcarrying OAM states in asymmetric oceanic turbulence.Using the weighted orthogonal characteristics of Bessel function,the establishment method of pulsed OAM states eigenfunction and the new oceanic turbulent refractive index power spectrum,an analytical expression of the complex phase perturbation of spatiotemporal wave that contains the outer scale and the asymmetric factors of weak oceanic turbulence was derived.Then the received probability model of signal OAM states for Lommel-Gaussian pulsed beams propagation through asymmetric oceanic turbulence was established.This model shows that Lommel-Gaussian pulsed beam carrying large OAM quantum number has a good anti-turbulence interference characteristic and realizes more information capacity communication. |
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