Theoretical And Experimental Researches On Optical Phase Dislocations In Oceanic Turbulence

In the fields of free-space optical communication and turbulence optical detection,it is essential to investigate the propagation characteristics of light.When the phase distortion of the beam caused by turbulence is strong enough,phase discontinuities,namely phase dislocations,appear in the optical field.Structured beams with discontinuous phases are widely applied to enhance the resistance of a beam to turbulence and measure the parameters of turbulence.Different forms of phase dislocation distributions will affect the propagation characteristics of the beams,and different conditions of turbulence will influence the distributions of phase dislocations in the beam.Therefore,the researches about the propagating characteristics of the beams containing phase dislocations in turbulence offer basic guidances for the construction of structured beams and the measurement of the statistic properties of turbulence.It is shown that the propagating characteristics of the beams containing various types of phase dislocations in free space and random media have been extensively discussed,while the interactions between different types of phase dislocations are not well understood when the beams propagate through turbulence yet.In order to explore the law of the interaction,based on the theory of light propagation through random media,the propagating properties of the beams containing phase dislocations with different combinations in weak turbulence have been studied and the relation between different phase dislocations and scintillation has been discussed.For further investigations of the relation between the second-order and fourth-order statistical properties of a beam in turbulence and phase dislocations,it is essential to measure the phase singularities accurately.In this thesis,a phase singularity measurement method based on dual shearing-type Sagnac interferometers is proposed,which realizes the simultaneous measurement of the positions of several randomly distributed screw dislocations.Due to the short underwater transmission distance of laser,when a large number of phase singularities in the beam are induced by turbulence,the correlation of the refractive index of turbulence in the optical transmission direction should be considered.To introduce the correlation,based on the sparse spectrum method,the impact of turbulence on multiply degenerate screw dislocations beams under the condition of perfectly correlated refractive index has been studied.First,we realized the combination of different types of phase dislocations through partially overlapping two coherent Laguerre-Gaussian beams with ±1 topological charges and adjusting the relative phase between two beams,then we investigated the distribution of the scintillation index of the beams through weak oceanic turbulence.It has been shown that the coexistence of screw dislocations and an infinitely extended edge dislocation induces scintillation enhancement by means of theoretical analysis and the phasescreen-based beam propagation simulation.To clarify the cause of this phenomenon,we measured the distribution of the scintillation index of the combined beam propagating through weak air turbulence.Comparing the distribution of scintillation index and the phase map measured by reference-wave-based method,the result clarifies that anisotropic screw dislocation can induce scintillation enhancement.The distributions of the scintillation index of a beam containing a screw dislocation with different anisotropies has been experimentally investigated.The results show that the larger anisotropy of the screw dislocation results in the larger peak value of scintillation enhancement.Second,based on the measuring principle of a multi-pinhole interferometer,we proposed a phase singularity measurement method based on dual shearing-type Sagnac interferometers,and the spatial positions of phase singularities with different densities in speckles were measured by this method.Compared with the spatial positions of phase singularities measured by the reference-wave-based interferometry,this method can realize the position measurement of phase singularities with highly precision.Although the anisotropy of screw dislocations will reduce the signal-to-noise ratio of the measurement results,this setup is still suitable for simultaneous measurement of a large number of screw dislocations with different densities.In addition,we have verified that the proposed method can realize the synchronous measurement of the spatial positions of the phase singularities in a beam propagating through oceanic turbulence by simulation as well.Finally,we have studied the impact of oceanic turbulence with different strength on the propagation of multiply degenerate screw dislocations beams by means of the sparse spectrum method.In order to verify the validity of the sparse spectrum method,we have studied the impact of turbulence on the orbital angular momentum carried by the beam when the refractive index of turbulence is correlated in the optical transmission direction,and the theoretical results are the same as those obtained by the traditional method.Based on that,we have investigated the impact of perfectly correlated turbulence on the orbital angular momentum carried by the beam.The results show that the introduction of the correlation of refractive index of the turbulence in the direction of beam transmission reduces the impact of turbulence on the orbital angular momentum carried by the beam.This effect is only related to the propagating distance,and has nothing to do with the strength of turbulence and beam wavelength.The research on the phenomenon of scintillation enhancement and anisotropic screw dislocations in this thesis helps people understand the fundamental characteristics of the propagation of the beams containing phase dislocations in random media.Furthermore,this work is significant for beam structure selection in the free-space optical communication and measurement of the parameters of oceanic turbulence in the field of optical detection of turbulence.As a new type of phase singularity measuring setup,the dual shearing-type Sagnac interferometer is used to measure the distribution of branch points generated when a beam propagates in moderate-to-strong turbulence and the distribution of phase singularities in speckle caused by strong scattering media.Using the sparse spectrum method to study the influence of perfectly correlated turbulence on the orbital angular momentum carried by the beam is helpful to investigate the propagating characteristics of the beams containing screw dislocations in deep oceanic turbulence.