Temporal And Spatial Coherence Properties Of Ultrafast Laser Speckle Fields And The Study Of Vorticity

Speckle is a random light intensity distribution formed when random light waves are scattered by rough random surface.The presentation of random light intensity and the study of the statistical properties of speckle are always the focus of research.From the analysis of the cause of speckle formation,the correlation of illumination light field and the random surface characteristics directly determine the properties of the speckle formed,but most of the current researches on speckle are confined to the illumination range of ordinary continuous light waves.Under the illumination condition of the traditional continuous laser light source,the time difference of the time variation of laser light wave and the time difference of the corresponding light propagation when passing through the scattering body is much slower than that of the traditional continuous laser light source.The dynamic variation form of speckle intensity produced by light wave at one point in space is basically consistent with the change of light intensity.In recent years,the rapid development of ultra-fast laser provides a new platform for the study of material changes in the objective world.The study of ultra-fast laser speckle field is an important frontier subject in the field of ultra-fast laser propagation and random light field.It has important application prospects in the field of ultra-fast dynamic physical process and ultra-fast interferometry.The important topic of the thesis has good scientific significance and potential application value.In this dissertation,the history and development of femtosecond laser propagation and some related light speckles are systematically and deeply summarized.Innovative results have been obtained in the following aspects: using the Hopkins formula of the surface light field with temporal coherence and its propagation,combining with the statistical properties of light waves in random media,The autocorrelation function of some coherent speckles on the observation surface under ultra-fast laser illumination is derived,and the ultra-fast laser speckle with obvious radial extension on the observation surface is obtained by using ultra-fast laser to illuminate the scatterers with different roughness.The interferogram of light field is recorded by Mach Zehnder interference system,and the radial extension of phase vortex is studied by using the Fourier transform of interferogram to extract the speckle field of ultra-fast laser.The causes of ultrafast laser speckle and its phase are analyzed.This article is divided into four chapters:In the first chapter,we give a comprehensive overview of the concept of speckle and its producing conditions,and then we discuss the classification of speckle and the theory of phase vortex and phase singularity.Some differences between femtosecond laser and conventional continuous light are described in detail.In this paper,two characteristics of the intensity distribution of the speckle,the coherence of the incident light and the characteristics of the rough random surface causing the scattering,are summarized in detail,and how these two aspects cause the distribution of the speckle is explained in detail.Then we discuss some characteristics of radial stretching of monochromatic light with different frequencies which are composed of polychromatic light through random diffractors.And the dependence of autocorrelation function of multi-color speckle and characteristic function of describing random surface roughness.In the second chapter,we obtain the speckle pattern of ultra-fast laser by designing experiments,and by controlling the light source,we obtain the speckle field and the speckle field formed by continuous light under the condition of the same scatterer and the same roughness.A series of ultrafast speckles are compared with the common continuous speckles,which are studied and introduced in detail from the point of view of speckle statistical theory and the coherence of light.For the fiber radiative structure in the ultrafast speckle field,The radial distance to the center of the speckle field is increased and the speckle particle is obviously stretched and the radiative distribution of the whole speckle plane from the center to the outside is analyzed theoretically in detail.The reason is that with the increase of radial distance from the center to the outside of the speckle field,the correlation of speckle decreases gradually.In the third chapter,we obtain the pattern of ultra-fast speckle interference field by setting up experimental light path,and obtain the nonlinearity under femtosecond pulse and ordinary continuous light irradiation under different roughness by controlling the light source and changing the scatterer.The same speckle pattern.The phase distribution of ultrafast speckle interference field is extracted successfully by Fourier transform method,and the difference between the speckle interferogram and the continuous light pattern produced by ultrashort pulse irradiation is obtained.For example,there are interference fringes in the middle part of the ultrafast speckle interferogram,which is completely different from the speckle pattern formed by the continuous light passing through the scattering body.This is due to the spatial correlation and partial temporal correlation caused by the broadening of femtosecond laser through the scatterer.Because of the special characteristics of ultralfast pulses,the speckles formed by ultra-fast pulses passing through rough random surfaces show a distribution of radiative structures from the center to the outside.On this basis,a series of tensile properties also occur in the distribution of ultra-fast speckle interference fringes produced by reference light.This feature can be observed intuitively by the phase diagram of the ultrafast speckle interferogram.The fourth chapter,we summarizes the main achievements and innovations of this dissertation,and briefly introduces the further research work to be carried out.