Research On Tight Focusing And OAM Properties Of Partially Coherent Vector Vortex Beams

Recently,a new type of spatially structured beam named vector vortex beams with both anisotropic spatial polarization state and helical phase distribution was born through the joint regulation to basic parameters of optical field such as phase and polarization state.Due to the unique polarization and phase characteristics,the beam focused by a high numerical aperture lens may also generate more novel orbital angular momentum(OAM)properties,which has gradually attracted the attention of researchers.Compared with the fully coherent beams,the partially coherent beams are less affected by the turbulent atmosphere.Therefore,the OAM of focused field for partially coherent vector vortex beams has greater application potential in the fields of space remote sensing detection and optical communication.In this paper,theoretical and experimental studies are conducted on the tight focusing and OAM properties of partially coherent vector vortex beams.The main work is as follows:(1)Combined with the partial coherence and vector diffraction integral theory,the distribution characteristics of tightly focused light intensity and OAM density for partially coherent radially polarized vortex beams carrying superposition topological charges are studied.The effects of the coherence length for incident beam and the numerical aperture for focusing lens on the light intensity distribution,the OAM density distribution and OAM of longitudinal component are investigated.The researches find that the intensity and OAM density distributions of focused field are petal-like.With the increase of the coherence length,the light intensity,the OAM density and OAM of longitudinal component all increase.When the coherence length increases to 0.5cm,the change of coherence length no longer affects them.As the numerical aperture increases,the light intensity does not change significantly,both the OAM density and OAM of longitudinal component become larger with the increase of the numerical aperture.(2)According to the Hybrid-order Poincaré sphere vector vortex optical field distribution described by the superposition of orthogonal left-handed and right-handed circularly polarized vortexes,the experimental program for generating vector vortex beams is designed based on the reflective spatial light modulator.The experimental optical path is built.The vector vortex light spots at the focal plane of different topological charges,polarization orders and polarization states are obtained by changing the phase distribution of left-handed and right-handed components loaded on the spatial light modulator.The light intensity distribution characteristics of focused field for vector vortex beams are analyzed.(3)The Hybrid-order Poincaré sphere vector vortex beams are constructed by the lefthanded and right-handed circularly polarized vortexes.Based on the vector diffraction integral theory,the OAM density distribution properties of longitudinal component after the beam is tightly focused are numerically calculated and analyzed.The effects of the topological charge and polarization order for vector vortex beams,as well as the initial phase difference for left-handed and right-handed components on the OAM of focused field for longitudinal component are studied.The researches find that the OAM density distribution of longitudinal component presents the inner and outer two layers on the whole,concentrating the corresponding left-handed and right-handed components,respectively.Meanwhile,the topological charge signs of the lefthanded and right-handed components determine the positive and negative distributions of the inner and outer OAM density layers,respectively.The OAM of the longitudinal component is independent with the initial phase difference of left-handed and right-handed components.Both the topological charge and polarization order of vector vortex beams affect the OAM.When the topological charge is greater than 2,the OAM increases with the increase of the polarization order.The results may provide references for the applications of partially coherent vector vortex beams and their OAM characteristics in the fields of space target detection and optical communication.