Measurement Of Rotational Doppler Effect Based On The Partially Coherent Vortex Beam

With the development of the research on the orbital angular momentum(OAM)characteristics of the vortex beam,the rotational Doppler effect associated with light’s OAM has been found,according to the Doppler frequency shift generated by the vortex beam incident on the surface of the rotating target,the rotational speed of rotating target can be calculated directly,which is of great significance to the optical remote sensing rotating target detection.However,the application of fully coherent vortex beam in remote sensing detection is easily affected by the atmospheric environment,compared with the fully coherent beam,the partially coherent beam is less affected by the turbulence,the longer propagation distance,the more obvious.Partially coherent vortex beam is a kind of partially coherent beam with OAM characteristics,compared with the fully coherent vortex beam or the general partially coherent beam,it has lower beam scintillation,wander and stronger self-repairing ability,which has the potential applications in optical remote sensing rotating target detection.Therefore,it is of practical significance to study the measurement of rotational Doppler effect of partially coherent vortex beams.The main contents of this thesis includes: in view of the generation and the characteristics of OAM of partially coherent vortex beams,the influence of the parameters of non-Gaussian random rough surface on the distribution of vortex beams after passing through its surface is discussed;combine with the double slit interference method for measuring the topological charge of vortex beams,the influence of slit parameters on the interference fringes of partially coherent vortex beams is studied;in the measurement of rotational Doppler effect of vortex beam,analyzed the speckle field and OAM characteristics of the partially coherent superposition state vortex beam based on the Goodman’s speckle theory and the OAM spectrum theory.The main work is as follows:1.Based on angular spectrum representation,the influence of the parameters of nonGaussian random rough surface,i.e.direction autocorrelation length,kurtosis,skewness and root mean square(RMS)roughness,on the distribution of the vortex beam field after passing through it is studied.The intensity distribution of vortex beam after passing through ground glass with different roughness(static ground glass)was collected by CCD.It is found that except the RMS roughness,the other parameters have little effect on the optical field distribution.When the autocorrelation length,skewness and kurtosis of non-Gaussian rough surface are 20 mm,0.001 and 6 respectively,and the RMS roughness is greater than 0.12 mm,the intensity of Laguerre Gaussian beam passing through the non-Gaussian random rough surface will no longer maintain the hollow distribution and the dark spot in the center of the vortex beam will disappear with the decrease of the grit of the ground glass in experiment.2.The topological charge of partially coherent vortex beams is measured in the near field by using the double slit interference method.The interference fringes of double slits reflect the coherence and topological charge of partially coherent vortex beams.Based on the cross spectral density function of partially coherent beam,combined with young’s double slit interference theory and experiment,the influence of different double slit separation on the interference fringes of partially coherent vortex beams in the near-field of the laboratory is analyzed.The results show that when the initial coherent length of the partially coherent vortex beam is 5 mm and the width of the beam spot at the cross section of the double slit is 5 mm,the distance between two slits is more than 1.5 mm,the interference fringes produced by double slits are better.In the experiment,by analyzing the interference fringes of the partially coherent vortex beams with different double slit separation,it can be seen that the interference fringes of the partially coherent vortex beams are better when the two slit separation is larger than the dark spot size of the beams.3.Based on Goodman’s speckle theory and OAM spectrum theory,the relationship between spatial coherent length(ground glass rotation speed),topological charge and speckle field of superposition state vortex beam is studied.The effect of the rotational speed of the ground glass on the OAM of the superposition state vortex beam is discussed.The results show that the speckle generated by the superposition state vortex beam after passing through the rotating ground glass gradually disappear with the increase of the rotating speed of the ground glass,and the corresponding OAM spectrum become more dispersive,and the more unstable the generated partially coherent superposition state vortex beam.The results of this thesis may provide some reference for the application of partially coherent vortex beam in rotational speed measurement and the optical remote sensing of target rotational speed detection.