Diffraction Properties And Speckle Characteristics Of The Vortex Beam

Vortex beams have attracted extensive attention due to their phase particularity and multi-dimensional controllable vector light field characteristics.Because of the unique helical phase,vortex beams have great potential applications in optical communication,quantum information,optical control,sensing and measurement.However,there are still some shortcomings in the study of vortex beams,so there are still many unsolved scientific and technical problems.For example,its generation scheme is more complex and its application in the sensing field is less.Therefore,we have carried out studies on the diffraction properties and speckle characteristics of vortex beams,including the generation of vortex beams based on fibers and silicon-based waveguides,the diffraction characteristics of vortex beams passing through apertures,and speckle characteristics of the vortex beams formed by roughness surface and turbulence.The main works of this dissertation are as follows.1.A vortex beam generator based on chiral fiber is proposed according to waveguide mode theory and coupling mode theory.By introducing a chiral coupling structure as perturbation,an optical orbital angular momentum modulator based on a multimode fiber structure is designed,and its modulation rule for orbital angular momentum mode is given.Through theoretical analysis and numerical simulation,the relationship between fiber structure and beam quality as well as mode loss is obtained.A silicon vortex beams generator based on ring resonator and grating is studied.The effects of structure on the mode conversion efficiency and mode purity are analyzed.2.The relationship between the topological charge and the diffraction field distribution of the Laguerre-Gaussian beam(LG beam)diffracted by different apertures is analyzed by diffraction theory.The topological charge invariance of the vortex beam diffracted by circular apertures is obtained.The relationship between the radial phase of LG beam and the major focus of the Fresnel zone plate is theoretically analyzed,and the focal red shift phenomenon of the focused LG beam by the Fresnel zone plate is found.The relevant results are verified through experiments.At the same time,it is proved that the focus characteristic is independent of topological charge.3.The speckle characteristics of vortex beam scattered by the rough surface are studied and compared with the Gaussian beam.It is concluded that the change of the LG beam speckle is more sensitive than the Gaussian beam under minor roughness.The adjustable roughness surface based on the spatial light modulator is designed,and related experiments are carried out.Through the analysis of the speckle pattern,the results that are consistent with the theory are obtained.The feasibility and advantages of measuring surface roughness by LG beam speckle are verified.4.The LG beam is used to study the speckle characteristics of vortex beams propagating in atmospheric turbulence,and the influence of atmospheric turbulence on LG beams with different topological charges and wavelengths is analyzed.According to the theory of atmospheric turbulence,a hot-air turbulence simulation device is designed and produced,and the calculation method of the atmospheric coherence length inside the device is given.Based on this,the speckle changes of LG beams under different intensities of atmospheric turbulence are analyzed.It is consistent with the theoretical results.It proves that the average size and contrast of speckles vary with the coherence length of atmospheric turbulence and verifies the feasibility of LG beams measuring the atmosphere turbulence.