Diffractive Focusing Effect Of Light Field And Its Application In Topological Charge Measurements

Optical field focusing is very important in the research of science and technology.There has been continuing interest in focusing optical waves within or beyond the diffraction limit.The diffraction phenomena of the light wave field have long been discovered,and it is widely believed that light and other waves will expand when they pass through an obstacle.However,recent studies have shown that light waves have the effect of diffractive focusing when they encounter obstacles.This will be very useful in the light beam micro-focusing,because the method is very simple and straightforward without the need to etch all kinds of complex structures.The vortex beam is a new type of beam with special helical wavefront structure.It has fixed topological charge and the phase singularity.Because of its unique properties,it has important application value in information coding,photon communication and biomedicine.The topological charges of vortex beam are an important property of the vortex beam,which is closely related with the orbital angular momentum.Therefore,it is very important to measure the topological charge of the vortex beam.In this paper,a new method of measuring topological charge is proposed based on the diffractive focusing effect of the optical field.This paper studies the diffractive focusing effect of one-dimensional and two-dimensional optical field,and the topological charge measurement based on it.It mainly includes the following aspects:1.The one-dimensional diffractive focusing effect of space optical field is introduced,which is to observe the diffractive focusing phenomenon of planar light waves through the slit.It is shown that optical plane waves first focus and only then expand.The demonstrations would advance our understanding of the conventional wisdom that a wave would become expanding when passing through a slit.And find that the focus of the maximum intensity is about 1.8 times of the incident light intensity.The focal length will increase when the slit width increases,so it can be modulated by slit.2.The focusing of one-dimensional slit is extended to the focusing of two-dimensional sharp-edge aperture.Firstly,the diffractive focusing properties of planar light waves through the rotationally symmetric sharp-edge aperture are studied.Study found that the beam will produce diffractive focusing effect after the round aperture,hexagonal aperture,square aperture and the triangle aperture,and it is concluded that the characteristics of diffractive focusing are closely related to the size of the aperture and its symmetry.Focusing light intensity of symmetry degree highest circular aperture is the largest,which is about four times of the incident light intensity.The larger the aperture diameter,the greater the focal length.Finally,unsymmetrical aperture and the thickness factors are analyzed.3.We firstly introduce the basic principle of vortex beam,and then introduce several methods of measuring the topological charge,finally introduce a new method to measure the topological charge of vortex beam based on the diffractive focusing effect of optical field,namely,the off-center light beams irradiate the aperture.After analyzing the splitting of the phase singularity of the beam,we can know the topological charge is equal to one add the number of dark core formed by splitting,and the sign of topological charge is determined by the splitting direction of singularity.