| Optical vortex beams refer to a specific subset of beams which have particular spiral phase structures and the central singularities.Different from conventional beams,the intensity distribution of the propagated vortex beams is a ring with orbital angular momentum.Because of their distinctive structure,vortex beams have a significant bearing on biomedicine,manipulation of particles as well as optical communications.In recent years,scientific researchers have exploited many methods to generate optical vortices,such as geometric transforming,computer generated hologram,using space light modulator or spiral phase plates.As the researches move along,new kinds of vortex beams have been proposed.In this paper,We do systematical research on the generation and applications of beams with sequence of focused optical vortices.The main points of this paper are as follows:1.Frist,we introduce the current state of researches of vortex beams,which leads to the topic,significance and content of this study.Then we analyze and compare several commonly used methods in vortex beams producing,and present basic features of vortex beams.2.We present new binary diffractive optical elements,called fractal spiral zone plates(FSZPs),which combine a fractal structure and spiral zone plates(SZPs)to generate a sequence of coaxial vortices in the focal volume along the propagation direction.The concepts are introduced and numerical simulations are carried out to verify the properties of FSZPs.In addition,a pilot experiment based on an spatial light modulator(SLM)was performed,and its results were in accord with the simulation results.3.Based on the FSZPs,we proposed modified azimuthal fractal spiral zone plates(AFSZPs)with particular fractal characteristics.The good performance of this fractal diffractive optical elements are verified experimentally based on a digital micromirror device(DMD).As we know,optical vortices are widely used to capture and spin particles.Beams with sequence of focused optical vortices will benefit for the powerful potential applications such as 3D trapping and manipulation,super resolution imaging and lithography. |
