The Theoretical And Experimental Study Of Double-vortex Beams

Beams with spiral phase wave-front are regarded as vortex beams. In1992, Allenet al. proved that vortex beams carry the orbital angular momentum (OAM), and theorbital angular momentum can be transferred to particles and rotate them. In addition,encoding data by the OAM of vortex beam is proposed, which may have potentialapplications in free space optical communication, etc. Because of its wideapplications, vortex beams were investigated quite intensively in recent years. Inparticular, double-vortex beam which carries two vortices have attracted muchattention because the beam can improve the data encoding capacity. On the otherhand, the measurement of OAM is of great significance. Measuring OAM of vortexbeam by different apertures has been developed. OAM of vortex beams can bedetermined by the diffraction pattern which is dependent on the OAM of the vortexbeams and the shape of the aperture.In this thesis, we will study the propagation characteristics of double-vortexbeams with different topological charges and perpendicular polarizationorientations. And to further explain orbital angular momentum information ofvortex beams, we study the far field diffraction pattern of vortex beamsformed by a rhombus aperture. In addition, the beams with both the radialpolarization and non-diffracting properties are also investigated. The maincontent of the thesis includes the following aspects:1. The propagation characteristics of double-vortex beams which aregenerated by coaxial superposition of two vortex beams with differenttopological charge of integer and (or) fractional order are studied theoreticallyand experimentally. It is found that each ring of the double-vortex beamcarries different OAM, and is propagating independently. In addition, whenthe internal topological charge of the double-vortex beam remain unchanged,the distance between the two rings of the double-vortex beam gets bigger withincreasing the external topological charge. 2. The diffraction pattern of vortex beam formed by a rhombus aperture isinvestigated theoretically and experimentally. It is found that the number ofthe dark stripes in the Fraunhofer diffraction intensity distribution is justequal to the topological charge value of the measured optical vortex, and thecentre of each dark spot or stripe is a phase singularity point. Based on thisproperty, it provides us a simple way to detect the orbital angular momentum(OAM) of an optical vortex beam.3. The properties of radially polarized non-diffracting beam are theoretically andexperimentally studied. The intensity distribution of the radially polarizednon-diffracting beam is theoretically simulated. The results show that the radiallypolarized non-diffracting beams have both the radial polarization property andnon-diffracting characteristic. The radially polarized non-diffracting beam isgenerated by the radial polarization converter and an axicon. The dependence of theintensity distribution of radially polarized non-diffracting beam on the polarizationangle of the polarizer is observed, being good agreement with the theoretical results.The beams with both the radial polarization and non-diffracting properties havemany important potential applications.