Propagation And Diffraction Properties Of Partially Coherent Vortex Beams In Atmospheric Turbulence

Orbital Angular Momentum(OAM)provides a new form of multiplexing for Free-Space Optical(FSO)communications,which increases the channel capacity of communication systems.In the OAM multiplex communication system,exploring the propagation mechanism of the vortex beam in atmospheric turbulence becomes one of the key scientific problems.In this dissertation,the intensity,polarization and diffraction properties of partially coherent vortex beams in atmospheric turbulence were deeply investigated,the main results are as follows:(1)The concept of partial coherence was introduced into the vortex beam.The Laguerre-Gaussian Schell-model Vortex(LGSMV)beam belonging to the partially coherent vortex source was introduced.The relationship between phase singularity and topological charge on the far-field plane was discussed.The influence of the Non-Kolmogorov turbulence on the intensity distribution and beam spreading of LGSMV beams were investigated through theoretical analysis,respectively.It was found that the number of phase singularities of the LGSMV beam on the far field plane is equal to the number of the topological charge.This provides an effective way to detect the topological charge.(2)The mathematical model of the radial partially coherent vortex beam array was established.The generalized Huygens-Fresnel principle was used to analyze the evolution law of the intensity of the radial partially coherent vortex beam array propagating along through the atmospheric turbulence.The effects of light source parameters and Non-Kolmogorov turbulence parameters on its propagation properties were discussed.The evolution of its intensity distribution was first converted by the array beam into a ring beam,then a Gaussian beam,until finally in the form of a standard Gaussian beam.(3)Based on the partially coherent vector beam,an electromagnetic Gaussian Schell-Model Vortex(EGSMV)beam belonging to a partially coherent vector vortex source was introduced.Using the unified theory of coherence-polarization and Stokes parameters,the expressions of Degree of Polarization(DoP)and Orientation Angle of Polarization(OAoP)of the EGSMV beam in atmospheric turbulence were derived.The effects of source parameters and atmospheric turbulence parameters on the DoP and OAoP of the EGSMV beam were analyzed.It was found that the OAoP distribution of the partially coherent vortex beam was petal-like,the number of petals was twice the number of the topological charge and the direction of rotation of the petal was related to the sign of the topological charge.(4)Using the Collins diffraction formula and ABCD transfer matrix,the analytical expression of the diffraction field of a vortex beam passing through an Maksutov-Cassegrain(MC)optical system was derived.The diffraction properties of the vortex beam passing through the MC optical system were discussed.It was confirmed that the vortex beam can improve the emission efficiency of the MC.The results show that compared to a Gaussian beam,the emission efficiency of MC was as high as 80%when the vortex beam was used as the reference light.The diffraction patterns of the vortex beam passing through the aperture diffraction element were studied theoretically and experimentally.The detection effects of different apertures on the topological charge of the vortex beam were compared and analyzed.The work of this dissertation lays a theoretical foundation for exploring the influence of atmospheric turbulence on the propagation mechanism of vortex beam and building the OAM multiplexing communication system.The relationship between phase singularity,orientation angle of polarization and aperture diffraction pattern and topological charge respectively provides an effective way to detect the OAM mode of the receiving end in OAM multiplex communication system.