Study On Propagation Properties Of Nonuniformly Polarized Beams

Nonuniformly polarized(NUP) beams possess spatially nonuniform states of polarization on cross sections, so the propagation properties of NUP beams are special. On the one hand, nonuniform polarization states may result in reduction of power density on target in far field. On the other hand,polarization states can be used as additional parameters to control the propagation properties of laser beams, just like the controlled phase being used in beam shaping. Recently, novel intensity distributions generated by tight-focused NUP beams, especially radially and azimuthally polarized beams, have drawn considerable attention for their potential applications in high-resolution optical imaging, optical trapping and manipulating, electron acceleration, optical data storage, and so on. And the decoherence effect of NUP beams under paraxial propagation has been used in beam smoothing. So the propagation property of NUP beams is a key research content. In the present paper, the intensity distribution properties of tight focused or paraxial propagating NUP beam issues are studied. The main contents of this thesis are included as follows:1. The basic concepts of NUP beams are introduced. The research and application status of NUP beams are summarized, and the conclusion is drown that the propagation property of NUP beams is a key research content. Then under the conditions of tight focusing or paraxial propagating,theoretical models that can be used to calculate the intensity distributions on target are presented.2. The current common generation methods of NUP beams are introduced. Then comparison and analysis of advantages and disadvantages of the methods are presented.3. Based on the Richards-Wolf vector diffraction theory, numerical calculation is performed to analyze two specific issues. Firstly, the influence of primary aberrations on the tight focusing properties of azimuthally polarized beams is analyzed. The results show that primary astigmatism and coma have great influence on the ring shape intensity distribution of azimuthally polarized beams. Secondly, the intensity distribution at focal plane of a tightly focused radially polarized coherent beams array(RPCBA) is calculated. The results show that RPCBA possesses the main tight focusing properties of ideal radially polarized beam. A longitudinal field appears at the focal region of a tightly focused RPCBA. And the main parameters of the longitudinal field, such as FWHM, purity parameter and generation efficiency, can be further optimized by appropriately arranging the array. The work above provides a reference and recommendations for applications of azimuthally polarized beams and RPCBA.4. Based on the paraxial propagation theory of NUP beams, the effect of nonuniform polarization angles on coherently combined fiber laser beams is analyzed numerically by using statistical methods. Beam propagation factor(BPF) is used to evaluate the quality of the combined beam. The numerical analysis shows that The reduction in beam quality caused by nonuniform polarization angles is relatively small, and 5% degradation in BPF corresponds to a possible polarization angle range of 0~0.26π. Effects of the vacancy factor and the array number are also studied, when the polarization angles of the array beams are nonuniform.5. Laser beam smoothing based on polarized beam combinatin is studied theoretically and experimentally. An experimental scheme is proposed, in which the separation between the two orthogonally polarized beams can be adjusted flexibly. The results show that the smoothing result has a close relationship with the separation between the two orthogonally polarized beams,and the optimal separation depends on the detailed speckle patten.