The Properties Of Radially Polarized Beam Propagating In Free Space And Turbulent Atmosphere

The thesis is based on the paraxial approximation of beam propagation of E.wolf and researches on properties of radially polarized beam propagating in free space and turbulence atmosphere by numerical analysis in both cases of completely coherent and partially coherent. The jobs mainly include:1. Based on the theory of paraxial approximation of beam propagation, in the case of completely coherent, the analytical expression of the intensity of the off axial radially polarized beam(OARPB) is derived and the effect of the off axial magnitude on the distribution of intensity of the OARPB is studied. Meanwhile, according to the definition of the first-order moment of centroid, the coordinate of centroid of the OARPB is derived and the variation of cenreoid of the OARPB is studied. Simulation result shows that the intensity distribution of the OARPB is different from that of the radially polarized beam. The intensity distribution of the OARPB is not uniform in the near-field. With increasing of propagation distance, the beam spreads and the uniformity of intensity of the OARPB is improved gradually. However, the intensity distribution of the radially polarized beam keeps the form of symmetric doughnut spot during propagation all the time.2. Owing to the beam has a certain width normally, the spectral width of monochromatic radially polarized beam which we talked about before is extended to polychromatic radially polarized beam(PRPB) in the case of completely coherent. We analysis the intensity and spectral properties while the PRPB propagating and compared the properties of PRPB with the case of monochrome and line polarized Gaussian beam respectively. Results show that the influence of spectral width increment of light source in near field or far field is different. As to spectral property, there always exists a critical position, inside which the observed spectrum is blue-shifted and outside of which, red-shifted. The increase of original spectral width will push the critical position away from the beam center wherever in the near field or far field. For the case of certain original spectral width, the observed spectral shift width in far field is bigger than that in the near field. Besides, while comparing the PRPB with the polychromatic linearly polarized Gaussian beam, it is found that at the same propagation distance, the radius of the critical position of the former is obviously bigger than that of the latter.3. We combined radially polarized beam with properties of partially coherent, and researched on the properties of partially coherent radially polarized beam(PCRPB) propagating in free space by theoretical analysis and numerical simulation. Discussion is made on the coherence influenced on spot shape, stocks parameters and degree of polarization during PCRPB propagating in free space. And we also compared the case of partially coherent with the case of completely coherent.4. Based on the theory of generalized Huygens and the unified theory of coherence and polarization, study is made on the module value of complex degree of coherence of partially coherent radially polarized beam(PCRPB) changes with transmission distance in different reference point. Results show that the module value distribution of the complex degree change with transmission distance is different in the different reference points while PCRPB propagates in free space.5. The transmission medium is extended to turbulent atmosphere and influence of the strength of turbulent atmosphere is adding to discuss. Distribution of the intensity influenced by source wavelength, and degree of polarization(DOP) of PCRPB influenced by coherence length, turbulent atmosphere, spot size and source wavelength, have been investigated while propagating in turbulent atmosphere. The numerical simulation results show that with the increase of propagation distance, the doughnut beam spot of the PCRPB becomes a solid beam, which is related to source wavelength. In addition, at a certain propagation distance, the rate of DOP with radius will increase, if the original coherence increases, or the atmosphere turbulence become weak, or the spot size become large, and or the source wavelength become short. Such tendency will become more obvious with longer propagation distance.6. We deeply investigate on the width and angular spreading of partially coherent radially polarized beam(PCRPB) in atmospheric communication. Based on the theory of generalized Huygens and the cross-spectral density matrix, the analytic expression of the mean-squared beam width of PCRPB and Gaussian Schell-mode(GSM) beam is derived and the influence of turbulence,coherence length and source wavelength on the spreading of width and angular between the two kinds of beams have been compared. The result shows that, with the same parameters the spreading of width of the GSM beams is larger than that of the PCRPB. The spreading of angular of the GSM beam not only relate to the three parameters above,but also is affected by the width of the source beam which is nothing to do with the spreading of angular of the PCRPB. All these show that PCRPB has superiority in atmospheric communication than GSM.