Theoretical And Experimental Study Of Scalar And Vectorial Partially Coherent Beams

The propagation and transformation of laser beams play an important role in laserphysics and laser applications. In the past decades, more and more studies have found thatlaser beams with lower coherence are more useful than general laser beams in free-spaceoptical communications, inertial confinement fusion and laser processing. Low coherentlaser beams not only reserve the general characteristics of laser beams, but also can reducedisadvantages caused by random turbulent medium and optical elements.Usually, people treat the light as completely coherent light and completely incoherentlight to investigate them. Strictly theory indicates that all light beams are partially coherentlight beams and the best way to deal with the laser beams is the partially coherent theory.Coherence and polarization are two basic properties of light field. The scalar theory weoften used ignores the polarization characteristics of light beams. In the past decades,scalar theory of partially coherent beams can’t completely characterize any practical beams,because of the polarization properties is lost. In2003, Wolf summarized previous studiesand gave out the partially coherent and partially polarized unified theory. This theoryclearly pointed out that coherence and polarization were closely related to each other, andpartially coherent electromagnetic beams can be characterized with the cross-spectraldensity(CSD). After then, lots of theoretical and experimental studies have shown that thecoherence and polarization properties of light beams affect each other.More and more studies have found that partially coherent beam of special shapes,such as hollow beams, flat-topped beams have important practical value in many areas. So,reasonable shaping of partially coherent beams are becoming very meaningful. With thehelp of phase elements, multi-mode fiber(MMF) and liquid crystal spatial light modulator, we experimentally generated partially coherent hollow beams, flat-topped beams, vortexbeams and measured the beam parameters.It’s well known that a light beam carrying vortex phase has orbital angularmomentum(OAM) determined by its topological charge, and vortex beam has importantapplications in optical tweezers, optical trapping, atom optics and free-space opticalcommunications. Besides the vortex phase, a general partially coherent beam carrying atwist phase proposed by Simon et al., may have OAM, as well. We derive the generalexpression for the OAM flux of an astigmatic partially coherent beam carrying twist phase(i.e., twisted anisotropic Gaussian Schell-model (TAGSM) beam) propagating through anastigmatic ABCD optical system. The numerical results show that we can modulate theOAM of a partially coherent beam by varying the parameter of the cylindrical thin lens. Weexperimentally produced partially coherent isotropic Gaussian-Schell model beam(GSM)and determined the radius of curvature by measuring the transverse beam widths and thetransverse coherence widths at two different planes. Moreover, we report the experimentalgeneration of an electromagnetic Gaussian-Schell-model (EGSM) beam and measure thebeam parameters based on the technique for measuring the fourth-order correlationfunction.In1986, it is revealed by Wolf that the spectrum of a partially coherent beamgenerally changes on propagation, even in free space, unless the degree of coherencesatisfies a certain scaling law. This phenomenon is also called “correlation-induced spectralshift”. Spectral shift is an important property of partially coherent beams and can be foundimportant applications in spectroscopy and medical diagnostic. We experimentallygenerated partially coherent hollow beams, flat-topped beams and radially polarized beamsby using MMF and radial polarization converter(RPC). Furthermore, we experimentallymeasured the spectral changes of such beam focused by a thin lens.Based on the unified theory of coherence and polarization, we investigated the beamparameters of partially coherent electromagnetic beams propagation through complexastigmatic optical system, Gaussian cavities, nonlinear media and human tissue. Especially, with the help of the tensor method proposed by our group before, the investigation ofpartially coherent beams passing through cavities became very convenient. All of thesetheoretical studies will help us in-depth understanding the propagation properties ofvarious partially coherent beams and finding the right partially coherent beams we need.Partially coherent beams have attracted much more attention in application offree-space optical communications. By using a tensor method, Wigner distribution functionand second-order moments, we studied the properties of spectral intensity, spectral shiftand the M2-factor of partially coherent TGSM beam and EGSM beam propagation throughturbulent atmosphere. Studies have shown that initial beam parameters have great impacton the properties of output beam. By choosing appropriate initial beam can effectivelyreduce the disadvantages of atmospheric turbulence bring to the optical signal.Radially polarized beams have been found useful for information encryption,high-density storage, super-resolution measurement, super-resolution imaging, materialthermal processing and particle trapping. Based on the generalized Collins formula of theastigmatic optical system, we analyzed the focusing properties of spectral intensity, degreeof polarization and degree of coherence of partially coherent radially polarized beam. It isdemonstrated that the astigmatism of the optical system can be used for modulating thestatistical properties of a focused partially coherent radially polarized beam. In addition,we also carry out theoretical and experimental study of Young’s double-slit interference ofpartially coherent radially polarized beam. It is shown that the initial coherencesignificantly affect the far-field interference pattern, degree of coherence and degree ofpolarization.