Theoretical And Experimental Studies On Partially Coherent Beams

Over the past decades, partially coherent beams have been extensively investigated and have found wide applications in many fields, such as free-space optical communications, material thermal processing by laser beam, laser scanning, inertial confinement fusion by laser, nonlinear optics and imaging applications. This thesis is mainly devoted to studying the generation of partially coherent Gaussian Schell-model (GSM) beam, the propagation of GSM beam through fractional Fourier transform (FRT) optical system, the coincidence FRT implemented with a partially coherent beam, the models for coherent and partially coherent dark hollow beams and their propagation properties.In chapter 1, the background and progresses in the field of partially coherent laser theory, fractional Fourier transform, coincidence imaging and dark hollow beam are reviewed. The main content and originality of this paper are presented.In chapter 2, the basic concepts and theories of lasers, such as Huygens-Fresnel diffraction theory, matrix method and higher order correlation of the field, are presented.In chapter 3, a partially coherent GSM beam whose intensity distribution and spectral degree of coherence are of Gaussian distributions is generated in experiment by scattering a coherent laser beam generated by a He-Ne laser with some modification. The coherence of the GSM beam can be controlled by the optical components in the experimental setup. One new way for measuring the transverse coherence length of the GSM beam is proposed based on the fourth-order correlation of laser beam, and we have successfully measured the coherence length of the GSM beam in experiment.In chapter 4, the experimental setup for measuring the properties of the GSM beam passing through fractional Fourier transform (FRT) optical system is built. The intensity distribution and transverse coherence length of GSM beam in the FRT plane after passing through the FRT optical system for different fractional order and their dependence on the transverse coherence length the GSM beam in the source plane are measured experimentally. The experimental results are in a good agreement with theoretical results. Furthermore, we derived the analytical propagation formula for a GSM beam passing through apertured FRT optical systems by expanding the hard aperture function into a series of Gaussian functions. The properties of GSM beam in the FRT plane after passing through apertured FRT optical systems are studied theoretically and experimentally. Our results show that we can control the intensity and coherence properties of a GSM beam using the FRT optical system conveniently.In chapter 5, we analyze the influence of the beam spot size and transverse coherence length of a GSM beam on the visibility and quality of the coincidence FRT pattern of a double slits implemented with a GSM beam based on the fourth-order correlation of laser beam. The coincidence FRT pattern of a double slits implemented with a GSM beam is observed successfully in experiment. Furthermore, we propose the lensless optical systems for implementing the coincidence FRT with a partially coherent or a entangled photon pairs, and the lensless optical system for implementing coincidence imaging is also proposed. Based on the optical systems proposed by theory, we build the lensless experimental setup and observe the successfully the coincidence FRT and image of a double-slits in experiment.In chapter 6, we propose a new modified model to describe a dark hollow beam. Furthermore, we extend the dark hollow beam to the partially coherent case, and we propose the model for describing the partially coherent anomalous hollow beam. Analytical propagation formulae for coherent and partially coherent dark hollow beam passing through a paraxial optical system are derived. The propagation properties such as the propagation factor and intensity distribution of coherent and partially coherent dark hollow beams are studied numerically.