Study On Propagation Properties Of A New Type Of Flat-topped Light Beams

Laser beam characterization, beam propagation and beam quality control are the fundamental research subject in laser optics and laser technology. It can be shown from the studies that the M2 factor is a very useful parameter for characterizing various laser beams. In some practical applications, it is a useful parameter for beam quality. On the other hand, laser beams generated from high-power lasers usually result in a complex multi-mode structure and possess partial coherence because of the high gain, amplification of spontaneous emission, etc. Studying on the theory of coherent-mode decomposition of laser beams would be of both theoretical significance and practical interest. Moreover, flat-topped beams are very useful beams in many practical cases of the high-power laser, such as in laser material processing and inertial confinement fusion (ICF). It would be of important significance to study the propagation properties of flat-topped beams.The goal of this thesis is to study the propagation properties, the beam propagation M factor and the theory of coherent-mode decomposition of flat-topped beams. The main results obtained in this thesis can be summarized as follow: 1. Based on the definition of generalized intensity second-order moments, the analytical expressions for the generalized M2 factor of flat-topped beams with andwithout aperture have been derived. The factors that affect the generalized M2 factor have been studied by numerical calculation examples.2. Theory of the coherent-mode decomposition of laser beams was built. The analytical expressions for the mode coherence coefficients of flat-topped beams withand without aperture have been derived, by means of which the mode correlation, mode structure, and coherent-mode decomposition can be analyzed.3. Making use of the Collins formula, the propagation properties of flat-topped light beams passing through a paraxial ABCD optical system have been studied, and the corresponding analytical propagation expressions have been derived. The focusing properties have been analyzed in detail. The propagation properties of flat-topped light beams passing through a lens series have been studied by the analytical propagation expression without aperture.4. The propagation expressions for the annular flat-topped light beams passing through a paraxial ABCD optical system have been derived by using the Collins formula. The factors that affect the Fresnel and Fraunhofer diffractions have been analyzed in detail.5. Making use of the Collins formula, the general propagation expression for flat-topped light beams passing through a paraxial ABCD optical system in gain and absorbing media have been derived. The M1 factor of flat-topped light beams propagating in gain and absorbing media have been analyzed in detail by means of the definition of generalized intensity second-order moments.