Study On Propagation Properties Of Partially Coherent Beams

Laser beam characterization, beam propagation, beam transformation and beam quality control are always the fundamental research subjects in laser optics and laser technology. Laser beams generated from high-power lasers usually result in a complex multi-mode structure and possess partial coherence because of the high gain, large aperture, amplification of spontaneous emission, nonlinear optical effects, diffraction of cavity mirror, as well as the variation of laser control parameters. It would be of important significance to study the laser beam characterization, beam propagation, mode decomposition and beam quality control.The goal of this thesis is to study the propagation properties, the beampropagation M2 factor and the theory of mode decomposition of the partially coherentbeams. The main results obtained in this thesis can be summarized as follow:1. Based on the theory of second-order moments and the treatment that the rectangularfunction can be expanded into the real part of an approximate sum of complexGaussian functions with finite numbers, an approximate method for calculating thesecond-order moments and the generalized M2-factor of partially coherent beamspassing through hard-edged apertures has been proposed. A laser beam withamplitude modulation and phase fluctuation was taken as a typical example ofpartially coherent beams. The analytical expression for the generalized M2-factorwas derived. Furthermore, the typical numerical examples have been given and the calculation precision of our method has been analyzed by numerical calculations.2. Considering of the complex wave number, the second-order intensity moments andthe M2-factor of partially coherent beams in gain or absorbing media have been modified. A Guassian Schell-model beam was taken as a typical example of partially coherent beams and the variations of the M2-factor of partially coherent beams in gain or absorbing media have been studied.3. The analytical expressions for the mode coefficients and M2-factor of partiallycoherent flat-topped beams, which can be combined by the independent Hermite-Gaussian beams, have been derived. The mode decomposition and mode composition of partially coherent flat-topped beams have been analyzed. It can be seen from our study that the M2-factor and the rectangular shape of the flat-topped beams combined by the independent Hermite-Gaussian beams are not available at the same time. If an approach rectangular shape of the intensity distribution of partially coherent flat-topped beams is desired, the M2-factor must be large. If the small value of M2-factor is required, the strictly rectangular shape of the intensity distribution is not available.4. The spectral density function of partially coherent flat-topped beams propagatingthrough ABCD optical system including the gain or absorbing media has been derived. The variations the optical spectrum of partially coherent flat-topped beams propagating in gain or absorbing media have been studied in detail. The influence of the parameter characterizing the gain or absorbing media, the real part of wave number, the correlation length and the beam shape on the spectral characteristics in the near zone or in the far zone has been investigated.5. Based on the theory of second-order intensity moments of truncated laser beams inspatial-frequency domain modified by Herrero et al., the theory of measuring the generalized M2-factor of truncated beams and the measuring method have been given. The measurement system has been set up and the experimental results have been analyzed.