Study On Propagation Characteristic Of Third Harmonics Of High Power Laser

In the research of Inertial Confinement Fusion (ICF), the absorption of fusion target to short-wavelength laser is more efficient than that of long-wavelength laser, and the third harmonic generation of large-aperture KDP crystal is an only accessible way to obtain high-power short-wavelength laser in high-power solid-state laser at present. Therefore, third harmonic generation of high power laser is a critical technology in the research of ICF. In order to satisfy the requirement of ICF to third harmonic converter, it is necessary to study on the physical factors affecting the conversion efficiency of third harmonic generation and the beam quality of tripling field. For these reasons, this thesis focuses on conversion efficiency and beam quality. The main results obtained here can be summarized as follows:1. The physical model of third harmonic generation, which includes paraxial diffraction, walkoff, the group-velocity mismatch, third-order nonlinear interaction, the variation of amplitude and phase of input field, and the absorption of crystal, has been built up.2. The numerical calculation code of third harmonic conversion of high-power laser has been built up and integrated in "SG-99" which is simulation software of the new generation of high-power solid-state laser driver "SG-III" of China. In addition, the separate numerical calculations software of third harmonic conversion of highpower laser, which has graphic user interface, has been built up. The comparisons between the results of our numerical calculation with the conclusions of the Lawrence Livermore National Laboratory and the perturbation theory for frequency tripling have been done, showing the validity and reliability of our code.3. Based on the research of the influence of variation of amplitude and phase of input field on the tripling conversion efficiency, the parameter optimization method for frequency converter has been proposed. Subsequently, the parameters of frequency converter have been optimized, and the transformation relation between the beam quality of fundamental field and tripling field has been studied quantitatively.4. Using type I/II angle-detuning schemes for third harmonics generation, the effect of third-order nonlinear effect of KDP crystal on the tripling efficiency and beam quality under high power condition has been studied.5 The model of wavefront phase perturbation has been built up, where the random phase screen characterized by RMS Gradient and the random phase perturbation have been introduced to describe the low and the high frequency phase noise, respectively. Taking the type I/II angle-detuning scheme for third harmonics generation as an example, the influence of low-frequency phase perturbation and the high frequency phase noise on the focal characterization of the tripling field has been studied.6. Using type I/II angle-detuning KDP crystal scheme for third harmonic generation, the effects of temporal phase modulation on conversion efficiency and spectrum under high-power condition have been studied, and then the principles of several schemes for broadband width third harmonic generation have been analyzed.