Influence Of Harmonic Generation On The Beam Quality And Focal Characterization Of High-Power Laser Beams

In the research of Inertial Confinement Fusion (ICF), the short-wavelength laser is usually used, because it can be absorbed by fusion target more efficiently than long-wavelength laser. At present, the only accessible way to obtain high-power short-wavelength laser in high-power solid-state laser is the third harmonic generation of large-aperture KDP crystal. Therefore, third harmonic generation of high-power laser is a critical technology in ICF. In addition, the distribution of focal spot is not only required to be with high power, but also with uniform central spot and little side lobes. Consequently, the beam quality and focal characterization of the tripling field are two important problems in the research of ICF. In order to satisfy the requirement of ICF on third harmonic converter, it is necessary to study on the physical factors affecting the beam quality and focal characterization of the tripling field. This thesis focuses on the beam quality and focal characterization of high-power tripling field. The main results obtained here can be summarized as follows:1. The general physical model of third harmonic generation has been built up on the basis of the coupling equation, which includes paraxial diffraction, walk-off, nonlinear interaction, the absorption of crystal, and the variation of phase. In addition, the numerical simulation code of third harmonic conversion and focus of high-power laser has been built up.2. Some physical factors affecting the beam quality of third harmonic, such as the variation of amplitude and phase of input fundamental field,nonlinear effect of KDP crystal and the fabrication quality of optical elements, has been studied in detail.3. The method for solving the inverse problem of harmonics generation has been proposed, namely, to determine the characterization of input fundamental field and the requirement for KDP from the required characterization of output tripling field. By using successive iteration method, the inverse problems of the type I angle-detuning scheme for double harmonics generation and the type I/II angle-detuning scheme for third harmonics generation have been solved numerically.4. 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. Furthermore, some technologies were summarized for controlling the beam quality and focal characterization of high-power laser beams.