Study On The Third Harmonic Generation Of High-power Broadband Laser

In the research of Inertial Confinement Fusion (ICF), 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. Therefore, third harmonic generation of high-power laser is an important content in the research of ICF. The technologies of beam smooth in space domain and beam smooth in time domain are usually combined to improve uniformity of the tripling field on the target. Consequently, the absorption of laser on target can be improved effectively. The spectrum width of laser is wider, the beam smooth of laser more available. However, the main difficulty of third harmonic generation of high-power broadband laser is how to achieve high conversion efficiency of third harmonic generation and also keep certain bandwidth of the tripling field at the same time. Because of the effect of dispersion of KDP crystal, the phase match and group-velocity match cannot fulfill at the same time, leading the high conversion efficiency of third harmonic generation with large-aperture KDP crystal can only be obtained for laser beam with narrow bandwidth. It is necessary to study on technology of high efficiency of third harmonic generation of high-power broadband laser.This thesis focuses on the analyzing and optimizing of the schemes of the third harmonic generation of the high-power broadband laser. The main results obtained here can be summarized as follows: 1. The physical model for third harmonic generation of high-power broadband laser has been built up, which included the paraxial diffraction, the walkoff effect, the group-velocity mismatch, the third-order nonlinear effects including the self- and cross- phase modulations, the amplitude and the phase, as well as the bandwidth of the input fundamental field, and the absorption of the harmonic generation crystals. The corresponding numerical simulation code has been developed and verified, showing the validity and reliability of our code.2. The effects of the bandwidth of the input fundamental field on the conversion efficiency of third harmonic generation have been studied. The schemes of third harmonic generation of the broadband laser by using the methods of the cascade crystals, the angular spectral dispersion, the chirp-matching, the new type thin nonlinear crystal, the quasi-phase-matching and the temperature-phase-matching have been analyzed and compared. Consequently, two methods of third harmonic generation of the broadband laser by using the schemes of the cascade crystals and the angular spectral dispersion have been confirmed to analyze and optimize.3. A scheme with cascaded crystals of type I/II/I has been proposed and optimized, and the effects of the intensity and the bandwidth of the input fundamental field, as well as the length of the KDP crystals on the conversion efficiency, the temporal shape and the spectral distribution of the third harmonic field has been analyzed quantitatively.4. For the scheme of third harmonic generation of the broadband laser with the technology of angular spectral dispersion (ASD), the effects of the grating dispersion parameter, the intensity and bandwidth of the incident fundamental field, as well as the length of the KDP crystal on the third harmonic conversion efficiency, the pulse shape and spectral distribution of the third harmonic field have been discussed. Finally, the parameters for the KDP crystals have been optimized.