Influence Of Phase Perturbation On Stimulated Rotation Raman Scattering And Focal Characterization Of High-Power Laser Beams

In the research of Inertial Confinement Fusion (ICF), the high-power fundamental fields from the main amplifier have a long air path to reach the frequency conversion system, and the high-power ultraviolet laser beams from the frequency conversion system also have a long air path to reach the target. However, the effect of stimulated rotational Raman scattering (SRRS) on high-power laser beams in the process of long air path propagation not only leads laser energy loss, but also the beam quality degrading. The degraded beam quality of fundamental field will influence not only the conversion efficiency of third harmonic generation, but also the beam quality of ultraviolet laser beams from the frequency conversion system, and the SRRS effect of high-power ultraviolet laser beams in long air paths will influence the optimum fuel compression in the target. Therefore, the study on the SRRS effect of high-power fundamental field and tripling field in long air paths would be of great importance in practice.This thesis focuses on the SRRS effect of high-power fundamental field and tripling field through long air paths. The threshold of SRRS effect, the conversion efficiency of Stokes field and the beam quality and phase properties of laser beams were discussed in detailed. Furthermore, the focal characterization of high-power ultraviolet laser beams was studied. The main contents can be summarized as follows:(1) The physical model of SRRS effect of high-power laser of different wavelengths through long air paths has been built upBased on high-power laser thorough long air paths, the physical model of SRRS effects was built up, which considered the diffraction effects of Stokes and pump lasers, Langevin noise sources, the nonlinear polarization of the medium, the depletion of pumped laser, the nonlinear amplification of the Stokes field and the Raman gain coefficient, ect.. The model of the phase perturbation was also given, where the root-mean-squared (RMS) Gradient and random phase perturbation were introduced to describe the low and the high frequency phase noise.(2) SRRS effects of high power laser beams through long air paths depend on several factors has been studiedThe SRRS effect of high-power fundamental field and tripling field through long air paths has been simulated and analyzed. The effects of the peak intensity and the pulse width of the pump laser beams on the threshold of SRRS effect and the conversion efficiency of Stokes field have been discussed in detail. The influence of initial phase perturbations on the intensity distribution of the pump and stokes light has also been analyzed.(3) Influence of phase perturbations on the phase of high-power laser beams has been studiedThe phase characteristics of high-power lasers with different wavelengths through long air paths have been studied. The phase of the high-power fundamental and tripling fields with different initial phase perturbations after long air paths has been discussed in detail. Subsequently, the variations of the phase perturbations of the high-power laser beams with the propagation distances have been analyzed quantitatively(4) Influence of phase perturbations on focal characterization of high-power ultraviolet laser beamsThe focal characterization of high-power ultraviolet laser beams from the frequency conversion system after long air path propagation has been studied. The shape of focal spot of high-power ultraviolet laser beams with different low-frequency phase perturbations through long air paths has been discussed in detail. Furthermore, the variation of the size of focal spot with the spatial phase perturbations and the propagation distances has been analyzed quantitatively.