Study On Near-field Properties Of SRRS After High-intensity Laser Propagationg Through A Long Path

In order to achieve inertial confinement fusion(ICF), ICF laser driver usually marshalled dozens even hundreds laser beams of high-power(GW/cm2) to irradiate the pellet uniformly and simultaneously. The single beam of large aperture(~40cm) and high-energy(100KJ) need to propagating in tens even hundreds meter’s path. These factors of ICF laser driver created objective conditions for the apperance of stimulated roational raman scattering(SRRS). The generation of SRRS leads to great harms to the output of the laser driver, such as energy lossing, near-field beam quality uniformity degrading, focal spot quality damaging in far-field. Currently, it has become an urgent problem to suppress SRRS in ICF laser beam development and beam quality controlling. In this thesis, laws of SRRS has been explored based on the characteristics of the beam near-filed in theory that eatablish numerical fundation for the coercion of SRRS.Firstly, the numerical three-dimension spatical model has been built based on beam near-field according to the SRS coupling wave equations. The numecial model has been optimized, including initial stokes light noise status, optical diffracted transmission, analysis of temporal and spatial sampling rate. Past experimental results and datas of SRRS based on ICF laser drivers has been collected, the numerical model has been checked according to them. Using the model, the basic characteristics of SRRS have been presented, some factors that impact the threshold of SRRS has been illustrated.Secondly, the threshold of SRRS dependent on the beam near-field, seriously. Various intial pump beam near-fields were constructed, which carry phase informations. flux modulation and beam flux contrast were used as the parameters to describe beam near-field, SRRS thresholdes have been computed under different parameter value, the tables of threshold criterion have been obtained, the formulae of SRRS threshold has been deduced.Thirdly, according to the static model of the amplitude type “defects” in optical elements, the intensity transmission rate function was given. took the beam that transmited the optical element as the intial laser beam of SRRS effect’s study. In order to study the diffraction in SRRS process how to impact the characteristics of beam near-field, two forms of diffraction transmission and Stokes intensity coupling amplification process without diffraction have been numerical computed. pump laser near-field and Stokes near-field of process above have been compared and analysed. The diffraction effect could abate the Uniformity’s damaging and fluctuation’s exacerbating caused by the spontaneous stokes noise and stokes’ amplification.Finally, the near-field power spectral density(PSD) was introduced to describe the evolution of the near-field characteristics in frequency domain. The intensity of pump beam near-field was detract in all range of frequency domain for SRRS effect. As the transformation rate of Stokes increase, intensity of the pump beam near-field kept decrease in the range of low-frequency region. But it’s almost unchanged when Stokes conversion efficiency reach up to a certain value in the range of high-frequency region.