| Inertial Confinement Fusion(ICF) is an important technique to solve the upcoming energy crisis. To realize the fusion ignition, several schemes have been proposed, among which, the Shock ignition is a novel method which can be achieved with the now available energy of ICF laser drivers. This method calls for laser pulses with ~100k J energy and ~200ps width. To obtain high energy hundred picosecond laser pulses, traditional approaches(e.g., CPA, MOPA etc.) are limited for their drawbacks. Based on stimulated Brillouin scattering(SBS), hundred picosecond pulses can be amplified with high energy nanosecond pulses efficiently. What’s more, this method has simple structure with low cost, becomes the ideal approach to generate ICF ignition pulses. In the Brillouin amplifier, higher pump energy leads to higher gain of the seed. However, when pump energy exceeds a certain value, self-excited SBS will happen. The self-excited SBS lowers the amplification efficiency and influences the output SNR(signal to noise ratio). Hence, studying the self-excited SBS threshold of nanosecond pulses has guiding significance to the design of Brillouin amplifier.In this thesis, according to the output pulse characteristics(3~5ns nearly flat-toped pulses) of the ICF laser driver in service, we conducted a systematic study of large-aperture-laser-pumped SBS threshold and pulse compression with a nonfocusing scheme. The fluorocarbon liquid(e.g. FC-40 and FC-770, etc.) is used as nonlinear medium.Firstly, starting from the coupled wave equations describing SBS and using the method of characteristic line, the numerical calculation model is established. The dependence of SBS threshold on pump parameters and medium parameters is simulated, and the critical factors that determine the SBS threshold are analyzed.Then, in the non-focusing scheme, combining with statistical theory, the threshold-evaluation method of SBS pumped by large-aperture laser with nonuniform near field distribution is proposed. The nonlinear hot-spot parameter is introduced, and the threshold of SBS pumped by laser with non-uniform near field is expressed as the product of the hot-spot parameter and the threshold of SBS pumped with ideal plane wave. The corresponding experiment is conducted, and the experimental results agree well with the theoretical results.Moreover, two methods of improving SBS threshold are studied:(1) In the Brillouin amplifier of hundred picosecond pulses, the existence of seed light could enhance the self-excited SBS threshold, the mechanism of which is gain competition between strong seed light and spontaneous scattering.(2) The phase modulation of pump light could divide the single spectral line into multiple lines, lower the power spectral density and enhance the SBS threshold. The mechanism of this method is theoretically stated, and the influence of spectral line spacing and modulation index on the threshold increasing degree is studied. The experiment of threshold improvement is conducted with three equal-amplitude spectral lines as pump light, and the experimental results agree well with the theoretical calculation.At last, the theory of self-pumped SBS with super-Gaussian-shaped pulse is proposed, and its application in the high energy pulse compression is studied. The numerical calculation model describing this effect is established, and the influence of super-Gaussian pulse order, medium parameters and structure parameters to self-pumped SBS is studied. The existence of self-pump SBS is experimentally proved, and then based on this effect, with the high power Nd:glass laser system self-built by Harbin Institute of Technology as light source, FC-40 as nonlinear medium, the output of 3J, 360 ps compressed pulses is obtained. |
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