Research On The Effect Of Laser Spatial Characteristics On Stimulated Raman Scattering

Stimulated rotational Raman scattering (SRRS) is occurred in the transmission ofhigh power laser, whose effective control has become one of the key problems in thedomain of development of ICF driver. Once SRRS occurs, it will grow rapidly andnonlinearly. It will not only leads to the loss of laser energy, but also reduces the beamquality. Thus, it restricts the output of laser driver. Study and validation of the effect ofvarious physical factors on SRRS is conducive to the control of SRRS. Therefore, inthis dissertation theoretical research was carried out about the influence of high powerlaser spatial characteristics on SRRS effect, which enriched the research content ofSRRS, aiming to provide theoretical support for effective control of SRRS.Firstly, three-dimensional numerical simulation program of SRRS was completed,which was based on Maxwell-Bloch-Langevin equation. The variations rule of SRRSgain with the incident laser pulse factors of super-Gauss spatial order, intensity,pulse-width, diameter, are simulated in case of absence of phase aberration.Secondly, considering the actual situation of laser intensity distribution and thephase aberration model, the influence of phase aberration characteristics on laserspace(flux modulation and flux contrast) were simulated, acquiring the law of fluxmodulation on the phase distortion and flux contrast on the phase distortion. As theStudies show, at the low frequencies, flux modulation and flux contrast increase slowlyas the propagation distance increases, and its variations change weakly with aberrationamplitude. At the high frequencies, flux modulation and flux contrast increase rapidly asthe propagation distance increases, and its variations change strongly with aberrationamplitude.Finally, under different phase aberration conditions, the relationship betweenSRRS threshold distance and flux modulation and flux contrast were investigated. Asthe studies show, at the low frequencies, SRRS threshold distance decreases asmodulation and contrast increase. At the high frequencies, SRRS threshold distanceincreases as modulation and contrast increase. At the low-high frequencies, thevariations of SRRS threshold distance are related to low and high distortion amplituderatio. When high frequency and low frequency ratio is greater than2:1, thresholddistance is preferred to high frequency trend. When high frequency and low frequencyratio is less than2:1, threshold distance is preferred to low frequency trend. Through discussing and analyzing on the numerical simulation results, the criterion of SRRS isgiven under the condition of phase distortion.