Research On The Effect On Srrs Of Phase Distortion Of The Laser Pump Beam

High power lasers is one of the important parts in the Inertial Confinement Fusion(ICF)project.It is easy to cause Stimulated Rotation Raman Scattering(SRRS)to destroy output beam quality because of the high power near-field beam and the long air path to reach the target.Near-filed beam quality has a very close connection with SRRS.One of the main factors to influence the near-field beam quality is the wave-front phase distortion.Inside the lasers,there are static phase distortions including cleanliness and the precision on the surface of the optics and dynamic phase distortions such as thermal pressure.We have analyzed the influence of phase distortion on the near-field beam quality and SRRS to give theoretical support for suppressing SRRS.Firstly,we have studied on the SRRS coupled wave transmission equations and established the SRRS numerical model.Then the evaluation of the wave-front phase distortion has been determined,and the Gaussian random phase screen and the sinusoidal phase modulation have been set as the phase distortion model respectively.It has been found: under the sinusoidal phase modulation,the RMS Gradient has increased as a sine with the modulation frequency increasing;under the Gaussian phase distortion,the RMS Gradient and PV have decreased as the the scale length increasing,and the near-field beam flux modulation and contrast have increased as RMS Gradient and PV increasing in a certain range.Secondly,based on the hundreds joule level laser system,the influence of the wave-front phase distortion on the SRRS has been simulated.The demarcation point of the high and low frequency phase distortion have been divided according to the curve of the SRRS threshold distance with the phase distortion scale length.Meanwhile,the influence of the phase distortion of the fundamental and third frequency pump beam on the SRRS threshold distance has been analyzed,and the range of PV value and RMS Gradient have been given to suppress the SRRS effect.Then the relationship between the phase distortion frequency and scale length has been analyzed,and the phase distortion frequency demarcation points have been given.Finally,it has been analyzed that the effect of two-dimensional phase distortion on near-field beam quality and SRRS effect.It has been found that the influence of two-dimensional and one-dimensional phase distortion on the near-field beam and SRRS effect is basically the same.The distortion scale length demarcation points under two-dimensional phase distortion are slightly larger.