A novel scheme for decreasing the scattering levels of stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), as well as for improving illumination uniformity of focal spots, is proposed. In this scheme, four beamlets in a laser quad are frequency shifted, spatial phase shaped by conjugate phase plates, and counter-rotating circularly polarized by polarization control plates before being focused by wedged lenses. Consequently, the intensity sweep and the polarization rotation of the focal spot in the timescale of a subpicosecond are achieved, resulting in the reduction of the scattering level of SRS and SBS, and also the improvement of the illumination uniformity when the frequency shift among beamlets is comparable to the parametric backscattering growth rate and the filamentation instability growth rate. The theoretical model of the novel smoothing scheme has been established, and the influences of some key parameters including spatial modulation type, peak-to-mean value of phase distortion, and frequency shift have been discussed. Results show that the uniformity of the focal spot of this novel scheme is improved much faster than that of the conventional temporal smoothing scheme, i.e., smoothing by spectral dispersion. The parametric instabilities are significantly reduced and both phase distortion and modulation depth of the laser quad have a large tolerance, opening a new dimension for controlling the incident light in laser plasma interaction.

中文翻译：

---

亚皮秒级的强度扫描和偏振旋转的平滑方案

提出了一种降低受激拉曼散射（SRS）和受激布里渊散射（SBS）的散射水平以及提高焦点照明均匀性的新方案。在这个方案中，激光四方体中的四个小光束被频移，由共轭相位板进行空间相位整形，并由偏振控制板反向旋转圆偏振，然后再由楔形透镜聚焦。因此，在亚皮秒级的时间范围内实现了焦点的强度扫描和偏振旋转，从而降低了SRS和SBS的散射水平，并改善了子束之间的频移为时的照明均匀性。可与参数反向散射增长率和细丝不稳定性增长率相媲美。建立了新的平滑方案的理论模型，讨论了空间调制方式，相位畸变峰均值，频移等关键参数的影响。结果表明，与传统的时间平滑方案（即通过频谱分散进行平滑）相比，该新颖方案的焦点均匀性得到了更快的改善。大大降低了参数不稳定性，并且激光四边形的相位失真和调制深度都具有较大的公差，从而为控制激光等离子体相互作用中的入射光开辟了新的维度。和频移已经讨论过了。结果表明，与传统的时间平滑方案（即通过频谱分散进行平滑）相比，该新颖方案的焦点均匀性得到了更快的改善。大大降低了参数不稳定性，并且激光四边形的相位失真和调制深度都具有较大的公差，从而为控制激光等离子体相互作用中的入射光开辟了新的维度。和频移已经讨论过了。结果表明，与传统的时间平滑方案（即通过频谱分散进行平滑）相比，该新颖方案的焦点均匀性得到了更快的改善。大大降低了参数不稳定性，并且激光四边形的相位失真和调制深度都具有较大的公差，从而为控制激光等离子体相互作用中的入射光开辟了新的维度。