Conjugate rotation smoothing scheme for laser quad based on dual-frequency laser and spiral phase plate
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摘要: 提出了一种基于双频光源和涡旋相位板实现光束快速互补旋转的集束匀滑方案。双频光源为集束中各子束提供频移，拓扑荷数相同但反号的涡旋相位板阵列用于将各个子束变换成拉盖尔—高斯(LG)光束，而通过偏振控制则可实现子束间两两的相干叠加。在此基础上，通过采用共轭连续相位板可使波长不同、偏振态不同的子束组合在靶面形成快速旋转且空间上互补填充的焦斑。结果表明，利用这一方案可实现子束散斑在靶面上快速旋转且散斑分布保持互补，进而有效改善靶面辐照均匀性，甚至为抑制激光等离子体不稳定性提供了一种潜在途径。
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关键词:
- 惯性约束聚变 /
- 集束匀滑 /
- 涡旋相位 /
- 辐照均匀性 /
- 激光等离子体不稳定性
Abstract: Conjugate rotation smoothing scheme for laser quad based on dual-frequency laser and spiral phase plate was proposed. The dual-frequency laser provides frequency shift among the beamlets, the spiral phase plates with same helical charge but opposite sign transform the beamlets into Laguerre-Gaussian beams, and the polarization control is applied to make these beamlets coherently superposed on the target plane. On this basis, the conjugate continuous phase plates are adopted to enable the beamlets with different central wavelength and orthogonal polarization form focal spots with rapid rotation. Moreover, the spatiotemporal focal spot of the laser quad looks like conjugate spin light because of the frequency beats. It is indicated that, the scheme enables the fine-scale speckles within the focal spot rotate in a period of a few picoseconds, and even exhibit different intensities and wavelengths at different time and different positions. Hence, the novel scheme can effectively smooth the irradiation uniformity of the laser quad and even has the potential to mitigate laser plasma interactions. -
图 1 互补旋转集束匀滑方案
Figure 1. Schematic illustration of conjugate rotation smoothing scheme for laser quad based on dual-frequency laser and spiral phase plate
图 2 不考虑CPP时，集束焦斑光强分布
Figure 2. Focused intensity distribution of the laser quad at different time (without continous phase plate), red areas denote the intensity distribution of beam 1 and beam 3, green areas denote the intensity of beam 2 and beam 4
图 3 旋转周期和相干长度随中心波长差的变化
Figure 3. Variations of the coherence length and the rotation period with the wavelength difference when the helical charge l＝±1
图 4 (a)原CPP和(b)共轭CPP的面型分布，(c)两光束分别经共轭CPP的焦斑光强分布
Figure 4. Surface shape of an original CPP (a) and its conjugate CPP (b). (c) is the far-field intensity distributions of a same laser beam after propagating through these two CPPs. Red regions shows the speckles generated by the original CPP, and green regions within the focal spot show the speckles generated by the conjugate CPP
图 5 焦斑内部散斑线速度随偏心距离的变化
Figure 5. Variation of the linear velocity of the speckles at different locations inside the focal spot
图 6 不同拓扑荷数时，光通量对比度随积分时间的变化以及积分时间为20 ps的FOPAI曲线
Figure 6. Comparison of the conjugate spin-light smoothing scheme at different combination of helical charges with smoothing by spectral dispersion.
图 7 不考虑CPP时激光集束焦斑光强分布
Figure 7. Focused intensity distribution of the laser quad at different time (without CPP), red areas denote the intensity distribution of beam 1 and beam 3, and green areas denote the intensity distribution of beam 2 and beam 4
图 8 不同振幅和位相畸变时，光通量对比度随积分时间的变化以及FOPAI曲线
Figure 8. (a) Variation of the contrast of the focal spot with the integral time when the beamlets have different beam quality. PV denotes the peak-to-mean value of the wavefront distortion, and a denotes the max-to-mean value of the amplitude modulation. (b) FOPAI curves of the focal spot when the integral time is 5 ps
图 9 不同时刻集束焦斑光强分布
Figure 9. Focal spots at different time without the use of continuous phase plates, when both spin-light smoothing scheme and radial smoothing are adopted
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