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 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
Fig. 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 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
Fig. 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 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
Fig. 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. (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
Fig. 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. Focal spots at different time without the use of continuous phase plates, when both spin-light smoothing scheme and radial smoothing are adopted
Fig. 9. Focal spots at different time without the use of continuous phase plates, when both spin-light smoothing scheme and radial smoothing are adopted
[7] Strozzi D J, Sepke S M, Bailey D S, et al. Inline modeling of crossbeam energy transfer Raman scattering in NIF hohlraums[R]. Lawrence Liverme National Labaty (LLNL), 2016.
钟哲强, 张彬. 基于双频光源和涡旋相位的互补旋转集束匀滑方案[J]. 强激光与粒子束, 2020, 32(1): 011012. Zheqiang Zhong, Bin Zhang. Conjugate rotation smoothing scheme for laser quad based on dual-frequency laser and spiral phase plate[J]. High Power Laser and Particle Beams, 2020, 32(1): 011012.