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时间低相干光由于其瞬时宽带的物理特性, 在激光惯性约束聚变中得到了广泛的关注. 然而其复杂的时间尖峰结构或将诱导非线性自聚焦效应的放大. 同时, 传统的非线性自聚焦特征数值的测试方法中, 多数材料的表面损伤先于体内自聚焦成丝损伤发生, 这为对比不同激光的非线性效应带来巨大影响. 本文利用短焦距透镜对熔石英进行紧聚焦, 通过调节入射激光能量, 在避免前表面损伤的前提下, 诱导熔石英产生自聚焦成丝损伤. 随后通过理论计算对光束在样品体内传输过程的光斑变化进行空间分辨处理, 并得到对应细分位置的非线性数值. 最终将各个位置对应的非线性相位变化值进行积分, 得到前表面无损条件下材料的非线性自聚焦特征数值. 测试结果表明时间低相干光的非线性自聚焦效应比传统单模脉冲激光更强. 本文不仅设计了一套更加精确的对比不同激光非线性效应的测试方法, 同时也探明了时间相干性对于非线性自聚焦效应的影响机制, 为高功率时间低相干激光器的设计提供理论依据和参考.The low-temporal coherence light (LTCL) has received extensive attention in the research of inertial confinement fusion due to its physical properties of instantaneous broadband. Recent reports demonstrated that the LTCL has significant suppression effects on laser plasma instability. However, the temporal spike structures of the LTCL will not only induce the amplification of the self-focusing effect, but also make its small-scale self-focusing characteristics and corresponding damage mechanism more complicated. Exploring the self-focusing characteristics of the LTCL will provide an important information for improving the output power of the LTCL. In this work, we design a more accurate test method for comparing the nonlinear self-focusing effects of different lasers, and compare the self-focusing effect of LTCL with single longitudinal mode (SLM) pulse. In the experiments, fused silica is tightly focused by a short focal length lens to avoid damaging the input surface. A spatially resolved test method is designed to measure the nonlinear I×L value (where I is the incident intensity, L is the distance from the head of filamentation damage to the input surface), which is accumulated from the input surface to the head of filamentation damage. The results show that the nonlinear I×L value obtained by the spatially resolved method is lower than by the traditional test method, since the energy loss caused by incident surface damage and backward stimulated Brillouin scattering (SBS) has been resolved. Furthermore, the nonlinear I×L values of the SLM pulse and the LTCL are also compared by the traditional test method and spatially resolved method. The test results show that due to the temporal spike structure, the LTCL has a lower nonlinear I×L value than the SLM pulse. The SBS effect and the different damage characteristics of the input surface are also analyzed. This study provides a more accurate test method for better analyzing the self-focusing effect of LTCL and laser pulses with different characteristics, and hence presenting a reference for designing high-power devices of low-temporal coherence light.
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Keywords:
- inertial confinement fusion /
- low-temporal coherence light /
- nonlinear self-focusing /
- critical value /
- spatial resolved method
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图 1 (a)传统非线性$ I\times L $测试法; (b)空间分辨测试法
Fig. 1. (a) Traditional nonlinear $ I\times L $ test method; (b) spatial resolved test method.
图 2 理论计算得到的 (a)激光在熔石英体内不同位置的光斑尺寸; (b)空间分辨法得到激光在熔石英体内不同位置的非线性$ I\times L $数值
Fig. 2. Theoretical calculation of (a) the beam size at different position of the fused silica, and (b) the nonlinear $ I\times L $ of different position obtained by the spatial resolution method.
图 3 非线性$ I\times L $测试结果 (a)传统测试方法; (b)空间分辨测试法
Fig. 3. Non-linear I × L test results: (a) Traditional test method; (b) spatially resolved test method.
图 4 SBS反射率随入射能量的变化 (a)传统测试法; (b)空间分辨测试法
Fig. 4. Reflectivity of stimulated Brillouin scattering as a function of the incident energy: (a) Traditional test method; (b) the spatial resolved test method.
图 5 (a)时间低相干光和传统单模脉冲激光的光谱测试图; (b)时间低相干光的时域测试图以及时间尖峰结构示意图
Fig. 5. (a) Spectrum of low-temporal coherence light and traditional single longitudinal mode pulse laser; (b) the temporal test pattern of low-temporal coherence light and schematic diagram of temporal spike structures.
图 6 激光诱导熔石英前表面损伤的形貌 (a)单模脉冲激光; (b)时间低相干光
Fig. 6. Laser-induced input surface damage morphologies: (a) Single longitudinal mode pulse laser; (b) the low-temporal coherence light.
表 1 传统测试法和空间分辨法测得的单模脉冲激光和时间低相干光的非线性$ I\times {L}_{{\mathrm{m}}{\mathrm{i}}{\mathrm{n}}} $
Table 1. Nonlinear $ I\times {L}_{{\mathrm{m}}{\mathrm{i}}{\mathrm{n}}} $ of single longitudinal mode pulse laser and the low-temporal coherence light measured by traditional test method and spatial resolved method.
$ I\times {L}_{{\mathrm{m}}{\mathrm{i}}{\mathrm{n}}} $ /(GW·cm–1) 单模脉冲激光 时间低相干光 传统测试法 157.56 49.70 空间分辨法 15.19 7.48 
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