Accurate model and performance analysis of broadband pulsed amplification in picosecond petawatt laser system
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摘要: 为准确分析皮秒拍瓦激光系统的频域放大特性, 通过引入钕玻璃实际受激发射截面, 建立了宽频带激光放大的精确模型, 对比分析了常用高斯线型近似的不足. 针对神光II高能拍瓦激光系统, 分析了不同线型下, 注入种子的光谱形状、中心波长以及能量稳定性对放大系统的影响. 结果表明: 实际线型会加剧增益窄化效应; 对于107 增益, 光谱将窄化为3 nm, 系统累积B积分增大至1.7; 窄化效应降低了注入种子中心波长的要求, 增益饱和会使输出能量稳定性提升近一倍. 在上述基础上, 进行了宽频带激光放大的实验研究, 对于注入的10 nm (FWHM)超高斯、1054 nm中心波长、3% (RMS)稳定性的参量放大种子, 实现了1900 J、中心波长1054.2 nm、谱宽3 nm的输出, 发次能量稳定性 < 1.8 %, 与分析结果一致. 本文结果将对国内基于钕玻璃的高能宽带激光装置建设和改进提供重要的参考依据.Abstract: In order to accurately analyze the broadband pulsed amplification performances of the domestic picosecond petawatt laser system, which uses large aperture N31 or N41 neodymium glass as gain medium, the broadband pulsed amplification model is improved by introducing the actual stimulated emission cross section (SECS) of neodymium glass. Comparing with the SECS under Gaussian approximation, the amplified pulsed spectrum gain narrowing effect with different SECSs are analyzed. It is found that in the actual SECS of N31 neodymium glass laser, the gain-narrowing effect is enhanced, the output energy decreases, gain’s saturation effect weakens, system’s accumulated B integral augments, but the laser system turns insensitive to the center wavelength simultaneously. Based on the Shenguang II high energy picosecond petawatt laser system which uses N31 neodymium glass, the spectral shape, center wavelength, and energy stability of amplified output pulse are simulated by using different SECSs. It is shown that the super-Gaussian spectral shape narrows more greatly than Gaussian spectral shape, the spectrum bandwidth narrows from 10 to about 3 nm with gain larger than 107, and the accumulated B integral increases to 1.7. Additionally, the gain-narrowing effect makes the output spectrum (with 1054 nm of center wavelength) less affected by changing the inputted center wavelength from 1052 to 1056 nm, and the gain saturation effect can improve output energy stability to less than 2% (root mean square (RMS)) with about 3% (RMS) inputted energy stability, which are beneficial to the subsequent pulse compression and physical experiment. Based on the above analysis, a broadband pulsed amplified experiment is conducted by using Shenguang II petawatt laser system, the injected seed is about 10 nm (full width at half maximum (FWHM)) with 5 order super Gaussian shape at 1054-nm center wavelength, and 1.2 mJ with 3% (RMS) energy stability from optical parametric chirped pulse amplification. The amplified pulse with 1900 J at 1054.2 nm (3 nm FWHM) and stability < 2% (shot to shot) is achieved, and the spectral shapes and bandwidths after bar and disk amplifiers are measured, which are consistent with theoretical analysis results. The results can provide a necessary reference for constructing high energy broadband laser system and improving its performances in the future.
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图 1 国内N31型磷酸盐钕玻璃实际SECS和高斯近似SECS对比
Figure 1. The compared SECSs between real N31 glass and Gaussian approximation.
图 2 小信号增益下, 10 nm (FWHM)高斯型光谱注入时, 不同SECS下增益窄化分析结果的对比 (a) 10 nm (FWHM)高斯光谱注入; (b) 高斯SECS和实际SECS下光谱窄化分析结果对比
Figure 2. In small-signal-gain regime and input of 10 nm (FWHM) Gaussian spectrum, the compared results of gain narrowing by different SECSs: (a) Input of 10 nm(FWHM) Gaussian spectrum; (b) the results of gain narrowing by Gaussian SECS and real SECS.
图 3 采用不同SECS时, 增益窄化对输出光谱宽度影响的对比分析 (a) 10 nm(FWHM), 5阶超高斯注入光谱; (b)采用高斯SECS和实际SECS时, 光谱窄化分析结果对比
Figure 3. The influence results of gain narrowing to spectrum bandwidth by different SECSs: (a) Input of 10 nm (FWHM), 5-order super-Gaussian spectrum; (b) the compared gain narrowing results between Gaussian SECS and real SECS.
图 4 不同SECS 下, 棒放(a), (b)和片放(c), (d)输出光谱形状及上能级粒子变化分析结果的对比
Figure 4. The compared numerical results of spectrum and upper state population after 70 (a), (b) and 350 (c), (d) amplifier, which influenced by different SECSs.
图 5 不同SECS下, 注入种子中心波长变化对放大光谱特性的影响, 其他参数与图4(c)相同 (a) 不同中心波长的注入光谱; (b) 高斯SECS下的放大光谱; (c) 实际SECS下的放大光谱
Figure 5. The influences of different inputted center wavelength spectrums to amplified spectrum by different SECSs: (a) Input spectrums of different center wavelength; the amplified spectrums by Gaussian SECS (b) and real SECS (c).
图 6 不同SECS 下, 宽频带激光放大输出和输入能量抖动性的分析曲线对比
Figure 6. The simulation relationship between input and output energy jitter by different SECSs.
图 7 神光Ⅱ拍瓦激光系统宽频带激光传输放大示意图
Figure 7. Block diagram of SG II PW laser amplification chain.
图 8 10 nm(FWHM), 5阶超高斯光谱注入, 输出1866 J时, 棒放和片放位置的实验数据与图4理论分析结果的对比 (a) 输入光谱实验及拟合数据; 棒放(b)和片放(c)位置光谱对比
Figure 8. The compared results between experiment and simulation results after bar and disk amplifiers: (a) The compared input spectrums of experiment and simulation; the compared spectrum results after rob amplifier (b) and disk amplifier (c).
表 1 注入1.2 mJ, 5 ns, 10 nm宽带种子, 不同SECS下, 棒放和片放输出位置主要参数分析结果对比.
Table 1. Input a 1.2 mJ, 5 ns, 10 nm broadband seed, and the main simulation parameters after bar and disk amplifier, which influenced by different SECSs.
位置 激光参数 实际SECS 高斯SECS 棒放 能量/J 24.43 34.23 光谱谱宽/nm 3.6 6 中心波长/nm 1054 1054.5 ∑B 0.24 0.22 片放 能量/J 1957 2369 光谱宽度/nm 3.1 4.5 中心波长/nm 1054.2 1055.5 ∑B 1.7 1.57 
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表 2 利用神光II高能拍瓦宽频带激光系统得到的实验数据.
Table 2. The experiment results of amplified broadband laser by using SG II PW laser amplification chain.
参数 发次1 发次2 发次3 发次4 实际输出/J 1482 1642 1711 1866 光谱宽度/nm 3.1 3.2 3.0 3.0 中心波长/nm 1054 1054 1054.1 1054.2 预估能量/J 1500 1650 1700 1900 偏差/% 1.2 0.5 0.6 1.8
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