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冲击点火方案具备低点火能量阈值、高增益以及更好的流体力学稳定性等优势, 已成为实现惯性约束聚变点火的核心方案之一. 在冲击点火方案中, 高质量的冲击脉冲是实现成功点火的必要条件. 本文基于光纤环相位调制时间透镜系统, 提出一种利用时域非对称相位调制结合频域线性色散补偿的方案产生对脉宽和峰值功率对比度高精度可控的冲击脉冲, 并构建了理论模型, 通过数值模拟详细分析了系统关键参数对冲击脉冲特性的影响. 模拟结果显示, 通过对斩波函数、相位调制函数、调制深度、调制频率以及啁啾补偿量等参数的组合优化设计, 可以实现对冲击脉冲的脉冲宽度、脉冲上升沿以及冲击脉冲峰值功率对比度等关键性能指标高精度主动调控. 这种对冲击脉冲峰值功率对比度与冲击脉冲宽度独立主动可调的新型设计思路, 不仅有利于加深对激光脉冲波形操控原理的理解, 而且对实验上如何获取高质量的冲击脉冲具有重要参考意义.
The shock ignition scheme has the advantages of low ignition energy threshold, high gain, and good hydrodynamic stability, which has become one of the key schemes for the potentially successful ignition of inertial confinement fusion. The crucial element of shock ignition is how to achieve a highly efficient shock laser pulse. We propose a new scheme based on a time-lens system combining the fiber-loop phase modulation and the grating-pair compression to generate a highly controllable shock pulse. Based on the asymmetric phase modulation in time-domain followed by linear dispersion compensation in frequency domain, the shock pulse can be actively controlled with high precision in both pulse duration and pulse contrast (peak power ratio of the compression part to the shock part of the pulse). We construct a theoretical model based on the nonlinear Schrödinger equation to simulate the evolution of the spectrum and temporal shape of the shock laser pulse. The influences of various key parameters of the proposed system on the characteristics of the generated shock pulse are analyzed in depth. The time lens system consists of three parts, i.e. the seed pulse carving part, the phase modulation loop, and the chirp-compensating grating pair. The operation principle of this system for generating shock pulse is as follows. First, a single-mode continuous wave 1053 nm distributed feedback seed laser is chopped into pulses with a Mach-Zehnder intensity modulator. Then the pulses enter into a fiber-loop for phase modulation. Owing to different modulation frequencies exerted on the left and right side of the pulse, the amount of spectral broadening of these two sides of the spectrum are also different after phase modulation. The spectrally broadened pulses are linearly chirped when the phase-modulation function has a parabolic shape. Finally, the pulse transits through a grating pair system for chirp compensating. Just like an anomalous dispersion delay line, the grating pair applies an anomalous group velocity dispersion to the passing optical pulse. When the chirp is compensated for appropriately, the pulse will be compressed. What the target pulse can be finally shaped into is dependent on the combined optimization of all the above processes. The simulation results show that by systematically designing the parameters such as chopping function, phase modulation function, modulation depth, modulation frequency, and chirp compensating, the target shock pulse can be actively controlled with high-precision in the pulse width, pulse rising edge, and peak-power contrast. In addition, we can also tune only one parameter (such as the pulse width) of the pulse, with the other parameters kept unchanged. This new design idea and the proposed system can actively and independently adjust the two key parameters (the peak power contrast and the pulse width) of the generated shock pulse, which is not only helpful in deepening our understanding of the principle of laser-pulse shaping, but also significant for the subsequent practical implement of shock ignition of inertial confinement fusion. -
Keywords:
- shock ignition /
- shock pulse /
- time-lens /
- electro-optic modulation
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图 1 时间透镜装置图(MZ, 马赫-曾德尔调制器; YDFA, 掺镱光纤放大器; AWG, 任意波形发生器; BPF, 带通滤波器; PM, 位相调制器; G1和G2, 光栅1和光栅2)
Fig. 1. Schematic setup of the time lens concept (MZ, Mach-Zehnder modulator; YDFA, ytterbium-doped fiber amplifier; AWG, arbitrary waveform generator; BPF, band-pass filter; PM, phase modulator; G1 and G2, grating1 and grating2).
图 2 强度调制器斩波出的脉冲频谱(a)与波形(b)
Fig. 2. Spectrum (a) and pulseshape (b) of the seed laser after the intensity modulator.
图 4 考虑色散与非线性效应时脉冲信号经过光纤环不同圈数相位调制之后的频谱(a)与脉冲(b) 的演化
Fig. 4. Evolution of the spectrum (a) and shape (b) of the pulse after different round trips of phase modulation (before compression) when the dispersion and nonlinear effects of fibers are included.
图 5 不同光栅对距离对最终输出脉冲的影响(光纤环环绕圈数设定为55圈, 光栅对角度设定为30°)
Fig. 5. Final output pulse shape with different grating-pair distance settings (the number of fiber loops is set to 20 turns and the grating pair angle is set to 30 degrees).
图 6 输入脉冲陡峭度对经过时间透镜系统之后的频谱展宽(a)和脉冲波形(b)的影响
Fig. 6. Influences of the input pulse shape (different orders of Gaussian function) on the spectrum (a) and pulse shape (b) of the output pulse after being operated by the time lens system.
图 7 压缩量一定时, 频谱展宽量不一样时被压缩输出后的脉冲 (a), (b) 表示调制深度不同的情况下, 频谱展宽与被压缩输出后的脉冲; (c), (d) 表示相位调制次数不同, 频谱展宽与被压缩输出后的脉冲
Fig. 7. Output pulse after different amount of spectrum broadening when the amount of compression is constant: (a), (b) Broadening spectrum and the output pulse after different modulation depth; (c), (d) the broadening spectrum and output pulse after different round trips.
图 8 不同参数设计下最终压缩输出的脉冲 (a) 控制冲击脉冲宽度不变, 改变冲击脉冲峰值功率对比度; (b) 控制冲击脉冲峰值功率之比不变, 改变冲击脉冲宽度
Fig. 8. Final output pulse under different combined-parameter design: (a) Tuning the ratio of the peak power of the shock pulse and the compress pulse while keeping the shock pulse width unchanged; (b) modifying the shock pulse width while keeping the ratio of the peak power of the shock pulse to the compress pulse unchanged.
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[1] Ongena J, Koch R, Wolf R, Zohm H 2016 Nat. Phys. 12 398
Google Scholar
[2] Lowdermilk W H 1997 Proc. SPIE 3047 16
Google Scholar
[3] Lindl J D 1995 Phys. Plasmas 2 3933
Google Scholar
[4] Lindl J D, Amendt P, Berger R L, Glendinning S G, Glenzer S H, Haan S W, Kauffman R L, Landen O L, Suter L J 2004 Phys. Plasmas 11 339
Google Scholar
[5] Hurricane O A, Callahan D A, Casey D T, Celliers P M, Cerjan C, Dewald E L, Dittrich T R, Doppner T, Hinkel D E, Hopkins L F B, Kline J L, Le Pape S, Ma T, MacPhee A G, Milovich J L, Pak A, Park H S, Patel P K, Remington B A, Salmonson J D, Springer P T, Tommasini R 2014 Nature 506 343
Google Scholar
[6] Wang W M, Gibbon P, Sheng Z M, Li Y T 2015 Phys. Rev. Lett. 114 015001
Google Scholar
[7] Tabak M, Hammer J, Glinsky M E, Kruer W L, Wilks S C, Woodworth J, Campbell E M, Perry M D 1994 Phys. Plasmas 1 1626
Google Scholar
[8] Betti R, Zhou C D, Anderson K S, Perkins L J, Theobald W, Solodov A A 2007 Phys. Rev. Lett. 98 155001
Google Scholar
[9] Cristoforetti G, Antonelli L, Atzeni S, Baffigi F, Barbato F, Batani D, Boutoux G, Colaitis A, Dostal J, Dudzak R, Juha L, Koester P, Marocchino A, Mancelli D, Nicolai P, Renner O, Santos J J, Schiavi A, Skoric M M, Smid M, Straka P, Gizzi L A 2018 Phys. Plasmas 25 012702
Google Scholar
[10] 袁强, 胡东霞, 张鑫, 赵军普, 胡思得, 黄文会, 魏晓峰 2011 物理学报 60 015202
Google Scholar
Yuan Q, Hu D X, Zhang X, Zhao J P, Hu S D, Huang W H, Wei X F 2011 Acta Phys. Sin. 60 015202
Google Scholar
[11] Moses E I, Boyd R N, Remington B A, Keane C J, Al-Ayat R 2009 Phys. Plasmas 16 041006
Google Scholar
[12] Theobald W, Nora R, Seka W, Lafon M, Anderson K S, Hohenberger M, Marshall F J, Michel D T, Solodov A A, Stoeckl C, Edgell D H, Yaakobi B, Casner A, Reverdin C, Ribeyre X, Shvydky A, Vallet A, Peebles J, Beg F N, Wei M S, Betti R 2015 Phys. Plasmas 22 056310
[13] Nora R, Theobald W, Betti R, Marshall F J, Michel D T, Seka W, Yaakobi B, Lafon M, Stoeckl C, Delettrez J, Solodov A A, Casner A, Reverdin C, Ribeyre X, Vallet A, Peebles J, Beg F N, Wei M S 2015 Phys. Rev. Lett. 114 045001
Google Scholar
[14] Casner A, Caillaud T, Darbon S, Duval A, Thfouin I, Jadaud J P, LeBreton J P, Reverdin C, Rosse B, Rosch R, Blanchot N, Villette B, Wrobel R, Miquel J L 2015 High Energy Density Phys. 17 2
Google Scholar
[15] Batani D, Koenig M, Baton S, Perez F, Gizzi L A, Koester P, Labate L, Honrubia J, Antonelli L, Morace A, Volpe L, Santos J, Schurtz G, Hulin S, Ribeyre X, Fourment C, Nicolai P, Vauzour B, Gremillet L, Nazarov W, Pasley J, Richetta M, Lancaster K, Spindloe Ch, Tolley M, Neely D, Kozlová M, Nejdl J, Rus B, Wolowski J, Badziak J, Dorchies F 2011 Plasma Phys. Controll. Fusion 53 124041
Google Scholar
[16] 袁强, 胡东霞, 张鑫, 赵军普, 胡思得, 黄文会, 魏晓峰 2011 物理学报 60 045207
Google Scholar
Yuan Q, Hu D X, Zhang X, Zhao J P, Hu S D, Huang W H, Wei X F 2011 Acta Phys. Sin. 60 045207
Google Scholar
[17] Batani D, Baton S, Casner A, Depierreux S, Hohenberger M, Klimo O, Koenig M, Labaune C, Ribeyre X, Rousseaux C, Schurtz G, Theobald W, Tikhonchuk V T 2014 Nucl. Fusion 54 054009
Google Scholar
[18] Perkins L J, Betti R, LaFortune K N, Williams W H 2009 Phys. Rev. Lett. 103 045004
Google Scholar
[19] 袁强, 魏晓峰, 张小民, 张鑫, 赵军普, 黄文会, 胡东霞 2012 物理学报 61 114206
Google Scholar
Yuan Q, Wei X F, Zhang X M, Zhang X, Zhao J P, Huang W H, Hu D X 2012 Acta Phys. Sin. 61 114206
Google Scholar
[20] Howe J V, Lee J H, Xu C 2007 Opt. Lett. 32 1408
Google Scholar
[21] Foster M A, Salem R, Geraghty D F, Turner-Foster A C, Lipson M, Gaeta A L 2008 Nature 456 81
Google Scholar
[22] Backus S, Durfee C G, Murnane M M, Kapteyn H C 1998 Rev. Sci. Instrum. 69 1207
Google Scholar
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