超短超强激光与稀薄等离子体相互作用背向受激Raman谱特性模拟

 利用LPIC++程序模拟了超短超强脉冲与稀薄等离子体相互作用产生的背向受激Raman谱。结果证明：在极端相对论条件下,背向受激Raman谱不再是通常所指的弱耦合模式,而进入强耦合模式。频谱加宽,并融合在一起;各谱峰之间的频移不再以等离子体波的频率为间隔,而是小于电子等离子体波的频率。模拟了各种条件下的背向散射Raman谱特性,结果显示：随着密度的提高,背向受激Raman谱的强度也将提高,这与理论结果符合得较好。     

超强激光在均匀等离子体中的背向拉曼散射放大机制

使用等离子体背向受激拉曼散射对激光进行放大时,等离子体的密度、温度和长度都会对激光的放大效果产生影响.为了探究等离子体密度对结果的影响,本文使用一维粒子模拟程序模拟了波长为800 nm的泵浦激光入射到均匀等离子体中,等离子体密度和泵浦光光强对散射光光谱的影响.模拟结果表明,等离子体密度降低会导致散射光的波长变短,而泵浦光的光强在一定范围内降低会增加散射光中背向散射光的比例.通过分析散射光的光强和等离子体的密度,发现前向拉曼散射是等离子体密度变化的原因.模拟结果对等离子体背向受激拉曼散射放大的实验研究具有重要的指导意义.     

高强度激光与等离子体相互作用中的受激Raman级联散射、光子凝聚以及大振幅电磁孤立子的产生与加速

应用一维相对论电磁粒子模拟程序，详细研究了线性极化强激光入射到无碰撞稀疏密度长等离子体中引起的受激Raman散射、Raman级联散射、级联散射到光子凝聚、以及大振幅电磁孤立子的产生与加速. 通过研究发现：在适当的激光振幅和等离子体状态下，强的光子凝聚现象会导致大振幅电磁孤立子的产生，电磁孤立子可以以静止、向后以及向前加速的形式存在；在密度均匀的等离子体中，电磁孤立子的加速不仅依赖于激光振幅而且依赖于等离子体的长度；电磁孤立子的电磁频率大约为未扰动电子等离子体振荡频率的二分之一左右，孤立子内电磁场的电场具有半周期结构，相应电磁场的磁场以及静电场则具有一个完整的周期结构. 关键词： 粒子模拟 受激Raman散射 Raman级联散射 光子凝聚 电磁孤立子     

黑腔等离子体中SRS与SBS过程的散射光谱分析

针对惯性约束聚变中激光通过等离子体时,在光路上激发的受激Raman散射(SRS)与受激Brillouin散射(SBS)过程进行了研究。介绍了用于分析两种不稳定性过程背向散射光谱特征的程序模型,数值模拟了不同等离子体状态参数条件下的SRS与SBS过程,讨论了黑腔内等离子体参数条件对两种不稳定性过程及其散射光谱特征的影响,提出了有益于抑制两种不稳定过程的等离子体参数条件。     

驱动激光束间相干性以及对背向散射影响的研究

早期的研究表明, 束匀滑技术能够有效地抑制高功率激光与等离子体相互作用中产生的各种参量不稳定性, 大大减少受激布里渊散射和受激拉曼散射. 但在NIF最近的实验研究中发现, 受激布里渊散射和受激拉曼散射份额远远高于预期, 原因可能与驱动激光束间的相干性有关. 本文利用"神光II"装置两路倍频纳秒激光辐照金平面靶, 研究了小口径取样情况下两者背向散射的能量和光谱在不同驱动激光相干条件下的变化情况. 初步结果表明, 激光束间存在着较强的相干性, 并且随着束间相干程度的增加, 背向散射也逐渐增强.     

神光Ⅲ原型全孔径背向散射测量系统研制及应用

为了测量激光与等离子体相互作用产生的散射光份额,获得黑腔耦合效率实验中的激光注入率,研制了基于神光Ⅲ原型装置的全孔径背向散射测量系统。该系统利用聚焦透镜收集散射光,通过离轴抛物面镜进行缩束,并采用二向色镜将散射光分为两个支路后分别进行受激布里渊散射和受激拉曼散射光的能量、时间过程和光谱测量。实验测量了有、无束匀滑条件下的散射光份额。结果表明,在当前实验条件下,通过束匀滑可有效降低散射光份额,提高激光注入率。     

掺杂材料对激光等离子体受激布里渊散射的影响

研究了激光等离子体背向受激布里渊散射光谱及反射率。当小能量短脉冲激光入射平面靶时，激光等离子体发展不充分，受激布里渊散射光谱结构相似，受激布里渊散射的增长主要受等离于体尺度的影响，随靶材料的有效原子序数的增加，等离子体尺度变小，受激布里渊散射反射率减少。     

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介绍了在神光Ⅱ装置上开展的长脉冲2ns三倍频激光与黑腔靶相互作用的实验。报道了采用PIN探测器阵列测量大角度受激Raman散射（SRS）角分布和采用激光卡计对背向SRS光能量积分测量的实验结果。相同实验条件下激光辐照缝靶产生的SRS光能量要强于激光与全腔靶作用产生的SRS光，小腔靶的SRS光能量要强于标准腔靶。对比长脉冲2ns及短脉冲1ns激光打靶实验结果可以看出：由于激光功率密度的下降，长脉冲激光打靶时SRS散射光能量要弱于短脉冲激光打靶。长脉冲2ns激光与标准腔靶相互作用时，等离子体堵腔比较严重。     

啁啾激光脉冲受激拉曼散射理论和粒子模拟

从入射光波、散射光波和等离子体波的三波耦合方程出发,推导小带宽频率啁啾对拉曼散射的影响.得到频率啁啾对拉曼背向散射有抑制作用,且背向散射不依赖于啁啾符号;正(负)啁啾对拉曼前向散射有微弱的增强(减弱)的作用.最后采用1D3V粒子模拟程序观测了不同啁啾参数下的受激拉曼散射现象,得到与理论分析一致的结果.     

利用一维Vlasov和Maxwell方程模拟激光等离子体相互作用

应用一维Vlasov和Maxwell耦合程序详细研究了激光等离子体相互作用中的基本问题——受激拉曼散射(SRS). 通过研究发现, SRS的产生与电子速度分布函数在相空间中的结构密切相关, 当电子速度分布函数形成相空间涡旋时,背向SRS光大幅增加,而当电子等离子体波相速度附近的电子速度分布函数曲线变平坦后, 背向SRS光基本停止发生. 在模拟中观测到了SRS的爆发、电子速度分布函数形成相空间涡旋、电子俘获等清晰的物理图像. 关键词： Vlasov-Maxwell模拟 受激拉曼散射     

三倍频Nd∶YAG激光抽运氧气中的受激拉曼和布里渊散射

报道了三倍频脉冲Nd∶YAG激光(355 nm)在两种不同带宽模式下抽运氧气中受激拉曼散射(SRS)和受激布里渊散射(SBS)的实验研究。在宽带(约1 cm-1)抽运模式下,只测到了前向受激拉曼散射,而没有观察到后向散射,其一级和二级斯托克斯最大能量转换效率可达22%和8%。在窄带(约0.003 cm-1)模式下,前向、后向受激拉曼散射和受激布里渊散射都测量到了,但大部分抽运能量都转换到受激布里渊散射,其转换效率可达67%。测量了两种带宽模式下各散射组分在它们最佳转换时的波形;窄带情况下后向受激拉曼散射和受激布里渊散射的脉宽分别可压窄至1.5 ns和2.3 ns,不到抽运脉宽的三分之一,使得受激布里渊散射峰值功率可大大高于抽运功率。对氧气中前向、后向受激拉曼散射和受激布里渊散射之间的竞争进行了讨论。     

三倍频Nd:YAG激光抽运氧气中的受激拉曼和布里渊散射

报道了三倍频脉冲Nd：YAG激光（355nm）在两种不同带宽模式下抽运氧气中受激拉曼散射（SRS）和受激布里渊散射（SBS）的实验研究。在宽带（约1cm^-1）抽运模式下,只测到了前向受激拉曼散射,而没有观察到后向散射,其一级和二级斯托克斯最大能量转换效率可达22％和8％。在窄带（约0．003cm^-1）模式下,前向、后向受激拉曼散射和受激布里渊散射都测量到了,但大部分抽运能量都转换到受激布里渊散射,其转换效率可达67％。测量了两种带宽模式下各散射组分在它们最佳转换时的波形;窄带情况下后向受激拉曼散射和受激布里渊散射的脉宽分别可压窄至1．5ns和2．3ns,不到抽运脉宽的三分之一,使得受激布里渊散射峰值功率可大大高于抽运功率。对氧气中前向、后向受激拉曼散射和受激布里渊散射之间的竞争进行了讨论。     

Enhanced stimulated Raman scattering in H<Subscript>2</Subscript> by seeding an SRS line into an H-ɛ Balmer line: Amplification-coupled laser wavelength shifter

The stimulated Raman scattering (SRS) in H₂ gas above the dissociation energy limit was recorded using a 266-nm UV laser. All of the observed Stokes and anti-Stokes SRS lines showed a normal behavior except the third Stokes SRS lines at 397.8 nm, which showed a substantial intensity enhancement of about a 36%-conversion efficiency of the pump energy. This enhancement in the SRS line is attributed to the seeding of the SRS line into the Balmer H-? line at 397 nm in molecular hydrogen. To the best of our knowledge, there is no report of any work on enhanced stimulated Raman scattering in H₂ by the seeding of the H-? Balmer line into the SRS line and attaining a very high intensity at the third Stokes SRS lines at 397.8 nm. The cell pressure and the laser pulse energy dependence of these SRS lines substantiate our explanation.     

On the effect of long-wavelength electron plasma waves onlarge-angle stimulated Raman scattering of short laser pulse in plasmas

Spectral features of a large-angle stimulated Raman scattering (LA SRS) of a short electromagnetic pulse in an underdense plasma, which are caused by the presence in a plasma of a given linear long-wavelength electron plasma wave (LW EPW), are investigated. It is shown that the LW EPW, whose phase velocity coincides with a group velocity of a pulse and a density perturbation normalized to a background electron density δn_LW/n₀ exceeds the ratio of the electron plasma frequency to the laser frequency ω_pe/ω₀ suppresses the well-known Stokes branch of the weakly coupled LA SRS. Under the same condition, the anti-Stokes band appears in the spectrum of the scattered radiation. Variation of a scattering angle and an electron temperature do not significantly modify qualitative features of the effect. In the case of strongly coupled LA SRS, the maximum of the increment is decreased by nearly one-half for δn_LW/n₀~(a₀ω_pe /ω₀)^2/3≫ω_pe /ω₀, where a₀ is an amplitude of an electron quiver velocity in the laser field normalized to a speed of light c, and it decreases further with an increase in plasma density perturbation in LW EPW     

Interaction of excess electrons with water molecules at the early stage of laser-induced plasma generation in water

Forward and backward stimulated Raman scattering (SRS) of OH stretching vibrations are measured when an intense 40 ps pulsed beam is focused into water. Characteristic features related to the interaction of OH groups with excess electrons generated by the strong laser irradiation are observed. The SRS spectra are indicative of the important role which excess electrons play in transient enhancement of the SRS at the initial stage of laser-induced plasma generation in water. The hydration structures around the excess electrons in water are also discussed.     

Frequency comb expansion in a monolithic self‐mode‐locked laser concurrent with stimulated Raman scattering

The discovery of a novel phase‐locked frequency comb generated from a monolithic laser with the concurrent processes of self‐mode locking (SML) and stimulated Raman scattering (SRS) is reported. It is experimentally shown that the width of the Raman gain can be exploited to considerably expand the frequency comb of a monolithic SML crystal laser via the SRS process. At a pump power of 6.5 W, an output power of 140 mW in the Stokes wave with a pulse width as narrow as 2.9 ps at a pulse repetition rate of 6.615 GHz is obtained. The present finding not only provides useful insights into the monolithic intracavity SRS process but also paves the way for generating mode‐locked pulses based on monolithic self‐Raman crystals.     

Transient stimulated Raman scattering in a two-component gas mixture

Generation of the first Stokes components of stimulated Raman scattering (SRS) in two-component gas mixtures, including the combination component (resulting from scattering of the first (second) Stokes component in the second (first) gas) is studied by numerical methods. The condition for efficient generation of the combination Stokes component is derived and interpreted. It is shown theoretically and by numerical simulation that the optimal ratio of the pressures for the transient SRS generation of the combination component is inversely proportional to the ratio of the Raman scattering cross sections of the two gases. The results of numerical simulation agree well with experimental data.     

非线性拉曼激光雷达测量CO2气体的研究

提出了利用气体的受激拉曼散射(SRS)效应激光雷达光源来探测大气中的CO2气体的新方法,设计出探测大气中CO2气体含量的非线性拉曼增益激光雷达,用Nd:YAG激光器(1064 nm)的三倍频光(354.7 nm)通过分别装有CO2气体和N2气体的拉曼管,分别得到CO2气体和N2气体的受激拉曼散射的一阶斯托克斯线(S1),并用S1线作为雷达的种子发射光源.通过实验得到拉曼管中的气压与S1能量的变化关系,对其优化条件和物理机制进行了分析.该实验方法已经成功测出了大气中CO2气体的回波电压信号.