激光等离子体受激散射的线性理论研究

利用包含两种阻尼(Landau阻尼和碰撞阻尼)成分的受激拉曼散射(SRS)、受激布里渊散射线性分析程序LIP,在给定的等离子体状态下分析了阻尼成分、激光强度以及等离子体组分对点火尺度等离子体中受激散射不稳定性发展的影响.详细评估了碰撞阻尼的效应,发现碰撞阻尼在接近1/4临界密度处能有效抑制受激散射的增长,从而造成SRS光谱上的"缝"现象.同时,电子温度的升高会在特定的密度区域促进SRS的发展.研究结果可为点火实验的设计提供参考.     

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     

密集波分复用石英光纤通信系统中受激Raman散射的稳态分析模型

假设石英光纤的Raman增益谱为线性谱,并给出了拟合直线.以此为基础,得到了前向N信道 受激Raman散射稳态耦合波方程的解析解.这个解析解是在考虑了N个信号光之间串话下得到 的,它适用于任意功率大小的信号光和任意信道间隔排列的情况.N个信号光在石英光纤中经 过受激Raman散射作用后,具有以下特点：在传输过程中,任意两信道的信号光光子通量的比 值随光纤的有效互作用长度、总的输入光子通量和两信道频率间隔按指数规律变化.解析解 与数值解进行了比较,两者取得了很好的一致. 关键词： 受激Raman散射 密集波分复用 石英光纤 Raman放大     

三角激光脉冲尾波加速粒子模拟

电子俘获是激光尾波场加速电子的主要机理，增大电子的初速度可以使更多的电子被尾波场俘获.提出三角脉冲激发尾波加速电子的方案，三角脉冲平缓上升沿激发受激Raman散射，用以初步加速电子，三角脉冲陡峭下降沿激发尾波场，将更多的电子加速到接近光速.2D3V粒子模拟结果证实了这一点.同时表明：脉冲长度为几个等离子体波长的超强激光在稀薄等离子体中传播时，还激发侧向Raman散射.在侧向受激Raman散射中，静电波增长最快的波矢模式为k_p=(2ω_p/ω₀ 关键词： 有质动力 电子俘获 前向受激Raman散射 侧向受激Raman散射     

金属蒸气受激喇曼散射的原子基态抽空效应

将受激Raman散射原子基态及终态粒子数变化的速率方程与聚焦的泵浦光束及喇曼光束功率变化的微分方程组联立并进行数值求解;研究了金属蒸气受激Raman散射过程中基态粒子数的抽空现象及其对Raman能量转换过程的影响。     

液芯波导中受激动力学散射与受激拉曼散射的竞争

研究了液芯波导中受激拉曼散射和受激动力学散射的阈值随泵浦激光脉宽的变化，发现受激动力学散射的阈值随泵浦脉冲前沿增长率增大而下降。研究了受激动力学散射和受激拉曼散射的竞争，结果表明受激动力学散射可分解为两种成分，其中与瞬态泵浦功率密度增长率相关的成分在大于阈值的泵浦脉冲前沿处于竞争优势，与瞬态泵浦功率密度相关的成分在高泵浦功率密度处于竞争优势，并讨论了其相应的机制。     

351nm激光入射大腔靶的受激Raman散射光谱

 报道了“神光 Ⅱ”装置上Au大柱腔靶产生的受激Raman散射光谱。通过分析实验条件和测量结果，排除了散射光谱来自增强非相干Thomson散射的可能性，发现用丝化不稳定性与受激Raman散射的耦合能合理解释观测到的Raman光谱。考虑到丝化不稳定性与SRS的耦合，测量到的散射光谱依然能用于密度诊断，其结果与对流SRS理论的计算值相差不到10%。     

部分离化激光等离子体的受激喇曼散射不稳定性

由于部分离化激光等离子体中存在束缚电子，它可显著改变其参量不稳定性。本文分析了部分离化激光等离子体的受激喇曼散射的非线性色散关系，计算了钕玻璃倍频和三倍频激光受激喇曼散射的增长率，结果表明，前向受激喇曼散射显著增强，后向受激喇曼散射影响不大。     

非Maxwell电子速度分布对受激散射的影响

从考虑动理学效应的受激散射不稳定性的线性理论出发,对于n=2的Maxwell分布函数直到n=5的饱和情况的超高斯分布函数,计算了电子等离子体波和离子声波的频率和阻尼率。对受激喇曼散射和受激布里渊散射进行分析,结果表明,在激光高Z等离子体中,或者在具有激光热斑的中等Z等离子体中,电子等离子体波的阻尼率降低很多,离子声波的频率比Maxwell分布情况升高约15%。这些结果可和实验进行比较。     

单模石英光纤中连续波泵浦SRS谱的演化

利用连续波掺Yb双包层光纤激光器为泵浦源,对单模石英光纤中受激Raman散射谱的形成过程进行了实验研究结果表明,由自发Raman散射向受激Raman散射演化的过程中Stokes谱宽度不断变窄当Stokes波信号功率较强时,在Raman光谱内部会出现能量红移现象,使Stokes光谱峰值相对于泵浦波的频移量从440cm^-1转化到490cm^-1.     

Effects of Landau damping and collision on stimulated Raman scattering with various phase-space distributions

Stimulated Raman scattering (SRS) is one of the main instabilities affecting success of fusion ignition. Here, we study the relationship between Raman growth and Landau damping with various distribution functions combining the analytic formulas and Vlasov simulations. The Landau damping obtained by Vlasov-Poisson simulation and Raman growth rate obtained by Vlasov-Maxwell simulation are anti-correlated, which is consistent with our theoretical analysis quantitatively. Maxwellian distribution, flattened distribution, and bi-Maxwellian distribution are studied in detail, which represent three typical stages of SRS. We also demonstrate the effects of plateau width, hot-electron fraction, hot-to-cold electron temperature ratio, and collisional damping on the Landau damping and growth rate. They gives us a deep understanding of SRS and possible ways to mitigate SRS through manipulating distribution functions to a high Landau damping regime.     

Saturation of backward stimulated scattering of a laser beam in the kinetic regime

Stimulated Raman (SRS) and Brillouin scattering (SBS) are examined in the kinetic regime using particle-in-cell simulations. Wave front bowing of electron-plasma waves (ion-acoustic waves) from trapped particle nonlinear frequency shift is observed in the SRS (SBS) regime for the first time. Self-focusing from trapped particle modulational instability (TPMI) is shown to occur in 2D and 3D SRS simulations. The key physics of SRS saturation is identified as a combination of wave front bowing, TPMI, and self-focusing: Bowing marks the beginning of SRS saturation and self-focusing terminates SRS. Ion-acoustic wave bowing also contributes to SBS saturation. Velocity diffusion by transverse modes and rapid loss of hot electrons in regions of small transverse extent formed from self-focusing dissipate wave energy and increase Landau damping, despite trapping that reduces Landau damping initially.     

Picosecond stimulated Raman scattering in crystals

The comparative values of the peak and integral cross sections of spontaneous Raman scattering and the optical dephasing time of molecular vibrations were determined for several oxide crystals by spontaneous Raman spectroscopy. The spectral, time, and energy parameters of stimulated Raman scattering (SRS) were measured for ten crystals using picosecond YLF: Nd laser pumping with a radiation wavelength of 1047 nm. An analysis of the experimental dependence of the threshold energy of pumping SRS on the integral and peak cross sections of spontaneous Raman scattering showed that the SRS gain increment explicitly depended on the integral cross section and was independent of the peak cross section of spontaneous Raman scattering as the ratio between the pumping pulse width (11 ps) and the time of optical dephasing of molecular vibrations changed from 0.42 to 9.3. The gain coefficients of steady-state stimulated Raman scattering under threshold stimulated Raman scattering conditions were determined for all the crystals studied on the basis of the measured threshold SRS pumping energies, the duration and width of the spectrum of pulses, the nonlinear interaction length, the intensity of pumping, and the theoretical dependences that relate the steady-state and transient SRS gain increments. The steady-state SRS gain coefficients obtained in this work fitted well a linear dependence on the peak cross sections of spontaneous Raman scattering, which substantiated the correctness of our analysis and measurements.     

拉曼增益系数可分离光纤中的受激拉曼散射

给出了拉曼增益系数分离光纤的定义,在考虑了N个单色光之间的两两受激拉曼散射,得到了这种光纤中单向传输的N个不同的单色光之间的受激拉曼散射（SRS）稳态耦合波方程的解析解。它适用于各单色光功率任意大小、光频率分布不均匀时的一般情况。这一结果可应用于光纤通信系统、拉曼光纤放大器和拉曼光纤激光器的设计与分析。最后把解析解与数值解进行了比较,两者取得了很好的一致。     

Raman Microscopy and Associated Techniques for Label-Free Imaging of Cancer Tissue

Abstract

Raman spectroscopy can identify cancerous from healthy tissue, with a chemical analysis from the measurement of vibrational bond frequencies. However, to detect small tumors a form of Raman imaging is required. Such imaging—by acquiring a Raman spectrum at each imaging pixel—can detect tumors but is rather slow. Multiphoton versions of Raman—anti-Stokes Raman scattering (CARS) microscopy and stimulated Raman scattering (SRS) microscopy—offer similar accuracies in identifying cancerous tissue and tumor margins but with a far higher speed, which is beneficial for diagnosis of small tumors in tissue. SRS microscopy can also be used to image extrinsic molecules in living cells, such as anti-cancer drugs at typical concentrations.     

Trapped-particle instability leading to bursting in stimulated Raman scattering simulations

Nonlinear, kinetic simulations of stimulated Raman scattering (SRS) under laser-fusion conditions present a bursting behavior. Different explanations for this regime have been given in previous studies: saturation of SRS by increased nonlinear Landau damping [K. Estabrook et al., Phys. Fluids B 1, 1282 (1989)]], and detuning due to the nonlinear frequency shift of the plasma wave [H. X. Vu et al., Phys. Rev. Lett. 86, 4306 (2001)]]. Another mechanism, also assigning a key role to the trapped electrons is proposed here: the breakup of the plasma wave through the trapped-particle instability.     

自受激拉曼晶体Nd3+:SrMoO4的光谱性质研究

通过拉曼散射光谱，吸收光谱，荧光发射寿命和808 nm LD激发下的红外荧光光谱的实验测量，系统研究了Nd³⁺:SrMoO₄晶体的自受激拉曼光谱性质.分析指认了拉曼散射光谱中各拉曼峰所对应的晶格振动模式，得出了其SRS活性最强的声子频率约为898 cm^-1，对应于(MoO^2-₄)离子团的完全对称光学伸缩振动A_g模；通过J-O理论对晶体的吸收谱进行了全面的光谱参数计算，得出⁴F_3/2→⁴I_11/2跃迁的积分发射截面达0.57×10^-18 cm²，自发辐射概率为141.06 s^-1；同时，实验测得该跃迁的荧光发射寿命约为0.2 ms.最后，结合808 nm LD激发下的红外波段荧光光谱，论证了SrMoO₄晶体中Nd³⁺离子1068 nm发射通过拉曼频移获得1180 nm一级斯托克斯激光发射的可能性，为Nd³⁺:SrMoO₄晶体的自受激拉曼激光器研究提供了理论依据. 关键词： 3+离子')" href="#">d³⁺离子 4 晶体')" href="#">SrMoO₄ 晶体 自受激拉曼散射     

Influencing factors of external fluorescence seeding enhancing stimulated Raman scattering in liquid‐core optical fiber

This paper reports an external fluorescence seeding technology enhancing the stimulated Raman scattering (SRS) in liquid‐core optical fiber (LCOF). By surrounding a small section of LCOF with a glass capillary and a solution of Rhodamine 6G filled between them to separate fluorescent dye and Raman medium, the initial intensity of SRS is linearly amplified by external fluorescence seeding, and then the SRS of LCOF can be enhanced effectively. Experimental results show that both the concentration of fluorescent dye and the seeding position have an influence on enhancement of SRS. The maximum enhancement of Stokes lines is obtained when the concentration of dye solution is optimized at ~10⁻⁶ mol/l and seeding position is located at the input end. Copyright © 2013 John Wiley & Sons, Ltd.     

Numerical analysis of stimulated inelastic scatterings in ytterbium-doped double-clad fiber amplifier with multi-ns-duration and multi-hundred-kW peak-power output

We investigate the stimulated Brillouin scattering (SBS) and the stimulated Raman scattering (SRS) in an ytterbium-doped double-clad fiber amplifier that outputs optical pulses with multi-ns-duration and multi-hundred-kW peak-power. The ytterbium-doped double-clad fiber amplifier is simulated by a model which is composed of a set of propagation-rate equations. The simulated results show that SBS and SRS will deform the output signal pulse in both the time domain and the spectral domain, and degrade the performance of the ytterbium-doped double-clad fiber amplifier seriously. It is shown in our simulation that the troublesome SBS can be effectively suppressed by broadening the signal linewidth to a critical value of 0.07 nm in our calculation, and the effect of SRS can be suppressed using a large-mode-area fiber with proper length. The model and the simulated results are very useful for designing an ultra-high-power ytterbium-doped double-clad fiber amplifier.     

斯托克斯束的时空分布

基于在自由空间光束的分丰是已知的条件下，推导出适用于任意束分布的广义化的耦合波方程并得到一种计算受激拉曼散射过程中泵浦束与斯托克斯束的时、空分布的简单方法。所得结果清楚地示明受激拉曼散射过程既能使斯托克斯信号得到放大，又能使脉宽受到压缩。