激光诱导等离子体开关控制脉宽实验研究

 利用激光诱导等离子体开关技术，在1 064 nm的Nd：YAG固体激光器上获得了脉宽4.4~6.4 ns的短脉冲激光输出。激光电离空气产生的等离子体开关控制脉冲宽度时，聚焦透镜焦距越短，压缩后的脉宽越窄，但激光能量损耗越大。压缩后的激光脉宽与激光能量近似成双曲线关系。在控制脉宽光路的焦点处放置带孔的Cu薄片可抑制等离子体的扩散，得到了脉宽最短可达4.4 ns的激光输出。     

光脉冲在密度随时间,空间变化的等离子体中的传播

本文定量地研究了光脉冲在密度随时间上升的等离子体中的频率上移,以及在密度随空间上升的等离子体中的脉宽压缩。通过求解波动方程,得到了描述光脉冲的时空演变特征的解析表达式,并在此基础上就脉冲形状、峰值位置、传播速度、脉冲宽度及光子数守恒等问题进行了讨论。     

Compton散射对等离子体中亚皮秒超高斯脉冲传输特性的影响

应用多光子非线性Compton散射模型、非线性薛定谔方程及龙格-库塔算法和变分法,研究了Compton散射对亚皮秒超高斯脉冲在等离子体传输的影响,给出了该脉冲传输演化的修正方程,并进行了数值计算,结果表明:与Compton散射前相比,Compton散射使脉宽压缩程度和非线性效应增大,脉冲展宽降低,非线性增强效应有效地削弱了色散效应;使脉宽趋于最大和最小时频率抖动,啁啾不为0;使超高斯脉冲振幅、啁啾和脉宽随传输距离增大振荡演化加快,频率和相位呈线性关系的抖动,脉冲中心位置呈准线性关系;Compton散射对超高斯脉冲保形传输有重要影响。     

飞秒孤子在色散渐增光纤中谱压缩的数值分析

通过数值求解修正了的广义非线性薛定谔方程,研究了孤子在色散渐增光纤中的谱压缩进程.详细分析了入射脉冲峰值功率对输出脉冲的谱宽和中心波长的影响,并描述了脉冲的脉宽、谱宽及啁啾在光纤中的演化过程.计算结果表明,脉宽200fs、中心波长1550nm的基孤子在群速度色散从-1ps2/km至-11ps2/km线性变化的长100m的色散渐增光纤中传输时,脉冲谱宽由12.6nm压缩至5.2nm,即可获得最大压缩比为2.42.     

飞秒孤子在色散渐增光纤中谱压缩的数值分析

通过数值求解修正了的广义非线性薛定谔方程,研究了孤子在色散渐增光纤中的谱压缩进程.详细分析了入射脉冲峰值功率对输出脉冲的谱宽和中心波长的影响,并描述了脉冲的脉宽、谱宽及啁啾在光纤中的演化过程.计算结果表明,脉宽200 fs、中心波长1 550 nm的基孤子在群速度色散从－1 ps2/km 至－11 ps2/km线性变化的长100 m的色散渐增光纤中传输时,脉冲谱宽由12.6 nm压缩至5.2 nm,即可获得最大压缩比为2.42.     

激光脉冲在等离子体中的压缩分裂

通过数值求解一维非线性薛定谔方程,研究了圆偏振入射激光脉冲在初始密度范围为1/4到略低于1倍临界密度的等离子体中的自压缩和分裂现象. 提高等离子体密度和入射激光强度以及减小脉冲宽度可以在更短的传输距离获得有效的激光脉冲压缩,压缩后的脉冲半高宽度可达到初始脉冲半高宽度的1/35,甚至更小. 这种压缩是激光脉冲在等离子体中形成高阶孤子的过程中产生的,可以获得比在稀薄等离子体中更好的压缩比例. 数值计算的结果给出了该情况下激光脉冲在等离子体中自压缩后形成的高阶孤子分裂. 利用一维粒子数值模拟程序(particle-in-cell,PIC)也观察到了脉冲的压缩和分裂现象,得到了与数值计算一致的结果. 关键词： 非线性薛定谔方程 自压缩 脉冲分裂 粒子模拟     

用变分法研究高阶色散和五阶非线性对高斯脉冲在光纤中传输特性的影响

从修正的非线性薛定谔方程出发,采用变分法,导出了在高阶色散和五阶非线性共同作用情况下高斯型脉冲参量随传输距离的演化方程组;求出了振幅与脉宽、频率与啁啾、脉宽与啁啾之间的三个重要约束关系;并进一步得出了脉宽随传输距离演化的解析解和脉冲中心位置随传输距离的演化规律;描绘了高阶色散和五阶非线性下,脉宽随传输距离演化的图形.结果表明:光纤中的高阶色散和五阶非线性都会影响高斯型脉冲各个参量的演化,但脉宽和振幅间的绝热关系并未改变.高阶色散使高斯型脉冲的脉宽展宽,五阶非线性使高斯型脉冲的脉宽压缩,它们对脉宽或初始啁啾的影响可以在一定程度上抵消,从而有可能使脉冲近似实现保形传输.     

200 Hz高重复频率SBS脉宽压缩实验研究

受激布里渊散射(SBS)脉宽压缩是实现高峰值功率、短脉冲激光输出的重要途径之一,然而,目前SBS脉宽压缩仅限于1~10 Hz低重复频率激光器,限制了高重频短脉冲激光器在激光雷达、空间碎片探测以及目标成像等领域的应用。基于此,开展了高重复频率下的SBS脉宽压缩实验研究。设计搭建了高重复频率的主振荡放大激光器,开展了SBS二次级联脉宽压缩和SBS振荡放大双池脉宽压缩实验。通过SBS二次级联压缩实现了脉冲宽度从~32 ns压缩到~1.9 ns,脉宽压缩比达16倍;而通过SBS振荡放大双池结构实现了脉冲宽度从~4 ns压缩到376 ps,脉宽压缩比达10倍。实验结果表明,采用该超净封闭型SBS相位共轭镜,在Stokes光输出能量达50 m J时,无光学击穿现象,实现了在200 Hz高重复频率下的SBS脉宽压缩。     

基于受激布里渊散射的200 ps激光脉冲放大的理论研究

利用受激布里渊散射(SBS)能量转移的方案可以实现高效的百皮秒激光脉冲放大,这种放大方案可以用于冲击点火中冲击脉冲的获取。通过在液体介质中激发声波场,实现了能量从长脉冲向短脉冲转移的过程。利用一个瞬态条件下SBS耦合波方程来对200ps脉冲的放大过程进行理论模拟。结果发现,利用SBS能量转移的方案可以将200ps的激光脉冲放大到较高的功率密度,同时这个放大过程也伴随着一定程度的脉宽压缩。     

利用等离子体非线性系数实现超强脉冲的压缩

超短脉冲压缩技术在强场物理研究中有非常重要的作用,但由于强场电离现象在惰性气体自相位调制脉冲压缩技术中限制了脉冲的能量。Tempea等人提出可以采用等离子体非线性系数对脉冲进行压缩,本文在考虑毛细管内表面电离的情况下,讨论能量为10mJ左右,脉宽为50fs的脉冲的压缩问题,发现可以将脉冲压缩至5fs左右。计算表明频谱展宽可以在气体密度很低的情况下进行,这样半可以减小电子对脉冲传输的影响。同时,由于毛细管内表面也处于电及状态,从而使脉冲能量不会受到电离阈值的.限制。     

Wakefield generation as the mechanism behind spectral shift of a short laser pulse

An equation describing the dynamics of plasma wave generation by a short intense laser pulse is analyzed to find a relation between the difference in mean-square pulse frequency before and after laser-matter interaction and the electric field amplitude in the wakefield plasma wave generated by the laser pulse. This relation can be effectively used in systems for wakefield diagnostics. The relation is applied to several geometries of interaction between a pulse and an ionizing gas or preformed plasma.     

On the focusing limit of high-power femtosecond laser pulse propagation in air

Focused propagation of high-power femtosecond laser radiation in air is considered. Based on numerical solution of the nonlinear Schr?dinger equation for complex envelope of light wave electric field, evolution of the beam effective radius is studied. The dependence of the effective (rms) size of a focal spot and the maximally achievable intensity of laser radiation at focal waist on the initial pulse power is established. It is shown that focal spot of tightly focused intensive ultrashort laser radiation can change its size during the pulse passage through the beam waist. This is the consequence of pulse intensity clamping in region of beam focusing caused by gas photoionization and plasma producing. This may prevent laser intensity from its further growth in the focal region and arrest the transversal compression of the beam in the linear focus as a whole.     

脉冲激光烧蚀Ge产生等离子体特性的数值模拟

 针对激光烧蚀半导体材料Ge初期的特点，建立了1维的热传导和流体动力学模型。对波长为248 nm、脉宽为17 ns、峰值功率密度为4×10⁸ W/cm²的KrF脉冲激光在133.32 Pa氦气环境下烧蚀Ge产生等离子体的特性进行了数值模拟。结果表明：单个激光脉冲对靶的烧蚀深度达到55 nm，蒸气膨胀前端由于压缩背景气体产生压缩冲击波, 波前的速度最大，温度很高。从不同时刻的电离率分布图中得出，在靶面附近区域，Ge的1阶电离始终占优势；在中心区域，脉冲作用时间内，Ge的2阶电离率比1阶电离率大，脉冲结束后，Ge的2阶电离率下降，1阶电离率逐渐变大。     

近共振区超短强激光脉冲激发的等离子体尾波场

 用一维相对论粒子模拟研究了相对论超短强激光脉冲在等离子体中传播时激发的尾波场，初步获得了近共振区尾波场的峰值幅度随激光脉冲宽度变化的特点，发现在近共振区等离子体波激发出现增强。通过准静态近似下尾波激发的一维非线性方程数值求解，并与粒子模拟结果比较，得到了该非线性方程的适用范围：当激光脉冲宽度小于等离子体波波长的4倍时，该方程所得结果与粒子模拟结果一致；而当激光脉冲宽度大于该数值时，该方程不再适用。     

在稀薄等离子体中激光传播的相对论自聚焦

讨论超强、超短脉冲激光在稀薄等离子体中传播过程的某些性质;用二维快速傅立叶变换求解波动方程.相对论自聚焦的基本方程包括非线性源项和衍射的影响.由于有质动力和相对论的影响,使等离子体的频率降低,从而使折射指数发生改变,影响激光在等离子体中的传播.在入射激光功率(p)超过临界功率(p_c)时,激光在传播过程中产生自聚焦;反之,激光在传播过程中逐渐衰减,不产生自聚焦.     

线性偏振激光在相对论等离子体中的调制不稳定性

 从相对论等离体中电磁波的非线性色散方程出发,利用Karpman方法获得了线性偏振波模所满足的非线性控制方程,在非线性色散方程和非线性控制方程的基础上对线性偏振激光在相对论等离体中传播的调制不稳定性进行分析,给出了调制不稳定的时间增长率与扰动态波数之间的函数关系。     

强激光等离子体中的自由电子波函数

借助激光等离子体的光学度规,严格求解了弯曲时空中的狄喇克方程,得到了强激光等离子体中的自由电子波函数。当激光场为零时,该波函数自然地过渡到相对论的自由电子波函数。简单地讨论了该波函数可能的应用。 关键词：     

Quadruple Gaussian Laser Beam Profile Dynamics in Collisionless Magnetized Plasma

This paper presents an investigation of self-focusing of a quadruple Gaussian laser beam in collisionless magnetized plasma. The nonlinearity due to ponderomotive force which arises on account of nonuniform intensity distribution of the laser beam is considered. The nonlinear partial differential equation governing the evaluation of complex envelope in the slowly varying envelope approximation is solved using a paraxial formalism. The self-focusing mechanism in magnetized plasma, in the presence of self-compression mechanism will be analyzed in contrast to the case in which it is absent. It can be observed that, in case of ponderomotive nonlinearity, the self-compression mechanism obstructs the pulse self-focusing above a certain intensity value. The effect of an external magnetic field is to generate pulses with smaller spot size and shorter compression length. The lateral separation parameter and the initial intensity of the laser beam play a crucial role on focusing and compression parameters. Also, the three-dimensional analysis of pulse propagation is presented by coupling the self-focusing equation with the self-compression one.     

Quadruple Gaussian Laser Beam Profile Dynamics in Collisionless Magnetized Plasma

This paper presents an investigation of self-focusing of a quadruple Gaussian laser beam in collisionless magnetized plasma. The nonlinearity due to ponderomotive force which arises on account of nonuniform intensity distribution of the laser beam is considered. The nonlinear partial differential equation governing the evaluation of complex envelope in the slowly varying envelope approximation is solved using a paraxial formalism. The self-focusing mechanism in magnetized plasma, in the presence of self-compression mechanism will be analyzed in contrast to the case in which it is absent. It can be observed that, in case of ponderomotive nonlinearity, the self-compression mechanism obstructs the pulse self-focusing above a certain intensity value. The effect of an external magnetic field is to generate pulses with smaller spot size and shorter compression length. The lateral separation parameter and the initial intensity of the laser beam play a crucial role on focusing and compression parameters. Also, the three-dimensional analysis of pulse propagation is presented by coupling the self-focusing equation with the self-compression one.     

Nonlinear Dynamics of Circularly Polarized Laser Pulse Propagating in a Magnetized Plasma with q ‐Nonextensive Velocity Distributions

The nonlinear dynamics of a circularly polarized laser pulse propagating in magnetized plasma contains hot nonextensive q ‐distributed electrons and ions is studied theoretically. A nonlinear equation which describes the dynamics of the slowly varying amplitude electromagnetic wave is obtained using the relativistic two‐fluids model. Some nonlinear phenomena include modulational instability, self‐focusing, soliton formation, and longitudinal and transversal evolutions of laser pulse in nonextensive plasma medium are investigated. Results show that the nonextensivity of particles can substantially change the nonlinearity of medium. The external magnetic field enhances the modulation instability growth rate of right‐hand polarization wave but for the left‐hand polarization the growth rate decreases. The spot size of the laser pulse is strongly affected by the plasma nonextensivity. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)