Self-Focusing/Defocusing of Chirped Gaussian Laser Beam in Collisional Plasma with Linear Absorption

This paper presents an investigation on the self-focusing/defocusing of chirped Gaussian laser beam in collisional plasma with linear absorption. We have derived the differential equation for the beam width parameter by using WKB and paraxial approximations and solved it numerically. The effect of chirp and other laser plasma parameters is seen on the behavior of beam width parameter with dimensionless distance of propagation. The results are discussed and presented graphically. Our simulation results show that the amplitude of oscillations decreases with the distance of propagation. Due to collisional frequency, the laser beam shows fast divergence which can be minimized by the introduction of chirp parameter. The chirp decreases the effect of defocusing and increases the ability of self-focusing of laser beam in collisional plasma.     

强激光束在等离子体通道中传输的变分法研究

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Electron Acceleration by Beating of Two Intense Cross-Focused Hollow Gaussian Laser Beams in Plasma

This paper presents propagation of two cross-focused intense hollow Gaussian laser beams(HGBs) in collisionless plasma and its effect on the generation of electron plasma wave(EPW) and electron acceleration process,when relativistic and ponderomotive nonlinearities are simultaneously operative. Nonlinear differential equations have been set up for beamwidth of laser beams, power of generated EPW, and energy gain by electrons using WKB and paraxial approximations. Numerical simulations have been carried out to investigate the effect of typical laser-plasma parameters on the focusing of laser beams in plasmas and further its effect on power of excited EPW and acceleration of electrons. It is observed that focusing of two laser beams in plasma increases for higher order of hollow Gaussian beams,which significantly enhanced the power of generated EPW and energy gain. The amplitude of EPW and energy gain by electrons is found to enhance with an increase in the intensity of laser beams and plasma density. This study will be useful to plasma beat wave accelerator and in other applications requiring multiple laser beams.     

Nonlinear Interaction of Elliptical Laser Beam with Collisional Plasma:Effect of Linear Absorption

In the present work,nonlinear interaction of elliptical laser beam with collisional plasma is studied by using paraxial ray approximation.Nonlinear differential equations for the beam width parameters of semi-major axis and semi-minor axis of elliptical laser beam have been set up and solved numerically to study the variation of beam width parameters with normalized distance of propagation.Effects of variation in absorption coefficient and plasma density on the beam width parameters are also analyzed.It is observed from the analysis that extent of self-focusing of beam increases with increase/decrease in plasma density/absorption coefficient.     

相干合成光束在湍流大气中的传输

分析了湍流大气对相干合成光束传输的影响,根据广义惠更斯-菲涅耳原理,对相干合成光束在不同强度湍流大气中传输进行计算,对接收平面处的光强分布的统计特性,如桶中功率(PIB)曲线、局部功率曲线进行比较.研究结果表明,较弱的湍流大气对相干合成光束的传输影响较小,接收平面的光强分布以及PIB曲线基本不变;随着湍流强度的增大,相干合成光束的光强分布和PIB曲线产生显著变化,光斑扩展和能量的弥散速度加快,光束的能量集中度显著降低.计算了湍流大气传输后光束的空间相干度,认为空间相干度下降是降低相干合成效果的原因,对如何降低湍流大气的影响进行讨论.     

Effects of relativistic and ponderomotive nonlinearities on the interaction of high-power laser beam with an inhomogeneous warm plasma

The influence of relativistic-ponderomotive nonlinearities and the plasma inhomogeneity on the nonlinear interaction between a high-power laser beam and a warm underdense plasma are studied. It is clear that the relativistic ponderomotive force and the electron temperature modify the electron density distribution and consequently change the dielectric permittivity of the plasma. Therefore, by presenting the modified electron density and the nonlinear dielectric permittivity of the warm plasma, the electromagnetic wave equation for the propagation of intense laser beam through the plasma is derived. This nonlinear equation is numerically solved and the distributions of electromagnetic fields in the plasma, the variations of electron density, and plasma refractive index are investigated for two different background electron density profiles. The results show that the amplitude of the electric field and electron density oscillations gradually increase and decrease, during propagation in the inhomogeneous warm plasma with linear and exponential density profiles, respectively, and the distribution of electron density becomes extremely sharp in the presence of intense laser beam. It is also indicated that the electron temperature and initial electron density have an impact on the propagation of the laser beam in the plasma and change the plasma refractive index and the oscillations' amplitude and frequency. The obtained results indicate the importance of a proper choice of laser and plasma parameters on the electromagnetic field distributions, density steepening, and plasma refractive index variations in the interaction of an intense laser beam with an inhomogeneous warm plasma.     

Effect of Critical Beam Radius on Self-focusing of cosh-Gaussian Laser Beams in Collisionless Magnetized Plasma

Effect of critical beam radius on self-focusing of cosh-Gaussian laser beams in collisionless magnetized plasma under ponderomotive nonlinearity forms the main core of present work. To investigate propagation dynamics of cosh-Gaussian laser beams in collisionless magnetized plasma, well established parabolic equation approach under WKB and paraxial approximations is employed. Our study is crucially pivoted on the concept of critical curve and subsequent determination of numerical interval for decentered parameter to sustain the competition between diffraction and self-focusing during the propagation of laser beam. Additionally, in the present study an interesting feature in the self-focusing region of the critical curve has been attempted for different values of decentered parameter.     

Investigation into absorption of the incident laser beam during Nd:YAG laser processing of metals

Laser materials processing is highly affected by the existence of surface plasma. The absorption of surface plasma during drilling alters the power intensity distribution of the incident laser beam across the irradiated spot. The present study is carried out to measure the electron number density and temperature using a Langmuir probe while a mathematical formulation is conducted for the absorption coefficients due to electron-ion, electron-neutral atom collisions, inverse Bremsstrahlung, and photoionization processes. Consequently, a computer program is developed to compute the relevant absorption coefficients as well as the overall absorption coefficient. The laser power intensity distribution before and after the plasma absorption is computed at a plane 2.6 mm above the workpiece surface. It is found that 13% of the reduction occurs in the incident laser output power intensity at this plane in the plasma.     

激光注入误差对多模光纤传能特性影响分析

采用ZEMAX软件对激光光纤注入系统进行了建模,仿真分析了激光注入光纤横向偏移、角度偏移对光纤传输激光能量特性的影响.结果表明,光纤输出激光能量分布与激光注入对准误差密切相关.注入误差引起光纤初始输入段激光峰值功率密度的剧烈波动,出现了一个激光峰值功率密度极大值,这个极大值是可以达到光纤截面内激光平均功率密度的数十倍;横向偏移激发大量斜光线产生,使光纤输出激光能量分布匀化;角度偏移仅影响光纤内子午光线与斜光线的传播方向,对光纤内激光能量分布的匀化作用较弱.     

Optical bistability in gold nano-colloid due to thermal lensing effect

In this study, experimental results are concerning “the optical bistability due to thermal lensing effect”, in colloidal solution of gold nanoparticles (Au Nps) which has been studied under CW laser illuminations. Two different CW laser beams (He–Ne and Nd-Yag) were used to interact with a vertical Fabry–Perot interferometer, containing colloid of the Au Nps. Hysteretic loops have been observed with regard to optical bistability and the results were fitted with theoretical curves. The radiated power to the resonator was elevated up to 50 mW for both the laser beams with the same spot size. Also, new method was defined to precise evaluation of the theoretical hysteretic loop which is describing the relation between nonlinear refractive index and the beam intensity inside the resonator. This dependency is due to thermal lensing effect and numerical simulations of this effect were also performed, in order to better understanding of it.     

激光与固体靶面烧蚀等离子体的能量耦合计算

 强激光辐照下固体靶表面迅速汽化产生靶蒸气等离子体,激光穿过等离子体区到达固体靶表面的过程就是激光束与等离子体的能量耦合与交换过程。采用具有五阶精度的WENO差分格式和简易等离子体状态方程模型对激光与等离子体相互作用的复杂物理过程进行了数值计算,分析了激光束能量在等离子体区中的吸收、屏蔽效应等动态耦合规律以及激光支持等离子体前驱冲击波传播。数值模拟结果表明：激光能量是支持靶面等离子体运动的唯一原因,能量屏蔽效应对激光与等离子体能量耦合有很大影响,通过控制激光脉冲宽度,可以合理调节屏蔽效应的影响。     

Self-Focusing of Quadruple Gaussian Laser Beam in an Inhomogenous Magnetized Plasma with Ponderomotive Non-Linearity: Effect of Linear Absorption

The propagation of quadruple Gaussian laser beam in a plasma characterized by axial inhomogeneity and nonlinearity due to ponderomotive force in the paraxial ray approximation is investigated. An appropriate expression for the nonlinear dielectric constant has been developed in the presence of external magnetic field, with linear absorption and due to saturation effects for arbitrary large intensity. The effects of different types of plasma axial inhomogeneities on self-focusing of laser beam have been studied with the typical laser and plasma parameters. Self-focusing of quadruple Gaussian laser beam in the presence of externally applied magnetic field and saturating parameter is found significantly improved in the case of extraordinary mode. Our results reveal that initially converging beam shows oscillatory convergence whereas initially diverging beam shows oscillatory divergence. The beam is more focussed at lower intensity in both cases viz. extraordinary and ordinary mode.     

热透镜控制激光光束质量技术研究

在端面抽运固体激光器中,就如何改善在高抽运功率时输出激光的光束质量,提出一种激光器的设计方法。在激光器谐振腔中放入多根掺杂浓度不同的激光介质,利用激光介质内部产生的热透镜控制抽运光和基模振荡光的空间分布,并且最大限度地使抽运光的分布区和基模振荡光分布区重叠,实现抽运光与基模振荡光在空间上高度匹配,进而提高抽运光能量的利用效率和振荡光的光束质量。实验表明,在不同抽运功率下,抽运光和基模振荡光在晶体内部的光斑的空间分布可通过热透镜加以控制。在端面抽运功率200 W附近时,实现了抽运光与基模振荡光较高程度匹配,光束质量因子M2由14.7改善为4.1。     

强激光在等离子体中的传播特性

分别讨论了非相对论条件下短脉冲和长脉冲激光在完全电离的等离子体中的传播特性。短脉冲情况下，激光的有效瑞利长度会增加，但不会出现自导引；长脉冲激光的传播取决于激光功率和等离子体温度之比，当该比值超过一定阈值时会出现自导引现象，激光束被限制在一个相对稳定的通道内，其半径对光束的初始半径有很强的依赖性。     

Analysis on X-Ray Laser Generation with EM-Wave Wiggler

The possibility of realizing SASE Free Election Laser in x-ray waveband with EM-wave wiggler is studied in this paper. SASE effect including saturation length, saturation power and nonlinear process is explored through a 3-D simulation code. A utilization of seed light from laser plasma x-ray is also analyzed, which demonstrates the feature of shortening the interaction length for saturation. The results show that sizeable output power of x-ray laser would be generated with a middle energy electron beam.     

激光感生等离子体特性的三维数值模拟

在激光焊接过程中,作用于金属工件表面的高强度激光会引起材料的强烈蒸发,金属蒸汽与入射激光相互怍用,又会引起金属蒸汽部分电离,形成激光感生等离子体。本文采用三维模拟方法,考虑保护气和侧吹气的影响,对激光感生等离子体中的温度与速度分布进行了研究。     

Laser plasma acceleration of quasi-monoenergetic beams of light multicharged ions

Experimental results on the spectra of residual impurity ions from a plasma created on the surface of a tungsten target that is irradiated by a femtosecond laser pulse and is subjected to preliminary pulsed laser cleaning are reported. It is shown that deep modulation corresponding to the formation of a quasi-monoenergetic ion beam appears in the energy spectra of protons and light multicharged carbon ions in this case. The position of a “dip” at the scale of velocities closely correlates with the position of the front of heavier ions.     

利用双折射透镜组实现激光束空间整形

为了提高惯性约束聚变高功率激光系统的整体效率和充分利用光能,需要将高斯分布的光束整形为空间均匀分布的平顶光束。本文从球面透镜的琼斯矩阵出发,利用光学传输矩阵对双折射透镜组空间光束整形系统进行了理论分析,数值模拟了整形效果,讨论了透镜组参量的选择以及中心加工厚度误差等因素带来的影响,并对该系统进行了实验研究。在实验中,利用该双折射透镜组整型系统实现了光束的均匀化输出。在神光Ⅱ第九路中,在近场静态工作条件下,可将光束填充因子从原来的66%提高到80%。     

Enhanced Raman Scattering of Elliptical Laser beam in a Collisional Plasma

This paper presents the Enhanced Raman scattering of a elliptical laser beam in a collisional plasma. We have considered the mechanism of non‐uniform heating of carriers along the wave‐front, which is important in collisional plasma. The nonlinearity arising through non‐uniform heating leads to redistribution of carriers, which modifies the background plasma density profile in a direction transverse to pump beam axis. This modification in density effects the incident laser beam, plasma wave and back‐scattered beam. Non‐linear differential equations for the beam width parameters of pump laser beam, plasma wave and back‐scattered beam are set up and solved numerically. Numerical results predict the effect of self‐focusing of waves on the back‐scattered beam (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

棒状钕玻璃热容激光器热透镜自平衡补偿方法分析

针对热容激光器的热透镜效应,给出了棒状钕玻璃热容激光器的热透镜公式表达式。理论上分析了热透镜焦距自平衡补偿方法的基本原理。在考虑了棒状钕玻璃热容激光器的具体特性的基础上,设计了一种简单的热透镜焦距自平衡补偿方法,使光抽运钕玻璃热容激光器在热透镜焦距变化情况下,可以使激光输出光斑大小稳定。为了解棒状热容激光器热透镜焦距的影响,利用自研的光栅型曲率传感器测量了热容激光器总的热透镜等效焦距的大小,结果表明热传导在抽运光发射完20 s左右后才建立热平衡产生了明显的热透镜效应; 随着抽运脉冲个数的增加,热透镜效应随之变大。