Laser shaping and optical power limiting of pulsed Laguerre–Gaussian laser beams of high-order radial modes in fullerene C60

We study the strong nonlinear optical dynamics of nanosecond pulsed Laguerre–Gaussian laser beams of high-order radial modes with zero orbital angular momentum propagating in the fullerene C60molecular medium. It is found that the spatiotemporal profile of the incident pulsed Laguerre–Gaussian laser beam is strongly reshaped during its propagation in the C60molecular medium. The centrosymmetric temporal profile of the incident pulse gradually evolves into a noncentrosymmetric meniscus shape, and the on-axis pulse duration is clearly depressed. Furthermore, the field intensity is distinctly attenuated due to the field-intensity-dependent reverse saturable absorption, and clear optical power limiting behavior is observed for different orders of the input pulsed Laguerre–Gaussian laser beams before the takeover of the saturation effect;the lower the order of the Laguerre–Gaussian beam, the lower the energy transmittance.

Phase Control of Transient Optical Properties of Double Coupled Quantum-Dot Nanostructure via Gaussian Laser Beams

We theoretically analyze the transient properties of a probe field absorption and dispersion in a coupled semiconductor double-quantum-dot nanostructure.We show that in the presence of the Gaussian laser beams,absorption and dispersion of the probe field can be dramatically influenced by the relative phase between applied fields and intensity of the Gaussian laser beams.Transient and steady-state behaviors of the probe field absorption and dispersion are discussed to estimate the required switching time.The estimated range is between 5-8 ps for subluminal to superluminal light propagation.

Cross-focusing of two chirped Gaussian laser beams and THz wave generation in plasmas of multi-ion species under a weakly relativistic and ponderomotive regime

The generation of terahertz(THz)waves via the beating of two high-intensity chirped Gaussian lasers in a multi-ion-species plasma is numerically studied by taking into account the weak relativistic and ponderomotive regime of interaction.The coupled differential equations for beamwidth parameters are extracted by introducing the dielectric function of such plasma and using WKB and paraxial ray approximations.The amplitude of THz radiation at beat frequency resulting from the nonlinear current density induced by the beat ponderomotive force of the cross-focusing of beams was obtained.The impacts of the chirp frequency parameter,initial laser intensity and initial ionic species density(specifically,the presence of singly and doubly charged ions)in the plasma on THz generation were discussed.Our numerical results reveal that THz radiation generation strongly depends on the chirp frequency parameter.A specific range of chirp frequencies exists for self-focusing as well as THz generation with a'turning point',where the THz emission reaches its maximum value.The results show that the strength of self-focusing and consequently the generated THz radiation are reduced by increasing the density of doubly charged ionic species in the plasma due to the suppression of the nonlinear effects.

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.

Laser transformation hardening on rod-shaped carbon steel by Gaussian beam

Laser transformation hardening(LTH) is one of the laser surface modification processes.The surface hardening of rod-shaped carbon steel(SM45C) was performed by lathe-based laser composite processor with Gaussian-beam optical head.The LTH characteristics by dominant processes,longitudinal and depth directional hardness distributions and behaviors of phase transformation in hardened zones were examined.Especially,two concepts of circumferential speed and theoretical overlap rate were applied.When laser power increased or circumferential speed decreased,the surface hardening depth gradually increases due to the increased heat input.Moreover,the longitudinal hardness distribution particularly shows periodicity of repetitive increase and decrease,which results from tempering effect by overlap.Finally,the feasibility of laser transformation hardening is verified by using the beam with Gaussian intensity distribution.

Relativistic Self-Focusing of Hermite-cosine-Gaussian Laser Beam in Collisionless Plasma with Exponential Density Transition

This work reveals an exploration of self-focusing of Hermite-cosine-Gaussian laser beam in a collisionless plasma under relativistic nonlinearity. Self-focusing along with self-trapping of Hermite-cosine-Gaussian laser beam are analyzed for different values of laser intensity, plasma density, and decentered parameters. Mathematical analysis displays that these parameters play a major role in achieving the stronger and earlier self-focusing. Further, a comparative study between self-focusing of Hermite-cosine-Gaussian laser beam with and without exponential density ramp profile is introduced. Plasma density transition with exponential profile is found to be more effective in order to have stronger self-focusing. The present analysis may lead to very useful applications in the field of efficient harmonic generation, laser driven fusion etc.

Study into the Applicability of Laser Beam Quality Factor M^2

The applicable condition of single-frequency laser beam quality factor M^2 is studied. Any real single-frequency laser beam can be classified as Gaussian mode and non-Gaussian mode according to the transverse field distribution. Non-Gaussian transverse field distribution can be analytically expressed as the sum of Hermite-Gaussian functions. The propagation function and M^2 factor expression for non-Gaussian mode can be obtained by the second moment definition of laser beam spot. The analytical results show, the same as that of Gaussian mode, that the propagation function follows the hyperbolic law and the value of M^2 factor is a constant for non-Gaussian mode. But, different non-Gaussian field distributions may have the same M^2 value. That means M^2 factor cannot reflect the quality of non-Gaussian laser beams correctly. We conclude that the M^2 factor is applicable only to ideal Gaussian laser beam generated by stable resonators.

Propagation of high-power partially coherent fibre laser beams in a real environment

The propagation performance of high-power partially coherent fibre laser beams in a real environment is inves- tigated and the theoretical model of a high-power fibre laser propagating in a real environment is established. The influence of a collimating system and thermal blooming is considered together with atmospheric turbulence and me- chanical jitter. The laser energy concentration of partially coherent beams in the far field is calculated and analysed based on the theoretical model. It is shown that the propagation performance of partially coherent beams depends on the collimating system, atmospheric turbulence, mechanical jitter and thermal blooming. The propagation performance of partially coherent beams and fully coherent beams is studied and the results show that partially coherent beams are less sensitive to the influence of thermal blooming, which results in that the energy degeneration for partially coherent beams is only 50% of that for fully coherent beams. Both partially coherent beams and fully coherent beams become less sensitive to thermal blooming when the average structural constant of the refraction index fluctuations increases to 1.7×10^-14 m^-2/3. The investigation presents a reference for applications of a high-power fibre laser system.

Electron acceleration by two crossed Bessel-Gaussian beams in vacuum

The direct acceleration of electrons by using two linearly polarized crossed Bessel-Gaussian （BG） beams with equal frequency and amplitude in vacuum is proposed and studied. It is shown that two linearly polarized BG beams of the same order （0 or 1） with a π-rad phase difference have a resultant non-zero longitudinal electric field on the z-axis and can be used, in principle, to accelerate electrons.

基于蓝光半导体激光器的水下光学天线

边缘发射半导体激光器在快慢轴方向发散角存在巨大差异,且具有固有像散,严重影响水下通信的传输距离,为此,本文基于高斯光束特性和准直扩束原理设计了一套由快慢轴准直器和伽利略扩束镜组成的水下激光光学天线系统,并对其进行了能量损失和发散角的仿真测试。根据测试结果表明,半导体激光器快慢轴方向的发散角分别从49°和9°压缩到了0.315 mrad和0.180 mrad,该天线系统可以实现水下传播100 m距离。降低了实际中快慢轴准直镜的校准和固定困难,快慢轴的Z轴公差在20μm内,探测器功率衰减<3.9%。此设计结构紧凑,便于加工装调,实际应用价值较高,有助于解决水下远距离传播时光能损耗严重和耦合效率偏低的问题。

高斯激光束在Fraunhofer圆孔衍射成像中的球差

基于标量衍射理论,利用Zernike多项式法构建像差函数,并将其应用到ZEMAX中,研究高斯型激光束在透镜与圆孔距离较大时的Fraunhofer圆孔衍射成像中的球差.实验和理论模拟结果表明,透镜及圆孔和透镜距离对产生球差的影响较大,当选择优化的非球面透镜、较高折射率的透镜材料和小视场时,可有效降低由圆孔和透镜距离较大而产生的球差,从而有效提升衍射成像质量.

外腔可调谐半导体激光器准直系统测试研究(特邀)

为了解决外腔可调谐半导体激光器波长调谐范围小的问题,对高斯光束在谐振腔内的传输特性进行了分析。结果表明,将增益芯片出射光束调整至发散角较小实现准直的方式不利于拓展波长调谐范围。针对一种涵盖1480~1640nm波段的Littman-Metcalf外腔可调谐半导体激光器,提出一种将光腰位置调整至与外腔长度相等的准直方案。经过仿真研究,相较于小发散角的准直方式,采用该方式准直后,可调谐半导体激光器的调谐范围得到大幅提升。

有限能量Olver-Gaussian光束通过电光晶体的光强分布特性

基于广义惠更斯-菲涅耳积分公式,研究了傍轴近似下有限能量Olver-Gaussian光束经过铌酸锂电光晶体的光强分布特性。得出了这类光束经单轴晶体垂直于光轴的电场和光强解析式,基于此,探究了一阶Olver光束在电光晶体不同横截面上的光强分布和光波侧面传输光强图,并得出了外加电场和光波中心位置及中心光斑相对光强变化的定量关系;对传统三类单轴晶体的光波演变特性也进行了分析。

基于数学形态学的小孔径高斯光束光斑半径测量方法

为减小现有的激光光束光斑半径测量方法的测量误差,设计基于数学形态学的小孔径高斯光束光斑半径测量方法。对小孔径光斑几何特征进行划分,计算探测误差,获取光斑能量在探测器光敏面上的均匀分布函数,得到质心点到光斑轮廓点的距离。建立高斯光束光斑轮廓模型,获取位置轮廓的灵敏度,并计算其在某光斑点位的能量值,通过质心点的误差函数,得到光斑点的轮廓函数。基于数学形态学设计光斑半径测量方法,计算不同方向与尺度元素在对角线上的梯度幅值,得到激光光束光斑的半径长度。分析光照强度与发射角对测量精度的影响,实验结果显示:在该激光光束下,光照强度为80且发射角度为1.8时,测量误差最小。对比4种测量方法的测量误差,由数据可知,数学形态学的测量方法误差最小,可见该方法优于其他测量方法。

基于高斯过程回归的线性激光束指向稳定优化

为解决激光应用中的光束指向抖动问题,提出一种基于高斯过程回归(Gaussian process regression)的激光光束指向稳定性优化方案。介绍了激光光束指向稳定系统装置构成及原理,论述了高斯过程回归方法的原理及其作为激光光束快速稳定控制算法的优势。经过该方法优化后,指向抖动达到水平方向2.3μrad,竖直方向3.3μrad,将激光系统指向稳定性提高了1个数量级以上。指向性抖动为已有线性反馈系统的20%,尤其对于高频噪声优化有显著效果。该研究对于激光光束指向性敏感的精密实验和精密加工具有重要意义。

大模场一维高阶厄米-高斯激光束产生

厄米-高斯光束在诸多前沿科学领域都有着重要的应用.不同于目前普遍采用的晶体端面离轴泵浦方式,本文提出了一种利用板条激光器产生厄米-高斯激光束的方法.采用半导体激光阵列大面正交泵浦板条激光介质,具有大模场特性.根据预先设计的谐振腔模场,在板条厚度和宽度方向分别采用尺寸可调的光阑限模.由于高阶模式对谐振腔腔镜不对准的灵敏度弱于低阶模式,可通过耦合输出镜倾斜量的控制,实现不同阶数模式腔内损耗的差异化调控,从而产生各阶次的高纯度厄米-高斯光束.利用Nd:YAG板条激光器,获得了0—9阶一维厄米-高斯光束,其光强分布与理论值的相关系数ρ高于0.95,光束质量因子M^(2)与理论值符合良好.最高阶HG09模式的输出功率为244 mW.在此基础上,进一步利用柱透镜对组成的像散模式转换器,实现了各阶厄米-高斯光束向对应拉盖尔-高斯光束的转换.结合板条放大器结构,基于本方案产生的厄米-高斯光束具备功率定标放大的前景.

圆偏振涡旋激光脉冲驱动等离子体产生飞秒磁脉冲的研究

飞秒磁场脉冲对研究超快磁化、超快退磁、超快磁存储和自旋超快动力学等过程具有重要意义。传统的脉冲磁场受限于脉冲电源性能无法获得毫秒量级以下的超短脉冲磁场,无法研究飞秒尺度的磁动力学过程。利用超短脉冲激光驱动等离子体产生旋转电流是目前产生飞秒磁场脉冲的有效方法。本文利用质点网格法模拟圆偏振拉盖尔高斯光束驱动等离子体中的电子运动从而产生光电流以及飞秒磁脉冲的过程,模拟产生了特斯拉量级的飞秒超短磁脉冲,并系统讨论了驱动激光强度与等离子体密度对磁脉冲的影响。结果表明,脉冲磁场的脉宽与驱动光一致,其强度随着激光场强度、等离子体密度增加而增加。通过本文研究寻找产生飞秒磁脉冲的优化实验参数,有望将超快磁动力学研究推进到飞秒时间尺度。

高斯光束传输特性的理论研究

高斯光束是一种重要的激光光束,在当前激光领域的研究和应用非常广泛。为研究高斯光束经过复杂光学系统的传输变化规律,本文通过求解电磁场的波动方程,系统推导了能够表征高斯光束传输特性的特解,并结合高斯光束q参数的ABCD矩阵定律,理论计算了高斯光束通过复杂光学系统的两个重要传输特性参数:像方高斯光束束腰半径及像方高斯光束束腰与光学系统的距离。该研究简化了传统的运算过程,并分别使用传统算法和新推导的简化算法对高斯光束经透镜系统的传输特性进行了验证,为高斯光束通过各类复杂光学系统的传输特性研究提供了参考。

相对论涡旋高次谐波的产生与调控理论研究

目前,具有螺旋相位波前和环状光强分布的涡旋光束已在光学领域获得了广泛应用,其产生与调控自然成了研究的热点。利用三维粒子模拟程序对双色拉盖尔高斯激光驱动固体等离子激发同时携带自旋角动量与轨道角动量的高次谐波的物理过程进行了研究,根据高次谐波产生过程中的光子能量与角动量守恒定律对其内在物理机制进行了理论分析,并讨论了对谐波阶次、偏振态(自旋角动量)以及拓扑荷数(轨道角动量)进行调控的方法。研究结果为开发高亮度、超短超快、短波长、自旋与轨道角动量可调控的涡旋光束辐射源提供了理论依据,在光学微操控、超分辨成像、光通信以及离子加速等领域具有较大的实际应用前景。

基于四瓣高斯飞秒激光的多丝阵列产生与调控

基于亥姆霍兹波动方程和非线性传输波动方程,模拟了四瓣高斯飞秒激光在空气中线性传输和非线性传输的光强空间分布,以期获得规则的多丝阵列稳定传输。研究结果表明,当初始入射激光功率相对较强时,基于四瓣高斯飞秒激光光束可获得规则的多丝阵列产生。通过改变初始束腰半径和光束阶数,实现对光丝阵列间距的调控。光丝阵列间距大于背景能量池尺寸时,光丝阵列将稳定传输且间距保持不变;小于背景能量池尺寸时,多丝相互融合进而形成稳定的单丝。当初始入射激光功率相对较弱且大于自聚焦阈值功率时,将会出现多次自聚焦现象,最终形成稳定的单丝传输。该研究提供了一种产生二维规则飞秒激光光丝阵列的方法,将为基于飞秒激光多丝阵列的实际应用,如太赫兹波增强、空气激光增强、遥感探测、微波通道以及微粒捕获等,提供理论依据。