非傍轴光束的小尺度自聚焦研究

基于非傍轴传输方程,利用线性扰动理论,得到了小尺度调制增长率的表达式.在此基础上,讨论了非傍轴对小尺度自聚焦的影响.通过对非傍轴和傍轴情形下小尺度调制的增益谱分析,发现非傍轴对小尺度调制的增益谱的影响程度与光束的强度有关,光强越强,影响越大.但总的趋势是非傍轴使小尺度调制增长的截止频率、最快增长频率和最大增长率减小,而非傍轴对截止频率和最快增长频率的影响比对最大增长率的更大 关键词：     

发散光束小尺度自聚焦特性的研究

研究了发散光束的小尺度自聚焦效应。从非线性傍轴波动方程出发,利用坐标变换,推导出发散光束小尺度扰动的传输方程,进而得到小尺度扰动增长的临界频率、最大增长频率和相应B积分值的变化规律。研究了发散光束初始曲率半径对小尺度自聚焦效应的影响。结果表明,对于一定的传输距离,随着发散光束初始曲率半径的减小,小尺度扰动的最大增长频率减小,相应的最大增益减小,即B积分值也减小。对于一定的初始曲率半径,随着传输距离的增大,B积分值增长变缓,并最终停止。利用局部能量守恒定律研究了发散光束的成丝距离,发现小的初始曲率半径可以延长成丝距离。     

增益介质中发散光束的小尺度自聚焦

将Bepalov-Talanov理论拓展为发散光束在增益介质中的情形,研究了介质小信号增益和饱和增益情况下小尺度扰动的自聚焦特性.在小信号增益情形下,对于确定的输入功率,不同的光束初始曲率半径,小尺度调制增长的临界频率、最快增长频率及其对应的最大增益随介质小信号增益或传输距离的变化有不同的变化性质.光束初始曲率半径表征着光束衍射发散的程度,它的减小在一定程度上能够使小尺度调制的最快增长频率和最大增益减小.小信号增益情形下,对于确定的输出功率,增加介质小信号增益,扰动的最大增益会随之减小,而最快增长频率相应向低频方向移动.相比于小信号增益情形,饱和增益介质情形下小尺度调制增长的最快增长频率及最大增益较小.在不同的系统设计中,需权衡放大器增益及光束参数,选择合适的系统参数及光束控制装置进行实际应用.     

激光带宽抑制光束小尺度自聚焦效应的条件

激光带宽有利于抑制光束的小尺度自聚焦效应,从而突破非线性效应的限制提高高能固体激光器的输出功率。研究表明,带宽抑制小尺度自聚焦效应的程度与带宽和脉宽之比密切相关,相同脉宽条件下带宽越大对空间小尺度自聚焦的抑制作用越明显,而带宽一定时随着脉冲宽度的增大,带宽的抑制作用则逐渐变弱。在实际的钕玻璃激光放大系统中,由有限的增益带宽以及增益窄化等因素所限制的带宽量,决定了在这类固体激光系统中需要脉冲宽度短至皮秒量级才能有效利用激光带宽抑制小尺度自聚焦效应,而对于长至数十皮秒,甚至纳秒量级的长脉冲而言,如此有限的带宽量对小尺度自聚焦的抑制几乎不起作用。     

片状放大器小信号噪声增益功率谱的理论计算

根据小信号的 Bespalov-Talanov理论 ,考虑光束的自由空间传输 ,计算了级联片状放大器小信号噪声的功率增益谱 ,通过比较不同参数的功率谱的变化 ,分析了影响功率谱强度及最大增益频率的因素。结果表明 ,以 B积分作为非线性自聚焦效应强弱的唯一判据是不够的 ,除了 B积分外 ,还应给出噪音的等级。对于级联片状放大器 ,考虑和不考虑光束的自由传输 ,极大增益的中心频率的差别很大 ,在空间滤波器小孔的截止频率的选择时必须考虑到这一点     

片状放大器小信号噪声增益功率谱的理论计算

 根据小信号的Bespalov-alanov理论,考虑光束的自由空间传输,计算了级联片状放大器小信号噪声的功率增益谱,通过比较不同参数的功率谱的变化,分析了影响功率谱强度及最大增益频率的因素。结果表明,以B积分作为非线性自聚焦效应强弱的唯一判据是不够的,除了B积分外,还应给出噪音的等级。对于级联片状放大器,考虑和不考虑光束的自由传输,极大增益的中心频率的差别很大,在空间滤波器小孔的截止频率的选择时必须考虑到这一点。     

多艾里光束合成自聚焦光束的实验实现

多光束合成和单光束聚焦一直是提高激光束功率和功率密度的两个重要方法. 结合艾里光束在自由空间中沿弯曲路径传播的特性, 从数值模拟和实验两个方面, 研究了利用多个一维艾里光束合成自由空间自聚焦光束的方法, 并对所得到的模拟和实验结果进行了对比. 采用分步束传播法, 分别模拟了由四个和八个一维艾里光束合成的自聚焦光束在自由空间中的传播过程, 给出了自聚焦光束在传播过程中横向和纵向的光强分布和变化趋势. 采用计算全息和空间光调制器技术实验, 实现了多个一维艾里光束合成的自聚焦光束. 实验中分别测量了四个和八个一维艾里光束合成的自聚焦光束的横向光强分布. 实验结果和理论结果符合得较好. 另外, 为了进一步增大自聚焦光束的功率, 可以增加参与合成的一维艾里光束的数量. 同时, 自聚焦光束的焦距可以通过调整各个一维艾里光束的相对位置进行调节.     

艾里光束通过负折射率介质的传输特性

为了研究艾里光束(Airy beams)通过负折射率介质中的传输特性,利用ABCD矩阵光学理论推导出了Airy光束通过负折射率介质的传输解析表达式.利用该解析表达式得到了Airy光束通过负折射率介质的传输特性.计算结果表明,Airy光束通过负折射率介质后的自加速和光强都可以通过负折射率介质的工作频率调控.Airy光束通过负折射率介质的横向偏转系数随传输距离z的增大而加速偏转;同时当传输距离z相同而负折射率介质的工作频率不同时,偏转系数也不相同.Airy光束的强度和偏转度都可以通过负折射率介质的工作频率调控.结果显示可以利用负折射率介质的工作频率方便有效地调控Airy光束,研究结果在光学器件设计和医学科学中都有潜在的应用价值.     

用高阶对比度研究光束的小尺度自聚焦

提出了高阶对比度这一新参数来描述光束的近场均匀性，并用之研究高功率激光装置中的小 尺度自聚焦现象.分析了高功率光束小尺度自聚焦发生的过程中填充因子、调制度、对比度 及高阶对比度随B积分值的变化，指出高阶对比度在光束出现自聚焦成丝前后有更显著 的变化，所以是描述小尺度自聚焦效应的理想参数. 关键词： 小尺度自聚焦 三阶对比度 四阶对比度 功率谱密度相位噪声     

强光束局部小尺度调制致多路成丝现象研究

基于非线性Schrdinger方程模拟了Kerr介质中强光束局部小尺度调制致光束多路成丝现象，发现在一定的B积分范围内，光束局部小尺度调制基本上只从该局部本底场中吸收能量获得增长，从而将该局部光束分裂成细丝.当光束不同部分的小尺度调制相隔较远时，各 细丝的演化基本上不相关. 关键词： 自聚焦 多路成丝 非线性Schrdinger方程     

强非线性吸收下高斯光束Z-扫描衍射理论模型

 摘要：在同时考虑非线性折射和非线性吸收的情况下，借助光的衍射理论，研究了Z-扫描技术的衍射理论模型，分析了非线性折射和非线性吸收同时存在时Z-扫描曲线的形状特征和变化规律。数值计算表明，非线性吸收相移和非线性折射相移的比值是影响接收小孔输出光功率的重要因素。要想得到好的光限幅效果，应选取非线性相移比值远小于1或远大于2的非线性材料。     

Nonlinear vibration of a double-walled carbon nanotube embedded in a polymer matrix

In the current work, the nonlinear vibration of an embedded double-walled carbon nanotube (DWCNT) aroused by nonlinear van der Waals (vdW) interaction forces from both surrounding medium and adjacent tubes is studied. Using both Euler–Bernoulli and Timoshenko beam models, the relation between deflection amplitudes and resonant frequencies of the DWCNT is derived through harmonic balance method. It is found that the nonlinear vdW forces from the surrounding medium result in noncoaxial vibration of the embedded DWCNT. The noncoaxial vibration includes both uni-directional and bi-directional vibration modes. It is found that the surrounding matrix has more prominent effect on the uni-directional vibration in comparison to the bi-directional vibration. The axial load effect on the vibrational behavior of the embedded DWCNT is also discussed. Due to the influence of the surrounding polymer, the prediction on the resonant frequencies of embedded CNTs is quite different from that for free-standing CNTs. A softening behavior for the deflection amplitude-resonant frequency relation is observed for the first time in the bi-directional vibration of the embedded DWCNT, which can only be obtained using the Timoshenko beam theory.     

Effect of defocusing thermal nonlinearity on Gaussian beam profile in Fabry-Perot cavity

We investigate numerically the steady-state transmission of a Gaussian beam through Fabry-Perot cavity containing liquid medium with the negative temperature nonlinearity. It is shown that multistability may appear in different ranges of the beam depending on the optical and geometric parameters of the resonator.     

环形斑激光束在非线性克尔介质中环半径的偏移现象

对于非线性克尔介质中的环形斑调制光束,通过数值求解非线性Schrdinger方程发现:当入射光强度超过一定值时,光束在传输过程中将发生环半径偏移现象.并对环半径偏移产生的条件、偏移的特点,以及自聚焦焦点上光强度的横向分布与环半径偏移之间的关系进行了详细的研究.     

环形斑光束在非线性克尔介质中的自聚焦效应

通过数值求解非线性Schrödinger方程,详细讨论了环形斑光束在非线性克尔介质中的传输特性.当入射光超过一定的功率时,光束产生多级自聚焦现象,自聚焦焦点出现在离轴的环上;自聚焦阈值与环形光束的半径有关;传输过程中光斑存在横向分裂现象,并且分裂与聚焦交替进行;当两个半径不同的同轴环状光束同时在非线性介质中传输时产生耦合效应,耦合效应使其自聚焦增强.     

Comparison of the Gaussian-decomposition and the Fresnel-Kirchhoff diffraction methods in circular and elliptic Gaussian beams

In this investigation, the Gaussian decomposition and the Fresnel-Kirchhoff diffraction methods are compared from different view points. The virtues and faults of each method and also the reliable region of each one are discussed. This work is based on a numerical method for both the circular and elliptic Gaussian beams and results are compared with experimental data. This study is useful for materials exhibit optical Kerr effect such as two photon absorption and thermo-optical nonlinearities.     

Self-trapped dynamics of a hollow Gaussian beam in metamaterials

We present a systematic investigation on the dynamics of a hollow Gaussian beam (HGB) in metamaterials. We predict self-trapped propagation of HGBs and evolution of the beam is highly influenced by dimensionless dispersion coefficient (κ), which determines the strength of dispersion over diffraction. The evolutions of HGBs such as disappearance of single ringed intensity pattern and appearance of patterns with a central bright spot are achievable with less propagation distance in metamaterials with higher values of κ. On the other hand, metamaterials with low values of κ can preserve single ring intensity distribution over a long propagation distance without focusing. When the strength of dispersion over diffraction increases, it significantly influences the evolution of the beam and may lead to the formation of tightly focussed beam with high peak intensity at the center. The phenomenon of tight focussing is found to have some applications in trapping of nanosized particles.     

温度对串联光折变晶体回路中高斯光束传播特性的影响

研究了温度对串联光折变晶体回路中高斯光束传播特性的影响.假设一束暗孤子光束和一束高斯光束分别入射到回路的两块晶体上,利用数值计算方法分析了改变暗孤子所在的晶体温度对高斯光束在另外一块晶体中传播特性的影响.结果表明,调节暗孤子的晶体温度能够影响另一块晶体中高斯光束的传播特性,可以决定高斯光束在晶体中是否能够演化为稳定传播的明孤子波.     

Nonlinear vibration of embedded SWBNNTs based on nonlocal Timoshenko beam theory using DQ method

In the present work, effect of von Kàrmàn geometric nonlinearity on the vibration behavior of a single-walled boron nitride nanotube (SWBNNT) is investigated based on nonlocal piezoelasticity theory. The SWBNNT is considered as a nanobeam within the framework of Timoshenko beam (TB). Loading is composed of a temperature change and an imposed axially electric potential throughout the SWBNNT. The interactions between the SWBNNT and its surrounding elastic medium are simulated by Winkler and Pasternak foundation models. The higher order governing equations of motion are derived using Hamilton's principle and the numerical solution of equations is obtained using Differential Quadrature (DQ) method. The effects of geometric nonlinearity, elastic foundation modulus, electric potential field, temperature change and nonlocal parameter on the frequency of the SWBNNT are studied in detail.     

A general model for nano-cantilever switches with consideration of surface effects and nonlinear curvature

A general model for nano-cantilever switches with consideration of surface stress, nonlinear curvature, the location and length of the fixed electrode is developed. Some representative cantilever switch architectures are incorporated into this model. The governing equation is derived by using Hamilton principal and solved numerical. Results show that the influence of nonlinear curvature and surface effect on the pull-in instability and free vibration is significant for a switch with a large gap-length ratio and a short fixed electrode (the length of the fixed electrode is smaller than that of the cantilever nanobeam). The length and position of the fixed electrode have a significant effect on the pull-in parameters.