移结构分布反馈激光器的静态及动态特性分析

分析了1/4相移分布反馈(QPS-DFB)激光器及三相移分布反馈(3PS-DFB)激光器的静态及动态特性.在分析中,考虑了空间烧孔及非线性增益的因素.在通用的传输矩阵法(TMM)的基础上,对TMM模型的输入进行向量处理,得出不同电流注入情况下两种DFB激光器的载流子密度及光子密度的轴向分布.给出了一个新的小信号动态响应模型并运用向量法进行了两种器件的动态响应分析.这个方法亦可用于其它DFB结构的分析. 关键词：     

一维光子晶体激光器中模场的空间分布及其对阈值的影响

采用有限时域差分（FDTD）法和传输矩阵法（TMM）计算了一维光子晶体微腔中模式的阈值和模场的空间分布,并用局域化长度和模面积描述了其空间分布特性.着重研究了模场的空间分布对阈值特性的影响.模拟结果显示,激光器的阈值依赖于模式的空间分布,局域化长度和模面积最小的模式具有最低阈值.通过调节包括缺陷宽度、光学厚度比、折射率比等介质参量,可以有效地调节模式的局域化长度和模面积,从而降低激光器的阈值.     

用传输矩阵法(TMM)研究二维光子晶体传输特性

将传输矩阵法(TMM)用于光子晶体传输特性的理论研究，采用Mur近似吸收边界和周期边界来截断计算区域，研究了以TM模正入射时，二维方格子光子晶体在完整周期结构下和有缺陷结构下的透过率谱；在微波波段制作了光子晶体模型，并设计了实验装置；实验与数值模拟计算取得了基本一致的结果。     

TEM_(np)模与空芯圆柱形介质波导模的耦合系数

本文讨论了大半径空芯圆柱形介质波导中存在的线偏振模EH_(1m)和LP_(nm)(n≥1)的场合分布,后者是一对简并模的线性组合.然后,我们计算了自由空间高斯-拉盖尔模TEM_(np)与这些线偏振模的耦合系数,得到的结果与已发表的实验资料符合得很好.我们还讨论了较小输入孔半径对光泵远红外激光器输出功率的影响.     

带有完美匹配层的光波导中模式的渐近解

对带有完美匹配层(PML)的光波导,通过W.K.B.方法,得到了关于波的模式的近似非线性方程.通过对该非线性方程进行渐近分析,得到了完美匹配层对波的各种模式的影响,推导出泄漏模的渐近解和Berenger模的渐近解.理论结果表明,对于光波导的横电(TE)模,泄漏模的渐近解和开放的波导是相同的,而对于横磁(TM)模,泄漏模的渐近解和开波导会有所不同;当上下包层的折射率不同时,Berenger模的轨迹会有两条,而折射率相同时Berenger模的轨迹只有一条.数值模拟表明,泄漏模及Berenger模的渐近解在其精确解附近,且随着模的增大.误差越来越小.     

相移超结构光纤布拉格光栅相位编/解码器的分析与设计

详细给出了采用传输矩阵法(TMM)分析的基于相移超结构光纤布拉格光栅(SSFBG)的相位编/解码器的理论,提出了设计方法.以长度为127切普(chip)的二相位和四相位编/解码对为例,对设计结果进行了仿真验证,通过与文献中的结果对比,进一步说明了分析方法和设计方法的正确性.仿真还比较了四相位码的A族序列和二相位码的Gold序列对光栅写入精度的灵敏度,发现在相同码长条件下,四相位码对光栅相移制作控制精度的要求要高于二相位码.     

离子温度梯度模湍流的带状流最小自由度模型

在离子温度梯度模(ITG)湍流背景中,通过最小自由度模型中模耦合方式产生带状流,对此模型做了动力学稳定性分析及数值求解.并在此基础上初步探讨了湍流中漂移波与带状流的能量转移,以及雷诺协强与带状流的关系. 关键词： 等离子体 离子温度梯度模 湍流 带状流     

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研究了模与模之间的相互作用对电阻壁模(RWM)稳定性的影响.当存在多个模的相互作用时,最不稳定的(3,1)模的线性增长率有所下降.经过线性演化后,因磁力线在壁上的挤压,电阻壁模进入非线性饱和状态.与单个模的演化相比,多个模存在时,(3,1)模的饱和度会下降,(5,2)模的磁能会有相应的增长,而(2,1)模磁能饱和度变化不大.     

LD泵浦、1W输出的Nd:GdVO4半导体可饱和吸收镜连续锁模激光器

报道了激光二极管泵浦、采用半导体可饱和吸收镜(SESAM)实现的Nd:GdVO4连续被动锁模运转激光器,得到了较大功率输出(接近1 W)的连续锁模激光.设计了四镜z型折叠激光谐振腔,在不同曲率半径的腔镜及不同的腔长条件下,分别获得了重复频率为250 MHz和100 MHz的稳定连续锁模脉冲输出.采用透过率1%的输出耦合镜,在泵浦功率为6.9 W时,得到连续锁模激光输出功率970 mW,光-光转换率达到13.7%.理论上讨论了调Q锁模的抑制措施,并结合实验结果进行了分析,得到了1063 nm稳定连续锁模激光输出,锁模脉冲光谱宽度1.2 nm(FWHM),用自相关仪测得脉宽为15.1 ps.     

磁声参量振荡的理论

在本文中,我们从磁-弹性耦合的宏观表达式,通过经典场论的方法,求得弹性振动和磁振璗的耦合方程,用来分析了伴随波长约等于铁氧体样品的线度的声振动而存在的磁振璗(磁声模)。文中指出,Spencer和LeCraw所发现的磁声效应是磁声模和静磁模在注入场的激发下产生的参量振璗现象(也可以说是热声子的电磁讯号的放大)。我们引用Berk等人在讨论一种半静磁操作放大器的文章中给出的公式,算出Spencer-LeCraw实验所需要的功率,其结果与记录的数据相接近。我们提出了使任一静磁模配合磁声模产生振璗的调谐条件以及降低激发功率和观测几十到几百兆赫的声频的办法。通过磁声模和静磁模的交变场向量的空间对称性的分析,我们推导出磁声参量振璗的选择定则:对于球体三个主要弹性振动模(旋转模、向径模和椭球模),(1)静磁模(n,m,r)的Walker指标n是偶数者不产生磁声效应;(2)指标m是奇数者不与旋转模产生磁声效应,m是偶数者不与向径模或椭球模产生磁声效应。我们也举出第一类本征振动中有只可能和n是偶数、m是奇数的静磁模产生参量振璗的例子。Spencer-LeCraw局限于使静磁场调谐在(110)模上,所观察到的现象仅仅是本文所给出理论预见的一个特殊情况。他们发现了椭球模和向径模的频率显著地出现,但并无旋转模的频率,这是上述的选择定则的具体验证。最后,我们指出,热声子的参量放大可形成铁氧体微波放大器的噪声的来源。     

The Effect of Temperature on the Operation of Quantum Well Laser: A Simulation Study Based on Three-Level Rate Equations

A temperature-dependent quantum well laser equivalent circuit model based on three-level rate equations is presented. The model is simulated using an ORCAD PSPICE circuit simulator. The characteristic properties such as threshold current, delay, and frequency response are simulated and compared with the available results. Included are temperature effect, leakage current, and heat dissipation effect on the chip. Threshold current increases, whereas turn on delay and optical output decrease with increasing temperature. An increase in temperature also reduces the operating frequency and resonance peak shifts to higher frequency. It is also observed that the modulation response of a QW laser with shorter separate confinement heterostructure region is more sensitive to the temperature variation.     

Experimental mathematical model of nanosecond laser interaction with material

Interaction of nanosecond pulsed lasers with material was studied from thermal point of view using experimental techniques and theoretical approach of dimensional analysis. Experimental data of laser heating, melting and plasma formation on metallic samples and silicon, presented mainly in previous publication and partly in this paper, were used for formation of experimental mathematical model in the form of criteria equations of laser-material interaction process. Four different criteria equations were created: for maximum surface temperature increase, melting threshold, melting duration and plasma formation threshold. Two equations account for changes of thermal properties with temperature. The value of latent heat of fusion was found to have almost no influence on melting duration. The presented model showed good agreement with the measured results. The criteria equations can be used for approximate prediction of laser pulse effects on materials without creating an exact mathematical or numerical model and for control of technological processes.     

A self-consistent Ornstein-Zernike approximation for the site-diluted Ising model

We propose a theory for the site-diluted Ising model which is an extension to disordered systems of the self-consistent Ornstein-Zernike approximation of Hoye and Stell. By using the replica method in the context of liquid-state theory, we treat the concentration of impurities as an ordinary thermodynamic variable. This approach is not limited to the weak-disorder regime or to the vicinity of the percolation point. A preliminary analysis using series expansion shows that it can predict accurately the dependence of the critical temperature on dilution and can reproduce the nonuniversal behavior of the effective exponents. The theory also gives a reasonable estimate of the percolation threshold.     

Fluxons interactions in a Josephson junction with a phase-shift

We consider a long Josephson junction with a discontinuity point characterized by a gauge phase-shift. The system is described by a modified sine-Gordon equation. We study, in particular, the interactions between a fluxon and a fractional fluxon. A perturbation theory is developed in the small phase-shift limit to understand the characteristics of the interaction. Finally, numerical computations of the threshold bias current and the threshold velocity for a fluxon running over a fractional fluxon are presented.     

纵向耦合对垂直腔面发射激光器的影响

考虑到垂直腔面发射激光器中的光子与载流子的纵向耦合,在行波速率方程中引入了纵向耦合因子。利用这个速率方程,讨论了阀值电流与量子阱数目的关系,计算结果表明光子与载流子的纵向耦合是不容忽视的。给出了阀值电流表达式,它表明纵向耦合因子越大,阀值电流越低。     

Self focusing of elliptic Gaussian laser beams: Saturable nonlinearity

An analysis of self focusing of an elliptic Gaussian laser beam, propagating through a medium with saturable nonlinearity has been presented. It has been established that stationary self trap propagation of the beam is not possible. Though stationary self trap propagation does not occur, one can define a virtual threshold power for self focusing. Above this threshold power value, but not far from it, the beam focuses; below this threshold value it defocuses. As a whole three different defocusing zones and two focusing zones have been identified. It is also shown that effective beam radius never reaches zero.     

The Taub solution again

A work, recently published by this journal, is discussed and the critique is presented in this same issue. Our revision of this critique is focused on two main topics: a) the suggested Taub solution and (b) the method followed to arrive at it.Reply to the comment by R. Jantzen, GRG 37 (2005)     

一种新的基于小波变换的图像去噪方法

根据图像小波分解的特点和小波分解后高频小波系数的统计特性,构造了一种新阈值函数的去噪算法。与传统的硬阈值、软阈值函数相比,新阈值函数考虑了图像能量分布的特点,对于每一小波系数乘以一个与自身大小相关的降噪因子,并且新阈值函数简单易于计算,具有较强的自适应性。实验结果表明,采用新阈值函数的去噪结果能够有效地抑制图像的马赛克效应,无论在视觉效果上,还是在信噪比增益上均优于传统的软、硬阈值方法。     

Laser-induced ionization and intrinsic breakdown of wide band-gap solids

A simple scaling of bulk laser-induced breakdown threshold for wide band-gap solids is derived on the basis of a recent modification of the Keldysh photo-ionization model [43, 46]. Contrary to most traditional models, the modification is based on rigorous energy dependence of reduced effective electron–hole mass. The dependence leads to a specific ionization regime with an extremely high ionization rate resulting in intensive generation of conduction-band electrons. The regime is characterized by a well-determined threshold intensity that is proposed to be associated with the threshold of bulk intrinsic laser-induced breakdown (LIB) by visible and near-infra-red laser radiation. That allows deriving dependence of LIB threshold on laser and material parameters. The presented model provides explanation for the experimental results on LIB thresholds that have not received theoretical interpretation. In particular, it reproduces empirical dependence of breakdown threshold on the average inter-atomic spacing derived from the experimental data. The LIB threshold evaluated from the presented model is very close to experimental data on bulk LIB by tightly focused beams in wide band-gap solids. PACS 78.47.+p; 42.50.Hz; 42.50.Ct     

Discrete phase space II. Relativistic lattice wave equations and divergence-free green's functions

Therelativistic lattice Klein-Gordon equation, Dirac equation, electromagnetic equations, and gauge field equations are presented as partialdifference equations. Various lattice Green's functions are constructed (except for non-abelian gauge fields). It is proved that many of the lattice Green's functions are non-singular or divergence-free. Moreover, it is conjectured that all lattice Green's functions are non-singular.