单轴晶体包层啁啾光纤光栅中电光效应和弹光效应的理论研究

提出了以单轴晶体材料为包层,光轴平行于光栅主轴(z轴)的新型啁啾光纤光栅模型,应用耦合模理论和传输矩阵方法在理论上分析了该类光纤光栅中的电光效应和弹光效应,理论研究发现在包层施加沿光栅轴向的电场和应变场可以改变布拉格波长和反射谱。得到了3种不同单轴晶体为包层时布拉格波长λB和反射光谱随外加电场和应变场变化的曲线。研究结果表明当轴向外加电场从1×107V/m变化到8×107V/m时λB减小0.12nm,当外加应变场从0变化到0.04时,λB减小0.45nm。     

联合梯度预测与导引滤波的图像运动模糊复原

针对由相机与所摄景物之间发生相对位置移动所导致的图像运动模糊,提出了一种鲁棒的基于单幅运动模糊图像的盲反卷积算法。该方法首先通过预测图像中的较强边缘信息,实现用简单、易于求解的优化问题在傅里叶域中快速、准确地估计出点扩散函数。然后利用得到的点扩散函数,使用基于梯度约束的非盲反卷积算法复原清晰图像,同时采用一种新的边缘保持滤波器-导引滤波来消除噪声并抑制振铃效应。实验结果表明：本文的算法能够快速地从单幅运动模糊图像复原出具有清晰边缘和纹理的高质量图像,并且运算时间不超过20 s。     

The single-longitudinal-mode operation of a ridge waveguide laser based on two-dimensional photonic crystals

An electrically driven, single-longitudinal-mode GaAs based photonic crystal (PC) ridge waveguide (RWG) laser emitting at around 850 nm is demonstrated. The single-longitudinal-mode lasing characteristic is achieved by introducing the PC to the RWG laser. The triangle PC is etched on both sides of the ridge by photolithography and inductive coupled plasma (ICP) etching. The lasing spectra of the RWG lasers with and without the PC are studied, and the result shows that the PC purifies the longitudinal mode. The power per facet versus current and current-voltage characteristics have also been studied and compared.     

金属包层对称平面单轴晶体波导的模式场(Ⅱ)

对于晶体光轴平行于波导界面的结构,讨论了光在金属包层对称平面单轴晶体波导(波导层是单轴晶体,两个波导界面均为金属)内的传输特性.解析地得到了这种结构下波导模式场的精确解.模式场的性质因单轴晶体的性质不同而异.对于正单轴晶体,波导的主模是横电波,任何频率的光波均可激励该模式;当频率满足一定条件时,波导内传输单模,否则,将激励起高阶模式.高阶模既非TE波,也非TM波,而是两者耦合而成的混合模.对于负单轴晶体,波导的主模是一种混合模,该模式同样可被任何频率的光波所激励;当频率满足一定条件时,波导内传输单模,否则 关键词： 平面金属波导 单轴晶体 模式场 混合模     

中能Abrasion-Ablation模型的核内核子密度分布修正

将具体的核内核子(包括中子、质子)密度分布引入到中能Abrasion-Ablation模型中,用修正后的模型计算了30MeV/u ⁴⁰Ar+^natAg反应中的弹核碎裂过程.结果表明,在非常周边的碰撞中,修正后的模型计算的碎片截面与修正前的计算相比有明显的差别,反映了核密度边缘弥散的影响.与实验结果的比较验证了中能弹核碎裂碎片能量相对于束流能量的降低是由摩擦作用引起的.     

Numerical simulations of the nonlinear propagation of femtosecond optical pulses in gases

A two-dimensional axisymmetric model of the propagation of intense femtosecond laser pulses through dispersion-free transparent media is described. The effects of diffraction, nonlinear Kerr effect (instantaneous and retarded) and multiphoton ionisation are included. Numerical results concerning air and other gases are discussed. In particular, time self-compression of femtosecond pulses is predicted. Stable self-guided pulses are simulated, in agreement with recent experimental observations. Received: 19 June 1998 / Received in final form: 14 January 1999     

弹丸爆炸驱动过程中层裂控制的研究

在弹丸爆炸驱动过程中,对厚度大于一定尺寸的弹丸,为降低弹丸内部负压以避免其层裂,通常对弹丸前端进行封装。研究发现,在爆炸产生的加载脉冲下,封装后的弹丸可能产生两次负压。一次负压的产生仅与选择的封装材料有关,二次负压的产生同时还与封装材料的厚度有关。通过理论推导得到了在给定弹丸材料和封装材料的情况下,为使弹丸不产生一次负压,加载冲击波和卸载稀疏波波后压力应满足的临界条件;同时导出了一次负压为零时,在弹丸不产生二次负压的条件下,封装材料厚度的临界值。采用AUTODYN有限元软件,对钢、铝弹丸封装材料不同的情况进行数值模拟,验证理论解的正确性;另外还对比研究了三角形冲击波加载的情况。研究结果可为弹丸爆炸驱动过程中封装材料的设计提供参考。     

大孔径双向聚能射孔弹的研究

 设计了一种双锥药型罩与双向装药结构相结合的聚能射孔弹模型,通过数值模拟方法研究其射流成型机理,并计算其射流参数。结果显示：双锥药型罩的小锥角部分形成聚能射流,大锥角部分形成翻转弹丸,射流头部和弹丸的速度分别为6 250 m/s和1 620.9 m/s,弹丸长度和平均直径分别为26.1 mm和8.6 mm。结合数值模拟结果,对射流侵彻公式进行了修正,并利用修正公式预测该射孔弹侵彻钢靶的深度,计算结果为69.6 mm。最后,按照该模型进行侵彻实验,实验回收弹丸的长度和平均直径分别为28.1 mm和8.8 mm,侵彻钢靶的深度和孔径分别为70 mm和17 mm。实验表明：数值模拟与理论计算方法相结合是可行的,能够有效地计算射孔弹的射流参数并预测其侵彻深度;该射孔弹侵彻性能优越。     

弹丸对钢筋混凝土中钢筋交汇处侵彻效应研究

考虑弹丸在钢筋交汇处与钢筋直接发生作用的情况,提出了弹丸侵彻钢筋混凝土的近似模型。利用该模型得到了弹丸侵彻钢筋混凝土过程中弹丸的加速度时间历程。计算结果与实验结果符合较好。用该模型分析了不同配筋结构、配筋尺寸和网眼尺寸对侵彻深度和侵彻过程的影响,结果表明:弹丸从钢筋交汇处侵彻时,当弹丸动能相对较小时,随着网眼尺寸的减小,弹丸的大部分能量均消耗在侵彻第一层配筋结构中,当弹丸动能较大时,不管网眼尺寸多大,第一层配筋均只消耗掉弹丸的部分能量;配筋直径和网眼尺寸对侵彻深度的影响较大。     

Review on second-harmonic generation of ultrasonic guided waves in solid media (I):Theoretical analyses

Considering the high sensitivity of the nonlinear ultrasonic measurement technique and great advantages of the guided wave testing method, the use of nonlinear ultrasonic guided waves provides a promising means for evaluating and characterizing the hidden and/or inaccessible damage/degradation in solid media. Increasing attention on the development of the testing method based on nonlinear ultrasonic guided waves is largely attributed to the theoretical advances of nonlinear guided waves propagation in solid media. One of the typical acoustic nonlinear responses is the generation of second harmonics that can be used to effectively evaluate damage/degradation in materials/structures. In this paper, the theoretical progress of second-harmonic generation (SHG) of ultrasonic guided wave propagation in solid media is reviewed. The advances and developments of theoretical investigations on the effect of SHG of ultrasonic guided wave propagation in different structures are addressed. Some obscure understandings and the ideas in dispute are also discussed.