Shaping of few-cycle laser pulses via a subwavelength structure

We theoretically investigate the propagation of few-cycle laser pulses in resonant two-level dense media with a sub- wavelength structure, which is described by the full Maxwell-Bloch equations without the frame of slowly varying envelope and rotating wave approximations. The input pulses can be shaped into shorter ones with a single or less than one optical cycle. The effect of the parameters of the subwavelength structure and laser pulses is studied. Our study shows that the media with a subwavelength structure can significantly shape the few-cycle pulses into a subcycle pulse, even for the case of chirp pulses as input fields. This suggests that such subwavelength structures have potential application in the shaping of few-cycle laser pulses.     

Image segmentation using adaptive loopy belief propagation

Loopy belief propagation (LBP) algorithm over pairwise-connected Markov random fields (MRFs) has become widely used for low-level vision problems. However, Pairwise MRF is often insufficient to capture the statistics of natural images well, and LBP is still extremely slow for application on an MRF with large discrete label space. To solve these problems, the present study proposes a new segmentation algorithm based on adaptive LBP. The proposed algorithm utilizes local region information to construct a local region model, as well as a local interaction region MRF model for image segmentation. The adaptive LBP algorithm maximizes the global probability of the proposed MRF model, which employs two very important strategies, namely, “message self-convergence” and “adaptive label pruning”. Message self-convergence can improve the reliability of a pixel in choosing a label in local region, and label pruning can dismiss impossible labels for every pixel. Thus, the most reliable information messages transfer through the LBP algorithm. The experimental results show that the proposed algorithm not only obtains more accurate segmentation results but also greater speed.     

Trapping of electromagnetic wave in single-negative metamaterial waveguides

We study the trapping properties of transverse magnetic (TM) waves in single-negative metamaterial waveguides including epsilon-negative (ENG) metamaterial and mu-negative (MNG) metamaterial. The relationship between effective refractive index and reduced core width is analyzed when the permittivities of ENG and MNG metamaterials are different, and the inflection on this curve can be regarded as the trapping point. Simulation results show that the properties in an ENG–MNG–ENG metamaterial waveguide are contrary to that in a MNG–ENG–MNG metamaterial waveguide. The sensitivity of trapping point to the change of permittivity makes the single-negative metamaterial waveguides to be an effective method to detect the permittivity variation and can be used as a new kind of waveguide sensor.     

光束成丝的非线性理论

在考虑调制场和本底场之间有能量交换的情况下,研究了小尺度调制场的非线性演化过程,结果表明,受到小尺度调制的光束在传输过程中有可能经历周期性成丝过程,通过与Bespalov-Talanov(B-T)关于成丝的线性理论比较,发现B－T理论给出的最快增长频率和临界频率等结果在非线性演化过程中仍然是适用的,但其给出的调制增长纺只在调制场的初始增长阶段与本文的非线性理论一致,此后,调制场的增长速度低于指数形式的增长速率,达到某个最大值后将经历周期性变化过程。     

Modeling the Effect of Injection Molding Process Parameters on Warpage Using Neural Network Theory

A back propagation artificial neural network (BPANN) prediction model for warpage of injection-molded polypropylene was developed based on an orthogonal design method. The BPANN model was trained by the input and output data obtained from the moldflow software platform simulations. It is proved that the BPANN model can predict the warpage with reasonable accuracy. Utilizing the BPANN model, the effects of the process parameters, packing pressure (Pp), melt temperature (T_me), mold temperature (T_mo), packing time (t_p), cooling time (t_c), and fill pressure (p_f), on the warpage were investigated. The most important process parameter affecting the warpage was Pp, and the second most important was T_me. The rest of the process parameters, T_mo, t_p, t_c, and p_f, were found to be relatively less influential. Warpage increased with elevating T_mo. In contrast, an increase in Pp and T_me caused the warpage to decrease.     

北极水声学研究的新进展和新动向

北极水声学作为水声学研究的一部分,起步要比达·芬奇所描述的声呐雏形晚很多年。第二次世界大战后北极水声学的研究开始受到发达国家(主要是美国)的重视。它的发展和研究重点带有明显的冷战烙印。冷战结束之后,随着北极持续变暖的趋势,北极及其毗邻海域的海洋水声环境受到特别的重视。环北极的8个国家组成排他性的北极理事会。我国政府于2018年1月26日发表北极政策白皮书,声明中国是近北极国家,是北极地区利益攸关方。本文介绍北极水声学研究的新进展,包括我国有关涉海单位近年来所做的科考和学术研究。指出,北极水声学的研究不局限于把传统水声学中的研究内容(如环境噪声、混响、传播等等)并行地在北极环境条件下加以重复探讨,而是要根据北极海洋环境的实际情况,进行有关领域的新研究。其中不乏传统浅海、深海水声学研究中所不具有的特色,如冰-水界面、冰下的半声道效应、冰盖下水下无人载器(UUV)的通信、定位及声呐对冰下环境的适应性研究等课题。     

Numerical propagation of partially coherent laser beams through optical systems

The propagation of partially coherent beams through optical systems is computed numerically in one transverse dimension. The optical system is divided into different elementary segments, through which the propagation of light can be calculated by appropriate operators, working on the coherence function or the Wigner distribution function respectively. For the necessary changes between these two functions describing the partially coherent beams, the use of the remarkable z-transform seems to be an advantage. With this algorithm the grid and the resolution in the spatial frequency domain can be arbitrarily chosen in contrast to the usual Fourier transform, the influence of phase aberrations on the focusability of Gauss-Schell model beams is discussed as an application example of the numerical model. With the help of this tool, practical beam guiding systems can be simulated for use with multimode laser radiation.     

圆柱对称光脉冲在负色散克尔介质中的传输

利用变分法得到了任意光脉冲传输的一般特性及高斯光脉冲压缩（或扩展）的必要条件，这说明在一定初始条件下时空非对称的光脉冲也可形成“光子弹”，在某些区域内，通过时空耦合作用，光束的空间会聚可以导至脉宽的压缩。     

生物组织光传播的时域特征分析

采用蒙特卡罗方法对脉冲光在组织中传播的时域特征作了研究，并把计算结果与漫射理论的解析解作了比较，文中详细分析了组织光学特性参数与组织表面时间分辨的漫反射光分布之间的关系。结果表明：散射系影响着漫反射光强到达峰值的时间，吸收系数影响着漫反射光强在峰值之后随时间下降的速度，而ｇ因子对响应曲线的前沿影响较大，在后沿阶段基本上没有影响。     

High-Q whispering gallery modes in a spherical shell dielectric resonator: Characterization at microwave frequencies

We present a numerical and experimental characterization of high-Q whispering gallery resonant modes excited in a fused quartz spherical shell operated in the 18-26.5 GHz band; easy coupling and selection of modes is a most advantageous feature of the proposed structure with respect to ordinary bulk resonators. The numerical results, based on a finite element algorithm, well match experimental data, which also show the capability of our device to perform gaseous spectroscopy measurements. Received 25 October 2002 Published online 4 February 2003