Induced Diffraction in Phase-Mismatched Second-Harmonic Generation

We show analytically that in phase-mismatched second-harmonic generation, an effective diffraction is induced at the second-harmonic （SH） frequency. Numerical simulation results agree with the analytical predictions. Compared to the case of linear propagation, the effect of the overall diffraction at the SH frequency becomes doubled due to the induced diffraction, which causes an interesting result that the SH beam width will be larger than that of the fundamental field.     

Audio watermarking based on psychoacoustic model and critical band wavelet transform

Watermark embedding algorithm based on critical band wavelet transform of digital audio signal is proposed in this paper. The masking threshold for each audio signal segment was calculated on the basic of psychoacoustic model. According to the similarity between critical band of human auditory system and critical band wavelet transform, a watermark was embedded into the low-band and mid-band coefficients of digital wavelet. The embedding strength was adaptively controlled by the masking threshold. The experiment results show that the embedded watermark signal is inaudible, and the watermarked audio signal has good robustness against many attacks such as compression, noise, re-sampling, low-pass filtering.     

An optical fiber methane gas sensing film sensor based on core diameter mismatch

<正>An optical fiber evanescent wave methane gas sensor based on core diameter mismatch is reported.The sensor consists of a multimode fiber in which a short section of standard single-mode fiber,coated with the inclusion of cryptophane molecules E in a transparent polysiloxane film,is inserted.The sensing principle is analyzed by optical waveguide theory.For different sensing film thicknesses and interaction lengths,the sensor signal is investigated within the methane concentration range of 0-14.5%(v/v).It is shown that the sensor signal with the thickness of 5μm and the interaction length of 3 mm strengthens linearly with the increasing concentration of methane,with a slope of 0.0186.The best detection limit of the sensor for methane is 2.2%(v/v) with a response time of 90 s.This sensor is suitable for the detection of methane concentration below the critical value of 5%.     

超宽带（UWB）技术新进展

本刊讯超宽带（UWB）技术是一种广泛应用于传感网络和短距离无线接入网的现代通信技术。超宽带信号近年来由于其较低的功耗，增强的安全性以及多径衰落的免疫性，在短距离无线接入网、高数据能力的传感网络中得到了广泛的应。美国通信联邦委员会（FCC）已经明确定义UWB技术规范。其最大的优点在于在允许频段内（3．1GHz-10．6GHz）可以免授权使用。传统的电域产生UWB信号的方法最大的缺点在于信号无中继传播距离受限于其最大允许功率密度，产生信号和对信号进行中继传输的申．     

新技术改善光纤通讯质量

新一期的《自然一光子学》报道了一种新技术可改善光纤通讯系统中的数据传输速率和发射距离。该技术可减少光纤长距离连接传输中的光学数据信号失真损失量，进而改善在此基础上的全球网络和声音数据的传输质量。与以往利用单个光束编码数据不同，XiangLiu等人采用了两种特制的“相位共轭”光束，这两种光束能有效地起到相互镜像的作用。因为任何失真在这两束光中将表现出相反的形式，     

Searching for Flavor-Changing Neutral-Current Signals via e＋e-（γγ） →tcht0 in Topcolor-Assisted Technicolor Model

In the context of the top-color-assisted technicolor （TC2） model, the flavor-changing neutral-current （FCNC） interaction induced by the top-higgs （hOt） are predicted at tree level. We study the relevant processes of e＋e-（γγ）→tcht0 at the International Linear Collider （ILC）. It is found that these rare processes production rates can be enhanced significantly in the TC2 model. Especially for the process γγ→tcht0, the total cross section will reach a few fb in the reasonable parameter space. We also calculate the distributions of transverse momenta, pseudo-rapid and invariant mass of the top-higgs. In view of the main decay modes of top-higgs for mho 〈 2mr, we find that there will be several hundreds FCNC events produced at the ILC with annually integral luminosity of 500 fb-1. Due to the clean background, such FCNC signals can possibly be detected at the ILC.     

严平稳谐波的上穿过期望次数以及最大值

本文着重讨论一般谐波过程(包括叠加谐波)严平稳的条件、上穿过的期望次数与最大值.     

Negative Resistance in a Two-Dimensional System with Entropic Barrier

A phenomenon of negative resistance is found in two-dimensional bistable and periodic potentials via Langevin simulation, where output quantities for noise and signal driven system, such as the power-spectrum density modulus and the signal power amplification, can become minima at finite temperatures. In such a system, the curvature of the potential along non-transport degree of freedom at the barrier is larger than that at the local minima. The temperature-dependent effective barrier, i.e. entropic barrier, is introduced via integration over the non-transport variables. The system shows the negative resistance because of the competence between the signal and the entropic barrier.