A Novel Coherent Multilevel Combined Phase and Polarization Shift Keying Modulation in Twisted Fibers

This article proposes a novel coherent multilevel modulation model that exploits polarization and phase of the field. This model is based on the possibility of coherent systems to use all the four degrees of freedom of the electromagnetic field. The proposed scheme is able, by applying a suitable twisting process to the optical fiber, to physically confine the polarization and the phase of the transmitted field within the phase sphere. In this way, with the same performances of existing systems, the receiver is considerably simplified because there is no longer need of birefringence tracking circuits.     

The effects of different dispersion compensation ratios on nonlinear single channel and WDM systems

The effects of the compensation ratio with dispersion post-compensation on nonlinear single channel and WDM systems with 10 Gb/s per channel are numerically investigated. When the transmission signal per channel is 10 Gb/s RZ pulse and the amplifier span is 50 km, proper undercompensation can enhance the performance of the nonlinear single channel system and the nonlinear WDM system. But the enhanced distance of the nonlinear WDM system is not as obvious as that of the single channel system, because the cross-phase-modulation effect plays an important role in the nonlinear WDM system.     

北京同步辐射装置X射线漫散射实验站及准周期非晶多层膜的X射线非镜面散射研究

讨论了北京同步辐射装置的４Ｗ１Ｃ光束线和Ｘ射线漫散射实验站的技术特点．利用在漫散射实验站上获得的能量分辨率为４．４×１０－４、光斑大小为０．５ｍｍ（水平）×０．３ｍｍ（垂直）的聚焦单色Ｘ射线光束，研究了准周期非晶多层膜的Ｘ射线漫散射──非镜面散射．实验结果表明，准周期多层膜的漫散射强度分布在倒空间中与表面平行的条纹上并形成散射强度的调制．对散射强度调制进行的模拟计算得到相当满意的结果．     

啁啾光脉冲的振幅调制和相位扰动对压缩光脉冲的影响

 通过采用分步傅里叶变换法求解非线性薛定谔方程,模拟了啁啾光脉冲有振幅调制和相位扰动下的自相位调制（SPM）对压缩光脉冲对比度和预脉冲宽度的影响。结果表明：啁啾光脉冲的振幅调制深度和相位扰动深度越大,则压缩后的光脉冲对比度越小;啁啾光脉冲的振幅调制周期和相位扰动周期越小,则压缩光脉冲的预脉冲宽度越大。     

A comparison between transitions induced by random and periodic fluctuations

It is shown that a certain class of nonlinear systems possesses a unique and stable stationary state when subjected to periodic dichotomous modulations of an external parameter. This result enables us to define a probability density for the system and to characterize its shape and support. We compare this probability density with the one obtained in the case that the external parameter fluctuates randomly like a Markovian dichotomous noise and discuss various fluctuation-induced transition phenomena. The effects of these two types of fluctuations are quite dissimilar: the random fluctuations give rise to a richer behavior. The results are applied to the Freedericksz transition in nematic liquid crystals.Fellow of the University of Texas at Austin.     

A high-temperature structure for Ta2O5 with modulations by TiO2 substitution

Solid solutions in the series (1−x)Ta₂O₅−xTiO₂ with x=0.0-0.1 were prepared by high-temperature ceramic processing methods, and the crystal structure was determined at room temperature by transmission electron microscopy, electron diffraction and high-resolution lattice imaging. A structural model is proposed for the oxygen-deficient tantalum oxide (Ta₂O₅) phase with high TiO₂ doping level (x=0.08). The model is based on edge sharing of an oxygen octahedron-hexagonal bi-pyramid-octahedron molecular building block unit that repeats four times per unit cell. Electron diffraction reveals a monoclinic distortion from a pseudo-tetragonal model structure that is modulated primarily along 〈110〉. The modulation length varies with increasing TiO₂ content. Furthermore, by quantitative HREM analysis and matching of lattice images by simulation, it is shown that the modulation is associated with small ionic displacements in specific lattice planes that coincide with Ta ions in the model structure coordinated by oxygen hexagonal bi-pyramids. Based on this evidence, it is suggested that the modulation comes from a replacement of Ta with Ti ions, and the loss of inversion symmetry in the modulated structure is related to the dielectric properties of the material.     

啁啾脉冲堆积宽带激光的时间与频谱特性分析

理论研究了啁啾堆积脉冲宽带激光时间与频谱特性与子脉冲参量的关系.结果表明,变换极限子脉冲堆积所得的脉冲平滑,频率啁啾型与时间啁啾型子脉冲堆积所得脉冲出现时间调制,平滑性变差,从频谱上看,前者出现单一谱线,而后者可看到多条谱线|随着子脉冲间延迟的增加,堆积脉冲由脉冲间干涉引起的时间调制增多,但幅度减弱,而频谱的谱线数量增加.所得结果可为适当地选择堆积脉冲参量以获得所需的时间与频谱形状的宽带激光提供参考.     

截断的有振幅调制和位相畸变光束的等效曲率半径

高功率激光通常具有振幅调制和相位畸变. 采用统计光学方法推导了截断的有振幅调制和位相畸变光束在大气湍流中传输 的等效曲率半径R的解析公式.研究表明:随着位相畸变参量、振幅调制参量和光 束截断参量的增大,光束在自由空间中的 R增大,但R受湍流的影响也会增大; 并且高斯光束在自由空间中的R最大, 但其受湍流影响也最大. 因此,在大气湍流中传输到足够远时, 截断的有振幅调制和位相畸变光束的R就要比高斯光束的大.特别地, 相对等效曲率半径R_r随传输距离为非单调变化, 存在一个最小值, 即在该位置处R受湍流的影响最大. 此外, 达到 R_r最小值所需传输距离随光束位相畸变和振幅调制的加剧而增大.     

Disturbing interference patterns in femtosecond stimulated Raman microscopy

Femtosecond stimulated Raman microscopy (FSRM) is an upcoming technique in nonlinear microscopy which facilitates rapid chemical mapping. It employs femtosecond white‐light pulses as probe pulses and intense picosecond pulses as pump pulses. Stimulated Raman scattering (SRS) occurs at the focus of a scanning microscope. Chemical constituents in the sample are identified via their Raman signatures. In this article, disturbing interference patterns in FSRM are reported. They are caused by a broadening of the pump pulse due to nonlinear interactions in the focal region of the microscope and reduce the signal‐to‐noise ratio. The properties of these modulations are explored, and the methods to suppress them are presented. Copyright © 2009 John Wiley & Sons, Ltd.     

A label swapping technique using synchronous phase modulation without wavelength conversion

We demonstrate a label swapping technique for differential-phase-shifted-keying (DPSK) encoded labels using synchronous phase modulation (SPM). For the first time label erasure and insertion are performed in a single step without a need for wavelength conversion. The receiver sensitivity penalty of label swapping is limited to 1 dB when the timing mismatch between the new label and old label is kept from −18% to 18%.