噪声对集合种群稳定性的影响

本文在Levins模型的基础上研究了噪声对集合种群的稳定性的影响. 应用Fokker-Planck方程得到了系统的稳态概率分布函数和平均灭绝时间. 经过数值分析,结果如下: 无关联(λ=0 λ为加性噪声和乘性噪声之间的关联强度)时, 加性噪声强度α和乘性噪声强度D均弱化集合种群的稳定性; 噪声之间关联(λ≠0)时, 随着λ的增大,系统的稳定性被增强. 当-(c-e-D)²/(4c√Dα)<λ<1时, λ诱导"共振抑制"现象. D存在一个临界值, 小于临界值时, D可以增强系统的稳定性.     

Free-space evolution of focused Gaussian beams transformed by conical diffraction in a biaxial crystal

The intensity and polarization pattern of circularly and linearly polarized Gaussian beams transformed by the effect of conical diffraction in a biaxial crystal has been studied experimentally, and compared with the predictions of paraxial theory. Some general regularities in the relative intensity and geometry of beam structures were analyzed. The formation of Mathieu beam profiles from linearly polarized input beams was demonstrated.     

双峰远场结构的激光二极管阵列光场传输特性

提出了一种描述激光二极管阵列(Laser Diode Array,LDA)双峰远场结构的光场新模型,该模型同时考虑了激光二极管阵列垂直于结平面大发散角的传输特性,基于亥姆霍兹方程的远场解,运用非傍轴光传输理论分析,得到描述激光二极管阵列远场光分布模型,其结果与实验测量结果吻合较好.     

基于Fournier Forand体积散射函数的水中激光脉冲后向散射特性分析模型

基于Fournier Forand体积散射函数,建立了一种水中激光脉冲后向散射仿真模型.运用该模型可用Monte Carlo方法模拟光子在水中的传播过程,并得到光波后向散射的冲击响应.将初始激光脉冲与冲击响应进行卷积并求其傅里叶谱,即可得到激光脉冲后向散射信号的时域和频域特征.利用该模型分析了入射为高斯型激光脉冲时,水中散射体的尺度分布、散射体与纯水的相对折射率以及水体衰减系数对激光脉冲后向散射特性的影响.结果表明：随着小尺度散射体相对数量的增多、散射体与纯水相对折射率的增加、水体衰减系数的增大,激光脉冲后向散射信号能量增强,宽度增加,低频分量显著增大.     

980 nm泵浦激光二极管与柱形透镜光纤耦合的角向容忍度

给出了一个计算980 nm泵浦激光二极管与柱形透镜光纤耦合角向容忍度的理论模型,并选择圆形的柱透镜光纤进行数值计算,得出了在水平和垂直方向上偏转加平移时的相对耦合效率及绕光纤轴向转动的相对耦合效率的数值计算结果并加以分析,同时也研究了光纤透镜的形状和激光二极管与光纤的间距对相对耦合效率的影响.     

Growth control of stoichiometry in LaMnO3 epitaxial thin films by pulsed laser deposition

We have studied structural, magnetic, and optical transport properties of LaMnO₃ (LMO) thin films grown on SrTiO₃. While the stoichiometric LMO is an insulating antiferromagnet, it tends to be a ferromagnetic insulator when grown as thin films. By exploring the majority of growth parameters, we have found that the bulk-like electronic and magnetic phases can be stabilized by growing thin films under reducing atmospheres and by using more energetic laser processes. These conditions are found to reduce the La deficiency in the film resulting in the greatly improved cation stoichiometry. Since oxides are prone to reduce the oxygen content and to alter the cation ratio under such growth conditions, it suggests that the cation and oxygen stoichiometries in complex oxide thin films can be improved by properly optimizing the growth parameters.     

Growth of Cr : LiCaAlF6 and Cr : LiSrAlF6 by the Czochralski method

Single crystals of chromium-doped LiCAF and LiSAF can be grown from nearly stoichiometric melts of the components LiF, AlF₃, CrF₃ and CaF₂ or SrF₂, respectively, by the Czochralski method. The optical quality of LiSAF crystals is usually better, as LiCAF contains more scattering particles. This different behavior can be attributed to different thermodynamic properties of both substances: The higher melting point of LiCAF leads to higher evaporation losses of volatile LiF and AlF₃. Moreover, LiCAF melts incongruently. The main problem during the growth and application of LiSAF crystals is the highly anisotropic thermal expansion that may lead to thermal cracking. The extreme hygroscopicity of the doping agent CrF₃ has to be considered for the growth of both substances.     

带ARMA有色噪声的广义系统信息融合Kalman平滑器

对于带自回归滑动平均(ARMA)有色观测噪声的多传感器广义离散随机线性系统,应用奇异值分解,提出了广义系统多传感器信息融合状态平滑问题。基于Kalman滤波方法,在线性最小方差信息融合准则下,给出了按矩阵加权融合降阶稳态广义Kalman平滑器。为了计算最优加权,提出了局部平滑误差协方差阵的计算公式。一个Monte Carlo仿真例子说明了所提方法的有效性。     

Effects of correlations between the real and imaginary parts of quantum noise on intensity fluctuation for a saturation laser model

This paper studies the effects of cross-correlations between the real and imaginary parts of quantum noise on the laser intensity in a saturation laser model. It derives the analytic expressions of the intensity correlation function C（τ） and the associated relaxation time T（C） in the case of a stable locked phase resulting from the cross-correlation λq between the real and imaginary parts of quantum noise. Based on numerical computations it finds that the presence of cross correlations between the real and imaginary parts of quantum noise slow down the decay of intensity fluctuation, i.e., it causes the increase of intensity fluctuation.     

The growth and dislocation characterization of Yb0.005Y0.848Lu0.147VO4 mixed laser crystal

A new mixed laser crystal, Yb_0.005Y_0.848Lu_0.147VO₄, has been successfully grown using the Czochralski method. An ICP‐OES was used to measure the concentrations of elements (Yb, Y, Lu, V) in the crystal, and the chemical formula was determined. X‐ray powder diffraction analysis shows that the crystal has ZrSiO₄ structure. A pair of edge dislocations with certain orientation was observed by the HREM, and the orientation was confirmed by the calibration of the diffraction spots. The formation mechanism of this type dislocation was given by analysis on structure and thermal expansion of crystal. The suggestion to avoid these dislocations was also given.