阶跃光纤的偏振色散

利用全反射光在光纤包层中的横向偏移和渡越时间,导出了各向同性光纤的全反射偏振色散公式及双折射光纤偏振色散的修正公式,通过模拟计算分析了偏振色散随入射角的变化规律.     

光子晶体光纤包层可见光及红外宽带色散波产生

对光子晶体光纤包层3个空气孔之间节区的传光特性进行了分析, 对比了纤芯与包层节区传光的模场面积、非线性系数及色散特性, 得到光子晶体光纤包层节区具有小芯、高非线性的特点.在包层空气孔较大的情况下, 得到了双零色散曲线.根据色散曲线, 分析了色散波产生的相位匹配特性, 得到了色散波中心波长随抽运波长及功率的变化规律.制备出了所设计的光子晶体光纤, 实验得到了在可见光及红外大于300 nm的宽带色散波, 并给出了色散波随抽运波长及功率的变化规律.实验和理论分析结果一致, 为波长变换及超连续宽带光源的研究奠定了基础. 关键词： 光子晶体光纤 色散波 相位匹配 非线性     

包层为负折射率介质的抛物型渐变光纤色散特性分析

研究了纤芯为各向同性非均匀材料,而包层为负折射率均匀介质光纤的色散特性.在纤芯满足弱渐变条件的假定下,按矢量场分析法,通过芯层与包层的介电常数比ξ和磁导率比η,讨论了导波模式色散特性对左手介质包层材料的依赖性.分析表明,ξ和 η对轴对称模色散曲线的低频段影响较大,但对非轴对称模影响不大.ξ和 η对导波模式的截止频率没有影响.由于该光纤中的HEm1模的截止频率都等于0,所以不存在理论上的单模区,但仍可近似实现无色散传输.     

薄包层啁啾长周期光纤光栅的色散补偿

为了提高啁啾长周期光纤光栅(LPFG)光纤通信的色散补偿能力,提出了利用薄包层啁啾LPFG进行包散补偿的方法.首先介绍了根据传输信号确定啁啾LPFG的啁啾系数、光栅长度等参数的方法.然后利用上述方法设计了对光纤中传输的中心波长为1550 nm,带宽为0.2nm的信号进行色散补偿的薄包层啁啾LPFG.利用耦合模理论及传输矩阵法计算了约1m长的此种啁啾LPFG的色散,结果表明可以补偿该光信号通过46 km光纤所产生的色散.进一步分析了切趾函数、啁啾系数、交叉耦合系数等参数对薄包层啁啾LPFG色散的影响.     

光子晶体光纤的导波模式与色散特性

利用有效折射率方法基于标量近似理论对光子晶体光纤的传播模式和色散特性进行了数值模 拟，发现通过调节光纤包层的空气填充率或包层空气穴节距及其有效芯径可以在很宽的波长 范围实现单模传播，可以设计零色散波长小于1.27μm的光子晶体光纤和在较宽的波段接近 于零色散的色散平坦光纤，以及具有较大的正常色散值的色散补偿光纤. 关键词： 光子晶体光纤 有效折射率 标量近似 导波模式     

基于薄芯光纤模态干涉技术的折射率传感特性实验研究

报道了一种具有微结构缺陷的折射率传感器,并对其折射率特性进行了实验研究.将一部分薄芯光纤熔接于标准单模光纤中,由于插入的薄芯光纤和单模光纤纤芯失配,导致包层的高次模被激发并与纤芯模在单模光纤内形成干涉仪.通过减小薄芯光纤的包层直径,以增强包层中的传输模在环境中的倏逝场,从而提高对环境折射率测量的灵敏度.实验表明,该折射...     

四包层光纤的归一化截止频率

本文从严格的场分布方程出发,导出了四包层光纤中LP模的特征方程,精确计算了几个低次模的归一化截止频率,给出了它们与结构参数的一般变化曲线.结果表明:四包层光纤的截止特性与单包层和W型光纤有较大不同;在所需波长要得到单模传输,第二包层的高度和宽度不宜太大.     

有限包层半径光纤Bragg光栅的理论研究

采用光纤波导三层模型,对有限包层半径光纤Bragg光栅导模有效折射率的改变进行了理论分析,结果表明:当包层直径小于16μm时,单模光纤Bragg光栅(纤芯直径为8.3μm)的导模有效折射率才开始发生明显变化.在包层外添加外包层,通过改变外包层的折射率可以实现对光栅Bragg反射波长的调谐,同时对不同芯子直径的光栅Bragg波长移动进行了数值计算.在保证光纤归一化频率不变的前提下,芯径越小Bragg波长调谐范围越大,当包层厚度为1μm时,芯径为a＝2.2μm的光栅Bragg波长调谐范围约为3.9μm.     

热光系数与长周期光纤光栅的温度灵敏度研究

利用受温度影响的光纤的本征方程和相位匹配条件,从理论上研究了长周期光纤光栅(LPFGs)的温度响应特性,给出了LPFGs的温度灵敏度的解析表达式。对利用低模序包层模的LPFG进行了实验研究。结果表明,利用不同包层模的LPFGs具有不同的温度灵敏度。分析了光纤的材料热光系数和模的热光系数的差别。单模光纤导模的热光系数接近纤芯的材料热光系数,而包层模的热光系数比包层的材料热光系数大,模序越大,其值越大。适当调整纤芯和包层的热光系数,并选用不同的包层模,可以得到对温度灵敏或不灵敏的LPFGs。     

含左手材料光纤的表面模特性

在弱导条件下,利用Maxwell方程组,对纤芯是左手材料,包层是右手材料的光纤表面模进行了研究,得到了TE (TM)、 EH和 HE表面模的色散方程,.根据色散方程,画出了相应表面模和含左手材料光纤导模的色散曲线.比较这些色散曲线,发现了含左手材料光纤表面模一些新的特性.     

光栅面不平行对压缩脉冲时空特性的影响

 推导了光栅对压缩器两光栅在垂直于刻线的平面(衍射面)内有一夹角时1至4阶色散变化量的表达式,以及脉冲超高斯光束经光栅对压缩后的场分布。并据此模拟分析了光栅表面平行性失调对输出脉冲时空特性的影响。结果表明：脉冲光束单次通过光栅对时其波形会产生扭曲,光栅对的失调会使脉冲扭曲更加严重;而两次通过光栅对时横向谱移动的影响会消除,但脉冲将出现旁瓣,光栅对的失调使得脉冲在时间上提前,且色散阶数越高色散变化量对压缩脉冲时空特性的影响越大。     

重离子弹性散射中的角分散研究

简要评述了重离子弹性散射角分散研究的内容、 方法及物理意义。通过前角区重离子弹性散射产物微分截面的角分布测量,作出角分散图ln(dσ/dθ)\|θ2。 分析经典偏转函数, 从而在实验上确定了反应系统的核虹角。 在低能、 重靶的重离子反应系统中, 核虹角远小于擦边角。 晕核及弱束缚核比稳定核具有更小的核虹角和更大的核相互作用范围。 经典偏转函数的计算有助于提供一套光学势参数, 以便于拟合弹性散射产物的微分截面。 In terms of the angular dispersion plot of ln(dσ/dθ) versus θ2, which can be obtained from the angular distribution of the elastic scattering differential cross sections in heavy ion collisions, systematic analysis on the angular dispersions is made by using classical deflection function for the available experimental data on the target of 208Pb. Our systematic analyses bring about some important results. Firstly, there is an angular dispersion turning angle at forward angular range beyond the grazing angle. Secondly, the nuclear rainbow angle for such reaction systems can be determined by measuring differential cross sections of elastic scattering at forward angular range and analyzing the angular dispersion. Thirdly, analysis of angular dispersion may provide a way to determine a set of optical potential parameters by means of fitting the experimental data of elastic scattering differential cross sections. Finally, for the halo nuclei as the projectiles, there is an exotic behaviour, i. e., smaller angular dispersion turning angle.     

Directional dispersion of phonons and polaritons in SnO

Using right angle Raman scattering we measured the directional dispersion for LO- and TO-phonons in ZnO. The experimental results are in good agreement with calculated dispersion curves. We also computed the directional dispersion from the measured dispersion branches of the A1-polariton. The scattering cross section of this polariton increases with polariton character.     

Transmission of light through a random small-angle scattering medium

A theory of random small-angle scattering is presented. The photon dispersion partial differential equation is derived and the dispersion coefficient characterizing the medium is introduced. The equation is solved for a spatial impulse. The modulation transfer function and the contrast loss are derived as a funcion of spatial frequency, dispersion coefficient, and object-to-image distance. The limitation on resolution is shown by an indeterminancy relation based on the dispersion coefficient. The dispersion angle (rms value of scattering angle) is calculated as an example.     

Sum rules for forward elastic photon scattering

Relations between integrals over forward elastic photon scattering amplitudes, forward elastic cross sections and total cross sections are derived from dispersion relations. A new photon-proton interaction sum rule is derived and evaluated.     

Boundary conditions for an electromagnetic field on the surface of linear and nonlinear crystals: Allowance for weak spatial dispersion and near-surface inhomogeneity of optical properties at the intermedium boundary

Boundary conditions for an electromagnetic field in the sharp-boundary model are derived for an arbitrary angle of incidence of the light with allowance made for the near-surface inhomogeneous layer actually existing and the spatial dispersion of the nonlinear medium. It is shown that the discontinuity of the tangential component of both the magnetic-field induction and electric-field intensity vectors should be taken into account. Formulas are derived for performing calculations of these discontinuities in nonlinear media.     

Linear theory of beam–wave interaction in double-slot coupled cavity travelling wave tube

A three-dimensional model of the double-slot coupled cavity slow-wave structure(CCSWS) with a solid round electron beam for the beam–wave interaction is presented. Based on the "cold" dispersion, the "hot" dispersion equation is derived with the Maxwell equations by using the variable separation method and the field-matching method. Through numerical calculations, the effects of the electron beam parameters and the staggered angle between adjacent walls on the linear gain are analyzed.     

Sum rules for forward elastic pion-nucleon scattering

A number of energy weighted sum rules relating amplitudes and differential cross sections for forward elastic and charge exchange scattering to the total pion-nucleon cross section are derived from dispersion relations.     

Identification of biphonons from prominent features of the angular and energy dependences of the inelastic-scattering cross section of biphonon-splitting neutrons

The dispersion relations and wave functions of biphonon and dissociated two-phonon states of anharmonic crystals are used to determine the cross section of inelastic scattering of neutrons that split biphonons into unbound phonons, which scatter each other as a result of their anharmonicity. Prominent features are found of the angular and energy dependences of the cross section, useful for analyzing experimental data to identify biphonons; it is possible that these features can also be used for subsequent, potentially major modification of the system of existing criteria, based solely on energy-balance considerations for the classification of series of spectral resonances, supposedly corresponding to bound multiphonon states of various multiplicities. For a fixed, large loss of neutron energy, the cross section is a maximum in a “nonhead-on” neutron-biphonon collision with a lobe-shaped angular scattering diagram; for intermediate energy losses the cross section has the largest of all possible values at all collision angles; and, for small energy losses, the cross section is a maximum for “head-on” collision in a narrow range of angles. For a fixed angle the energy dependence of the cross section has a resonance peak, which exists at the low-energy edge of a finite energy band for large angles and, as the angle decreases, gradually increases as it shifts toward the high-energy edge of the band, which becomes narrower and shifts into the low-energy region. However, when the angle decreases below a critical value, the still-increasing resonance maximum changes direction and shifts back toward the low-energy edge. It is shown that, despite strong oscillations of the biphonon wave function in the presence of negative phonon dispersion, the cross section does not depend on the sign of the dispersion, i.e., the universal law of independence from this sign, established previously for the dispersion relation and the biphonon damping constant, appears to carry over to the cross section. Fiz. Tverd. Tela (St. Petersburg) 40, 728–734 (April 1998)