顺电相KTN中反射型全息相位栅的衍射效率分析

在顺电相光折变晶体中存储全息相位光栅是研制新型电控全息光开关的有效途径。为了提高光栅的衍射效率,将Kogelnik耦合波理论与Kukhtarev方程相结合,得出了顺电相钽铌酸钾晶体中反射型相位栅衍射效率的解析式,对影响光栅衍射效率的各因素进行了计算分析。结果表明,使写入光的入射角远离45°、增加晶体厚度和增大外加电场等措施都可以使反射型全息相位栅的衍射效率提高。45°入射角附近出现较宽的低衍射区。衍射效率对入射角、晶体厚度以及外加电场的变化敏感,其中任一个参量的较小改变都可以引起衍射效率的较大变化。晶体的吸收对衍射效率的影响可以忽略。分析结果有助于在实践中调整参量以获得较高的读出光衍射效率。     

采用全息干涉法的纯扩散机制光折变相移研究

基于光折变两波耦合(TWC)理论，导出纯扩散机制情况下衍射再现光波和直透光波间的相位差与光折变相移的关系；把光折变晶体(PRC)作为全息干涉记录介质，基于光折变TWC机制和全息干涉技术，提出了一种可直观验证光折变相移的方法；以纯扩散情况下Ce：SBN晶体TWC为例，给出了光折变相移的直观观测实验结果。     

双光子光折变晶体中全息屏蔽空间孤子

为了得到双光子光折变晶体中全息屏蔽空间孤子的结果,基于双光束耦合耗散系统和双光子光折变效应,证明了外加偏压的双光子光折变晶体中存在一种新的全息屏蔽空间孤子。此类孤子需要满足两种平衡,一是损耗与增益之间的平衡,增益由抽运光通过两波耦合为信号光提供;二是衍射与非线性之间的平衡,非线性包括外加电场的非均匀屏蔽和两波耦合对折射率的周期性调制。在信号光远小于抽运光的条件下得出了全息亮和暗孤子的解析表达式。研究了全息屏蔽孤子的演化特性。     

LiNbO3晶体中光折变光栅的各向异性衍射

光折变晶体中光栅的各向异性衍射可用来提高光折变存储器读出信号的信噪比。本文通过极化率张量对LiNbO3晶体中光栅形成和衍射的偏振特性进行了理论分析,并关重对光栅矢量与晶体光轴共面的情形进行了实验研究。观察到的各向异性衍射读出角度的测量结果与理论计算相符。     

基于FlexPDE研究顺电相KTN晶体中大调制度全息光栅的形成

在用带输运模型研究光折变效应时,大多数情况下都限定入射的相干光调制度远小于1,对方程做线性化近似以便于求解,这在求解小调制度问题时可以达到满意的精度。但在很多实际情况下光调制度接近于1,此时线性理论不再适用。基于FlexPDE程序提出了一种新的求解方法,适用于大调制度时的情况。采用有限元算法,严格求解光折变带输运方程,研究了外加电场时顺电相KTN晶体中大调制度全息光栅的形成,给出了晶体中的空间电荷场、光激发电子、离化施主、晶体折射率等的分布情况和时间演化曲线。计算结果表明,采用FlexPDE程序不仅可以大大降低带输运模型的求解难度,而且能够实时动态显示全息光栅的形成过程,具有直观性。     

温度对小振幅双光子光折变屏蔽孤子的影响

研究了温度变化对双光子光折变晶体中屏蔽孤子稳定性和演化特性的影响.其结果表明,当双光子光折变晶体的温度发生变化时,由于光折变效应与衍射效应抵消后导致的在光折变晶体中稳定传输的屏蔽光折变孤子光束的强度轮廓随之变化;晶体中在一定温度下形成的稳定孤子,当温度在一个较小的范围内变化时,孤子能够克服较小的微扰而保持其稳定性;而当晶体温度发生比较大的变化而超出了孤子稳定温度范围时,孤子将变得不稳定甚至瓦解;改变温度而使晶体中不稳定的孤子变成稳定孤子.     

光折变立方晶体中的双光束正交耦合

本文报导光折变立方晶体中的双光束耦合的理论分析和实验研究,讨论了具有闪锌矿结构和具有非中心对称的43m立方晶体中的双光耦合,立方晶体具有(ε_w)_(ij)=n~2δ_j和非零电光系数的张量元γ_(ijk)=γ_(kij)=γ_(41)因此,由相干光场激发的载流子通过漂移和扩散产生空间电荷和电场,又通过线性电光效应(普克尔效应)形成折射率或电介常数空间变化的相位光枷     

光学自增强效应与衍射效率的提高

利用含时光折变双光耦合方程数字模拟计算了掺铁铌酸锂晶体中双光耦合的动态过程,分析了空间电荷场及其位相、光折变光栅位相等光折变参量的变化过程,解释了光折变自增强效应,提出了提高光折变光栅衍射效率的有效方法．     

全息存储中的栅增强读出

基于波的耦合理论,给出了实际写入光折变晶体中光栅振幅的空间分布函数,通过数值解耦分析了单光束读栅和双面交替读栅时全息光栅的增强特性。提出了一种易于实现双面交替读栅增强衍射效率的存储结构,此结构中包括一个存储晶体和一个动态全息刷新器,给出了实现栅增强的最佳结构参数。     

30 ns电子全息光学开关

国家光纤光学工程师会议(2000年8月,丹佛)宣称,Trellis Photonics的研究人员证实,他们已在为高速光学交叉连接而设计的光学开关中获得亚微秒的开关速度。该开关基于钽铌酸锂钾(KLTN),这是一种光折变材料,在这种光折变材料中能写入波长专用的全息图,然后使其有电气开关的作用。将KLTN晶体组装到行列矩阵中,其列代表输入波长,而行代表输出光纤,任何波长路由到任何光纤。信号也能作动态试验、测量和衰减。 KLTN为顺电材料,具有二次电光效应。外加电场将空间电荷转换成双折射,从而引起衍射。由每块晶体有4个全息图的三块晶体组成4×4开关结构,其开关速度为30 ns,总衰减为3.5 dB。在1.25 G b/s的速度下进行了比特误差速率测量,测量表明由电子全息机制产生的功率损失小于1 dB。该器件能实现光学信息开关。     

The quasi-static approximation for moving and finite temperature plasmas

A stationary plasma in the presence of a magnetic field may be represented by an equivalent dielectric tensor. The equivalent dielectric tensor of a moving plasma, valid within the quasi-static approximation, is found by a Lorentz transformation of the dielectric tensor from a reference frame in which the plasma is at rest to one in which the laboratory is at rest. The range of validity of the dielectric tensor is investigated, and the relation between this tensor and the equivalent dielectric tensor found from Maxwell's curl equation is determined. An infinite number of moving plasma streams with a Maxwellian velocity distribution are summed to obtain the dielectric tensor of a finite temperature plasma. The dielectric tensor leads directly to the dispersion relation for waves on a bounded plasma.     

Resonant modes in shielded cylindrical ferrite and single-crystaldielectric resonators

The Galerkin-Rayleigh-Ritz method is applied for computing the first few lowest resonant frequencies of cylindrical anisotropic resonators in a cylindrical cavity. The resonators are allowed to have gyromagnetic and uniaxial dielectric anisotropy with respect to the z-axis of the cylinder. Results of computations of the resonant frequencies are compared to exact solutions for many simple resonant structures and to results of experiments for more complicated structures, with good agreement. A new method of measuring permeability tensor components is presented. The method utilizes two parallel-plate cylindrical resonators operating in the HE⁺-/₁₁₁ and H₀₁₁ modes. A method of measuring the permittivity tensor components of single crystals using one parallel-plate cylindrical resonator operating in two different modes is proposed     

Photorefractive effects in cubic crystals: explicit treatment ofthe piezoelectric contribution

The influence of piezoelectrically induced strains on the electrooptical coefficient governing the photorefractive behavior of cubic crystals is reexamined. A general direction is assumed for the space charge grating. The analysis is carried out in a grating-adapted system of axes, which is directly linked to the actual experimental configuration. The appropriate photorefractive parameters, e.g., the electrooptical tensor, the dielectric constant, the optical indicatrix, and the space charge density, are put in analytical forms which are easy to compute and interpret. A practical example of experimental interest is worked out in detail     

四频差动激光陀螺中水晶片在应力作用下的光学性质

建立了四频差动激光陀螺水晶片弹光效应和旋光性交互作用的理论模型,并指出了该模型的求解方法和步骤。首先,计算在应力作用下新旧主轴坐标系的变换矩阵以及在新主轴坐标系中水晶片的逆介电张量和旋光张量,然后计算实验室坐标系与新主轴坐标系之间的变换矩阵,求出实验室坐标系下水晶片的逆介电张量和旋光张量,根据水晶片的简正模方程和物质方程就可以求出其应力作用下沿波矢方向的简正模和相应的折射率。通过数值实例计算了四频差动激光陀螺中波矢与主轴不同位置关系时水晶片在波矢方向简正模椭圆度和相应折射率随应力的变化,对分析四频差动陀螺温变零漂的产生机理和水晶片的安装具有一定的指导意义。     

结构参量对二维三角晶格光子晶体带隙影响规律的研究

本文利用了平面波展开法分别计算了二维三角晶格空气中介质柱和介质中空气孔两种形状的光子晶体,研究了填充率、介电常数对带隙宽度和中心频率的影响规律,研究结果对光子晶体的制作提供了理论依据.     

Ta2O5单晶的激光法生长

采用激光加热基座生长法(LHPG)生长出Ta_2O_5单晶,测量了所生长晶体的介电系数和介电损耗随晶格方向的关系。Ta_2O_5晶体表现出很强的介电各向异性,并且其介电系数的平均值比 Ta_2O_5陶瓷的大许多。Ta_2O_5单晶的成功生长证明了LHPG技术在生长高熔点晶体方面所具有的独特能力,同时也表明它作为一种特殊的激光材料加工技术在材料科学与工程中将发挥重要作用。     

用平面波展开法计算二维方形光子晶体的带隙结构

用平面波展开法计算二维正方晶格光子晶体的带隙结构，对二维光子晶体的电磁波理论及周期介质中的Bloch波解进行了详细的推导，得出TE模和TM模下无缺陷时光子晶体的色散曲线，并设计了低频区域内具有较大带隙宽度的两种二维光子晶体的空间周期结构. 经过大量的计算，发现硅中的空气柱型光子晶体在红外波段TE模和TM模存在重叠的绝对光子带隙，并分别研究了空气中的硅介质柱和硅中的空气柱的TM模带隙宽度随空气柱半径和填充比变化的规律.     

Electromagnetic scattering from nonspherical particles: a new polarisability model with application to agglomerating spheres

A new model for the static polarisability tensor elements of small particles is proposed and investigated. The static polarisability tensor for nearly agglomerated and agglomerated homogeneous dielectric spheres is investigated using a numerical procedure. Excellent agreement is obtained with a recent theoretical formulation for the slightly separated case.<>     

An Analysis of the Tensor Dielectnc Constant of Sea Ice at Microwave Frequencies

The bilocal approximation of strong fuctuation theory is used to develop a model for the tensor dielectric constant of sea ice that is treated as a random anisotropic medium. The application is a twofold one: first the individual ice grains are treated as pure ice with embedded elongated brine cells with random spacings and sizes and secondly as the combined ice-brine system, which has tensor dielectric properties, with random orientations of the principal axes and random embedded air bubbles. Comparisons with measurements on a variety of ice types over the temperature range 0 to -32 ° C and frequencies from 0.1 to 40 GHz show that the theory is able to account quantitatively for large parts of the published data including expected scattering corrections to the quasi-static dielectric constant at low salinities and high frequencies.     

Propagation in Rectangular Waveguide Containing Inhomogeneous, Anisotropic Dielectric

The Wentzel, Kramers and Brillouin (WKB) approximation is used to solve the wave equations for propagation of guided waves in rectangular waveguide containing an inhomogeneous dielectric. The simplest form of anisotropy is used to characterize the relative dielectric constant, i.e., it is assumed that the relative permittivity tensor is diagonalized with respect to the waveguide coordinants. Each of the elements of the relative permittivity tensor is allowed to vary continuously across the broad dimension of the waveguide. The TE/sub nm/ and TM/sub nm/ cases are analyzed for the instance of completely filled guide, while the TE/sub no/ modes are considered for slab-loaded guide.