Numerical calculations of two-beam energy coupling gains inphotorefractive BaTiO3 crystal

Steady-state two-beam energy coupling gains are numerically obtained in photorefractive BaTiO₃ crystal. The energy coupling gains are calculated by changing the angle subtended by two incident beams and by rotating the crystal relative to the grating wave vector. Polarizations of the two incident beams are assumed to be (ordinary, ordinary), (extraordinary, extraordinary), (ordinary, extraordinary), or (extraordinary, ordinary). The wavelength of the beams is assumed to be 515 nm. The maximum coupling gain is found to be 4.8/cm when both beams are ordinary and 74.7/cm when the beams are extraordinary. It is also shown that energy can transfer between ordinary (extraordinary) and extraordinary (ordinary) beams. In this case the maximum energy coupling gain is found to be 9.2/cm     

LiNbO3晶体中光折变光栅各向异性衍射的定量研究

对LiNbO3晶体中光折变光栅的各向异性衍射进行了定量研究。基于有效电光系数非零和布拉格相位匹配条件,对在LiNbO3晶体中各向异性衍射的衍射效率与光束偏振态和写入几何的关系进行了较为详细的理论计算和分析,根据建立的数学模型进行了数值计算,发现光轴垂直于写入平面时,写入的光栅可发生显著的各向异性衍射。实验测量与理论计算结果基本符合。     

双曲正弦平方-高斯光束在单轴晶体中垂直于光轴的传输

基于光束在单轴晶体中的傍轴矢量传输理论,对双曲正弦平方-高斯光束在单轴各向异性晶体中垂直于光轴的传输做了研究,得到了一般的解析传输公式,并用数值计算方法分析了该光束的传输特性。研究表明:由于晶体的各向异性,其原来的偏振特性将不再保持,仅沿x方向偏振的初始场分量衍生出y分量;随着传输距离的增加,非寻常光沿x轴和z轴的偏振场慢变振幅均减小,光强峰值也随之减小。此外,双曲正弦平方-高斯光束在单轴晶体中垂直于光轴传输时,光束参数β取值不同,光强呈现不同的分布状态。     

矢量光场激发三阶非线性光学效应的研究进展（特邀）

具有偏振结构分布的强激光与非线性光学材料相互作用导致了多种新颖的非线性光学效应,反映了材料的非线性光学特性,调制了光场的传播行为。笔者概述了矢量光场激发三阶非线性光学效应的研究进展。首先简要介绍了任意偏振光激发三阶非线性光学效应的理论,如非线性薛定谔方程、光束传播方程、各向同性和各向异性三阶非线性光学系数。也简要介绍了表征三阶非线性光学系数的 Z-扫描技术。在弱聚焦条件下,给出了诸如径向偏振光、杂化偏振光和柠檬型庞加莱光束这三种类型矢量光场的焦场表达式。其次,重点回顾了多种矢量光场激发的各向同性/各向异性三阶非线性光学效应,包括径向偏振光激发的各向异性非线性光学效应、杂化偏振光激发的各向同性和各向异性非线性光学效应、柠檬型庞加莱光束激发的各向同性/各向异性非线性光学效应。最后,简要讨论了矢量光场在非线性偏振旋转、光束整形、可控光场塌缩与成丝和光限幅方面的应用。     

读出光偏振态控制的Ce：KNSBN光折变开关

理论分析了以读出光偏振态控制Ce：KNSBN晶体衍射光强从而实现光折变光开关操作的基本原理：在o光和e光读出条件下实验测量了o光两波耦合写入体光栅的衍射效率与写入光光强比的关系。结果表明，e光读出时最大光栅衍射效率为o光读出的13．6倍；e光读出与o光读出衍射效率之比随写入光强比的变化近似为一常数；Ce：KNSBN晶体具有良好的开关性能。     

IR birefringence in artificial crystal fabricated by anisotropic etching of silicon

Periodic structures of deep narrow grooves with vertical walls have been fabricated by liquid anisotropic etching of silicon. It has been shown experimentally that the medium obtained behaves as a negative uniaxial crystal with an optical axis parallel to the wafer plane and exhibits an extremely high optical anisotropy in the mid-IR spectral range. The difference in the effective refractive indices for the ordinary and extraordinary rays, Δn, is about 1.5.     

电光角度信息传递装置中光学器件定位误差研究

电光角度信息传递装置是利用铌酸锂晶体的线性电光效应,实现2.5S的角度信息传递和检测。装置由两部分组成：发射部分包括铌酸锂晶体、起偏器、λ/4波片和LD激光器;接收部分包括渥拉斯顿棱镜和两个光检测器。导出了出射的两束光的光强差与发射部分和接收部分之间相对转角的关系,从理论上分析了光学器件定位误差对电光角度信息传递装置性能的影响,并通过实验进行了验证,理论计算和实验结果符合得很好。     

部分相干月牙形光束在非Kolmogorov谱中的漂移

为了探究部分相干月牙形光束在非Kolmogorov谱中漂移的演化规律, 采用拓展Huygens-Fresnel原理, 得到了相应的解析表达式, 并运用MATLAB进行了数值模拟。结果表明, 在非Kolmogorov谱中, 部分相干月牙形光束的漂移分别随着各向异性参量的增大、湍流内尺度的增大、湍流外尺度的减小、结构常数的减小而降低; 与各向同性湍流相比, 各向异性湍流对漂移的影响较小; 月牙形光束的最大光强位置的离轴距离分别随着波长、光束阶数的增大而增大, 随着相干长度的增大而减小。月牙形光束由于最大光强位置的离轴特性, 有利于绕过障碍物传输, 所得结论对实际光通信有一定参考价值。     

Analysis of the stimulated Raman effects in an anisotropic crystal KDP

The semiclassical model of Shen and Bloembergen is applied to discuss the stimulated Raman effects in an anisotropic medium. In crystal KDP when the linear electrooptic effects are suppressed by proper orientation of the laser polarization with respect to the crystal axes, the coupled-wave analysis for Stokes generation give rise to solutions of ten composite Stokes-vibrational waves in the (anisotropic) Raman active medium. These composite waves may be identified as two ordinary forward-traveling waves, two ordinary backward-traveling waves, three extraordinary forward-traveling waves, and three extraordinary backward-traveling waves. When the phonons are nonpropagating, the solutions reduce to only one ordinary and one extraordinary wave in each direction. The thresholds and Stokes generations in this situation are discussed. Owing to the anisotropy of the crystal, it is found that the dispersion of KDP in general allows the momentum-matching condition for the coupling between Stokes and anti-Stokes waves to be nearly satisfied in five different situations.     

环状平顶光束在单轴晶体中垂直于光轴的传输特性

基于光束在各向异性介质中的傍轴矢量传输理论,对环状平顶光束在单轴晶体中垂直于光轴的传输作了研究,导出了偏振场的慢变振幅表达式,并通过数值计算分析了光束的传输特性。结果表明,环状平顶光束在单轴晶体中垂直于光轴传输时,晶体的各向异性对其光强分布有较大影响,在正单轴晶体远场处ne/no值越大,光强越小,光束在xy平面内扩散越小;在负单轴晶体中传输性质则相反;随传输距离z的增加,中心光强逐渐加强,光强分布不再保持原来暗中空分布的特性。     

Index of Refraction and Absorption Coefficient Spectra of Paratellurite in the Terahertz Region

Index of refraction and absorption coefficient spectra of pure paratellurite (α-TeO₂) crystal as a potential material for terahertz (THz) applications were determined in the 0.25–2 THz frequency range at room temperature by THz time domain spectroscopy (THz-TDS). The investigation was performed with beam polarization both parallel (extraordinary polarization) and perpendicular (ordinary polarization) to the optical axis [001] of the crystal. Similarly to the visible spectral range, positive birefringence was observed in the THz range as well. It was shown that the values of the refractive index for extraordinary polarization are higher and show significantly larger dispersion than for the ordinary one. The absorption coefficient values are also larger for extraordinary polarization. The measured values were fitted by theoretical curves derived from the complex dielectric function containing independent terms of Lorentz oscillators due to phonon-polariton resonances. The results are compared with earlier publications, and the observed significant discrepancies are discussed.     

Hermite-Cosh-Gaussian光束在单轴晶体中的传输

基于光束在各向异性介质中传输的傍轴矢量理论,对沿x方向偏振的Hermite—Cosh—Gaussian（HChG）光束在单轴各向异性介质中的传输作了研究,得到了解析的传输公式,并借助于数值方法讨论了各向异性介质对x方向偏振的HChG光束偏振特性的影响。结果表明,对于初始沿x方向偏振的入射光束,在晶体中其场强因参量取不同值而有不同分布。     

包层为单轴晶体的光纤中基模功率分配的分析

研究了纤芯为均匀材料，包层为单轴各向异性晶体材料且其主轴沿光纤轴线的单包层光纤模型，针对HE11模，运用矢量精确解法分析了单轴晶体材料主轴折射率比参数kcl对光纤芯层和包层间功率分配的影响．研究结果表明kcl对该种光纤中芯层和包层间的功率分布影响显著，且其影响的大小和规律与kcl的取值有关．     

Ca0.28Ba0.72Nb2O6单晶的光学性质的研究

研究了室温下铌酸钙钡单晶的光学性质。分别用分光光度计和椭偏光谱仪测量了铌酸钙钡晶体的透射率和折射率随波长的变化关系，结果表明铌酸钙钡晶体具有正常色散关系，且寻常光折射率‰大于异常光折射率ne，说明该晶体为负的单轴晶。折射率之差在短波区达到0．12。透射率光谱显示该晶体在400～900nm波段是透明的。根据透射率计算了该波段晶体的吸收系数以及它的平方根。通过对该曲线的研究，发现铌酸钙钡晶体吸收边以下对应的跃迁为间接跃迁，计算出间接跃迁的禁带宽度E为2．94eV以及声子能量E为0．17eV。此外，通过改变系统光路中偏振片的透振方向，获得了寻常光和异常光的透射率，并进一步计算了它们的吸收系数。     

一种新的各向异性锥形散射

一束异常偏振的Ar+激光束(λ=488 nm,I=3.4 W/cm2)斜入射到一块沿主坐标轴切割的Cu掺杂的钾钠铌酸锶钡(KNSBN)晶体上,在后面的观察屏上,观察到一种新型的各向异性散射环,同时还出现了强烈的非对称扇形效应和非对称各向同性及向向异性锥形散射以及法线方向的各向同性散射振荡,理论分析表明前者由后者引起的,且理论值和实测值吻合得非常好.     

洛伦兹光束通过一阶轴对称光学系统的传输变换特性

从广义惠更斯-菲涅耳衍射积分出发,导出了洛伦兹光束通过一阶轴对称光学系统传输变换的解析公式.给出了洛伦兹光束的束宽、瑞利长度和发散角的表达式,讨论了这些参量通过自由空间的变换规律.结果表明,洛伦兹光束的瑞利长度大于高斯光束的瑞利长度,洛伦兹光束光斑半径的扩展速度慢于高斯光束,洛伦兹光束的远场发散角小于高斯光束的远场发散角.结果还表明,自由空间中洛伦兹光束的束宽、瑞利长度和发散角的变化规律与高斯光束相应参量的变化规律相同.     

Dynamic self-diffraction effects in liquid crystals

The dynamic self-diffraction effects due to large optical nonlinearities of nematic liquid crystals via molecular reorientation are reviewed. The laser-induced gratings are shown to arise because two waves with ordinary and extraordinary polarizations propagate in a nonlinear anisotropic medium. Both threshold and thresholdless effects as well as nonlinear scattering in an initially nonhomogeneous medium are discussed. Direction of further work in this field as well as potential applications are indicated.     

Optical and electrical properties of two-dimensional anisotropic materials

Two-dimensional(2D) anisotropic materials, such as B-P, B-As, GeSe, GeAs, ReSe2, KP15 and their hybrid systems, exhibit unique crystal structures and extraordinary anisotropy. This review presents a comprehensive comparison of various 2D anisotropic crystals as well as relevant FETs and photodetectors, especially on their particular anisotropy in optical and electrical properties. First, the structure of typical 2D anisotropic crystal as well as the analysis of structural anisotropy is provided. Then, recent researches on anisotropic Raman spectra are reviewed. Particularly, a brief measurement principle of Raman spectra under three typical polarized measurement configurations is introduced. Finally, recent progress on the electrical and photoelectrical properties of FETs and polarization-sensitive photodetectors based on 2D anisotropic materials is summarized for the comparison between different 2D anisotropic materials. Beyond the high response speed, sensitivity and on/off ratio, these 2D anisotropic crystals exhibit highly conduction ratio and dichroic ratio which can be applied in terms of polarization sensors, polarization spectroscopy imaging, optical radar and remote sensing.     

Acoustooptic interaction of two light beams in a paratellurite crystal

Double acoustooptic diffraction of light in a paratellurite crystal is investigated experimentally and theoretically. The anisotropic Bragg interaction with three diffraction maxima is considered in the case when two monochromatic light beams are incident on the crystal simultaneously. For each beam, double diffraction is realized, so that the propagation direction of one of the beams coincides with the propagation direction of the second-order diffracted ray for the other beam. It is shown that the diffraction pattern consists of three diffraction maxima: two of these are formed by the zero- and second-order diffracted rays, while the third maxima is formed by the first-order diffracted rays. The possibility of summation of the radiation intensities at the diffraction maxima is analyzed.     

EEMGSM光束在各向异性湍流中的光束质量因子研究

为了探究矢量椭圆多高斯-谢尔模(EEMGSM)光束的传输特性, 基于广义惠更斯-菲涅耳原理和维格纳分布函数的二阶矩理论, 理论推导了EEMGSM光束在各向异性湍流中传输的质量因子解析表达式。通过数值计算和分析, 探究了初始偏振度、初始相干度、腰宽、波长和湍流结构常数等与质量因子的变化规律。结果表明, EEMGSM光束的质量因子随着阶数、波长和腰宽的增大而减小; 随着初始相干度和湍流结构常数的减小而减小; 在相同条件下, 具有大初始偏振度的EEMGSM光束的光束质量受各向异性的影响小于具有小初始偏振度的EEMGSM光束的光束质量; 且在相同条件下, EEMGSM光束的质量因子比标量椭圆高斯-谢尔模(EGSM)光束小, 即EEMGSM光束具有缓解各向异性湍流影响的优点。所得结论对于自由空间光通信的研究具有一定的理论参考价值。