Multiple-wave diffraction in high energy resolution back-reflecting x-ray optics

We have studied the effects of multiple-wave diffraction in a novel optical scheme recently published by Shvyd'ko et?al. utilizing Bragg diffraction of x?rays in backscattering geometry from asymmetrically cut crystals for achieving energy resolutions beyond the intrinsic width of the Bragg reflection. By numerical simulations based on dynamic x-ray diffraction and by experimentation involving two-dimensional angular scans of the back-reflecting crystal, multiple-wave diffraction was found to contribute up to several tens percent loss of efficiency but can be avoided without degrading the energy resolution of the original scheme by careful choice of azimuthal orientation of the diffracting crystal surface and by tilting of the crystal perpendicular to the dispersion plane.     

Effect of parametric refraction upon acousto-optic interaction

The interaction of optical and acoustic beams is studied for the case of a violated Bragg matching condition. The optical beam experiences self-action due to a cascade nonlinearity. Acousto-optic interaction can result in total reflection of the optical rays from the ultrasound beam.     

Diffraction of X rays at a Bragg angle of π/2 (Back reflection) with consideration of multiwave effects

The energy dependence of the back reflectivity in the dynamical diffraction of x rays at a Bragg angle of π/2 (back diffraction) in perfect crystals of cubic symmetry (silicon) is investigated theoretically. In this case strict backscattering is realized only under the conditions of multiple diffraction. The features of the influence of multiple diffraction on back reflection in the energy range near the nuclear resonance radiation energy of 14.41 keV for ⁵⁷Fe nuclei, specifically in the six-wave case, including the silicon (1,9,9) reflection (with an energy of 14.57 keV), which can be investigated experimentally with high energy resolution (1 meV) using synchrotron radiation and a monochromator developed for nuclear resonant absorption, are thoroughly studied. It is shown that the back reflectivity observed under the conditions of multiple diffraction has several maxima on the plot of its energy dependence with a value at each maximum smaller than half, in contrast to two-wave diffraction, where there is one maximum with a value close to unity. Zh. éksp. Teor. Fiz. 116, 940–952 (September 1999)     

Soft X-ray resonant magnetic diffraction

We have conducted the first soft x-ray diffraction experiments from a bulk single crystal, studying the bilayer manganite La2-2xSr1+2xMn2O7 with x=0.475 in which we were able to access the (002) Bragg reflection using soft x rays. The Bragg reflection displays a strong resonant enhancement at the L(III) and L(II) manganese absorption edges. We demonstrate that the resonant enhancement of the magnetic diffraction of the (001) is extremely large, indeed so large that it exceeds that of the nonresonant Bragg diffraction. Resonant soft x-ray scattering of 3d transition metal oxides is the only technique for the atomic selective measurement of spin, charge, and orbital correlations in materials, such as high temperature superconductors, colossal magnetoresistance manganites, and charge stripe nickelates.     

利用遗传算法优化线性啁啾光栅性能

针对制作町用于40 Gb/s全光色散补偿的宽带线性啁啾光栅时出现带内群时延纹波波动较大等问题.提出了一种通过设计和改变切趾函数的参量来优化线性啁啾光栅的新方法.该方法实现简单.只需根据需要设计具有不同滚降特性的切趾函数,同时利用遗传箅法来优化切趾参量.结合传输矩阵法经过200代获得了低带内时延纹波的线性啁啾光栅.数值结果验证了采取非对称分段切趾法在保持反射谱宽和平坦性的同时可以抑制带内群时延纹波的优越性.利用该方法制作了反射谱工作带宽为1.06 nm、时延纹波不超过45.60 ps、可用于大容量密集波分复用系统(DWDM)色散补偿的线性啁啾光纤光栅.     

Anomalous dispersion behavior of multiple-wave X-ray diffraction at absorption edges: determination of phase change at resonance

The effects of anomalous dispersion (resonance) on multiple reflection of x rays and their interference in crystals at atomic absorption edges are studied. Intensity ratios of two inversion-symmetry-related multiple diffractions at or near absorption edges exhibit highly phase-sensitive profiles with strong asymmetric characteristics, unlike those far from the edges. A new resonance perturbation Bethe approach is developed to explain this behavior. This leads to direct determination of the phase change for x-ray reflections at resonance.     

Right handed or left handed? Forbidden x-ray diffraction reveals chirality

Enantiomers, or stereoisomers, have crystal structures that are mirror images of each other and are thus handed, like our right and left hands. The physical properties of enantiomers are identical except for optical activity, which rotates linearly polarized light by equal amounts but in opposite directions. While conventional x-ray Bragg diffraction can determine crystal structures, it does not distinguish between right- and left-handed crystals. We show resonant Bragg diffraction using circularly polarized x rays reveals the handedness of crystals by coupling x-ray helicity to a crystal screw axis. The intensity of resonantly allowed reflection of alpha-quartz is well described by an admixture of a parity-even and a parity-odd process. Our results are of general importance and demonstrate a new method to directly study chiral motifs in structures that include biomaterials, liquid crystals, magnets, multiferroics, etc.     

Sinc切趾布拉格光栅谱合成特性

 谱合成是获得高功率激光输出的有效方法。反射布拉格光栅衍射旁瓣是影响谱合成效率的主要因素。建立了sinc切趾布拉格光栅谱合成理论模型,采用传输矩阵法,分析了光栅参数对切趾光栅衍射特性的影响,以及入射光束光谱宽度和发散角对谱合成效率的影响。计算结果表明：sinc切趾布拉格光栅可有效抑制衍射旁瓣的影响,其一级衍射旁瓣和二级衍射旁瓣的峰值分别由62%和36%下降为0.57%和0.12%。通过优化光栅参数,利用sinc切趾布拉格光栅可实现窄光谱间距、高谱合成效率的多光束谱合成。切趾后,在10 nm的带宽内,参与谱合成光束的数目由7束增加为25束。对于波长为1 064 nm和1 064.4 nm的两束光谱合成,当入射光束光谱宽度小于0.15 nm,且发散角小于0.8 mrad时,谱合成效率达90%以上。     

吸收限附近非对称反射条件下X射线的异常透射

研究了原子吸收限附近非对称布拉格条件下完整平板晶体的X射线异常透射．当衍射主要由原子散射因子的虚部引起时，在严格的布拉格角处，晶体内部驻波的波节位于衍射原子面上，从而导致异常透射的发生．透射波主要来源于晶体内部坡印廷矢量指向晶体下表面（入射面为上表面）的波场．该波场的有效吸收系数随非对称因子a的增大而减小，所以整个晶体的透射系数随a的增大而增大．当原子散射因子的实虚部对衍射的贡献之比一定时，晶体内坡印廷矢量偏离色散面实部法向的程度随反射的非对称程度的增大而增大． 关键词：     

Asymmetrically Apodized Fiber Bragg Gratings for Applications in Dispersion-Less Fabry-Pérot Fiber Cavities

Abstract

This article presents a theoretical study of the dispersion properties of the asymmetrically apodized fiber Bragg gratings. It is shown that the dispersive behavior of these gratings may be changed from normal to anomalous, depending on the apodization function and the grating strength. The asymmetrical Bessel functions were chosen in order to demonstrate these properties of the gratings. The application of this effect in a dispersion-less Fabry-Pérot fiber cavity formed by two asymmetrically apodized fiber Bragg gratings is discussed.     

基于布拉格反射镜的X射线多色单能成像谱仪

报道了一种基于布拉格反射镜的多色单能成像谱仪研制工作,谱仪由针孔阵列、布拉格反射镜和CCD相机组成.大约有300个微孔的针孔阵列板置于布拉格镜前用于空间成像,通过布拉格反射镜的单色化,投射到CCD上的数百个小孔成像沿色散方向获得了能量分辨.经过图像处理,可以还原得到目标的多色单能二维成像.根据采用的布拉格分光元件和图像还原方法,谱仪的能量分辨达到了50—200（λ/Δλ）;针孔成像的空间分辨优于10 μm.同时还为该谱仪开发了专门的单能图像重建软件及图像数据后处理软件,可以在任意选择的窄能带内还原准单能图像.并重点介绍了该谱仪的优化设计、获得的技术指标以及专门研制的超短周期（2.5 nm）X射线W/B₄C多层镜. 关键词： X射线光学 诊断技术 布拉格反射镜 X射线多层镜     

Parametric x-ray radiation from 500-MeV electrons in a tungsten single crystal with a surface mosaicity less than 1.5′

The yield of parametric x-ray radiation from a tungsten single crystal irradiated with a E=500 MeV electron beam from the Tomsk Sirius synchrotron was studied experimentally in Bragg geometry. The tungsten sample was 1.7 mm thick and had a surface mosaicity less than 1.5′; it was oriented in such a way that the (111) face was at the Bragg angle of θ _B=45° with respect to the electron-beam direction. The x-ray photons were detected at the angle of 2θ _B=90° with respect to the electron beam. The angular distributions of parametric x rays are compared with those calculatedwith allowance for actual experimental conditions.     

Characterization of x‐rays emerging from between reflector and sample carrier in reflector‐assisted TXRF analysis

The possible application of an Si reflector, which is placed just above the sample carrier in total reflection x‐ray fluorescence (TXRF) analysis, was investigated. The x‐rays that were emitted from an Mo tube and passed between the Si reflector and the Si sample carrier were analyzed with an Si drift detector. In our experimental setup, the angle between the reflector and the sample carrier can be changed by adjusting the inclination of the reflector. The intensity of the x‐rays that emerged from between the two Si surfaces drastically changed depending on the reflector angle. At a proper reflector angle, this intensity showed a maximum and, in addition, the Compton peak in the x‐ray spectrum was suppressed. When this x‐ray beam was used for excitation of TXRF signals, the highest intensity of x‐ray fluorescence emitted from the sample was detected, indicating that these experimental conditions are useful for the enhancement of TXRF intensities. Copyright © 2004 John Wiley & Sons, Ltd.     

体布拉格光栅用于高功率光谱组束的研究

使用透射型体布拉格光栅组束两束光纤激光,实现了856 W光谱组束输出。总的光谱组束效率为73.7%,组束光束的横向质量因子为7.9,纵向质量因子为2.7。研究结果显示,虽然体光栅的角色散严重影响衍射光束的光束质量,但其并不影响透射光束的光束特性。由于当前宽谱光纤激光器的输出功率远大于窄线宽输出,使用宽谱光纤激光器(光谱带宽超过4nm)作为透射光束,能够在不降低组束效率和组束光束质量的前提下,有效提升使用体布拉格光栅进行光谱组束的总输出功率。     

体布拉格光栅用于高功率光谱组束的研究

使用透射型体布拉格光栅组束两束光纤激光,实现了856 W光谱组束输出。总的光谱组束效率为73.7%,组束光束的横向质量因子为7.9,纵向质量因子为2.7。研究结果显示,虽然体光栅的角色散严重影响衍射光束的光束质量,但其并不影响透射光束的光束特性。由于当前宽谱光纤激光器的输出功率远大于窄线宽输出,使用宽谱光纤激光器（光谱带宽超过4 nm）作为透射光束,能够在不降低组束效率和组束光束质量的前提下,有效提升使用体布拉格光栅进行光谱组束的总输出功率。     

Engineering Optimisation: An Introduction with Metaheuristic Applications,by Xin-She Yang

When a beam of high intensity laser light is scattered by the fluctuations spontaneously existing within a fluid medium a spatial modulation of the dielectric constant may be induced. The role of such a modulation in causing stimulated scattering of the intense laser beam or Bragg reflection of any weak independent probe beam is discussed and theoretically analyesd. It is shown that such Bragg reflection provides a convenient technique for the study of the modulation. Experimental observation and investigation of Bragg reflection from such a modulation is described. A related technique has been used to investigate the development of light induced periodic structures on surfaces.     

SHG in Bragg reflection waveguides for enhanced bandwidth applications

A novel scheme for second harmonic generation in asymmetric Bragg reflection waveguides is presented for enhancing the bandwidth of non-linear interaction. The strong dispersion exhibited by the Bragg reflection waveguide is exploited for countering the rapidly falling efficiency as one moves away from the phase-matching wavelength. The role of the periodic cladding on the efficiency and bandwidth of second harmonic generation in a lithium niobate based Bragg reflection waveguide is analyzed. Such an idea could be used in other parametric processes for enhancing the bandwidth and for easing fabrication tolerances.     

Broadening of the phase-matching bandwidth in quasi-phase-matched second-harmonic generation using GaN-based Bragg reflection waveguide

We present an analysis of a high index core symmetric Bragg reflection waveguide (BRW) design based on a GaN/AlxGa1-xN system for efficient quasi-phase-matched second-harmonic generation for broadband applications. By choosing the fundamental frequency to be a BRW mode and suitably tailoring the overall dispersion characteristics, the strong dispersion of the second-harmonic mode is partially canceled, leading to phase matching between the fundamental and second-harmonic over a broad range of wavelengths. The crucial interplay between the dispersive behavior of the fundamental and second-harmonic wave manifests as a broad acceptance bandwidth of approximately 33 nm accompanied with appreciable conversion efficiency (22.8%/W) for a 10 mm long waveguide. The impact of tailoring the dispersion characteristics on the conversion efficiency is also discussed.     

Broadband fiber Bragg gratings for dispersion management

This paper provides an overview and technology update of a dispersion management component made from chirped fiber Bragg gratings. The history and technology of fiber Bragg gratings (FBGs) have been extensively covered in several excellent review articles, [1-6] book chapters, [7] and books. [8,9] We give a brief overview of fiber Bragg grating technology in this section and then focus on the details concerned with construction and performance of dispersion management devices in optical communications systems with a single broadband long-length grating used in reflection. Sampled chirped grating are sometimes referred to as broadband devices, but they actually have a narrow bandwidth of operation that is periodically repeated across a given spectral range. Although these periodic devices have shown promise as dispersion management devices in communications systems, they will not be discussed in this paper.     

Stochastic extraction of periodic attosecond bunches from relativistic electron beams

Intense laser waves can form a time-dependent gate, which transmits or reflects particles depending on their initial phases. When faced by a relativistic electron beam, such a barrier slices it by randomly scattering all but some particles, which nearly conserve their velocity. Subfemtosecond or attosecond periodic electron bunches are then formed downstream and can be used, for example, to generate coherent x rays via Thomson backscattering of the laser light.