Short Communication

The fidelity of reconstruction of 3-dimensional images by reflection and transmission holograms is investigated, concentrating on the measurement and theoretical modelling of large-scale geometric distortions in holograms illuminated by white or monochromatic light. Variable parameters include: recording and replay wavelengths; object position at recording; emulsion thickness and refractive index; replay light-source angle and distance; presence of a glass substrate, its thickness and refractive index. Holograms were recorded on Agfa 8E56HD photographic plates, and measurements made of image-point directions as a function of position on the hologram, revealing considerable image distortions even for modest changes in parameter values between recording and replay. The results are accurately modelled by the theory, which gives numerical values for point-by-point image positions, without any resort to paraxial approximations.     

Determination of the refractive index and the chiral parameter of a chiral solution based on chiral reflection equations and heterodyne interferometry

This study develops a method for determining the chiral parameter and the refractive index of an isotropic chiral medium using chiral reflection equations and critical angle phenomena. Linearly polarized light propagates back and forth in a parallelogram prism between two parallel compartments with chiral solutions. A beam splitter then divides the light that emerges from the prism into a reflected light beam and a transmitted light beam. The two beams pass through a compensator and an analyzer, respectively, to cause phase compensation and interference of s and p polarizations. The phase difference between the two interference signals are initially optimized by a suitable optical arrangement and subsequently measured by heterodyne interferometry. Additionally, the refractive index of the solution is determined from the critical angle that occurred at the discontinuity of the phase difference between the two interference signals. These results are substituted into derived equations to calculate the chiral parameter. The approach has the merits of both common-path interferometry and heterodyne interferometry.     

Modelling the asymmetry of the recorded transition zone in digitalmagnetic recording

A vector model of the recorded transition zone for digital magnetic recording is used to incorporate asymmetric transition profiles, non-constant recording contours and both longitudinal and perpendicular components of magnetization into a channel model. These factors all contribute to the asymmetry of the recording zone and hence the asymmetry of replay pulses and the associated nonlinear channel phase response. The use of the model for evaluation of channel spectra is examined. In particular it is found that good fits to experimental results for a typical recording channel are provided by a linear recording contour in conjunction with an arctangent transition profile. This model is useful for inclusion in a full digital magnetic recording system model since analytic expressions are available for both the replay pulse and its spectrum     

Optimizing replay intensity and resolution in aberration-compensated off-axis holograms by ambient humidity control

Abstract

In hologrammetry it is desirable to reconstruct the real image rather than the virtual image as the latter must be viewed at a distance through the window of the holographic plate itself. When a scene is located in water but the image is replayed into air, it is necessary to correct for the refractive index difference by reconstructing the image with shorter wavelength illumination and changing the beam angle to satisfy the grating equation. However, this means that the Bragg condition may no longer be satisfied during replay, reducing the diffraction efficiency and decreasing the signal-to-noise ratio of the reconstructed images. Changing the replay beam angle to satisfy better the Bragg condition makes the images brighter but also renders them unusable by increasing the optical aberrations. Our solution is to alter the Bragg properties of the hologram by altering the humidity of the surrounding atmosphere. This approach has been experimentally demonstrated for Agfa 8E56HD emulsions by measuring the brightness and resolution of a reconstructed real image from an off-axis hologram over a humidity range from 6 to 93 per cent. The emulsion swelling and its effect on the Bragg properties of the hologram were modelled using the Flory-Huggins theory of polymer swelling.     

Switching time of single magnetic particle and maximum recording frequency of perpendicular magnetic recording

We investigated the switching time of a magnetic particle-which may impose limits on magnetic recording frequency-by computer simulation. We found that the difference in switching time between low and high temperatures decreases with increasing the angle /spl xi/ between the applied field and the easy direction, and scarcely changes with temperature if the angle is larger than a few degrees. This can be explained by considering the energy contour and locus of magnetization in switching. The switching times derived by using divided models, in which the particle is divided into 2/spl times/2/spl times/2 or 4/spl times/4/spl times/4 cells, are smaller than those derived by using an undivided model. But the difference between the switching times derived with the divided and undivided models is less than 1% if the angle /spl xi/ is larger than 1/spl deg/. We derived equations to express the switching times of the particle for the divided models at /spl xi/=0, and we used the equations to estimate the maximum recording frequency in perpendicular recording.     

An interferometric method for simultaneously determination the phase retardation and fast-axis azimuth angle of a wave plate

In this study, an interferometric method was proposed for the simultaneous measurement of phase retardation and fast-axis azimuth angle of a wave plate by using a Mach–Zehnder interferometer. The wave plate to be tested is placed in one of the light passages in the interferometer, and two analyzers with transmission axes at horizontal and vertical orientations are arranged at the two output regions. When a linearly polarized laser light beam is passed through the interferometer, two interference light beams are simultaneously generated. Through an analysis of the intensities of the two light beams, the phase retardation and fast-axis azimuth angle of the tested wave plate can be simultaneously determined using specially derived equations. The feasibility of the proposed method was demonstrated using measurements of zero-order half-wave and zero-order quarter-wave plates. The proposed method is easy to operate, enables rapid measurement, and has high stability and accuracy.     

Off-Bragg analysis of the diffraction efficiency of reflection photorefractive holograms

Formulas are given for the calculation of diffraction efficiency of reflection-type gratings recorded in a photorefractive medium. The analysis incorporates the coupled-wave theory that was developed for photorefractive hologram gratings. This analysis takes into account grating slant with respect to the medium surface, light absorption during reconstruction, any incident angle of the reference beam, and any photorefractive phase shift. General solutions for signal and reference wave functions are given in a closed-form expression by use of a hypergeometric function. The optimum media parameters and recording conditions for high diffraction efficiency are obtained by the derived formulas. The diffraction properties for off-Bragg conditions are also discussed.     

弹性曲梁几何非线性精确模型及其数值解

基于直法线假设,采用轴线可伸长梁的几何非线性理论,建立了弹性曲梁在任意荷载(保守和非保守)作用下的静态大变形数学模型。其中包含了轴线弧长、轴线位移、横截面转角、内力等七个独立未知函数。通过引进变形后的弧长为未知函数,使得问题的求解区间为未变形梁的轴线长度。该模型不仅考虑了轴线伸长,同时精确地考虑了梁的初始曲率对变形的影响以及轴向变形与弯曲变形之间的相互耦合效应。作为应用,采用打靶法计算了悬臂半圆形曲梁在沿轴线均布的切向随动载荷作用下的非线性平面弯曲问题,给出了随载荷参数大范围变化的平衡路径曲线及平衡构形。     

Optical compensation of hologram distortion avoiding interpage crosstalk on reconstructed image in angle-multiplexed holograms

We are studying a form of holographic data storage with phase conjugation, and we compensated for hologram distortion due to shrinkage of photopolymer materials in the holographic medium by controlling the wavefront of the reference beam. When a high NA lens and narrow angle interval of angle multiplexing are employed to obtain a high data recording density, some wavefronts cause interpage crosstalk on the reconstructed image. We tried to determine the moving range of actuators in a deformable mirror for controlling the wavefront. As a result, we found that the distortion in the hologram could be compensated while avoiding interpage crosstalk and that the bit error rates of the reproduced data could be decreased. We also found that the optimized wavefront could compensate for distortions in several neighboring data pages. This method can ensure a high data recording density in holographic data storage.     

Transient Energy Transfer during Hologram Formation in Photorefractive Crystals

The temporal and spatial variation of the beam intensities and space charge electric field during hologram formation in a photorefractive crystal are determined by solving the relevant partial differential equations. Starting with the equations of Kukhtarev, Markov and Odulov [1], analytic and numerical solutions are presented for a range of parameters for the time evolution of the energy transfer between the optical beams, the appearance of the space charge electric field, the phase angle between the interference pattern and the space charge field, and the bending of the grating.     

Off-axis Holographic Zone Plates Recorded and Reconstructed by Cylindrical Wavefronts

Theoretical aspects of off-axis holographic zone plate recording using two cylindrical waves, with arbitrary angled focal lines, and replay of such holograms with a cylindrical or spherical wave are discussed in the narrow-beam approximation. Expressions for the distances and orientations of focal lines of generally reconstructed astigmatic beams are given, and conditions for obtaining spherical waves are analysed. The results have been verified by experiments using photographic emulsion (Agfa-Gevaert Holotest 10E56 plates) as the recording material. In particular, experimentally obtained parameters of the reconstructed astigmatic beam and the spherical wave are found to be in good agreement with theoretical results.     

一种基于横摇稳定的多波束测深方法

针对阵列姿态变化造成深水多波束测深声纳测深误差问题,给出了一种基于横摇稳定的多波束测深方法。其基本思路是利用姿态数据实时地改变接收波束相控角,使得合成波束指向角稳定在较小的变化区间内,确保了波束形成后信号来自有效的波束宽度覆盖范围。通过发射时刻三维姿态和接收时刻横摇姿态解算得出波达时刻对应的合成波束指向角,进行声速修正和归位计算得到最终测深点的深度与位置。MATLAB仿真和实际海试数据处理结果表明,该方法能够获得期望方向附近的海底脚印覆盖并且达到较好的精度。     

Dynamic micromagnetic field measurement by stroboscopic electronbeam tomography

A stroboscopic electron beam tomography system for measuring the dynamic micromagnetic field of recording heads is presented. A pulsed electron beam, which is synchronized with the recording head driver, is scanned along the recording head surface from all directions. Integration of the magnetic field intensity along the beam path is calculated from the electron beam deflection angle. Intensity distributions of the dynamic magnetic field are calculated using a tomographic reconstruction algorithm. To obtain enough current even in pulsed electron beam operation, a high-brightness Ti/W thermal field emitter is used. This system was successfully applied in measuring the field distributions of a thin-film recording head, with 0.1 μm spatial resolution and 1 ns time resolution at an operation frequency of 30 MHz     

Optical properties of a slightly absorbing film for oblique incidence

Approximate equations are derived for calculating the transmittance and reflectance of a slightly absorbing film when radiation is incident at an arbitrary angle. These formulas are compared with those derived from wave optics. Examination of the real and the imaginary parts of the complex phase change and the complex angle of refraction shows the simple equations to be consistent with the wave-optics formulation under the assumption that the imaginary part of the refractive index of the film is much smaller than its real counterpart.     

Polarization of holographic grating diffraction. I. General theory

The full polarization property of volume holographic grating diffraction is investigated theoretically. With a simple volume grating model, the diffracted fields and Mueller matrices are first derived from Maxwell's equations by using the Green's function algorithms. The formalism is derived for the general case that the diffraction beam and the grating wave vector are not in the plane of incidence, where s waves and p waves are not decoupled. The derived photon-momentum relations determine the Bragg angle selectivity. The parameters of diffraction strength related to the hologram-writing process and material are defined and are not necessarily small in general. The diffracted-beam profiles are analytically calculated by using the known grating shape function. This theory has provided a fundamental understanding of the polarization phenomena of a real holographic diffraction grating device. The derived algorithm would provide a simulation-analysis tool for the engineering design of real holographic beam combiner/splitter devices.     

利用波动法研究曲梁结构中的波形转换和能量传递

基于Flugge理论,建立了薄壁均质常曲率曲梁面内运动的6阶微分控制方程,得到了曲梁的频散特性曲线和6种波的轴向位移和径向位移的比值,推导了位移和内力响应的表达式以及物理域和波数域的变换矩阵。利用波的传递和反射矩阵对曲梁和半无限长直梁耦合时的能量传递系数和反射系数进行了求解分析。对于半无限长直梁中给定的拉伸波或弯曲波入射,得到了和频率,曲率半径和伸展角度相关的各种波传递和反射的能量系数表达式。数值结果表明,纵波和弯曲波在经过曲梁结构之后发生了波形转换,并研究了能量传递和反射系数随频率,伸展角度,曲梁曲率半径和截面尺寸比的变化。结果表明,无限长直梁和曲梁耦合系统中,低频时,经过曲梁反射和传递后的弯曲波和纵波会相互转化;高频时弯曲波和纵波都能够没有散射地通过曲梁而进行传播。为改善高频时曲梁中的能量衰减效果,研究了在曲梁结构中插入单个、多个中间支撑或阻振质量块时的能量传递和反射系数。结果表明,阻振质量块能够很好地阻止高频时曲梁中能量的传递,对于周期分布的多个阻振质量块,能量传递系数随频率的变化存在周期结构的阻带特征。这些研究结果为曲梁结构的设计提供定性的理论基础。     

A new protein crystallography station on the SRS wiggler beamline for very rapid Laue and rapidly tunable monochromatic experiments: I. Design principles,ray tracing and heat calculations

A new instrument is being established on the SRS wiggler beamline at Daresbury. The design consists of a toroid focussing mirror to produce a point focussed white beam of X radiation of an intensity sufficient for millisecond and sub-millisecond Laue exposures from protein crystals. The wavelength band available for focussed Laue experiments will be variable by means of a plane mirror with an adjustable graze angle and by adjustable thickness beryllium filters. Unfocussed white beam will also be available to provide the full spectral range of the wiggler source. New, larger film cassettes and changing mechanisms are being designed to use the available data more efficiently (by decreasing spatial overlap and increasing the number of recording ‘singlet’ reflections). A double crystal monochromator option will be available for rapidly tunable monochromatic data collection. Design principles based on theoretical work and ray-tracing are dealt with in the paper.     

The dependence of recording characteristics of thin metal tapes on their magnetic properties and on the replay head

A theory is derived to explain the recording properties of thin metal tapes in terms of their magnetic properties and in terms of the losses within the replay head. The tape losses are considered as being due to self-demagnetization and head losses resulting from gap and separation effects. The reproduction of isolated pulses is first considered and then modified to the condition of pulse crowding. A comparison of theory with experiment shows that the theory is sufficient in its prediction of head losses but has some limitations in its prediction of tape losses at high packing densities.     

Spectral shifts and spectral switches of a pulsed Gaussian beam from a rectangular aperture in the far field

The far-field anomalous spectral behaviours of a space–time-dependent Gaussian pulsed beam passing through a rectangular aperture are studied. By expanding a hard aperture function into a finite sum of complex Gaussian functions and starting from the Fresnel diffraction integral, the approximate analytical expression for the spectral intensity of a space–time-dependent Gaussian pulsed beam passing through a rectangular aperture is derived. Meanwhile, the corresponding closed-forms for the slit and the unapertured cases are also given as special cases of the general results. The red and blue shifts and the spectral intensity distribution are studied and illustrated with numerical calculations. Specifically, it is shown that the spectral switch takes place when the truncation parameter is equal to particular values or the observation position is at the critical diffraction angle. The possibility of tunable spectral switching in the far field with an apertured pulsed beam by varying the size of the rectangular aperture is highlighted.     

索-曲梁组合结构的面内振动

研究了索-曲梁组合结构的面内动力特性。首先利用Hamilton变分原理得到索-曲梁的线性运动方程以及索-曲梁组合结构的边界条件,然后在给定位移边界条件下通过解析求解得到索、曲梁各自的特征值,进而可以确定组合结构的固有频率。最后讨论了索的倾角和曲梁半径以及曲梁的不同截面对组合结构固有频率的影响,并且与有限元结果进行了对比。结果表明固有频率与索与曲梁的质量比、曲梁半径以及索的倾角有关。