Two-wave mixing and energy transfer in BaTiO3 application to laser beamsteering

Energy transfer between the two recording beams in a BaTiO₃ photorefractive crystal is analyzed as a function of the following parameters: incident beams ratio, spatial frequency, pump beam intensity. Exponential gain coefficients Γ ≈ 20 cm^-1 are reached for optimized holographic recording conditions. Application of the energy transfer to a new method of laser beam deflection is proposed.     

大角带宽高衍射效率体全息光栅的设计和制备

高性能体全息光栅是全息波导的重要耦合元件,角带宽小、平均衍射效率不高是制约体全息光栅性能的重要因素.以不对称倾斜记录为出发点,设计并制备了大角带宽高衍射效率的体全息光栅.首先讨论在横电模式光和横磁模式光下体全息光栅的记录参数与其衍射效率的关系,找到平均衍射效率较高的记录参数范围,随后进一步分析在此范围内的记录参数与体全...     

90°全息记录衍射效率分析与提高

对光折变全息记录特别是双中心全息记录中90°记录结构下较低的衍射效率进行了研究,采用局域衍射理论对90°记录结构的衍射进行了分析,表明在同样的折射率变化和2 mm的光束宽度的情况下,只有当折射率光栅振幅大于10^-4时,90°记录结构衍射效率才能够与小角度透射记录结构的衍射效率大致相当。针对环境干扰导致的干涉条纹振动影响光栅记录,提出了有效调制度概念,根据分析90°记录结构的干涉条纹间距很小,容易受外界环境干扰而导致低的折射率变化率,因此应采用主动条纹锁定系统。此外在双中心全息记录中,微观光学参量的优化对衍射效率影响也很重要。     

High-density holographic recording parameters under different wavelength of a two dyes-sensitized photopolymer

The high-density holographic recording parameters of a novel two dyes-sensitized photopolymer under different exposure wavelengths are studied. The results show that the maximum diffraction efficiency, exposure sensitivity, maximum refraction index modulation, dynamic range, and the exposure time constant increases with the increase of the exposure wavelength. The analysis indicates that the scattering has an important role in the forming of the holographic grating.     

I型全息凹面光栅制作误差对光谱像的影响分析

通过对I型全息凹面光栅制作参量误差对光谱像的影响进行数值计算发现|1)两记录臂长的相对误差而不是绝对误差决定光谱像的展宽程度,即使绝对误差较大,只要两记录臂长的误差值相同,像宽也没有明显改变|2)由于I型光栅的记录臂一般较长,记录角度误差对像宽的影响不大,但会影响光栅的刻线密度,导致光谱成像位置的偏移|3)曲率半径误差对像宽的影响较大.通过数值模拟明确了I型全息凹面光栅制作的误差容许范围,找到了对光谱像宽度影响较大的误差来源,从而为此类光栅的制作提供理论指导,有助于制作出高质量光栅,降低罗兰圆光谱仪的调节难度.     

使用辅助光栅的平面变间距全息光栅加工理论研究

变间距全息光栅具有自聚焦和消像差功能,是高分辨率光谱仪与同步辐射单色器中的重要元件。研究了使用平面等间距光栅产生非球面波,记录平面变间距全息光栅的方法。根据几何光学的光线追迹理论,推导了光栅参量的四阶解析表达式。并基于费马原理,提出了记录光路的光线追迹数值算法。应用所推导的光栅参量四阶表达式,仿真设计了变间距全息光栅。通过合理选择记录参量,可以避免光栅基底受到零级及高阶衍射光场的影响。设计结果表明,理论光栅线密度与要求值相当符合;经光线追迹数值算法验证,解析表达式的展开误差在整个记录区域内小于1.5线;考虑到实际加工工艺允许误差,使用辅助光栅的记录光路对记录参量的误差并不敏感;设计实例证明了解析表达式的有效性,以及使用辅助光栅的记录光路的优越性。     

Conjugate Image Plane Correlator with Holographic Disk Memory

A conjugate image plane correlator with holographic disk memory is proposed. Optical correlation between conjugate images reconstructed from a holographic disk and an input image on liquid-crystal television is executed with the rotation of the disk. Regardless of Fourier hologram recording with the pseudorandom diffuser, it is found possible to take out the diffuser from the original hologram recording scheme using an image reconstruction process and to get correlation signals between input and reconstructed conjugate images in the output plane of a two-lens imaging system. Generation of conjugate replicas with high contrast causes exact matching with an input image which results in high recognition performance for autocorrelation signals. The transfer function of an optical system can be controlled by adjustment of either hologram size or hologram area illuminated with a laser beam. Hence, the output intensity distribution can be adjusted by selecting a proper pupil function and the size of an output pupil defined by the input pupil size and the optical system magnification factor. The real-time character recognition by optical parallel high-speed processing for two dimensional images with position normalization is demonstrated.     

Systematic analysis of the invariance properties of plane holograms

A general equation of condition is derived for holographic setups for which recording and reconstruction are invariant with respect to given translations or rotations of the hologram. The solutions are stated and discussed. Using an astigmatic reference wave, there is a setup which combines invariances in two degrees of freedom of translation with two of rotation. This is of particular interest for the holographic recording of data on a moving carrier.     

Diffraction behavior of an azo-dye-doped nematic liquid crystal without applied electric field

We investigate the dependence of the two-beam coupling diffraction efficiency on the experimental parameters, and hence achieve the optimum experimental conditions for holographic recording in planar Methyl Red-doped nematic liquid crystal film without external electric field. Under the optimum experimental conditions, the diffraction efficiency of a holographic grating is improved to 21.4%, and holographic image with long-term stability is stored in the film. These results provide an experimental basis for holographic data storage in this liquid crystal film.     

（Ce，Cu）：LiNbO3晶体非挥发全息记录理论研究与优化

根据双中心带输运模型，对（Ce，Cu）：LiNbO3晶体双中心非挥发全息记录进行了理论研究与优化。推导了（Ce，Cu）：LiNbO3晶体的微观参量，采用数值方法通过严格求解模拟双中心带输运方程来模拟全息记录过程。分析了记录过程中，记录与敏化光强、Ce和Cu掺杂浓度以及晶体微观参量对（Ce，Cu）：LiNbO3晶体双中心全息记录的影响。发现（Ce，Cu）：LiNbO3晶体非挥发全息记录中实现高衍射效率与固定效率的主导因素是深中心Cu，在记录过程中，深中心Cu建立起了很强的空间电荷场。数值模拟的结果经过实验验证，最高饱和与固定衍射效率别为60．5％和53．8％。     

Study on the simplified phase-shifting digital holographic microscopy

In this paper, a new simplified technique for effectively eliminating the zero order and the conjugate virtual image in digital holographic microcopy, which makes use of two-step phase-shifting method of just recording two holograms and an intensity image of object wave, is proposed. Meanwhile, combined with the principle of making full use of spatial bandwidth of the CCD sensor by in-line lens-less Fourier holographic recording geometry, the theory and experimental methods to increase the resolution of the reconstructed image in digital holography by using phase-shifting technique are detailedly analyzed. At end, the validity and availability of this technique has been demonstrated through the off-axis and in-line Fourier transform recording geometry. The study provides some theoretical and experimental guidance for the design and operation of a digital holographic microscopy system.     

Features of the dynamic self-action of light beams during holographic recording in photopolymers

The influence of changes in the average refractive index and thickness of a working medium layer on the holographic record of sloped transmission gratings in photopolymerizable compositions (PPCs) was studied. The parameters of holographic gratings change with increasing slope of grating lines. It is shown that shrinkage of the PPC layer is the main factor influencing the direction and rate of the energy exchange between recording beams and medium during the holographic recording in PPCs.     

An approach for sensing marine plankton using digital holographic imaging

The goal of this paper is to use digital holographic imaging for sensing marine plankton in recording sampling volume. The process stage of this approach includes: wavefront recording using in-line holographic recording set up and numerical reconstruction using Fresnel approximation and convolution algorithm. So, by capturing hologram of marine plankton and reconstructing hologram, the recorded optical field of marine plankton is retrieved. Digital holographic imaging is an extremely powerful technique for the study of marine plankton fields as it allows instantaneous, noninvasive, high-resolution recording of substantial volumes. Finally, this paper presents that it is possible for digital holographic imaging system to sense marine plankton according to laboratory results.     

Comparison of two-color holography among different doped LiNbO3 crystals at low intensities

The effects of material and experimental parameters on the nonvolatile two-color holographic recording space charge field and sensitivity for different doped LiNbO₃:Fe crystals have been studied theoretically based on a two-center model. When the direct electron transfer between the deep-trap centers and the shallow-trap centers was considered, the near-stoichiometric LiNbO₃:Fe is confirmed theoretically to be of bigger space charge field and higher recording sensitivity than the LiNbO₃:Fe:Mn and LiNbO₃:Cu:Ce in the low intensity region. A further improvement of the recording sensitivity can be achieved by doping concentration, thermal reduction treatment of Fe, appropriate gating and recording wavelengths with large photo-excitation cross sections.     

Reconstruction resolution enhancement of EPISM based holographic stereogram with hogel spatial multiplexing

We investigate how the splicing mode of a holographic element (hogel) affects the reconstruction of a 3D scene to improve the reconstruction resolution of a holographic stereogram fabricated using the effective perspective image segmentation and mosaicking method (EPISM). First, the effect of hogel spatial multiplexing on holographic recording and reconstruction is studied based on the mechanism of recording interference fringes in the holographic recording medium. Second, combined with the influence of multiple exposures on the hologram's diffraction efficiency, the diffraction efficiency of the holographic stereogram is analyzed in the spatial multiplexing mode. The holographic stereogram is then regarded as a special optical imaging system. The theory of spatial bandwidth product is adopted to describe the comprehensive resolution of the holographic stereogram, which explains why hogel spatial multiplexing can significantly improve the reconstruction resolution of a holographic stereogram. Compared with the traditional printing method under the same parameters in optical experiments, hogel spatial multiplexing has a lower diffraction efficiency but a higher quality of reconstructed image, consistent with the theoretical analysis.     

杂环偶氮染料薄膜的全息光存储

采用含杂原子的偶氮染料TADEB、DMTAM、DMTAA和DMTAMB掺杂高分子聚合物PMMA制成薄膜,利用四波混频全息光路系统,研究了它们的全息光存储性质。实验结果显示DMTAA样品的衍射效率最高,可达到22%,是一种存储性能较为优良的永久性全息光存储介质材料。     

Holographic grating recorded by He–Ne laser operating at 632.8 nm in polymer film containing push–pull azo dye

Holographic gratings were optically recorded by two beams from a He–Ne laser operating at 632.8 nm in poly (methyl methacrylate) (PMMA) film containing push–pull azo dye. The holographic characteristics of the recorded gratings were dependent on the polarization direction of the recording beams and the relationship between the diffractive signals and the power density of recording beams was investigated. The formation of holographic gratings accompanied by 532 nm (double-frequency of Nd:YAG laser) irradiation was studied in detail. It was found that the effect of 532 nm laser to the holographic grating lay in two aspects. The acceleration effect of 532 nm laser to the formation of holographic grating is predominant when the power density of 532 nm laser is low. While at relatively high power density of 532 nm laser, the erasure is the main factor to the holographic grating. Moreover, the holographic grating was probed by 532 nm laser at low power density and the dependence of the first order diffractive signal on the recording beam power density was also presented.     

LiNbO3:Cu:Ce晶体非挥发全息存储性能的理论研究

以双中心模型为基础, 理论研究了LiNbO₃:Cu:Ce晶体在稳态情况下的非挥发双光双步全息存储性能. 研究中考虑了在晶体深能级中心Cu⁺/Cu²⁺ 与浅能级中心Ce³⁺/Ce⁴⁺ 之间由隧穿效应引起的电荷直接交换过程. 结果表明, 总的空间电荷场大小主要由深能级上的空间电荷场所决定, 并且非挥发全息存储性能主要由隧穿效应引起的深能级中心Cu⁺/Cu²⁺ 与浅能级中心Ce³⁺/Ce⁴⁺ 之间的电荷直接交换过程所决定. 与隧穿效应相关的材料参数对于非挥发双光双步全息存储的性能起到了至关重要的作用.     

Reflection mode holographic recording in methylene blue-sensitized polyvinyl alcohol acrylamide films

Photopolymer systems can produce good image quality holograms that does not require any post-processing and are environmentally stable with good diffraction efficiency. The present work reports the development of a methylene blue-sensitized polyvinyl alcohol acrylamide (MBPVA/AA) photopolymer system for recording white light reflection holograms. Reflection gratings were recorded in the photopolymer films with different concentrations of methylene blue (MB). Various parameters affecting the holographic properties of the samples were also studied. The holographic performance of the material is found to depend on its chemical composition and the recording parameters.     

Simultaneous occurrence of beam deflection, holographic recording, and self-interference in one lithium niobate crystal

Coupling of the acousto-optic effect and the photorefractive effect in a magnesium-doped lithium niobate crystal was studied by a holographic recording technique. The process of self-interference that is due to the coupling of these two effects was observed. Our results demonstrate that beam splitting and deflection, holographic recording, and self-interference can occur simultaneously in one crystal.