Deterministic Phase Encoded Holographic Data Storage Using Lenticular Lens Array

This work presents a novel optical holographic encrypted data storage approach based on a phase encoding multiplexed scheme. In the proposed data storage scheme, patterns to be encrypted are stored holographically in a photorefractive LiNbO₃:Fe crystal using a lenticular lens array (LLA) sheet phase-encoded multiplexing. Experimental results reveal that rotating an LLA placed as a phase modulator in the path of the reference beam is a simple but effective method of increasing the phase addresses for holographic memory in a crystal. Combining this rotational multiplexing with two-axis rotating multiplexing provides further data storage and data encryption capacity.     

Signal and reference wave dually encrypted holographic memory with shift multiplexing

We propose a secure holographic storage system with signal and reference waves dually encrypted for shift multiplexing. Two random phase masks are used to encrypt the images in the input and the Fourier planes. The reference beam is phase encoded by a fiber optic faceplate. The encrypted data is stored in a LiNbO₃:Fe crystal and decrypted during the readout process. We experimentally demonstrate encryption and decryption of multiple images and the results show high quality and good fault tolerance.     

Multiplexing encryption-decryption via lateral shifting of a random phase mask

We present an holographic memory optical arrangement based on the successive shifting of a random pure-phase mask to achieve encrypted images multiplexing. The input images are encrypted to a stationary white noise using the usual double random encoding in the Fresnel domain. The encrypted information is imaged in a photorefractive crystal where also a reference beam impinges. In the holographic memory, a BSO crystal is used to provide both a recording medium and a phase conjugate mirror. The combination of these two features supplies at the same time the necessary exact cancellation of the random pure-phase mask as well as allows a real-time decryption process. Successive images are encrypted and position-encoded by speckle patterns arising from the random pure-phase mask in-plane shifting between exposures. We include experimental results to corroborate the multiplexing capability and the read-out fidelity of the proposed arrangement.     

光折变晶体均匀多重全息图存储研究

本文推导了光折变晶体角度编码多重全息存储中写入均匀光栅时的曝光时间递推公式.分析了散射效应对光栅写入时间常数的影响和写入光耦合对光栅振幅的影响,给出了两种因素影响下曝光时间计算的修正递推公式.数值计算结果表明,按照这种修正公式计算所得多重存储中各幅全息光栅振幅不仅均匀性好,而且振幅相对较大,这种曝光方法有利于提高晶体的存储容量.实验中以递推公式所得时间进行曝光记录,在厚度为0.6mm的Fe:LiNbO₃晶体中采用角度编码很容易存储了30幅全息图.     

Holographic recording in Fe:Ce:Ti doped LiNbO3 crystal

High diffraction efficiency measurements in a triply doped Fe:Ce:Ti-LiNbO₃ crystal are reported. The crystal shows an improved recording sensitivity, weak light-induced scattering and extremely slow readout erasure which makes it highly desirable for non-volatile data storage applications. The suitability of the crystal was demonstrated by high quality holographic recording and retrieval of several astronomical images using shift and rotational multiplexing scheme.     

Super-parallel holographic correlator with optical fixing

Seven thousand five hundred holograms are stored in 15 locations within a bar of Zn:Mn:Fe:LiNbO₃, using a signal beam that propagates along the long axis of the bar. At each location, angular multiplexing is performed with the reference light changing its orientation in two-dimensional space. The same angular multiplexing is repeated at different locations along the long axis of the Zn:Mn:Fe:LiNbO₃ bar. When operating as a holographic correlator, an input image is compared simultaneously with the records stored in all locations within the bar and is recalled accurately.     

Reconstruction of parasitic holograms to characterize photorefractive materials

Photoinduced light scattering is a serious drawback that limits the applicability of thick holographic recording media but provides valuable information on the recording medium. As long as there is no correlation between the scattering centers in the crystal, photoinduced light scattering may be explained to result from the interference pattern of the incident beam and the field scattered from a single point-like scattering center. The hologram of this ellipsoidally scattered wave field will have practically the same structure in the reciprocal space modified by a response function which reflects the anisotropic properties of the recording medium. We studied photoinduced light scattering in LiNbO₃:Fe, a model system for photorefractive materials. The transmitted intensity in the stationary state of the scattering process is investigated as a function of the reconstruction angle at different wavelengths and polarizations of the reconstructing beam. The experimental results are analyzed by a simple phenomenological model based on the Ewald construction and can be used to choose suitable conditions at which holographic scattering can be minimized as well as to extract some physical parameters of the crystal. Received: 27 September 2000 / Revised version: 1 December 2000 / Published online: 21 February 2001     

Fast electrically switchable holographic optical elements in LiNbO3

We present here the realization of fast electrically switchable holographic optical elements based on electric field multiplexing of volume holograms in lithium niobate crystals. We demonstrate the electrical control of holographic lenses and holographic mirrors for fast switching of the focal length and the direction of the reconstructed light beam, respectively. The switching time in the range of few hundred microseconds has been demonstrated using this technique.     

Denisiuk-type reflection holography display with sillenite crystals for imaging and interferometry of small objects

A novel setup for imaging and interferometry through reflection holography with Bi₁₂TiO₂₀(BTO) sillenite photorefractive crystals is proposed. A variation of the lensless Denisiuk arrangement was developed resulting in a compact, robust and simple interferometer. A red He-Ne laser was used as light source and the holographic recording occurred by diffusion with the grating vector parallel to the crystal [0 0 1]-axis. In order to enhance the holographic image quality and reduce noise a polarizing beam splitter (PBS) was positioned at the BTO input and the crystal was tilted around the [0 0 1]-axis. This enabled the orthogonally polarized transmission and diffracted beams to be separated by the PBS, providing the holographic image only. The possibility of performing deformation and strain analysis as well as vibration measurement of small objects was demonstrated.     

Folded shift multiplexing

Shift multiplexing is a holographic recording method that uses a spherical reference wave. We extend the principle to a thin slab of holographic material that acts as a waveguide. Total internal reflection folds the reference spherical beam in one dimension. We demonstrate that the shift selectivity with the folded spherical beam is independent of the slab thickness but depends instead on the numerical aperture of the coupled spherical wave. A shift selectivity of 0.5 microm has been achieved with a 1-mm-thick LiNbO3 crystal and 50 high-definition data pages are recorded with this method.     

Holographic digital data storage using phase-modulated pixels

We propose and demonstrate the use of phase images for holographic data storage. Use of phase images as input leads to uniform diffraction efficiency of multiplexed data pages. Use of binary phase-based data pages with 0 and π phase changes produces uniform spectral distribution at the Fourier plane. This in turn facilitates better recording of higher spatial frequencies. We experimentally demonstrate a phase-based holographic data storage system using shift multiplexing in a Fe:LiNbO₃ crystal, and use it for associative retrieval. Preliminary studies indicate high discrimination capabilities of phase-based holographic data storage system over the amplitude-based system in a content-addressable memory.     

Holographic angle multiplexing combined with peristrophic multiplexing in a photorefractive LiNbO3 crystal

We have investigated angle multiplexing combined with peristrophic (rotational) multiplexing in a photorefractive LiNbO₃ crystal of cylindrical shape. The peristrophic multiplexing was achieved by rotating the recording medium while the angle multiplexing, by varying the incident angle of the reference beam. Angle multiplexing in synchronization with rotation of the sample made it possible to continuously record and retrieve holograms. In the combined multiplexing, holograms were stored at different peristrophic positions and thus their angular selectivity could be much improved in comparison to that of only angle-multiplexed holograms. The theoretical angular selectivity was derived, along with its dependence on the sample rotation. The experimentally measured selectivity was in agreement with the theoretical one. The angular selectivity of angle-multiplexed holograms was measured to be 0.03° in the absence of sample rotation and it became less than 0.0075° with a sample rotation by 5.4°.     

彩色全息显示中液晶空间光调制器位相调制偏差的矫正方法

采用三基色激光照明并结合时分复用或空间复用技术,可实现基于液晶空间光调制器(LC-SLM)的彩色全息显示.但由于在不同激光波长入射条件下LC-SLM的位相调制特性曲线不同,难以同时满足多波长入射条件下2π线性位相调制的要求,致使彩色全息再现结果受到共轭像和零级斑的干扰.针对该问题,本文通过实验测试获得LC-SLM在不同波长入射时的位相调制特性曲线,并分析了RGB激光器各分量的位相调制特性曲线非线性偏差和调制幅度偏差对多阶位相型傅里叶变换相息图再现效果的影响.根据"查表法"建立了各波长入射条件下满足2π线性位相调制的灰度映射关系,并对RGB分量相息图进行修正.通过对修正前后RGB分量相息图的数值模拟再现和光电再现实验与分析,结果表明:该方法有效地克服了LC-SLM位相调制特性曲线偏差的不利影响,从而改善了彩色全息显示中各分量相息图的再现质量.     

Photonic ratchet superlattices by optical multiplexing

We present a method based on incremental holographic multiplexing to create a refractive index ratchet distribution into a photorefractive crystal as an example for the generation principle of such complex multiperiodic lattices. The implemented technique follows a finite optical series expansion of the desired index modulation. To analyze the induced lattice, we determine the phase retardation of a probe beam at the back face of the crystal by digital holography analysis. Our result depicts a first example to optically explore the fascinating phenomena of ratchet resembling systems.     

Color holographic display based on azo-dye-doped liquid crystal (Invited Paper)

A multiplexed holographic display video has been achieved by using a passive azo-dye-doped liquid crystal （LC） cell. Holograms formed in this cell can be refreshed in the order of several milliseconds. By angular multiplexing technique, dynamically multiplexed holographic videos are realized. Moreover, the reconstructed RGB images are merged into a color image, which illustrates the possibility of a color holographic three-dimensional （3D） display by holographic multiplexing of the LC cell.     

利用菌紫质薄膜进行角度复用和偏振复用全息存储实验研究

全息光存储以其高密度、大容量、高速并行数据存取而成为光存储领域的一个重要研究方向。生物光致变色材料———菌紫质是一种新型可擦重写全息记录介质。实验证明了使用菌紫质薄膜进行角度复用和偏振复用全息存储的可行性。利用菌紫质的光致变色特性,采用90°角度复用全息存储光路,在BR-D96N薄膜样品同一位置上实现了6幅全息图存储,并分别读出了无串扰的再现像。利用菌紫质薄膜的光致各向异性进行了偏振复用全息存储,在BR-D96N薄膜样品的同一位置上存储了两幅正交偏振光记录的图像,用原参考光再现和偏振片选择,可分别读出这两幅图像。     

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.     

Holographic recording in LiNbO3 using various polarized lights

Holographic recordings in a LiNbO₃ crystal using ordinary and extraordinary light in transmission-type geometry were performed. The system parameters of storage capacity (M/#), sensitivity, and signal-to-noise ratio were measured. Although extraordinary light will induce stronger fanning light (nonlinear enhanced scattered light) in LiNbO₃, our experimental results indicate that the system using extraordinary light is superior to that using ordinary light. Our demonstrations and results can help to optimize the holographic recording in some applications of volume holographic optical element (VHOE), such as wavelength division multiplexing (WDM) filters in optical communication.     

Ce:KNSBN光折变晶体光栅衍射特性研究

研究了Ce:KNSBN光折变晶体两波耦合作用写入体光栅的衍射效率与写入光偏振态和光强比的关系, 分析了写入光偏振态造成光栅衍射效率差别的原因, 并用修正耦合波理论对实验结果进行了拟合。 实验结果为Ce:KNSBN晶体在全息记录和光学信息处理领域的应用提供了依据。     

Phase-modulated multigroup volume holographic correlator

We propose a hybrid multiplexing method for volume holographic correlators based on the combination of orthogonal random phase modulation of an object beam and two-dimensional angular multiplexing of a reference beam. This method implements storing multiple groups of library images in a single storage location and outputting multiple groups of correlation spots by switching the modulation phase masks. Three experiments are designed to test the validity of this method.