Sparse-wavelength angle-multiplexed volume holographic memory system: analysis and advances

We present and analyze our hybrid wavelength-and-angle-multiplexed volume holographic memory system. The hybridization of wavelength and angle multiplexing relaxes demands on spectral-tuning sources, angle-tuning devices, and optical system numerical apertures while maintaining a large K-space addressability. We consider realistic properties of our volume holographic memory system, addressing practical issues such as storage density and material-dependent photon-limited information throughput. Finally, we present experimental results of the storage of 2000 sparse-wavelength angle-multiplexed volume holograms in a 1.86-cm3 volume of lithium niobate.     

Inorganic materials for optical data storage

Recyclable holographic (optical) storage in inorganic materials is nowadays possible due to the advent of laser. Various performance parameters of the state-of-the-art of optical storage are discussed in detail with reference to the well-established case of ferroelectric lithium niobate (LiNbO₃). Various physicochemical techniques are employed in understanding the microscopic mechanisms responsible for optical storage in LiNbO₃. A short summary of other inorganic materials capable of holographic storage is also presented.     

White Light Phase-conjugate Lensless Image Projection

Abstract

The lensless projection in white light of images with ～ 30 µ;m resolution has been demonstrated using phase-conjugate reflections from a self-pumped BaTiO₃ crystal which was simultaneously illuminated with red, green and blue laser light. The relative beam incident intensities were crucial in reducing holographic photoerasure and cross-talk effects.     

Intensity-dependent beam-fanning in high-gain photorefractive barium titanate crystal

Abstract

A phenomenological multi-wave mixing model has been used along with a more general solution of Kukhtarev's model to explain intensity-dependent beam-fanning in photorefractive (PR) BaTiO₃ crystal. The variation of trap density owing to the photoexcitation of charge carriers has been considered to explain the intensity-dependent effect. Theoretical results predicted by the model show a confirmation with experimental results obtained from PR crystal, and also show some of the controlling mechanisms that enable use of beam-fanning for some potential applications.     

Cross-talk Noise and Storage Capacity of Holographic Memories with a LiNbO(3) Crystal in the Open-Circuit Condition

A solution to the Kukhtarev equations is obtained for a typical holographic memory system in which multiplexed holograms, including the effects owing to a nonuniform beam profile in the focal regions, are used. The various noise mechanisms and storage capacity are analyzed on the basis of this solution. The cross-talk noise of a typical 4f holographic memory configuration with defocus is compared with that of a phase mask. It is shown that the memory capacity and the signal-to-noise can be significantly improved by design of an optimal phase mask. The experimental results with defocus and an eight-level phase mask are presented.     

Temperature dependence of F → X reaction based holographic recording in electrolytically coloured KBr crystals

Abstract

The temperature dependence of the F → X reaction based holographic recording of the electrolytically coloured KBr crystals is studied. It was observed that the maximum diffraction efficiency of the volume gratings varied with the recording temperature in the range 160–220°C. The optimal temperature was found to depend on the initial optical density of the F-centres. It decreased with increasing optical density.     

超高密度光存储技术的现状和今后的发展

文章综述了光存储领域的研究进展 ,主要包括体全息存储、近场光学存储和双光子双稳态存储技术 .在介绍各种存储技术发展现状的同时 ,分析了各自的优势和存在的问题 .从整个光存储学科发展的角度给出了未来的趋势 .     

Incoherent-to-coherent Conversion Via Photorefractive Fan-out and Two-wave Mixing

Abstract

We report a novel scheme for an incoherent-to-coherent converter by means of the selective erasure of the random dynamic gratings created by the beam-fanning process in BaTiO₃. Furthermore, the spatial resolution properties of a two-wave mixing incoherent-to-coherent converter are discussed in the geometrical limit, and using the two-dimensional two-wave mixing theory.     

Maximum information capacity of a three-dimensional hologram

This Letter is devoted to the problem of the maximum information capacity of a three-dimensional hologram with optimal utilization of the dynamic range of the storage medium. A hologram is treated as an object of information theory. Diffraction-limited holographic writing is analyzed on the basis of the reciprocal-lattice formalism. The calculation of the information capacity of the three-dimensional hologram is reduced to analysis of a set of multiplexed holograms each of which possesses a finite signal/noise ratio determined by the dynamic range of the holographic medium. The optimal number of pages which give the maximum information capacity with angular multiplexing is found. Pis’ma Zh. Tekh. Fiz. 23, 37–43 (September 26, 1997)     

Interferometric Measurement of Holographic Lenslets

Abstract

A holographic optical element can either be characterized by the wave aberrations or by the point spread function or alternatively by the modulation transfer function. Here we propose the measurement of the wave aberrations with the help of a Twyman-Green interferometer adapted to the requirements for testing holographic optical elements. The evaluation of the interferograms is done with the phase-shifting technique. The resulting wave aberrations are expressed by Zernike coefficients. In addition to this evaluation, the point spread function and the modulation transfer function are calculated from the wave aberrations. The set-up, the evaluation method and also exemplary results of a tested holographic optical element are presented.     

Interleaving and error correction in volume holographic memory systems

We study the use of error-correction coding (ECC) and two-dimensional interleaving for volume holographic memory (VHM) systems suffering from both random and systematic errors. The bit-error rate (BER) is used as the data-fidelity measure and as a design metric for optical 4f systems. The correlated error patterns arising from both lens aberrations and misalignment errors are analyzed, and we discuss the information theoretic storage capacity of VHM in the presence of such correlated error patterns. The performance of interleaving and ECC is analyzed from both BER and storage-capacity perspectives. Magnification, rotation, tilt, and defocus errors are also studied, and an experimental demonstration that combines ECC with two-dimensional interleaving is included.     

吲哚俘精酸酐实现三值偏振全息光学存储器

针对三值光计算机需要直接存储线偏振光束的问题,利用光致各向异性材料吲哚俘精酸酐对偏振态敏感的特性,提出了一种基于吲哚俘精酸酐/PMMA 薄膜的三值偏振全息数字存储方法,并建立了相应的光学存储器模型.该存储器系统采用He-Ne 激光器为记录和读出光源,以双层液晶和偏振器为核心的编码器作为三值数据的输入部件,以双CCD 为核心的解码器作为数据输出器件,采用傅里叶变换全息记录的方法,在吲哚俘精酸酐/PMMA薄膜上实现三值数字光学存储.该存储器系统可望实现直接并行存储用光束的正交线偏振态和无光态表示的三值数字信息,以及以页面为单位的并行寻址和读写操作.     

Holographic memory based on the angular speckle-selectivity of volume holograms

The possibility of information storage using the angular speckle-selectivity of volume holograms is analyzed. It is demonstrated that such holograms can be used to construct high-capacity memory elements. Pis’ma Zh. Tekh. Fiz. 24, 88–93 (April 12, 1998)     

Holographic data storage using an optimized phase mask

Abstract

A method for reducing the reconstruction error and improving the spectral distribution in holographic data storage has been proposed. An optimized phase mask is introduced into the optical system. By using the phase mask, the reconstruction error and uniformity of the spectral distribution can be optimized. The relative importance between reconstruction error and uniformity can be emphasized with an adjustable weight factor. The phase mask can be designed by using amplitude-phase retrieval algorithms. Simulation results have shown the merits of the proposed method. The feasibility of optical realization is investigated briefly.     

Optimal composition of an acrylamide and N,N′-methylenebisacrylamide holographic recording material

Abstract

We study the effect of the addition of a cross-linking agent (N,N′-methylenebisacrylamide) to a photopolymerizable matrix for real-time holography. Optimization of the concentration of this component has been achieved, paying attention to holographic parameters such as energy sensitivity and diffraction efficiency. Diffraction efficiencies of around 88% have been obtained with energy exposures of 12 mJ cm^?2. At the same time considering a nonlinear response of the material we have carried out a theoretical study of the experimental results. We observed a nonlinear response of the material with regard to the storage intensity. This is a very interesting point in reflection holographic optical elements.     

Information theoretic limits to the capacity of volume holographic optical memory

We derive the information theoretic limit to storage capacity in volume holographic optical memories for the limiting cases of dominant intensity noise (Gaussian noise) and dominant field noise (Rician noise). These capacity bounds are compared with the performance achievable using simple Reed-Solomon error-correcting codes.     

Optically rewritable Bragg gratings in Mn2+ doped RbCdF3

Optical absorption, optically stimulated luminescence, and thermo-luminescence measurements have been made on RbCdF₃:Mn²⁺ single crystals with the aim of characterizing this compound for holographic storage. We show that there are stable traps that are filled after deep ultra-violet irradiation (<270 nm) and they can be emptied after ∼365 nm irradiation. The photo-induced changes in the optical properties can be used to create optically rewritable Bragg gratings in these crystals.     

Simultaneous determination of the refractive index and wedge angle of an optical wedge plate using a photorefractive holographic interferometer

Abstract

A simple and effective method for simultaneously determining the refractive index and the wedge angle of an optical wedge plate is described. The method is based on a real-time holographic interferometer which uses a photorefractive crystal as the recording and reconstruction medium. The wedge sample under test is inserted into a rectangular cell that is placed in the object light beam of the holographic interferometer. The interference patterns produced before and after a reference liquid is poured into the cell are received by a CCD camera and stored in a computer, respectively. The refractive index and the wedge angle of the wedge sample are determined by measuring the number of fringes falling inside a fixed aperture. The principle of the method is analysed and some experimental results with adequate accuracy are given.     

Immersion holography based on amorphous chalcogenide films

A solid immersion holographic method for the recording of refractive index and surface-relief modulated gratings with a period of 0.1–1 μm in amorphous films of chalcogenide semiconductors As₂S₃ and As–S–Se has been developed and studied. The laser immersion interference lithography can be used as a low-cost method for the exposure of large surfaces with regular patterns like subwavelength-gratings and microsieves. The polarization sensitive properties of the subwavelength refractive-index modulated transmission gratings were studied. The possibility to use the amorphous chalcogenide films as a media for holographic recording and storage of information with high density is discussed.     

Recording of holographic diffraction gratings in photopolymers: Theoretical modelling and real-time monitoring of grating growth

Abstract

Recording of holographic volume diffraction gratings in Du Pont's photopolymer HRS-150 is studied theoretically and experimentally. Particular attention is paid to the dynamics of the recording process. The temporal evolution of holographic gratings is monitored for a range of illumination intensities in two ways: (i) by means of a multiple-exposure approach when intensity profiles of recording beams are uniform, many holographic exposures are necessary and each exposure is carried out for a particular value of the total illumination intensity; (ii) by means of a single-exposure approach when intensity profiles of recording beams are strongly Gaussian and a volume grating with spatially distributed diffraction efficiency arises, depending on a local (average) value of the total illumination intensity. The second approach proves to be a useful tool providing us very quickly with qualitative information about the dynamics of the recording process while the first one, which is much more time-consuming, is more reliable for quantitative evaluations. The grating evolution is analysed theoretically by means of the well known photopolymerization-diffusion model. Discrepancies are found between the model and experimental results for low illumination intensities while qualitative agreement is found for higher intensities. For a given grating period, the discrepancies are interpreted in terms of insufficient change of the refractive index due to photopolymerization for low illumination intensities.