Three-dimensional shift selectivity in reflection-type holographic disk memory with speckle shift recording

Three-dimensional shift selectivity of a reflection-type hologram with speckle shift recording is investigated experimentally and numerically. We build an experimental setup consisting of lenses with numerical apertures of 0.28 and an iron-doped LiNbO(3) with a thickness of 0.5 mm. The experimental results show that three-dimensional selectivity has a size of 0.97 microm x 0.97 microm x 8.8 microm in diffraction efficiency. We also develop a volume holographic memory simulator to evaluate the experimental results. The simulator can quantitatively evaluate bit error rate, signal-to-noise ratio, and diffraction efficiency. Numerical results are in good agreement with the experimental results. The experimental and numerical results indicate that three-dimensional shift multiplexing can increase the storage capacity.     

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.     

Spatioangular-multiplexing scheme for dense holographic storage

A novel multiplexing scheme for dense holographic storage in photorefractive crystals, spatioangular multiplexing, is described in detail. Compared with spatial multiplexing, spatioangular multiplexing increases the storage capacity by fully utilizing the volume of the storage medium. On the other hand, spatioangular multiplexing reduces the number of holograms overlapping any one hologram in a given volume and so increases the diffraction efficiency achievable as compared with angular multiplexing. Using this scheme, we succeeded in storing 756 high-resolution patterns in an Fe:LiNbO(3) crystal of volume 1 cm(3), with an average diffraction efficiency of 0.5%. This large database is designed for practical use in a novel associative-memory system, called a high-order feedback neural network.     

Frequency and phase swept holograms in spectral hole-burning materials

A new hologram type in spectral hole-burning systems is presented. During exposure, the frequency of narrow-band laser light is swept over a spectral range that corresponds to a few homogeneous linewidths of the spectrally selective recording material. Simultaneously the phase of the hologram is adjusted as a function of frequency-the phase sweep function. Because of the phase-reconstructing properties of holography, this recording technique programs the sample as a spectral amplitude and phase filter. We call this hologram type frequency and phase swept (FPS) holograms. Their properties and applications are summarized, and a straightforward theory is presented that describes all the diffraction phenomena observed to date. Thin FPS holograms show strongly asymmetric diffraction into conjugated diffraction orders, which is an unusual behavior for thin transmission holograms. Investigations demonstrate the advantages of FPS holograms with respect to conventional cw recording techniques in freq ncymultiplexed data storage. By choosing appropriate phase sweep functions, various features of holographic data storage can be optimized. Examples for cross-talk reduction, highest diffraction efficiency, and maximal readout stability are demonstrated. The properties of these FPS hologram types are deduced from theoretical considerations and confirmed by experiments.     

Experimental evaluation of user capacity in holographic data-storage systems

An experimental procedure for determining the relation between the number of stored holograms and the raw bit-error rate (BER) (the BER before error correction) of a holographic storage system is described. Compared with conventional recording schedules that equalize the diffraction efficiency, scheduling of recording exposures to achieve a uniform raw BER is shown to improve capacity. The experimentally obtained capacity versus the raw-BER scaling is used to study the effects of modulation and error-correction coding in holographic storage. The use of coding is shown to increase the number of holograms that can be stored; however, the redundancy associated with coding incurs a capacity cost per hologram. This trade-off is quantified, and an optimal working point for the overall system is identified. This procedure makes it possible to compare, under realistic conditions, system choices whose impact cannot be fully analyzed or simulated. Using LiNbO(3) in the 90 degrees geometry, we implement this capacity-estimation procedure and compare several block-based modulation codes and thresholding techniques on the basis of total user capacity.     

Determination of threshold exposure and intensity for recording holograms in thick green-sensitive acrylamide-based photopolymer

For optical data storage applications, it is essential to determine the lowest intensity (also known as threshold intensity) below or at which no data page or grating can be recorded in the photosensitive material, as this in turn determines the data capacity of the material. Here, experiments were carried out to determine the threshold intensity below which the formation of a simple hologram--a holographic diffraction grating in a green-sensitized acrylamide-based photopolymer--is not possible. Two main parameters of the recording layers--dye concentration and thickness--were varied to study the influence of the density of the generated free radicals on the holographic properties of these layers. It was observed that a minimum concentration per unit volume of free radicals is required for efficient cross-linking of the created polymer chains and for recording a hologram. The threshold intensity below which no hologram can be recorded in the Erythrosin B sensitized layers with absorbance less than 0.16 was 50 μW/cm(2). The real-time diffraction efficiency was analyzed in the early stage of recording. It was determined that the minimum intensity required to obtain diffraction efficiency of 1% was 90 μW/cm(2), and the minimum required exposure was 8 mJ/cm(2). It was also determined that there is an optimum dye concentration of 1.5 × 10(-7) mol/L for effective recording above which no increase in the sensitivity of the layers is observed.     

Coding of Many Phase Objects Using Computer-generated Binary Holograms

A method for coding many phase objects by means of a binary synthetic hologram is presented. The paper gives a vectorial analysis of the spectrum of multi-exposure synthetic holograms and the analysis of the storage capacity of the method. On the basis of theoretical considerations and experimental results, the errors caused by the overlapping of the first diffraction order reconstructed wavefront with beat frequencies are discussed.     

Comparison of diffraction efficiencies for single-exposure and unity-contrast multiple-exposure holograms

The diffraction efficiency for a single-exposure holographic recording is compared with that of a multiple-exposure hologram recorded with maximum signal-to-bias ratio. For the multiple-exposure recording the object is illuminated point by point, and the reference wave amplitude for each subexposure is adjusted to produce a unity-contrast interferogram in the holographic recording plane. It is shown that, for the same M-point object and the same average exposure, the ratio of the single-exposure diffraction efficiency to the mulitple-exposure diffraction efficiency is proportional to M/R, where R is the reference-to-object beam ratio in the single-exposure case. This result is compared with that obtained in the multiple-exposure case when the reference wave amplitude assumes a constant value. Application areas include optical interconnects, holographic data storage, and volumetric multiplexed holograms for display.     

Selectivity and tolerance calculations with half-cone-shaped reference beam in volume holographic storage

When developing a compact holographic storage system it is beneficial to use a reflection-type arrangement, where the entire optical system is on the same side of the storage material. For reflection type holographic discs, it is important to use half-cone-shaped spherical reference beams to avoid the ghost images caused by phase conjugate readout. The goal of this paper is to look for appropriate engineering tools to model diffraction efficiency of finite volume holograms created by half-cone-shaped reference beams. Two numerical methods – volume integral and beam propagation – were applied to calculate the shift selectivity curves. Simulation results show significant discrepancies between the shift selectivity curves corresponding to the approximated analytical equation and the numerically calculated shift selectivity curves; there are no Bragg zeros and there are no selective and nonselective directions. Beside the shift selectivity curves, track, focus, tilt and wavelength tolerance values are shown for finite volume holograms.     

Optimal recording wavelength for maximum diffraction efficiency of thermal fixing in LiNbO3:Fe

Optimal recording wavelength for maximum diffraction efficiency of thermal fixing in LiNbO(3):Fe crystal is investigated. Holographic gratings are recorded using three typical recording wavelengths including 488, 514, and 633 nm. Optimal switching from recording to thermal fixing is taken into consideration. The fixed holograms are developed by an original recording setup. Diffraction efficiencies of recording and thermal fixing are measured by a two-wave coupling technique. The theoretical and experimental results are analyzed and compared. With a blue beam, the nonvolatile hologram with maximum fixing efficiency is achieved. This work can obtain high persistent diffraction of the nonvolatile holographic storage in LiNbO(3):Fe crystals.     

Reflection-type holographic disk memory with random phase shift multiplexing

A reflection-type holographic disk memory system with random phase shift multiplexing is proposed. The experimental results show that a binary data page of 18x17 bits is recorded successfully at intervals of 4 mum in a Fe:LiNbO3 crystal with a thickness of 0.5 mm when six data pages are superimposed. Numerical results show that random phase modulation can improve the shift selectivity in shift multiplexing recording as well as in data security. Experimental and numerical results show that reflection-type holographic disk memory has a high potential for terabyte storage capacity as in transmission-type memory.     

Estimation of insertion loss of a holographic switch with an optically addressed spatial light modulator

We investigate the cause of the insertion loss of our holographic switch by determining the relation between the diffraction efficiency and the hologram patterns generated by the control beams. According to the calculation the theoretical insertion loss is 7.4 dB with loss distribution of 0.03 dB under the conditions of a previous experiment. We find that incomplete storage of the interference pattern on the optically addressed spatial light modulator is the strongest factor determining the insertion loss.     

阶梯形浮雕和折射率位相光栅的衍射性质比较

位相全息图一般既有浮雕调制又有折射率调制,它们都会对衍射特性产生影响。利用以严格电磁波理论为基础的模式匹配法（ＭＭＴ）,从理论上比较了阶梯形浮雕位相调制光栅和折射率位相调制光栅对ＴＥ波的各级衍射效率。尽管它们同一周期内的相应部分所引起的位相变化相同,但随着光栅层厚度的加大两种位相调制光栅的各级衍射效率会出现差别。这种差别与周期有关。并将矢量理论的结果与标量理论的结果进行了比较,研究结果对研究全息术     

Holographic image storage and multiple hologram storage in a planar Methyl Red-doped liquid crystal film

Holographic images are recorded and the diffraction characteristic of the hologram is investigated in a planar Methyl Red-doped liquid crystal film without an external electric field. Multiple holograms are stored in this film with no external electric field, and peristrophic multiplexing, angular multiplexing, and spatial multiplexing are demonstrated to be suited for the implementation of azo-dye doped liquid crystal films among the multiplexing techniques. The diffraction efficiency is theoretically studied for multiple storage. Optical reconstruction for peristrophic multiplexed holograms and angular multiplexed holograms is discussed, which involves the peristrophic rotation angles and the recording angles, respectively.     

Method of hologram multiplexing by use of a fiber bundle with rotary movement

A simple method of rotating an optical fiber bundle is presented to apply the random-pattern referencing scheme to hologram multiplexing. In the theoretical study, a dependence of the diffraction efficiency on the number of spatial frequencies in the reference pattern is estimated with the Bragg diffraction theory. In the experiment, hologram multiplexing is performed in which 30 holograms are recorded in a LiNbO3 crystal and are read out by rotating a fiber bundle. The result shows that this simple approach enables us to perform the hologram multiplexing and also contributes to the building of a compact optical setup.     

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)     

Chaos in Aso Dyes with Weigert's Effect

In the process of recording a hologram of linearly polarized plane waves in azo-dye-colored films, a chaotic time-dependent oscillation of the diffraction efficiency of the hologram was observed. The oscillation was detected under conditions of constant intensities of the hologram recording waves.     

Mirror hologram recording with a phase-change medium

We propose mirror hologram recording with a phase-change material, which has a large refractive-index difference between its amorphous and crystalline states. It offers excellent diffraction efficiency and is erasable and nonvolatile. We designed an optimum multilayer structure for high diffraction efficiency by simulating the effect of each film thickness on diffraction efficiency. Experiments with a germanium tellurium alloy as the phase-change material show a high diffraction efficiency close to the calculated value. This medium can also be used for directly drawing computer-generated holograms. Lee-type computer-generated holograms were drawn on this medium with optical-disk technology.     

Computer-generated microcooled reflection holograms in silicon for material processing with a CO(2) laser

We report on the design, fabrication, and testing of multilevel computer-generated reflection holograms in Si for CO(2) laser material processing for laser intensities of <2 kW/cm(2). The holograms are designed with an iterative method based on scalar diffraction theory. In this case the reconstructed intensity distribution is independent of the incident high-power laser mode. For achieving high diffraction efficiencies, multilevel staircase surface topologies are fabricated by multimask and reactive ion-etching technology on the front side of a polished Si wafer. For efficient hologram cooling, a gratinglike structure of microchannels is chemically etched on the back side of the Si wafer. Absorption and deformation measurements have been carried out on both a microcooled flat mirror and a reflection hologram. The maximum deformation amounts to 200 nm and is 10 times smaller than comparable conventional uncoated Cu mirrors. A diffraction efficiency of 88% is achieved with an eight-level reflection hologram and a 30-mm-diameter CO(2) laser beam with a power of 5 kW.