Volume Holographic Storage Research Articles

Reliability of associative recall based on data manipulations in phase encoded volume holographic storage systems

We investigate the characteristics of correlation signals accomplished by content addressing in a phase encoded volume holographic storage system under different realistic conditions. In particular, we explore two crucial cases with respect to the structure of the data. The first one deals with a scenario where only partial or defective data are available for content addressing. The second case takes similarities among the stored data sets into account, which significantly differ from their statistical correlation. For both the cases we provide, for the first time, a theoretical approach and present experimental results when employing phase-code multiplexing. Finally, we discuss the reliability of the employed methods.

High optical power dependence of the EL2 recovery in GaAs

In this work, a new way of achieving the recovery from the EL2 metastable state is reported and analyzed theoretically. Despite being an old problem, no definitive picture of the EL2 center has been established to date. For this reason, long past the days of effervescent research on the EL2, new models and investigations keep appearing in the literature as, for example, the recently proposed autocatalytic model to describe the inter-defect correlation during the thermal recovery process [A. Fukuyama, T. Ikari, Y. Akashi, M. Suemitsu, Phys. Rev. B 67 (2003) 113202]. In the course of a re-evaluation of the EL2 for nanosecond volume holographic storage, we found that a strong laser pulse is capable of destroying the metastable state and decided to investigate further this effect. The experiment reported here consists of monitoring the transmission of a λ=1.05μm continuous-wave (CW) laser, used to populate the metastable state, while subjecting the sample to the incidence of a strong λ=1.06μm laser pulse. A full simulation of the problem has been carried out and the results could be fit very well by assuming a recovery induced by electron–hole recombination and a nonlinear free-carrier production mechanism. It is perhaps worth noting that such a fast recovery induced by the nanosecond laser may prove to be an interesting tool to initiate a recovery process (even at low temperature) in a controlled way to check the predictions of the recently proposed autocatalytic recovery process.

Analysis of volume holographic storage allowing large-angle illumination

Advanced technological developments have stimulated renewed interest in volume holography for applications such as information storage and wavelength multiplexing for communications and laser beam shaping. In these and many other applications, the information-carrying wave fronts usually possess narrow spatial-frequency bands, although they may propagate at large angles with respect to each other or a preferred optical axis. Conventional analytic methods are not capable of properly analyzing the optical architectures involved. For mitigation of the analytic difficulties, a novel approximation is introduced to treat narrow spatial-frequency band wave fronts propagating at large angles. This approximation is incorporated into the analysis of volume holography based on a plane-wave decomposition and Fourier analysis. As a result of the analysis, the recently introduced generalized Bragg selectivity is rederived for this more general case and is shown to provide enhanced performance for the above indicated applications. The power of the new theoretical description is demonstrated with the help of specific examples and computer simulations. The simulations reveal some interesting effects, such as coherent motion blur, that were predicted in an earlier publication.

Multiplexed holographic memory by use of fiber bundle referencing

We propose a volume holographic storage technique, in which a fiber bundle is used to guide the reference beam. Multiplexing is implemented by changing the incident direction of the laser beam upon the fiber bundle in the reference arm. In the technique, we make the system more compact by using a wedge prism to change the direction of the laser beam. This method has a large accessible angular scanning range and a large geometric storage bandwidth. Multiple images are stored in a LiNbO3:Fe crystal with an angular separation of 0.05° between successive holograms using a wedge prism with a deviation angle of 8°. This method is useful as a new image storage technique because of its compactness and simplicity.

Miniaturized volume holographic optical data storage and correlation system with a storage density of 10 Gb/cm3

The general idea of holographic optical data storage (HODS) is briefly introduced. Based on the recent advances of HODS, the key techniques and the challenges of HODS are discussed. Some new techniques are proposed to improve the system. A miniaturized volume holographic data storage and correlation system is presented. It can achieve a density of 10 Gb/cm3 and a fast correlation recognition rate of more than 2000 images per second. It shows the attracting potential advantages over other conventional storage methods in the information storage as well as information processing.

Volume holographic storage and animation based on addressing with an elastomer phase mask

A novel technique for phase-multiplexed holographic storage is reported. Phase codes for recording individual images are generated with an elastomer mask fabricated from silicone. The mask is imprinted with a random phase distribution by using a mold with a nonuniform surface profile. By displacing the elastomer from equilibrium, the optical path length traversed by each point on the reference beam wavefront can be altered, creating a unique phase address for a particular amount of strain. Images are stored in a photorefractive crystal at 0.21 mm (0.22% strain) intervals. Phase selectivity of different masks is variable, with a minimum strain sensitivity for sufficient phase mismatch at 0.06%. Prestrain of the elastomer at 7.1% leads to lower phase selectivity at small strain intervals. Holographic animation at 8–25 frames per second is demonstrated for two sequences at recording intervals of 0.105 and 0.21 mm displacements.

Design of Dye Concentrations in Azobenzene-Containing Polymer films for Volume Holographic Storage

Diffraction efficiencies of holograms in thick azo polymer films were improved by designing optical densities of the films. A nonphotosensitive polymer was used as a binder for controlling the dye concentrations. We fabricated 250-µm-thick films by changing the concentrations. While the optically induced birefringence decreased with decreasing concentration, reduction of absorption loss of the reading beam increased the diffraction efficiency. Thus, owing to the competition between the decrease in birefringence and the increase in efficiency, there exist appropriate concentrations for improving both sensitivity and magnitude of the diffraction efficiency. Consequently, the diffraction efficiency reached 25% without degradation of the sensitivity. We also demonstrated holographic multiplexing using the film. A set of 20 data pages (66 kbits per page) was successfully multiplexed and retrieved with a low bit-error rate of 1.3×10-3. The use of an error correction coding would allow error-free recovery of the data pages.

Polytopic multiplexing.

Polytopic multiplexing is a new method of overlapping holograms that, when combined with other multiplexing techniques, can increase the capacity of a volume holographic data storage system by more than a factor of 10. This is because the method makes possible the effective utilization of thick media. An experimental demonstration of this technique is also presented.

Experimental study on polarization holographic storage in genetic mutant BR-D96N film

Recording with both parallel and orthogonal linearly polarized lights, polarization holographic storage in genetic mutant BR-D96N film is reported with both transmission type geometry and reflection type geometry. Polarization properties of diffraction light and scattering light are discussed for two different cases, parallel polarization recording and orthogonal polarization recording. It shows that, compared with recording with parallel polarization lights, orthogonal polarization holography can separate the diffraction light from the scattering noise, therefore improving the signal-to-noise ratio. It also shows that, compared with reconstruction with reference light, reconstruction with phase conjugated wave of the reference light can improve the signal-to-noise ratio of the reconstructed diffraction image, and also the wave-front aberration of the object light introduced by irregular phase object in the optical pass-way can also be corrected effectively, which ensures that the reconstructed diffraction image has a better fidelity. The preliminary angle-multiplexed volume holographic storage multiplexed by transmission type geometry and reflection type geometry is demonstrated in the BR-D96N film. Experiment shows that there is no cross-talk between the two pages of images except for some scattering noises.

Discrete magnitude-squared channel modeling, equalization, and detection for volume holographic storage channels

As storage density increases, the performance of volume holographic storage channels is degraded, because intersymbol interference and noise also increase. Equalization and detection methods must be employed to mitigate the effects of intersignal interference and noise. However, the output detector array in a holographic storage system detects the intensity of the incident light's wave front, leading to loss of sign information. This sign loss precludes the applicability of conventional equalization and detection schemes. We first address channel modeling under quadratic nonlinearity and develop an efficient model named the discrete magnitude-squared channel model. We next introduce an advanced equalization method called the iterative magnitude-squared decision feedback equalization (IMSDFE), which takes the channel nonlinearity into account. The performance of IMSDFE is quantified for optical-noise-dominated channels as well as for electronic-noise-dominated channels. Results indicate that IMSDFE is a good candidate for a high-density, high-intersignal-interference volume holographic storage channel.

Improvement of photorefractive properties and holographic applications of lithium niobate crystal

Based on analyzing and synthesizing the effects of In doping, Fe doping and oxidation state, we propose an efficient way to improve photorefractive properties of LiNbO3 crystal. According to the proposed way, a sample of In:Fe:LiNbO3 crystal is grown and achieves larger dynamic range, higher sensitivity and better signal-to-noise ratio thanFe:LiNbO3 crystal (Fe:0.03wt.%) that is generally considered as a preferable storage medium for high-density holographic data storage. In a coherent volume 0.074cm3 of this crystal, 2030 holograms have been successfully multiplexed and exactly identified in a compact volume holographic data storage and correlation recognition system.

Miniaturized volume holographic optical data storage and correlation system with a storage density of 10 Gb/cm3

Miniaturized volume holographic optical data storage and correlation system with a storage density of 10 Gb/cm3

Exposure-schedule study of uniform diffraction efficiency for DSSM holographic storage

An exposure-schedule theory of uniform diffraction efficiency for a Dynamic-Static speckle multiplexing (DSSM) volume holographic storage system is proposed. The overlap-factor (? overlap) is introduced into the system to compensate for the erasure effect of the static speckle multiplexing scheme. The exposure-schedule which is an inverse recursion formula is determined. Experimental results are obtained in a LiNbO(3):Fe crystal and 400 holograms with uniform diffraction efficiency are achieved by the use of the new exposure-schedule.

Study of self-erasing of recording process in Zn:Fe:LiNbO3 with the double-carrier four-trap model

Photorefractive properties of Zn:Fe:LiNbO3 crystals were compared with Fe:LiNbO3 crystals. We described the photoconduction and diffraction efficiency as a function of incident light intensity. The self-erasing of recording process in the Zn:Fe:LiNbO3 crystal is observed. We found that the diffraction efficiency of the crystal increased to a saturation value and then decreased as recording time increases. The phenomena were explained by the double-carrier four-trap model. A suit able exposure schedule is chosen to realize the angle-multiplexed volume holographic storage. We have stored 30 images in the same volume of the crystal.

Send a hologram - photorefractive materials and devices are becoming viable alternatives for information systems

As computers and the Internet become faster and faster, more and more information is transmitted, received, processed, and stored everyday. The demand for high-speed, large-capacity information systems is pushing scientists and engineers to explore all possible approaches, including electrical and optical means. Photorefractive materials and devices are becoming viable alternatives for information systems. Photorefractive materials, including traditional electrooptic photorefractive crystals as well as photopolymers and photosensitive glasses, have demonstrated their potential in information systems. In this article we describe several applications of various photorefractive materials in information storage, processing, and communication systems. Specifically, we briefly discuss the applications of the traditional electrooptic photorefractive crystals and photopolymers in volume holographic data storage (VHDS) and information processing. Then, we discuss our recent works on the applications of photopolymers, holographic polymer dispersed liquid crystals (H-PDLC), and photosensitive glasses in photonic devices for optical fiber communications.

Diarylethene Materials for Rewritable Volume Holographic DataStorage

The photochromic diarylethene,1,2-bis(2-methyl-5-(4-formylphenyl)-thien-3-yl)perfluorocyclopentene(1a) is studied and its applicable potential in rewritable volumeholographic data storage is verified. Holographic recording films of10-µm thickness have been fabricated. The refractive index modulation(Δn = 1.15×10-3) between the open- and close-ring formsis detected to be large enough so that the films are suitable for theproduction of volume holographic storage. The experiments of anglemultiplexing and rewriting holograms show that the materials are fitfor volume holographic data storage.

Insensitivity of Speckle Multiplexing to Multi-longitudinalModes of Laser in Volume Holographic Storage

If a diode pumped solid state laser is used in a holographicstorage system, its multi longitudinal modes may damage the angularselectivity of the hologram and introduce more cross talk in thesystem. By theoretical analysis, we found that with adopting thespeckle multiplexing scheme, holographic systems are no longersensitive to the multi longitudinal modes of the laser source, andconsequently the damage described above could be well suppressed.Moreover, the following high density storage experimental results alsoexpress strong advocacy of this conclusion. This result may greatlyprompt the miniaturization of a holographic storage system.

Experimental characterization of phenanthrenequinone-doped poly(methyl methacrylate) photopolymer for volume holographic storage

We present an experimental study on the photopolymer poly- (methyl methacrylate) doped with phenathrenequinone molecules. Mate- rial characteristics for holographic data storage, including optical charac- teristics, scattering effect, relation between diffraction efficiency and number of holograms, and exposure schedule for multiple storage, are investigated. An experimental demonstration of multiple storage of digital data pages in a polymer cube is presented. © 2003 Society of Photo-Optical

Applications of photorefractive materials in information storage, processing and communication

Photorefractive materials, including traditional electro-optic photorefractive crystals as well as photopolymers and photosensitive glasses, have demonstrated their potential in information systems. In this paper, we describe several applications of various photorefractive materials in information storage, processing, and communication systems. Specifically, we briefly review the applications of the traditional electro-optic photorefractive crystals in optical information processing and volume holographic data storage, then discuss our recent works on the applications of photopolymers, holographic polymer dispersed liquid crystals, and photosensitive glasses in photonic devices for optical fiber communications.

Dynamic speckle multiplexing scheme in volume holographic data storage and its realization

Dynamic speckle multiplexing scheme in volume holographic data storage is proposed, since it offers a novel multiplexing geometry, and could be combined with other schemes to make the full use of the dynamic ranges. In this scheme, a random diffuser is added in the original reference path of the classical 90 masculine setup. In this paper, we analyzed the propagation of the speckle field in the holographic system and established the related theoretical model based on the dynamic speckle auto-correlation function and diffraction theory. We successfully realized the dynamic speckle multiplexing in our experimental system and reached a storage density of 4.6 Gigapixels/cm3 based on the DPL laser source.