Digital Data Pages Research Articles

Readout fidelity of coaxial holographic digital data page recording in nanoparticle–(thiol–ene) polymer composites

We report on an experimental investigation of nanoparticle-concentration and thiol-to-ene stoichiometric ratio dependences of symbol error rates (SERs) and signal-to-noise ratios (SNRs) of digital data pages recorded at a wavelength of 532 nm in thiol–ene based nanoparticle–polymer composite (NPC) films by using a coaxial holographic digital data storage method. We show that SERs and SNRs at the optimized material condition can be lower than 1 × 10−4 and higher than 10, respectively, without error correction coding. These results show the usefulness of thiol–ene based NPCs as coaxial holographic data storage media.

Holographic nanoparticle-polymer composites based on radical-mediated thiol-yne photopolymerizations: characterization and shift-multiplexed holographic digital data page storage

We investigate volume holographic recording in a photopolymerizable nanoparticle-polymer (NPC) composite film that employs radical-mediated thiol-yne step-growth photopolymerizations. Because each alkyne functional group can react consecutively with two thiol functional groups in thiol-yne photopolymerizatins, the thiol-yne based NPC system dispersed with inorganic nanoparticles has the potentiality to overcome the drawback of low crosslinking densities but to retain the advantage of low shrinkage that is possible by use of thiol-ene photopolymerizations. We show that a thiol-yne based NPC film dispersed with 25 vol.% SiO2 nanoparticles and 15 wt.% single functional co-monomer gives the saturated refractive index change as large as 0.008 and the material recording sensitivity as high as 2005 cm/J at a recording and readout wavelength of 532 nm. We find that while the shrinkage of a volume hologram recorded in a thiol-yne based NPC dispersed with organic nanopartices can be as low as 0.5%, it is approximately 1% with the dispersion of SiO2 nanoparticles due to the plasticizing effect of the doped co-monomer. On the other hand, the thermal stability is improved better with the dispersion of SiO2 nanoparticles. We also demonstrate shift-multiplexed holographic storage of 80 digital data pages in a thiol-yne based NPC film with high readout fidelity.

Effects of symbol modulation coding on readout fidelity of shift-multiplexed holographic digital data page storage in a photopolymerizable nanoparticle-(thiol-ene)polymer composite film.

We report on shift-multiplexed holographic storage of 250 digital data pages in a photopolymerizable SiO₂ nanoparticle-polymer composite film being capable of step-growth thiol-ene polymerization in the green. Various two-dimensional symbol modulation codes for the digital data page format were employed to examine the dependence of the readout fidelity on modulation coding schemes. It is found that, as compared to 1:2 and 2:4 modulation codes, higher-order 5:9, 9:16, and 13:25 modulation codes possessing reduced white rates and higher coding efficiencies give lower symbol-error rates of ~1×10⁻³ and higher signal-to-noise ratios (>4).

Resisting shrinkage properties of volume holograms recorded in TiO2 nanoparticle-dispersed acrylamide-based photopolymer

A novel organic—inorganic nanoparticle—photopolymer composite system is developed, and its fundamental holographic recording characteristics are studied. In this hydrophilic TiO2-nanoparticle-dispersed acrylamide photopolymer composite system, the counter-diffusion of monomers and nanoparticles plays a fundamental and key role in hologram grating formation. The experimental results indicate that the volume shrinkage of the nanoparticle—photopolymer film during the holographic recording can be drastically reduced compared with the undoped photopolymer film. It is also found that the diffraction efficiency of the grating recorded in the nanoparticle—photopolymer film depends strongly on the concentration of the TiO2-nanoparticles, and there exists an optimal TiO2-nanoparticle-doping concentration to make the diffraction efficiency and the refractive index modulation reach their maxima. Additionally, the digital data page is stored and reconstructed in the nanoparticle—photopolymer film.

Shift-multiplexed holographic digital data page storage in a nanoparticle-(thiol–ene) polymer composite film

We demonstrate shift-multiplexed holographic storage of 180 digital data pages with low symbol-error rates in a thick (250 μm) SiO2 nanoparticle-polymer composite film using step-growth thiol-ene photopolymerization. A two-dimensional 2:4 modulation code was employed for formatting digital data pages in order to reduce the average intensity of code block without decreasing the coding efficiency. This study clearly shows the feasibility of the thiol-ene based nanoparticle-polymer composite system as a holographic data storage medium.

Content-addressable holographic data storage system for invariant pattern recognition of gray-scale images

Conventionally a holographic data storage system uses binary digital data as the input pages. We propose and demonstrate the use of a holographic data storage system for the purpose of invariant pattern recognition of gray-scale images. To improve the correlation accuracy for gray-scale images, we present a coding technique, phase Fourier transform (phase-FT) coding, to code a gray-scale image into a random and balanced digital binary image. In addition to the fact that a digital data page is obtained for incorporation into a holographic data storage system, this phase-FT coded image produces dc-free homogenized Fourier spectrum. This coded image can also be treated as an image for further processing, such as synthesis of distortion-invariant filters for invariant pattern recognition. A space-domain synthetic discriminant function (SDF) filter has been synthesized using these phase-FT coded images for rotation-invariant pattern recognition. Both simulation and experimental results are presented. The results show good correlation accuracy in comparison to correlation results obtained for SDF filter synthesized using the original gray-scale images themselves.

Overloaded phase-code multiplexing for volume holographic storage

Overloaded phase codes for volume holographic data storage are introduced. In contrast to any previous phase-code design, overloaded phase codes enable multiplexing of a number of data pages that exceeds the number of utilized reference beams. In this way the achievable data capacity can be augmented. Overloaded codes are generated by extending multilevel phase codes based on the discrete Fourier transform. We demonstrate multiplexing of 70 analog pages by means of 64 reference beams. The analysis of reconstructed digital data pages suggests that a capacity gain of up to 15% is reasonable.

An input-data page modulation scheme for content-addressable holographic digital data storage

We implement a new modulation scheme for inputting digital data pages for content-addressable holographic data storage. This scheme, which is a modification of the hybrid ternary modulation (HTM) scheme, exploits the advantages of both pure phase modulation and hybrid ternary modulation. The technique of using binary phase data pages with equal number of 0 and π using twisted-nematic liquid crystal spatial light modulator reduces strong dc component and produces a more homogeneous spectral distribution at the recording plane. This technique should facilitate better recording of all spatial frequencies, and thus improve the discrimination capability of a content-addressable memory. In HTM, there are three states to represent binary bits, whereas in the proposed code, we use the two bright states with a difference of π for coding the binary data. The third state, which is a dark state, is used to code the undesired regions of the spatial light modulator (SLM) while performing a search operation using a small search argument. We also introduce a new balanced modulation code which is a modified version of the 8:12 modulation code to address the coding subtleties associated with phase-based data pages. The proposed technique enhances the discrimination capability of content search using small search arguments, thereby giving better results in associative recall in a holographic memory system, with very low number of false hits compared to the existing schemes. A comparative study is presented of discrimination ratio and SNR for the different modulation schemes.

Non-volatile volume holograms in bismuth tellurite crystals

We investigate the durability of volume holograms in bismuth tellurite crystals, Bi2TeO5. This material enables self-fixing of holograms during the recording process rendering any further fixing technique unnecessary. Therefore the material overcomes the vulnerability of holograms which is a typical inconvenient property of inorganic storage crystals. The quality reduction of reconstructed analogue and digital data pages under permanent read-out is studied. In terms of digital page-oriented storage, the change of the bit error rate is investigated. For comparison, all experiments are also performed on iron doped lithium niobate crystals, LiNbO3:Fe.

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

Digital Data Storage and Computing by a Vector Holographic Memory System

We have applied polarization multiplexing to digital data storage and logic operation. Two digital data pages with mutually orthogonal polarization directions were recorded with a reference beam within the same volume of polyester containing cyanoazobenzene units in the side chain. Illuminating the polyester with the reference beam simultaneously yielded the two data pages. The polarization states of the two retrieved images were mutually orthogonal, because the polyester film is capable of recording arbitrary polarization states. Thus, there was no significant cross talk between the two reconstructed data pages. The bit error rates of the two retrieved pages were estimated to be 10-9 and 10-6. Furthermore, by detecting an appropriate polarization component of the diffracted beams, we have successfully obtained subtraction and addition signals between the two multiplexed pages. Since this system is also applicable to logic operation, it would take advantage of the high-density storage, fast transfer rates, and fast processing speeds.

Recording media that exhibit high dynamic range for digital holographic data storage

A general strategy for fabricating thick, optically flat photopolymer recording media with high dynamic range (M/#) that exhibit low levels of recording-induced Bragg detuning for holographic data storage is presented. In particular, media with M/# values as high as 42 in 1-mm-thick formats are obtained. We believe that these results are the first demonstration of a holographic storage medium with a dynamic range of this magnitude. In addition, we report the holographic recording and recovery of high-capacity (480-kbit) digital data pages in these media, further illustrating their data-storage capabilities.

Holographic storage of multiple high-capacity digital data pages in thick photopolymer systems

Multiple digital data pages (480 kbits per page) were holographically recorded and retrieved with low bit-error rates in thick (~250- and ~500-mum) photopolymer media. The photopolymer systems were fabricated with the optical quality and low level of scatter required for digital data storage. We believe that these results represent the first demonstration of holographic storage of high-capacity digital data pages in photopolymer media with the thickness that will be required for such storage densities.

Digital holographic storage system incorporating optical fixing

Digital holograms have been written in stoichiometric Pr:LiNbO(3) in a two-color recording scheme, demonstrating what is to our knowledge the first all-optical nondestructive readout of digital data. Using writing light at 800nm and gating light at 476 nm, we stored and retrieved 256-kbit digital data pages with a raw bit-error rate BER of <10(-4) .

Holographic digital data storage in a photorefractive polymer

We report high-contrast storage of 64-kbit digital data pages in a photorefractive polymer material. Singlepage writing, reading, and erasing operations were demonstrated with a dual-function-dopant polymeric material having a dark lifetime of several days. Data were reconstructed without error by use of several simple readout algorithms.

A review of aspects relating to the improvement of holographic memory technology

Results of studying a holographic memory to write/read digital data pages are presented. The research has been carried out in Novosibirsk, Russia. Great attention was paid to methods of improving recording density and the reliability of data reading, the development of ‘dry’ photopolymers that provide recording of superimposed three-dimensional phase holograms, and the designing of parallel optic input large-scale integration (LSI) for reading and logical processing of data arriving from the holographic memory.