Volume Holographic Storage Research Articles

Holographic Grating Enhancement Induced by a Dual-Photo-Initiator System in PMMA Substrate Polymers.

Polymer systems induced by the reaction between monomers and photo-initiators play a crucial role in the formation of volume-phase gratings. In this paper, we fabricated a dual-photo-initiator photopolymer by doping EY (Eosin Yellow) molecules into a TI (Titanocene, Irgacure 784@BASF) dispersed PMMA (poly-[methyl methacrylate]) substrate system, with the aim of promoting the diffusion and polymerization processes in volume holographic storage. The two-wave interference system is adopted to record a permanent grating structure in our materials. The temporal diffraction variations of photopolymerization (during the interference exposure) and dark diffusion (after the interference exposure) processes have been investigated and analyzed. Aiming to analyze the influence of EY doping ratios on holographic performances, some key parameters were examined in the experiment. We first measured the temporal evolution of diffraction efficiency, then an exponential fitting was adopted to obtain the response time. Finally, the angular selectivity was evaluated by the Bragg condition after holographic recording. Also, the temporal evolution of each component is described by the nonlocal polymerization-driven diffusion model with a dual-photo-initiator composition, theoretically. Furthermore, we experimentally achieved the holographic grating enhancement in both the dark diffusion and photopolymerization processes by doping appropriate EY concentrations, respectively. This work provides a foundation for the acceptability of TI&EY/PMMA polymers in further holographic storage research.

The Interplay of Processing-Related Influences on the Formation of Volume Holographic Gratings in a Free-Surface Epoxy-Based Recording Material

Understanding the formation processes of holographic gratings in polymers as a function of material composition and processing is important for the development of new materials for holography and its associated applications. Among the processing-related factors that affect grating formation in volume holographic recording material, pre-exposure, prebaking and dark storage, as well as the associated variations in layer thickness and composition, are usually underestimated. This study highlights the influence and interaction of these factors and shows that they should not be neglected. This is of particular importance for samples with a free surface. Here, one such epoxy-based free-surface material is investigated. To determine the influence of prebaking on the holographic grating formation, as well as on the achieved refractive index contrast, angular resolved analysis of volume holographic phase gratings is applied through point-by-point scanning of the local material response. Grating characteristics are determined by comparison with simulations based on rigorous coupled wave theory. Thus, the optimal dose for prebaking can be determined, as well as the optimal exposure time, depending on the dose. The influence of dark storage on the material response is investigated over a period of 12 weeks and shows a strong dependence on the deposited energy density.

Theoretical and Experimental Investigations of Diffraction Characteristics Influenced by Holographic Reciprocity Effect in PQ/PMMA Polymers.

We propose the holographic reciprocity effect (HRE) to describe the relationship between the exposure duration (ED) and the growth rate of diffraction efficiency (GRoDE) in volume holographic storage. The HRE process is investigated experimentally and theoretically in order to avoid the diffraction attenuation. Herein, introducing the medium absorption, we present a comprehensive probabilistic model to describe the HRE. PQ/PMMA polymers are fabricated and investigated to reveal the influence of HRE on the diffraction characteristics through two recording approaches: pulsed exposure with nanosecond (ns) level and continuous wave (CW) exposure at the millisecond (ms) level. We obtain the holographic reciprocity matching (HRM) range of ED in PQ/PMMA polymers with 10-6~102 s level and improve the response time to microsecond (μs) order with no diffraction deficiency. This work can promote the application of volume holographic storage in high-speed transient information accessing technology.

Optimization of Blue Photorefractive Properties and Exponential Gain of Photorefraction in Sc-Doped Ru:Fe:LiNbO3 Crystals

Sc:Ru:Fe:LiNbO3 crystals were grown from congruent melt by using the Czochralski method. A series of LiNbO3 crystals (Li/Nb = 48.6/51.4) with 0.1 wt% RuO2, 0.06 wt% Fe2O3 and various concentrations of Sc203 were prepared. RF1 and RF4 refers to the samples containing 0 mol% Sc203 and 3 mol% Sc203, respectively. The photorefractive properties of RF4 were measured by Kr+ laser (λ = 476 nm blue light): ηs = 75.7%, τw = 11 s, M/# = 19.52, S = 2.85 cmJ−1, Γ = 31.8 cm−1 and ∆nmax = 6.66 × 10−5. The photorefractive properties of five systems (ηs, M/#, S, Γ and ∆nmax) under 476 nm wavelength from RF1 to RF4 continually increased the response time, while τw was continually shortened. Comparing the photorefractive properties of Sc (1 mol%):Ru (0.1 wt%):Fe (0.06 wt%): LiNbO3 measured by Kr+ laser (λ = 476 nm blue light) with Sc (1 mol%):Fe (0.06 wt%):LiNbO3 measured by He-Ne laser (633 nm red light), ηs increased by a factor of 1.9, Vw (response rate) increased by a factor of 13.9, M/# increased by a factor of 1.8 and S increased by a factor of 32. The ∆nmax improved by a factor of 1.4. A strong blue photorefraction was created by the two-center effect and the remarkable characteristic of being in phase between the two gratings recorded in shallow and deep trap centers. The photorefractive properties (ηS, τw, M/#, S, ∆nmax) were increased with an increase in Sc3+ ion concentration. Damage-resistant dopants such as Sc3+ ions were no longer resistant to damage, but they enhanced the photorefractive properties at the 476 nm wavelength. The experimental results clearly show that Sc-doped two-center Ru:Fe:LiNbO3 crystal is a promising candidate blue photorefraction material for volume holographic storage. Sc-doped LiNbO3 crystal can significantly enhance the blue photorefractive properties according to the experimental parameters. Therefore, the Sc:Ru:Fe:LiNbO3 crystal has better photorefractive properties than the Ru:Fe:LiNbO3 crystal.

Volume holographic storage using sync-angular multiplexing by rotating material in thick photopolymer

The sync-angular multiplexing by rotating medium is a simple but seldom used technique in recent years. While compared with the regular angular multiplexing, sync-angular multiplexing has the advantage to alleviate the cumulative scattering noise during recording in thick materials. Based on the experimental parameters, the available rotational angle range and spatial selectivity are studied by simulating the recording spot. Experimentally, 150 grey images are sync-angular multiplexed in a 2.0 mm thick PQ/PMMA photopolymer, resulting in an average diffraction efficiency of 0.05% and a storage density of 41.1 GB/cm3.

Reduction of phase error on phase-only volume-holographic disc rotation with pre-processing by phase integral.

In this paper, we present a study of observation of phase error of a volume holographic storage disc during the reading process when the disc is rotated or displaced in the theoretical calculation and the corresponding experiment. This additional phase error will dramatically decrease the bit error rate of a phase-only signal, even applying double-frequency shearing interferometry to retrieve the stored phase signal. Then we propose a novel approach to solve the problem. The stored signal is pre-processed by phase integral along the shearing direction so that applying the integral process to decode the phase signal is not necessary in the readout process. The proposed approach effectively reduces the error in phase retrieval and will be useful when applying double-frequency shearing interferometry in the readout process for volume holographic storage.

Cationic photo-initiator titanocene dispersed PMMA photopolymers for holographic memories

Bulk polymer composites have been investigated in a two-beam interference exposure system of titanocene (Irgacure 784, BASF) (TI) molecules dispersed in poly(methyl methacrylate) (PMMA) substrates, which are created through an optimized three-step thermo-polymerization method, at a wavelength of 532 nm. As a property of PMMA photopolymers, the dark enhancement of grating formation is observed, which mainly corresponds to the diffusion of TI molecules. We also examine the influences of dark enhancement on Bragg angular selectivity that shifts after a long-time dark reaction. Single grating under continuous exposure was recorded in TI/PMMA photopolymers with a diffraction efficiency of 74% and response time of 20 s. An enhanced multiplexed grating was also obtained after a completely dark reaction with the cumulative grating strength of 6.88. Compared to bulk PQ/PMMA photopolymers in millimeter magnitude, TI/PMMA photopolymers are more competitive for further research on volume holographic memory storage.

Holographic memory performances of titanocene dispersed poly (methyl methacrylate) photopolymer with different preparation conditions

This work proposed a new kind of photosensitizer doped photopolymer and an optimized fabrication method. Holographic gratings are recorded in a glass-like photopolymer based on methyl methacrylate (MMA) as monomer, 2,2-azo-bisisobutyrolnitrile (AIBN) as thermo-initiator and a cationic initiator with high sensitivity and solubility, titanocene (Irgacure 784, BASF) (TI), as photo-initiator. In our fabrication, an optimized three-step thermal-polymerization method is investigated and depicted. 3 mm thick TI/PMMA photopolymers with different concentrations of AIBN and TI molecules are examined in detail. The photo-physical and photo-chemical processes inside the sample during continuous exposure are analyzed. In our photopolymers, permanent volume holographic gratings with diffraction efficiency approaching 74% and response time close to 20s, which makes this photopolymer available in volume holographic data storage.

Ultrafast volume holographic storage on PQ/PMMA photopolymers with nanosecond pulsed exposures.

Ultrafast holographic recording in bulk phenanthrenequinone dispersed poly (methyl methacrylate) photopolymers is experimentally examined under nanosecond pulsed exposure. A modified interference optical system is set to investigate the dark enhancement effect and real-time diffraction grating strength. Single transmission diffraction grating is recorded in a 6 nanosecond pulse exposure. Grating enhancement formation with different pulse quantity, repetition rate and spatial frequency are also measured. Diffraction efficiency is enhanced by increasing the pulse number as well as the single-pulse energy. The grating strength of 0.58 within 1.8 μs cumulative exposure time is obtained. Moreover, holographic reciprocity failure occurring in the ultrafast holographic storage is analyzed. This paper presents a practical support for PQ/PMMA photopolymers in applications of transient information holographic storage.

One-shot and aberration-tolerable homodyne detection for holographic storage readout through double-frequency grating-based lateral shearing interferometry.

A simple method to decode the stored phase signal of volume holographic data storage with adequate wave aberration tolerance is highly demanded. We proposed and demonstrated a one-shot scheme to decode a binary-phase encoding signal through double-frequency-grating based shearing interferometry (DFGSI). The lateral shearing amount is dependent on the focal length of the collimated lens and the frequency difference between the gratings. Diffracted waves with phase encoding were successfully decoded through experimentation. An optical model for the DFGSI was built to analyze phase-error induction and phase-difference control by shifting the double-frequency grating longitudinally and laterally, respectively. The optical model was demonstrated experimentally. Finally, a high aberration tolerance of the DFGSI was demonstrated using the optical model.

First-principles study on the electronic structures and optical properties of Cu, Fe doped LiNbO_3 crystals

The binding energies, electronic structures and optical properties of LiNbO3 and Cu/Fe doped LiNbO3 crystals are investigated by first principles based on the density functional theory in this paper. The supersell structures of crystals are established each with 60 atoms, including five models: pure LiNbO3, LN1 (Cu2+ occupy Li+ site), LN2 (Fe3+ occupy Li+ site), LN3 (Cu2+ occupy Li+site and Fe3+ occupy Li+ site) and LN4 (Cu2+ occupy Li+ site and Fe3+ occupy Nb5+ site). The optimized results show that the total energies of all models can achieve certain stable values, which means that the models accord with the actual crystal structures. The impurity energy levels of Cu and Fe doped LiNbO3 crystals appear within the band gaps, which are contributed by Cu 3d orbital, Fe 3d orbital and O 2p orbital; in co-doped LiNbO3, Cu offers deep energy level and Fe offers shallow energy level within the band gaps. There are two wide absorption peaks appearing respectively at 445 nm and 630 nm in co-doped LiNbO3 crystal, which correspond to the electron transitions from Eg orbital of Cu to Nb 4d orbital and T2g orbital of Fe to Nb 4d orbital respectively; the absorption edge of Cu, Fe mono and co-doped LiNbO3 crystals are red-shift successively, which coincides with the variation of band gape. The light absorption intensity of co-doped LiNbO3 crystal is stronger than that of mono-doped LiNbO3 crystal. The co-doped sample light absorption property is related to Fe site occupation. In this paper, it is suggested that the co-doped sample with Fe at Nb site is more competitive than that with Fe at Li site in optical volume holographic storage applications, and that reducing properly [Fe2+]/[Fe3+] value may be conducible to the formation of this advantage.

The optical polarization properties of phenanthrenequinone-doped Poly(methyl methacrylate) photopolymer materials for volume holographic storage

We present an experimental study on the optical polarization properties of the phenanthrenequinone-doped poly(methyl methacrylate) photopolymer materials. We discuss the diffraction efficiency with different weight ratios of phenanthrenequinone that was dissolved in the materials. In addition, we observe the diffraction efficiency difference between polarization holography and traditional holography.

Influence of fabrication conditions on characteristics of phenanthrenequinone-doped poly(methyl methacrylate) photopolymer for holographic memory

In this paper, we experimentally investigate the influence of the fabrication conditions on holographic characteristics in phenanthrenequinone-doped poly(methyl methacrylate) (PQ:PMMA) bulk photopolymer. In our investigation, the PQ:PMMA bulk samples are fabricated by use of a two-step thermo-polymerization method. We firstly propose to monitor relative viscosity of the monomer solution during the sample preparation to obtain a reliable criterion for material fabrication. We then compare experimentally characteristics of 2-mm thick samples fabricated with different conditions for holographic memory. The results show that the conditions in the first step play a important rule for fabricating bulk PQ:PMMA samples with good optical uniformity. In addition, the conditions in the second step play the rule for controlling the concentration of residual monomer and determine holographic characteristics. These results can provide a useful rule for fabricating bulk PQ:PMMA photopolymers for further applications on volume holographic data storage.

Random phase encoding in holographic optical storage with energy-effective phase modulation by a phase plate of micro-lens array

A study of phase multiplexing for volume holographic storage with use of a phase plate of micro-lens array is proposed and demonstrated. We calculate the shifting tolerance of the recorded hologram encoded by a phase plate, which serves as a phase encoder as well as a beam modulator so that high shifting selectivity and high energy efficiency can be achieved at the same time. The experimental results show that a truly-random phase encoding can be performed by the phase plate of micro-lens array.

Review of Random Phase Encoding in Volume Holographic Storage

Random phase encoding is a unique technique for volume hologram which can be applied to various applications such as holographic multiplexing storage, image encryption, and optical sensing. In this review article, we first review and discuss diffraction selectivity of random phase encoding in volume holograms, which is the most important parameter related to multiplexing capacity of volume holographic storage. We then review an image encryption system based on random phase encoding. The alignment of phase key for decryption of the encoded image stored in holographic memory is analyzed and discussed. In the latter part of the review, an all-optical sensing system implemented by random phase encoding and holographic interconnection is presented.

Photorefractive properties for holographic application of Ce:Fe:LiNbO3 crystals with various [Li]/[Nb] ratios

Ce:Fe:LiNbO3 crystals with various [Li]/[Nb] ratios were grown by the Czochralski method from melts having compositions varying between 48.6 and 58 mol% Li2O. The Ce, Li and Nb concentrations in the grown Ce:Fe:LiNbO3 crystals were analyzed by the inductively coupled plasma atomic emission spectrometer (ICP-AES). It was found that as the [Li]/[Nb] ratio increases in the melt, the [Li]/[Nb] ratio in the crystal and the distribution coefficients of Ce ions increase also. The photorefractive properties of the Ce:Fe:LiNbO3 crystals were experimentally studied by the two-wave coupling method. The results show that as the [Li]/[Nb] ratio increases, the dynamic range decreases, but the photorefractive sensitivity and the signal-to-noise ratio improve. In a coherent volume 0.192 cm3 of a Ce:Fe:LiNbO3 crystal with [Li]/[Nb] ratio of 1.2, 3800 holograms with 800×600 pixels have been successfully multiplexed in a compact volume holographic data storage system.

基于LiNbO3晶体的透射式共光轴体全息存储技术

基于LiNbO₃晶体的透射式共光轴体全息存储技术

A two-dimensional constant-weight sparse modulation code for volume holographic data storage

In order to simplify the threshold determination, reduce the inter-pixel cross-talk, and improve the storage density for high-density volume holographic data storage, a two-dimensional constant-weight sparse modulation code is proposed. The evaluation criteria and design rules are investigated based on the page-oriented optical data storage system. Coding parameters are optimized to achieve large channel capacities. An 8:16 modulation code is designed to reduce the raw bit error rate and its performances are experimentally evaluated. A raw bit error rate of the magnitude of 10−4 is obtained with a single-data-page storage and 10−3 with multiplexing.

Shifting selectivity of collinear volume holographic storage

A paraxial equation for calculating pixel-based 2-D shifting selectivity of collinear holographic storage system is proposed. The equation shows that the broader reference pattern makes high shifting selectivity. In addition, both amplitude and phase modulation make no obvious change in shifting selectivity so shifting selectivity is not affected through advanced design of reference pattern. Besides, a simplified formula based on a point signal is proposed, which shows a clear physical insight that the shifting selectivity is the square of Fourier transform of the intensity distribution of reference pattern and does not depend on thickness of the disc when a point signal is applied.

Nonlinear response studies and corrections for a liquid crystal spatial light modulator

The nonlinear response of light transmission characteristics of a liquid crystal (LC) spatial light modulator (SLM) is studied. The results show that the device exhibits a wide range of variations with different control parameters and input settings. Experiments were performed to obtain intensity modulation that is best described by either power-law or sigmoidal functions. Based on the inverse transformation, an appropriate pre-processing scheme for electrically addressed input gray-scale images, particularly important in several optical processing and imaging applications, is suggested. Further, the necessity to compensate the SLM image nonlinearities in a volume holographic data storage and retrieval system is demonstrated.