Optimization of a thick polyvinyl alcohol-acrylamide photopolymer for data storage using a combination of angular and peristrophic holographic multiplexing

The capability of polyvinyl alcohol-acrylamide photopolymer materials to obtain angularly multiplexed holographic gratings has been demonstrated [Appl. Phys. B 76, 851 (2003)]. A combination of two multiplexing methods--peristrophic and angular multiplexing--is used to record 60 holograms. An exposure schedule method is used to optimize the capability of the photopolymerizable holographic material and obtain holograms with a higher, more uniform diffraction efficiency. In addition, because of this exposure schedule method, the entire dynamic range (M#) of the material will be exploited, obtaining values of approximately M# approximately 9 in layers approximately 800 microm thick.     

Multiplexed holographic data page storage on a polyvinyl alcohol/acrylamide photopolymer memory

Holographic data pages were multiplexed in different thickness layers of a polyvinyl alcohol/acrylamide photopolymer. This material is formed of acrylamide photopolymers, which are considered interesting materials for recording holographic memories. A liquid crystal device was used to modify the object beam and store the data pages. A peristrophic multiplexing method is used to store a large number of data pages in the same spot in the material. The bit error rate was calculated fitting the histograms of the images to determine what parameters improve the quality of the images.     

Optimization of holographic storage with modulated recording beams in a thick polyvinyl alcohol/acrylamide photopolymer

In a holographic photopolymer system, the storage properties were often limited due to the attenuation in depth of light during the recording step. To obtain smaller values of the depth attenuation profiles in 1 mm thick polyvinyl alcohol/acrylamide (PVA/acrylamide) photopolymers, we used a triangle prism, sitting one face tilted at 13.7 degrees to the axis within the focus of a lens, to modulate the distribution of recording beams. Doing this permitted larger refractive index modulation depth to be achieved, and larger dynamic range (M#=9.2) was obtained in the PVA/acrylamide photopolymers.     

Improvement of photopolymer materials for holographic data storage

Photopolymer materials are practical materials for use as holographic recording media due to the fact that they are inexpensive, self-processing materials with the ability to record low loss, highly diffraction efficient volume holographic gratings. In general these materials absorb light of an appropriate wavelength, causing photo-polymerization of the local monomer, inducing a change in the material’s refractive index. These small changes in refractive index enable the storage of large quantities of data using holographic techniques. In an attempt to further develop the data storage capacity and quality of the information stored, i.e., resolution, in such materials, a deeper understanding of the photochemical mechanisms present during the formation of holographic gratings has become ever more crucial. From this understanding the response of an acrylamide/polyvinylalcohol based photopolymer to high spatial frequency information is improved through the addition of a chain transfer agent, i.e., sodium formate, HCOONa.     

Single-beam data encoding using a holographic angular multiplexing technique

We propose a novel configuration for angular multiplexing holographic encoding in which the signal beam and the reference beam are combined into a single beam. By using a spatial light modulator based on twisted nematic liquid crystals, the signal and the reference beams are modulated in amplitude mode and phase mode, respectively. The multiplexed interference patterns with the reference beams of different incident angles are recorded near the Fourier transform plane, and then the signals are selectively reconstructed by the corresponding reference beam. Both the simulation and the experiment of single-beam angular multiplexed holography are performed with consistent results. Compared with the traditional angular multiplexing holographic recording system, the single-beam configuration is more compact, easier to adjust, and less sensitive to the vibration of the environment. Therefore, it will be more attractive for potential applications in many fields, such as high-density signal recording and data encryption.     

Optical response of photopolymer materials for holographic data storage applications

We briefly review the application of photopolymer recording materials in the area of holographic data storage. In particular we discuss the recent development of the Non-local Polymerisation Driven Diffusion model. Applying this model we develop simple first-order analytic expressions describing the spatial frequency response of photopolymer materials. The assumptions made in the derivation of these formulae are described and their ranges of validity are examined. The effects of particular physical parameters of a photopolymer on the material response are discussed.     

High efficiency panchromatic photopolymer recording material for holographic data storage systems

Studies carried out to gauge the potential of a metal-ion doped panchromatic photopolymer recording material for application in real-time holographic data storage is presented. The photopolymer films are spin coated on glass plates to ensure better surface uniformity. Volume holographic transmission gratings with peak diffraction efficiency of 80% could be stored in the photopolymer films of 100 μm thickness. An efficiency of 70% is achievable even for gratings recorded with exposure energy as low as 10 mJ/cm². A checkerboard pattern data page recorded in the photopolymer film using a defocused 4-f recording geometry could be reconstructed with good image quality. The experimental results illustrate the competency of the developed photopolymer for holographic data storage applications.     

Characterization of the DuPont photopolymer for three-dimensional holographic storage

DuPont's HRF-150 photopolymer film is investigated for use in three-dimensional holographic memories. Measurements of sensitivity, hologram persistence, the lateral spread of the photoinitiated reaction, and the variation of diffraction efficiency with modulation depth, spatial frequency and tilt angle, and intensity are reported. We observed that the diffraction efficiency of the HRF-150 photopolymer for a given exposure decreases with increases in intensity and grating tilt angle. The holograms were nondestructively reconstructed for long periods of time at room temperature. The photoinitiated reaction spread less than 100 μm over a period of 16 h.     

一种新型绿敏光致聚合物及全息特性研究

报道了一种新型的对绿光敏感的光致聚合物材料,对环境湿度不敏感,材料的干燥时间短。该材料以丙烯酰胺为单体,N-苯基甘氨酸作为引发剂,藻红B作为光敏剂,并加入成膜剂聚乙烯醇及增塑剂二甲亚砜。对材料中光引发剂、光敏剂及增塑剂浓度进行了优化,发现其有一个最佳值。本材料记录的全息图衍射效率可达50％以上,感光强度为60mJ／cm^2。在光聚物介质上能够记录信噪比比较高的图像,说明该材料可用于全息存储。     

Half-data-page insertion method for increasing recording density in angular multiplexing holographic memory

We have developed a method to use a half-size data page between two full-size data pages to increase the recording density in angular multiplexing holographic memory up to 1.5× as much as the conventional angular multiplexing sequence. In our recording sequence, the full- and half-size data pages are alternately multiplexed. This is because each plane wave from various points in a data page has different angular selectivity. A half-size data page has higher angular selectivity than a full-size data page. The required angular intervals were estimated by numerical simulation taking holographic medium tilt into account. Also, an angular multiplexing experiment using the half-data-page insertion method resulted in a low bit error rate of the order of 10(-3), which is sufficient for practical use.     

对绿光敏感的全息存储光致聚合物性能的研究

报道了一种对绿光敏感的以丙烯酰胺为单体的光致聚合物,该光聚物由单体、光引发剂、共引发剂和成膜物组成。可以得到材料的最大衍射效率为55％。将光聚物用不同温度进行烘烤加热,可以看到材料的折射率调制度会增加。使用角度复用技术在光聚物中存储10幅全息图,说明所研究材料有希望于大容量体全息存储。     

The shift of Bragg angular selectivity curve in darkness in glass-like photopolymer for holographic recording

The shift of Bragg angular selectivity curve after exposure was studied by recording single hologram and multiplexed holograms, respectively. The evolution of shift with probe time after recording, as well as the saturated shift with exposure energy, was investigated in detail. The mechanism of Bragg mismatch was analyzed. By optimizing parameters of material synthesis and holographic recording, the shift of Bragg matched angle was alleviated in PQ/PMMA photopolymer after hologram recording.     

Three-dimensional optical data storage in a fluorescent dye-doped photopolymer

We propose a new, to our knowledge, monolithic multilayer optical storage medium in which data may be stored through the diffusional redistribution of fluorescent molecules within a polymer host. The active portion of the medium consists of a photopolymer doped with a fluorescent dye that is polymerized at the focal point of a high-numerical-aperture lens. We believe that as fluorescent molecules bond to the polymer matrix they become more highly concentrated in the polymerized regions, resulting in the modulated data pattern. Since data readout is based on detection of fluorescence rather than index modulation as in other photopolymer-based memories, the problems of media shrinkage and optical scatter are of less concern. An intensity threshold observed in the recording response of this material due to the presence of inhibitor molecules in the photopolymer allows for the three-dimensional confinement of recorded bits and therefore multilayer recording. The nonlinear recording characteristics of this material were investigated through a simple model of photopolymerization and diffusion and verified experimentally. Both single-layer and multilayer recordings were demonstrated.     

Application of a photopolymer to a holographic reflector for reflective liquid-crystal displays

A photopolymer for fabrication of volume-type transmission holograms, which are used primarily on the holographic reflector for reflective LCD's, is reported. This photopolymer consists of bisphenol-type epoxy resin and an acrylic monomer with diaryliodonium salt and 3-ketocoumarin as a complex initiator. The chemistry of the imaging formation is based on the radical polymerization of the monomer initiated by a holographic exposure, followed by the cationic polymerization of epoxy resin by UV exposure after postexposure baking. The yellowish color of the hologram derivatives that resulted from the sensitizing dye and from the photospeed were improved to satisfy the specifications for the holographic reflector. The holographic reflector now gives brighter images.     

Digital holographic data storage in a high-performance photorefractive polymer composite

Binary information in the form of a 256 x 256 pixel array has been stored holographically within a polymeric photorefractive composite of 130-mum thickness. Devices used consisted of 55-wt. % organic chromophore 1-(2?-ethylhexyloxy)-2, 5-dimethyl-4-(4?nitrophenylazo) benzene and up to 11.2-wt. % 2, 4, 7-trinitro-9-fluorenone (TNF) dispersed within a poly(N-vinyl carbazole) matrix. In a degenerate four-wave mixing arrangement, the refractive-index modulation and speed were 10(-3) and 200 ms, respectively. The high TNF concentration leads to short digital hologram recording times of 2 s without detriment in optical quality. Although not yet optimized, a figure of merit, M#, for holographic storage in this composite has been measured to be 0.017, and this is estimated to reach 0.18 in a revised experimental geometry.     

Phase-to-amplitude data page conversion for holographic storage and optical encryption

A new phase-to-amplitude data page conversion method is proposed for efficient recovery of the data encoded in phase-modulated data pages used in holographic storage and optical encryption. The method is based on the interference between the data page and its copy shifted by an integral number of pixels. Key properties such as Fourier plane homogeneity, bit error rate, and positioning tolerances are investigated by computer modeling, and a comparison is provided with amplitude-modulated data page holographic storage with and without static phase masks. The feasibility and the basic properties of the proposed method are experimentally demonstrated. The results show that phase-modulated data pages can be used efficiently with reduced system complexity.     

Application of linear minimum mean-squared-error equalization for volume holographic data storage

When target densities of volume holographic data storage systems are increased, the systems experience increased interference from adjacent pixels and noise. Here we present a method for designing and applying linear minimum mean-squared-error (LMMSE) equalization to improve the bit error rates (BER's) and hence the storage densities achievable. Numerical results with five defocused data pages indicate that a significant improvement in the BER is possible with LMMSE equalization.     

Channel modeling and estimation for intrapage equalization in pixel-matched volume holographic data storage

We present two different channel models (the magnitude model and the intensity model) for a pixel-matched volume holographic data storage system that employs the 4-focal-length architecture. First, a framework to describe the channel models is developed. We evaluate the linearity of the channel models by comparing data values obtained from diffraction-limited interference with data values predicted by the channel models. The models are evaluated for linearity and equalization gain under different storage and read-back conditions, such as fill factors, apertures, and contrast ratios. Bit error rate results obtained by use of linear equalization methods in conjunction with the channel models developed are also presented. Our results suggest that the magnitude model leads to better performance when the fill factors are small, whereas the intensity model appears to be more appropriate for the high-fill-factor cases. The magnitude model, when suitable, appears to provide a storage density improvement of as great as 65%, whereas the intensity model seems capable of providing as much as 15% density gain through deconvolution. The optimum aperture for storage seems to be close to the Nyquist aperture.     

Holographic characteristics of a 1-mm-thick photopolymer to be used in holographic memories

Poly(vinyl alcohol-acrylamide) photopolymers are materials of interest in the field of digital information storage (holographic memories). We analyzed the behavior of a 1-mm-thick photopolymer. Using a standard holographic setup, we recorded unslanted diffraction gratings. The material has high angular selectivity (0.4 degrees), good sensitivity (88 mJ/cm2), and small losses caused by absorption and scattering of light. It also has a high maximum diffraction efficiency (70%). A significant induction period was seen in the material. The authors hypothesize that, during most of this induction period, polymerization does in fact take place but is not reflected in the appearance of the diffracted light until a certain threshold value of exposure is reached.     

Analysis and simulations of two-wave mixing in photopolymer holographic recording media

Abstract

The process of two-wave mixing in photopolymer recording materials was investigated theoretically. The diffraction grating already forms during exposition and it may influence the original interference field distribution through diffraction of waves on the refractive index modulation. In order to show this, Kogelnik's coupled wave theory was extended to demonstrate the possibility of energy transfer from one recording wave to the other. The energy transfer and the intensity distribution during the recording process were systematically analysed depending on the boundary conditions. As a next step, the first harmonic model of the transmission grating recording, based on a simple material model, was implemented and solved. The ratio of the input intensities was found to be a crucial parameter and thus extensive simulations for various ratios of intensities were carried out. Modelling implies that the interference field and the refractive index grating just coincide for equal intensities. For intensities differing from unity they do not overlap themselves during the recording process. It has also turned out that the diffraction efficiency of the recorded grating drops against the case where the effects of two-wave mixing are not considered. The results of our analysis and simulation help give a better understanding of the physics of the recording process and proper adjustment of recording parameters in such applications as optical holography and holographic memories.