Many kinds of recording techniques have been proposed for holographic data storages (HDS). Multiplexing recording technique is a primary contributor to determining the recording density in HDS. The method that utilizes spherical reference waves is characterized by the ability to enable multiplexing recording only by displacing (shifting or rotating) the recording medium. In this study, we propose a theoretical diffraction model of peristrophic multiplexing with spherical reference wave for HDS.
Along with the wider use of high-speed information networks and multimedia, it is increasingly necessary to have
higher-density and higher-transfer-rate storage devices. Therefore, research and development into holographic memories
with three-dimensional storage areas is being carried out to realize next-generation large-capacity memories. The
mainstream in the world is the angle multiplexing method, however, its beam position control is quite severe. In such
situation, we study about shift multiplexing method because it costs not much and its control is easier than the angle
multiplexing. In this experiment, we examined shift selectivity of track direction, radial direction and vertical direction of
the medium. As a result, combining these different kinds of selectivity, we found a possible multiplexing way to achieve
several tera bits per inch square density recording.
The diffractive optical element (DOE) has the transformation function of wavefront, and its applications are forming or
homogenization of beam, and aberration correction, and so on. In this study, we evaluate possibility as storage
application of the DOE. The optical data storage using the DOE is thought of as a kind of holographic data storage
(HDS). In the HDS, digital data is recorded and read out as modulated 2-dimensional page data, instead of bit-by-bit
recording in conventional optical storages. Therefore, HDS actualize high data transfer rate. We design and optimize
phase distribution of the DOE using the iterative method with regularization. In the optimization process, we use
iterative Fourier transform algorithm (IFTA) that is known as Gerchberg–Saxton (GS) algorithm. At this time, the
regularization method is adopted to suppress minute oscillation of the diffraction pattern. Designed and optimized DOE
is fabricated by ultraviolet (UV) nanoimprinting technology. High productivity can be expected by adopting
nanoimprinting technology. DOEs are duplicated on the silicon (Si) substrate as reflection-type elements. Fabricated
DOE is evaluated in the experiment. We verify that DOE for optical data storage can be actualized through our approach.
Holographic Data Storage (HDS) is one of the next generation storage technologies that can actualize high data capacity and high data transfer rate. Since information is recorded 3-dimensionally in a thick medium, data capacity of the HDS is not constrained by diffraction limit. However, behavior of wavefront in an inhomogeneous thick medium is highly complex, and it is hard to handle propagation of wavefront in the medium analytically. Therefore, we establish a numerical technique for analysis of volume holograms. The proposed technique is based on the scalar diffraction theory, which is described as the volume integral equation. By applying Born approximation and angular spectrum method to the volume integral equation, the technique can be applicable for various problems. We analyze characteristics of the volume hologram with spherical reference wave, and confirm effectiveness of the proposed technique. Compared to conventional techniques such as coupled wave analysis, beam propagation method, and finite-difference time domain method, the proposed technique has application potentiality for various problems, and it is easy to implement. In this study, we show effectiveness of the proposed technique by applying to analysis of the volume hologram with spherical reference wave. It can be expected that the proposed technique may become a tool for design of HDS systems.
Today, along with the wider use of high-speed information networks and multimedia, it is increasingly necessary to have higher-density and higher-transfer-rate storage devices. Therefore, research and development into holographic memories with three-dimensional storage areas is being carried out to realize next-generation large-capacity memories. The mainstream in the world is the angle multiplexing method, however, it costs too much and its control is quite severe. In such situation, we study about shift multiplexing method because it costs not much and its control is easier than the angle multiplexing. In this experiment, we examined shift selectivity of track direction, radial direction and vertical direction of the medium. As a result, combining these different kinds of selectivity, we found a possible multiplexing way to achieve several tera bits per inch square density recording.
We examined the possibility of high-density recording using shift-multiplexed holographic memory with a spherical
reference beam. The use of a spherical reference beam is considered to make it possible to realize a multi-dimensional
multiplex system that uses the disk track direction (x-axis), radial direction (y-axis), and disk thickness direction (z-axis);
this would clearly improve the recording density when compared with the conventional angle multiplex recording. The
experimental results confirm the possibility of multiple recording by 3 dimensional medium shift. Furthermore, the
results indicate that a large capacity memory system of over 1 Tb/in2 can be obtained if a thick medium (about 1.5 mm)
is used.
The research and development of the holographic data storage (HDS) is advanced, as one of the high-speed, mass storage systems of the next generation. Recently, along the development of the write-once system that uses photopolymer media, large capacity ROM type HDS which can replace conventional optical discs becomes important. In this study, we develop the ROM type HDS using a diffractive optical element (DOE), and verify the effectiveness of our approach. In order to design DOE, iterative Fourier transform algorithm was adopted, and DOE is fabricated with electron beam (EB) cutting and nanoimprint lithography. We optimize the phase distribution of the hologram by iterative Fourier transform algorithm known as Gerchberg–Saxton (GS) algorithm with the angular spectrum method. In the fabrication process, the phase distribution of the hologram is implicated as the concavity and convexity structure by the EB cutting and transcribed with nanoimprint lithography. At this time, the mold is formed as multiple-stage concavity and convexity. The purpose of multiple-stage concavity and convexity is to obtain high diffraction efficiency and signal-to-noise ratio (SNR). Fabricated trial model DOE is evaluated by the experiment.
Gradient-index (GRIN) lenses have excellent optical properties that are not generally observed in the case of
homogeneous lenses. Hence, GRIN lenses are used to fabricate new optical elements that have promising applications in
optical information processing and optical communication. For example, it is widely used for scanner, fax machines and
copiers etc. One of the low cost fabricating methods of these lenses involves pulling up the core fiber vertically from a
polymer solution whose refractive index has been adjusted to the desired value. But in fact, the refractive-index
distribution is not ideal because of several factors in manufacturing. When a GRIN lens has the refractive-index
distribution which is not ideal, it degrades modulation transfer function (MTF) extremely. In this paper, we studied the
picture reconstruction by using Bayes' theorem. Bayes' theorem is applied to the degraded picture obtained in an
experiment with the plastic rod lens, and as a result MTF has extremely improved. First, spatial distribution of point
spread function (PSF) is calculated from the refractive index distribution inside a rod lens. The 4096 PSFs of spatial
distributions are obtained by this calculation. By applying image processing using the Bayes' theorem, MTF becomes
about 92.9% after the application, even if MTF is 23.3%. These researches show that Bayes' theorem is very effective in
image restoration.
In this article, we introduce the quasi-Monte Carlo (QMC) method that applies a low-discrepancy sequence (LDS) for
the evaluation of illumination optical systems. LDS is used in sequence-generation methods for numerical calculation,
with high-order integration, and a characteristic of the QMC method that adopts a LDS is that it can deliver faster
convergence than the Monte Carlo (MC) method, which uses random numbers. In this study, we have applied a LDS to
the evaluation of illumination optical systems, which are conventionally evaluated by using the MC method, and verified
its effectiveness. By assuming the evaluated system to have a gradient-index (GRIN) lens and comparing its illuminance
distribution with the theoretical illuminance distribution, we confirmed that by using the QMC method, the evaluation
process could be sped up by approximately five times compared to the MC method at the equivalent precision level.
Furthermore, we established a method to reconstruct the image by using the QMC method that applies a LDS to evaluate
the image-forming characteristics of the lens system and compared its results with those of the conventional MC method.
It was found that the QMC method that applies a LDS was superior to the MC method even in this case, in both precision
and conversion speed. On the basis of these results, it is evident that the QMC method that applies a LDS is extremely
useful in the evaluation of illumination optical systems.
A major barrier to the practical use of holographic memories is deterioration of reconstructed signals due to
shrinkage of the medium. It is reported that a photopolymer medium shrinks mainly in the thickness direction
during recording or as a result of variations in ambient temperature, and the optimal reconstruction angle shifts
and deteriorates. To avoid the influence of medium shrinkage, we propose co-axial, dual-reference-beam
recording and reconstruction. The recording and reconstruction characteristics of this method were examined
through simulation and experiments, and the results confirmed that in holographic multiplex recording, the
reference-beam angle is not affected by medium shrinkage, and stable recording and reconstruction are achieved.
KEYWORDS: Modulation transfer functions, Monte Carlo methods, GRIN lenses, Optical fibers, Refractive index, Light sources, Ray tracing, Quasi-Monte Carlo methods, Diffusers, Optical transfer functions
We propose a numerical analysis method for evaluating gradient-index (GRIN) optical fiber using the Monte Carlo
method. GRIN optical fibers are widely used in optical information processing and communication applications, such as
an image scanner, fax machine, optical sensor, and so on. An important factor which decides the performance of GRIN
optical fiber is modulation transfer function (MTF). The MTF of a fiber is swayed by condition of manufacturing process
such as temperature. Actual measurements of the MTF of a GRIN optical fiber using this method closely match those
made by conventional methods. Experimentally, the MTF is measured using a square wave chart, and is then calculated
based on the distribution of output strength on the chart. In contrast, the general method using computers evaluates the
MTF based on a spot diagram made by an incident point light source. But the results differ greatly from those by
experiment. In this paper, we explain the manufacturing process which affects the performance of GRIN optical fibers
and a new evaluation method similar to the experimental system based on the Monte Carlo method. We verified that it
more closely matches the experimental results than the conventional method.
In this paper, we examine whether the use of a two-dimensional finite impulse response (FIR) filter in
two presently available recording/reproduction methods, the Off-axis method, results in an improvement
in the signal quality. The linear minimum mean square error (LMMSE) method is first tested to examine
its effectiveness in optimizing the coefficients of the FIR filter. Subsequently, the real-coded genetic
algorithm (RCGA), which has the capability of searching a wide range of coefficients, is applied and the
result obtained is compared with that for the LMMSE method to see if the coefficient evaluation leads to
a local solution or the minimum solution. We also applied quadratic minimum mean-square-error
(QMMSE) Equalizer as a nonlinear equalizer.
The linear minimum mean square error (LMMSE) method is tested to examine its effectiveness in optimizing the
coefficients of the FIR filter. The real-coded genetic algorithm (RCGA), is also applied and the result is compared with
that for the LMMSE.
The rapid progress in computer performance and widespread use of broadband networks has facilitated the
transmission of huge quantities of digital information, thus increasing the need for high-speed, large-capacity
storage devices and leading to studies on holographic data storage (HDS). Compared with laser disks where the
recording density is limited by optical diffraction, HDS provides ultrahigh capacity with multiplex recording and
high-speed transfer greater than 1 Gbps; it has excellent potential for optical memory systems of the future [1].
To develop HDS, a design theory for element technologies such as signal processing, recording materials and
optical systems is required. Therefore, this study examines technology for simulating the recording and
reproduction for HDS. In simulations thus far, the medium for the recording process has usually been approximated
as laminated layers of holographic thin films. This method is suitable for systematic evaluation because the
computational cost is low and it allows simulation in the true form of data, that is, in two-dimensional digital data
patterns. However, it is difficult to accurately examine the influence of film thickness with a two-dimensional
lamination simulation. Therefore, in this study, a technique for analyzing thick-film holograms is examined using
the beam propagation method. The results of a two-dimensional simulation assuming laminated, holographic thin
films and a three-dimensional simulation using the beam propagation method are compared for cases where the
medium need not be treated as a thick film.
Diffractive optical elements (DOE) are used for various purposes, such as a beam shaper, beam splitter and so on. In
order to design DOEs, iterative Fourier transform algorithm (IFTA) is widely used. IFTA is fast and effective
optimization method and can handle large size data. In the IFTA, propagation in the far field and near field is described
by Fourier transform and Fresnel transform, respectively. Iterative angular spectrum approach (IASA) is one of iterative
methods based on angular spectrum technique. IASA adopt angular spectrum technique as expression of propagation in
near field instead of Fresnel transform. Angular spectrum technique is based on scalar diffraction theory and satisfies
Helmholtz equation; therefore angular spectrum technique is the accurate method for the calculation of diffraction.
We designed a DOE by using IASA and evaluated its diffraction characteristics based on vector diffraction theory. In
this paper, we report on the outcome.
Photopolymers are used popularly as a recording medium for holograms. In this study, we simulate the formation of diffraction grating in photopolymers using the model based on diffusion. Moreover we analyze diffraction characteristics in detail with beam propagation method. As a result, diffraction characteristics which are difficult to analyze using coupled-wave analysis and rigorous coupled-wave analysis have been obtained. It is our expectation that the result of this analysis will provide profitable knowledge for development of holographic data storages.
In order to clarify the hologram recording process for the photopolymer media, we analyzed both the creation process of multiplexed hologram and diffraction characteristics of reproduction process using a Beam Propagation Method (BPM). We also conducted experiment and evaluated the multiplexed hologram recording
characteristics. By the simulation using BPM and experimental results of hologram multiplexed recording, we made clear the evaluation process of photopolymer media and recording / reproduction conditions of multiplexed holograms recording.
KEYWORDS: Signal to noise ratio, Multiplexing, Holograms, Filtering (signal processing), Spatial filters, Signal processing, Near field diffraction, Optical simulations, 3D modeling, Wavefronts
To improve the recording density of Polytopic multiplexed recording, we studied the spatial filtering effect from
the view points of the filter size and signal-to-noise-ratio (SNR) of the reproduced signal. The relationship
between SNR and spatial filter size is evaluated by using three dimensional model and Fresnel diffraction analysis.
By applying the Wiener filter to the reproduced signal, we can improve SNR for small spatial filter size.
In holographic memories, photopolymer is a hopeful material as a recording medium. To use a photopolymer for
holographic memories as practical recording media, it is necessary to clarify the design condition of
recording/reproduction characteristics. The coupled-wave analysis (CWA) and the rigorous coupled-wave analysis
(RCWA) are widespread methods to analyze diffraction characteristics of volume holographic gratings. However,
holographic grating is more complex than simple grating that is presumed in CWA and RCWA. In this study, we
analyzed the index change of photopolymer based on a diffusion model and clarified the diffraction characteristics by
using the finite-difference time-domain (FDTD) method.
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