| 1. Very high
capacity optical disk with multilayer holographic storage
Protected by the Hungarian Patent Application P0301354
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I. Targets
Increasing the data capacity is probably the main factor that drives
the progress of optical data storage technology. The currently available
DVD technology enables to store 4-20 Gbyte information in a single
disk depending on the number of sides and layers used. The next
step of the progress is the Blu-Ray-Disc that can store 25-100 Gbyte
in a single disk. Today it is not clear which technology comes after
the Blu-Ray-Disc that can store 100-1000 Gbytes in single disk.
Our research work focuses on the topic: How to
store 100-1000 Gbytes in single disk like the CD?
We think that this is best realizable with our storage concept based
on multiple layers of holograms recorded in an either stratified
or bulky storage material. Our storage concept provides significant
technological advantages against the alternative research approaches
of multilayer spot based and holographic multiplexed solutions.
We are financing this research from our own resources
and from some Hungarian Government Funds.
We are looking for investors and/or partners
who feel interest to participate in this research.
2. Optical system
for multi-layer thin film holographic storage with confocal filtering
Our research team developed a thin film polarization
holographic storage system [2,3]. Compared with volume holographic
storage our solution has some benefits. Data carrier works also
in reflection mode, high diffraction efficiency is achievable. Because
of the polarization holography, except for the 0th order only one
diffraction order (+1 or -1) is present. This increases the storage
density by a factor of 2. In most cases there is no need for using
complicated servomechanisms. It is possible to read and write with
different wavelengths because of the lack of Bragg selectivity.
The material shrinkage causes small geometrical distortion that
can be compensated by software. The storage material is 2-10 µm
thick providing easier fabrication technology. The volumetric data
density can be as high as »1.3 bit/µm3. Beside these
advantages an important drawback of the thin film holographic storage
is its limited multiplexing possibility as upper boundary of the
storage capacity is fully determined by the NA of the objective
and the applied wavelength.
The question is whether instead of multiplexing
is there any way to increase data density while retaining the advantages
of both thin-film polarization holography and volume holographic
storage? Because of the limiting factors of thin film two-dimensional
storage the solution must be any type of volumetric storage. Adapting
digital bit oriented volumetric storage concept, i.e. multi-layer
structure and confocal filtering, a new idea has been developed
for thin-film polarization holography that is multi-layer thin-film
holography with confocal filtering of the addressed micro-hologram.
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Fig. 1 shows the concept
of our method.The optical layout is divided into two parts:
a writing and a reading section. The writing section is above
the multi-layer material. It is a simple Fourier transforming
arrangement consisting of the SLM and the 1st Fourier objective.
The reading section is a 6f system with three Fourier objectives.
In our solution the entire thickness of the multiple-layer
material is 1-2 mm containing 10-200 2-5µm thick storage
layers, separated by ˜0.01-0.1 mm rigid spacers. Using
special beam separation techniques the optical system allows
suitable suppression of the reference wave (0th diffraction
order) at reading out. In Fig. 1 the object beam has an inner
prohibited area. The reference beam goes at the axis of the
object beam cone in the prohibited volume and is suppressed
by the beam stop after the 2nd Fourier objective. |
Figure. 1
Optical layout of a prospective implementation of the multi-layer
micro-holographic storage system (8f linear arrangement) with
on-axis reference wave and transmissive storage medium. The
addressed hologram is in confocal position with the spatial
filter. |
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Reading out is performed by a 6f system equipped
with a suitable spatial filter. Only light diffracted from one
single hologram located on one specific layer of the storage medium
is allowed to reach the detector array (see Fig. 1). For this
purpose the addressed hologram and the spatial filter are placed
in confocal positions. With appropriate setting of the hologram
size, the NA of the objective and the layer distance only one
reconstructed image can go through the filter. The confocal Fourier
filtering reduces inter-layer as well as inter-hologram crosstalk
and ensures layer and hologram addressing.
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3. Results of data density estimation
Fig. 2 shows the scheme used for data density and capacity estimation.
During that procedure we used the hologram size as free parameter.
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| The requirement of confocal
filtering determines the layer thickness from the hologram
size and the hologram size determines the number of pixels
in a single hologram through the optical resolution limits.
Based on these requirements one can construct a simple formula
for estimating the disk capacity. Figs. 3 and 4 show the results
of capacity estimation.
The model reveals that the capacity of a disc grows monotonically
when the hologram size is reduced (see Fig. 3). The limit
of this model is the case when the reconstructed pixel diameter
equals to the width of storage ring (R-r, see Fig. 2). In
this case the pixel number in a single hologram is 6 pieces,
the number of layers is about one hundred (see Fig. 4).
The achievable storage capacity is about 400 Gbyte for a
1-2mm thick 120mm diameter disk. It is similar to the capacity
of volumetric holographic storage disks. |
Figure. 2
Scheme for calculating raw storage capacity
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Figure. 3
Hologram diameter
and layer thickness versus disk raw capacity (diffraction
approach).
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Figure.
4
Number of pixels per hologram and number of layers per
disc versus raw storage capacity of the disk (diffraction
approach). |
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IV Multilayer and multiplexed
holographic storage
In order to achieve higher than 400 Gbyte disk capacity with thin
film multilayer holographic storage information should be stored
in more than 100 layers (see Fig. 4). An alternative approach to
store that high capacity with using a smaller number of layers is
to combine multilayer holographic storage with holographic multiplexing.
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The optical system for such a MUX-ed multilayer holographic
storage is the same as shown in Fig. 1 with the only difference
that the object and reference “pixels” are arranged
differently on the SLM surface. As Fig. 5 shows in the MUX-ed
setup multiple non-axial reference beams are used that are
angularly separated from each other and from the object beams.
The multiple reference beams are needed to realize holographic
multiplexing in contrast with the thin film multilayer setup
in which a single axial reference beam is used (see Fig. 2).
This optical setup is able to realize angular or phase-coded
multiplexing depending on the modulation capability of the
SLM applied.
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Figure. 5
Arrangement of reference and object
beams on the SLM surface. |
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Figure 6 shows the cross section of a single volume hologram
created as the intersection of a single reference beam with the
object beams. In an angular multiplexing arrangement different holograms
can be stored in the same volume simultaneously by recording with
different reference beams.
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The small individual “volume holograms”
can be stored side by side and on top of each other to fill
the whole volume of the bulky storage material. In holographic
reconstruction the confocal filter suppresses the noise originating
from the not addressed “volume holograms”.
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Figure. 6
Cross section of a single volume hologram. |
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Figure. 7 shows the calculation of the total disk capacity versus
the diameter of the single volume holograms (dholo in Fig. 6). The
total disk capacity is about 8 terrabits at all possible hologram
diameters. |
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Fig. 7 also shows the number of layers and
the number of multiplexing used at two points of the curve.
E.g. at a hologram diameter of 20mm 21 layers and 53-times
multiplexing are used to achieve 8 terrabits disk capacity.
At a hologram diameter of 100mm only 4 layers but 266-times
multiplexing are applied to achieve the same capacity. If
hologram diameters much smaller than 20mm were used, the system
would approach the thin film multilayer storage, thus the
layer number would exceed while the multiplexing would decay.
We do not think that either end of the curve is technologically
advantageous, since both the very large number of layers and
the large number of multiplexing raise serious technological
problems.
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Figure. 7
Total capacity of disk versus diameter
of volume hologram (dholo). |
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| In contrast with that we do think that middle area of the curve
with a moderate number of layers and a moderate number of multiplexing
is technologically best feasible. One such system is e.g. the 20mm
hologram diameter with 21 layers and 53-times multiplexing. |
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References
[1] Holographic Data Storage, H. J. Coufal, D. Psaltis,
G.T. Sincerbox (Eds.); Springer series in optical sciences, 2000.
[2] G. Erdei, G. Szarvas, E. Lorincz, J. Fodor, F. Ujhelyi, P. Koppa,
P. Várhegyi, P. Richter, “Optical system of Holographic
Memory Card writing/reading equipment”,
Proc. SPIE, Vol. 4092, p. 109-118, Novel
Optical System Design and Optimization III, Ed. Jose M. Sasian,
2000.
[3] E. Lorincz, F. Ujhelyi, P. Koppa, G. Szarvas, G. Erdei, J.Fodor
A. Süto, , P. Varhegyi, , P.S. Ramanujam, S. Hvilsted, "Read/write
demonstrator of rewritable
holographic memory
card system", in Optical Data Storage 2001, Terril Hurst, Seiji
Kobayashi, Proc. of SPIE ,Vol. 4342, p. 566-573, 2001.
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A presentation held
at the EOS Advanced Imaging Techniques
20-23 October 2003, Delft, The Netherlands |
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Multilayer Thin-Film Holographic Storage: a New Aproach |
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Shift Selectivity Calculation for Finite-Volume Holograms with Half-Cone Reference Beams
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