Abstract
The enhanced capability of advanced computer designing has enabled us to push beyond the present level of computer technology and to attain substantial advancesin the understanding of design processes. The potential speed breakthrough for mass storage offered by volume Holographic storage technology (Holostore) is discussed along with the possibility of moleculer memory.Introduction
Computing technologies have consistently high rates of change and
transformation.Since the IC was developed, the number of transistors
that engineers can pack on a chip has increased at a phenomenal rate.
ICs are made by the Photolithography process
in which the patterns of metal or chemically treated
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| Photolithography |
silicon are
layered one atop another, on to a die of silicon.Current lithographic
processes employ a mercury light source whose 0.365 micron wavelength
creates the 0.35 micron features.Building even smaller chip features
requires using light sources with even shorter wavelength.That means
designers had to move from visible light , to ultraviolet light, and
finally to the X-Rays territory.But using the X rays for the
photolithographic introduces a new set of problems. For example the
issue of having a reliable X ray source, the X rays cannot be
focused with optical lenses and therefore the mask, which produces the
required pattern on the silicon, must be the size of the features
themselves and furthermore the materials opaque to light are not
necessarily opaque to X rays.
The computationally intensive problems requiring tremendous computing
speed and volume (storage capacity) motivated a new field of optical
computing.
Optical disks are an alternative to magenetic devices for high density, low cost secondary memory requirements.Read only (ROM), Write once read many (WORM) and Eraseable/Rewritable are the three catagories of optical disks.
The capacities of today's mass storage devices cannot satisfy the demands of new processes which will be developed near future.To achieve a full scale opticle computing environment (with large computing power), it is necessary to have memories with rapid access time and large storage capacity.To meet these needs holographic memories have emergrd.
Using the Holostore technology bits can be accessed in micro second instead of the miliseconds required for magenetic or optical disk.Moreover, instead of getting a serial stream of bits, entire arrays of bits, up to 1MBit, are delivered simultaneously.Storage capacities are very competitive with magenetic or optical disk, and the media volumetric storage density is significantly greater since it is a 3-D storage media.
Today's research has shown that even smaller objects might serve as storage devices thus giving the idea of biological memory.The data stored in this way can be stable for five years.
Optical disks are an alternative to magenetic devices for high density, low cost secondary memory requirements.Read only (ROM), Write once read many (WORM) and Eraseable/Rewritable are the three catagories of optical disks.
The capacities of today's mass storage devices cannot satisfy the demands of new processes which will be developed near future.To achieve a full scale opticle computing environment (with large computing power), it is necessary to have memories with rapid access time and large storage capacity.To meet these needs holographic memories have emergrd.
Using the Holostore technology bits can be accessed in micro second instead of the miliseconds required for magenetic or optical disk.Moreover, instead of getting a serial stream of bits, entire arrays of bits, up to 1MBit, are delivered simultaneously.Storage capacities are very competitive with magenetic or optical disk, and the media volumetric storage density is significantly greater since it is a 3-D storage media.
Today's research has shown that even smaller objects might serve as storage devices thus giving the idea of biological memory.The data stored in this way can be stable for five years.
What is Holostore Technology?
A volume holographic storage (holostore) devive is a page oriented devive
that writes and reads data in an optical form.The holography technology
achieves the necessary high storage densities as well as fast access times.
This capability occurs because a holographic image, or hologram, encodes
a large block of data as a single entity in a single write operation.
Conversely, the process of reading a hologram retrieves the entire data
block simultaneously.
Why Holostore Technology?
Practically, researchers believe that Holographic data storage system in
which thousands of pages (blocks of data), each containing million bits,
can be stored within the volume of a sugar cube, have a storage capacity of
10 GB per cubic centimeter.This figure is still very impressive compared to
today's magenetic storage densities, which are around 100 Kb per square
centimeter (not including the derive mechanism).
At this density a block of optical media roughly the size of a deck of playing cards would be able to house a terabyte of data.Because such system can have no moving parts and its pages are accessed in parallel, it is estimated that data throughput on such system can hit 1 Gbps or higher. In holographic recording applications, longer interaction lengths imply increased angular selectivity and also higher data storage capacity . These advantages are in addition to the ability to synthesize a much larger cross sectional area then is currently attainable using bulk materials.
At this density a block of optical media roughly the size of a deck of playing cards would be able to house a terabyte of data.Because such system can have no moving parts and its pages are accessed in parallel, it is estimated that data throughput on such system can hit 1 Gbps or higher. In holographic recording applications, longer interaction lengths imply increased angular selectivity and also higher data storage capacity . These advantages are in addition to the ability to synthesize a much larger cross sectional area then is currently attainable using bulk materials.
How Holographic storage works?
Holostore leverages the imaging properties of light and its ability to
launched. The reading out of images instead of single bits serially
provides a tremendous improvement in the bandwidth. The ability for
light to be launched through space and deflected easily will eliminate
the need for rotation of the medium. The capability of coherent light to
interfere and to form holograms provides a convenient way to address a
storage medium in three dimensions, while only scanning the beams in
two dimensions.
Holography records the information from a three-dimensional object in such
a way that a three dimensional image may subsequently be constructed.
Holographic memory uses lasers for both reading and
writing the blocks of data into the photosensetive material. A digital
hologram is formed by recording the interference pattern between a
discretely modulated coherent wave front and a reference beam on a
photosensitive material.
Holographic data storage materials
Despite decades of research in holographic-storage materials, iron
dopped lithium niobate is still the medium of choice for all
demonstrations of holographic-storage system.Despite its well known
shortcomings, such as destructive readout of data and relatively low
sensitivity, its the only material that currently has the optical
quality that is critical for a system application.
Problems with the Hologarphic storage system
A difficulty with the holostore technology had been the destructive readout.
The re-elluminated reference beam (i.e the read beam, see How holographic
storage system works), used to retrieve the recorded information, also exites
the donor electrons and disturbs the equilibrium of the space charge field
in a manner that produces a gradual erasure of the recording. In the past,
this has limited the number of reads that can be made before the signal-to
-noise ratio becomes too low. Morover, wrtes in the same fashion can
degrade previous writes in the same region of the medium. This restricts the
ability to use the three-dimensional capacity of a photorefractive for
recording angle-multiplexed holograms.
The other challenge has been the geometry of the crystal medium. It is
difficult to grow large crystals of good optical quality and of limited size.
As a consequence of this problem, widespread application of the bulk
photorefrective technology has not been occured, despite an initial
surge of development in the decade of the 1970s.
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| Holographic Memory |
Moleculer Memory
With the advances in Moleculer electronics, it is possible to implement
a prototype memory subsystem that uses molecules to store digital bits.
The molecule in question here is the protein called bacteriorhodopsin
. Its photocycle,
the sequence of structural changes, a molecule undergoes in reaction to
light, makes it an ideal AND data storage gate, or flip-flop.
According to the today's research, the bR (where the state is
0) and the Q (where the state is 1) intermediates are both stable for
many
years.
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| Moluculer Memory & Photocycle(inset) |
The reason for considering the moleculer memory is that it is protein
based and therfore is inexpensive to produce in quantity. Secondly, the
system has ability to operate over a wider range of temperatures than
semiconductor memory.
How Protein Memory works?
In a prototype memory system, bacteriorhodopsin stores data in a 3-D matrix.
The matrix can be build by placing the protein into a cuvette (a transparent
vessel) filled with a polyacrylamide gel. The protein, which is in the bR state
, gets fixed in by the polymerization of the gel. A battery of Kypton lasers
and a charge-injection devive (CID) array surround the cuvette and are used
to write and read data.
While a molecule changes states within microseconds, the combined steps
to
read or write operation take about 10 milliseconds. However like the
holographic storage, this device obtains data pages in parallel, so a 10
Mbps is possible. This speed is similar to to that of slow
semiconductor memory.
References
R.K. Grygier, "Holographic data storage materials," MRS Bulletin,
Vol. 21, Iss. 9, Sept 1996, PP. 51-60
Steve Redfield and Jerry Willenbring "Holostore technology for higher
levels of memory hierarchy," IEEE potentials, 1991, PP. 155-159
Najeeb Imran, "Optical computing," IEEE potentials, Dec 1992, PP. 33-36
Tom Thomson, "What's Next, "Byte, April 1996, PP. 45-51.