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Holography enables storage densities that can far surpass the
super-paramagnetic and diffraction limits of traditional magnetic and
optical recording. Holography can break through these density limits
because it goes beyond the two-dimensional approaches of conventional
storage technologies to write data in three dimensions. In addition,
unlike conventional technologies which record data bit by bit,
holography allows a million bits of data to be written and read out in
single flashes of light, enabling data transfer rates as high as a
billion bits per second. This would be comparable to a speed fast enough to transfer a DVD movie in about 30
seconds.
A powerful combination of high storage densities and rapid data
transfer rates makes it possible for holography to become a
compelling choice for next-generation storage needs.
In holographic data
storage, light from a coherent laser source is split into two beams,
signal (data-carrying) and reference beams. Digital data to be stored
are "encoded" onto the signal beam via a spatial light modulator. The
data or strings of bits are first arranged into pages or large arrays.
The 0's and 1's of the data pages are translated into pixels of the
spatial light modulator that either block or transmit light. The light
of the signal beam traverses through the modulator and is therefore
encoded with the "checkerboard" pattern of the data page. This encoded
beam then interferes with the reference beam through the volume of a
photosensitive recording medium, storing the digital data pages.
The interference pattern induces modulations in the refractive
index of the recording material yielding diffractive volume
gratings. The reference beam is used during readout to diffract off
of the recorded gratings, reconstructing the stored array of bits.
The reconstructed array is projected onto a pixilated detector that
reads the data in parallel. This parallel readout of data provides
holography with its fast data transfer rates.
The readout of data depends
sensitively upon the characteristics of the reference beam. By
varying the reference beam, for example by changing its angle of
incidence or wavelength, many different data pages can be recorded
in the same volume of material and read out by applying a reference
beam identical to that used during writing. This process of
multiplexing data yields the enormous storage capacity of
holography.
In the past,
the realization of holographic data storage has been frustrated by
the lack of availability of suitable system components, the
complexity of holographic multiplexing strategies, and perhaps most
importantly, the absence of recording materials that satisfied the
stringent requirements of holographic data storage.
Recently the development of practical
components for holographic systems has rekindled interest in this
technology. While the development of the needed components has been
accomplished largely in fields outside the storage industry, the
volume of these markets is expected to lead to low-cost, reliable
components for holographic data storage. Frequency-doubled,
diode-pumped Nd:YAG green lasers, used in the medical, cable TV, and
printing industries, are attractive recording sources due to their
small size, ruggedness, and low cost. Digital micro-mirror devices
appearing in new types of displays are ideal spatial light
modulators with their large numbers of pixels (~ 1 million), fast
frame rates (2000 Hz), and high optical contrast. The CMOS active
pixel detector arrays emerging in digital photography exhibit the
rapid access and data transfer properties required for holography.
At Bell Labs, we invented a multiplexing
geometry that yielded a simple, easily-implement able architecture
for holographic storage systems. Spurred by this development, we
focused on the long-standing problem of the lack of suitable storage
materials and invented new high-performance recording media with
demonstrated high density data storage capabilities. Our work serves
as the foundation for a practically realizable, high capacity
storage system with fast transfer rates and low-cost, removable
recording media.
The methods used to overlap or multiplex holograms determine the
complexity and architecture of the recording system. In the past,
multiplexing methods have required large optical systems and moving
optical parts. We have developed a method known as correlation
multiplexing where an optically complex reference beam, created by a
fixed set of optics, encodes the position of the hologram in the
recording medium. Large numbers of holograms can therefore be
multiplexed in essentially the same volume of the recording medium
through only micron-size spatial translations of the medium relative
to the reference beam. This "fixed optics" method enables
construction of a simple holographic storage system based on a
spinning disk architecture used throughout much of the storage
industry.
One of the major
challenges in the area of holographic data storage has been the development
of suitable storage materials. Holographic media must satisfy stringent
criteria, including high dynamic range, high photosensitivity, dimensional
stability, optical clarity and flatness, nondestructive readout, millimeter
thickness, and environmental and thermal stability.
To meet the needs of high-density
holographic data storage, researchers at Bell Laboratories have
designed a new type photopolymer, a "two-chemistry" system,
which yields high response, high photosensitivity media in
millimeter-thick, optically flat formats. The media exhibit the
some of the highest dynamic range of any holographic material
and currently represent one of the few recording systems
appropriate for high density digital holographic storage
applications.
The media are fabricated from mixtures
of two independently polymerizable yet compatible chemical
systems. Recording disks are formed by an in-situ polymerization
of one of the components to form the matrix or support of the
medium. The other component, which is photosensitive, remains
unreacted and dissolved in this matrix. Recording of holograms
occurs through a spatial pattern of polymerization of the
photosensitive species that mimics the optical interference
pattern generated during holographic writing The concentration
gradient that results from this patterned polymerization leads
to diffusion of the unpolymerized species which creates a
refractive index modulation that is determined by the difference
between the refractive indices of the photosensitive component
and the matrix. Our approach allows us flexibility in tailoring
the media to the particular needs of high density holographic
data storage.
In these materials,
a storage densities of 31.5 channel Gbits/in2 (a density that
would yield ~45 Gbytes on a 5 ¼" disk) have been demonstrated by
recording and retrieving >3000 digital data pages. Newer
"two-chemistry" materials we have developed have the capability
to store densities at least five times higher. With these
photopolymer materials meeting the critical performance
requirements for holographic mass storage, we believe they have
removed much of the risk associated with the development of
holographic technology.
The substantial advances in recording media, recording methods, and the
demonstrated densities of greater than thirty channel gigabits/pairs coupled with
the recent commercial availability of system components remove many of the
obstacles that previously prevented the practical consideration of
holographic data storage and greatly enhance the prospects for holography to
become a next-generation storage technology.
Bell Labs
has recently entered into an agreement with Imation
Corporation, one of the leading
data storage companies, to jointly further develop our work in
holographic data storage. We are currently working with the Lucent Technologies New Ventures Group
to explore avenues that would lead to commercialization of the
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