Tuesday, November 26, 2013
Abstract of 3d
Optical Data Storage
3D optical data
storage is the term given to any form of opticaldata storage in which
information can be recorded and/or read with three dimensionalresolution (as
opposed to the two dimensional resolution afforded, for example, by CD). This
innovation has the potential to provide petabyte-level mass storage on
DVD-sized disks. Data recording and readback are achieved by focusing lasers
within the medium. However, because of the volumetric nature of the data
structure, the laser light must travel through other data points before it
reaches the point where reading or recording is desired. Therefore, some kind
of nonlinearity is required to ensure that these other data points do not
interfere with the addressing of the desired point. No commercial product based
on 3D optical data storage has yet arrived on the mass market, although several
companies are actively developing the technology and claim that it may become
available soon.
The origins of the field date
back to the 1950s, when Yehuda Hirshberg developed the photochromicspiropyrans
and suggested their use in data storage. In the 1970s,ValeriBarachevskii
demonstrated that this photochromism could be produced by two-photon
excitation, and finally at the end of the 1980s Peter T. Rentzepis showed that
this could lead to three-dimensional data storage. This proof-of-concept system
stimulated a great deal of research and development, and in the following
decades many academic and commercial groupshave worked on 3D optical data
storage products and technologies. Most of the developed systems are based to
some extent on the original ideas of Rentzepis. A wide range of physical
phenomena for data reading and recording have been investigated, large numbers
of chemical systems for the medium have been developed and evaluated, and
extensive work has been carried out in solving the problems associated with the
optical systems required for the reading and recording of data. Currently,
several groups remain working on solutions with various levels of development
and interest in commercialization.
Optical Recording Technology
Optical storage
systems consist of a drive unit and a storage medium in a rotating disk form.
In general the disks are pre-formatted using grooves and lands (tracks) to
enable the positioning of an optical pick-up and recording head to access the
information on the disk. Under the influence of a focused laser beam emanating
from the optical head, information is recorded on the media as a change in the
material characteristics. The disk media and the pick-up head are rotated and
positioned through drive motors controlling the position of the head with
respect to data tracks on the disk. Additional peripheral electronics are used
for control and data acquisition and encoding/decoding.
As an example, a
prototypical 3D optical data storage system may use a disk that looks much like
a transparent DVD. The disc contains many layers of information, each at a
different depth in the media and each consisting of a DVD-like spiral track. In
order to record information on the disc a laser is brought to a focus at a
particular depth in the media that corresponds to a particular information
layer. When the laser is turned on it causes a photochemical change in the
media. As the disc spins and the read/write head moves along a radius, the
layer is written just as a DVD-R is written. The depth of the focus may then be
changed and another entirely different layer of information written. The
distance between layers may be 5 to 100 micrometers, allowing >100 layers of
information to be stored on a single disc.
In order to read
the data back (in this example), a similar procedure is used except this time
instead of causing a photochemical change in the media the laser causes
fluorescence. This is achieved e.g. by using a lower laser power or a different
laser wavelength. The intensity or wavelength of the fluorescence is different
depending on whether the media has been written at that point, and so by
measuring the emitted light the data is read.
The size of
individual chromophoremolecules or photoactive color centers is much smaller
than the size of the laser focus (which is determined by the diffraction
limit). The light therefore addresses a large number (possibly even 109) of
molecules at any one time, so the medium acts as a homogeneous mass rather than
a matrix structured by the positions of chromophores.
Comparison with Holographic
Data Storage:
3D optical data
storage is related to (and competes with) holographic data storage. Traditional
examples of holographic storage do not address in the third dimension, and are
therefore not strictly "3D", but more recently 3D holographic storage
has been realized by the use of microholograms. Layer-selection multilayer
technology (where a multilayer disc has layers that can be individually
activated e.g. electrically) is also closely related.
Holographic data
storage is a potential replacement technology in the area of high-capacity data
storage currently dominated by magnetic and conventional optical data storage.
Magnetic and optical data storage devices rely on individual bits being stored
as distinct magnetic or optical changes on the surface of the recording medium.
Holographic data storage overcomes this limitation by recording information
throughout the volume of the medium and is capable of recording multiple images
in the same area utilizing light at different angles.
Additionally,
whereas magnetic and optical data storage records information a bit at a time
in a linear fashion, holographic storage is capable of recording and reading
millions of bits in parallel, enabling data transfer rates greater than those
attained by traditional optical storage.
The stored data
is read through the reproduction of the same reference beam used to create the
hologram. The reference beam’s light is focused on the photosensitive material,
illuminating the appropriate interference pattern, the light diffracts on the
interference pattern, and projects the pattern onto a detector. The detector is
capable of reading the data in parallel, over one million bits at once,
resulting in the fast data transfer rate. Files on the holographic drive can be
accessed in less than 200 milliseconds.
