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Holographic Memory


Devices that use light to store and read data have been the backbone of data storage for nearly two decades. Compact discs revolutionized data storage in the early 1980s, allowing multi-megabytes of data to be stored on a disc that has a diameter of a mere 12 centimeters and a thickness of about 1.2 millimeters. In 1997, an improved version of the CD, called a digital versatile disc (DVD), was released, which enabled the storage of full-length movies on a single disc.

CDs and DVDs are the primary data storage methods for music, software, personal computing and video. A CD can hold 783 megabytes of data. A double-sided, double-layer DVD can hold 15.9 GB of data, which is about eight hours of movies. These conventional storage mediums meet today's storage needs, but storage technologies have to evolve to keep pace with increasing consumer demand. CDs, DVDs and magnetic storage all store bits of information on the surface of a recording medium. In order to increase storage capabilities, scientists are now working on a new optical storage method called holographic memory that will go beneath the surface and use the volume of the recording medium for storage, instead of only the surface area. Three-dimensional data storage will be able to store more information in a smaller space and offer faster data transfer times.

Holographic memory is developing technology that has promised to revolutionalise the storage systems. It can store data upto 1 Tb in a sugar cube sized crystal. Data from more than 1000 CDs can fit into a holographic memory System. Most of the computer hard drives available today can hold only 10 to 40 GB of data, a small fraction of what holographic memory system can hold. Conventional memories use only the surface to store the data. But holographic data storage systems use the volume to store data. It has more advantages than conventional storage systems. It is based on the principle of holography.

Scientist Pieter J. van Heerden first proposed the idea of holographic (three-dimensional) storage in the early 1960s.

A hologramis a block or sheet of photosensitive material which records the interference of two light sources. To create a hologram, laser light is first split into two beams, a source beam and a reference beam. The source beam is then manipulated and sent into the photosensitive material. Once inside this material, it intersects the reference beam and the resulting interference of laser light is recorded on the photosensitive material, resulting in a hologram. Once a hologram is recorded, it can be viewed with only the reference beam. The reference beam is projected into the hologram at the exact angle it was projected during recording. When this light hits the recorded diffraction pattern, the source beam is regenerated out of the refracted light. An exact copy of the source beam is sent out of the hologram and can be read by optical sensors.

Holography was invented in 1947 by the Hungarian-British physicist Dennis Gabor (1900-1979), who won a 1971 Nobel Prize for his invention m
Holographic Memory
Susan S6 IT and Parvathy S6 CS
Mohandas College of Engineering and Technology, Anad


At high density inside crystals Holographic memory is developing technology that has promised to revolutionalise the
storage systems Holographic memory is a technique that can store information. Data from more than 1000 CDs can fit
into a holographic memory System. . Holographic storage has the potential to become the next generation of storage
media Conventional memories use only the surface to store the data. But holographic data storage systems use the
volume to store data. It has more advantages than conventional storage systems. It is based on the principle of
holography .This paper provides a description of Holographic data storage system (HDSS), a three dimensional data
storage system which has a fundamental advantage over conventional read/write memory systems.

With its omnipresent computers, all connected via the Internet,
the Information Age has led to an explosion of information
available to users. The decreasing cost of storing data, and the
increasing storage capacities of the same small device footprint,
have been key enablers of this revolution. While current storage
needs are being met, storage technologies must continue to
improve in order to keep pace with the rapidly increasing
demand. However, both magnetic and conventional optical data
storage technologies, where individual bits are stored as distinct
magnetic or optical changes on the surface of a recording
medium, are approaching physical limits beyond which
individual bits may be too small or too difficult to store. Storing
information throughout the volume of a medium—not just on its
surface— offers an intriguing high-capacity alternative.
Holographic data storage is a volumetric approach which,
although conceived decades ago, has made recent progress
toward practicality with the appearance of lower-cost enabling
technologies, significant results from longstanding research
efforts, and progress in holographic recording materials. Hence
the holographic memory has become a great white whale of
technology research.

What Is Holography?
While a photograph has an actual physical image, a hologram
contains information about size, shape, brightness and contrast of
object being recorded. This information is stored in a very
microscopic and complex pattern of interference. The
interference pattern is made possible by the properties of light
generated by LASER. In order to record the whole pattern, the
light used must be highly directional and must be one of one
color. Such light is called coherent, because the light from a
LASER is one colors and leaves the LASER with one wave in
perfect one-step with all others, it is perfect for making
When we shine a light on the hologram the information that is
stored as an interference pattern takes the incoming light and re-
creates the original optical wave front that was reflected off the
object hence the eyes and brain now perceives the object as being
in front of us once again.

Concept Of Holographic Memory
Holography is a technique, which allows recording and playback
of 3-dimensional images. This is called a hologram unlike other
3-dimensional picture hologram provide what is called parallax.
Parallax allows the viewer to move back and forth up and down
and see different perspective as if the object were actually there.
In holography, the aim is to record complete wave field( both
amplitude and phase) as it is intercepted by recording medium
the record in plane may not even be an image plane. The
scattered or reflected light by object is intercepted by the
recording medium the recorded completely in spite of the fact
that the detector is insensitive to the phase difference among the
various part of the optical field.
In holography, interference between the object wave and
reference wave is formed and recorded on a holographic material.
The record known as hologram (whole record) captures the
complete wave, which can be viewed at the later time by
illuminating the hologram with an appropriate light beam.
To this day holography continues to provide the most accurate
depiction of 3-dimensional image in the world.
In a holographic memory device, a laser beam is split in two, and
the two resulting beams in a crystal medium to store a
holographic recreation of a page of data

Technique Of Storing Data On A Holographic

To record on the hologram, a page composer converts the data in
the form of electric signals to optical signal the controller
generate the address to access the desired page. This results in the
exposure of a small area of the recording medium through an
aperture. The optical output signal is directed to the exposed area
by the deflector.
When the Blue-Argon laser is focused, a beam splitter splits it
into two, a reference beam and a signal beam. The signal beam
passes through spatial light modulators (SLM) where digital
information organized in a page like format of ones and zeros, is
modulated onto the signal beam as a two-dimensional pattern of
brightness and darkness. This signal beam is then purified using
different crystals. When the signal beam and reference beammeets the interference pattern created stores the data that is
carried by the Different data pages are recorded over the surface
depending on the angle at which the reference beam meet the
signal beam a holographic data storage system is fundamentally
page oriented with each block of data defined by the no. of data
bits that can be spatially impressed onto the object the total
storage capacity of the system is then equal to the product of the
paper size (in bits) and the no. of pages that can be recorded. .
The theoretical limits for the storage density of this technique is
approximately several tens of Terabytes (1 terabyte = 1024
gigabytes) per cubic centimeter. In 2006, In Phase Technologies
published a white paper reporting an achievement of 500 Gb/in2
From this figure we can deduce that a regular disk (with 4 cm
radius of writing area) could hold up to a maximum of 3895.6Gb

Spatial Light Modulator (Slm)
Spatial light modulator is used for creating binary information
out of laser light. The SLM is a 2-D plane, consisting of pixels,
which can be turned on and off to create 1â„¢s and 0â„¢s. An
illustration of this is a window and a window shade. It is possible
to pull the shade down over window to block incoming sunlight.
If sunlight is desired again, the shade can be raised. A spatial
light modulator contains a two dimensional array of windows
which are only microns wide. These windows block some parts
of the incoming laser light and let other parts go through. The
resulting cross section of the laser beam is a two dimensional
array of binary data, the same as what was represented in SLM.
After the laser beam is manipulated, it is sent into hologram to be
recorded. This data is written into the hologram as page form. It
is called this due to its representation in
Two dimensional plane or page of data. Holographic memory
reads data in the form of pages instead. For example, if a stream0f 32 bit is sent to a processing unit by a conventional read head,
a holographic memory system would in turn send 32*32 bits, or
1024 bits due to its added dimension this provides very fast
access times in volumes for greater than serial access methods.
The volume could be one Megabit per page using a SLM
resolution of 1024*1024 bits at 15-20 microns per sec

Reading Data
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


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