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Holographic memory is a technology that can store
information at high density inside crystals or
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
Unlike magnetic storage mechanisms which store data on
their surfaces, holographic memories store information
throughout their whole volume. After a page of data is
>>Devices that use light to store and read data has always become the
backbone of our data storage system for nearly two decades. Each
time you access towards the large repository of digital information
the harddisc the mainstay of personal and digital information has
always Obeyed the law.
>> The rate of advancement in storage technology has been truly
amazing. From the first instance of digital storage with IBM’s 5 MB
RAMAC hard disk drive invented in 1956, all the way to the mammoth
180 GB hard disk drives available today, there has been a 36,000-fold
increase in storage capacities over the last 50-odd years. However, we
are fast approaching the physical limit for storing information on media
such as the magnetic platters of hard disks or the chemical layers in
optical devices such as CDs and DVDs.
>> With the promise of tomorrow's operating systems incorporating
stunning graphical interfaces that offer truly immersive virtual reality
and next generation games that will blur the line between fiction and
reality, the demands of being able to quickly store and retrieve
enormous quantities of data are ever increasing.
BASIC PRINCIPLE OF HDSS
>>A hologram is a block or sheet of photosensitive material which records the
diffraction 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 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 and 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. For example a
hologram that can be obtained from a toy store illustrates this idea. Precise
laser equipment is used at the factory to create the hologram..
BASIC COMPONENTS OF HDSS
>> BLUE-GREEN ARGON LASER
>>POLARIZING BEAM SPLITTERS
>>MIRRORS TO DIRECT THE LASER BEAMS
>>SPATIAL LIGHT MODULATOR (SLM)
>> LENSES TO FOCUS THE LASER BEAMS
>> PHOTO POLYMER
>>CHARGE-COUPLED DEVICE (CCD )
In a holographic memory device, a laser beam is split in two,
and the two resulting beams interact in a crystal medium to
store a holographic recreation of a page of data.
WORKING OF HDSS
RECORDING DATA ON MEDIUM
>>Light from a single laser beam is split into two beams, the signal
beam (which carries the data) and the reference beam. The
hologram is formed where these two beams intersect in the
recording medium. The object beam, gets expanded so that it
fully illuminates a spatial light modulator (SLM)
>>The object beam finally interacts with the reference beam inside
a photosensitive crystal causing a holograph to be stored
READING DATA FROM HOLOGRAM
>>When reading out the data, the reference beam has to hit the crystal
at the same angle that's used in recording the page. To read out the
data, the reference beam again illuminates the crystal. The stored
interference pattern diffracts the reference beam's light so that it
reconstructs the checkerboard image of the light or dark pixels.
>> The image is directed upon a charge-coupled device (CCD) sensor
array that reads the data in parallel, and it instantly captures the
entire digital page. The binary information can now be read from
this CCD and the originally stored data is retrieved. This parallel
read out of data provides holography with its fast data transfer rates
>>The method of storing multiple pages of data in the
hologram is called multiplexing.
> PERISTROPHIC MULTIPLEXING
> SHIFT MULTIPLEXING
PHASE ENCODED MULTIPLEXING
>>When a laser beam is split up (for example, through a SLM), the generated
light bleeds into places where light was meant to be blocked out. Areas where
zero light is desired might have minuscule amounts of laser light present
which mutates its bit representation. For example, if too much light gets
recorded into this zero area representing a binary 0, an erroneous change to a
binary 1 might occur.
PAGE-LEVEL PARITY BITS
>>When a page of data is written to the holographic media, the page is
separated into smaller two dimensional arrays. These subsections are
appended with an additional row and column of bits. The added bits calculate
the parity of each row and column of data. An odd number of bits in a row or
column create a parity bit of 1 and an even number of bits create a 0 .
>>Smart interfacing is a method of controlling the way data
is sent to the processor from the holographic memory by
a predefined set of logical commands.
>>intelligent interfacing is different in one important way.
Intelligent interfacing has external control signals which
can be manipulated to transform incoming data in a non-
>>Photorefractive crystals are suitable for random
access memory with periodic refreshing of data
and can be erased and written many times.
Photo polymers have been developed that can
also be used as a holographic storage medium.
>>An example of photo polymer is Dupont’s HRF-
150. This film can achieve 12bits/µm2 with a
100µm thickness, which is grater than DVD-ROM
by a factor of two.
>>Stored holograms are permanent and do not
degrade over time or by read out of the
hologram, so photopolymers are suited for ROM.
ADVANTAGES OF HDDS
>> With three-dimensional recording and
parallel data readout, holographic memories
can outperform existing optical storage
techniques. In contrast to the currently
available storage strategies, holographic
mass memory simultaneously offers high
data capacity and short data access time
(Storage capacity of about 1TB/cc and data
transfer rate of 1 billion bits/second).
>> While a defect in the medium for disk or
tape storage might garble critical data, a
defect in a holographic medium doesn't wipe
DISADVANTAGES OF HDDS
>> Manufacturing cost HDSS is very high and there is a
lack of availability of resources which are needed to
>> A difficulty with the HDSS technology had been the
destructive readout. The re-illuminated reference
beam used to retrieve the recorded information, also
excites the donor electrons and disturbs the
equilibrium of the space charge field in a manner
that produces a gradual erasure of the recording.
>> You would be unable to locate the data if there’s an
error of even a thousandth of an inch.
>> Holographic memory systems can potentially provide the high
speed transfers and large volumes of future computer system.
>>One possible application is data mining. Data mining is the
processes of finding patterns in large amounts of data. Data
mining is used greatly in large databases which hold possible
patterns which can’t be distinguished by human eyes due to
the vast amount of data.
>> The many advances in access times and data storage capacity
that holographic memory provides could exceed conventional
storage and speedup data mining considerably
>> Another possible application of holographic memory is in
Holographic Versatile Disc (HVD) is
an optical disc technology still in the
research stage which would hold up
to 3.9 terabytes (TB) of information
The Holographic Versatile Card (HVC)
is a data storage format by Optware;
the projected date for a Japanese
launch is 1st half 2007
Holographic Versatile Disc structure
1. Green writing/reading laser (532 nm)
2. Red positioning/addressing laser (650 nm)
3. Hologram (data)
4. Polycarbon layer
5. Photopolymeric layer (data-containing layer)
6. Distance layers Picture of a HVD by Optware
7. Dichroic layer (reflecting green light)
8. Aluminium reflective layer (reflecting red light)
9. Transparent base
>> The future of HOLOGRAPHIC DATA STORAGE
SYSYEM is very promising. The page access of data
that HDSS creates will provide a window into next
generation computing by adding another dimension
to stored data
>> It will most likely be used in next generation
supercomputers where cost is not as much of an
>> However, many advances in optical technology and
photosensitive materials need to be made before we
find holograms in our computer systems