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Project Report
On
HOLOGRAPHIC
MASS STORAGE SYSTEM
Submitted To Submitted By
Mrs. Jyoti Kaushik Ankit Bansal
ECE Dept. 11082021MMEC ECE (D2)
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Acknowledgment
Firstly I would like to express my sincere gratitude to the Almighty for His solemn presencethroughout the seminar study. I would also like to express my special thanks to the HOD Mr.
Naveen Hemrajani for providing an opportunity to undertake this seminar. I am deeply indebted to
our seminar coordinators for providing me with valuable advice and guidance during the course of
the study.
I would like to extend my heartfelt gratitude to the Faculty of the Department of Computer
Science and Engineering for their constructive support and cooperation at each and every juncture
of the seminar study.
Finally I would like to express my gratitude to Gyan Vihar College of Engineering and
Technology for providing me with all the required facilities without which the seminar studywould not have been possible.
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Table of Contents
Table of contents1.Abstract..............................................................................................................................32.Introduction........................................................................................................................43.Technical Aspect...............................................................................................................54.Holograms..........................................................................................................................85.Underlying Technology.....................................................................................................106. Working.............................................................................................................................167. Application......................................................................................................................178. Advantages & Disadvantages of HDSS..........................................................................219. Comparison.....................................................................................................................2210.HVD at a glance..............................................................................................................2311..Bibliography.....................................................................................................................25
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1.Abstract
Holographic Versatile Disc (HVD) is an optical disc technology still in the research stage which
would greatly increase storage over Blue-ray Disc and HD DVD optical disc systems. It employs
a technique known as collinear holography, whereby two lasers, one red and one blue-green, arecollimated in a single beam. The blue-green laser reads data encoded as laser interference fringes
from a holographic layer near the top of the disc while the red laser is used as the reference beam
and to read servo information from a regular CD-style aluminium layer near the bottom. Servo
information is used to monitor the position of the read head over the disc, similar to the head,
track, and sector information on a conventional hard disk drive. On a CD or DVD this servo
information is interspersed amongst the data.
A dichroic minor layer between the holographic data and the servo data reflects the blue-green
laser while letting the red laser pass through. This prevents interference from refraction of the
blue-green laser off the servo data pits and is an advance over past holographic storage media,
which either experienced too much interference, or lacked the servo data entirely, making them
incompatible with current CD and DVD drive technology. These discs have the capacity to hold
up to 3.9 terabyte(TB) of information, which is approximately 6,000 times the capacity of a CD-
ROM, 830 times the capacity of a DVD, 160 times the capacity of single-layer Blu-ray Discs, and
about 8 times the capacity of standard computer hard drives as of 2006. The HVD also has a
transfer rate of 1 gigabit/s. Optware has released a 200 GB disc in early June 2006 and Maxell in
September 2006 with a capacity of 300 GB and transfer rate of 20 MB/s.
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2.Introduction
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, has been key enablers of this revolution.
While current storage needs are being met, storage technologies must continue to improve in orderto keep pace with the rapidly increasing demand.
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 CDscan 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. 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
mediumnot just on its surfaceoffers 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, significantresults from longstanding research efforts, and progress in holographic recording materials.
In holographic data storage, an entire page of information is stored at once as an optical
interference pattern within a thick, photosensitive optical material (Figure 1). This is done by
intersecting two coherent laser beams within the storage material. The first, called the object
beam, contains the information to be stored; the second, called the reference beam, is designed to
be simple to reproducefor example, a simple collimated beam with a planar wavefront. The
resulting optical interference pattern causes chemical and/or physical changes in the
photosensitive medium: A replica of the interference pattern is stored as a change in the
absorption, refractive index, or thickness of the photosensitive medium. When the stored
interference grating is illuminated with one of the two waves that were used during recording[Figure 2(a)], some of this incident light is diffracted by the stored grating in such a fashion that
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the other wave is reconstructed. Illuminating the stored grating with the reference wave
reconstructs the object wave, and vice versa [Figure 2(b)]. Interestingly, a backward-propagating
or phase-conjugate reference wave, illuminating the stored grating from the back side,
reconstructs an object wave that also propagates backward toward its original source [Figure 2(c)].
A large number of these interference gratings or patterns can be superimposed in the same thickpiece of media and can be accessed independently, as long as they are distinguishable by the
direction or the spacing of the gratings. Such separation can be accomplished by changing the
angle between the object and reference wave or by changing the laser wavelength. Any particular
data page can then be read out independently by illuminating the stored gratings with the reference
wave that was used to store that page. Because of the thickness of the hologram, this reference
wave is diffracted by the interference patterns in such a fashion that only the desired object beam
is significantly reconstructed and imaged on an electronic camera. The theoretical limits for the
storage density of this technique are around tens of terabits per cubic centimeter.
.
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3.Technical Aspects
Like other media, holographic media is divided into write once (where the storage medium
undergoes some irreversible change), and rewritable media (where the change is reversible).
Rewritable holographic storage can be achieved via the photorefractive effect in crystals:
Mutually coherent light from two sources creates an interference pattern in the media. Thesetwo sources are called the reference beam and the signal beam.
Where there is constructive interference the light is bright and electrons can be promoted fromthe valence band to the conduction band of the material (since the light has given the electrons
energy to jump the energy gap). The positively charged vacancies they leave are
called holes and they must be immobile in rewritable holographic materials. Where there is
destructive interference, there is less light and few electrons are promoted.
Electrons in the conduction band are free to move in the material. They will experience twoopposing forces that determine how they move. The first force is the Coulomb force between
the electrons and the positive holes that they have been promoted from. This force encourages
the electrons to stay put or move back to where they came from. The second is the pseudo-
force ofdiffusion that encourages them to move to areas where electrons are less dense. If the
coulomb forces are not too strong, the electrons will move into the dark areas.
Beginning immediately after being promoted, there is a chance that a given electron willrecombine with a hole and move back into the valence band. The faster the rate of
recombination, the fewer the number of electrons that will have the chance to move into thedark areas. This rate will affect the strength of the hologram.
After some electrons have moved into the dark areas and recombined with holes there, there isa permanent space charge field between the electrons that moved to the dark spots and the
holes in the bright spots. This leads to a change in the index of refraction due to the electro-
optic effect.
http://en.wikipedia.org/wiki/Coherenthttp://en.wikipedia.org/wiki/Interference_patternhttp://en.wikipedia.org/wiki/Reference_beamhttp://en.wikipedia.org/wiki/Signal_beamhttp://en.wikipedia.org/wiki/Interference_(wave_propagation)http://en.wikipedia.org/wiki/Electronshttp://en.wikipedia.org/wiki/Valence_bandhttp://en.wikipedia.org/wiki/Conduction_bandhttp://en.wikipedia.org/wiki/Electron_holehttp://en.wikipedia.org/wiki/Coulomb_forcehttp://en.wikipedia.org/wiki/Diffusionhttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Refractive_indexhttp://en.wikipedia.org/wiki/Electro-optic_effecthttp://en.wikipedia.org/wiki/Electro-optic_effecthttp://en.wikipedia.org/wiki/Electro-optic_effecthttp://en.wikipedia.org/wiki/Electro-optic_effecthttp://en.wikipedia.org/wiki/Refractive_indexhttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Diffusionhttp://en.wikipedia.org/wiki/Coulomb_forcehttp://en.wikipedia.org/wiki/Electron_holehttp://en.wikipedia.org/wiki/Conduction_bandhttp://en.wikipedia.org/wiki/Valence_bandhttp://en.wikipedia.org/wiki/Electronshttp://en.wikipedia.org/wiki/Interference_(wave_propagation)http://en.wikipedia.org/wiki/Signal_beamhttp://en.wikipedia.org/wiki/Reference_beamhttp://en.wikipedia.org/wiki/Interference_patternhttp://en.wikipedia.org/wiki/Coherent8/3/2019 29659592 Holographic Data Storage System Seminar Report
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When the information is to be retrieved or read out from the hologram, only the reference beam is
necessary. The beam is sent into the material in exactly the same way as when the hologram was
written. As a result of the index changes in the material that were created during writing, the beam
splits into two parts. One of these parts recreates the signal beam where the information is stored.
Something like a CCD camera can be used to convert this information into a more usable form.
Holograms can theoretically store one bit per cubic block the size of the wavelength of light in
writing. For example, light from a helium-neon laser is red, 632.8 nm wavelength light. Using
light of this wavelength, perfect holographic storage could store 4 gigabits per cubic millimeter. In
practice, the data density would be much lower, for at least four reasons:
The need to add error-correction The need to accommodate imperfections or limitations in the optical system Economic payoff (higher densities may cost disproportionately more to achieve) Design technique limitationsa problem currently faced in magnetic Hard Drives wherein
magnetic domain configuration prevents manufacture of disks that fully utilize the theoretical
limits of the technology.
Unlike current storage technologies that record and read one data bit at a time, holographic
memory writes and reads data in parallel in a single flash of light.
http://en.wikipedia.org/wiki/Hologramhttp://en.wikipedia.org/wiki/Charge-coupled_devicehttp://en.wikipedia.org/wiki/Bithttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Helium-neon_laserhttp://en.wikipedia.org/wiki/Nanometrehttp://en.wikipedia.org/wiki/Error-correctionhttp://en.wikipedia.org/wiki/File:Hologram_lezen.svghttp://en.wikipedia.org/wiki/Error-correctionhttp://en.wikipedia.org/wiki/Nanometrehttp://en.wikipedia.org/wiki/Helium-neon_laserhttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Bithttp://en.wikipedia.org/wiki/Charge-coupled_devicehttp://en.wikipedia.org/wiki/Hologram8/3/2019 29659592 Holographic Data Storage System Seminar Report
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HOLOGRAPHIC MEMORY LAYOUT
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4.Holograms
A hologram is a recording of the optical interference pattern that forms at the intersection of two
coherent optical beams. Typically, light from a single laser is split into two paths, the signal path
and the reference path. The beam that propagates along the signal path carries information,
whereas the reference is designed to be simple to reproduce. A common reference beam is aplanewave: a light beam that propagates without converging or diverging. The two paths are overlapped
on the holographic medium and the interference pattern between the two beams is recorded. A key
property of this interferometric recording is that when it is illuminated by a readout beam, the
signal beam is reproduced. In effect, some of the light is diffracted from the readout beam toreconstruct a weak copy of the signal beam. If the signal beam was created by reflecting light
off a 3D object, then the reconstructed hologram makes the 3D object appear behind the
holographic medium. When the hologram is recorded in a thin material, the readout beam can
differ from the reference beam used for recording and the scene will still appear.
4.1 Volume Holograms
To make the hologram, the reference and object beams are overlapped in a photosensitive
medium, such as a photopolymer or inorganic crystal. The resulting optical interference pattern
creates chemical and/or physical changes in the absorption, refractive index or thickness of the
storage media, preserving a replica of the illuminating interference pattern. Since this pattern
contains information about both the amplitude and the phase of the two light beams, when the
recording is illuminated by the readout beam, some of the light is diffracted to reconstruct a
weak copy of the object beam .If the object beam originally came from a 3D object, then the
reconstructed hologram makes the 3D object reappear. Since the diffracted wave front
accumulates energy from throughout the thickness of the storage material, a small change in eitherthe wavelength or angle of the readout beam generates enough destructive interference to make
the hologram effectively disappear through Bragg selectivity.
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As the material becomes thicker, accessing a stored volume hologram requires tight tolerances on
the stability and repeatability of the wavelength and incidence angle provided by the laser and
readout optics. However, destructive interference also opens up a tremendous opportunity: a small
storage volume can now store multiple superimposed holograms, each one distributed throughout
the entire volume. The destructive interference allows each of these stored holograms to be
independently accessed with its original reference beam. To record a second, angularly
multiplexed hologram, for instance, the angle of the reference beam is changed sufficiently so that
the reconstruction of the first hologram effectively disappears. The new incidence angle is used to
record a second hologram with a new object beam. The two holograms can be independently
accessed by changing the readout laser beam angle back and forth. For a 2-cm hologram
thickness, the angular sensitivity is only 0.0015 degrees. Therefore, it becomes possible to store
thousands of holograms within the allowable range of reference arm angles (typically 2030
degrees). The maximum number of holograms stored at a single location to date is 10,000.
Figure 4
Reconstruction of an image from a hologram
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5.Underlying Technology
5.1 HOLOGRAPHY
Holographic data storage refers specifically to the use of holography to store and retrieve digitaldata. To do this, digital data must be imposed onto an optical wave front, stored holographically
with high volumetric density, and then extracted from the retrieved optical wav front with
excellent data fidelity. A hologram preserves both the phase and amplitude of an optical wave
front of interest called the object beamby recording the optical interference pattern between it
and a second coherent optical beamthe reference beam. Figure shows this process.
The reference beam is designed to be simple to reproduce at a later stage (A common reference
beam is a plane wave a light beam that propagates without converging or diverging). These
interference fringes are recorded if the two beams have been overlapped within a suitable
photosensitive media, such as a photopolymer or inorganic crystal or photographic film. The
bright and dark variations of the interference pattern create chemical and/or physical changes in
the media, preserving a replica of the interference pattern as a change in absorption, refractiveindex or thickness.
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Though holography is often referred to as 3D photography, this is a misconception. A better
analogy is sound recording where the sound field is encoded in such a way that it can later be
reproduced. In holography, some of the light scattered from an object or a set of objects falls on
the recording medium. A second light beam, known as the reference beam, also illuminates the
recording medium, so that interference occurs between the two beams. The resulting light field is
an apparently random pattern of varying intensity which is the hologram. It can be shown that if
the hologram is illuminated by the original reference beam, a light field is diffracted by the
reference beam which is identical to the light field which was scattered by the object or objects.
Thus, someone looking into the hologram "sees" the objects even though they are no longer
present. There are a variety of recording materials which can be used, including photographic
film.
5.2 Interference and diffraction
Interference occurs when one or more wavefronts are superimposed. Diffraction occurs whenever
a wavefront encounters an object. The process of producing a holographic reconstruction is
explained below purely in terms of interference and diffraction. It is somewhat simplistic, but is
accurate enough to provide an understanding of how the holographic process works.
5.3 Plane wavefronts
A diffraction grating is a structure with a repeating pattern. A simple example is a metal plate with
slits cut at regular intervals. Light rays travelling through it are bent at an angle determined by ,
the wavelength of the light and d, the distance between the slits and is given by sin = /d.
A very simple hologram can be made by superimposing two plane waves from the same light
source. One (the reference beam) hits the photographic plate normally and the other one (the
object beam) hits the plate at an angle . The relative phase between the two beams varies across
the photographic plate as 2 y sin/ where y is the distance along the photographic plate. The two
beams interfere with one another to form an interference pattern. The relative phase changes by 2
at intervals of d = /sin so the spacing of the interference fringes is given by d. Thus, the relative
phase of object and reference beam is encoded as the maxima and minima of the fringe pattern.
When the photographic plate is developed, the fringe pattern acts as a diffraction grating and when
the reference beam is incident upon the photographic plate, it is partly diffracted into the same
angle at which the original object beam was incident. Thus, the object beam has been
reconstructed. The diffraction grating created by the two waves interfering has reconstructedthe
"object beam" and it is therefore a hologram as defined above.
http://en.wikipedia.org/wiki/Sound_recordinghttp://en.wikipedia.org/wiki/Interference_(wave_propagation)http://en.wikipedia.org/wiki/Diffractionhttp://en.wikipedia.org/wiki/Interference_(wave_propagation)http://en.wikipedia.org/wiki/Wavefrontshttp://en.wikipedia.org/wiki/Diffractionhttp://en.wikipedia.org/wiki/Diffraction_gratinghttp://en.wikipedia.org/wiki/Ray_(optics)http://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Plane_wavehttp://en.wikipedia.org/wiki/Phase_(waves)http://en.wikipedia.org/wiki/Interference_(wave_propagation)http://en.wikipedia.org/wiki/Waveshttp://en.wikipedia.org/wiki/Interference_patternhttp://en.wikipedia.org/wiki/Interference_patternhttp://en.wikipedia.org/wiki/Waveshttp://en.wikipedia.org/wiki/Interference_(wave_propagation)http://en.wikipedia.org/wiki/Phase_(waves)http://en.wikipedia.org/wiki/Plane_wavehttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Ray_(optics)http://en.wikipedia.org/wiki/Diffraction_gratinghttp://en.wikipedia.org/wiki/Diffractionhttp://en.wikipedia.org/wiki/Wavefrontshttp://en.wikipedia.org/wiki/Interference_(wave_propagation)http://en.wikipedia.org/wiki/Diffractionhttp://en.wikipedia.org/wiki/Interference_(wave_propagation)http://en.wikipedia.org/wiki/Sound_recording8/3/2019 29659592 Holographic Data Storage System Seminar Report
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5.4 Point sources
A slightly more complicated hologram can be made using a point source of light as object beamand a plane wave as reference beam to illuminate the photographic plate. An interference pattern
is formed which in this case is in the form of curves of decreasing separation with increasing
distance from the centre.
The photographic plate is developed giving a complicated pattern which can be considered to be
made up of a diffraction pattern of varying spacing. When the plate is illuminated by the reference
beam alone, it is diffracted by the grating into different angles which depend on the local spacing
of the pattern on the plate. It can be shown that the net effect of this is to reconstruct the object
beam, so that it appears that light is coming from a point source behind the plate, even when the
source has been removed. The light emerging from the photographic plate is identical to the light
that emerged from the point source that used to be there. An observer looking into the plate from
the other side will "see" a point source of light whether the original source of light is there or not.
This sort of hologram is effectively a concave lens, since it "converts" a plane wavefront into a
divergent wavefront. It will also increase the divergence of any wave which is incident on it in
exactly the same way as a normal lens does. Its focal length is the distance between the point
source and the plate.
http://en.wikipedia.org/wiki/Point_sourcehttp://en.wikipedia.org/wiki/Plane_wavehttp://en.wikipedia.org/wiki/Photographic_platehttp://en.wikipedia.org/wiki/Point_sourcehttp://en.wikipedia.org/wiki/Concave_lenshttp://en.wikipedia.org/wiki/Concave_lenshttp://en.wikipedia.org/wiki/Point_sourcehttp://en.wikipedia.org/wiki/Photographic_platehttp://en.wikipedia.org/wiki/Plane_wavehttp://en.wikipedia.org/wiki/Point_source8/3/2019 29659592 Holographic Data Storage System Seminar Report
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5.5 Complex objects
To record a hologram of a complex object, a laser beam is first split into two separate beams of
light using a beam splitter of half-silvered glass or a birefringent material. One beam illuminates
the object, reflecting its image onto the recording medium as it scatters the beam. The second
(reference) beam illuminates the recording medium directly.
According to diffraction theory, each point in the object acts as a point source of light. Each of
these point sources interferes with the reference beam, giving rise to an interference pattern. The
resulting pattern is the sum of a large number (strictly speaking, an infinite number) ofpoint
source + reference beam interference patterns.
When the object is no longer present, the holographic plate is illuminated by the reference beam.
Each point source diffraction grating will diffract part of the reference beam to reconstruct the
wavefront from its point source. These individual wavefronts add together to reconstruct the
whole of the object beam.
The viewer perceives a wavefront that is identical to the scattered wavefront of the object
illuminated by the reference beam, so that it appears to him or her that the object is still in place.
This image is known as a "virtual" image as it is generated even though the object is no longer
there. The direction of the light source seen illuminating the virtual image is that of the original
illuminating beam.
http://en.wikipedia.org/wiki/Beamsplitterhttp://en.wikipedia.org/wiki/Silveringhttp://en.wikipedia.org/wiki/Birefringenthttp://en.wikipedia.org/wiki/Diffractionhttp://en.wikipedia.org/wiki/Diffractionhttp://en.wikipedia.org/wiki/Birefringenthttp://en.wikipedia.org/wiki/Silveringhttp://en.wikipedia.org/wiki/Beamsplitter8/3/2019 29659592 Holographic Data Storage System Seminar Report
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This explains, albeit in somewhat simple terms, how transmission holograms work. Other
holograms, such as rainbow and Denisyuk holograms, are more complex but have similar
principles.
When the recording is illuminated by a readout beamsimilar to the original reference beam, some
of the light is diffracted to reconstruct a copy of the object beam as shown in Fig if the object
beam originally came from a 3-D object, then the reconstructed hologram makes the 3-D object
reappear.
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6.Working
A holographic data storage system consists of a recording medium, an optical recording system,
and a photodetector array. A beam of coherent light is split into a reference beam and a signal
beam which are used to record a hologram into the recording medium. The recording medium is
usually a photorefractive crystal such as LiNbO3 or BaTiO3 that has certain optical characteristics.These characteristics are high diffraction efficiency, high resolution, permanent storage until
erasure, and fast erasure on the application of external stimulus such as UV light. A hologram is
simply the three-dimensional interference pattern of the intersection of the reference and signal
beams at 90degree to each other. This interference pattern is imprinted into the crystal as regions
of positive and negative charge. To retrieve the stored hologram, a beam of light that has the same
wavelength and angle of incidence as the reference beam is sent into the crystal and the resulting
diffraction pattern is used to reconstruct the pattern of the signal beam.
Many different holograms may be stored in the same crystal volume by changing the angle of
incidence of the reference beam. One characteristic of the recording medium that limits the
usefulness of holographic storage is the property that every time the crystal is read with the
reference beam, the stored hologram at that location is disturbed by the reference beam and
some of the data integrity is lost. With current technology, recorded holograms in Fe- and Tb-
doped LiNbO3 that use UV light to activate the Tb atoms can be preserved without significant
decay for two years.
A series of spectral memory demonstration experiments have been conducted at the University of
Oregon. These experiments employ a 780-nm commercial semiconductor diode laser as the light
source, acrystal of Tm3+:YAG as the frequency selective recording material, and an avalanchephotodiode as a signal detector. The diode laser was stabilized to an external cavity containing a
grating and an electro optic crystal. The intracavity electro optic crystal provides for microsecond
timescale sweeping of the laser frequency over roughly one gigahertz. Two storage (reference and
data) beams and one reading beam, are created from the output of the single laser source using the
beam splitter and the acousto-optic modulators shown in figure. The beams are focused to a 150
m2
spot in a Tm3+:YAG crystal. The reference and data beams are simultaneous as are the read
and signal beams.
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The most common holographic recording system uses laser light, a beam splitter to divide the
laser light into a reference beam and a signal beam, various lenses and mirrors to redirect the light,
a photorefractive crystal, and an array of photodetectors around the crystal to receive the
holographic data. To record a hologram, a beam of laser light is split into two beams by a mirror.
These two beams then become the reference and the signal beams. The signal beam interacts withan object and the light that is reflected by the object intersects the reference beam at right angles.
The resulting interference pattern contains all the information necessary to recreate the image of
the object after suitable processing. The interference pattern is recorded onto the photoreactive
material and may be retrieved at a later time by using a beam that is identical to the reference
beam (including the wavelength and the angle of incidence into the photoreactive material). This
is possible because the hologram has the property that if it is illuminated by either of the beams
used to record it, the hologram causes light to be diffracted in the direction of the second beam
that was used to record it, thereby recreating the reflected image of the object if the reference
beam was used to illuminate the hologram. So, the reflected image must be transformed into a real
image with mirrors and lenses that can be sent to the laser detector array.
There are many different volume holographic techniques that are being researched. The most
promising techniques are angle-multiplexed, wavelength multiplexed, spectral, and phase
conjugate holography. Angle- and wavelength- multiplexed holographic methods are very similar,
with the only difference being the way data is stored and retrieved, either multiplexed with
different angles of incidence of the reference beam, or with different wavelengths of the reference
beam. Spectral holography combines the basic principles of volume holography using a
photorefractive crystal with a time sequencing scheme to partition holograms into their own
subvolume of the crystal using the collision of ultrashort laser pulses to differentiate between the
image and the time-delayed reference beam. Phase-conjugate holography is a technique to reducethe total volume of the system (the system includes recording devices, storage medium, and
detector array) by eliminating the need for the optical parts between the spatial light modulator
(SLM) and the detector.
The SLM is an optical device that is used to convert the real image into a single beam of light that
will intersect with the reference beam during recording. Phase-conjugate holography eliminates
these optical parts by replacing the reference beam that is used to read the hologram with a
conjugate reference beam that propagates in the opposite direction as the bam used for recording.
The signal diffracted by the hologram being accessed is sent back along the path from which itcame, and is refocused onto the SLM, which now serves as both the SLM and the detector.
There are two main classes of materials used for the holographic storage medium. These are
photorefractive crystals and photopolymers (organic films). The most commonly used
photorefractive crystals used are LiNbO3 and BaTiO3. During hologram recording, the refractive
index of the crystal is changed by migration of electron charge in response to the imprinted three
dimensional interference patterns of reference and signal beams. As more and more holograms are
superimposed into the crystal, the more decay of the holograms occurs due to interference from
the superimposed holograms. Also, holograms are degraded every time they are read out because
the reference beam used to read out the hologram alters the refractive nature of the crystal in that
region.
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7.Application
7.1 Holographic Versatile Disc
An HVD (holographic Versatile Disc), a holographic storage media, is an advanced optical discthats presently in the development stage. Polaroid scientist J. van Heerden was the first to come
up with the idea for holographic three-dimensional storage media in 1960. An HVD would be a
successor to todays Blu-ray and HDDVD technologies. It can transfer data at the rate of 1 Gigabit
per second. The technology permits over 10 kilobits of data to be written and read in parallel with
a single flash. The disc will store upto 3.9 terabyte (TB) of data on a single optical disk.
Holographic data storage, a potential next generation storage technology, offers both high storage
density and fast readout rate. In this article, I discuss the physical origin of these attractive
technology features and the components and engineering required to realize them. I conclude by
describing the current state of holographic storage research and development efforts in the context
of ongoing improvement to established storage technologies.
7.2 FEATURES
Data transfer rate: 1 gbps.The technology permits over 10 kilobits of data to be written and read in parallel with a
single flash.
Most optical storage devices, such as a standard CD saves one bit per pulse.HVDs manage to store 60,000 bits per pulse in the same place.1 HVD5800 CDs830 DVD160 BLU-RAY Discs.
7.3 STRUCTURE
HVD STRUCTURE
HVD structure is shown in fig 3.1 the following components are used in HVD.
1. Green writing/reading laser (650 nm).
2. Red positioning/addressing laser (650 nm).
3. Hologram (data).4. Polycarbon layer.
5. Photopolymeric layer (data-containing layer).
6. Distance layers.
7. Dichroic layer (reflecting green light).
8. Aluminum reflective layer (reflecting red light).
9. Transparent base.
10. PIT.
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HVD STRUCTURE
7.4 HVD READER PROTOTYPE
To read data from an HVD reader. The following components are used to make a reader.
A blue-green laser, beam splitters to split the laser beams, mirrors to direct the laser beams, LCD
panels (spatial light modulator), lenses to focus the beams, lithiumniobate crystals or
photopolymers, and charge-coupled device (CCD) cameras.
HVD READER PROTOTYPE
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RECORDING DATA
A simplified HVD system consists of the following main components:Blue or green laser (532-nm wavelength in the test system)Beam splitter/mergerMirrorsSpatial light modulator (SLM)CMOS sensorPolymer recording medium
The process of writing information onto an HVD begins with encoding the information into binary
data to be stored in the SLM. These data are turned into ones and zeroes represented as opaque or
translucent areas on a "page" -- this page is the image that the information beam is going to pass
through.
When the blue-green argon laser is fired, a beam splitter creates two beams. One beam, called the
object or signal beam, will go straight, bounce off one mirror and travel through a spatial-light
modulator (SLM). An SLM is a liquid crystal display (LCD) that shows pages of raw binary data
as clear and dark boxes.
The information from the page of binary code is carried by the signal beam around to the light-
sensitive lithium-niobate crystal. Some systems use a photopolymer in place of the crystal.
A second beam, called the reference beam, shoots out the side of the beam splitter and takes a
separate path to the crystal. When the two beams meet, the interference pattern that is createdstores the data carried by the signal beam in a specific area in the crystal -- the data is stored as a
hologram.
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8.ADVANTAGES and Disadvantages Of HDSS
8.1 Advantages Of HDSS
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).
Holographic data storage has the unique ability to locate similar features stored within a crystal
instantly. A data pattern projected into a crystal from the top searches thousands of stored
holograms in parallel. The holograms diffract the incoming light out of the side of the crystal, with
the brightest outgoing beams identifying the address of the data that most closely resemble the
input pattern. This parallel search capability is an inherent property of holographic data storage and
allows a database to be searched by content.
Because the interference patter ns are spread uniformly throughout the material, it endows
holographic storage with another useful capability: high reliability. While a defect in the medium
for disk or tape storage might garble critical data, a defect in a holographic medium doesn't wipe
out information. Instead, it only makes the hologram dimmer. No rotation of medium is required as
in the case of other storage devices. It can reduce threat of piracy since holograms cant be easily
replicated.
8.2 DISADVANTAGES OF HDSS
Manufacturing cost HDSS is very high and there is a lack of availability of resources which areneeded to produce HDSS. However, all the holograms appear dimmer because their patterns must
share the material's finite dynamic range. In other words, the additional holograms alter a material
that can support only a fixed amount of change. Ultimately, the images become so dim that noise
creeps into the read-out operation, thus limiting the material's storage capacity.
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. In the past, this has limited the number of reads that can be made before the signal-to -
noise ratio becomes too low. Moreover, writes 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 aphotorefractive for recording angle-multiplexed holograms. You would be unable to locate the data
if theres an error of even a thousandth of an inch.
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9 .COMPARISON
9.1 Comparison between DVD, Blue-Ray and HVD
Parameters DVD BLU-RAY HVD
Capacity 4.7 GB 25 GB 3.9 TB
Laser wave length650 nm
(red)
405 nm
(blue)
532 nm
(green)
Disc diameter 120 mm 120 mm 120 mm
Hard coating No yes Yes
Data transfer rate
(rawdata)
11.08 mbps36 mbps 1 gbps
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10.HVD AT A GLANCE
Media type : Ultra-high density optical disc.
Encoding : MPEG-2, MPEG-4 AVC (H.264), and VC-1.
Capacity : Theoretically up to 3.9 TB.
Usage : Data storage, High-definition video, & he possibility of ultra High-definition
video.
STANDARDS
On December 9, 2004 at its 88th General Assembly the standards body Ecma International created
Technical committee 44, dedicated to standardizing HVD formats based on Optwares technology.
On June 11, 2007, TC44 published the first two HVD standards ECMA-377, defining a 200 GB
HVD recordable cartridge and ECMA-378,defining a 100 GB HVD-ROM disc. Its next stated
goals are 30 GB HVD cards and submission of these standards to the International Organization
for Standardization for ISO approval.
POSSIBLE APPLICATION FIELDS
There are many possible applications of holographic memory. Holographic memory systems can
potentially provide the high speed transfers and large volumes of future computer system. One
possible application is data mining.
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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 cant be distinguished by human eyes
due to the vast amount of data. Some current computer system implement data mining, but the
mass amount of storage required is pushing the limits of current data storage systems. The many
advances in access times and data storage capacity that holographic memory provides could exceed
conventional storage and speedup data mining considerably. This would result in more locatedpatterns in a shorter amount of time.
Another possible application of holographic memory is inpetaflop computing. A petaflop is athousand trillion floating point operations per second. The fast access extremely large amounts of
data provided by holographic memory could be utilized in petaflop architecture. Clearly advances
are needed to in more than memory systems, but the theoretical schematics do exist for such a
machine. Optical storage such as holographic memory provides a viable solution to the extreme
amount of data which is required for a petaflop computing.
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11. Bibliography
www.holopc.com www.wikeipedia.com www.engeeniringseminars.com www.computer.howstuffworks.com www.tech-faq.com/hvd.shtml www.ibm.com - IBM Research Press Resources Holographic Storage