Download Helography memory

BHARATHIYAR COLLEGE OF ENGINEERING & TECHNOLOGY
KARAIKAL – 609609 (U.T OF PUDUCHERRY)
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
A Seminar on
Holographic Memory
Submitted by:
Name
:
Reg no
:
Year
: 2021 - 2022
Under the Guidance of : Mr.P.SHANKARANARAYANAN
ACKNOWLEDGEMENT
I express my sense of gratitude to my seminar guide Prof. Shankar Narayanan sir, EC
Department, Bharathiyar college of engg & Technology, Pondicherry University, for all of
his guidance, encouragement and patience which enable me to carry out my seminar work.
He inspired me with his original and innovative ideas and provided his technical guidance in
the seminar.
Appreciation is expressed for all those who directly or indirectly helped me inthe course of
my seminar work. I am also thankful to Bharathiyar college of engg & Technology
Management for providing me an opportunity.
T.Hariharan
(18TC0001)
BTech (ECE)
ABSTRACT
In data storing technologies, high data density per volume and high data transfer speed, both
are highly demanded. There are two types of data storing devices that are magnetic and
optical. Now a day optical devices are more preferable as they support more data transfer
speed.
A Holographic Memory is an optical data storing technology which gives these both features
that are high data density per volume and high data transfer speed. In this report it is
explained that how it is getting these much data density and high data transfer speed by using
components like spatial light modulator (SLM), multiplexing agent, lenses and charge
coupled device (CCD).
Holographic Versatile Disc (HVD) is an example of holographic memory. In this entire
volume of disc is used to store data as interference pattern of signal beam and reference
beam is to be stored which increases data density per volume.
INDEX
SI No.
1
2
3
4
5
6
7
8
9
10
Title
PageNo.
Acknowledgement
-
Abstract
-
Index
-
List of Figures
-
List of Tables
-
Introduction
1
1.1
Brief History
2
1.2
What is Holography?
2
1.3
Recording and Reading of Hologram
2
1.4
Holography Vs. Photography
3
1.5
Applications of Holography
4
Holographic Memory
5
2.1
Roadmap of Memory
5
2.2
Recording to HVD
6
2.3
Reading from HVD
7
Holographic Versatile Disc (HVD) Components
8
3.1
Spatial Light Modulator (SLM)
8
3.2
Multiplexing Agent
9
3.3
Storage Medium
10
3.4
Charge Coupled Device (CCD)
10
Advanced HVD
11
4.1
Comparison
11
4.2
Advantages and Drawbacks
11
4.3
Potential Competitors
12
4.4
Companies working on Holographic Memory
12
HOLOGRAPHIC MEMORY vs. EXISTING MEMORY TECHNOLOGY
HOLOGRAPHIC MEMORY
MOLECULAR MEMORY
COLLINEAR SYSTEM
COLLINEARHOLOGRAPHIC MEDIA EVALUATION SYSTEM S-VRD
CONCLUSION
TM
13
14
19
24
25
26
LIST OF FIGURES
Fig. No.
Fig. Name
Page No
1
Recording of Hologram
3
2
Reading of Hologram
3
3
Recording of data to HVD
6
4
Reading data from HVD
7
5
Spatial Light Modulator
9
6
Common CCD Image Sensor Formats (Dimensions in Millimeters)
10
LIST OF TABLES
Table No.
Table Name
Page No.
1
Roadmap of Memory
5
2
Comparison of HVD with other discs
11
1. INTRODUCTION
We know that data storing capacity of processors and buses roughly double every three years,
data storage has struggled to close the gap. CPU is shaving capacity of executing instructions
at every nanosecond. Single magnetic disk is required almost six times more time than this
CPU execution speed. There are so many research been done to lose this gap between CPUs
and data storage. Pipelining, cache, optimizing compilers and RAM are some advances of
these type.
For almost two decades for data storing devices are used that use light for storing and reading
data. In the early 1980’s revolution happens in data storing field as a Compact Disc (CD)
which is having 12 centimetres of diameter and 1.2 millimetres of thickness. These CDs
allows multimegabit of data to be stored on disc. Digital Versatile Disc (DVD) is an
improved version of CD which was invented in 1997. The full-length movie can be stored a
single DVD.
At initial level CDs and DVDs are famous to store data like music, personal computing,
software and video. A CD is having capacity of data storing up to 783 megabytes, which is
almost equal to one hour and 15 minutes of music. DVDs which are double-sided, doublelayer having data storing capacity of 15.9 GB, which is approximately 8 hours of movie.
Today’s needs are met by these conventional storage mediums, but consumer demands also
increases so companies working on storage technologies have to make changes to keep pace.
In these all technologies data is stored bit by bit on a recording medium.
However, technology is either optical or magnetic, data is combination of bits and each bit is
stored as distinct changes of magnetic or optical recording medium on its surface. This
technique of storing data bit by bit is approaching to physical limits because bit is a very
small part of whole data. If data can be stored on entire volume of storing material, it will
increase high density of data storing.
With a purpose of increasing storage capabilities, researchers and scientists are now majorly
working on a new optical storage technique, called Holographic Memory. Holographic
memory is technology which uses entire volume of storing material to store data instead of
only the surface area. So, this three dimensional data storing technique will increase data
density per volume and also offers faster data transfer rate. Actually, volumetric approach is
used for holographic memory which was introduced almost a decade ago.
Holographic memory is having capacity storing data up to 1 Terabyte in a single sugar cube
sized crystal. The main advantage of holographic memory is that it is storing data page by
page not bit by bit and also same for retrieving. Holographic memory system’s capacity is
almost equal to 1,000 CDs.
1|Pa ge
1.1 Brief History
In 1971, the noble prize in physics was given to Dennis Gabor for his invention and
development of the holographic method. The optical holography highly depending on laser,
so more research and invention in this was started after development of laser in 1960. At
initial stage in 1962 after invention of laser the first optical hologram was recorded at the
University of Michigan, USA. Before that for recording holograms silver halide photographic
emulsions was used as a recording medium.
1.2 What is Holography?
The word “Holography” comes from Greek words which are having meaning of “whole” and
“Drawing”. In normal photography we store only intensity level of image but in holography
with intensity we also store phase for image while construction an image. So while
reconstructing of an image we can get 3D view of image. But here one thing is to be noticed
that we can get feel of 3D image only in horizontal direction that means if we either move our
eyes or an image horizontally then only we can feel 3D image. If we move our head in
vertical direction then it will image will be looked like rainbow i.e. all seven colors.
1.3 Recording and Reading of Hologram
As we discussed earlier that in Holography, information is stored as page by page. At one
instance entire page is to be stored by an optical interference pattern within a thick,
photosensitive optical material as shown in fig.1. This pattern will be generated by
intersection of two laser beams which are coherent which are object beam and reference
beam within the storage material. Here object beam is having data what to be stored and
reference beam is for perfect generation of hologram at reading. The photosensitive storing
medium’s chemical and physical properties will be changed according to the interference
pattern.
According to the interference pattern, the changes occur in properties of the photosensitive
medium like the refractive index, absorption or thickness. Either object beam or reference
beam is illuminated on the interference pattern and from that we can get either reconstructed
reference beam or reconstructed object beam which is shown in fig. 2. Here one thing is to be
noticed that at time of reconstruction position, angle and wavelength of laser beam should be
same as time of construction, otherwise there may be possibility that we will get a different
3D pattern or say some portion of pattern which is unexpected. So, these requirements make
holography very complex and costly because proper arrangements of all components are very
time worthy.
2|Pa ge
Fig. 2 Reading of Hologram[1]
Fig. 1 Recording of Hologram[1]
1.4 Holography Vs. Photography




In a normal photography information is only from one direction whereas in
holography information is from different direction and due to that only it generates a
3D view of image so from different direction observer can observe it.
A photograph may be recorded using normal light sources like sunlight or electric
lighting whereas a hologram is recorded using laser lights.
To store a normal photograph a lens is required whereas to store a hologram storing
medium required on which interference pattern will be stored by scattering a light
from object.
In normal photography only light from object is required whereas in holography
object bema as well as reference beam is also required.
3|Pa ge


An observer or viewer can observe photograph in any light condition whereas
hologram can be observed only in very specific illumination’s form.
If a photograph is cut from half, each piece will show half image whereas if hologram
is cut from half, each piece will show full hologram. This happens because in normal
photography each point related with specific point of scene or image only whereas in
holography each point is having information of all points of scene or image because in
this laser is scattered. So it is like window either you see from 4 ft * 4 ft window or 8
ft * 8 ft window, you can see the same scene but range of horizontal and vertical
visibility changes that you have move your head more to see same scene from 4 ft
window compare to 8 ft window.
1.5 Applications of Holography

Holographic Interferometry
It is a one of the technique to identify static and dynamic change of object’s position.
In which two interference patterns of object are again interfaced and gives another
interference pattern which indicates the displacement of object.

Security
Holograms are very useful in security and it is already used by many countries with a
purpose of security on their currency notes. It is also used by banks for security of
their credit cards. Now a days passports are also having holograms on them and with
that sports equipment, books, ID cards are also having holograms on them.

Dynamic Holography
Holography is of two types that are static and dynamic. Till now we have seen only
static holography where hologram is constructed, stored and reconstructed. But in
dynamic holography some type of materials are used that is having capacity of
recording hologram very quickly and instantly we can reconstruct it using proper
arrangements. These can be used in image processing to recognize pattern images
which are time varying.

Data Storage
Holography is having capacity of storing different data using entire volume of storing
material like crystals or photopolymers. Now a day researchers are majorly working
in the field of minimizing size of data storage devices which means they want to
increase data density within specific volume of material.
So, now we will focus more on holography’s application of data storing which is having
special features of high volume density and high data transfer speed.
4|Pa ge
2. HOLOGRAPHIC MEMORY
There are many ways of storing data like optical memory, flash memory, magnetic memory
etc. But next will be Holographic Memory. Holographic memory is a technology that allows
holograms containing millions of bits of data to be written or read in a single flash of light.
Thousands of overlapping holograms can be stored in a common volume of recording
medium which increases storage capacity per volume. Holography breaks through the density
limits of conventional 2D storage by recording throughout the full depth of a 3D medium.
Holography can write and read millions of bits of data in parallel, enabling significantly
higher data transfer rates than current optical storage devices which gives high data transfer
rate.
2.1 Roadmap of Memory
Initially in 1970s to store data LASER discs were used which is having capacity of storing 30
minutes or 60 minutes per side in size if 30 cm. After that they were replaced by Compact
Discs and Magneto Optical Discs which are having different storage capacity. The 12 cm
sized CD can store 700 MB data while 8 cm CD can store 200 MB of data. The 13 cm sized
Magneto optical disc can store from 650 MB to 9.2 GB. In 1990s Mini discs were invented
which is having storing capacity of 650 MB at initial stage. At present it is having capacity of
1 GB data storing. These all were then replaced by Digital Versatile Discs (DVDs) which is
having storage capacity up to 8.5 GB. In these CDs data were written only on single layer
which restrict storing capacity so BLURAY discs were invented in 2000s. BLURAY discs
are having layers. The single layer disc can store data of 25 GB while dual layer can store
data of 50 GB. This also restricts data storing capacity because still data is to be stored on
surface of disc. Whole volume of disc is not utilized to store data. So, solution for that is
Holographic Versatile Disc (HVD) in which to store data entire volume is to be utilized
which increases data storing capacity in small volume.
Year
1978
1982
1985
1992
1995
2006
The Future
TABLE 1 Roadmap of Memory
Memory Type
LASER Disc
Compact Disc
Magneto Optical Disc
Mini Disc
Digital Versatile Disc (DVD)
Blu-Ray Disc
Holographic Versatile Disc (HVD)
5|Pa ge
2.2 Recording to HVD
Till now we have seen how to store hologram of an image. But, now we are talking about
holographic memory which mean there are much more then only an image. Holographic
memory is optical type of memory which means we are using lasers in a procedure of storing
data. So we are having information in a form a LASER which will be split in to two laser
beams by lens. These two beams are signal beam and reference beam. Signal beam is having
information to be stored. Now this signal beam pas through the spatial light modulator (SLM)
which converts this beam into pixels of black and white as SLM is a 1024*1024 pixels square
where each pixel will either allow or stop the laser. On basis of this pixel charge coupled
device (CCD) generates a string of 1’s and 0’s and that entire page is stored on hologram
with a help of reference beam. On basis of data physical and/or chemical characteristics of
storing material will change. This all procedure of data recording is shown in fig. 3.
Fig. 3 Recording of data to HVD[3]
6|Pa ge
2.3 Reading from HVD
Now data reading is also following the same method used for recording. As shown in fig 4
reference beam is used for reading data from storing material. Once again these data passes
through CCD for conversion and Detector will detect that data.
Fig. 4 Reading data from HVD[3]
Till now we have seen all basic concepts of Holography, how hologram is constructed and
reconstructed. With help of this how data can be stored to HVD and can be read from HVD.
Now, In next chapter we will see each component of HVD in detail and structure of HVD..
7|Pa ge
3. HOLOGRAPHIC VERSATILE DISC (HVD) COMPONENTS
An HVD is hot topic for researchers at present as it can support more data density per volume
and with that high data transfer speed which are highly required for any storage device. So
there is possibility that in a few years all CDs and DVDs will be replaced by HVDs. Now a
day two sided blu-ray discs are becoming famous because it can store data on more than on
surface i.e. it is having multi-layer structure so data storing capacity increases with same area.
HVD is having capacity of transferring data at speed of 1 Gigabit per second. Till now
researchers have reached up to level that DVD’s same sized HVD can store data up to 1
terabyte of data and they are claiming that they will able to store 1 terabyte of data in just one
cube sugar size.
We can say that HVD is having 60 times more capacity of storing data than DVD and can
support 10 times more data transfer speed than DVD. So, we can say that 1 HVD is
equivalent to 830 DVDs and 160 Bluray discs in comparison of data storing capacity. With a
purpose of increasing capacity of data storing, holographic memory uses laser beams for
storing data in whole volume of HVD. The HVD process uses a blue green laser beam, used
for reading and writing data, collimated (made parallel) with a red laser beam, which is used
for servo and tracking. HVD uses the concept of holographic memory.
For writing to and reading data from HVD, proper setup should be made. For that main
components are required that are
 Spatial Light Modulator (SLM)
 Multiplexing Agent
 Storage Medium
 Charge Coupled Device (CCD)
3.1 Spatial Light Modulator (SLM)
An SLM - Spatial Light Modulator is used to convert electrical or optical signal into image. It
can create 1000 of images in a second which gives an entire system high speed. It is used in
many various applications like optical devices like televisions, projectors. Normally, the
medium used in SLM is liquid crystal. It is also used as variable electro-optic mirror which is
used in optical systems for beam splitting, shutters and mirrors. SLM is having feature of
working at different frequencies. To analyse the image, SLM can be used as it gives output as
pixel by pixel of an image. SLM is having capacity of precisely controlling light which is
very much required for holographic memory and optical computing as light is just modified
in pixels.
Basically, SLM is having two types of addressing modes. Addressing mode is depended on
the type of signal.
 Optically-addressed
From name it is indicating that when SLM is having optical input so it is optically8|Pa ge
addressed mode.

Electrically-addressed
From name it is indicating that when SLM is having electrical input so it is
electrically-addressed mode.
As we have discussed earlier to use holography as a storage technology, digital data must be
in a form of a laser as an object beam while recording and reading. A device, having this type
of feature is SLM. SLM is nothing but made up of pixels and each pixel is an independent
microscopic shutter which is having capacity of either pass or block light.
Fig. 5 Spatial Light Modulator (SLM)[2]
As we know that holographic memory is having data transfer speed of Gbit per second. So, if
we want to use SLM as one of the component of HVD, it should also have ability t transfer
data at that rate and yes it is can refresh itself 1000 times per second.
3.2 Multiplexing Agent
As we have discussed earlier that on basis of interference pattern storing material properties
will be changed and here by changing angle, position or wavelength we can create different
holograms so for that multiplexing agents are used.
 Angular Multiplexing
In this type of multiplexing, angle of reference beam is changed to generate different
interference pattern and same angle is used at time of reading data. Here it should be
noticed that a very small change in angle will generate different data or say wrong
data. So, on basis of sensitivity of storing material angle is changed while recording
of data for multiplexing.
9|Pa ge


Wavelength Multiplexing
This type of multiplexing is used majorly when conjunction is there with other
multiplexing methods. In this different wavelength’s lasers are used while recording
the data so that on same medium more than one pattern can be stored.
Spatial Multiplexing
In this type of multiplexing, position of point source is changed so that from same
two sources different interference pattern will be generated. But in this more
mechanical work is required in comparison of other two multiplexing.
3.3 Storage Medium
As we have discussed earlier that on basis of interference pattern physical and/or chemical
properties of storing medium is changed. There are mainly two storage mediums are used that
are
1. Lithium-niobate crystals
2. Photopolymers
3.4 Charge Coupled Device (CCD)
CCD is nothing but an array of sensors which gives responses on basis of pixels of SLM. It is
mainly used to read interference pattern at time of recording and reading of data. It is able to
read 1 Mb of data at one instance. In general CCDs are of one square centimeter and having
typical access rate of 1000 frames per seconds or say 1 Gigabit per second.
Fig. 6 Common CCD Image Sensor Formats (Dimensions in Millimeters) [7]
For CCD there are two types of resolution which are
 Temporal Resolution
CCD is having specific resolution but it is with specific speed. If image rate per
second higher than specific rate then resolution of CCD will decrease. As we all
know that in our mobile phones or cameras images are having more resolution than
videos. It is because of this problem only that is for video image rate per second will
increases which will affect the resolution.
 Spatial Resolution

For same resolution at high speed scan and slow speed scan, there should be spatial resolution for
10 | P a g e
CCD so that we can get same resolution image and video.
4. ADVANCED HVD
4.1 Comparison
TABLE 2 Comparison of HVD with other Discs
DVD
BLU-RAY
HVD
Capacity
9.4 GB
50GB
Up to 3.9 TB
Laser wave length
650 nm
405 nm
407 nm
Disc diameter
120 mm
120 mm
120 mm
Read/write speed
11.08 Mbps
36.5Mbps
1
Gbps
4.2 Advantages and Drawbacks
Advantages




High Data Transfer Speed
As we have discussed that holographic memory is giving data transfer speed up o 1
Gbps which is highest till date for any storage device.
High Data Density
Holographic memory can store data up to 3.9 TB in in 120 mm diameter sized disc
and researchers are claiming that they will reach up to 1 TB data in just 1 inch sized
sugar cube.
Data Stability and Reliability
One survey says that within a three year of period, 26.5% of seagate’s drive failed,
5.2% of WD’s (Western Digital) drives failed and 3.1% of Hitachi’s drives failed
whereas scientists of holographic memory are claiming that Holographic memory is
reliable till 50 years which is very higher than these hard drives.
Data Security
Holographic memory is of WORM (Write Once Read Many) type of memory so
once data is written then nobody can edit it which gives high data security.
Drawbacks

Not guaranteed market leader
Holographic memory is offering all required features but still it is not that much
popular because recording and reading of data is very hard. If any other storing
technique will come in market then it is very hard to find components to record and
11 | P a g e
read data form HVD.

Expensive Development
As we have discussed for recording and reading data, whole setup should be very much
perfect. If there is any small displacement in any component, it will cause major errors
in data.
4.3 Potential Competitors


Blu-ray Discs
UV or X-rays
4.4 Companies working on Holographic Memory






Akonia Holographics
IBM
Intel
Musion-The world leaders in holographic technology
Zebra Imaging
Realview medical holography
12 | P a g e
5 HOLOGRAPHIC MEMORY vs. EXISTING MEMORY TECHNOLOGY
In the memory hierarchy, holographic memory lies somewhere between RAM and magnetic storage in
terms of data transfer rates, storage capacity, and data access times. The theoretical limit of the number
of pixels that can be stored using volume holography is V2/3/
medium and
2
where V is the volume of the recording
is the wavelength of the reference beam. For green light, the maximum theoretical
storage capacity is 0.4 Gbits/cm2 for a page size of 1 cm x 1 cm. Also, holographic memory has an
access time near 2.4
s, a recording rate of 31 kB/s, and a readout rate of 10 GB/s. Modern magnetic
disks have data transfer rates in the neighborhood of 5 to 20 MB/s [8]. Typical DRAM today has an
access time close to 10 – 40 ns, and a recording rate of 10 GB/s.
Storage Medium
Access Time
Holographic Memory
2.4 s
Main Memory (RAM)
10 – 40 ns
Magnetic Disk
8.3 ms
Data Transfer Rate
Storage Capacity
10 GB/s
400 Mbits/cm2
5 MB/s
5 – 20 MB/s
4.0 Mbits/cm2
100 Mbits/cm2
Holographic memory has an access time somewhere between main memory and magnetic disk, a data
transfer rate that is an order of magnitude better than both main memory and magnetic disk, and a
storage capacity that is higher than both main memory and magnetic disk. Certainly if the issues of
hologram decay and interference are resolved, then holographic memory could become a part of the
memory hierarchy, or take the place of magnetic disk much as magnetic disk has displaced magnetic
tape for most applications.
13 | P a g e
6 HOLOGRAPHIC MEMORY
Wide possibilities in this case are provided by technology of optical recording, it's known as holography:
it allows high record density together with maximum data access speed. It's achieved due to the fact that
the holographic image (hologram) is coded in one big data block, which is recorded at one access. And
while reading this block is entirely extracted out of the memory. For reading and recording of the blocks
kept holographically on the light-sensitive material (LiNbO3 is taken as the basic material) they use
lasers. Theoretically, thousands of such digital pages, which contain up to a million bits each, can be
put into a device measuring a bit of sugar. And theoretically they expect the data density to be 1 TBytes
per cubic cm (TBytes/cm3). In practice, the developers expect around 10 GBytes/cm3, what is rather
impressive when comparing with the current magnetic method that allows around several MBytes/cm2 and this without the mechanism itself. With such recording density an optical layer which is approx 1
cm in width will keep around 1TBytes of data. And considering the fact that such system doesn't have
any moving parts, and pages are accessed parallel, you can expect the device to be characterized with 1
GBytes/cm3 density and higher.
FIGURE-1 SYSTEM OF HOLOGRAPHIC MEMORY
14 | P a g e
Exceptional possibilities of the topographic memory have interested many scientists of universities and
industrial research laboratories. This interest long time ago poured into two research programs. The first
of them is PRISM ( Photorefractive Information Storage Material), which is targeted at searching of
appropriate light-sensitive materials for storing holograms and investigation of their memorizing
properties. The second program is HDSS (Holographic Data Storage System). Like PRISM, it includes
fundamental investigations, and the same companies participate there. While PRISM is aimed at
searching the appropriate media for storing holograms, HDSS is targeted at hardware development
necessary for practical realization of holographic storage systems.
How does a system of holographic memory operate? For this purpose we will consider a device
assembled by a task group from the Almaden Research Center.
At the first stage in this device a beam of cyan argon laser is divided into two components - a reference
and an object beam (the latter is a carrier of data). The object beam undergoes defocusing in order it
could entirely illumine the SLM (Spatial Light Modulator) which is an LCD panel where a data page is
displayed in the form of a matrix consisting of light and dark pixels (binary data).
The both beams go into the light-sensitive crystal where they interact. So we get an interference pattern
which serve a base for a hologram and is recorded as a set of variations ofthe refractive exponent and the
reflection factor inside this crystal. When reading data the crystal is illuminated with a reference beam,
which interacts with the interference factor and reproduces the recorded page in the image of "chessboard" of light and dark pixels (the holograms converts the reference wave into the copy of the object
one). After that, this image is transferred into the matrix detector where the CCD (Charge-Coupled
Device) serves a base. While reading the data the reference beam must fall at the same angle at which
the recording was made; alteration of this angle mustn't exceed 1 degree. It allows obtaining high data
density: measuring the angle of the reference beam or its frequency you can record additional pages of
data in the same crystal.
However, additional holograms change properties of the material, and such changes mustn't exceed the
definite number. As a result, the images of holograms become dim, what can lead
15 | P a g e
to data corruption when reading? This explains the limitation of the volume of the real memory that
belongs to this material. The dynamic area of the medium is defined by the number of pages which can
be virtually housed, that's why PRISM participants are investigating limitations to the light sensitivity
of substances.
The procedure in 3-dimensional holography which concludes in enclosure of several pageswith data
into the same volume is called multiplexing. Traditionally the following multiplexing methods are used:
of angle of dip of the reference beam, of wavelength and phase; but unfortunately they require
complicated optical systems and thick (several mm) carriers what makes them unfit for commercial use,
at least in the sphere of data processing. But lately Bell Labs have invented three new multiplexing
methods: shift, aperture and correlative, which are based on the usage of change in position of the carrier
relative to the beams of light. The shift and aperture multiplexing use a spherical reference beam, and
the correlative uses a beam of more complicated form. Besides, considering the fact that the correlative
and shift multiplexing enable mechanical moving elements, the access time will be the same as that of
the usual optical discs. Bell Labs managed to build an experimental carrier on the same LiNBO3 using
the technology of correlative multiplexing but this time with 226 GBytes per square inch.
Another problem standing on the way of development of holographic memory devices is a search of the
appropriate material. The most of the investigations in the sphere of holography were carried out with
usage of photoreactive materials (mainly the mentioned LiNBO3), but they are not suitable for data
recording especially for commercial use: they are expensive, weak sensitive and have a limited dynamic
range (frequency bandwidth). That's why they developed a new class of photopolymer materials facing a
good perspective in terms of commercial use. Photopolymers are the substances where the light cause
irreversible changes expressed through fluctuation of the composition and density. The created material
have a longer life circle (in terms of storing data) and are resistant to temperature change, besides, they
have improved optical characteristics and are suitable for WORM (write-once/read many).
One more problem concludes in the complexity of the used optical system. For holographic memory the
LEDs based on semiconductor lasers used in traditional optical devices are not suitable, since they have
insufficient power, give out a wide beam angle, and at last it's too difficult to get a semiconductor laser
generating radiation in the middle range of the visible
16 | P a g e
spectrum. There you need as powerful laser as possible which gives the most exact parallel beam. The
same we can say about the SLM: yet some time ago there were no any such devices which could be used
in the holographic memory systems. But time flies and today you can get inexpensive solid-state lasers;
besides, there appeared the MEM technology (Micro- Electrical Mechanical). The devices on its base
consist of the arrays of micromirrors around 17 micron in size, they suit very much for the role of SLM.
Since the interference patterns fill up the whole substance uniformly, it gives another useful property to
the holographic memory - high reliability of the recorded information. While a defect on the surface of
the magnetic disc or tape destroy important data, a defect in holographic medium doesn't cause a loss of
information, it leads only to tarnish of the hologram. The small desktop HDSS-devices are to appear by
2003. Since the optical discs. Bell Labs managed to build an experimental carrier on the same LiNBO3
using the technology of correlative multiplexing but this time with 226 GBytes per square inch.
Another problem standing on the way of development of holographic memory devices is a search of the
appropriate material. The most of the investigations in the sphere of holography were carried out with
usage of photoreactive materials (mainly the mentioned LiNBO3), but they are not suitable for data
recording especially for commercial use: they are expensive, weak sensitive and have a limited dynamic
range (frequency bandwidth). That's why they developed a new class of photopolymer materials facing a
good perspective in terms of commercial use. Photopolymers are the substances where the light cause
irreversible changes expressed through fluctuation of the composition and density. The created material
have a longer life circle (in terms of storing data) and are resistant to temperature change, besides, they
have improved optical characteristics and are suitable for WORM (write-once/read many).
One more problem concludes in the complexity of the used optical system. For holographic memory the
LEDs based on semiconductor lasers used in traditional optical devices are not suitable, since they have
insufficient power, give out a wide beam angle, and at last it's too difficult to get a semiconductor laser
generating radiation in the middle range of the visible spectrum. There you need as powerful laser as
possible which gives the most exact parallel beam. The same we can say about the SLM: yet some time
ago there were no any such devices which could be used in the holographic memory systems. But time
flies and today you
17 | P a g e
can get inexpensive solid-state lasers; besides, there appeared the MEM technology (Micro- Electrical
Mechanical). The devices on its base consist of the arrays of micromirrors around 17 micron in size,
they suit very much for the role of SLM.
Since the interference patterns fill up the whole substance uniformly, it gives another useful property to
the holographic memory - high reliability of the recorded information. While a defect on the surface of
the magnetic disc or tape destroy important data, a defect in holographic medium doesn't cause a loss of
information, it leads only to tarnish of the hologram. The small desktop HDSS-devices are to appear by
2003. Since theHDSS equipment use an acoustooptical deflector for measuring angle of dip (a crystal
which properties change with a sound-wave passing through it), the extraction time for adjacent data
pages will constitute 10ms. Any traditional optical or magnetic memory device needs special means for
data access of different tracks, and this access time constitutes several milliseconds.
The holographic memory is not a completely new technology since its basic conceptions were
developed about 30 years ago. The only that has changed is availability of the key components - the
prices considerably fell down. The semiconductor laser, for example, is not unusual. On the other hand,
SLM is a result of the same technology which is used in production of LCD-screens for notebooks and
calculators, and the CCd detector array is taken right from a digital video camera.
Well, the new technology has more than enough highlights: apart from the fact that information is stored
and recorded parallel, you can reach very high data rate, and in some cases high speed of random access.
And the main advantage is that mechanical components are practically absent (those that typical for
current storage devices). It ensures not only a fast data access, less probability of failures, but also lower
power consumption, since today a hard disc is one of the greatest power-consuming elements of a
computer. However, there are problems with adjustment of optical devices, that's why at the
beginning the data of the device will
probably "fear" exterior mechanical effects.
18 | P a g e
7 MOLECULAR MEMORY
Another approach in creation of storage devices is a molecular method. A group of researchers of the
"W.M. Keck Center for Molecular Electronic" with Professor Robert R. Birge as a head quite a long
time ago received a prototype of memory subsystem which uses digital bits of a molecule. These are
protein molecules which is called bacteriorhodopsin. It's purple, absorbs the light and presents in a
membrane of a microorganism called halobacterium halobium. This bacterium lives in salt bogs where
the temperature can reach +150 °C. When a level of oxygen contents is so low in the ambient that to
obtain power breathing (oxidation) is not enough, it uses protein for photosynthesis.
Bacteriorhodopsin was chosen because a photocycle (a sequence of structural changes undergone by a
molecule when reacting with light) makes this molecule an ideal logically storing element of "&" type or
a type of a switch from one condition into another (trigger). According to Birge's investigation, bR-state
(logical value of the bit "0") and Q-state (logical value of the bit "1") are intermediate states of the
molecule and can remain stable during many years. This property (which in particular provides a
wonderful stability of protein) was obtained by an evolutional way in the struggle for survival under
severe conditions of salt bogs.
Birge estimated that the data recorded on the bacteriorhodopsin storage device must "live" around 5
years. Another important feature of the bacteriorhodopsin is that these both states have different
absorption spectra. It allows easily defining the current state of the molecule with the help of a laser set
for the definite frequency.
They built a prototype of memory system where the absorption spectrum stores data in 3- dimensional
matrix. Such matrix represents a cuvette (a transparent vessel) filled up with polyacryde gel, where
protein is put. The cuvette has an oblong form 1x1x2 inch in size. The protein which is in the bR-state is
fixed in the space with gel polymerization. The cuvette is surrounded with a battery of lasers and a
detector array based on the device using a principle of CID (Charge Injection Device), they serve for
data recording and reading.
19 | P a g e
When recording data first you need switch on a yellow-wave "page" laser - for converting the molecules
into Q-state. The SLM which represents an LCD-matrix creating a mask on the beam way stimulates
appearing of an active (excited) plane in the material inside the cuvette. This poweractive plane is a page
of data which can house 4096x4096 bit array. Beforereturning of protein into the quiescent state (in such
state it can remain quite a long time keeping the information) a red-wave recording laser lights on; it's
positioned at the right angleto the yellow one. The other SLM displays binary data, and this way creates
the corresponding mask on the way of the beam, that's why only definite spots (pixels) of the page will
be irradiated. The molecules in these spots will convert into Q-state and will represent a binary one. The
remaining part of the page will come into the initial bR-state and will represent binary zeros. In order to
read the data you will need again the "page" laser which converts the read page into Q-state. It's
implemented so that in the future one can identify binary one and zero with the help of difference in
absorption spectra. 2ms later the page is plunged into low- intensive light flux of the red-wave laser.
Low intensity is necessary to prevent jumping into Q-
20 | P a g e
state. The molecules that represent a binary zero absorb red light, and those that represent a binary one
let the beam pass by. It creates a "chess" picture of light and dark spots on the LCD-matrix which takes
a page of digital information.
For erasing information a short impulse of a cyan laser is enough in order to convert themolecules
from Q-state back into bR-state. This beam can be obtained not necessary with thelaser: you can erase
the whole cuvette with a usual ultraviolet lamp. In order to ensure theentirety of data when erasing
only the definite pages there used caching of several adjacentpages. For read/write operations two
additional parity bits are also used to prevent errors. Thedata page can be read without corruption up to
5000 times. Each page is traced by a meterand after 1024 reading the page gets refreshed (regenerated)
with a new recording operation. Considering that a molecule changes its states within 1 ms, the total
time taken for read orwrite operations constitutes around 10 ms. But similar to the system of a
holographic memorythis device makes a parallel access in the read-write cycle, what allows counting on
the speedup to 10 Mbps. It is assumed that combining 8 storing bit cells into a byte with a parallel
access, you can reach, then, 80 Mbps, but such method requires a corresponding circuitrealization
of the memory subsystem. Some versions of the SLM devices implement a pageaddressing, which in
cheap constructions is used when sending the beam to the required pagewith the help of rotary system of
galvanic mirrors. Such SLM provides 1ms access but costsfour times more.
Birge states that the system suggested by him is close to the semiconductor memory in operating speed
until a page defect is come across. On detecting of a defect it's necessary to resend the beam for
accessing these pages from the other side. Theoretically, the cuvette can accommodate 1TBytes of data.
Limitation of capacity is mainly connected with problems oflens system and quality of protein.
21 | P a g e
Will the molecular memory be able to compete against the traditional semiconductor memory? Its
construction has undoubtedly some advantages. First, it's based on protein which is produced in large
volumes and at low price. Secondly, the system can operate in the wider range of temperatures than the
semiconductor memory. Thirdly, data are stored constantly - even in case of power switching off, it
won't cause data loss. And at last, bricks with data whichare rather small in size but contain gigabytes of
data can be placed into an archive for storing copies (like magnetic tapes). Since the bricks do not have
moving parts, it's more convenient than usage of portable hard discs or cartridges with magnetic tape.
Recently, holographic data storage technologies have again been in the limelight as a next- generation
storage system that achieves large storage capacities and high transfer rates simultaneously. This shift
was mainly shaped by two USA national projects, the PhotoRefractive Information Storage Materials
(PRISM) consortium and the Holographic Data Storage System (HDSS) consortium, launched and
conducted from 1994 to 1999. In PRISM, it was shown that photopolymer has entered a commercially
practicable realm as a holographic recording material other than crystals. In HDSS, rotating a diskshaped storage medium to continuously
[2]
even
record
and
reconstruct
data
was
demonstrated
[1]
[3]. However, a large number of subjects still remain to be tackled regarding commercialization
after
the
completion
of
these
large-scale
USA
national
projects. We have
been developing a holographic memory that has compatibility with the conventional optical disk
technologies. Since the reference beam and information beam are coaxially arranged to perform
recording and reconstruction, the holographic memory is called a coaxial read/write holographic
memory or collinear systemThere have been a number of proposals for holographic memories in which
holographic recording media were shaped into a disk[4]. However, simply shaping a recording medium
into a disk does not assure practicability and a number of problems have to be tackled from the
perspective of commercialization
22 | P a g e

For the two-beam interference method as shown in Fig.1, the reference beam and information
beam are configured in two different optical axes. This makes the optical system complex.

There are no precise standards or addresses for the holographic recording media, resulting in
very poor removability

There is no interchangeability between the holographic recording devices and recording
media of the same type

Insufficient consideration is given to measures to combat plane deflection or eccentricity
during rotation of the disk.

Holographic recording media require flatness of the order of the optical wavelength, which is
unsuitable for volume production

No consideration is given to maintaining upward compatibility with existing storage media
such as CDs and DVDs
23 | P a g e

Utilization of the existing production infrastructure has not been considered. This means that
the production of holographic memories requires investments in new equipment
8 COLLINEAR SYSTEM
By studying the conventional problems noted above, it becomes especially important to fundamentally
reconsider a holographic recording and reconstruction optical system using the two-beam
interference
method
noted
in
(1).
The collinear system is based on the coaxial read/write type in which the reference and information
beams are handled as a pencil of coaxial light, rather than the two-beam interference method that was
widely used in the past. This enables the comprehensive optical disk technologies to be easily fused to
realize large storage capacity, and high transfer rate memories of a new concept while exploiting the
advantages of the holographic memory.
The following shows the concept of collinear holographic memories
 Thick volume-recording media are used to increase the recording capacities
 A batch of two-dimensional page data are recorded and reconstructed as a hologram to
improve the transfer rates.
 The information beam and reference beam are collinearly optically arranged on the same
axis without angles to perform holographic recording and reconstruction.
 Disk construction with a coated reflection film is employed to configure an optical
system that completes on a single side of the disk
 An optical disk is preformatted with addresses and optical servo information
 The optical servo technique is applied so that interference patterns are recorded even in the
presence of disk rotation, eccentricity, or plane deflection
 A two-wavelength optical system is configured to read out addresses at a wavelength that
does not photosensitize a holographic recording material, and perform optical servo
operations.
24 | P a g e

A beam for the optical servo is utilized to provide upward compatibility with the
existing CDs or DVDs
9 COLLINEAR HOLOGRAPHIC
MEDIA EVALUATION
SYSTEM S-VRD TM
Fig.2 Collinear Holographic Media Evaluation System S-VRD
To accelerate the commercialization of collinear holographic memories, it is necessary to promote
the
research
and
development
of
holographic
recording
media. Fig.2 shows the
appearance of a recording media evaluation system (abbreviated product name:
VRD)
employing
the
collinear
S-
system. This evaluation
system can record and reconstruct two-dimensional digital page data employing the collinear system
and evaluate the characteristics of the recording material such as bit error rates and signal-to-noise
ratios (SNR) from different angles. Moreover, it can measure small-piece samples to disk samples
(option) using a 6-axes control universal sample holder.
25 | P a g e
CONCLUSION
Holographic memory is using lasers for recording and reading of data in which signal beam
and reference beam are used and interfering pattern of these two beams is to be stored in
memory. As interfering pattern is stored, by changing angle, position or wavelength of
reference beam different patterns can generated which increases data density. Holographic
Memory is having all required features of any storing device like high storage densities – 1
TB in just sugar sized cube, fast data transfer rate - 1 Gbps and with that 3D representation of
image or video. So these all make Holographic memory a perfect suitable choice for storing
data.
26 | P a g e