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18-12-2009, 03:51 PM
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 revolutionize the storage systems. It can store data up to 1 Tb in a sugar cube sized crystal. Data from more than 1000 CDs can fit into a holographic memory System. Most of the computer hard drives available today can hold only 10 to 40 GB of data, a small fraction of what holographic memory system can hold. Conventional memories use only the surface to store the data. But holographic data storage systems use the volume to store data. It has more advantages than conventional storage systems. It is based on the principle of holography
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HOLOGRAPHIC MEMORY
Seminar by
Vatsala
8th Sem,CS
Introduction
To address rapidly increasing requirements for Storage critical enabling technology for many new multimedia applications is the key to bring forward this new technology.
Holographic memory is a promising technology for data storage.
Increase in storage density and access speed.
Holographic Data storage is an alternative to magnetic disks data storage.
Holographic Data Storage uses the depth of the medium instead of just the surface
The 3 features of holographic memory are
Redundancy of data stored
-because of the nature of the interfernce pattern.
Parallelism
-because tha data is recorded as an optical wave front.
Multiplexing
-because it allows many different patterns to be stored.
Holograms
A hologram is a block or sheet of photosensitive material which records the diffraction of two light sources.
It is simply the 3-D interference pattern of the intersection of the reference beam and signal beams at 90â„¢ eachother.
Compact disks hold between 500 and 600 megabytes or approximately 250,000 times less information than those holograms hold.
Implementation
Consists of a recording medium and a photodetector array
A beam of coherent light is split into a reference beam and signal beam used to record a hologram into the recording medium.
The recording medium is usually a photorefractive crystal “LiNb03,BaTiO3.
The most common holographic recording system uses are
1.Laser light
2.Beam splitter to divide the laser light into a reference beam and a signal beam.
3. Various lenses and mirrors to redirect the light
4. A photorefractive crystal
5. An array of photodetectorâ„¢s around the crystal to receive the holographic data.
The optical characteristics of the refractive crystals are
1. High diffraction efficiency
2. High resolution
3. Permanent storage until erasure
4. Fast erasure on the application of external stimulus such as UV light.
Application to binary
An order for holographic technology to be applied to computer systems, it must store data in a form that a computer can recognize. In current computer systems, this form is binary.
A spatial light modulator contains a two-dimensional array of windows, which are only microns wide.
The resulting cross section of the laser beam is a two dimensional array of binary data
After the laser beam is manipulated, it is sent into the hologram to be recorded. This data is written into the hologram as page form
Page Data Access
Page data access is the method of reading stored data in sheets, not serially as in conventional storage systems
Holographic memory reads data in the form of pages.
Provides very fast access times in volumes far greater than serial access methods.
The volume could be one Megabit per page using a SLM resolution of 1024 x 1024 bits at 15-20 microns per pixel.
Multiplexing
After a page of data is recorded in the hologram, a small modification to the source beam before it reenters the hologram will record another page of data in the same volume. This method of storing multiple pages of data in the hologram is called multiplexing.
There are different holographic techniques that are being researched.
Angular multiplexing
wavelength multiplexing
Spatial multiplexing
Phase encoded multiplexing
Angular multiplexing
Multiplexed with different angles of incidence of the reference beam.
When the reference beam recreates the source beam ,it needs to be at the same angle it was during recording.
Wavelength multiplexing
Multiplexed with different wavelenghts of the reference beam.
It is limited to the small tuning range of lasers.
Spatial multiplexing
Is a method of changing the point of entry of source and reference beams into the recording medium.
It is combined with other multiplexing to maximize the amount of data stored in the holographic volume.
2 common methods are
1. Peristrophic multiplexing
2. Shift multiplexing
Peristrophic multiplexing
Rotates the recording medium as the light source beams remain in fixed positions.
To be taken care during implementing
-The rotational axes need to be posotioned appropriately.
-Bringing the recording media for data retrieval need to be very precise.
Shift multiplexing
Alters the point of entry on one surface of the recording media.
The points of entry the source beam takesinto it can be immense depending on the size of the laser beam and the sensitivity of the recording media.
This when combined with peristrophic cover a very large percentage of the hologram
Phase encoded multiplexing
Change the phase of individual parts of the reference beam.
The main reference beam is split up into many smaller partial beams which cover the same area as the original reference beam.
Speeds up access times.
Error Correction
It is inevitable that storing massive amounts of data in a small volume will be error prone. Factors exist in both the recording and retrieval of information which will be covered in the following subsections, respectively. In order for holographic memory systems to be practical in next generation computer systems, a reliable form of error control needs to be created.
Recording Errors
When a laser beam is split up ( for example, through a SLM ), the generated light bleeds into places where light was meant to be blocked out. Areas where zero light is desired might have minuscule amounts of laser light present which mutates its bit representation.Improvements must take into consideration the cost-effectiveness of a holographic memory system. These limitations to current laser beam and photosensitive technology are some of the main factors for the delay of practical holographic memory systems.
Page-Level Parity Bits
Current error control methods concentrate on a stream of bits.
Steps are as follows:
1. An odd number of bits in a row or column create a parity bit of 1 and an even number of bits create a 0.
2. calculate the parity of each row and column of data
3. A parity bit where the row and column meet is also created which is called an overall parity bit.
4. When data is read back from storage, another row and column are added called parity check bits.
5. The overall parity check bit becomes a one and the place of error is calculated
6. Erroneous bit is flipped and the data is read out error free.
HOLOGRAPHIC MEMORY vs. EXISTING MEMORY TECHNOLOGY
Applications
There are many possible applications of holographic memory. Holographic memory systems can potentially provide the high-speed transfers and large volumes of future computer systems. One possible application is data mining.
Another possible application of holographic memory is in petaflop computing. A petaflop is a thousand trillion floating point operations per second. The fast access in extremely large amounts of data provided by holographic memory systems could be utilized in a petaflop architecture.
Future of holographic memory
Today holographic memory is very close to becoming a reality.
Finding holograms in personal computers might be a bit longer off, however .
Holographic memory will most likely be used in next generation super computers where cost is not as much of an issue.
A read only version of holographic data storage is certainly feasible with some of the photopolymer films
Hologram decay must become negligible, and hologram recording time must be reduced.
"Whoever delivers a SOLUTION to the current storage capacity problem, will dominate the information storage market of the future"
John Stockton, Tamarack Storage Technologies
"Hot Technology for the 21st Century"
Holographic Storage - Datamation - August 1996
"The next cannibal is the displacement of magnetic and solid-state storage by holographic media ... and it is going to happen on or before the millenium."
- G.J. Butters, President, North America, Lucent Technologies,
"We lok forward toward an exotic improvement called holographic memory , which can hold terabytes ... in less than a cubic inch"
- Bill Gates, Chairman, Microsoft, The Road Ahead, 1995
Bibliography
howstuffswork.com
Stephanie Boyles white paper Analysis
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[attachment=6788]
Holographic Memo
Submitted by:
Manas Ranjan Pattanayak
MCA 4th sem
Regd.no:0505209074
Introduction
As computer systems continue to become faster, they will need a way to access larger amounts of data in shorter periods of time.
Holographic memory is a three-dimensional data storage system that can stored information at high density inside the crystal or photopolymer.
Hence holographic storage system has the potential to became the next storage generation over conventional storage system.
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ABSTRACT ON HOLOGRAPHY MEMORY
A hologram is a block or sheet of photosensitive material which records the interference of two light sources. To create a hologram, laser light is first split into two beams, a source beam and a reference beam. The source beam is then manipulated and sent into the photosensitive material. Once inside this material, it intersects the reference beam and the resulting interference of laser light is recorded on the photosensitive material, resulting in a hologram. Once a hologram is recorded, it can be viewed with only the reference beam. The reference beam is projected into the hologram at the exact angle it was projected during recording. When this light hits the recorded diffraction pattern, the source beam is regenerated out of the refracted light. An exact copy of the source beam is sent out of the hologram and can be read by optical sensors. For example, a hologram that can be obtained from a toy store illustrates this idea. Precise laser equipment is used at the factory to create the hologram. A recording material which can recreate recorded images out of natural light is used so the consumer does not need high-tech equipment to view the information stored in the hologram. Natural light becomes the reference beam and human eyes become the optical sensors.
IMPLEMENTATION
The components of Holographic data storage system is composed of
Blue-green argon laser
Beam splitters to spilt the laser beam
Mirrors to direct the laser beams
LCD panel (spatial light modulator)
