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.
-because tha data is recorded as an optical wave front.
-because it allows many different patterns to be stored.
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.
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 â€œLiNb03,BaTiO3.
The most common holographic recording system uses are
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.
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.
Phase encoded 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.
Multiplexed with different wavelenghts of the reference beam.
It is limited to the small tuning range of lasers.
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
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.
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.
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.
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
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
Stephanie Boyles white paper Analysis