HOLOGRAPHIC MEMORYTHE MEMORY THAT ROCKS THE FUTURE

PRESENTED BY

K. Ashwin kumar K. Ramesh BabuIII/IV B.Tech-CSE III/IV B.Tech-CSE JNTU , ANANTAPUR. JNTU , ANANTAPUR

Email ID : Email ID :kavaliashwinkumar@gmail.com rameshbabu1062@gmail.comContact no:9440870414 Contact no:8125859596

ABSTRACTThe theme of our presentation is mainly based upon an overall view of a three-dimensional memory that offers possibility of storing 1 TB (terabyte=1000 GB)

of data in a sugar-cube-sized crystal called the holographic memory. Most computers hard drive only hold 10 to 40 or 80 GB of data, only a small fraction of what a holographic memory system might hold. It is a storage system that stores more information in smaller space and offer faster data rates of about 1 GB per second. The hardware is based upon optical interference patterns. High-speed recording would be possible with holographic recording because of parallel signal processing using a spatial light modulator. Holography enables storage densities that can far surpass the super paramagnetic and diffraction limits of traditional magnetic and optical recording. In addition, unlike conventional technologies that store data bit by bit, holography allows a million bits of data to be written and read out in single flashes of light, enabling data transfer rates as high as a billion bits per second (fast enough to transfer a DVD movie in about 30 seconds). The paper now deals with the hardware concepts of the holographic memory system, coding and signal processing, a comparison with existing memory systems, concluded with its applications and technical problems faced in this regard are also dealt upon along with the future of this storage system.

1. HISTORY

Holography (from the Greek, Όλος-holos whole + γραφή-graphe writing) is the science of producing holograms, an advanced form of photography that allows an image to be recorded in three dimensions, which can also be used to optically store and retrieve information.

Holography was a discovery of an unexpected result of research into improving electron microscopes in 1948 by Hungarian physicist Dennis Gabor (1900-1979), for which he received the Nobel Prize in physics in 1971.

The very first holograms were "transmission holograms", which were viewed by shining laser light through them. A later refinement, the "rainbow transmission" hologram allowed viewing by white light and is commonly seen today on credit cards as a security feature and on product packaging. Another kind of common hologram is the true "white-light reflection hologram" which is made in such a way that the image is reconstructed naturally using light on the same side of the hologram as the viewer.

2. INTRODUCTION

Holographic memory offers the possibility of storing 1 terabyte (TB) of data in a sugar-cube-sized crystal. A terabyte of data equals 1,000 gigabytes, 1 million megabytes or 1 trillion bytes. Data from more than 1,000 CDs could fit on a holographic memory system. Most computer hard drives only hold 10 to 40 GB of data, a small fraction of what a holographic memory system might hold. 3. COMPONENTS-Blue-green argon laser -Beam splitters to spilt the laser beam

-Mirrors to direct the laser beams -LCD panel (spatial light modulator) -Lenses to focus the laser beams -Lithium-niobate crystal or photopolymer -Charge-coupled device (CCD) camera 4. WORKING

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 created stores the data carried by the signal beam in a specific area in the crystal -- the data is stored as a hologram. The device first splits a blue argon laser beam into separate reference and object beams. The object beam that carries the information gets expanded so that it fully illuminates a spatial light modulator (SLM). An SLM is simply an LCD panel that displays a page of raw binary data as an array of clear or dark pixels.

4.1 RETREIVAL OF DATA

To read the stored data, the hologram is illuminated with the reference beam. Each page of the hologram is recorded separately. To record on the hologram, the data in the form of electric signal is converted to optical signals by a page composer. The controller generates the address to access the desired page. This results in the exposure of a small area of the recording medium through an aperture. The optical output signal is directed to the exposed area by the deflector. Using this beam deflecting mechanism, the light (which carries the information) and the reference beam are made to interact. The interference

pattern is thus recorded on the hologram. To record a different page, the aperture is moved and the above process is repeated.

For data retrieval, the laser (reference beam) is focused on the appropriate page according to the address generated. A photo detector array on the other side of the hologram records the image of that sub4.2 DATA STORAGE

To produce a recording of the phase of the light wave at each point in an image, holography uses a reference beam which is combined with the

light from the scene or object (the object beam). Optical interference between the reference beam and the object beam, due to the superposition of the light waves, produces a series of intensity fringes that can be recorded on standard photographic film. These fringes form a type of diffraction grating on the film, which is called the hologram.

5. CODING AND SIGNAL PROCESSING

In a data-storage system, the goal of coding and signal processing is to reduce the BER to a sufficiently low level. This is accomplished by stressing the physical components of the system well beyond the point, at which the channel is error-free, and then introducing coding and signal processing schemes to reduce the BER to levels acceptable to users. Coding and signal processing can involve several qualitatively distinct elements. The cycle of user data from input to output can include interleaving, error-correction-code (ECC) and modulation encoding, signal preprocessing, data storage in the holographic system, hologram retrieval, signal post processing, binary detection, and decoding of the interleaved ECC.

5.1 ECC ENCODERThe ECC encoder adds redundancy to the data in order to provide

protection from various noise sources. The ECC-encoded data are then passed on to a modulation encoder which adapts the data to the channel: It manipulates the data into a form less likely to be corrupted by channel errors and more easily detected at the channel output. The modulated data are then input to the SLM and stored in the recording medium.5.2 BINARY DETECTION

The simplest detection scheme is threshold detection, in which a threshold T is chosen: Any CCD pixel with intensity above T is declared a 1, while those below T are assigned to class 0. However, it is not at all obvious how to choose a threshold, especially in the presence of spatial variations in intensity, and so threshold detection may perform poorly. The following is an alternative.

Within a sufficiently small region of the detector array, there is not much variation in pixel intensity. If the page is divided into several such small regions, and within each region the data patterns are balanced (i.e., have an equal number of 0s and 1s), detection can be accomplished without using a threshold. Thus, sorting detection combined with balanced modulation coding provides a means to obviate the inaccuracies inherent in threshold detection. One problem with this scheme is that the array detected by sorting may not be a valid codeword for the modulation code; in this case, one must have a procedure which transforms balanced arrays into valid code words. A more complex but more accurate scheme than sorting is correlation detection in which the detector chooses the codeword that achieves maximum correlation with the array of received pixel intensities5.3 INTERPIXEL INTERFERENCE

Interpixel interference is the phenomenon in which intensity at one particular pixel contaminates data at nearby pixels. Physically, this arises from optical diffraction or aberrations in the imaging system.

Deconvolution has the advantage that it incurs no capacity overhead (code rate of 100%). However, it suffers from mismatch in the channel model (the physics of the intensity detection makes the channel nonlinear); an alternative approach to combating interpixel interference is to forbid certain patterns of high spatial frequency via a modulation code. A code that forbids a pattern of high spatial frequency (or, more generally, a collection of such patterns of rapidly varying 0 and 1 pixel) is called a low-pass code. Such codes constrain the allowed pages to have limited high spatial frequency content.5.4 ERROR CORRECTION

Error correction incorporates explicit redundancy in order to identify decoded bit errors. An ECC code receives a sequence of decoded data (containing both user and redundant bits) with an unacceptably high raw BER, and uses the redundant bits to correct errors in the user bits and reduce the output user BER to a tolerable level (typically, less than 1012). The simplest and best-known error-correction scheme is parity checking, in which bit errors are identified because they change the number of 1s in a given block from odd to even,

5.4 PREDISTORTIONA unique novel preprocessing technique developed to holographic data

storage called "predistortion”, works by individually manipulating the recording exposure of each pixel on the SLM, either through control of exposure time or by relative pixel transmission (analog brightness level on the SLM). Use of the predistortion technique is to increase the contrast between the 1 and 0 pixel states provided by the SLM.

AFTER BEFORE PRE-DISTORTION PRE-DISTORTION

5.5 GRAY SCALEThe predistortion technique described in the previous section makes it

possible to record data pages containing gray scale. Since we record and detect more than two brightness levels per pixel, it is possible to have more than one bit of data per pixel. The histogram of a hologram with six gray-scale levels made possible by the predistortion technique

If pixels take one of g brightness levels, each pixel can convey log2 g bits of data. Gray scale also divides the system's signal-to-noise ratio (SNR) into g 1 parts, one for each transition between brightness levels. Because total SNR depends on the number of holograms, dividing the SNR for gray scale (while requiring the same error rate) leads to a reduction in the number of holograms that can be stored

5.6 CAPACITY ESTIMATION

To quantify the overall storage capacity of different gray-scale encoding options, an experimental capacity-estimation technique has been developed in which the dependence of raw BER on readout power is first measured experimentally. The capacity-estimation technique then produces the relationship between M, the number of holograms that can be stored, and raw BER

Capacity estimation techniques begins with simple experimental measurement of raw BER of a few holograms as a function of reconstructed intensities and produces an estimation of number of holograms that could be superimposed as a function of raw BER that the system is asked to maintain. Without this technique, one would need to perform repeated multiple-hologram techniques to obtain these data.

6. ASSOCIATIVE RETREIVAL

Volume holographic data storage conventionally implies that data imprinted on an object beam will be stored volumetrically to be read out at some later time by illumination with an addressing reference beam. However, the same hologram (the interference pattern between a reference beam and a data-bearing object beam) can also be illuminated by the object beam .This reconstructs all of the angle-multiplexed reference beams that were used to record data pages into the volume. The amount of power diffracted into each "output" beam is proportional to the 2D cross-correlation between the input data page (being displayed on the SLM) and the stored data page (previously recorded with that particular reference beam). Each set of output beams can be focused onto a detector array, so that each beam forms its own correlation "peak." Because both the input and output lenses perform a two-dimensional Fourier transform in spatial coordinates [5], the optical system is essentially multiplying the Fourier transforms of the search page and each data page and then taking the Fourier transform of this product (thus implementing the convolution theorem optically). Because of the volume nature of the hologram, only a single slice through the 2D correlation function is produced (the other dimension has been "used" already, providing the ability to correlate against multiple templates simultaneously).

7. 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/l2 where V is the volume of the recording medium and l 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 [7]. Also, holographic memory has an access time near 2.4 ms, a recording rate of 31 kB/s, and a readout rate of 10 GB/s [3]. 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 Data Transfer Rate Storage Capacity ------------------------------------------------------------------------------------------------------------

Holographic Memory 2.4 ms 10 GB/s 400 Mbits/cm2 Main Memory

(RAM) 10 – 40 ns 5 MB/s 4.0 Mbits/cm2

Magnetic Disk 8.3 ms 5 – 20 MB/s 100 Mbits/cm2 ------------------------------------------------------------------------------------------------------------

This table shows the comparison of access time, data transfer rates (readout), and storage capacity (storage density) for three types of memory; holographic, RAM, and magnetic disk.

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.

8. IMPLEMENTATION

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 sub volume of the crystal using the collision of ultra

short laser pulses to differentiate between the image and the time-delayed reference beam [6]. Phase-conjugate holography is a technique to reduce the 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 beam used for recording. The signal diffracted by the hologram being accessed is sent back along the path from which it came, and is refocused onto the SLM which now serves as both the SLM and the detector [5].

9. PROPERTIES

Digital holography:The angularly selective property of holograms recorded in thick materials

enables a unique form of high-capacity data storage distinguished by its parallel data access capability. A holographic data storage system is fundamentally page-oriented, with each block of data defined by the number of data bits that can be spatially impressed onto the object beam. The total storage capacity of the system is then equal to the product of the page size (in bits) and the number of pages that can be recorded

The coherence length of the beam determines the maximum depth the image can have. A laser will typically have a coherence length of several meters, ample for a deep hologram.

An alternate method to record holograms is to use a digital device like a CCD camera instead of a conventional photographic film. This approach is often called digital holography. In this case, the reconstruction process can be carried out by digital processing of the recorded hologram by a standard computer. A 3D image of the object can later be visualized on the computer screen.

High capacity, high transfer rate, random access memory systems are needed to archive and distribute the tremendous volume of digital information being generated in many applications

Metro Laser is developing an innovative, ultra-high density holographic data storage system. Holographic storage extends the high density of an optical disk (CD-ROM) into a true three-dimensional random access.

In volume holographic storage, a two dimensional image (Data Page) is recorded using a coded reference beam. The beam encoding can be by virtue of its wavelength, propagation angle, or by amplitude/phase modulation of its wave front. By re-illuminating the hologram with the original reference beam, the entire data page is recalled, what brings a high data flow rates.

In practice, the number of holograms that can be stored and reliably retrieved from a common volume of material is limited to less than 10,000 so that spatial multiplexing techniques must be used. Although solid-state designs are possible, it is easiest to envision a storage material formed as a volume disk in which holograms in a particular cell are stored and retrieved by angular multiplexing and where random access to arbitrary cells is enabled by rotation of the disk.

Stability is a desirable property for any data storage system. In the case of holographic storage, the response of the recording medium, which converts the optical interference pattern to a refractive index pattern (the hologram), is generally linear in light intensity and lacks the response threshold found in bistable storage media such as magnetic films

10. ADVANTAGES

Using currently available SLM's can produce about 1000 different images a second at 1024 X 1024 bit resolution. With the right type of media (probably polymers rather than something like LiNbO3), this would result in about 1 Gigabit per second writing speed. Read speeds can surpass this and experts believe 1 Terabit per second readout is possible.The advantages of the proposed hologram memory architecture are:-Ultra-high storage density - up to 1 TB/cm3 -High retrieval rate ~ 1GB/sec-Secure data access

An advantage of a holographic memory system is that an entire page of data can be retrieved quickly and at one time

With the fuzzy coding techniques introduced, volume holographic content-addressable data storage is an attractive method for rapidly searching vast databases with complex queries. Areas of current investigation include implementing system architectures which support many thousands of simultaneously searched records, and quantifying the capacity reliability

tradeoffs. Holograms are common in science-fiction, most notably Star Trek, Star Wars, and Red Dwarf.

By storing and reading out millions of bits at a time, a holographic disc could hold a whole library of films. Movies, video games, and location-based services like interactive maps could be put on postage-stamp-size chips and carried around on cell phones. A person's entire medical history, including diagnostic images like x-rays, could fit on an ID card and be quickly transmitted to or retrieved from a database

Many of the remarkable advances in consumer electronics over the last few years--and much of the economic health of the industry--are directly traceable to the explosion in storage capacity. Web e-mail services routinely offer each of their customers a gigabyte of memory for free. Apple's newest iPod is only possible because of small, cheap hard drives that can hold a staggering 60 gigabytes of data--a storage capacity that just five years ago would have been a lot for a desktop PC.

Eventually, if the hardware becomes affordable for consumers, holographic storage could supplant DVDs and become the dominant medium for games and movies. Portable movie players and phones that download multimedia from the Web would take off. Holographic storage could even compete with the magnetic hard drive as the computer's fundamental storage unit. And on a larger scale, corporate and government data centers could replace their huge, raucous storerooms of server racks and magnetic-tape reels with the quiet hum of holographic disc drives

Likewise, cell phones now come with flash memory chips easily able to store address books, calendars, photos, and the like. Indeed, the theoretical promise of holographic storage has been talked about for 40 years. But advances in smaller and cheaper lasers, digital cameras, projector technologies, and optical recording materials have finally pushed the technology to the verge of the market. And the ability to cram exponentially more bits into infinitesimal spaces could open up a whole new realm of applications. The benefits of exploiting the third dimension could go beyond storage to include more efficient ways to search ultra dense databases, like those that store satellite images for mapping and surveillance; new kinds of displays; and even ultra fast processors whose logic circuits are carved into holographic materials.

11. TECHNICAL PROBLEMS

Storage technologies such as CD’s and DVD’s have drawbacks. The density of magnetic materials in hard drives is fast approaching a fundamental physical limit. Flash memory is slow, and a DVD is barely large enough to hold a full-length movie.

Storing data in three dimensions would overcome many of these limitations.

The upcoming problems requiring very huge computing power make us today looking properly for new technical solutions not only in terms of CPU enhancement but also in terms of other PC components. Regardless of the technology used for CPU production, the data number transferred for processing is determined also by possibilities of other subsystems. Capacity of modern devices of mass memory reflects this tendency. CDs discs allow storing up to 700 MBytes, the developing technology of DVD-ROM - up to 17 GBytes. Technology of magnetic recording develops quickly as well - for the last year the typical capacity of a hard disc in the desktop computers has increased up to 15-20 GBytes and higher. But in the future computers are to process hundreds of gigabytes and even terabytes - much more than any current CDs or hard discs can accommodate. Servicing of such data volumes and their transfer for processing by ultra speed processors requires completely new approaches when creating storage devices.

12. FUTURE OF HOLOGRAPHIC MEMORY

Today holographic memory is very close to becoming a reality. The basic theory behind it has been shown to be reliable and has been implemented in numerous experiments. Materials research has yielded some promising results in photorefractive crystals such as LiNbO3 and BaTiO3, especially for use with rewritable, refreshed random access memory. Also, a read only version of holographic data storage is certainly feasible with some of the photopolymer films available today. For holographic memory to truly become the next revolution in data storage, data transfer rates must be improved, hologram decay must become negligible, and hologram recording time must be reduced. Then it will be economical for holographic memories to be produced for mass consumption.

It's likely to be one of the first commercial systems to use "holographic storage," in which bits are encoded in a light-sensitive material as the three-dimensional interference pattern of lasers.

Three-dimensional memory could dramatically change how we use microelectronics.

But if and when holographic storage will come to dominate the market is still an open question. InPhase's initial product launch is slated for late 2006, but industry experts, while optimistic, are also cautious. "They have made numerous contributions on the hardware side, in media and materials, and in error correction," says Hans Coufal, manager of science and technology strategy at IBM's Almaden Research Center in San Jose, CA, and an expert on holographic storage. "It's very impressive but still some ways away from a viable product. Not a long ways, but some ways."

Over the next four years, the Bell Labs team got its holographic material to work in conjunction with the latest miniaturized lasers, cameras, and optical components to read and write data. This also required advances in software to correct for errors in storing and retrieving digital bits.

13. CONCLUSION

But whoever wins, holographic storage could change the rules for information technology by opening up the possibilities of working in three dimensions.

14. References

1.  "Holographic data storage.". IBM journal of research and development. Retrieved 2008-04-28.

2. ^ "High speed holographic data storage at 500 Gbit/in.2". Archived from the original on 2008-04-27.

Retrieved 2008-05-05.

3. ^ a b Robinson, T. (2005, June). The race for space. netWorker. 9,2. Retrieved April 28, 2008 from

ACM Digital Library.

4. ^ "Maxell Introduces the Future of Optical Storage Media With Holographic Recording Technology",

(2005) retrieved January 27, 2007

5. ^ a b "Update: Aprilis Unveils Holographic Disk Media". 2002-10-08.

6. ^ "Holographic-memory discs may put DVDs to shame". New Scientist. 2005-11-24.

7. ^ "Aprilis to Showcase Holographic Data Technology". 2001-09-18.

8. ^ Sander Olson (2002-12-09). "Holographic storage isn't dead yet".

9. ^ Engadget, “InPhase delays Tapestry holographic storage solution to late 2009”

10. ^ Television Broadcast, “Holographic Storage Firm InPhase Technologies Shuts Down”

11. ^ GE Unveils 500-GB, Holographic Disc Storage Technology

12. ^ "as of when?".

13. ^ "Could Holography Cure Nintendo's Storage Space Blues? News".

14. ^ Inphase Technologies, Inc. (Longmont, CO, US) and Nintendo Co., Ltd. (Kyoto, JP) (2008-02-

26). "Miniature Flexure Based Scanners For Angle Multiplexing Patent".