Holographic data storage system- Seminar Report

Gyan Vihar School of Engineering & Technology

ASeminar Report on

“ HOLOGRAPHIC DATA STORAGE SYSTEM ”

Submitted in partial fulfillment for the award for

degree of Bachelor of Technology (Computer Engineering)

Session

(2009-2010)

Submitted To Submitted ByMr. Naveen Hemrajani Anoop NairHead of Department Roll No. - 10Computer Science Computer Science

Computer Science EngineeringGyan Vihar School of Engineering and Technology

Jagatpura, Jaipur

Certificate

This is to certify that the Final Seminar Report entitled “Holographic Data Storage System” is an authentic record of the work carried out by Anoop Nair, Computer Science, VIII Semester. The report is submitted to Gyan Vihar School of Engineering and Technology, Jaipur, India, in partial fulfillment of the requirements for the award of the degree Bachelor of Technology in Computer Science Engineering during the academic year 2009-2010, VIII Semester.

It is further certified that the work embodied in the report has not been submitted to any other University or Institute for the award of any degree or diploma.

Naveen Hemrajani Head of Department

(Computer Science)G.V.S.E.T

Computer Science Department Holographic Data Storage System (HDSS) 1

Acknowledgment

Firstly I would like to express my sincere gratitude to the Almighty for His solemn presence throughout the seminar study. I would also like to express my special thanks to the HOD Mr. Naveen Hemrajani for providing an opportunity to undertake this seminar. I am deeply indebted to our seminar coordinators for providing me with valuable advice and guidance during the course of the study.

I would like to extend my heartfelt gratitude to the Faculty of the Department of Computer Science and Engineering for their constructive support and cooperation at each and every juncture of the seminar study.

Finally I would like to express my gratitude to Gyan Vihar College of Engineering and Technology for providing me with all the required facilities without which the seminar study would not have been possible.


Table of Contents

Certificate Acknowledgment Table of contents

1.Abstract............................................................................8

2.Introduction....................................................................9

3.Technical Aspect...........................................................14Holography Memory layout..................................................................16

4.Removable Media Storage Devices...............................17

Floppy Disk................................................................................................................17

Optical Formats.........................................................................................................17

CD-ROM...................................................................................................................18

DVD-ROM................................................................................................................18

DVD-R.......................................................................................................................20


DVD-RAM................................................................................................................20

DVD-RW..................................................................................................................21

+RW..........................................................................................................................21

Magneto-Optical Format..........................................................................................21

5.Holograms..................................................................23Volume

Holograms..................................................................................................23


6.Underlying Technology...........................................25

Holography...............................................................................................................25

Interference & Diffraction.........................................................................................26

Plane Wavefronts......................................................................................................26

Point Sources............................................................................................................27

Complex Objects......................................................................................................28

7. Working.....................................................................31

8.Application to Binary...............................................34

Spatial Light Modulator...........................................................................................34

Page Data Access.....................................................................................................34

9.Application to Binary...............................................34

Angular Multiplexing...............................................................................................36

Wavelength Multiplexing.........................................................................................37

Spatial Multiplexing..................................................................................................37


Phase-Encoded Multiplexing...................................................................................37

Combining Multiplexing Methods...........................................................................38

10. Error Correction.....................................................38

Recording Errors....................................................................................................38

Page-Level Parity Bits.............................................................................................39

Smart Interfacing....................................................................................................40

Intelligent Interfacing.............................................................................................40

11. Implementation.................................................................4112. Holographic Memory Vs Existing Memory

Technology......43

13. Hardware For Holographic Data Storage...............................44

14. HDSS Testers...........................................................................46

Prism Tester.............................................................................................................46

Demon I...................................................................................................................48

Demon II.................................................................................................................49

Innovative Optics.....................................................................................................52


Axicon......................................................................................................................52

Aspherical Apodizer................................................................................................53

Phase-Conjugate Readout.......................................................................................55

15. Coding and Signal Processing..................................................59

Binary Detection.....................................................................................................60

Interpixel Interference.............................................................................................61

Error Correction.....................................................................................................62

Predidtortion...........................................................................................................64

Gray Scale................................................................................................................65

Capacity Estimation................................................................................................66

16. Associative Retrieval................................................................67

17. Recording Materials................................................................72

18. Outlook...................................................................................86

19. Application...............................................................88

Holographic Versatile Disv...................................................................................88


20. Advantages & Disadvantages of HDSS...............97

21. Comparison...............................................................98

22. HVD at a glance.....................................................99

23. References................................................................101


Abstract

Holographic Versatile Disc (HVD) is an optical disc technology still in the research stage which would greatly increase storage over Blue-ray Disc and HD DVD optical disc systems. It employs a technique known as collinear holography, whereby two lasers, one red and one blue-green, are collimated in a single beam. The blue-green laser reads data encoded as laser interference fringes from a holographic layer near the top of the disc while the red laser is used as the reference beam and to read servo information from a regular CD-style aluminium layer near the bottom. Servo information is used to monitor the position of the read head over the disc, similar to the head, track, and sector information on a conventional hard disk drive. On a CD or DVD this servo information is interspersed amongst the data.

A dichroic minor layer between the holographic data and the servo data reflects the blue-green laser while letting the red laser pass through. This prevents interference from refraction of the blue-green laser off the servo data pits and is an advance over past holographic storage media, which either experienced too much interference, or lacked the servo data entirely, making them incompatible with current CD and DVD drive technology. These discs have the capacity to hold up to 3.9 terabyte(TB) of information, which is approximately 6,000 times the capacity of a CD-ROM, 830 times the capacity of a DVD, 160 times the capacity of single-layer Blu-ray Discs, and about 8 times the capacity of standard computer hard drives as of 2006. The HVD also has a transfer rate of 1 gigabit/s. Optware has released a 200 GB disc in early June 2006 and Maxell in September 2006 with a capacity of 300 GB and transfer rate of 20 MB/s.


IntroductionWith its omnipresent computers, all connected via the Internet, the Information Age has led to an explosion of information available to users. The decreasing cost of storing data, and the increasing storage capacities of the same small device footprint, has been key enablers of this revolution. While current storage needs are being met, storage technologies must continue to improve in order to keep pace with the rapidly increasing demand.

Devices that use light to store and read data have been the backbone of data storage for nearly two decades. Compact discs revolutionized data storage in the early 1980s, allowing multi-megabytes of data to be stored on a disc that has a diameter of a mere 12 centimeters and a thickness of about 1.2 millimeters. In 1997, an improved version of the CD, called a digital versatile disc (DVD), was released, which enabled the storage of full-length movies on a single disc.

CDs and DVDs are the primary data storage methods for music, software, personal computing and video. A CD can hold 783 megabytes of data. A double-sided, double-layer DVD can hold 15.9 GB of data, which is about eight hours of movies. These conventional storage mediums meet today's storage needs, but storage technologies have to evolve to keep pace with increasing consumer demand. CDs, DVDs and magnetic storage all store bits of information on the surface of a recording medium. In order to increase storage capabilities, scientists are now working on a new optical storage method called holographic memory that will go beneath the surface and use the volume of the recording medium for storage, instead of only the surface area. Three-dimensional data storage will be able to store more information in a smaller space and offer faster data transfer times.

Holographic memory is developing technology that has promised to revolutionalise the storage systems. It can store data upto 1 Tb in a sugar cube sized crystal. Data from more than 1000 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. However, both magnetic and conventional optical data storage technologies, where individual bits are stored as distinct magnetic or optical changes on the surface of a recording medium, are approaching physical limits beyond which individual bits may be too small or too difficult to store. Storing information throughout the volume of a medium—not just on its surface— offers an intriguing high-capacity alternative. Holographic data storage is a volumetric approach which, although conceived decades ago, has made recent progress toward practicality with the appearance of lower-cost enabling technologies, significant results from longstanding research efforts, and progress in holographic recording materials.


Figure 2:Reading of holographic information by (a) Illumination with the reference beam, which is diffracted by the stored interference pattern to reconstruct the original spherical wavefront of the object beam. This beam can be imaged to a single small detector, resulting in the retrieval of a single bit. (b) Illumination with the diverging object beam, which is diffracted by the stored interference beam. This beam can be focused to a detector, representing an optical measurement of the correlation between the stored data and the illuminating object beam, allowing content-addressable searching. (c) Illumination with a counter-propagating (or “phase-conjugate”) reference beam, which is diffracted by the stored interference pattern to reconstruct a phase-conjugate copy of the original beam. This phase-conjugate object beam returns to its original point of origin, where the stored bit value can be read without requiring a high-quality imaging system.

In holographic data storage, an entire page of information is stored at once as an optical interference pattern within a thick, photosensitive optical material (Figure 1). This is done by intersecting two coherent laser beams within the storage material. The first, called the object beam, contains the information to be stored; the second, called the reference beam, is designed to be simple to reproduce—for example, a simple collimated beam with a planar wavefront. The resulting optical interference pattern causes chemical and/or physical changes in the photosensitive medium: A replica of the interference pattern is stored as a change in the absorption, refractive index, or thickness of the photosensitive medium. When the stored interference grating is illuminated with one of the two waves that were used during recording [Figure 2(a)], some of this incident light is diffracted by the stored grating in such a fashion that the other wave is reconstructed. Illuminating the stored grating with the reference wave reconstructs the object wave, and vice versa [Figure 2(b)]. Interestingly, a backward-propagating or phase-conjugate reference wave, illuminating the stored grating from the “back” side, reconstructs an object wave that also propagates backward toward its original source [Figure 2(c)].

A large number of these interference gratings or patterns can be superimposed in the same thick piece of media and can be accessed independently, as long as they are distinguishable by the direction or the spacing of the gratings. Such separation can be accomplished by changing the angle between the object and reference wave or by changing the laser wavelength. Any particular data page can then be read out independently by illuminating the stored gratings with the reference wave that was used to store that page. Because of the thickness of the hologram, this reference wave is diffracted by the interference patterns in such a fashion that only the desired object beam is significantly reconstructed and imaged on an electronic camera. The theoretical limits for the storage density of this technique

In addition to high storage density, holographic data storage promises


Figure 1:Storage of one bit of information as a hologram: (a) Superposition of the spherical wave from one bit with a coherent plane wave reference beam forming an interference pattern. (b) Exposure of a photosensitive medium to the interference pattern. (c) Record of the interference grating, stored as changes in the refractive properties of the medium.

fast access times, because the laser beams can be moved rapidly without inertia, unlike the actuators in disk drives. With the inherent parallelism of its pagewise storage and retrieval, a very large compound data rate can be reached by having a large number of relatively slow, and therefore low-cost, parallel channels.

The data to be stored are imprinted onto the object beam with a pixelated input device called a spatial light modulator (SLM); typically, this is a liquid crystal panel similar to those on laptop computers or in modern camcorder viewfinders. To retrieve data without error, the objectbeam must contain a high-quality imaging system—one capable of directing this complex optical wavefront through the recording medium, where the wavefront is stored and then later retrieved, and then onto a pixelated camera chip (Figure 3).

The image of the data page at the camera must be as close as possible

to perfect. Any optical aberrations in the imaging system or misfocus of the detector array would spread energy from one pixel to its neighbors. Optical distortions (where pixels on a square grid at the SLM are not imaged to a square grid) or errors in magnification will move a pixel of the image off its intended receiver, and either of these problems (blur or shift) will introduce errors in the retrieved data. To avoid having the imaging system dominate the overall system performance, near-perfect optics would appear to be unavoidable, which of course would be expensive. However, the above-mentioned readout of phase-conjugated holograms provides a partial solution to this problem. Here the reconstructed data page propagates backward through the same optics that were used during the recording, which compensates for most shortcomings of the imaging system. However, the detector and the spatial light modulator must still be properly aligned. A rather unique feature of holographic data storage is associative retrieval: Imprinting a partial or search data pattern on the object beam and illuminating the stored holograms reconstructs all of the reference beams that were used to store data. The intensity that is diffracted by each of the stored interference gratings into the corresponding reconstructed reference beam is


Figure 3:Basic holographic data system. Data are imprinted onto the object beam with a pixelated input device called a spatial light modulator (SLM). A pair of lenses images the data through the storage material onto pixelated detector array such as a charge-coupled device (CCD). A reference beam intersects the object beam in the storage material, allowing the storage and later retrieval of holograms

proportional to the similarity between the search pattern and the content of that particular data page. By determining, for example, which reference beam has the highest intensity and then reading the corresponding data page with this reference beam, the closest match to the search pattern can be found without initially knowing its address.

Because of all of these advantages and capabilities, holographic storage has provided an intriguing alternative to conventional data storage techniques for three decades. However, it is the recent availability of relatively low-cost components, such as liquid crystal displays for SLMs and solid-state camera chips from video camcorders for detector arrays, which has led to the current interest in creating practical holographic storage devices. A team of scientists from the IBM Research Division have been involved in exploring holographic data storage, partially as a partner in the DARPA-initiated consortia on holographic data storage systems (HDSS) and on photorefractive information storage materials (PRISM). In this paper, we describe the current status of our effort.

The overall theme of our research is the evaluation of the engineering tradeoffs between the performance specifications of a practical system, as affected by the fundamental material, device, and optical physics. Desirable performance specifications include data fidelity as quantified by bit-error rate (BER), total system capacity, storage density, readout rate, and the lifetime of stored data. This report begins by describing the hardware aspects of holographic storage, including the test platforms we have built to evaluate materials and systems tradeoffs experimentally, and the hardware innovations developed during this process. Phase-conjugate readout, which eases the demands on both hardware design and material quality, is experimentally demonstrated. The second section of the report describes our work in coding and signal processing, including modulation codes, novel preprocessing techniques, the storage of more than one bit per pixel, and techniques for quantifying coding tradeoffs.

Then we discuss associative retrieval, which introduces parallel search capabilities offered by no other storage technology. The fourth section describes our work in testing and evaluating materials, including permanent or write-once read-many-times (WORM) materials, read–write materials, and photon-gated storage materials offering reversible storage without sacrificing the lifetime of stored data.

Technical Aspects

Like other media, holographic media is divided into write once (where the

storage medium undergoes some irreversible change), and rewritable media

(where the change is reversible). Rewritable holographic storage can be

achieved via the photorefractive effect in crystals:


Mutually coherent light from two sources creates an interference pattern in

the media. These two sources are called the reference beam and the signal

beam.

Where there is constructive interference the light is bright

and electrons can be promoted from the valence band to the conduction

band of the material (since the light has given the electrons energy to

jump the energy gap). The positively charged vacancies they leave are

called holes and they must be immobile in rewritable holographic

materials. Where there is destructive interference, there is less light and

few electrons are promoted.

Electrons in the conduction band are free to move in the material. They will

experience two opposing forces that determine how they move. The first

force is the Coulomb force between the electrons and the positive holes

that they have been promoted from. This force encourages the electrons to

stay put or move back to where they came from. The second is the

pseudo-force of diffusion that encourages them to move to areas where

electrons are less dense. If the coulomb forces are not too strong, the

electrons will move into the dark areas.

Beginning immediately after being promoted, there is a chance that a

given electron will recombine with a hole and move back into the valence

band. The faster the rate of recombination, the fewer the number of

electrons that will have the chance to move into the dark areas. This rate

will affect the strength of the hologram.

After some electrons have moved into the dark areas and recombined with

holes there, there is a permanent space charge field between the electrons

that moved to the dark spots and the holes in the bright spots. This leads

to a change in the index of refraction due to the electro-optic effect.


http://en.wikipedia.org/wiki/Electro-optic_effect

http://en.wikipedia.org/wiki/Refractive_index

http://en.wikipedia.org/wiki/Electric_field

http://en.wikipedia.org/wiki/Diffusion

http://en.wikipedia.org/wiki/Coulomb_force

http://en.wikipedia.org/wiki/Electron_hole

http://en.wikipedia.org/wiki/Conduction_band

http://en.wikipedia.org/wiki/Conduction_band

http://en.wikipedia.org/wiki/Valence_band

http://en.wikipedia.org/wiki/Electrons

http://en.wikipedia.org/wiki/Interference_(wave_propagation)

http://en.wikipedia.org/wiki/Signal_beam

http://en.wikipedia.org/wiki/Signal_beam

http://en.wikipedia.org/wiki/Reference_beam

http://en.wikipedia.org/wiki/Interference_pattern

http://en.wikipedia.org/wiki/Coherent

When the information is to be retrieved or read out from the hologram, only

the reference beam is necessary. The beam is sent into the material in

exactly the same way as when the hologram was written. As a result of the

index changes in the material that were created during writing, the beam

splits into two parts. One of these parts recreates the signal beam where the

information is stored. Something like a CCD camera can be used to convert

this information into a more usable form.

Holograms can theoretically store one bit per cubic block the size of

the wavelength of light in writing. For example, light from a helium-neon

laser is red, 632.8 nm wavelength light. Using light of this wavelength, perfect

holographic storage could store 4 gigabits per cubic millimeter. In practice,

the data density would be much lower, for at least four reasons:

The need to add error-correction

The need to accommodate imperfections or limitations in the optical

system

Economic payoff (higher densities may cost disproportionately more to

achieve)

Design technique limitations—a problem currently faced in magnetic Hard

Drives wherein magnetic domain configuration prevents manufacture of

disks that fully utilize the theoretical limits of the technology.

Unlike current storage technologies that record and read one data bit at a

time, holographic memory writes and reads data in parallel in a single flash of

light.


http://en.wikipedia.org/wiki/Error-correction

http://en.wikipedia.org/wiki/Nanometre

http://en.wikipedia.org/wiki/Helium-neon_laser

http://en.wikipedia.org/wiki/Helium-neon_laser

http://en.wikipedia.org/wiki/Wavelength

http://en.wikipedia.org/wiki/Bit

http://en.wikipedia.org/wiki/Charge-coupled_device

http://en.wikipedia.org/wiki/Hologram

http://en.wikipedia.org/wiki/File:Hologram_lezen.svg

HOLOGRAPHIC MEMORY LAYOUT


REMOVABLE MEDIA STORAGE DEVICES (RMSDs)

Let us have a glance on the different RMSDs.

Floppy Disk

Floppy disk drives provide faster data access because they access data randomly. Floppy drives provide an average data access speed of less than 100 milliseconds (ms). The 1.44-MB, 3.5-inch floppy is useful for storing and backing up small data files, can be used to boot computer systems, and has been the standard for data interchange between PCs. However it provides only a fraction of the storage capacity required for many files and most software programs in use today. Storing data on floppy drives also is slow. Data transfer rates average around 0.06 MB/sec.

Floppy disk

Optical Formats

Optical RMSD formats use a laser light source to read and/or write digital data to disc. CD and DVD are two major optical formats. CDs and DVDs have similar compositions consisting of a label, a protective layer, a reflective layer (aluminum, silver, or gold), a digital-data layer molded in polycarbonate, and a thick polycarbonate bottom layer

CD formats include:


Compact disc-read only memory (CD-ROM) Compact disc-recordable (CD-R) Compact disc-rewritable (CD-RW) DVD formats include: Digital versatile disc-read only memory (DVD-ROM) Digital versatile disc-recordable (DVD-R) DVD-RAM (rewritable) Digital versatile disc-rewritable (DVD-RW) +RW (rewritable)

CD-ROM

CD-ROM Standard was established in 1984.They quickly evolved into a low cost digital storage option because of CD-audio industry Data bits are permanently stored on a CD as a spiral track of physically molded pits in the surface of a plastic data layer that is coated with reflective aluminum. Smooth areas surrounding pits are called lands. CDs are extremely durable because the optical pickup (laser light source, lenses and optical elements, photoelectric sensors, and amplifiers) never touches the disc. Because data is read through the thick bottom layer, most scratches and dust on the disc surface are out of focus, so they do not interfere with the reading process. One CD-ROM (650-700 MB) storage capacity can store data from more than 450 floppy disks. Data access rate ranges from 80 to 120 ms. Data transfer rates are approximately 6 MB/sec.

DVD-ROM

The DVD-ROM standard, introduced in 1995 came over as a result of a DVD consortium. Like CD drives, DVD drives read data through the disc substrate reducing interferences from surface dust and scratches. However DVD-ROM technology provides seven times the storage capacity of CDs and accomplishes most of this increase by advancing the technology used for CD systems. The distance between recording tracks is less than half that is used for CDs. The pit size also is less than half that of CDs, which requires a reduced laser wavelength read the smaller sized pits. These features alone give DVD-ROM discs 4.5 times the storage capacity of CDs. DVD drives can also store on both sides of the disc; manufacturers deliver the two-sided structure by bonding two thinner substrates together, providing the potential to double a DVD's storage capacity. Single sided DVD discs have the two fused substrates, but only one side contains data.

In a DVD, storage of data in the data layers can be: Single-sided, single layer (4.7 GB) Double-sided, single layer (9.4 GB) Single-sided, double layer (8.5 GB)


Double-sided, double layer (17 GB)

DVD Data Storage Versions

DVD-R

DVD-R drives were introduced in 1997 to provide write-once capability on DVD-R discs used for producing disc masters in software development and for multimedia post-production. This technology sometimes referred to as DVD-R for authoring, is limited to niche applications because drives and media are expensive. DVD-R discs employ a photosensitive dye technology similar to CD-R media. At 3.95 GB per side, the first DVD-R discs provided a little less storage capacity than DVD-ROM discs. That capacity has now been extended to the 4.7-GB capacity ofDVD-ROM discs. The 1X DVD-R data transfer rate is 1.3 MB/sec. Most DVD-ROM drives and DVD video players read DVD-R discs. Slightly modified DVD-R drives and discs have recently become available for general use.


DVD-RAM

DVD-RAM (rewritable) drives were introduced in 1998. DVD-RAM devices use a phase change technology combined with some embossed land/pit features. Employing a format termed "land groove", data is recorded in the grooves formed on the disc and on the land between the grooves. The initial disc capacity was 2.6 GB per side, but a 4.7 GB- per-side version is now available. The 4.7-GB DVD-RAM discs come in cartridges that protect the medium from handling damage, such as fingerprints and scratches. A single-sided disc is expected to be removable from the cartridge so it can also be played in DVD-ROM drives that support DVD-RAM. The double-sided disc, providing 4.7GB of storage capacity per side, is not removable from the cartridge. Each DVD-RAM disc is reported to handle more than 100,000 rewrites. DVD-RAM is specifically designed for PC data storage; DVD-RAM discs use a storage structure based in sectors, instead of the spiral groove structure used for CD data storage. This sector storage is similar to the storage structure used by hard drives. Sector storage results in faster random data access speed. Because of their high cost relative to CD-RW technology, current consumer-oriented DVD-RAM drives and media are not a popular choice for PC applications. Slow adoption of DVD-RAM reading capability in DVD-ROM drives has also limited DVD-RAM market acceptance.


DVD-RW

The DVD-RW drive format is similar to the DVD-R format, but offers rewritability using a phase-change recording layer that is comparable to the phase change layer used for CD-RW. DVD-RW is intended for consumer video (non-PC) use, but PC applications are also expected for this technology. The first DVD-RW drives based on this format, which also recorded DVD-R discs, were introduced in early 2001

+RW

Sony and Philips were founding members of the DVD consortium, but broke away to introduce the DVD+RW (now called +RW) phase change, rewritable technology in 1997. Discs can be written approximately 1000 times, which makes them a good option for video recording, but not optimal for data storage. +RW technology's strongest feature is its backward compatibility with DVD-ROM drives and DVD video players.

Magneto-Optical Formats

Magneto-optical (MO) technology combines the strengths of magnetic and optical technologies by using a laser to read data and the combination of a laser and magnetic field to write data. The top (label side) of the disk is exposed to a magnetic field to write data, and a laser light source targets the data layer through the bottom substrate to read data.

There are 3.5- and 5.5-inch disk formats that contain a magnetic alloy layer. Magnetic particles in the alloy are very stable and resist changing polarity at room temperature. Data bits re recorded on this magnetic layer by heating it with a focused laser beam in the presence of magnetic field. Changes in the magnetic orientation of the data bits along a track represents Os and I s much like on hard disks and other magnetic media. The magnetic layer also changes the rotation or polarization of reflected laser light depending on the 0 or 1 polarity of the magnetic bits. This property called the "Kerr Effect" and is used to read the data. MO systems also increase the data bits vertically rather than horizontally.


The 3.5-inch disks are available in 128-, 230-, and 640- MB storage capacities. The 5.25-inch disks come in 650-MB and 1.3-, 2.6-, and 5.2-GB sizes. A 9.1 - GB size is expected soon. At less than 25ms, data access times faster than the average 100ms of phase change CD and DVD technologies. MO drives are widely used in Japan for general-purpose storage, similar to the way Zip drives are used in the U.S. Outside of Japan; applications for MO drives typically have been in niche markets for Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM), document imaging, and high-capacity archives.

Magneto-Optical disk


Holograms

A hologram is a recording of the optical interference pattern that forms at the intersection of two coherent optical beams. Typically, light from a single laser is split into two paths, the signal path and the reference path. The beam that propagates along the signal path carries information, whereas the reference is designed to be simple to reproduce. A common reference beam is a plane wave: a light beam that propagates without converging or diverging. The two paths are overlapped on the holographic medium and the interference pattern between the two beams is recorded. A key property of this interferometric recording is that when it is illuminated by a readout beam, the signal beam is reproduced. In effect, some of the light is diffracted from the readout beam to “reconstruct” a weak copy of the signal beam. If the signal beam was created by reflecting light off a 3D object, then the reconstructed hologram makes the 3D object appear behind the holographic medium. When the hologram is recorded in a thin material, the readout beam can differ from the reference beam used for recording and the scene will still appear.

Volume HologramsTo make the hologram, the reference and object beams are overlapped in a photosensitive medium, such as a photopolymer or inorganic crystal. The resulting optical interference pattern creates chemical and/or physical changes in the absorption, refractive index or thickness of the storage media, preserving a replica of the illuminating interference pattern. Since this pattern contains information about both the amplitude and the phase of the two light beams, when the recording is illuminated by the readout beam, some of the light is diffracted to “reconstruct” a weak copy of the object beam .If the object beam originally came from a 3–D object, then the reconstructed hologram makes the 3–D object reappear. Since the diffracted wave front accumulates energy from throughout the thickness of the storage material, a small change in either the wavelength or angle of the readout beam generates enough destructive interference to make the hologram effectively disappear through Bragg selectivity.


As the material becomes thicker, accessing a stored volume hologram requires tight tolerances on the stability and repeatability of the wavelength and incidence angle provided by the laser and readout optics. However, destructive interference also opens up a tremendous opportunity: a small storage volume can now store multiple superimposed holograms, each one distributed throughout the entire volume. The destructive interference allows each of these stored holograms to be independently accessed with its original reference beam. To record a second, angularly multiplexed hologram, for instance, the angle of the reference beam is changed sufficiently so that the reconstruction of the first hologram effectively disappears. The new incidence angle is used to record a second hologram with a new object beam. The two holograms can be independently accessed by changing the readout laser beam angle back and forth. For a 2-cm hologram thickness, the angular sensitivity is only 0.0015 degrees. Therefore, it becomes possible to store thousands of holograms within the allowable range of reference arm angles (typically 20–30 degrees). The maximum number of holograms stored at a single location to date is 10,000.


Figure 4Reconstruction of an image from a hologram

Underlying Technology

HOLOGRAPHY

Holographic data storage refers specifically to the use of holography to store and retrieve digital data. To do this, digital data must be imposed onto an optical wave front, stored holographically with high volumetric density, and then extracted from the retrieved optical wav front with excellent data fidelity. A hologram preserves both the phase and amplitude of an optical wave front of interest called the object beam – by recording the optical interference pattern between it and a second coherent optical beam – the reference beam. Figure shows this process.

The reference beam is designed to be simple to reproduce at a later stage (A common reference beam is a plane wave a light beam that propagates without converging or diverging). These interference fringes are recorded if the two beams have been overlapped within a suitable photosensitive media,


such as a photopolymer or inorganic crystal or photographic film. The bright and dark variations of the interference pattern create chemical and/or physical changes in the media, preserving a replica of the interference pattern as a change in absorption, refractive index or thickness.

Though holography is often referred to as 3D photography, this is a

misconception. A better analogy is sound recording where the sound field is

encoded in such a way that it can later be reproduced. In holography, some of

the light scattered from an object or a set of objects falls on the recording

medium. A second light beam, known as the reference beam, also illuminates

the recording medium, so that interference occurs between the two beams.

The resulting light field is an apparently random pattern of varying intensity

which is the hologram. It can be shown that if the hologram is illuminated by

the original reference beam, a light field is diffracted by the reference beam

which is identical to the light field which was scattered by the object or

objects. Thus, someone looking into the hologram "sees" the objects even

though they are no longer present. There are a variety of recording materials

which can be used, including photographic film.

Interference and diffraction

Interference occurs when one or more wavefronts are superimposed. Diffraction occurs whenever a wavefront encounters an object. The process of producing a holographic reconstruction is explained below purely in terms of interference and diffraction. It is somewhat simplistic, but is accurate enough to provide an understanding of how the holographic process works.

Plane wavefronts

A diffraction grating is a structure with a repeating pattern. A simple example

is a metal plate with slits cut at regular intervals. Light rays travelling through

it are bent at an angle determined by λ, the wavelength of the light and d, the

distance between the slits and is given by sinθ = λ/d.

A very simple hologram can be made by superimposing two plane waves from

the same light source. One (the reference beam) hits the photographic plate

normally and the other one (the object beam) hits the plate at an angle θ. The

relative phase between the two beams varies across the photographic plate

as 2π y sinθ/λ where y is the distance along the photographic plate. The two


beams interfere with one another to form an interference pattern. The relative

phase changes by 2π at intervals of d = λ/sinθ so the spacing of the

interference fringes is given by d. Thus, the relative phase of object and

reference beam is encoded as the maxima and minima of the fringe pattern.

When the photographic plate is developed, the fringe pattern acts as a

diffraction grating and when the reference beam is incident upon the

photographic plate, it is partly diffracted into the same angle θ at which the

original object beam was incident. Thus, the object beam has been

reconstructed. The diffraction grating created by the

two waves interfering has reconstructed the "object beam" and it is therefore

a hologram as defined above.

Point sources

A slightly more complicated hologram can be made using a point source of

light as object beam and a plane wave as reference beam to illuminate the

photographic plate. An interference pattern is formed which in this case is in

the form of curves of decreasing separation with increasing distance from the

centre.


The photographic plate is developed giving a complicated pattern which can

be considered to be made up of a diffraction pattern of varying spacing. When

the plate is illuminated by the reference beam alone, it is diffracted by the

grating into different angles which depend on the local spacing of the pattern

on the plate. It can be shown that the net effect of this is to reconstruct the

object beam, so that it appears that light is coming from a point source

behind the plate, even when the source has been removed. The light

emerging from the photographic plate is identical to the light that emerged

from the point source that used to be there. An observer looking into the plate

from the other side will "see" a point source of light whether the original

source of light is there or not.

This sort of hologram is effectively a concave lens, since it "converts" a plane

wavefront into a divergent wavefront. It will also increase the divergence of

any wave which is incident on it in exactly the same way as a normal lens

does. Its focal length is the distance between the point source and the plate.

Complex objects

To record a hologram of a complex object, a laser beam is first split into two

separate beams of light using a beam splitter of half-silvered glass or

a birefringent material. One beam illuminates the object, reflecting its image

onto the recording medium as it scatters the beam. The second (reference)

beam illuminates the recording medium directly.

According to diffraction theory, each point in the object acts as a point source

of light. Each of these point sources interferes with the reference beam,

giving rise to an interference pattern. The resulting pattern is the sum of a

large number (strictly speaking, an infinite number) of point

source + reference beam interference patterns.

When the object is no longer present, the holographic plate is illuminated by

the reference beam. Each point source diffraction grating will diffract part of

the reference beam to reconstruct the wavefront from its point source. These

individual wavefronts add together to reconstruct the whole of the object

beam.


The viewer perceives a wavefront that is identical to the scattered wavefront

of the object illuminated by the reference beam, so that it appears to him or

her that the object is still in place. This image is known as a "virtual" image as

it is generated even though the object is no longer there. The direction of the

light source seen illuminating the virtual image is that of the original

illuminating beam.

This explains, albeit in somewhat simple terms, how transmission holograms

work. Other holograms, such as rainbow and Denisyuk holograms, are more

complex but have similar principles.


When the recording is illuminated by a readout beam similar to the original reference beam, some of the light is diffracted to “reconstruct” a copy of the object beam as shown in Fig if the object beam originally came from a 3-D object, then the reconstructed hologram makes the 3-D object reappear.

To reconstruct the object exactly from a transmission hologram, the reference

beam must have the same wavelength and curvature, and must illuminate

the hologram at the same angle as the original reference beam (i.e. only the

phase can be changed). Departure from any of these conditions will give a

distorted reconstruction. While nearly all holograms are recorded using lasers,

a narrow-band lamp or even sunlight is enough to recognize the reconstructed

image.


WorkingA holographic data storage system consists of a recording medium, an optical recording system, and a photodetector array. A beam of coherent light is split into a reference beam and a signal beam which are used to record a hologram into the recording medium. The recording medium is usually a photorefractive crystal such as LiNbO3 or BaTiO3 that has certain optical characteristics. These characteristics are high diffraction efficiency, high resolution, permanent storage until erasure, and fast erasure on the application of external stimulus such as UV light. A ‘hologram’ is simply the three-dimensional interference pattern of the intersection of the reference and signal beams at 90degree to each other. This interference pattern is imprinted into the crystal as regions of positive and negative charge. To retrieve the stored hologram, a beam of light that has the same wavelength and angle of incidence as the reference beam is sent into the crystal and the resulting diffraction pattern is used to reconstruct the pattern of the signal beam.

Many different holograms may be stored in the same crystal volume by changing the angle of incidence of the reference beam. One characteristic of the recording medium that limits the usefulness of holographic storage is the property that every time the crystal is read with the reference beam, the stored hologram at that “location” is disturbed by the reference beam and some of the data integrity is lost. With current technology, recorded holograms in Fe- and Tb- doped LiNbO3 that use UV light to activate the Tb atoms can be preserved without significant decay for two years.

A series of spectral memory demonstration experiments have been conducted at the University of Oregon. These experiments employ a 780-nm commercial

semiconductor diode laser as the light source, a crystal of Tm3+:YAG as the frequency selective recording material, and an avalanche photodiode as a signal detector. The diode laser was stabilized to an external cavity containing a grating and an electro optic crystal. The intracavity electro optic crystal provides for microsecond timescale sweeping of the laser frequency over roughly one gigahertz. Two storage (reference and data) beams and one reading beam, are created from the output of the single laser source using the beam splitter and the acousto-optic modulators shown in figure. The beams are focused to a 150 m2 spot in a Tm3+:YAG crystal. The reference and data beams are simultaneous as are the read and signal beams.


The most common holographic recording system uses laser light, a beam splitter to divide the laser light into a reference beam and a signal beam, various lenses and mirrors to redirect the light, a photorefractive crystal, and an array of photodetectors around the crystal to receive the holographic data. To record a hologram, a beam of laser light is split into two beams by a mirror. These two beams then become the reference and the signal beams. The signal beam interacts with an object and the light that is reflected by the object intersects the reference beam at right angles. The resulting interference pattern contains all the information necessary to recreate the image of the object after suitable processing. The interference pattern is recorded onto the photoreactive material and may be retrieved at a later time by using a beam that is identical to the reference beam (including the wavelength and the angle of incidence into the photoreactive material). This is possible because the hologram has the property that if it is illuminated by either of the beams used to record it, the hologram causes light to be diffracted in the direction of the second beam that was used to record it, thereby recreating the reflected image of the object if the reference beam was used to illuminate the hologram. So, the reflected image must be transformed into a real image with mirrors and lenses that can be sent to the laser detector array.


There are many different volume holographic techniques that are being researched. The most promising techniques are angle-multiplexed, wavelength multiplexed, spectral, and phase conjugate holography. Angle- and wavelength- multiplexed holographic methods are very similar, with the only difference being the way data is stored and retrieved, either multiplexed with different angles of incidence of the reference beam, or with different wavelengths of the reference beam. Spectral holography combines the basic principles of volume holography using a photorefractive crystal with a time sequencing scheme to partition holograms into their own subvolume of the crystal using the collision of ultrashort laser pulses to differentiate between the image and the time-delayed reference beam. Phase-conjugate holography is a technique to 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 bam 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.

There are two main classes of materials used for the holographic storage medium. These are photorefractive crystals and photopolymers (organic films). The most commonly used photorefractive crystals used are LiNbO3 and BaTiO3. During hologram recording, the refractive index of the crystal is changed by migration of electron charge in response to the imprinted three dimensional interference patterns of reference and signal beams. As more and more holograms are superimposed into the crystal, the more decay of the holograms occurs due to interference from the superimposed holograms. Also, holograms are degraded every time they are read out because the reference beam used to read out the hologram alters the refractive nature of the crystal in that region. Photorefractive crystals are suitable for random access memory with periodic refreshing of data, and can be erased and written to many times. Photopolymers have been developed that can also be used as a holographic storage medium.


Typically the thickness of photopolymers is much less than the thickness of photorefractive crystals because the photopolymers are limited by mechanical stability and optical quality. An example of a photopolymer is DuPont’s HRF-150. This film can achieve 12 bits/nm2 with a 100 nm thickness, which is greater than DVD-ROM by a factor of two. When a hologram is recorded, the interference pattern is imprinted into the photopolymer by inducing photochemical changes in the film. The refractive index modulation is changed by changing the density of exposed areas of the film. Stored holograms are permanent and do not degrade over time or by readout of the hologram, so photopolymers are suited for read-only memory (ROM).

APPLICATION TO BINARY

In 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. For this the source beam is manipulated. In computer applications, this manipulation is in the form of bits. The next section explains the spatial light modulator, a device that converts laser light into binary data.

Spatial Light Modulator (SLM)

A spatial light modulator is used for creating binary information out of laser light. The SLM is a 2D plane, consisting of pixels which can be turned on and off to create binary 1’s and 0’s. An illustration of this is a window and a window shade. It is possible to pull the shade down over a window to block incoming sunlight. If sunlight is desired again, the shade can be raised. A spatial light modulator contains a two dimensional array of windows which are only microns wide. These windows block some parts of the incoming laser light and let other parts go through. The resulting cross section of the laser beam is a two dimensional array of binary data, exactly the same as what was represented in the SLM. 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. It is called this due to its representation as a two dimensional plane, or page, of data. Figure below shows a Spatial Light Modulator implemented with a LCD panel.


Spatial Light Modulator (SLM)

Page Data AccessBecause data is stored as page data in a hologram, the retrieval of this data must also be in this form. Page data access is the method of reading stored data in sheets, not serially as in conventional storage systems. Conventional storage was reaching its fundamental limits. One such limit is the way data is read in streams. Holographic memory reads data in the form of pages instead. For example, if a stream of 32 bits is sent to a processing unit by a conventional read head, a holographic memory system would in turn send 32 x 32 bits, or 1024 bits due to its added dimension. This 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

Once one can store a page of bits in a hologram, an interface to a computer can be made. The problem arises, however, that storing only one page of bits is not beneficial. Fortunately, the properties of holograms provide a unique solution to this dilemma. Unlike magnetic storage mechanisms which store data on their surface, holographic memories store information throughout their whole volume. 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. The thicker the volume becomes, the smaller the modifications to the source beam can be.

Angular Multiplexing

When a reference beam recreates the source beam, it needs to be at the same angle it was during recording. A very small alteration in this angle will make the regenerated source beam disappear. Harnessing this property, angular multiplexing changes the angle of the source beam by very minuscule amounts after each page of data is recorded. Depending on the sensitivity of the recording material, thousands of pages of data can be stored in the same hologram, at the same point of laser beam entry. Staying away from conventional data access systems which move mechanical matter to obtain data, the angle of entry on the source beam can be deflected by high-frequency sound waves in solids. The elimination of mechanical access methods reduces access times from milliseconds to microseconds.

Figure above shows a compact module that uses angular multiplexing. The module is composed of a photorefractive crystal in which holograms are stored, a pair of liquid crystal beam steerers (oneof which is hidden behind the crystal) that is responsible for angularly multiplexing holograms in the crystal, and an Opto Electronic Integrated Circuit (OEIC) that merges the functions of a reflective spatial light modulator (SLM) for recording holograms and a detector array for readout. One is aligned at unit magnification with the photo detectors that sense it, because


of the conjugate nature of the readout process and because the detectors are located within the same OEIC pixels as the modulators used to record the holograms. Furthermore, the OEIC provides a solution to the volatility of holograms stored in a read–write photorefractive memory.

Wavelength Multiplexing

Used mainly in conjunction with other multiplexing methods, wavelength multiplexing alters the wavelength of source and reference beams between recordings. Sending beams to the same point of origin in the recording medium at different wavelengths allows multiple pages of data to be recorded. Due to the small tuning range of lasers, however, this form of multiplexing is limited on its own.

Spatial Multiplexing

Spatial multiplexing is the method of changing the point of entry of source and reference beams into the recording medium. This form tends to break away from the non-mechanical paradigm because either the medium or recording beams must be physically moved. Like wavelength multiplexing, this is combined with other forms of multiplexing to maximize the amount of data stored in the holographic volume. Two commonly used forms of spatial multiplexing are peristrophic multiplexing and shift multiplexing.

Phase-Encoded Multiplexing

The form of multiplexing farthest away from using mechanical means to record many pages in the same volume of a holograph is called phase-encoded multiplexing. Rather than manipulate the angle of entry of a laser beam or rotate/translate the recording medium, phase encoded multiplexing changes the phase of individual parts of a reference beam. The main reference beam is split up into many smaller partial beams which cover the same area as the original reference beam. These smaller beamlets vary by phase which changes the state of the reference beam as a whole. The reference beams intersects the source beam and records the diffraction relative to the different phases of the beamlets. The phase of the beamlets can be changed by nonmechanical means, therefore speeding up access times.

Combining Multiplexing Methods

No single multiplexing method by itself is the best way to pack a hologram full of information. The true power of multiplexing is brought out in the combination of one or more methods. Hybrid wavelength and angular multiplexing systems have been tested and the results are promising. Recent tests have also been formed on spatial multiplexing methods which create a hologram the size of a compact disc, but which hold 500 times more data.


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 data is recorded in holographic medium, certain factors can lead to erroneously recorded data. One major factor is the electronic noise generated by laser beams. 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. For example, if too much light gets recorded into this zero area representing a binary 0, an erroneous change to a binary 1 might occur. Changes in both the quality of the laser beam and recording material are being researched, but these 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 BitsOnce error-free data is recorded into a hologram, methods which read data back out of it need to be error free as well. Data in page format requires a new way to provide error control. Current error control methods concentrate on a stream of bits. Because page data is in the form of a two dimensional array, error correction needs to take into account the extra dimension of bits. When a page of data is written to the holographic media, the page is separated into smaller two dimensional arrays. These subsections are appended with an additional row and column of bits. The added bits calculate the parity of each row and column of data. An odd number of bits in a row or column create a parity bit of 1 and an even number of bits creates a 0. A parity bit where the row and column meet is also created which is called an overall parity bit. The sub-sections are rejoined and sent to the holographic medium for recording. When data is read back from storage, another row and column are added called parity check bits. Because the row of parity bits evens out the data, the addition or subtraction of a bit of 0 stored data will cause two of the parity check bits to become a one. The overall parity check bit becomes a one and the place of error is calculated. The calculation occurs by finding where the column parity check bit and the row parity check bit meet up in the original data. This erroneous bit is flipped and the data is read out error free. If there happens to be two or more errors in the original data, the overall parity check bit becomes a zero and the page is re-read.

Like error control, the I/O interface to modern computer systems needs to be tailored to data retrieval in page format. Bits are no longer read from a


stream, they are sent to the computer as sheets. Clearly the I/O interface needs to be changed to accommodate for this. One of the problems with such large amounts of data being fed to a processor is that the incoming data may exceed the processor’s throughput. This is where interfacing needs to bridge the data in a coherent fashion between memory and processor. In the following subsections, two kinds of interfacing are covered which vary in a unique way.


Smart Interfacing

Smart interfacing is a method of controlling the way data is sent to the processor from holographic memory by a pre-defined set of logical commands. These logical commands come from outside the stored memory and are provided to control the way data is managed before going to the processor. An example of these pre-defined instructions are the fixed set of rules used by error detection and correction. Because these rules stay the same throughout memory retrieval, they can be hard coded into the smart interfacing agent.

Intelligent Interfacing

Seemingly the same as smart interfacing by name, intelligent interfacing is different in one important way. Intelligent interfacing has external control signals which can be manipulated to transform incoming data in a non-static manner. These signals create a way for the intelligent interfacing agent to reduce the incoming data in a meaningful way. For example, a data mining system could utilize these control signals to ignore certain data which is not a part of the pattern being searched for. Intelligent interfacing agents can contain the functionality of smart interfaces such as error control, but have the added feature of dynamically changing the way data passes through it.


Implementation

A holographic data storage system consists of a recording medium, an optical recording system, a photo detector array. A beam of coherent light is split into a reference beam and a signal beam which are used to record a hologram in the recording medium. The recording medium is usually a photo refractive crystal.

A ‘hologram’ is simply the three-dimensional interference pattern of the intersection of the reference and signal beams are perpendicular to each other. This interference pattern is imprinted into the crystal as regions of positive and negative charges. To retrieve the stored holograms, a beam of light that has the same wavelength and angle of incidence as the reference beam is sent into the crystal and the resulting diffraction pattern is used to reconstruct the pattern of the signal beam. Many different holograms may be stored in the same crystal volume by changing the angle of incidence of reference beam.

The most common holographic recording system uses laser light, a beam splitter to divide the laser light into reference beam and signal beam, various lenses and mirrors to redirect the light, a photo reactive crystal, and an array of photo detectors around the crystal to receive the holographic data. To record a hologram, a beam laser light is split into two beams by mirror. These two beams then become the reference and the signal beams. The signal


beam interacts with an object and the light that is reflected by the object intersects the reference beam at right angles. The resulting interference pattern contains all the information necessary to recreate the image of the object after suitable processing. The interference pattern is recorded on to a photo reactive material and may be retrieved at a later time by using a beam that is identical to the reference beam. This is possible because the hologram has the property that if it is illuminated by either of the beams used to record it, the hologram causes light to be diffracted in the direction of the second beam that was used to record it, there by recreating the reflected image of the object if the reference beam was used to illuminate the hologram. So the reflected must be transformed into the real image with mirrors and lenses that can be sent to the laser detector array

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 V2/3/2 where V is the volume of the recording medium and 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 ms, 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. Typical DRAM today has an access time close to 10 – 40 ns, and a recording rate of 10 GB/s

Table 1: The table on the next page 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.


Storage Medium Access TimeData Transfer

RateStorage Capacity

Holographic Memory 2.4 ns 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

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.


Hardware for holographic data storage

Figure shows the most important hardware components in a holographic storage system: the SLM used to imprint data on the object beam, two lenses for imaging the data onto a matched detector array, a storage material for recording volume holograms, and a reference beam intersecting the object beam in the material. What is not shown in Figure is the laser source, beam-forming optics for collimating the laser beam, beam splitters for dividing the laser beam into two parts, stages for aligning the SLM and detector array, shutters for blocking the two beams when needed, and wave plates for controlling polarization.

Assuming that holograms will be angle-multiplexed (superimposed yet accessed independently within the same volume by changing the incidence angle of the reference beam), a beam-steering system directs the reference beam to the storage material. Wavelength multiplexing has some advantages over angle-multiplexing, but the fast tunable laser sources at visible wavelengths that would be needed do not yet exist.

The optical system shown in Figure, with two lenses separated by the sum of their focal lengths, is called the “4-f” configuration, since the SLM and detector array turn out to be four focal lengths apart. Other imaging systems such as the Fresnel configuration (where a single lens satisfies the imaging condition between SLM and detector array) can also be used, but the 4-f system allows the high numerical apertures (large ray angles) needed for high density. In addition, since each lens takes a spatial Fourier transform in two dimensions, the hologram stores the Fourier transform of the SLM data, which is then Fourier transformed again upon readout by the second lens. This has several advantages: Point defects on the storage material do not lead to lost bits, but result in a slight loss in signal-to-noise ratio at all pixels; and the storage material can be removed and replaced in an offset position, yet the


data can still be reconstructed correctly. In addition, the Fourier transform properties of the 4-f system lead to the parallel optical search capabilities offered by holographic associative retrieval. The disadvantages of the Fourier transform geometry come from the uneven distribution of intensity in the shared focal plane of the two lenses, which we discuss in the axicon section below.


Holographic digital data storage testers

In order to study the recording physics, materials, and systems issues of holographic digital data storage in depth, we have built three precision holographic recording testers. Each of these platforms is built around the basic design shown in above figure, implementing mapping of single SLM pixels to single detector pixels using precision optics in the object beam, and angle multiplexing in the reference beam. In addition, care has been taken in the design and assembly of the components listed above but not shown in figure, in order to allow experimental access to a wide range of holographic data storage parameters with minimal instrumental contributions to the raw error rate. The three testers, described in the following sections, are called the PRISM tester, the DEMON I platform, and the DEMON II platform.

PRISM tester

The PRISM tester, built as part of the DARPA Photo Refractive Information Storage Materials consortium, was designed to allow the rigorous evaluation of a wide variety of holographic storage materials. This tester was designed for extremely low-baseline BER performance, flexibility with regard to sample geometry, and high stability for both long recording exposures and experimental repeatability. The salient features of the PRISM tester are shown in Figure.

The SLM is a chrome-on-glass mask, while the detector array is a lowframe- rate, 16-bit-per-pixel CCD camera. Custom optics of long focal length (89 mm) provide pixel matching over data pages as large as one million pixels, or one megapel. A pair of precision rotation stages directs the reference beam, which is originally below the incoming object beam, to the same horizontal plane as the object beam. By rotating the outer stage twice as far as the inner, the reference-beam angle can be chosen from the entire 360-degree angle range, with a repeatability and accuracy of approximately one microradian. (Note,


Figure 4:Primary feature of the PRISM holographic materials test apparatus. The SLM is chrome-on-glass, and the detector array a 1024 x 1024 portion of a large CCD camera. A pair of precision rotation stages allows the reference beam to enter the storage material under test at any horizontal incidence angle.

Figure 5:Histogram of received data values for an error-free one-million-pixel data page reconstructed with a one-million second pulse of read-out light. There are two measured distribution of received intensities: dark (binary “0”) pixels on the left, and bright (“1”) pixels on the right. The intensity scale covers the 12-bit range of CCD camera. In the absence of noise, each distribution would be a single spike (all detector pixel which were supposed to receive a bright pixel would measure exactly the same intensity). If the two distributions were to blur into each other, a simple intensity threshold would not be able to assign binary values without error.

however, that over two 30-degree-wide segments within this range, the reference-beam optics occlude some part of the objectbeam path.) The storage material is suspended from a three-legged tower designed for interferometric stability (better than 0.1 mm) over time periods of many seconds. The secondary optics occupy approximately 2 feet by 4 feet of optical table space, and the tower and stages approximately 4 feet by 4 feet. The system is equipped with an argon (514.5-nm) and a krypton (676-nm) laser, and all optics are optimized to work at both wavelengths. Beam-forming optics and shutters control the power and polarization of the object and reference beams, and relay optics over expand the object beam to ensure a uniform illumination of the data mask. Precision linear stages control the position of the data mask in two axes (allowing selection from a set of multiple patterns), the Fourier lenses in one axis each (to control magnification), and the crystal position in three axes. In addition, the crystal can be rotated about two axes, and the camera position controlled in three linear axes and one rotational axis. All stages and shutters are under computer control, allowing direct operator control of the system as well as unsupervised execution of long experiments. While the camera uses 1024 x 1024 detector pixels on 9-mm centers, data masks are available with pixel pitch of 36 mm (resulting in 65 536 data pixels), 18 mm (262 144 pixels), and 9 mm (1 048 576 data pixels, also known as a “megapel”). The baseline BER performance of the system without a storage material (limited only by the imaging system) was estimated to be 1 x 10218 with the low resolution mask, 1 x 10212 with the medium-resolution mask, and 1 x 1027 with the megapel data mask.

Figure 5 shows the experimental demonstration of holographic storage and retrieval of a 1Mb data page, with object and reference beams entering orthogonal faces (90-degree geometry) of a Fe-doped lithium niobate (LiNbO3) crystal.

This histogram shows the occurrence of intensity levels in the data page detected by the camera. Since the data mask pattern of bright (“1”) and dark


(“0”) pixels is known, the intensity levels of each of these classes can be plotted separately. In the absence of random noise and deterministic variations, all bright pixels would have the same detected intensity, which would be well separated from the intensity of all dark pixels, resulting in two spikes. Instead, the distribution of intensities makes it more difficult to apply a single threshold and separate the bright and dark pixels in the real data-retrieval scenario (for which the data mask pattern is, by definition, unknown). While this particular page has no detected errors, the distributions can be fitted with Gaussian approximations to provide a BER estimate of 2.4 x 1026. Since this hologram was retrieved using a readout pulse of 1 ms, this experiment implements the optical signal (but not the subsequent fast electronic readout) of a system with a readout rate of 1 Gb/s.

DEMON I

While PRISM was designed to handle any conceivable material testing requirement, the DEMON I platform, shown in Figure 6, was built to be a platform for evaluating coding and signal processing techniques. The reference/object-beam geometry was restricted to the 90-degree geometry, and the reference beam deflected with a galvanometrically actuated mirror through a simple 4-f system, limiting the variation of the angle to 610 degrees. A transmissive liquid crystal SLM, capable of displaying arbitrary data patterns, was pixel-matched onto a small, 60-Hz CCD camera in two stages. First, a precision five element zoom lens demagnified the SLM (640 3 480 pixels with 42-mm pitch) to an intermediate image plane (same pixel count on 18-mm pitch). Then a set of Fourier lenses identical to those in the PRISM imaged this plane 1:1 onto the detector array (640 3 480 pixels, but 9-mm pitch). Because of the finer pitch on the CCD, only the central 320 3 240 field of the SLM was detected. To implement true pixel matching, the detector was aligned so that light from each SLM pixel fell squarely on a single detector pixel (thus ignoring three of every four pixels on the CCD). Laser light from the green 514.5-nm line of an argon-ion laser was delivered to the platform with a single-mode polarization-preserving optical fiber, which produces a clean Gaussian intensity profile.


Optical power

delivered to the apparatus prior to the object/reference beamsplitter was as much as 400 mW. Simple linear stages move the SLM in two axes and the CCD in three axes for alignment. The entire system, not including the laser, occupies 18 3 24 inches of optical table space.

The first experiment performed on the DEMON I tester was the demonstration of multiple hologram storage at low raw BER (BER without error correction) using modulation codes, which allow decoding over smaller pixel blocks than the global thresholding described above. Using an 8-mm-thick LiNbO3:Fe crystal storage medium and a strong modulation code (8:12), 1200 holograms were superimposed and read back in rapid succession with extremely low raw BER (,2 3 1028) . In addition, the DEMON I platform has been used to implement both associative retrieval and phase-conjugate readout, as described below.


Figure 6:Salient feature of the DEMON I holographic digital data storage engine. A five-element zoom lens demagnifies the SLM to an intermediate image plane, which is then imaged to the CCD detector with a pair of lenses. The reference beam and object beams enter orthogonal faces of a LiNbO3 crystal; a galvanometrically actuated scanner changes the reference beam angle over ±10 degrees about the normal.

Figure 7:Primary features of the DEAMON II holographic digital data storage engine. Utilizing 30-mm-focal-length. Fourier transform lenses in the 90-degree geometry with a one-million-pixel SLM, this system has demonstrated areal storage densities in excess of 100 bits/µm2

DEMON II

The DEMON II holographic storage platform, shown in Figure 7, was designed to achieve high-density holographic data storage using short-focal-length optics, while including aspects of the previous two test platforms. DEMON II combines the large data pages of the PRISM tester with the dynamic SLM and the 90-degree geometry configuration of the DEMON I platform. Here, the SLM is a reflective device fabricated by IBM Yorktown, containing 1024 x 1024 pixels and illuminated via a polarizing beam splitter cube. A novel apodizer, described in the next section, provides uniform illumination over the entire data page without sacrificing input power. The magnification from the 12.8-mm pitch of the SLM pixels to the 12-mm pitch of the 41-Hz CCD camera (1024 x 1024 pixels, 41 frames per second) is built into the Fourier optics (effective focal length 30 mm). A pair of scan lenses provides an improved relay of the reference beam from the galvanometrically actuated mirror to the LiNbO3 crystal, providing diffraction-limited performance over an angular scan range of 615 degrees.


The laser light is provided by a diode-pumped solidstate laser (532 nm, doubled Nd-YAG); waveplates and polarizing beamsplitters provide control over the power in the reference beam and object beam. The use of two separate elements in the back Fourier lens (between the storage material and the detector array) allows the magnification of the optical system to be varied over a range of 60.5%. Linear stages provide two axes of motion for the storage material and three axes of motion for the detector array. The entire system, including the laser, occupies 2 feet x 2 feet. As with the PRISM and DEMON I systems, all stages and shutters are under computer control, allowing both direct operator control of the system and unsupervised execution of complex scripted experiments.

The short focal length of the DEMON II optics allows the system to demonstrate high areal storage densities (the storage capacity of each stack of holograms, divided by the area of the limiting aperture in the object beam). Since the lenses in the object beam implement a two dimensional spatial Fourier transform, an aperture placed in the central focal plane of the 4-f system (just in front of the storage material) can be described as a spatial lowpass filter. The smaller the volume allocated to each stack of holograms, the larger the capacity of a given large block of storage material. However, if the aperture is decreased too far, some of the information from the SLM fails to pass through the aperture. The size of the smallest tolerable aperture corresponds to the spatial equivalent of the Nyquist sampling condition, in which the spatial frequency sampling on the SLM (one over its pixel pitch) is twice the maximum spatial frequency allowed to pass the limiting aperture. Only for apertures equal to or larger than this so-called “Nyquist” aperture is the information from all pixels of the SLM guaranteed to pass to the detector array. Since both “positive” and “negative” spatial frequencies are represented in a centered aperture, the Nyquist aperture turns out to be equal to the inverse of the pixel pitch of the SLM, scaled by the wavelength and the focal length of the lenses. The design of the imaging optics is then complicated by this need for short focal length, since the maximum ray angle (and thus the potential for optical aberrations) is greatly increased. The optical distortion (displacement of pixel centers from a rectangular grid) in the DEMON II platform is consequently much larger than in the other two testers, reaching approximately 0.03% (0.3 pixels) in the corners of the received data page. The development of signal-processing algorithms to compensate for his mis registration between SLM and CCD pixels is a research topic that we are currently pursuing, with some initial success.


Innovative opticsIn the course of development of PRISM, DEMON I, and DEMON II, a number of challenging optical design problems arose. Here we describe two innovative hardware solutions that have been developed.

AxiconAs previously noted, the Fourier transform process used to focus the object beam into the storage media has the side effect of producing an undesired high-intensity peak on the optical axis. This intensity spike can easily saturate the photosensitive response of the storage media, resulting in severe degradation of both transmitted images and stored holograms. It has been known for many years that a potential solution to this problem can be implemented by superimposing a random phase distribution on the pixels of the SLM. In work performed by M.-P. Bernal et al. at IBM Almaden, it was shown that although such a “random phase mask” does redistribute the intensity in this spike, the alignment of such a phase mask is critical, and new optical artifacts (dark lines and interference fringe effects) are introduced in the transmitted image. These artifacts, along with the difficulty of maintaining the alignment of yet another pixelated component, have made it improbable that random phase masks will be the solution to the coherent saturation problem.


Figure 8:A convex axicon deflects the Fourier components of the pixelated data mask into a toroid. This spreads out the rays, which would otherwise focus to a single large spike at the optical axis of the focal plane and distort holographic data pages

As an alternative, we have developed several optical structures which also spread the energy in the undesired intensity spike across the Fourier transform plane, without requiring precision alignment. One particular structure of interest is the axicon, a simple cylindrically symmetric cone of glass, typically with an oblique vertex angle. Introducing the axicon in the illumination beam of the SLM distributes the undesired intensity spike along a ring in the Fourier plane. The diameter of the ring depends on the vertex angle of the conical optic, the index of refraction, and the focal length of the Fourier lens. The axicon can either be placed directly behind the data mask or SLM, as shown in Figure 8, or, preferably, imaged onto the SLM using some relay optics. In the latter case, there is some slight longitudinal alignment sensitivity (but little sensitivity to transverse position). These relay optics can double as the beam expander used to fill the SLM aperture, with the axicon placed at its input focal plane. The axicon has been shown to slow down the degradation of the objectbeam imaging path with optical exposure to the same degree as the random phase mask, without requiring precision alignment or increasing interpixel crosstalk.

Aspherical apodizer

Typical laser beams have a spatial profile dictated by the oscillation mode of the laser resonator, with the simplest mode having a Gaussian or bell-shaped profile. The simplest method for generating a beam with a uniform (or flat) spatial profile is to simply expand a Gaussian beam and use only the center portion. The power efficiency then trades off directly with the desired flatness of illumination: If an illumination flatness of 5% is required over a certain area, only 5% of the incident beam power can actually be used. It has long been desirable in laser physics to be able to efficiently generate a laser beam with a uniform cross section. Although many ingenious solutions have been proposed, the few that have been implemented generally work only over the first 1/e field points of the original Gaussian beam, and commonly suffer from poor flatness, severe diffraction effects, and distortion of the wavefront quality of the apodized beam. In addition, many solutions, including diffractive optics, create a beam which attains uniform intensity in one plane in space, but then diverges and distorts away from that plane.

As part of the design of DEMON II, the creation of “flat-top” beams was studied. This was germane not only to DEMON II, but also to ongoing work in deep-UV lithography. A new insight was obtained after a review of historical efforts in this field. A two-element telescope with transmissive optical elements was designed that produces a highly efficient flat-top laser beam with the capability of propagating for several meters with little distortion and diffraction-limited wavefront quality. The Gaussian-beam-to-flat-top converter utilizes a convex aspheric lens to introduce aberrations into the beam, redistributing the laser power from a particular incident Gaussian profile to the desired flat-top profile with a rapid-intensity roll-off at the edge. A second aspheric optic recollimates the aberrated beam, restoring the wavefront quality and allowing it to propagate for long distances without spreading. As a result, the central 60% of the output power will be uniform in intensity to 2%,


Figure 9:A pair of optical elements with aspheric surfaces distributes the power from an input beam with a Gaussian profile, resulting in an output beam of uniform intensity within a given region.

and 99.7% of the incident laser beam power is used in the output apodized beam. The roll-off of the intensity profile was carefully crafted to minimize diffraction effects from the edge of the beam during propagation. Although the input and output beam dimensions are fixed for a given apodizer, it was discovered that a single apodizer could be used from the deep UV into the far IR with only a simple focus adjustment.Fabrication of such aspheric elements has long been very difficult and costly. Recently, new computer controlled polishing technology has become available which can make the fabrication of such aspheric surfaces routine. Working closely with the vendor who developed these fabrication capabilities has allowed the DEMON II design team to build such an apodizer and to demonstrate that it works. Figure 9 shows an example of input and output intensity profiles (not showing the roll-off) measured using the apodizer. A second design will achieve tighter specifications through the use of more sophisticated optical testing devices (computer-generated holograms) during

fabrication. This apodizer represents a real step forward in the area of laser illumination control, and many potential applications in a variety of areas have already surfaced.

Phase-conjugate readoutAs described in the previous sections on tester platforms, the need for both high density and excellent imaging requires an expensive short-focal-length lens system corrected for all aberrations (especially distortion) over a large field, as well as a storage material of high optical quality. Several authors


have proposed bypassing these requirements by using phase-conjugate readout of the volume holograms. After the object beam is recorded from the SLM with a reference beam, the hologram is reconstructed with a phase-conjugate (time reversed copy) of the original reference beam. The diffracted wavefront then retraces the path of the incoming object beam in reverse, canceling out any accumulated phase errors. This should allow data pages to be retrieved with high fidelity with a low-performance lens, from storage materials fabricated as multimode fibers, or even without imaging lenses for an extremely compact system.

Most researchers have relied on the visual quality of retrieved images or detection of isolated fine structure in resolution targets as proof that phase-conjugate retrieval provides high image fidelity. This, however, is no guarantee that the retrieved data pages will be correctly received by the

detector array. In fact, the BER of pixel-matched holograms can be used as an extremely sensitive measure of the conjugation fidelity of volume holograms. Any errors in rotation, focus, x-y registration, magnification, or residual aberrations will rapidly increase the measured bit-error rate (BER) for the data page. Using the pixel-matched optics in both the DEMON I platform and the PRISM tester, we have implemented low-BER phase-conjugate readout of large data pages. Onthe PRISM tester, phase conjugation allowed the readout of megapel pages through much smaller apertures than in the original megapel experiment mentioned above, which was performed without phase conjugation. This demonstrates a thirtyfold increase in areal density per hologram.

Figure 10 shows a simplified diagram of the PRISM tester, modified for this phase-conjugate experiment. The Fourier lenses were removed, and the


Figure 10:Modified IBM PRISM test stand, used to implement a pixel-matched phase-conjugate readout of data pages containing 1024 x 1024 pixels, preented to the system on a fixed mask.

object beam was focused by a lens through the megapel mask onto a mirror placed halfway between the mask and CCD. After deflection by this mirror, the object beam was collected by a second lens, forming an image of the mask. Here an Fe-doped LiNbO3 crystal was placed to store a hologram in the 90-degree geometry. After passing through the crystal, the polarization of the reference beam was rotated and the beam was focused into a self-pumped phaseconjugate mirror using a properly oriented, nominally undoped BaTiO3

crystal. In such a configuration, the input beam is directed through the BaTiO3

crystal and into the far corner, creating random backscattering throughout the crystal. It turns out that counter-propagating beams (one scattered upon input to the crystal, one reflected from the back face) are preferentially amplified by the recording of real-time holograms, creating the two “pump” waves for a four-wave-mixing process. Since momentum (or wave vector) must be conserved among four beams (energy is already conserved because all four wavelengths are identical), and since the two “pump” beams are already counter-propagating, the output beam generated by this process must be the phase-conjugate to the input beam.

The crystal axes of the LiNbO3 were oriented such that the return beam from the phase-conjugate mirror wrote the hologram, and the strong incoming reference beam was used for subsequent readout. (Although both mutually phase-conjugate reference beams were present in the LiNbO3 during recording, only the beam returning from the phase-conjugate mirror wrote a hologram because of the orientation of the LiNbO3 crystal axes. For readout, the phase-conjugate mirror was blocked, and the incoming reference beam read this hologram, reconstructing a phase-conjugate object beam.) By turning the mirror by 90 degrees, this phase-conjugate object beam was deflected to strike the pixel-matched CCD camera. We were able to store and retrieve a megapel hologram with only 477 errors (BER = 5 x 10-4) after applying a single global threshold. The experiment was repeated with a square aperture of 2.4 mm on a side placed in the object beam at the LiNbO3

crystal, resulting in 670 errors. Even with the large spacing between SLM and CCD, this is already an areal density of 0.18 bits per mm2 per hologram. In contrast, without phase-conjugate readout, an aperture of 14 mm 3 14 mm was needed to produce low BERs with the custom optics. The use of phase conjugate readout allowed mapping of SLM pixels to detector pixels over data pages of 1024 pixels x 1024 pixels without the custom imaging optics, and provided an improvement in areal density (as measured at the entrance aperture of the storage material) of more than 30.


In a second experiment, we modified the DEMON I platform in an analogous manner, using a BaTiO3 crystal for phase conjugation and LiNbO3 for recording data bearing holograms of 320 pixels x 240 pixels. To demonstrate the phase-conjugation properties, the two retrieved pages of Figure 11 illustrate the results of passing the object beam through a phase aberrator (a 1-mm-thick plastic plate). Figure 11(a) shows the data page with only one pass through the plastic plate, demonstrating conventional, non-phase-conjugate readout, while Figure 11(b) demonstrates phase-conjugate readout, where the object beam passes through the plate once during hologram storage and then again upon readout with the phase-conjugate reference beam, correcting the phase aberrations.

One of the practical issues affecting the use of phase conjugate readout is the need to multiplex the reference beam in order to attain meaningful capacities. Instead of the single pair of reference beams shown in Figure 10, a practical system would require as many as a thousand pairs of reference-beam angles. If the two reference beams are not true phase-conjugate pairs, the differences between them will distort the resulting reconstructed data page. It is not yet clear how a practical system would be able to guarantee this phase-conjugation relationship among many reference beams.

Having discussed the optical components that imprint and detect information,

we move to a discussion of coding and signal processing, and the best possible use of these components to record and retrieve digital data from a holographic data storage system.


Portions of data pages holographically reconstructed through a phase aberration (a) without phase-conjugate readout (BER: 5 x 10-2); (b) with phase-conjugate readout (BER < 10-5), thus canceling out accumulated phase errors.

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 while achieving such important figures of merit as high density and high data rate. 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. Although the system retrieves raw data from the storage device with many errors (a high raw BER), the coding and signal processing ensures that the user data are delivered with an acceptably low level of error (a low user BER).

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.

The 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. On the retrieving side, the CCD returns pseudo-analog data values (typically camera count values of eight bits) which must be transformed back into digital data (typically one bit per pixel). The first step in this process is a post-processing step, called equalization, which attempts to undo distortions created in the recording process, still in the pseudo-analog domain. Then the array of pseudo-analog values is converted to an array of binary digital data via a detection scheme. The array of digital data is then passed first to the modulation decoder, which performs the inverse operation to modulation encoding, and then to the ECC decoder. In the next subsections, we discuss several sources of noise and distortion and indicate how the various coding and signal-processing elements can help in dealing with these problems.


Binary detectionThe 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. For instance, in sorting detection, letting N denote the number of pixels in a region, one declares the N/ 2 pixels with highest intensity to be 1s and those remaining to be 0s. This balanced condition can be guaranteed by a modulation code which encodes arbitrary data patterns into codewords represented as balanced arrays. Thus, sorting detection combined with balanced modulation coding provides a means to obviate the inaccuracies inherent in threshold detection. The price that is paid here is that in order to satisfy the coding constraint (forcing the number of 0s and 1s to be equal), each block of N pixels now represents only M bits of data. Since M is typically less than N, the capacity improvement provided by the code must exceed the code rate, r= M/N. For example, for N = 8, there are 70 ways to combine eight pixels such that exactly four are 1 and four are 0. Consequently, we can store six bits of data (64 different bit sequences) for a code rate of 75%. The code must then produce a >33% increase in the number of holographic pages stored, in order to increase the total capacity of the system in bits.

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. This is not much of a problem when most balanced arrays of size N are code words, but for other codes this process can introduce serious errors. A more complex but more accurate scheme than sorting is correlation detection, as proposed in. In this scheme, the detector chooses the codeword that achieves maximum correlation with the array of received pixel intensities. In the context of the 6:8 code described above, 64 correlations are computed for each code block, avoiding the six combinations of four 1 and four 0 pixels that are not used by the code but which might be chosen by a sorting algorithm.

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. The extent of interpixel interference can be quantified by the point-spread function, sometimes called a PSF filter. If the channel is linear and the PSF filter is known, the interpixel interference can be represented as a convolution with the original (encoded)


data pattern and then “undone” in the equalization step via a filter inverse to the PSF filter (appropriately called deconvolution).

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), inaccuracies in estimation of the PSF, and enhancementof random noise. An alternative approach to combating interpixel interference is to forbid certain patterns of high spatial frequency via a modulation code. According to the model in, for certain realistic and relatively optimal choices of system parameters (in particular at the Nyquist aperture described above), if one forbids a 1 surrounded by four 0s (in its four neighbors on the cardinal points of the compass), areal density can be improved provided that the modulation code has a rate .0.83. Such a code at rate 8:9 = 0.888 . . . is described in; in fact, describes such codes of much higher rate, but at the expense of increased complexity.

A code that forbids a pattern of high spatial frequency (or, more generally, a collection of such patterns of rapidly varying 0 and 1 pixels) is called a low-pass code. Such codes constrain the allowed pages to have limited high spatial frequency content. A general scheme for designing such codes is given in, via a strip encoding method in which each data page is encoded, from top to bottom, in narrow horizontal pixel strips. The constraint is satisfied both along the strip and between neighboring strips. Codes that simultaneously satisfy both a constant-weight constraint and a low-pass constraint are given in.


Error correctionIn contrast to modulation codes, which introduce a distributed redundancy in order to improve binary detection of pseudo-analog intensities, 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 10-12). 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, for instance. Most of the work on ECC for holographic storage has focused on more powerful Reed–Solomon (RS) codes. These codes have been used successfully in a wide variety of applications for two reasons: 1) They have very strong error-correction power relative to the required redundancy, and 2) their algebraic structure facilitates the design and implementation of fast, low-complexity decoding algorithms. As a result, there are many commercially available RS chips.

In a straightforward implementation of an ECC, such as an RS code, each byte would be written into a small array (say 2 times 4 for 8-bit bytes), and the bytes in a codeword would simply be rastered across the page. There might be approximately 250 bytes per codeword. If the errors were independent from pixel to pixel and identically distributed across the page, this would work well. However, experimental evidence shows that the errors are neither independent nor identically distributed. For example, interpixel interference can cause an error event to affect a localized cluster of pixels, perhaps larger than a single byte. And imperfections in the physical components can cause the raw BER to vary dramatically across the page (typically, the raw BER is significantly higher near the edges of the page).

Assume for simplicity that our choice of ECC can correct at most two byte errors per codeword. If the codewords are interleaved so that any cluster error can contaminate at most two bytes in each codeword, the cluster error will not defeat the error-correcting power of the code. Interleaving schemes such as this have been studied extensively for one-dimensional applications (for which cluster errors are known as burst errors). However, relatively little work has been done on interleaving schemes for multidimensional applications such as holographic recording. One recent exception is a class of sophisticated interleaving schemes for correcting multidimensional cluster errors developed in.

For certain sources of error, it is reasonable to assume that the raw-BER distribution is fixed from hologram to hologram. Thus, the raw-BER distribution across the page can be accurately estimated from test patterns. Using this information, codewords can then be interleaved in such away that not too many pixels with high raw BER can lie in the same codeword (thereby lowering the probability of decoder failure or miscorrection). This technique, known as matched interleaving, can yield a significant improvement in user BER.



Predistortion

The techniques we have described above are variations on existing coding and signal-processing methods from conventional data-storage technologies. In addition, a novel preprocessing technique unique to holographic data storage has been developed at IBM Almaden. This technique, 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). Deterministic variations among the ON pixels, such as those created by fixed-pattern noise, nonuniformity in the illuminated object beam, and even interpixel crosstalk, can be suppressed (thus decreasing BER). Many of the spatial variations to be removed are present in an image transmitted with low power from the SLM directly to the detector array. Once the particular pattern of nonuniform

brightness levels is obtained, the recording exposure for each pixel is simply calculated from the ratio between its current brightness value and the desired pixel brightness. At low density, raw-BER improvements of more than 15 orders of magnitude are possible. More significantly, at high density, interpixel crosstalk (which is deterministic once each data page is encoded) can be suppressed and raw BER improved from 10-4 to 10-12 . Figure 12 shows this experimental result, implemented on the DEMON I platform with a square aperture of 2.8 mm x 2.8 mm placed at the Fourier transform plane of the imaging optics. Another use of the predistortion technique is to increase the contrast between the 1 and 0 pixel states provided by the SLM. By using interferometric subtraction while recording the hologram, the amount of light received at the 0 detector pixels can be reduced.


Intensity histogram for high-areal-density holograms (a) before and (b) after applying the predistortion technique. Before, interpixel crosstalk broadens the brightness distribution; after, these deterministic variations are reduced, improving the BER of the system

Gray scaleThe previous sections have shown that the coding introduced to maintain acceptable BER comes with an unavoidable overhead cost, resulting in somewhat less than one bit per pixel. The 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 is shown in Figure 13. To encode and decode these gray-scale data pages, we also developed several local-thresholding methods and balanced modulation codes.

If pixels take one of g brightness levels, each pixel can convey log2 g bits of data. The total amount of stored information per page has increased, so gray-scale encoding appears to produce a straightforward improvement in both capacity and readout rate. However, gray scale also divides the system’s signal-to-noise ratio (SNR) into g - 1 part, 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. The gain in bits per pixel must then outweigh this reduction in stored holograms to increase the total capacity in bits.


Histogram of a hologram with six-scale levels recorded on the DEMON I platform using the predistortion technique.

Capacity estimation

To quantify the overall storage capacity of different grayscale encoding options, we developed an experimental capacity-estimation technique. In this technique, the dependence of raw BER on readout power is first measured experimentally. A typical curve is shown in Figure 14(a). The capacity-estimation technique then produces the relationship between M, the number of holograms that can be stored, and raw BER [Figure 14(b)]. Without the capacity-estimation technique, producing Figure 14(b) would require an exhaustive series of multiple hologram experiments.

In general, as the raw BER of the system increases, the number of holograms, M, increases slowly. In order to maintain a low user BER (say, 10212) as this raw- BER operating point increases, the redundancy of the ECC code must increase. Thus, while the number of holograms increases, the ECC code rate decreases. These two opposing trends create an “optimal” raw BER, at which the user capacity is maximized. For the Reed–Solomon ECC codes we commonly use, this optimal raw BER is approximately 1023. By computing these maximum capacities for binary data pages and grayscale data pages from g = 2 to g = 6, we were able to show that gray-scale holographic data pages provide an advantage over binary encoding in both capacity and readout rate. The use of three gray levels offered a 30% increase in both capacity and readout rate over conventional binary data pages.


Associative retrieval

As mentioned in the Introduction, volume holographic data storage conventionally implies that data imprinted on an object beam will be stored volumetrically [Figure 15(a)], to be read out at some later time by illumination with an addressing reference beam [Figure 15(b)]. 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 [Figure 15(c)]. 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, 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 theoremoptically). Because of the volume nature of the hologram, only a single slice through the 2D correlation function is produced (the other dimension has been


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

“used” already, providing the ability to correlate against multiple templates simultaneously).

The center of each correlation peak represents the 2D inner product (the simple overlap) between the input page being presented to the system and the associated stored page. If the patterns which compose these pages correspond to the various data fields of a database, and each stored page represents a data record, the optical correlation process has just simultaneously compared the entire database against the search argument . This parallelism gives content-addressable holographic data storage an inherent speed advantage over a conventional serial search, especially for large databases. For instance, if an un-indexed conventional “retrieve-from-disk-and compare” software-based database is limited only by sustained hard-disk readout rate (25 MB/s), a search over one million 1 KB records would take ~40 s. In comparison, with off-the-shelf, video-rate SLM and CCD technology, an appropriately designed holographic system could search the same records

in ~30 ms — a 1200x improvement. Custom components could enable 1000 or more parallel searches per second.

For this optical correlation process to represent a database search, the spatial patterns of bright (ON) pixels on the holographic data pages must somehow represent the digital data from fixed-length database fields. The SLM is divided into separate regions, each dedicated to a particular fixed-length field of the database. For example, a two-bit data field might be encoded by four blocks of pixels at a particular point within the SLM page. Such an encoding implements an exact search through the database. By thresholding the detected optical signal (essentially an analog quantity), any matching records are identified. Thresholding becomes commensurately more difficult, however, when many fields are being searched simultaneously. And when the


Holographic data storage system: (a) two coherent beams, one carrying a page of information, interfere within a photosensitive material to record a hologram. (b) Illuminating the hologram with the reference beam reconstructs a weak copy of the original information-bearing beam for capture with a detector array. (c) Illuminating multiple stored holograms with a new page of search information reconstructs all of the reference beams, computing in parallel the correlation between the search data and each of the stored pages.

threshold does not work correctly, completely unrelated records are identified as matches because near matches between pixel block patterns do not

represent near matches in encoded data value.

We have developed a novel data-encoding method which allows similarity or fuzzy searching, by encoding similar data values into similar pixel block patterns. As shown in Figure 16(a), data values are encoded by the position of a block of ON pixels within a vertical track, creating a “slider” (like the control found on a stereo’s graphic equalizer, for instance). As an example, the data value 128 might be encoded as a pixel block of height hs, centered within a column of 256 pixels. During the search for data values near 128, the partial overlap between the input slider block [Figure 16(b)] and the stored slider block causes the resulting correlation peak to indicate the similarity between the input query and the stored data. The holographic content-addressable system is

optically measuring the inner product between an input data page (containing a pixel block at some

position along this slider column), and each stored page (possibly containing a pixel block at the same position in the same slider column). This is the same result that would be produced by cutting holes at nearly the same spot on two sheets of black cardboard, aligning their

edges, and then holding them up to a light. The holographic system is merely condensing this partial overlap into a single intensity result, and is performing the same test on a large number of holograms simultaneously. More compact data representations can be realized by combining both fuzzy and exact search encodings. The higher-order bits would be encoded


Data encoding for fuzzy searching: (a) when storing a hologram, a small block of SLM pixels are turned ON at same location within a predefined rectangular portion (“slider” track) of the data page. (b) for correlation readout, an input query is encoded as a similar block within the same track. (c) any offset between the two blocks causes the brightness of the correlation peak to decrease. By encoding data values with the center position of the similarity between data records and the input query, implementing fuzzy searching.

compactly with binary type encoding, while the low-order bits remained available for fuzzy searching. This trades search flexibility for more capacity (in terms of fields per database record). By adding a correlation camera to the DEMON I platform, we experimentally demonstrated this fuzzy search encoding. Figure 16(c) shows results from a search of a single fuzzy-encoded data field as the input data value approached and then exceeded the stored value. The amplitude response (the square root of measured power as a function of the relative position of the input slider block) formed a triangularly shaped function. The correlation of identical rectangles creates the triangle; the signals add in field amplitude yet are detected in intensity; thus, this triangle shows up after taking the square root of the measured signals. With this fuzzy encoding technique, the analog nature of the optical output becomes an enabling feature instead of a drawback.

To demonstrate high-fidelity parallel searching of a holographic content-addressable memory, we stored a small multimedia database in our modified DEMON I system. Each hologram represented one record from an IBM query-by-image-content (QBIC) database. In the QBIC system, searches are performed across feature vectors previously extracted from the images, rather than on the images themselves. Each record included several alphanumeric fields (such as image description and image number) encoded for exact searches, and 64 fuzzy sliders containing the color histogram information (percentage of each given color within the associated image). A separate portion of the SLM page, pixel-matched onto a CCD detector for conventional address-based holographic readout, was encoded with the binary data for the small binary image. One hundred holograms were recorded in a 90-degree-geometry LiNbO3 crystal, with the reference angles chosen so that each reference beam was focused to a unique portion of the correlation camera.

Each search, initiated by a user query, ran under computer control, including display of the appropriate patterns, detection of the correlation peaks (averaging eight successive measurements to reduce detector noise), calibration by hologram strength, identification of the eight highest correlation scores, mapping of correlation bins to reference-beam angle, address-based recall of these eight holograms, decoding of the pixel-matched data pages, and, finally, display of the binary images on the computer monitor. The optical readout portion occupied only 0.25 s of the total 5-s cycle time. To find images based on color similarity, the 64 sliders were used to input the color histogram information for the upper left image in Figure 17(a). The slider patterns for this color histogram were input to the system on the SLM, resulting in 100 reconstructed reference beams. After detection, calibration, and ranking of these 100 correlation peaks, the reference beams for the brightest eight were input to the system again, resulting in eight detected data pages and thus eight decoded binary images. Figure 17(a) shows the first four of these images, indicating that the holographic search process found these images to be those which most closely matched the color histogram query. Figure 17(b) quantifies the search fidelity by plotting the detected correlation peak intensity as a function of the overlap between the


object-beam search patterns. Perfect system performance would result in a smooth monotonic curve; however, noise in the real system introduces deviations away from this curve. As expected, the feature vector for the left-hand image correlated strongly with itself, but the system was also able to correctly identify the images with the highest cross-correlation.

These sliders could also be used to select images by color distribution. Figures 17(c) and 17(d) correspond to a search for images containing 20% white and 20% light gray. Although several images were ranked slightly higher than they deserved (red circle), the system performance was impressive, considering that the background “dark” signal was twice as large as the signal. In Figures 17(e) and 17(f), the alphanumeric description field was used to search for the keyword shore. Note that because many characters are involved, both the expected and measured scores are large. However, we obtained similar results for exact search arguments as small as a single character.

With the fuzzy coding techniques we have 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.


Recording materials

Materials and media requirements for holographic data storage

Thus far, we have discussed the effects of the hardware, and of coding and signal processing, on the performance of holographic data storage systems. Desirable parameters described so far include storage capacity, data input and output rates, stability of stored data, and device compactness, all of which must be delivered at a specified (very low) user BER. To a large extent, the possibility of delivering such a system is limited by the properties of the materials available as storage media. The connections between materials properties and system performance are complex, and many tradeoffs are possible in adapting a given material to yield the best results. Here we attempt to outline in a general way the desirable properties for a holographic storage medium and give examples of some promising materials.

Properties of foremost importance for holographic storage media can be broadly characterized as “optical quality,” “recording properties,” and “stability.” These directly affect the data density and capacity that can be achieved, the data rates for input and output, and the BER.

As mentioned above, for highest density at low BER, the imaging of the input data from the SLM to the detector must be nearly perfect, so that each data pixel is read cleanly by the detector. The recording medium itself is part of the imaging system and must exhibit the same high degree of perfection. Furthermore, if the medium is moved to access different areas with the readout beam, this motion must not compromise the imaging performance. Thus, very high standards of optical homogeneity and fabrication must be maintained over the full area of the storage medium. With sufficient materials development effort and care in fabrication, the necessary optical quality has been achieved for both inorganic photorefractive crystals and organic photopolymer media. As discussed above, phase-conjugate readout could ultimately relax these requirements.

A more microscopic aspect of optical quality is intrinsic light scattering of the material. The detector noise floor produced by scattering of the readout beam imposes a fundamental minimum on the efficiency of a stored data hologram, and thus on the storage density and rate of data readout [38]. Measurements on the PRISM tester have shown that, in general, the best organic media have a higher scattering level than inorganic crystals, by about a factor of 100 or more.

Because holography is a volume storage method, the capacity of a holographic storage system tends to increase as the thickness of the medium increases, since greater thickness implies the ability to store more independent diffraction gratings with higher selectivity in reading out individual data pages without crosstalk from other pages stored in the same volume. For the storage densities necessary to make holography a


competitive storage technology, a media thickness of at least a few millimeters is highly desirable. In some cases, particularly for organic materials, it has proven difficult to maintain the necessary optical quality while scaling up the thickness, while in other cases thickness is limited by the physics and chemistry of the recording process.

Holographic recording properties are characterized in terms of sensitivity and dynamic range. Sensitivity refers to the extent of refractive index modulation produced per unit exposure (energy per unit area). Diffraction efficiency (and thus the readout signal) is proportional to the square of the index modulation times the thickness. Thus, recording sensitivity is commonly expressed in terms of the square root of diffraction efficiency,

S2 = (1/2)/(Iιt) (1)

Where I is the total intensity, is the medium thickness, and t is the exposure time; this form of sensitivity is usually given in units of cm/J. Since not all materials used are the same thickness, it is a more useful comparison to define a modified sensitivity given by the usual sensitivity times the thickness:

S'2 = S2 x ι (2)

This quantity has units of cm2/J and can be thought of as the inverse of the writing fluency required to produce a standard signal level. The unprimed variable, S2, might be used to convey the potential properties of a storage material, given that the particular sample under test is extremely thin; in contrast, S'2 quantifies the ability of a specific sample to respond to a recording exposure.

For high output data rate, one must read holograms with many pixels per page in a reasonably short time. To read a megapixel hologram in about 1 ms with reasonable laser power and to have enough signal at the detector for low error rate, a diffraction efficiency around = 3 x 10-5 is required. To write such a hologram in 1 ms, to achieve input and output data rates of 1 Gb/s, the sensitivity for this example must be at least S'2 =20 cm2/J

The term dynamic range refers to the total response of the medium when it is divided up among many holograms multiplexed in a common volume of material; it is often parameterized as a quantity known as M# (pronounced “M-number” [39]), where

M# = Σ 1/2 (3)

and the sum is over the M holograms in one location. The M# also describes the scaling of diffraction efficiency as M is increased, i.e,

= (M#/M) 2 (4)

Dynamic range has a strong impact on the data storage density that can be achieved. For example, to reach a density of 100 bits/mm2 (64 Gb/in.2) with


megapixel data pages, a target diffraction efficiency of 3 x 10-5, and area at the medium of 0.1 cm2 would require M# = 5, a value that is barely achievable with known recording materials under exposure conditions appropriate for recording high fidelity data holograms.



Three experimental search results from an all-holographic search-and-retrieve engine, operating on a database of 100 feature vectors from the IBM Query-by-image-content (QBIC) image database. (a) The best four images found when the search query was the color feature vector for the leftmost image. (b) Measured correlation score (ratio of the detected signal to the “dark” calibration value), for each of the 100 database records, as a function of the expected response (number of SLM pixels in common between the input and each stored page). (c)The best four images found when the color sliders for 20% white and 20% light grey were input. (d) Measured vs. expected correlation score. (e) The best four images found when searching for the key word shore, encoded into five characters with three nonbinary subfields per character. (f) Measured vs. expected correlation score.

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. In the case of write-once-read many (WORM) media such as photopolymers, the material response is irreversible; once the material has been fully exposed, further optical irradiation produces no further response, and the data can be interrogated by the readout beam without erasing it or distorting it. Much basic research in holographic storage has been performed using photorefractive crystals as storage media. Of these crystals, Fe-doped lithium niobate has been the workhorse. Its sensitivity is sufficient for demonstration purposes, but lacks a factor of 100 for practical application. Since photo refractives are reversible materials, they suggest the possibility of a rewritable holographic storage medium. However, because they are linear and reversible, they are subject to erasure during readout. Several schemes have been investigated for stabilizing or “fixing” the recording so that the data can be read without erasure. One scheme that does this without compromising the ability to erase the data, known as two-color recording, has received a good deal of attention recently. Recording is enabled by simultaneous irradiation of the crystal by a gating beam of different wavelength than the usual object and reference beams. In the absence of the gating wavelength, the data can be read without causing erasure. More details are given in the next section.

Stability in the dark over long periods is also an issue; organic photopolymer materials are often subject to aging processes caused by residual reactive species left in the material after recording or by stresses built up in the material during recording. Erasure may occur because of residual thermal diffusion of the molecules which record the hologram. Index modulation in photo refractives results from a space charge that is built up by the optical excitation and migration of mobile charge carriers. Stability in the dark depends on the trapping of these carriers with trap energies that are not thermally accessible at room temperature. Many kinds of materials have been investigated as holographic storage media. Table 1 is a comparison of the


properties of several that are among the best available as data storage media. Five materials are compared on the basis of optical imaging quality, scattered light level, hologram fidelity, sensitivity, M#, stability, and available thickness. These include the much-studied Fe-doped lithium niobate, two-color recording in reduced stoichiometric lithium niobate [41], and three organic materials that were chosen to typify the range of properties available from various organic materials systems. Photopolymers are very promising because of their high sensitivity and dynamic range; they are discussed in more detail below. Phenanthrenequinonedoped polymethylmethacrylate (PQ/PMMA) has excellent optical quality and is based on a photoreaction between the dopant and polymer followed by diffusion of unreacted chromophore; this requires a long thermal treatment, which is a disadvantage from a system perspective. Finally, photo-addressable polymers are also promising but are still at an early stage of development.


Summary of polymer work

Polymer materials are important candidates for holographic storage media. They promise to be inexpensive to manufacture while offering a wide variety of possible recording mechanisms and materials systems. The opportunity for fruitful development of polymer holographic media is thus very broad, and a variety of approaches to using organic materials for holography have been pursued, including organic photorefractive materials, triplet-sensitized photo chromic systems, photo-addressable polymers, and materials which produce index modulation via material diffusion. Of the latter class, PQ/PMMA is a polymer glass in which a photoreaction binds the phenanthrenequinone chromophore to the PMMA. During a thermal treatment, typically for about 24 hours, unbound PQ diffuses, and the resulting concentration gradients are frozen in place by a final uniform illumination that binds the remaining unreacted chromophore to the PMMA backbone, leading to a fixed hologram. This material has the excellent optical quality of the PMMA matrix, it is available in reasonable thickness, and its sensitivity, while somewhat low, is reasonably good. However, the current need for lengthy thermal treatment makes it unacceptable for most storage applications.

The diffusion-driven photopolymer systems offer very high sensitivity and need no such post exposure processing. The basic mechanism is a photosensitized polymerization, coupled with diffusion of monomer and other components of the material formulation under influence of the resulting concentration gradients. The medium is usually partially prepolymerized to produce a gel-like matrix, allowing rapid diffusion at room temperature. Refractive index modulation and recording of holograms result from both the density change and the difference in polarizability of the polymerized material. The magnitude of this refractive index modulation can be very high, resulting in a high dynamic range. For simple plane-wave holograms, an M# as high as 42 has been observed. For digital data holograms, the contrast of the interference pattern between object and reference beams is lower than in the plane-wave case and the recording conditions do not produce as large an index modulation. Even so, the M# observed for digital holograms on the PRISM materials tester is around 1.5, one of the highest yet observed; this value can undoubtedly be improved by optimization of the recording conditions.

The recording mechanism for photopolymers also leads to some disadvantages, including the shrinkage of the material with polymerization and the possibility of nonlinear response. Both of these distort the reconstructed holograms and thus cause errors in decoding the digital data. For some photopolymers, significant advances have been made toward eliminating these undesired properties; for example, shrinkage has been reduced to less than 0.1% while sufficient useful dynamic range for recording of data has been retained. There are additional problems in increasing the thickness of these materials to the millimeter scale that is desirable for holography, and even then the Bragg angle selectivity is not sufficient to allow enough holograms to be written in a common volume to achieve high


data density. However, through the use of nonselective multiplexing methods, it is possible to increase the density to a competitive level. One of these methods, known as peristrophic multiplexing, involves the rotation of the medium about an axis normal to its plane such that the reconstructed hologram image rotates away from the detector, allowing another hologram to be written and read. We have recently demonstrated the recording and readout with very low error rate of 70 holograms of 256 Kb each on the PRISM tester, using a combination of Bragg angle and peristrophic multiplexing.

Photopolymer materials have undergone rapid development and show great potential as write-once holographic media. Because of this rapid development, there is relatively little research addressing the issue of long-term data integrity and stability after recording. Work in this area is ongoing.

Another class of organic materials undergoing rapid development is the photo-addressable polymer systems [49]. These systems incorporate azo-dye chromophores that are highly optically anisotropic and that undergo optically induced reorientation. Thus, optical irradiation produces a large refractive index change through the birefringence induced by this reorientation process. The index change can be stabilized by incorporating the chromophores into a polymer matrix containing liquid crystal components. At this point, these materials lack a convenient means of desensitization once the data have been written, so that they do not saturate and overwrite the holograms during readout. However, the index change available via this mechanism is very large; a recording medium of this type could have very high dynamic range, and thus the potential for high data storage density, and perhaps be reversible, thus enabling rewritable storage.

The best of the photopolymers are promising as storage media for WORM data storage. The photorefractive crystals have traditionally been the favorite candidates for reversible, rewritable storage; recent work on two-color recording has shown the way to a possible solution of the volatility of reversible media during readout. The following section describes this concept.


Two-color or photon-gated holography

Two main schemes for providing nondestructive readout have been proposed, both in lithium niobate, although the concepts are applicable to a broader range of materials. The first was thermal fixing, in which a copy of the stored index gratings is made by thermally activating proton diffusion, creating an optically stable complementary proton grating. Because of the long times required for thermal fixing and the need to fix large blocks of data at a time, thermally fixed media somewhat resemble reusable WORM materials. Another class of fixing process uses two wavelengths of light. One approach uses two different wavelengths of light for recording and reading, but for storage applications this suffers from increased crosstalk and restrictions on the spatial frequencies that can be recorded. The most promising two-color scheme is “photon-gated” recording in photorefractive materials, in which charge generation occurs via a two-step process. Coherent object and reference beams at a wavelength l1 record information in the presence of gating light at a wavelength l2 . The gating light can be incoherent or broadband, such as a white-light source or LED. Reading is done at l1 in the absence of gating light. Depending on the specific implementation, either the gating light acts to sensitize the material, in which case it is desirable for the sensitivity to decay after the writing cycle, or the gating light ionizes centers in which a temporary grating can be written at the wavelength l1 . Figure 18 shows a schematic of energy levels comparing the two-color and one-color schemes for a photorefractive material with localized centers in the bandgap. A very important and unique figure of merit for photon-gated holography is the gating ratio, the ratio between the sensitivity of the material in the presence and absence of gating light.


Reduced stoichiometric lithium niobate shows both one-color sensitivity in the blue-green spectral region and two-color sensitivity for writing in the near IR and gating with blue-green light . From this it can be seen that the gating light also produces erasure. This is a consequence of the broad spectral features of reduced or Fe-doped lithium niobate. Considerable progress is envisaged if a better separation of gating and erasing functions can be achieved by storing information in deeper traps and/or using wider-bandgap materials. Figure 19 compares one-color and two-color writing in a sample of reduced, near-stoichiometric lithium niobate to illustrate the nondestructive readout that can be achieved. The gating ratio in this case was in excess of 5000.


Schematic level diagram of the one-color and two-color photorefractive effects. In stoichiometric lithium niobate, level 1 is attributed to a Nb bipolaron state or Fe2+/Fe3+ state, level 2 to a Nbli

antisite polaron, and level 3 to an Fe3+ trap. The single center modal for one-color recording is appropriate for low-power continous-wave writing.

Conventionally, lithium niobate is grown in the congruent melting composition, expressed by the quantity CLi = [Li]/([Li] + [Nb]) = 48.5%, because the identical compositions of the melt and the crystal promote high optical quality and large boules. Crystals of nominally undoped lithium niobate, grown with a stoichiometry (SLN) of 49.7% by a special double-crucible technique, were compared with those of the congruent composition (CLN). Strong differences were observed,


Typical write-read-erase curve for holographic grating in LiNbO3 crystals; (a) One-color scheme, in which an argon ion laser at 488nm, 1 W/cm2 is used for both writing (two beams) and reading (one beam). (b) Two-color scheme, in which a laser diode at 852nm(4 W/cm2 total intensity) is used for writing and an argon ion laser at 488nm, 1 W/cm2 is used for the gating step. Nondestructive reading was done with one of the unattenuated writing beams (2 W/cm2) and erasing with the gating light.

As shown in Table 2. Materials were evaluated in a plane wave geometry in which two collimated 852-nm beams from a single-frequency diode laser were incident on the sample at an external crossing angle of 20 degrees. Gating light was provided either by an Ar1 laser at 488 nm or by several GaN LEDs. Further details of the experimental setup were recently published.

Reduction of lithium niobate (heat treatment in an oxygen-poor atmosphere) induces a broad visible absorption band. This band is attributed primarily to absorption by a bipolaron consisting of an electron trapped on a regular Nb site and another trapped at a NbLi antisite, together with a strong lattice distortion. In addition, there is some contribution to the band from residual impurities such as Fe21. Irradiating with bluegreen light is the gating or sensitizing step, which produces a transient absorption around 1.6 eV. This absorption is assigned to a small polaron, or electron trapped at NbLi, produced by dissociation of the bipolaron, and is responsible for the sensitivity at 852 nm.

As we have seen, the most important photorefractive properties for two-color holographic data storage are the gating ratio (measuring the degree of nonvolatility), sensitivity, M# or dynamic range, dark decay, and optical quality. Table 2 shows most of these properties for stoichiometric and congruent compositions compared to the behavior of conventional one-color Fe-doped lithium niobate. Photorefractive sensitivity for two-color recording in lithium niobate is linear in the gating light intensity, Ig, only at low values of Ig because of competition between gating and erasing. Hence, the sensitivity in terms of incident intensities Sh2 is defined similarly to that for onecolor processes [see Equation (2)], but for a fixed and reasonably low value of Ig = 1 W/cm2.


The sensitivity in terms of absorbed power is S1 = S2/, where a is the absorption coefficient at the writing wavelength. In terms of this sensitivity, all samples studied, including the single photon Fe-doped material written at 488 nm, are almost equally sensitive. This suggests that the sensitivity is determined by the amount of light that can be absorbed at the writing wavelength. So far, the maximum absorption of writing light that we have found in reduced SLN is 6% for Ig = 1 W/cm2.

Summarizing the results of Table 2, the sensitivity gains for two-color recording in reduced, nearly stoichiometric lithium niobate with respect to the congruent material are 153 for increased stoichiometry and 203 for degree of reduction. In addition, lowering the gating wavelength from 520 nm to 400 nm gains a further factor of 10, and cooling from 208C to 08C a factor of 5.

There is an interesting difference in the behavior of one- and two-color materials with regard to dynamic range. In a one-color material, the M# is proportional to the modulation index or fringe visibility of the optical interference pattern, m = 2(I1I2)1/2/(I1 + I2). However, in a two-color material, the writing light (I1 + I2) does not erase the hologram, and the M# is proportional to (I1I2)1/2 . As a result, for object and reference beams of equal intensity, the M# is proportional to the writing intensity. While this provides a general way of increasing the dynamic range in a two-color material, the writing power requirements in the present material system become rather high in order to achieve a substantial increase in M#.

Instead of amplifying the role of the intrinsic shallow levels with stoichiometry, an alternative scheme for implementing two-color holography in lithium niobate is the introduction of two impurity dopants. One trap, such as Mn, serves as the deep trap from which gating occurs, while a more shallow trap, such as Fe, provides the more shallow intermediate level for gated recording. While this scheme provides more opportunities for tuning through choice of dopants, in general it is difficult in LiNbO3 to separate the two absorption bands enough to provide high gating ratios and thus truly nonvolatile storage. In addition, while M# improves monotonically with writing intensity for stoichiometric lithium niobate, with the two-trap method M# is maximized at a particular writing intensity, thus creating an undesirable tradeoff between recording rate and dynamic range.


Two-color, photon-gated holography provides a promising solution to the long-standing problem of destructive readout in read/write digital holographic storage. In lithium niobate, optimization of the sensitivity requires control over stoichiometry (or doping), degree of reduction, temperature, gating wavelength, and gating intensity. Two-color materials differ fundamentally from one-color materials in that the dynamic range or M# can be increased by using higher writing intensity, andthe sensitivity can be increased with higher gating intensity. Another route to increasing the M# would be to find a material which exhibits a two-color erase process. Substantial progress has been made in recent years in the field of two-color holography, and further progress can be expected on this complex and challenging problem.


Four scenarios highlighting the properties of holographic data storage: an all-solid-state memory module, which takes advantage of the potential for short access times; two rotating-disk geometrics, with either erasable or WORM-type media and finally, a data warehouse media. With its high volumetric density, holographic data storage has the potential to affect all types of data storage.

OutlookHolographic data storage has several characteristics that are unlike those of any other existing storage technologies. Most exciting, of course, is the potential for data densities and data transfer rates exceeding those of magnetic data storage. In addition, as in all other optical data storage methods, the density increases rapidly with decreasing laser wavelength. In contrast to surface storage techniques such as CD-ROM, where the density is inversely proportional to the square of the wavelength, holography is a volumetric technique, making its density proportional to one over the third power of the wavelength. In principle, laser beams can be moved with no mechanical components, allowing access times of the order of 10 ms, faster than any conventional disk drive will ever be able to randomly access data. As in other optical recording schemes, and in contrast to magnetic recording, the distances between the “head” and the media are very large, and media can be easily removable. In addition, holographic data storage has shown the capability of rapid parallel search through the stored data via associative retrieval.

On the other hand, holographic data storage currently suffers from the relatively high component and integration costs faced by any emerging technology. In contrast, magnetic hard drives, also known as direct access storage devices (DASD), are well established, with a broad knowledge base, infrastructure, and market acceptance. Are there any scenarios conceivable for holographic data storage, where its unique combination of technical characteristics could come to bear and overcome the thresholds faced by any new storage technology?

Four conceivable product scenarios are shown in Figure 20. The first two scenarios use read/write media, while the latter two are designed for WORM materials, which are much easier to develop but must support data retention times as long as tens of years. The first scenario [Figure 20(a)] takes advantage of rapid optical access to a stationary block of media, resulting in a random-access time of the order of 10 ms. The capacity is limited to about 25 GB by the size of the block of media that can be addressed by simple, inexpensive optics. Such a device could bridge the gap between conventional semiconductor memory and DASD, providing a nonvolatile holographic cache with an access time that is between DASD and dynamic random-access memory (DRAM).

Using the same optical components but replacing the stationary block of media with a rotating disk results in performance characteristics similar to those of a disk drive, albeit with terabytes (1012 bytes) of capacity per platter [Figure 20(b)]. In the CD-ROM type of embodiment [Figure 20(c)], holographic data storage takes advantage of the fact that single-exposure full-disk replication has been demonstrated. The player for the holographic ROM is conceptually very simple: The photodiode from a conventional ROM player is replaced by a CMOS camera chip, and the reconstructed data page is then imaged with suitable optics onto that camera. Combining one of the DASD-type R/W heads and possibly a number of CD-ROM-type readers, a robotic


picker, and sufficient tiles of media, a data warehouse with petabyte (1015 bytes) capacity in a standard 19-inch rack is conceivable [Figure 20(d)]. While the access time to any of the stored files is determined by the robotic picker and will be of the order of tens of seconds, the aggregate sustained data rate could be enormous. In this scenario, the relatively high component cost of a read/write holographic engine is amortized over a large volume of cheap media to obtain competitive cost per gigabyte. Will one of these scenarios with data stored in holograms materialize and become reality in the foreseeable future? In collaboration and competition witha large number of scientists from around the globe, we continue to study the technical feasibility of holographic storage and memory devices with parameters that are relevant for real-world applications. Whether this research will one day lead to products depends on the insights that we gain into these technical issues and how well holography can compete with established techniquesin the marketplace.


Application

Holographic Versatile Disc

An HVD (holographic Versatile Disc), a holographic storage media, is an advanced optical disc that’s presently in the development stage. Polaroid scientist J. van Heerden was the first to come up with the idea for holographic three-dimensional storage media in 1960. An HVD would be a successor to today’s Blu-ray and HDDVD technologies. It can transfer data at the rate of 1 Gigabit per second. The technology permits over 10 kilobits of data to be written and read in parallel with a single flash. The disc will store upto 3.9 terabyte (TB) of data on a single optical disk. Holographic data storage, a potential next generation storage technology, offers both high storage density and fast readout rate. In this article, I discuss the physical origin of these attractive technology features and the components and engineering required to realize them. I conclude by describing the current state of holographic storage research and development efforts in the context of ongoing improvement to established storage technologies.

BRIEF HISTORY

Although holography was conceived in the late 1940s, it was not considered a potential storage technology until the development of the laser in the 1960s. The resulting rapid development of holography for displaying 3-D images led researchers to realize that holograms could also store data at a volumetric density of as much as 1/ where is the wave-length of the light beam used. Since each data page is retrieved by an array of photo detectors, rather than bi-by-bit, the holographic scheme promises fast readout rates as well as high density. If a thousand holograms, each containing a million pixels, could be retrieved every second, for instance, then the output data rate would reach 1 Gigabit per second.

In the early 1990s, interest in volume-holographic data storage was rekindled by the availability of devices that could display and detect 2-D pages, including charge coupled devices (CCD), complementary metal-oxide semiconductor (CMOS) detector chips and small liquid-crystal panels. The wide availability of these devices was made possible by the commercial success of digital camera and video projectors. With these components in hand, holographic-storages researchers have begun to demonstrate the potential of their technology in the laboratory. By using the volume of the media, researchers have experimentally demonstrated that data can be stored at equivalent area densities of nearly 400 bits/sq. micron. (For


comparison, a single layer of a DVD disk stores data at ~ 4.7 bits/sq. micron) A readout rate of 10 gigabit per second has also been achieved in the laboratory.

FEATURES

Data transfer rate: 1 gbps. The technology permits over 10 kilobits of data to be written and read in

parallel with a single flash. Most optical storage devices, such as a standard CD saves one bit per

pulse. HVDs manage to store 60,000 bits per pulse in the same place. 1 HVD – 5800 CDs – 830 DVD – 160 BLU-RAY Discs.


STRUCTURE

HVD STRUCTURE

HVD structure is shown in fig 3.1 the following components are used in HVD.

1. Green writing/reading laser (650 nm).2. Red positioning/addressing laser (650 nm).3. Hologram (data).4. Polycarbon layer.5. Photopolymeric layer (data-containing layer).6. Distance layers.7. Dichroic layer (reflecting green light).8. Aluminum reflective layer (reflecting red light).9. Transparent base.

10. PIT.

HVD STRUCTURE


HVD READER PROTOTYPE

To read data from an HVD reader. The following components are used to make a reader.A blue-green laser, beam splitters to split the laser beams, mirrors to direct the laser beams, LCD panels (spatial light modulator), lenses to focus the beams, lithiumniobate crystals or photopolymers, and charge-coupled device (CCD) cameras.

HVD READER PROTOTYPE

STORAGE DATA

RECORDING DATA

A simplified HVD system consists of the following main components: Blue or green laser (532-nm wavelength in the test system) Beam splitter/merger Mirrors Spatial light modulator (SLM) CMOS sensor Polymer recording medium

The process of writing information onto an HVD begins with encoding the information into binary data to be stored in the SLM. These data are turned


into ones and zeroes represented as opaque or translucent areas on a "page" -- this page is the image that the information beam is going to pass through.

When the blue-green argon laser is fired, a beam splitter creates two beams. One beam, called the object or signal beam, will go straight, bounce off one mirror and travel through a spatial-light modulator (SLM). An SLM is a liquid crystal display (LCD) that shows pages of raw binary data as clear and dark boxes.

The information from the page of binary code is carried by the signal beam around to the light-sensitive lithium-niobate crystal. Some systems use a photopolymer in place of the crystal.

A second beam, called the reference beam, shoots out the side of the beam splitter and takes a separate path to the crystal. When the two beams meet, the interference pattern that is created stores the data carried by the signal beam in a specific area in the crystal -- the data is stored as a hologram.


RECORDING DATA

DATA IMAGE

PAGE DATA (LEFT) STORED AS HOLOGRAM (RIGHT)


READING DATA

To read the data from an HVD, you need to retrieve the light pattern stored in the hologram.

In the HVD read system, the laser projects a light beam onto the hologram – a light beam -- a light beam that is identical to the reference beam.

An advantage of a holographic memory system is that an entire page of data can be retrieved quickly and at one time. In order to retrieve and reconstruct the holographic page of data stored in the crystal, the reference beam is shined into the crystal at exactly the same angle at which it entered to store that page of data. Each page of data is stored in a different area of the crystal, based on the angle at which the reference beam strikes it.

The key component of any holographic data storage system is the angle at which the reference beam is fired at the crystal to retrieve a page of data. It must match the original reference beam angle exactly. A difference of just a thousandth of a millimeter will result in failure to retrieve that page of data.

During reconstruction, the beam will be diffracted by the crystal to allow the recreation of the original page that was stored. This reconstructed page is then projected onto the CMOS, which interprets and forwards the digital information to a computer.


READING DATA

PAGE DATA STORED AND RECREATED BY CMOSIN AN HVD (LEFT) SENSOR (RIGHT)


MORE ON HVD

High Storage capacity of 3.9 terabyte (TB) enables user to store large amount of data.

Records one program while watching another on the disc.

Edit or reorder programs recorded on the disc.

Automatically search for an empty space on the disc to avoid recording over a program.

Users will be able to connect to the Internet and instantly download subtitles and other interactive movie features

Backward compatible: Supports CDs and DVDs also.

The transfer rate of HVD is up to 1 gigabyte (GB) per second which is 40 times faster than DVD.

An HVD stores and retrieves an entire page of data, approximately 60,000 bitsof information, in one pulse of light, while a DVD stores and retrieves one bitof data in one pulse of light.


ADVANTAGES OF HDSSWith three-dimensional recording and parallel data readout, holographic memories can outperform existing optical storage techniques. In contrast to the currently available storage strategies, holographic mass memory simultaneously offers high data capacity and short data access time (Storage capacity of about 1TB/cc and data transfer rate of 1 billion bits/second).

Holographic data storage has the unique ability to locate similar features stored within a crystal instantly. A data pattern projected into a crystal from the top searches thousands of stored holograms in parallel. The holograms diffract the incoming light out of the side of the crystal, with the brightest outgoing beams identifying the address of the data that most closely resemble the input pattern. This parallel search capability is an inherent property of holographic data storage and allows a database to be searched by content.

Because the interference patter ns are spread uniformly throughout the material, it endows holographic storage with another useful capability: high reliability. While a defect in the medium for disk or tape storage might garble critical data, a defect in a holographic medium doesn't wipe out information. Instead, it only makes the hologram dimmer. No rotation of medium is required as in the case of other storage devices. It can reduce threat of piracy since holograms can’t be easily replicated.

DISADVANTAGES OF HDSS

Manufacturing cost HDSS is very high and there is a lack of availability of resources which are needed to produce HDSS. However, all the holograms appear dimmer because their patterns must share the material's finite dynamic range. In other words, the additional holograms alter a material that can support only a fixed amount of change. Ultimately, the images become so dim that noise creeps into the read-out operation, thus limiting the material's storage capacity.

A difficulty with the HDSS technology had been the destructive readout. The re- illuminated reference beam used to retrieve the recorded information also excites the donor electrons and disturbs the equilibrium of the space charge field in a manner that produces a gradual erasure of the recording. In the past, this has limited the number of reads that can be made before the signal-to -noise ratio becomes too low. Moreover, writes in the same fashion can degrade previous writes in the same region of the medium. This restricts the ability to use the three-dimensional capacity of a photorefractive for recording angle-multiplexed holograms. You would be unable to locate the data if there’s an error of even a thousandth of an inch.


COMPARISON

Parameters DVD BLU-RAY HVD

Capacity 4.7 GB 25 GB 3.9 TB

Laser wave length650 nm

(red)405 nm(blue)

532 nm (green)

Disc diameter 120 mm 120 mm 120 mm

Hard coating No yes Yes

Data transfer rate(rawdata)

11.08 mbps 36 mbps 1 gbps

INTERESTING FACTS

It has been estimated that the books in the U.S. Library of Congress, the largest library in the world, could be stored on Six HVDs. The pictures of every landmass on Earth - like the ones shown in Google Earth - can be stored on two HVDs.With MPEG4 ASP encoding, a HVD can hold anywhere between 4,600-11,900 hours of video, which is enough for non-stop playing for a year.


HVD AT A GLANCE

Media type : Ultra-high density optical disc.

Encoding : MPEG-2, MPEG-4 AVC (H.264), and VC-1.

Capacity : Theoretically up to 3.9 TB.

Usage : Data storage, High-definition video, & he possibility of ultra High-definition video.

STANDARDSOn December 9, 2004 at its 88th General Assembly the standards body Ecma International created Technical committee 44, dedicated to standardizing HVD formats based on Optware’s technology. On June 11, 2007, TC44 published the first two HVD standards ECMA-377, defining a 200 GB HVD “recordable cartridge” and ECMA-378,defining a 100 GB HVD-ROM disc. Its next stated goals are 30 GB HVD cards and submission of these standards to the International Organization for Standardization for ISO approval.

POSSIBLE APPLICATION FIELDS

There are many possible applications of holographic memory. Holographic memory systems can potentially provide the high speed transfers and large volumes of future computer system. One possible application is data mining.


Data mining is the processes of finding patterns in large amounts of data. Data mining is used greatly in large databases which hold possible patterns which can’t be distinguished by human eyes due to the vast amount of data. Some current computer system implement data mining, but the mass amount of storage required is pushing the limits of current data storage systems. The many advances in access times and data storage capacity that holographic memory provides could exceed conventional storage and speedup data mining considerably. This would result in more located patterns in a shorter amount of time.

Another possible application of holographic memory is in petaflop computing. A petaflop is a thousand trillion floating point operations per second. The fast access extremely large amounts of data provided by holographic memory could be utilized in petaflop architecture. Clearly advances are needed to in more than memory systems, but the theoretical schematics do exist for such a machine. Optical storage such as holographic memory provides a viable solution to the extreme amount of data which is required for a petaflop computing.


References E. Chuang, W. Liu, J.J. Drolet, and D. Psaltis, “Holographic

Random Access Memory (HRAM),” Proceedings of the IEEE, vol. 87, no. 11, pp. 1931-1940, 1999.

“Literature review”, www.entelky.com/holography/letrew.htm, 2000.

P.S. Ramanujam, S. Hvilsted, and R.H. Berg, “New polymer materials for erasable holographic storage,” Risc National Laboratory, Solid State Physics Department, 2000.

E. Chuang, J.J. Drolet, G. Barbasthathis, W. Liu, and D. Psaltis, “Compact Phase Conjugate Holographic Memory,” website, 2000.

G. Barbasthathis and D.J. Brady, “Multidimensional Tomographic Imaging Using Volume Holography,”Proceedings of the IEEE, vol. 87, no. 12, pp. 2098-2120, 1999.

D. Psaltis and F. Mok, “Holographic Memories,” Sci.Amer. 273, No. 5, 70 (1995).

J. F. Heanue, M. C. Bashaw, and L. Hesselink, “Volume Holographic Storage and Retrieval of Digital Data,”Science 265, 749 (1994).

J. H. Hong, I. McMichael, T. Y. Chang, W. Christian, and E. G. Paek, “Volume Holographic Memory Systems: Techniques and Architectures”, Opt. Eng. 34, 2193–2203 (1995).

D. Psaltis and G. W. Burr, “Holographic Data Storage”, Computer 31, No. 2, 52– 60 (1998).

J. W. Goodman, Introduction to Fourier Optics, McGraw-Hill Book Co., Inc., New York, 1968.

M.-P. Bernal, H. Coufal, R. K. Grygier, J. A. Hoffnagle, C. M. Jefferson, R. M. Macfarlane, R. M. Shelby, G. T. Sincerbox, P. Wimmer, and G. Wittman, “A Precision Tester for Studies of Holographic Optical Storage Materials and Recording Physics,” Appl. Opt. 35, 2360 –2374 (1996).


R. M. Shelby, J. A. Hoffnagle, G. W. Burr, C. M. Jefferson, M.-P. Bernal, H. Coufal, R. K. Grygier, H. Guenther, R. M. Macfarlane, and G. T. Sincerbox, “Pixel- Matched Holographic Data Storage with Megabit Pages,” Opt. Lett. 22, 1509 (1997).

G. W. Burr, J. Ashley, H. Coufal, R. Grygier, J. Hoffnagle, C. M. Jefferson, and B. Marcus, “Modulation Coding for Pixel-Matched Holographic Data Storage,” Opt. Lett. 22, 639 – 41 (1997).

J. L. Sanford, P. F. Greier, K. H. Yang, M. Lu, R. S. Olyha, Jr., C. Narayan, J. A. Hoffnagle, P. M. Alt, and R. L. Melcher, “A One-Megapixel Reflective Spatial Light Modulator System for Holographic Storage,” IBM J. Res. Develop. 42, No. 3/4, 411– 426 (1998).

M.-P. Bernal, G. W. Burr, H. Coufal, R. K. Grygier, J. A. Hoffnagle, C. M. Jefferson, E. Oesterschulze, R. M. Shelby, G. T. Sincerbox, and M. Quintanilla, “Effects of Multilevel Phase Masks on Interpixel Crosstalk in Holographic Data Storage,” Appl. Opt. 36, No. 14,3107–3115 (1997).

Web references:

www.holopc.com www.wikeipedia.com www.engeeniringseminars.com www.computer.howstuffworks.com www.tech-faq.com/hvd.shtml www.ibm.com - IBM Research Press Resources Holographic

Storage


Date post:	27-Apr-2015
Category:	Documents
Upload:	anoopwise
View:	1,492 times
Download:	12 times

Holographic data storage system- Seminar Report

Documents