Holographic data storage is a potential technology in the area of high-capacity data storage currently dominated by magnetic data storage and conventional optical data storage. Magnetic and optical data storage devices rely on individual bits being stored as distinct magnetic or optical changes on the surface of the recording medium. Holographic data storage records information throughout the volume of the medium and is capable of recording multiple images in the same area utilizing light at different angles.

Additionally, whereas magnetic and optical data storage records information a bit at a time in a linear fashion, holographic storage is capable of recording and reading millions of bits in parallel, enabling data transfer rates greater than those attained by traditional optical storage.[1]

Devices that use light to store and read data have been the backbone of data storage for nearly two

decades. Compact discsrevolutionized 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, which is equivalent to about one hour and 15 minutes of music,

but Sony has plans to release a 1.3-gigabyte (GB) high-capacity CD. 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. In this article, you will learn how a holographic storage system might be built in the

next three or four years, and what it will take to make a desktop version of such a high-density storage

system.

Recording data[edit]

Holographic data storage contains information using an optical interference pattern within a thick,

photosensitive optical material. Light from a single laser beam is divided into two separate optical

patterns of dark and light pixels. By adjusting the reference beam angle, wavelength, or media

position, a multitude of holograms (theoretically, several thousand) can be stored on a single

volume.

Reading data[edit]

The stored data is read through the reproduction of the same reference beam used to create

the hologram. The reference beam's light is focused on the photosensitive material, illuminating the

appropriate interference pattern, the light diffracts on the interference pattern, and projects the

pattern onto a detector. The detector is capable of reading the data in parallel, over one million bits

at once, resulting in the fast data transfer rate. Files on the holographic drive can be accessed in

less than 0.2 seconds.[2]

Longevity[edit]

Holographic data storage can provide companies a method to preserve and archive information. The

write-once, read many (WORM) approach to data storage would ensure content security, preventing

the information from being overwritten or modified. Manufacturers[who?] believe this technology can

provide safe storage for content without degradation for more than 50 years, far exceeding current

data storage options[dubious  –  discuss]. Counterpoints to this claim are that the evolution of data reader

technology has – in the last couple of decades – changed every ten years. If this trend continues, it

therefore follows that being able to store data for 50–100 years on one format is irrelevant, because

you would migrate the data to a new format after only ten years. However, claimed longevity of

storage has, in the past, proven to be a key indicator of shorter-term reliability of storage media.

Current optical formats – such as CD – have largely lived up to the original longevity claims (where

reputable media makes are used) and have proved to be more reliable shorter-term data carriers

than the floppy disk and DAT media they displaced.[2]

Terms used[edit]

Sensitivity refers to the extent of refractive index modulation produced per unit of exposure.

Diffraction efficiency is proportional to the square of the index modulation times the effective

thickness.

The dynamic range determines how many holograms may be multiplexed in a single volume data.

Spatial light modulators (SLM) are pixelated input devices (liquid crystal panels), used to imprint

the data to be stored on the object beam.

Technical aspects[edit]

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 beamand the signal beam.

Where there is constructive interference the light is bright and electrons can be promoted from

the valence band to the conduction bandof 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.

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 CCDcamera 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 500 megabytes per cubic millimeter. At the

extreme end of the laser spectrum, fluorine excimer laser at 157 nm could store 30 gigabytes 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.

Despite those limitations, it is possible to optimize the storage capacity using all-optical signal

processing techniques[3]

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.[4]

Two-color recording[edit]

Set up for holographic recording

For two-color holographic recording, the reference and signal beam fixed to a

particular wavelength (green, red or IR) and the sensitizing/gating beam is a separate, shorter

wavelength (blue or UV). The sensitizing/gating beam is used to sensitize the material before and

during the recording process, while the information is recorded in the crystal via the reference and

signal beams. It is shone intermittently on the crystal during the recording process for measuring the

diffracted beam intensity. Readout is achieved by illumination with the reference beam alone. Hence

the readout beam with a longer wavelength would not be able to excite the

recombined electrons from the deep trap centers during readout, as they need the sensitizing light

with shorter wavelength to erase them.

Usually, for two-color holographic recording, two different dopants are required to promote trap

centers, which belong totransition metal and rare earth elements and are sensitive to certain

wavelengths. By using two dopants, more trap centers would be created in the lithium

niobate crystal. Namely a shallow and a deep trap would be created. The concept now is to use the

sensitizing light to excite electrons from the deep trap farther from the valence band to

the conduction band and then to recombine at the shallow traps nearer to the conduction band. The

reference and signal beam would then be used to excite the electrons from the shallow traps back to

the deep traps. The information would hence be stored in the deep traps. Reading would be done

with the reference beam since the electrons can no longer be excited out of the deep traps by the

long wavelength beam.

Effect of annealing[edit]

For a doubly doped lithium niobate (LiNbO3) crystal there exists an optimum oxidation/reduction state

for desired performance. This optimum depends on the doping levels of shallow and deep traps as

well as the annealing conditions for the crystal samples. This optimum state generally occurs when

95 – 98% of the deep traps are filled. In a strongly oxidized sample holograms cannot be easily

recorded and the diffraction efficiency is very low. This is because the shallow trap is completely

empty and the deep trap is also almost devoid of electrons. In a highly reduced sample on the other

hand, the deep traps are completely filled and the shallow traps are also partially filled. This results

in very good sensitivity (fast recording) and high diffraction efficiency due to the availability of

electrons in the shallow traps. However during readout, all the deep traps get filled quickly and the

resulting holograms reside in the shallow traps where they are totally erased by further readout.

Hence after extensive readout the diffraction efficiency drops to zero and the hologram stored

cannot be fixed.

Development and marketing[edit]

Developed from the pioneering work on holography in photorefractive media and holographic data

storage of Gerard A. Alphonse. At the National Association of Broadcasters 2005 (NAB) convention

in Las Vegas, InPhase conducted public demonstrations of the world's first prototype of a

commercial storage device at the Maxell Corporation of America booth.

The three main companies involved in developing holographic memory, as of 2002,

were InPhase and Polaroid spinoff Aprilis in the United States, and Optware in Japan.[5]Although

holographic memory has been discussed since the 1960s,[6] and has been touted for near-term

commercial application at least since 2001,[7] it has yet to convince critics that it can find a viable

market.[8] As of 2002, planned holographic products did not aim to compete head to head with hard

drives, but instead to find a market niche based on virtues such as speed of access.[5]

InPhase Technologies, after several announcements and subsequent delays in 2006 and 2007,

announced that it would soon be introducing a flagship product. InPhase went out of business in

February 2010 and had its assets seized by the state of Colorado for back taxes. The company had

reportedly gone through $100 million but the lead investor was unable to raise more capital.[9][10]

In April 2009, GE Global Research demonstrated their own holographic storage material that could

allow for discs that utilize similar read mechanisms as those found on Blu-ray Disc players.[11]

Video game market[edit]

Nintendo filed a Joint Research Agreement with InPhase for holographic storage in 2008.[12]

Nintendo is also mentioned in the patent as a joint applicant: "... disclosure is herein made that the

claimed invention was made pursuant to a Joint Research Agreement as defined in 35 U.S.C. 103

(c)(3), that was in effect on or before the date the claimed invention was made, and as a result of

activities undertaken within the scope of the Joint Research Agreement, by or on the behalf of

Nintendo Co., and InPhase Technologies, Inc." [13]

Although the offices of IBM and Hewlett-Packard are nearby, Longmont, CO, is decidedly not Silicon Valley chic. But in this Denver suburb, a radical experiment in data storage is under way. At the headquarters of InPhase Technologies, where the conference rooms are named after ski resorts, chief executive Nelson Diaz holds up a clear plastic disc, about the size of a DVD but thicker, and pops it into a disc drive. A laptop connected to the drive downloads streaming video of an old episode of Seinfeld as the drive writes it to the disc.

But this is no ordinary recording process. The disc has more than 60 times the storage capacity of a standard DVD, while the drive writes about 10 times faster than a conventional DVD burner. That means the disc can store up to 128 hours of video content – almost twice enough for the full nine seasons of Seinfeld – and records it all in less than three hours.

It’s likely to be one of the first commercial systems to use “holographic storage,” in which bits are encoded in a light-sensitive material as the three-dimensional interference pattern of lasers. Unlike CDs and DVDs, which store data bit by bit on their surfaces, holographic discs store data a page at a time in three dimensions, enabling huge leaps in capacity and access speed. And InPhase, a 70-person startup spun out of Lucent Technologies’ Bell Labs in Murray Hill, NJ, is leading a handful of companies racing to commercialize this optical storage breakthrough.

Three-dimensional memory could dramatically change how we use microelectronics. Many of the remarkable advances in consumer electronics over the last few years – and much of the economic health of the industry – are directly traceable to the explosion in storage capacity. Web e-mail services routinely offer each of their customers a gigabyte of memory for free. Apple’s newest iPod

is only possible because of small, cheap hard drives that can hold a staggering 60 gigabytes of data – a storage capacity that just five years ago would have been a lot for a desktop PC.

Likewise, cell phones now come with flash memory chips easily able to store address books, calendars, photos, and the like. Meanwhile, CDs and DVDs have already transformed how people listen to music and watch movies. But each of these storage technologies has drawbacks. The density of magnetic materials in hard drives is fast approaching a fundamental physical limit. Flash memory is slow, and a DVD is barely large enough to hold a full-length movie.

Storing data in three dimensions would overcome many of these limitations. Indeed, the theoretical promise of holographic storage has been talked about for 40 years. But advances in smaller and cheaper lasers, digital cameras, projector technologies, and optical recording materials have finally pushed the technology to the verge of the market. And the ability to cram exponentially more bits into infinitesimal spaces could open up a whole new realm of applications.

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

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

InPhase’s competitive edge lies in its partnerships with Hitachi Maxell, a leading producer of computer tapes and CD-ROMs, and – as of this May – Bayer MaterialScience, one of the world’s largest makers of plastics used in optical discs. These large corporations see holographic techniques as the next step in the evolution of storage. “Our collaboration with InPhase gives us a tremendous opportunity,” says Hermann Bach, head of technologies for the Americas at Bayer MaterialScience.

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

holographic storage might actually become viable with suitably small and cheap optical components. In discussing the technical issues with their colleagues, they realized the key to making it viable was the material that stored the data.

In holographic storage, a “data beam” holding information is crossed with a “reference beam” to produce an interference pattern that’s recorded in a light-sensitive material. To retrieve data from a particular spot, a reference beam is shone onto it, and the combination of the reference beam and the patterned material reconstructs the original data beam, which is read by a digital-camera detector that translates the beam into a series of electrical signals. The recording material is typically either an inorganic crystal or a polymer. Polymers are more sensitive and require less powerful

lasers, but they have their own flaws. For instance, when you hit a photosensitive polymer with a laser, it tends to deform, which messes up the data.

In 1994, a materials team at Bell Labs led by chemist Lisa Dhar worked with Wilson and Curtis to produce a “two-chemistry” photosensitive polymer. The researchers mixed one scaffoldlike polymer, which stayed rigid and preserved its structure, with another polymer that reacted to light and stored data. Decoupling the recording material’s optical and structural properties let the researchers fine-tune each independently, arriving at a combination of sensitivity and stability that had eluded previous efforts.

Over the next four years, the Bell Labs team got its holographic material to work in conjunction with the latest miniaturized lasers, cameras, and optical components to read and write data. This also required advances in software to correct for errors in storing and retrieving digital bits. In 1998, as a proof of concept, they built a prototype holographic recorder and recorded MP3 digital audio in real time. It was a bulky contraption and not particularly efficient. But at that point, says Wilson, “we realized we could build the darn thing.”

So in mid-2000, the researchers contacted Nelson Diaz about starting up a company. Diaz had made his name in the storage industry, working as an engineer for nearly 20 years at Digital Equipment Corporation and most recently as a general manager at StorageTek in Louisville, CO, a leading maker of disk and tape drives. When first told of the researchers’ focus on holographic storage, he was skeptical: he had heard the hype for years. But the closer he looked at the Bell Labs design, the more he believed. Five months later, he signed on as chief executive of InPhase.

The first order of business, says Diaz, was getting rights to the underlying intellectual property. InPhase negotiated a deal with Bell Labs that gave it ownership of the core patents for the holographic storage system. Then, of course, the company needed funding. In late 2000, before the tech bubble collapsed, InPhase raised $15 million in three weeks “without a business plan,” says Diaz. (Storage giant Imation was a first-round investor.) So in December 2000, six researchers from Bell Labs, including Wilson, Curtis, and Dhar, moved out of the suburbs of New Jersey and joined their new CEO in Colorado.