Data Storage in 3-D

Companies developing holographic data storage systems have made enormous technical advances toward their goal of inventing an optical data storage system with far greater storage capacity and faster access to data than even blue-laser digital versatile discs (DVDs). The first commercial applications, though modest, are feasible in the short term. And polymer companies, recognizing a nascent market, are keen on leveraging their expertise for the technology.

But it will be years before the average person owns a holographic disc drive. Conventional DVDs only recently became commonplace. Blue-laser DVDs, now being commercially introduced, have been slow to take off because the mass market doesn't need them yet. Holographic data storage will have to wait its turn.

The shorter wavelength of the blue-laser DVD versus the conventional red-laser DVD enables smaller and closer pits--representing bits of data--to be stored along the two-dimensional plane of the disc. This advance leads to a fivefold increase in the amount of data that the laser can read on the disc.

Holographic data storage works on a completely different principle. In it, a signal--or object--beam is encoded with digital information using a spatial light modulator. It makes an optical interference pattern with a reference beam, originating from the same laser, to form the data-encoded hologram. A light-sensitive material, usually a photopolymerizable medium, records the hologram. The data can be retrieved later by scanning with a beam identical to the reference beam.

Lisa Dhar, vice president of media development with InPhase Technologies, one of the leading developers of the technology, explains that instead of encoding a 2-D surface as in conventional DVDs, a holographic storage system can write information throughout the entire volume of a storage medium such as a disc. "Here, what you are doing is writing macroscopic spots on the order of 1 to 2 mm in diameter," she says. "But within that spot you are writing diffraction gradients that have patterns of nodes and peaks of a few hundred nanometers. So you are actually storing within that spot something on the order of a hundred to a thousand data pages."

InPhase started out as a research group within Lucent Technologies' Bell Laboratories and was spun off in 2000. Although the exact chemistry of InPhase's media technology is being kept as a trade secret, Dhar says the company uses a two-component system. One is a photopolymerizable monomer; the other is a cross-linked polymer matrix that provides structure.

IN APRIL, the company demonstrated a prototype that holds 200 gigabits per square inch of storage capacity. The company is planning to commercially launch a disc next year that has a capacity of 300 gigabytes. By the end of the decade, it hopes to have a 1.6-terabyte disc ready. A blue-laser DVD, by contrast, maxes out at 100 GB of capacity, Dhar says.

Moreover, InPhase claims a data transfer advantage, with its first commercial product boasting a data transfer rate of 20 megabytes per second. It is planning to improve this to 120 MB. Blue-laser DVDs are capable of transferring about 12 MB per second, Dhar says.

In April, Bayer MaterialScience signed a joint development agreement with InPhase and purchased an equity stake in the company for $5 million. Hermann Bach, head of technologies for the Americas at Bayer MaterialScience, says holographic data storage is a natural fit for the firm, which provides polycarbonate resin to the DVD market. "We are very much in the business of providing materials for optical data storage," he says, "and consequently, we have a lot of expertise in the area."

There's a chemistry fit for Bayer as well. "These are materials that we are also familiar with. We have a polymer platform that is very much at the heart of the InPhase technology," Bach says, although Bayer doesn't currently sell InPhase's exact materials.

Bayer even has a history with holographic data storage in the form of a research program it ran in the 1990s focusing on polymers with azobenzene side chains. The problem was that Bayer focused on rewritable materials--a subtler chemistry requiring reversible polymerization reactions--right from the start. "It was too many technology jumps at the same time," Bach says.

Another company that has been developing media for holographic data storage is Aprilis of Maynard, Mass. The company, which was spun off from Polaroid in 1999, is using epoxy-modified siloxy silane chemistry, according to Glenn Horner, vice president of business development. Cationic ring-opening polymerization is induced in the siloxy silanes through a photoacid generator. The medium also contains a siloxy silane binder and is sandwiched between two glass substrates.

In 2002, the company unveiled a 120-mm disc capable of 200 GB of storage capacity and a 200-MB-per-second data transfer rate.

Dow Corning signed a supply agreement with Aprilis in 2003 and is also one of the major investors in the company. Dow Corning runs a ventures arm that develops new technologies for silicon-based materials, as well as a photonics program that targets next-generation technologies such as optical circuitry. "We are always on the lookout for new photonics applications," says Peter Lo, R&D manager of the firm's photonics program. "And, of course, the Aprilis technology fits quite well."

Like Bayer, Dow Corning has chemical expertise applicable to holographic data storage, namely epoxy-modified siloxy silanes, which it already uses for paper release coatings and optical waveguides. "The way the Aprilis materials are made is a core competency," Lo says.

THE CHEMISTRY of holographic data storage has been challenging and, in the opinion of Hans J. Coufal, a researcher at IBM Almaden Research Center, still needs refinement for large-scale commercial applications. "We are getting close," he says.

IBM has been involved with holographic data storage since the 1990s as part of two consortia working with the U.S. military's Defense Advanced Research Projects Agency (DARPA). Aprilis was also involved with the projects. "We established a protocol for testing materials: measuring bit-error rates, for example, so you can quantify the quality of the holographic data storage," Coufal says.

Coufal says the optical media has to be long lasting--perhaps up to 100 years for data archiving applications--and able to withstand the same conditions as magnetic tape. Also, the materials have to be cost-effective. Some of the earlier holographic data storage experiments used lithium niobate crystals, which aren't very sensitive and can only be written by lasers that cost upward of $50,000.

The slight shrinkage that typically comes with polymerization poses a problem for today's holographic data storage photopolymers, Coufal says. "If you illuminate it with the same light that you use for recording, you reconstruct an object beam that is not quite what you used for recording," he explains.

Coufal considers the approaches taken by Aprilis and InPhase to be the two best solutions to this problem so far. InPhase uses the cross-linked matrix to maintain the integrity of the system. Aprilis, Horner explains, uses the cationic ring- opening polymerization, which chemically compensates for the increase in density that typically accompanies polymerization.

Another horizon for the chemistry of holographic data storage is the development of rewritable media. The product that InPhase is launching next year will not be rewritable. In 2002, though, the firm received a grant from the National Institute of Standards & Technology's Advanced Technology Program to develop a rewritable disc.

The real limitation of holographic data storage is not the technology, however, but the marketplace. Holographic technology is meant to eventually replace blue-laser DVD. And, according to IMS Research, only about 61 million blue-laser recorders are expected to be shipped globally by 2009. "If it is taking a long time for Blu-ray to catch on, it is an indication that consumers don't have a demand for writing high quantities of data yet," Horner acknowledges.

Already, one developer of holographic data storage, a U.K. firm named Polight that used an inorganic chemistry, has gone out of business for want of funding.

This is why InPhase will work on archival markets in the meantime, Dhar says. "The information technology market is looking for something better than tape for archiving," she says. "You can see the technology starting out in the archiving market and then moving into the consumer market."

Aprilis has a similar strategy. "It did not make a lot of sense for us to get something out there, have a market that is relatively limited, and then struggle along until 2010," says Horner, who considers mass-market use of holographic data storage to be in "a holding pattern."

TO GENERATE revenue, Aprilis is leveraging the holographic technology for other applications. Horner says the firm is taking advantage of "searching by optical correlation," whereby the signal and the reference beams are switched and holographic information matching the data coded on the signal beam gives the reference beam as the output, allowing the search to be done quickly on the media itself.

Aprilis is working on a system using holographic data storage and optical correlation that could conduct a 100-GB search in five seconds. The same search could take half an hour by scanning information from tape memory or a hard drive and loading it into a computer for processing. Aprilis is using this technology to scan biometric data like fingerprints and is approaching users such as the Department of Homeland Security.

It's possible, however, that companies working on holographic technology may not need such ancillary applications for long. Given the unpredictable nature of technology, Dow Corning's Lo points out, the momentum could suddenly shift in holographic data storage's favor. "You need a killer application," he says.

Lo is confident that the technology can be ramped up when the need arises. "We have the capability built," he says, "so we are all waiting."