For nearly four decades, holographic memory has been the great white whale of technology research. Despite enormous expenditures, a complete, general-purpose system that could be sold commercially continues to elude industrial and academic researchers. Nevertheless, they continue to pursue the technology aggressively because of its staggering promise.

Theoretical projections suggest that it will eventually be possible to use holographic techniques to store trillions of bytes—an amount of information corresponding to the contents of millions of books—in a piece of crystalline material the size of a sugar cube or a standard CD platter. Moreover, holographic technologies permit retrieval of stored data at speeds not possible with magnetic methods. In short, no other storage technology under development can match holography's capacity and speed potential.

These facts have attracted name-brand players, including IBM, Rockwell, Lucent Technologies and Bayer Corporation. Working both independently and in some cases as part of research consortia organized and co-funded by the U.S. Defense Advanced Research Projects Agency (DARPA), the companies are striving to produce a practical commercial holographic storage system within a decade.

Since the mid-1990s, DARPA has contributed to two groups working on holographic memory technologies: the Holographic Data Storage System (HDSS) consortium and the PhotoRefractive Information Storage Materials (PRISM) consortium. Both bring together companies and academic researchers at such institutions as the California Institute of Technology, Stanford University, the University of Arizona and Carnegie Mellon University. Formed in 1995, HDSS was given a five-year mission to develop a practical holographic memory system, whereas PRISM, formed in 1994, was commissioned to produce advanced storage media for use in holographic memories by the end of this year.

With deadlines for the two projects looming, insiders report some significant recent advances. For example, late last year at Stanford, HDSS consortium members demonstrated a holographic memory from which data could be read out at a rate of a billion bits per second. At about the same time, an HDSS demonstration at Rockwell in Thousand Oaks, Calif., showed how a randomly chosen data element could be accessed in 100 microseconds or less, a figure the developers expect to reduce to tens of microseconds. That figure is superior by several orders of magnitude to the retrieval speed of magnetic-disk drives, which require milliseconds to access a randomly selected item of stored data. Such a fast access time is possible because the laser beams that are central to holographic technologies can be moved rapidly without inertia, unlike the actuators in a conventional disk drive.

Although the 1999 demonstrations differed significantly in terms of storage media and reading techniques, certain fundamental aspects underlie both demonstration systems. An important one is the storage and retrieval of entire pages of data at one time. These pages might contain thousands or even millions of bits. Each of these pages of data is stored in the form of an optical-interference pattern within a photosensitive crystal or polymer material. The pages are written into the material, one after another, using two laser beams. One of them, known as the object or signal beam, is imprinted with the page of data to be stored when it shines through a liquid-crystal-like screen known as a spatial-light modulator. The screen displays the page of data as a pattern of clear and opaque squares that resembles a crossword puzzle.

A hologram of that page is created when the object beam meets the second beam, known as the reference beam, and the two beams interfere with each other inside the photosensitive recording material. Depending on what the recording material is made of, the optical-interference pattern is imprinted as the result of physical or chemical changes in the material. The pattern is imprinted throughout the material as variations in the refractive index, the light absorption properties or the thickness of the photosensitive material.

When this stored interference pattern is illuminated with either of the two original beams, it diffracts the light so as to reconstruct the other beam used to produce the pattern originally. Thus, illuminating the material with the reference beam re-creates the object beam, with its imprinted page of data. It is then a relatively simple matter to detect the data pattern with a solid-state camera chip, similar to those used in modern digital video cameras. The data from the chip are interpreted and forwarded to the computer as a stream of digital information.

Researchers put many different interference patterns, each corresponding to a different page of data, in the same material. They separate the pages either by varying the angle between the object and reference beams or by changing the laser wavelength.

Rockwell, which is interested in developing holographic memories for applications in defense and aerospace, optimized its demonstration system for fast data access, rather than for large storage capacities. Thus, its system utilized a unique, very high speed acousto-optical-positioning system to steer its laser through a lithium niobate crystal. By contrast, the demonstration at Stanford, including technologies contributed by IBM, Bayer and others, featured a high-capacity polymer disk medium about the size of a CD platter to store larger amounts of data. In addition, the Stanford system emphasized the use of components and materials that could be readily integrated into future commercial holographic storage products.

According to Hans Coufal, who manages IBM's participation in both HDSS and PRISM, the company's strategy is to make use of mass-produced components wherever possible. The lasers, Coufal points out, are similar to those that are found in CD players, and the spatial-light modulators resemble ordinary liquid-crystal displays.

Nevertheless, significant work remains before holographic memory can go commercial, Coufal says. He reports that the image of the data page on the camera chip must be as close to perfect as possible for holographic information storage and retrieval to work. Meeting the exacting requirements for aligning lasers, detectors and spatial-light modulators in a low-cost system presents a significant challenge.

Finding the right storage material is also a persistent challenge, according to Currie Munce, director of storage systems and technology at the IBM Almaden Research Center. IBM has worked with a variety of materials, including crystal cubes made of lithium niobate and other inorganic substances and photorefractive, photochromic and photochemical polymers, which are in development at Bayer and elsewhere. He notes that independent work by Lucent and by Imation Corporation in Oakdale, Minn., is also yielding promising media prospects. No materials that IBM has tested so far, however, have yielded the mix of performance, capacity and price that would support a mainstream commercial storage system.

Both Munce and Coufal say that IBM's long-standing interest in holographic storage intensified in the late 1990s as the associative retrieval properties of the medium became better understood. Coufal notes that past applications for holographic storage targeted the permanent storage of vast libraries of text, audio and video data in a small space. With the growing commercial interest in data mining—essentially, sifting through extremely large warehouses of data to find relationships or patterns that might guide corporate decision making and business process refinements—holographic memory's associative retrieval capabilities seem increasingly attractive.

After data are stored to a holographic medium, a single desired data page can be projected that will reconstruct all reference beams for similarly patterned data stored in the media. The intensity of each reference beam indicates the degree to which the corresponding stored data pattern matches the desired data page. "Today we search for data on a disk by its sector address, not by the content of the data," Coufal explains. "We go to an address and bring information in and compare it with other patterns. With holographic storage, you could compare data optically without ever having to retrieve it. When searching large databases, you would be immediately directed to the best matches."

While the quest for the ideal storage medium continues, practical applications such as data mining increase the desirability of holographic memories. And with even one business opportunity clearly defined, the future of holographic storage systems is bright indeed.