To make a hologram, a laser beam is aimed at the object or data array, and another is focused on the storage medium. The area where the rays overlap contains all the information about the object. dark and light spots representing binary computer information. Like light passing through a picture slide, the pattern is projected into the crystal. The points where the two beams intersect (interference pattern) are recorded in the crystal as a hologram. Subsequent pages are added by moving the reference beam to a slightly different angle. To retrieve a page, the reference beam is aimed at the crystal at precisely the same angle used to store the information. So while the information itself is twodimensional, the 3-D capability of holography allows data to be, in effect, piled up slightly askew like a stack of papers. "This is a very interesting and complicated set of processes that depends on the symmetry of the crystal, the physical mechanisms used to move the charge, and the electro-optic effect in the material," Gaylord says. "How the crystal responds is extremely important, and if we don't understand the underlying physical processes and learn to control them, we can't produce the commercial systems people need." For instance, engineers must learn how a holographic pattern already stored in the crystal is affected by the process of storing another pattern. There are also obstacles concerning the retrieval of information in <i- highfidelity, useable form. Georgia Tech' scientists are also concerned with the long-term stability of the crystals, and are studying samples that spent six years in orbit aboard NASA's Long Duration Exposure Facility. Researchers at Bellcore, a consortium of Bell companies based in 1 Livingston, N.J., have been working with lithium niobate and gallium 18
GEORGIA TECH • Fall 1992
arsenide crystals measuring one centimeter on a side. Theoretically, each crystal can hold 10 million holographic pages—that's roughly equivalent to a medium-size library of 200,000 novels. The limits of holographic storage will probably never be fully realized, Glytsis notes, "but even if you could use only a fraction of that capacity, it would be an enormous gain not only for optical computing, but also for memory applications." Holographic systems don't use any moving parts, and while serial computers retrieve data one bit at a time, the holographic technique processes in increments of an entire page or image. They can retrieve in one second what would take a magnetic disk drive five hours. Holographic techniques can also be used to optically process the information they store. "Various types of associative processing can be done holographically by finding matches between patterns you have received, and patterns that are stored," says Gaylord. "This can be used for identifying objects or signatures, or in data processing."
A Technology Link
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he next step is the development of optical/electronic interfaces—devices that bridge today's serial computers with the faster parallel holographic data storage systems. Here, too, holography plays an important role. "The holographic technique is more oriented to very high-speed applications, and applications where there is parallel data present," says Gaylord. "Holographically produced gratings are used to interconnect optical channels in communications
or data processing structu Professors Carl Verber ndJohn Uyemura of the School of Electrical Engineering are involved LASER in the research effort. "Electronics is serial—it does one thing at a time," Uyemura explains. By contrast, optical computing is parallel, processing many thousands of bits simultaneously. "But no matter how fast you can pn xvss something optically, you still have to get it into an electronic system so someone can use it." Verber and Uyemura use holographic techniques to direct optical beams at light detectors and memory cells embedded on an electronic circuit, while downloading it with data from several optical signals. Uyemura believes that such a hybrid approach is the most practical technological direction, and that alloptical computing—if it is ever possible—would be far into the future. "We can intercept the optical beam, process it electronically, and shoot it back out. This represents an intermediate step between electronic computing and optical computing. It's really characterizing the [optical] interconnects and what they mean."
Sottd-State of the Art
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he most familiar applications of holography are the threedimensional images that have been popping up on everything from cereal boxes to trading cards. The process for making display holograms is essentially the same as for storing data, except that instead of passing through an array of data,