In the era of the Internet, massive amounts of information and multimedia have become easily accessible in every corner of the world. The decreasing cost of storing data, and the increasing storage capacities of ever smaller devices, have been key enablers of this revolution. Current storage needs are being met because improvements in conventional technologies - such as magnetic hard-disk drives, optical disks and semiconductor memories - have been able to keep pace with the demand for greater and faster storage.
However, there is strong evidence that these surface-storage technologies are approaching fundamental limits that may be difficult to overcome as the ever-smaller regions that store bits of information become less thermally stable and harder to access. Exactly when this limit will be reached remains an open question: some experts predict these barriers will be encountered in 2-3 years, while others believe they will not be reached for at least another five years. In either case, one or more successors to current data-storage technologies will be needed in the near future.
An intriguing approach for the next generation of data-storage systems uses optical holography to store information throughout the three-dimensional volume of a material. And by superimposing many holograms within the same volume of the recording medium, it should be possible to achieve far greater storage densities than current technologies can offer.
The rapid development of holography for displaying 3-D images led to the realization that holograms could potentially store data at a volumetric density of one bit per cubic wavelength. Given a typical laser wavelength of around 500 nm, this density corresponds to 1012 bits (1 terabit) per cubic centimetre or more.
In holographic storage, data are transferred to and from the storage material as 2-D images composed of thousands of pixels, each of which represents a single bit of information. Since an entire "page" of data can be retrieved by a photodetector at the same time, rather than bit-by-bit, the holographic scheme promises fast read-out rates as well as high storage densities. If a thousand holograms, each containing a million pixels, could be retrieved every second, for example, then the output data rate would reach 1 gigabit per second. (In comparison, a DVD optical-disk player reads data 100 times slower.) Despite this attractive potential, however, research into holographic data storage all but died out in the mid-1970s due to the lack of suitable devices that could transfer 2-D pixelated images.
Interest in volume-holographic data storage was rekindled in the early 1990s by the availability of charge coupled devices (CCD), semiconductor detectors, small liquid-crystal panels and other devices that can display and detect 2-D pages of data. The wide availability of these devices was made possible by the commercial success of hand-held camcorders, digital cameras and video projectors.
With these components at their disposal, researchers have begun to demonstrate the potential of holographic storage and have shown that data can be stored at densities equivalent to 390 bits per square micron. This density exceeds the storage capabilities of DVD disks by a factor of almost 20, and of magnetic disks by a factor of five.
The potential of holographic storage has generated numerous research efforts at large multinational companies, including Lucent, Imation, Panasonic, Sony and NEC, as well as independent start-up companies such as Holoplex, Tamarak and Accuwave. Several consortia of universities and industries are funded by the Defense Advanced Research Project Agency (DARPA) in the US with industrial partners including IBM, Siros, Rockwell, Kodak, Bayer and Aprilis.
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