The use of holographs to store data has taken a step closer to becoming a commercial reality. InPhase Technologies and Bayer MaterialScience have announced plans to launch a holographic storage device and recording media by the end of next year.
Similar in size to today's magneto optical drives, this holographic storage system will initially feature removable 130 mm discs with a capacity of 300 Gbytes—or about 50 times as much as a DVD and 460 times as much as a CD. With further development, the same holographic technology could produce discs capable of storing far more data. "Our roadmap is for 1.6 terabyte discs," says Lisa Dhar, InPhase's vice president for media development.
To record data holographically, InPhase starts by splitting a laser beam in two. One of the beams passes through a device called a "spatial light modulator," an array of pixels that can selectively block or transmit light. These devices encode the data-carrying beam with pages of data in the form of a "checkerboard" pattern of light and dark elements. According to Dhar, each data page would typically carry one million bits based on a modulator array of 1,000 × 1,000 pixels. That number, she adds, would vary with the size of the modulator.
The other half of the split beam, meanwhile, acts as a reference beam. It intersects the data-carrying beam within the storage medium, creating an interference pattern of light and dark regions. This pattern registers as localized changes in the refractive index of a photo-sensitive polymer inside the storage medium. The same reference beam later reconstructs the data pages by diffracting off the stored interference pattern and creating a hologram of the data pages. A special detector finally displays these representations of the data.
The advantages of such a system boil down to capacity and speed. Dhar points out that the system stores data through the entire volume of the disc, "not just in grooves and pits of DVDs and CDs." What's more, InPhase has developed multiplexing techniques that allow data pages to overlap within the same disc volume. These involve making subtle changes in the angle of the data and reference beams during recording and readout. As for speed, the system offers transfer rates of 20 Mbytes/sec, in part because the system handles such large amounts of data every time it creates a hologram.
Much of the technology behind holographic storage hardly qualifies as new. InPhase, which traces its roots back to Bell Labs, has been pursuing the technology since December 2000. Some of its researchers have been working on the technology for more than a decade. But some of the system's hardware components have cost too much to make high-volume holographic systems practical in the past. In Dhar's view, this cost picture has changed over the past couple of years. She cites blue-wavelength lasers and modulators borrowed from digital projectors as two examples of components that are becoming more affordable. "All the pieces have started to fall into place," she says.
The same goes for availability of the speciality polymers used in the system's recording media. InPhase and Bayer MaterialScience have signed an agreement to jointly develop polymers used for the holographic storage medium. These include materials for the medium's photo-sensitive layer, which consists of a photo-polymerizable compound embedded in a thermoset matrix. Hermann Bach, who directs Bayer's new technology introductions in the Americas, notes that this proprietary light-sensitive polymer differs from photo-sensitive technology that Bayer announced last fall. That earlier polymer, which may turn up in other types of holographic systems, is based on acrylic with azobenzene and liquid-crystal side chains.
Bayer will also collaborate with InPhase on the thermoplastic material that sandwiches the storage disc's photo-sensitive layer, creating a three-layer disc structure. In its prototype systems, InPhase employed a polyolefin for the outer layers because it provided a close match of the refractive index of the active layer. Commercial versions of the system, however, may use new polycarbonate grades developed by Bayer.
InPhase's holographic storage technology creates and stores holograms that represent large pages of data. It does so by first splitting a laser beam in two. One of the resulting beams passes through a pixelated modulator, which encodes it with data. The other serves as a reference beam which intersects with the data-carrying beam to form a hologram in the system's storage medium. That same reference beam later reconstructs the data pages.