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Getting Holographic

5/1/2004

Data storage on optical discs has been of critical importance to electronic musicians and consumers ever since CDs first appeared two decades ago. Since then, the capacity of those 12 cm plastic discs has increased from 650 MB to 4.7 GB on DVD to 25 GB on blue-laser discs, which will hit the U.S. market within a year or two.

While 25 GB sounds like a lot, experience teaches us that we will soon outgrow even that capacity, and the basic idea of representing bits of information as pits and lands in a spiral track on the disc probably can't be extended much further. However, there is hope for creating optical discs with much greater capacities, thanks to a different approach: holographic encoding. What seemed like a sci-fi dream just a few years ago will soon be an actual commercial product, thanks in large part to a company called InPhase Technologies (www.inphase-tech.com).

For those who aren't familiar with holographic storage, it begins with a laser beam that passes through a beam splitter, thus forming two identical beams (see Fig. 1a). One beam, called the signal beam, passes through a spatial light modulator (SLM), which is an array of pixels that represents one “page” of data. In the simplest SLM, each pixel is binary, either blocking the light or letting it pass. In more sophisticated SLMs, each pixel could exhibit a gray scale, allowing more or less light to pass through, which would allow it to represent, say, an 8- or 16-bit value.

Once the light passes through the SLM and reaches the recording medium, it is recombined with the other laser beam, called the reference beam, which creates an interference pattern that is recorded in a light-sensitive material. In fact, many different interference patterns can be recorded at the same physical location by changing the angle of the reference beam with respect to the signal beam and medium for each page of data. To read a page of data, the reference beam is directed to the medium at a certain angle, and the interference pattern is picked up by a detection array (see Fig. 1b).

InPhase has been working on holographic storage for several years, first introducing a medium that reacted to green laser light. Now they've introduced a medium that reacts to shorter-wavelength blue light, which allows more data to be packed into a given region. Called Tapestry, the new medium will first be implemented in the form of a 12 cm write-once disc that will come in a light-tight cartridge, much like a magneto-optical disc of today. These discs should be available later this year for developers of holographic-storage hardware, including most current manufacturers of CD and DVD drives.

The company is also working on its own recording drive that has an SLM composed of on/off LCD pixels in a 1,280×1,024 array, yielding 1.3 Mb of data per page. Each storage location on the disc will hold 500 to 1,000 pages of data. With thousands of separate storage locations, the first generation system is slated to have a total capacity of 200 GB per disc and a data-transfer rate of 20 MBps. Compare that to blue-laser DVD with 25 GB at 4.5 MBps, and you can see where this is going. Future generations of holographic discs could easily have capacities in the terabytes.

Other form factors, such as memory cards and chips, will also emerge. Imagine a slice of plastic the size of a postage stamp with a capacity of 1 GB. That's still a few years down the road, though the company's first-generation disc drive could reach the market by 2006. It's all good for musicians, whose insatiable thirst for capacity and bandwidth just might be slaked by holographic storage — at least for a while.

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