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Holographic storage company InPhase Technologies announced this week that it has broken a storage density record by writing 64.3 gigabytes of data onto a single square inch of disc space. This advance could eventually lead to a holographic disc that can hold more than 100 DVD-quality movies, according to the company. By comparison, magnetic disks, such as those in the hard drives of computers, can manage a storage density of about 37.5 gigabytes per square inch of disk.

“We’ve now demonstrated that we’re more than two years ahead of magnetic storage,” says Kevin Curtis, chief technology officer of InPhase, a Longmont, CO, company. Curtis explains that while magnetic storage – the leading option for storing large amounts of data – is quickly approaching its physical limit, holographic storage will grow rapidly without major technological overhauls for at least another five to six years.

InPhase plans to release its first two products at the end of this year: a holographic disk drive and a 300-gigabyte disc. This rollout will be followed by 800-gigabyte discs in 2008 and 1.6 terabyte (1,600 gigabyte) discs in 2010, which, Curtis says, will be compatible with the holographic disk drive released in 2006.

The concept of using a hologram – a three-dimensional image generated by lasers – to store information dates back to the 1960s (see Holographic Memory). But it’s only been within the last five years that the optical storage technology has become feasible, says Demetri Psaltis, electrical engineering professor at the California Institute of Technology. One reason the technology is now more commercially viable is because the lasers needed to read and write data have become smaller and cheaper.

Other types of optical storage disks – CDs, DVDs, Blu-Ray, HD DVD – also use lasers to read and write information. However, they can keep data only on their surfaces, whereas holographic products store data in three dimensions – the key to holding an entire movie library on a single disc.

To write three-dimensional information to a disc, engineers exploit the fact that beams of light, when crossed in a certain way, generate holograms. The process begins as a single blue laser beam is split into the signal beam and reference beam. Information is encoded into the signal beam as a pattern of light and dark pixels, representing 0s and 1s. When the signal beam is crossed with the reference beam, a three-dimensional hologram of the array of pixels is imprinted into a photosensitive medium in the disc.

This array is called a “page” of data; InPhase’s discs contain about 1.3 million bits of data per page, says Curtis. Furthermore, he explains, multiple pages can be placed at slightly different angles to each other, to make up what he calls a “book.” Angling the pages allows the engineers to place numerous pages within the same three-dimensional disc space, maximizing the amount of data that can be stored within a disc.

The record storage density was achieved by creating 320 data pages per book. Each page was offset from the next one by an angle of just 0.067 degrees. Three tracks were then created, spaced 700 micrometers apart. In order to further increase capacity, Curtis says, the company will increase the number of pages per book by reducing the angle between pages even more, and also by reducing the spacing between the books to 480 micrometers.

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