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Definition: holographic storage

A unique optical technology that records data as digital holograms in DVD-sized discs holding terabytes of data. Although research in holographic storage dates back to the 1960s, the first commercial product debuted in 2010 (see Tapestry). Whether this dramatically different technology has a future remains to be seen.

Two Lasers Write the Hologram
The first "data" laser is beamed through a matrix of LCD shutters, called a "spatial light modulator," into an optical region. The shutters are opened and closed based on the binary pattern of the data. For example, using a matrix of 1,000 by 1,000 bits, the data page would hold one million bits.

The second "reference" laser is angled into and intersects the data laser at the optical site. If the angle or frequency of the reference laser is changed, another hologram can be written into that area, overlapping and filling the exact same three-dimensional volume as the first hologram. Theoretically, thousands of pages can be written into the same optical space; however, the first commercial drive recorded 330 holograms.

One Laser Reads the Hologram
The page is read by directing just the reference laser back into the hologram. The light is diffracted into a copy of the binary data that is sensed by a matrix of CCD sensors. See micro-holographic, PRISM, HVD and optical disc.

The Spatial Light Modulator
Although this example uses cylinders as the optical medium rather than sections on a disc, the concept is the same. The LCD shutters are opened and closed to create the data page being stored.

The Interference Pattern
A unique interference pattern (the hologram) is created at the intersection of the two lasers in the optical material. To read the binary pattern, only the reference laser is used to output the data.

Early Prototype From IBM
The green laser beams are directed through lenses to the optical storage unit. The bottom magnification of the storage area shows the intersection where the hologram is created. The red arrow is the reference laser; the blue is the data laser. (Images courtesy of IBM Almaden Research Center.)