holographic storage

An optical technology that records data as digital holograms that fill up the entire volume of a small optical cylinder one millimeter in diameter. It truly is an amazing technology.

Two Lasers Write the Hologram
The first laser (the data laser) is beamed through a matrix of LCD shutters, called a "spatial light modulator," into an optical cylinder or section of an optical disc. The shutters are opened or closed based on the binary pattern of the data matrix (data page) being stored. For example, using a matrix of 1,200 pixels on each side, the page would hold one 1.44 megabits.

The second laser (the reference laser) is angled into and intersects the data laser at the optical site. If the angle and/or frequency is changed, another hologram can be written into that same area, overlapping and filling the same three-dimensional volume as the first hologram. In fact, as hard as it is to imagine, each bit occupies the full volume of the material.

Theoretically, thousands of holograms (pages) can be written into and overlap each other in the same optical space; however, the first devices on the market are limited to only a few hundred overlapping 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 data that is sensed by a matrix of CCD sensors.

Optical - Yes, But Not Like CDs and DVDs
Although lasers are used in both holographic and CD/DVD technologies, the storage media are intrinsically different and function differently. The holographic medium stores wavelengths of light as a 3-D hologram that occupies the entire volume of a particular region of the material. For reading, the hologram is decoded back into its binary page. On CDs and DVDs, lasers alter the molecules on the disk's surface to represent a 0 or 1 digit, which are themselves the binary data.

Back to the Sixties
Holographic devices that hold terabytes of data on a single CD-sized disk are expected to become the high-capacity storage medium by 2015. Although research in this area dates back to the 1960s, holographic storage was not announced as a commercial product until 2002, when InPhase demonstrated its Tapestry holographic disk drive and media. See Tapestry, PRISM, HVD and optical disc.

The Spatial Light Modulator

LCD shutters open and close to create a binary pattern, typically a million bits. The data laser is beamed through the modulator and intersects with the reference beam, creating a unique interference pattern.

Create an Interference Pattern

The combination of data and reference lasers create the unique interference pattern, which is the hologram stored in the optical materal. To read back the binary pattern, only the reference laser is used to output the data.

An Early Prototype

This was a prototype of holographic storage from IBM's Almaden labs. The laser beams (green) are directed through various lenses to the optical storage unit. The bottom picture is a magnification of the storage area, showing 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.)