STORAGE OF DATA

In holographic data storage, an entire page of information is stored at once as an optical interference pattern within a thick, photosensitive optical material (Figure 1).

Figure 1

Intersecting two coherent laser beams within the storage material does this. The first, called the object beam, contains the information to be stored; the second, called the reference beam, is designed to be simple to reproduce—for example, a simple collimated beam with a planar wavefront. The resulting optical interference pattern causes chemical

and/or physical changes in the photosensitive medium: A replica of the interference pattern is stored as a change in the absorption, refractive index, or thickness of the photosensitive medium.

A large number of these interference gratings or patterns can be superimposed in the same thick piece of media and can be accessed independently, as long as they are distinguishable by the direction or the spacing of the gratings. Such separation can be accomplished by changing the angle between the object and reference wave or by changing the laser wavelength.


When the blue-green argon laser is fired, a beam splitter creates two beams. One beam, called the object or signal beam, will go straight, bounce off one mirror and travel through a spatial-light modulator (SLM). An SLM is a liquid crystal display (LCD) that shows pages of raw binary data as clear and dark boxes. The signal beam around to the light-sensitive lithium-niobate crystal carries the information from the page of binary code. Some systems use a photopolymer in place of the crystal. A second beam, called the reference beam, shoots out the side of the beam splitter and takes a separate path to the crystal. When the two beams meet, the interference pattern that is created stores the data carried by the signal beam in a specific area in the crystal -- the data is stored as a hologram.