Holographic Data Storage Digital Data Storage Using Volume Holograms Offers High Density and Fast Readout. Current Research Concentrates on System Design, Understanding and Combating Noise, and Developing Appropriate Storage Materials. Possible Applications Include Fast Data Servers and High-capacity Optical Disks

52 Computer H as the rapid increase in available storage capacity fueled the demand for storage, or vice versa? It's hard to say: Computer users' hard disk drives are perpetually overflowing with data, even though a year earlier the same-size disk seemed more than sufficient. Research into and development of data storage devices is a race to keep up with this continuing demand for more capacity, more density, and faster readout rates. Improvements in conventional memory technolo-gies—magnetic hard disk drives, optical disks, and semiconductor memories—have managed to keep pace with the demand for bigger, faster memories. However, strong evidence indicates that these two-dimensional surface-storage technologies are approaching fundamental limits that may be difficult to overcome, such as the wavelength of light and the thermal stability of stored bits. An alternative approach for next-generation memories is to store data in three dimensions. In this article we discuss recent developments in holographic 3D memories, 1-4 in which researchers achieve high density by superimposing many holo-grams within the same volume of recording material. (For a brief look at other techniques, see the sidebar, " Methods of Optical Storage. ") Data are encoded and retrieved as two-dimensional pixelated images, with each pixel representing a bit. The inherent parallelism enables fast readout rates: If a thousand holograms can be retrieved each second, with a million pixels in each, then the output data rate can reach 1 Gbit per second. For comparison, an optical digital videodisk (DVD) outputs data at about 10 Mbits per second. The surface density that has been experimentally demonstrated in holographic memories is 100 bits per squared micron in a 1-mm-thick material. 5 This can reach in excess of 350 bits per squared micron with thicker materials. 6 By comparison, the surface density of a DVD disk is 20 bits per squared micron; of magnetic disks, 4 bits per squared micron. The potential of this storage technology has generated a large research effort. The participants range from Bell Labs to a large consortium funded by the US Defense Advanced Research Projects Agency (which includes industrial partners such as IBM, Optitek, Rockwell, Kodak, and Polaroid), to independent start-up companies such as Holoplex. HOLOGRAMS A hologram is a recording of the optical interference pattern that forms at the intersection of two coherent optical beams. Typically, light from a single laser is split into two paths, the signal path and the reference path. Figure 1 …