Holographic memory

A postage-stamp-sized plastic media prototype with a 1-Gbyte memory capacity has been created using Information-Multi-layered Imprinted Card (info-MICA) media technology from NTT. Able to store up to 1.7 Gbits/in. per layer, storage media made with the technology promise to replace semiconductor ROM in internal applications and read-once media in external data distribution and storage applications.

The technology is based on thin-film holography, where the hologram is of a thickness equal to or less than the light wavelength so that the required condition to trigger diffraction is not difficult to attain, even under fluctuating reference light (enabling the use of economical semiconductor lasers). Named for its similarity in structure to mica stone, the multilayered construction consists of alternating layers of material with a high refractive index and a low refractive index, creating a waveguide structure that confines light and prevents crosstalk, allowing an unlimited number of layers.

In operation, digital data are encoded into a 2-D image that is then converted into a computer-generated hologram and recorded as a submicron concave-convex pattern in each waveguide layer of the media. For data retrieval, a read-laser propagates in the waveguide and is scattered by the hologram to generate the 2-D image on a plane parallel to the waveguide for capture by an image sensor and decoding.

An additional benefit of the technology is that it is difficult to reproduce illegal info-MICA copies, making the technology suitable for the release of copyrighted multimedia content.

A different type of holographic storage media will soon be commercially available. The Tapestry HDS500 blue-laser holographic media from InPhase Technologies can store up to 200 Gigabytes on a single disk. The media is sensitive to blue-laser wavelengths from 400 to 410 nanometers—the same used in next-generation blue-laser DVD devices. The disks are intended to be used with user-developed systems as well as the company's HDS-200R drive, which is capable of a 20-Megabyte/second transfer rate and is expected to reach the market in 2006.