Holographic Data Storage System

Lore & Background

Holographic storage was proposed in the 1960s. It records data by capturing interference patterns between a modulated optical field and a reference field within a storage medium. Data is retrieved by diffracting the reference field off the hologram, reconstructing the original optical field. The HDSS program was created with initial goals of developing several key components, including a high-capacity, high-bandwidth spatial light modulator for data input, optimized sensor arrays for data output, and a high-power red semiconductor laser. Researchers also explored issues relating to optical systems architecture, multiplexing schemes, access modes, data encoding and decoding methods, signal processing techniques, and target application requirements.

Large amounts of data can be stored holographically because lasers store pages of electronic patterns. Holographic storage is sometimes referred to as 3D storage within special optical materials, as opposed to just on the surface. In traditional holography, each viewing angle gives a different aspect of the same object; with holographic storage, a different 'page' of information is accessed. The system uses two laser beams—a reference and a data beam—to create an interference pattern at a medium where they intersect, causing a stable physical or chemical change. During reading, the reference beam and the stored interference pattern recreate the data beam, sensed by a detector array. The medium may be a rotating disk containing a polymeric material, or an optically sensitive single crystal. The key to making the system work is the second laser beam, which must exactly match the original reference beam angle; a difference of a thousandth of a millimeter will fail to retrieve the data.

Into the program's final year, progress was such that consortium member IBM Research Division believed holograms could hold the key to high-capacity data storage in the next millennium. Holography is expected to be of value in archival or library storage applications where large quantities of data need to be retained at the lowest costs possible.

Reader's Guide

The Holographic Data Storage System program represents a significant government-industry-academic collaboration aimed at realizing a long-proposed storage technology. Its significance lies in addressing the fundamental challenge of retrieving data from holograms, requiring precise alignment of the reference beam. The program's exploration of multiplexing schemes, data encoding, and signal processing laid groundwork for practical implementations. A prototype demonstrated at the Consumer Electronics Show in 2006 achieved a storage capacity of 300 GB, compared to 100 GB for Blu-ray discs at the time, suggesting holographic disks could serve as a successor to Blu-ray. The technology's potential for high reliability, due to having no moving parts, and its ability to store up to 1 TB in a sugar-cube-sized crystal with data transfer rates of one trillion bits per second, highlight its promise. However, a significant hurdle remains in developing a rewritable form of holographic storage. The legacy of the HDSS program includes advancing the understanding of optical systems architecture and demonstrating that holographic storage could compete with and potentially surpass existing optical disc technologies for archival and library applications where low-cost retention of large data quantities is paramount.

Did You Know?

• The HDSS program was funded by DARPA and created in 1995 by Teledyne Technologies, IBM and Stanford University.
• A difference of a thousandth of a millimeter in the reference beam angle will fail to retrieve data from the hologram.
• IBM research demonstrated the potential to store up to 1 TB of data within a crystal approximately the size of a sugar cube.
• A prototype holographic drive demonstrated at CES in 2006 achieved a storage capacity of 300 GB.