HDSS
Encyclopedia
In late 1995 a joint university, industry and government consortium initiated the Holographic Data Storage System (HDSS) programme, with the initial goals of developing several key components for the system, including a high-capacity, high-bandwidth spatial light modulator
Spatial light modulator
A spatial light modulator is an object that imposes some form of spatially-varying modulation on a beam of light. A simple example is an overhead projector transparency. Usually when the phrase SLM is used, it means that the transparency can be controlled by a computer. In the 1980s, large SLMs...

 used for data input; optimised sensor arrays for data output; and a high-power red-light, semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

 laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

. At the same time, the HDSS researchers were to explore issues relating to the optical systems architecture (such as multiplexing
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...

 schemes and access modes), data encoding and decoding methods, signal processing
Signal processing
Signal processing is an area of systems engineering, electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time...

 techniques, and the requirements of target applications. Into the programme's final year, progress has been such that consortium member - IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

 Research Division - believes that holograms could hold the key to high-capacity data storage in the next millennium.

Large amounts of data can be stored holographically because lasers are able to store pages of electronic patterns 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, however, a different 'page' of information is accessed.

Holographic storage uses two laser beams, a reference and a data beam to create an interference pattern at a medium where the two beams intersect. This intersection causes a stable physical or chemical change which is stored in the medium. This is the write sequence. During reading, the action of the reference beam and the stored interference pattern in the medium recreates this data beam which may be sensed by a detector array. The medium may be a rotating disk containing a polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

ic material, or an optically sensitive single crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

. The key to making the holographic data storage system work is the second laser beam which is fired at the crystal to retrieve a page of data. It must match the original reference beam angle exactly. A difference of just a thousandth of a millimeter will result in failure to retrieve the data. Holography is expected to be of value in archival or library storage applications where large quantities of data are required to be retained at the very lowest costs possible.

Since it involves no moving parts, holographic data storage will be far more reliable than existing hard disk technologies. IBM has already demonstrated the possibility of holding 1 TB
Terabyte
The terabyte is a multiple of the unit byte for digital information. The prefix tera means 1012 in the International System of Units , and therefore 1 terabyte is , or 1 trillion bytes, or 1000 gigabytes. 1 terabyte in binary prefixes is 0.9095 tebibytes, or 931.32 gibibytes...

 of data in a crystal the size of a sugar cube and of data access rates of one trillion bits per second. The major challenge ahead is expected to be the development of a rewritable form of holographic storage.

During CES 2006, a workable holographic drive was tested and stored 300 GB of memory compared to blu-ray
Blu-ray Disc
Blu-ray Disc is an optical disc storage medium designed to supersede the DVD format. The plastic disc is 120 mm in diameter and 1.2 mm thick, the same size as DVDs and CDs. Blu-ray Discs contain 25 GB per layer, with dual layer discs being the norm for feature-length video discs...

's 100 GB. It has been announced that hologram disks will be a post-blu-ray storage device.
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