Holographic storage: Virtual reality?

$120 per disk
The Tapestry media is being produced by Maxell at their high technology media manufacturing plant in Tsukuba, Japan, using chemicals supplied by Bayer of Germany. According to Maxell's D'Ambrise, "Disk prices have not yet been set but we expect them to be about 25 cents per gigabyte, or about $120 per disk".

Recording of the holographic data occurs through a spatial pattern of polymerisation of the photosensitive polymer that mimics the optical interference pattern generated during holographic writing. The concentration gradient that results from this patterned polymerisation leads to a diffusion of the unpolymerised polymer. This creates a refractive index modulation that is determined by the difference between the refractive indices of the photosensitive component and the matrix.

The holograms are created within this special photopolymer medium, like any holographic image. This involves taking the light from a coherent laser source — in the Tapestry drive this is a 407nm blue laser of the sort developed for Blu-Ray DVD drives — and splitting it into two beams, a signal or data-carrying beam and a reference beam. The reference beam shines directly onto the recording medium that will be used to store the hologram while the signal beam passes through a spatial light modulator (SLM), before reaching the medium.

Two-dimensional matrix
The function of the SLM is to convert the light beam into an image of the data to be stored. The data is stored in blocks, each block forms a two dimensional matrix of information, typically containing one million bits and can be thought of as a chessboard. Measuring 1000x1000 bits on each side, this is referred to as a page. The SLM produces an optical image of this chessboard data block, which can then be recorded with a single laser flash.

The SLM is, in fact, a ferroelectric liquid crystal on silicon (FLCOS) microdisplay manufactured by Displaytech Inc. and is a technology that has been developed for digital TV projectors. The two beams are then recombined to form an interference pattern within the photosensitive medium, this is the pattern that constitutes the hologram of a data page within the photosensitive recording medium. By varying the reference beam's angle of incidence many different data pages can be recorded within the same volume of material. This allows the data to be multiplexed and accounts for the enormous storage capacity of holographic data storage systems.

The servo system, designed and developed by InPhase, regulates both radial and rotational movement of the media and the angle of the reference beam. During a read operation, feedback from the hologram provides information to the servo system to optimise the recovery of the data with the best signal to noise ratio.

On the Tapestry drive each data page is located at a unique address within the material and several hundred pages of data, each with their own unique address, are recorded in the same location of the medium. Multiple pages of data are referred to as a book, approximately 12MB of data is stored in each book location.

Chessboard
To read the data from a holographic disk the reference beam is used to diffract off of the recorded holograms and thus reconstruct the image of the stored chessboard array of bits. This reconstructed array can then be projected onto a CCD photo-detector that reads the data in parallel.

In the Tapestry drive this photo-detector is an ultrasensitive and ultrafast CMOS image sensor manufactured by Cypress which has a pixel count of 1696x1710, a pixel pitch of 8 microns and most importantly is capable of very high-speed reading of data, over 500 frames per second. This allows the drive to have a data transfer rate of 20MB per second.

This high data transfer rate means that holographic drives are particularly well suited to applications such as the storing and replaying of video in real time, the professional film and video industry is seen as a major market for such drives.

Real-time video
"We have the need to archive high-definition movies as large files and, yet, be able to retrieve them quickly when needed for air. Until now, there has been no cost-effective, practical way to meet our volume and throughput requirements. Holographic storage appears to be the perfect answer," says Ron Tarasoff, vice-president of Broadcast Technology & Engineering at Turner Entertainment Networks.

To make the removable disks easier to use, a special media cartridge has been developed by InPhase. The media cartridges can be loaded and unloaded automatically using a mechanism designed and developed by ALPS Electric Co. Ltd.

The Tapestry drive has a SCSI interface that uses the Pegasus Disk Technologies Windows device driver and InPhase is working with Pegasus to provide file system connectivity for the Windows operating system...

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