Advertisment

Data @ Speed of Light

author-image
PCQ Bureau
New Update

Imagine saving 1 TB (approximately 1500 CDs) of data on a DVD-sized disk or a 1 cm3 crystal. Amazing? What about accessing all that data in a few microseconds, literally at the speed of light. Remember that the fastest hard disk takes a few milliseconds (1 millisecond =1000 microseconds). These twin promises have driven research in holographic data storage for four long decades. 

Advertisment

Beam it up



Holography is an optical method of storing very large amounts of data in relatively small areas by writing the data in pages, as three-dimensional holograms on a photosensitive crystal or polymer medium. 

Each page of data is stored as an optical-interference pattern (hologram) in a photosensitive polymer disk or a crystal cube. To write data, a laser beam is split into two beams using a beam splitter. The first beam, known as the object beam is then passed through a series of lenses and mirrors before being sent through a SLM (Spatial-light Modulator). The SLM is a small LCD, which displays the data as pages of binary data–a pattern of clear and opaque squares very much like a crossword puzzle. On passing through the SLM, the object beam is imprinted with the data. 

Advertisment

A second beam also called the reference beam now meets the data-imprinted object beam inside a photosensitive material (media), which records the optical interference pattern or the hologram created by the intersection of the two beams. (see picture on previous page). Once a page of data is stored a mechanical scanner changes the angle of the reference beam so that another hologram can be recorded at the same location. 

To read the stored data, the media is illuminated by the original reference beam. This reference beam diffracts off the recorded stored hologram to recreate the original object beam. The different bits of data (in binary form) in the object beam are now detected in parallel by a CCD camera (solid state video chip). The recovered page is now converted from binary code into the original data.

Not just fast-n-vast



Holographic data is stored throughout the depth of the media as opposed to just the surface area in conventional magnetic and optical disks. Up to a few thousand holograms can be stored in the same location (multiplexing) by simply changing the angle of the reference beam. Repeat this throughout the entire medium and you have a phenomenal increase in storage capacity. 

Advertisment
Beginnings





Hungarian scientist Dennis Gabor developed the theory of holography in 1947, while trying to increase the resolution of electron microscopes. However, theory was not put into practice till the 1960’s when the first laser was invented. Gabor was finally awarded the Nobel Prize for Physics in 1971 for inventing holography. 

The high volume apart and speed of data retrieval apart, associative memory capability is another advantage. Let’s see how this works. Consider the old story of the blind men who went to ‘see’ an elephant. The one who felt a sharp tusk with his hands thought that it was a sword. Another who felt the brush-like tail thought it to be a broom while the third man thought the ‘wriggly’ trunk to be a snake. Conventional data storage and retrieval systems are like this. However, if anybody with normal vision sees only a trunk or a pair of tusks, he will immediately associate it with an elephant. Similarly, in a holographic storage system, while a hologram can be illuminated with a reference beam to get the stored data, a hologram can also be illuminated with a beam indicating a certain data pattern to recreate the corresponding reference beam and angle. Thus you can identify the location of the data and retrieve the full data, even if you have only a part of it. This capability can be used to search holographic memories quickly and efficiently.

Advertisment

End of the tunnel 



Holograpic data storage is today close enough to commercialization thanks to miniaturization of components. In the early 1960’s when the idea of holographic storage was first mooted, a typical laser was about 6 feet long, scientists hadn’t realized the potential of polymers and LCDs existed only in science fiction. However, development of CMOS and LCD technologies, and the reduction in both cost and size of laser units have made this very much feasible. The idea has been aggressively pursued in the last decade by companies like IBM, Bayer, Rockwell, InPhase (spun-off from Lucent), Aprilis (spun off from Polaroid) and Optware a little-known Japanese firm. Bayer, IBM and Aprilis have all claimed similar success in developing a photo-polymer media to replace the less sensitive Lithium Niobate crystals that are currently used. 

InPhase has already developed a holographic storage system code-named “Tapestry”. Slightly larger than a DVD drive, it claims to be able to store 100 GB data on a single DVD-like disc. InPhase plans to push this up to 1 TB in the coming decade. The first few pieces are expected to be in the market by mid 2003 and volume shipments by 2004. 

Benoy George Thomas

Advertisment

Tech that’s similar

The latest advance in memory technology comes not from a rare metal, fiber or even a polymer, but from a bacterial protein–bR or

bacterio-rhodopsin. Dr Robert Birge and his team from the University of Connecticut derived bR from Halobacterium

Salinarum, a bacterium found in salt bogs. bR is kept in a small transparent plastic cube (1”x1”x2”) called a

cuvette. When illuminated with a green laser it takes one shape. If subsequently illuminated with red laser, it takes another shape. Now if this new shape is targeted with a blue laser, bR returns back to its original shape. The bRs reaction to green and red lasers was used to write data in the form of binary code. A low power red laser can later be used to read the data, while a blue laser can reset (rewrite over) the data. Each cuvette is claimed to hold about 7 GB of data for up to 12 years. Though the technology is quite expensive, at roughly $25,000 per device, the US Air Force is funding further research to commercialize it.

FMDs (Fluroscent multiple-layer disks) from Constellation 3D is based on a similar technology. Using multiple layers of fluorescent coating to create more layers per disk to store data instead of the usual two layers in a conventional

DVD, Constellation 3D has already demonstrated a 10-layer disk that holds up to 140 GB. Researchers hope to eventually come out with a 100-layer disk that can hold 1 TB of data. FMDs are (WORM) write-once read-many disks that are currently touted as the answer to HDTV’s needs. More information at www.c-3d.net.

Advertisment