(This article is sponsored by The Boston Group)

Most of us, in our daily lives, have come across holograms - from those simple, shiny holograms on our credit cards, known as surface relief holograms, to high quality holograms demonstrating truly 3-dimensional images, known as volume-phase holograms, displayed in museums and some collector's items such as super-bowl tickets. One feature common to both these types of holograms is that in same physical location multiple images are stored. Hence, you see at least two different images when you view the silvery hologram on your credit card at different angles. In volume-phase holograms, you also see a tremendous visual depth (that hanging-in-the-space feeling) because hologram is recorded throughout the thickness of the recording medium.

While Denis Gabor, the Hungarian Scientist, first did a first significant invention in 1947 and later received the Nobel Prize in 1971 for holography, it was Pieter van Heerden, a researcher at Polaroid, who in 1963 first predicted immense potential of holography for data storage. As it happens with many great inventions, a number of other technical advances had to happen as well as growing demand for data storage (so go ahead, take more digital pictures and shoot more digital videos!) had to develop before holographic data storage could be commercially pursued. Fast forward to mid-1990's and we now, have, available high quality lasers (similar to ones used in DVD players), excellent detectors (also needed for digital cameras) and high contrast, fast refresh Spatial Light Modulator (SLM) (display screen of your laptop is an example of SLM). Most importantly, thanks to advances in Chemistry and Materials Science, we have optimum recording medium - a critical missing component until now.

While everyday holograms are analog images, the holograms used for high-density data storage are digital images, representing analog information. Figure below illustrates schematically the elements of holographic recording and readout of data. Data are stored holographically by recording in a polymerizable storage medium the optical interference pattern that is created by mixing an encoded data beam with a coherent reference beam. The data are conveniently encoded as two-dimensional binary pages of bright and dark spots (corresponding to the "1s" and "0s" of the digital data) using an SLM, a device that acts as a collection of independent "light valves." For readout, a laser beam equivalent to the reference beam is projected onto the recorded interference pattern in the media, and the transmitted beam carrying the reconstructed data page is imaged onto a detector similar to the SLM.

What can holographic data storage (HDS) do for you?
As a recent article in the Economist (July 31, 2003) best put it, it can store data in memories, which are vast and fast. There are three primary differences between HDS and all other types of storage such as magnetic hard drive, tape, CD and DVD that make HDS a compelling proposition.

Store more data in smallest Form Factor: All other technologies store data on the surface of the recording medium and hence they are two-dimensionally limited. Holographic recording medium can store the data in three dimensions, i.e. it uses not only surface of the medium but also the thickness of the medium. Thus, data density is tremendously high. Following comparison briefly illustrates the storage potential of holographic disk.

Find data fast: All other technologies write and read data serially (one-bit-at-a-time for a single drive head). In HDS, entire data page is written and read at the same time (transferring hundreds of thousands of bits in parallel) and thus giving rise to massive parallel data transfer rate from a single head. A state-of-the art DVD will transfer data at max. rate of ~5 MB/sec, Aprilis has demonstrated sustained system data rate of 125 MB/Sec and raw data rate of 1000 MB/sec. Most of the companies developing HDS are estimating first generation products to have data rate of ~75 MB/sec.

Find data you really want: Since the data are stored as optical images of digital pages, then optical correlation methods can be used to sort the stored data for relevance with respect to a search characteristic. Most important, this sorting can occur by comparison to the direct written record, without requiring the transfer of data to system memory, dramatically accelerating the search. In addition, since each data page record can hold many thousands of bits of data, then the entire stored database can be searched very rapidly. Data stored using HDS can be searched at a rate of up to about 15 GB/sec simply by encoding the reference beam with the pattern of the data one is looking for. Searching data stored in conventional memories will take far longer. For example, at GHz processor speeds such a search might take about one minute to complete from a 100 Gigabytes database stored in a typical semi-conductor RAM. The same search might take more than 30 minutes if data from a hard disk storage system must be loaded into memory.

With these attributes, HDS combines superior performance with robustness of optical storage (no head-crashes), random search capabilities and archivability. While success in market place is governed by a number of business parameters, as with any new revolutionary technology, there is a considerable momentum and pragmatic optimism worldwide in developing HDS for a number of consumer and enterprise-class applications.

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