HOLOGRAMS will be storing movies, pictures, sound and computer data much sooner than anyone thought, according to Imation, a company based in Oakdale, Minnesota, which specialises in optical storage systems. Imation says it can combine several cheap off-the-shelf technologies to make a disc recording system that will store six times as much data as a DVD.
The firm is in a race with many other companies, including Bayer of Germany, which is developing holographic discs that use liquid crystal displays to construct data "pages" that are then stored in the layers of a hologram (New Scientist, 23 January 1999, p 6). Bayer hopes to store 1000 gigabytes-1 terabyte-on a holodisc by 2004. Imation says its holographic CD-ROM recorder will be on the market in 2002.
George Purrio, an engineer with Imation in Europe, says his company's holodiscs will initially store around 125 gigabytes. Today's DVDs store less than 20 gigabytes. He says later versions have a capacity of 1 terabyte, and will deliver data at up to 1 gigabit per second-at least 25 times as fast as a DVD can manage.
The off-the-shelf technologies that Imation is exploiting include a cheap gas laser and a micro-mirror array designed for digital video projectors. The 200-milliwatt gas laser, which Purrio says will soon be selling for just $300, shines through a half-silvered mirror to create a reference beam and data beam (see Diagram). The reference beam strikes a light-sensitive polymer disc spinning on a modified CD-ROM drive.
The data beam travels via the digital micro-mirror array, which was developed by Texas Instruments and carries a million or more tiny mirrors on the surface of a chip. The mirrors flip-flop between an "on" position that lets light pass through, and an "off" position that reflects it away. Driven by a PC, or any other digital system, the array allows bitmapped "pages" of data to be constructed at much higher resolution than is possible with LCDs.
The data beam generated by the micro-mirrors falls on the polymer disc at the same spot as the reference beam, but at a different angle. This creates an optical interference pattern corresponding to the bitmapped page of information built up by the micro-mirrors. This is recorded as a permanent change to the chemistry of the polymer. The disc stores a sequence of these bitmaps along a spiral track.
The disc is read by shining light through the polymer onto a microchip image sensor. Devices of this type are now routinely built into digital still picture cameras, and cost $500 or less. Data is transferred very rapidly because the sensor can receive whole pages or blocks of data at a time.
The data on the discs will not be erasable. Imation thinks this won't matter because the capacity of the discs is so large. The advantage of this write-once, read-many-times technology, Imation says, is that it will avoid the need for expensive and unstable erasable hologram technology.
Author: Barry Fox
New Scientist issue: 15th April 2000
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