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 鈥減ages鈥� that are then stored in the layers of a hologram
(快猫短视频, 23 January 1999, p 6).
Bayer hopes to store 1000
gigabytes鈥�1 terabyte鈥攐n 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鈥檚
holodiscs will initially store around 125 gigabytes. Today鈥檚 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鈥攁t least 25 times as fast
as a DVD can manage.
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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 鈥渙n鈥� position that lets light pass
through, and an 鈥渙ff鈥� position that reflects it away. Driven by a PC, or any
other digital system, the array allows bitmapped 鈥減ages鈥� 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鈥檛 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.