An SLM is simply an LCD panel that displays a page of raw binary data as an array of clear or dark pixels.
The
object beam finally interacts with the reference beam inside a photosensitive crystal. The ensuing interference pattern--the substance of the hologram--gets stored as a web of varying optical characteristics inside this crystal. To read out the data, the reference beam again illuminates the crystal. The stored interference pattern diffracts the reference beam's light so that it reconstructs the checkerboard image of the light or dark pixels. The image is directed upon a charge-coupled device (CCD) sensor array, and it instantly captures the entire digital page.
When reading out the data, the reference beam has to hit the crystal at the same angle that's used in recording the page. The beam's angle is crucial, and it can't vary by more than a fraction of a degree.
This apparent flaw in the recording process is actually an asset. It's how holographic storage achieves its high data densities. By changing either the angle of the reference beam or its frequency, you can write additional data pages in
to the same volume of crystal.
However, all the holograms appear dimmer because their patterns must share the material's finite dynamic range. In other words, the additional holograms alter a material that can support only a fixed amount of change. Ultimately, the images become so dim that noise creeps into the read-out operation, thus limiting the material's storage capacity.
The dynamic range of the medium determines how many pages it can hold reliably; therefore, the PRISM project examines the limitations in a variety of photosensitive materials. Current work uses iron-doped lithium niobate, strontium barium niobate, or barium titanate crystals. "We're also looking into polymers and other organic materials," says Glenn T. Sincerbox, the principal investigator from IBM.
Because the interference patterns are spread uniformly throughout the material, it endows holographic storage with another useful capability: high reliability. "While a defect in the medium for disk or tape storage might
garble critical data, a defect in a holographic medium doesn't wipe out information. Instead, it only makes the hologram dimmer," he says.
The PRISM consortium has stored up to 200 holograms composed of 37.5-KB data pages (640 by 480 bits) into a crystal with less than 1 centimeter on a side, achieving a storage density of 48 MB per cubic cm. This is far short of the goal of a practical storage density of 10 GB per cubic cm, but it's sufficient to pursue the development of Holographic Data Storage System (HDSS) hardware.
Sincerbox believes that it will take several more years to refine the technology enough to build small desktop HDSS units. Such devices might be ready by about the year 2003.
Because HDSS hardware uses an acoustoptical light deflector (i.e., a crystal whose refractive properties change according to sound waves traveling through it) to modify the beam angle, Sincerbox estimates that an HDSS system can retrieve adjacent data pages in under 100 microseconds. "Any convention
al optical or magnetic storage unit will require some sort of mechanical means to access different data tracks, which takes on the order of milliseconds to accomplish," he explains. "A gigabit-per-second data rate appears reasonable for holographic storage, and this should make it a cost-competitive leader with whatever exists."
While holographic storage appears to be a radically new technology, actually it's not. The basic concepts were worked out almost 30 years ago. What's changed, according to Sincerbox, is the availability of key low-cost components. "Consumer electronics has played a large part in making holographic storage feasible today," he says. "Thirty years ago, lasers were made of glass tubes that were 6 feet long and had unreliable output. Now they consist of small, reliable, semiconductor junctions, similar to those mass-produced for CD players. The SLM is the result of fabrication techniques that make LCD screens for laptop computers and calculators. The CCD sensor array comes straight
from a digital video camera. Neither of these were available 30 years ago--perhaps not even 10 years ago."
illustration_link (48 Kbytes)
-- To read data out,
the reference beam illuminates the crystal, and an image of the pattern gets projected onto a CCD array (e).
-- The object beam
(a) passes through an LCD (b) that displays the data pattern. The object beam interferes with the reference beam (c) inside a crystal to make a hologram of the pattern (d).
Creating Holographic Storage
