Crystal memory for optical storage offers high efficiencies and densities

July 1, 1995

Researchers at the University of Oregon (Eugene, OR) have developed a time-domain frequency-selective optical storage system, or crystal memory, that currently achieves record information densities of 8 Gbit/in2. The recording medium is a Tm:YAG crystal; a 30-mW CD-ROM type diode laser writes data. The amount of information stored in a given volume is enhanced by the ability to "color code" data (Tm:YAG absorbs different wavelengths in different ways).

Most storage techniques have a trade-off between speed and density. The high density of the crystal memory is accessed at high speed (making it efficient), so the density bandwidth product is on the order of 1017 bit/in.2 s, according to physics professor Thomas Mossberg. The maximum capacity of magnetic media, the most widely used current storage method, is theoretically too low at 1016 bit/in.2 s to handle expected memory requirements. Mossberg adds that even though their method holds the record for storage density, "within a year our work should yield results of an order of magnitude greater in both density and speed for a combined hundredfold increase in the density/speed level we have already achieved."

Crystal memory development is not without problems, including long-term memory stability, material dependency in erasing and rewriting, and functioning at low temperatures. Currently, liquid-helium temperatures (3 K-5 K) are needed, but the goal is to function with liquid nitrogen (77 K). Mossberg claims that these are "technical hurdles, not fundamental physical roadblocks. For example, refrigeration units already exist that address temperature and stability issues."