since the days of the magnetic or paper tape, data-storage technology has come long way. Magnetic disks, compact discs and now, even optical storage is fast becoming a reality. Of these, hologra-phic memory appears the most promising way to store huge amounts of data.
With ultra-high storage densities, faster data transfer and short access time, this technology is already finding uses in several specialised fields.
One of the disadvantages of existing holographic technology has been its long recording rate. But now, a group at the Molecular Physics Laboratory, sri International in California, usa, has developed a new technique which combines a rapid recording and read out with low error rates. Xiao Shen and collaborators have used an optical storage technique based on time-domain holography which could have far-reaching applications.
Conventional holography relies on the interference of two laser beams (the reference beam and the data beam). The interference pattern is then recorded on to a storage material, a process which considerably slows down the transfer rate. Thus conventional technologies like photo-refractive memories are limi-ted to applications which do not require much reading or writing onto the memory device. Time-domain holography, on the other hand, incorporates the advantages of conventional holography (like high capacity) with some additional features like fast recording rates and low power requirement. The difference between the two techniques is that unlike the conventional technique, the two laser beams do not interact at one point in time but appear in the form of short pulses which follow one another. Shen and his team used a rare earth doped crystal kept at a temperature of almost - 270 c . Using pulses a microsecond long, they were able to achieve recording rates of about 300 megabits pre second. The performance of the memory with regard to error rate was also commendable. Even without the use of error-correcting codes, the device was able to give a very low bit error rate. With this amazing per-formance, very fast optical data storage with dynamic access could become a practical reality soon if certain problems, most notably related to the low temperature requirement, are solved (Science , Vol 278, No 5345).