Low-Power, Time-Domain Holography Holds Promise

Kate Leggett

MENLO PARK, Calif. -- A recent demonstration of time-domain holography could dispel beliefs that holographic memories need high-power lasers to function.
Holographic memories, characterized by ultrahigh storage densities, rapid data transfer rates and exceptional reliability, have the potential to supplant today's commercially available storage devices. True parallel processing capabilities have led to prototype devices for image storage and comparisons. Ravinder Kachru and his team from SRI International have demonstrated a device that uses ion-doped crystals and low-power pulsed lasers to store more than 100 Mb/cm³.
Conventional holographic techniques record data in a spatial interference pattern generated by two laser beams overlapping in a medium. Although this kind of holographic memory offers the high capacities associated with optical memories, it suffers from slow recording rates and a need for high-power lasers.
Kachru and his colleagues have circumvented these problems using time-domain holography -- a technique based on photon echoes. Like conventional holography, information is stored using reference and data beams that interfere in the recording medium, causing a change in its optical properties. Unlike spatial holography, however, these laser beams are low-power, pulsed and do not overlap in time.

Albert in lights
Using this technique, Kachru's team stored a picture of Albert Einstein in a 7-mm-thick Eu³⁺ doped Y₂SiO₅ crystal. The ions, once excited by a laser pulse, store information as a population modulation of their ground state. The population modulation is the result of resonant excitation, he said, which is instantaneous and works well with low laser powers. Kachru's group stored information in the crystal for up to several days by keeping the material at cryogenic temperatures.
A third read pulse retrieves the data from the crystal. This reconstructed image, typically known as a photon echo, appears from the crystal as an emitted laser pulse, which is separate in time from the read pulse. A charge-coupled device camera captures the emitted data, and a frame grabber digitizes it. Kachru's group used a modulated ring dye laser from Coherent Laser Group of Santa Clara, Calif., to create the read, data and write pulses; read and write pulses required peak powers of only 200 mW.
Using this setup, the crystal stored a 356-kb high-resolution black-andwhite photograph at recording speeds of 300 Mb/s. The reconstructed photograph looked as good as the original image.
The size of this stored image represents 1 percent of the storage capacity of the crystal, Kachru said. This means that in principle 100 such photographs would fit on a single crystal, which translates to a storage capacity of about 6 x 10⁸ bits/cm³.
Future developments could concentrate on eliminating the cryogenic requirement for this type of time-domain holographic storage system.