Abstract

Optical data storage has been interested in several decades because of the potential of fast access time and high-density storage. Recently volume holographic data storage technique has drawn much interest as new materials and improved techniques have developed.1 However, the large of hologram storage is restricted by low diffraction efficiency and mainly focused to read only memory because of the material's characteristic difficult to rewrite in short time interval. Another optical memory technique (photon echo optical memory)2 based on time-domain has developed since 1980's and made big progress for the area of real time image processing and dynamic random access memory. In this technique, data pulses interfere with timely separated write pulse (reference), and their interference pattern is written as population gratings. A read pulse (reference) retrieves each data pulse information as a four-wave mixing process. The write and read pulse separations are restricted by phase and population decay times of the material, respectively, and storage density is determined by the ratio of inhomogeneous to homogeneous linewidths. Even though photon echo technique has advantages of fast access time (≥ ns) and high-density (≥ Mbits/laser spot), its operating temperature is kept extremely low around liquid helium temperature. This is because that homogeneous decay time rapidly shortens as temperature increases due to phonon interactions, i.e., the ratio of inhomogeneous to homogeneous decay rates is ~ 1 at liquid nitrogen temperature (77 K) in a rare-earth doped solid, so that restricts its applications.

© 1998 Optical Society of America