Photorefractive Materials for Optical Storage and Display

Real -time data storage and processing using optical techniques have been considered in recent years. Of particular interest are photosensitive electro-optic crystals which permit volume storage in the form of phase holograms, by means of a charge transfer process. A survey of the state of the art of such holographic memories is presented. The physical mechanism responsible for the formation of phase holograms in such crystals is discussed. Attention is focused on various aspects of materials characterization, development and utilization. Experimental reversible holographic read-write memory systems with fast random access and high storage capacity employing this new class of photosensiitive materials have already been demonstrated. Introduction The increasing use of electronic computers in recent years has considerably increased the demands on computer memories and memory technologies. Current and projected memory requirements define the goals for such a technology: reliable, flexible, random access, read -write memories with high storage densities, fast access times and high data transfer rates, all at an acceptable cost. Several new technologies are emerging to satisfy these memory requirements.i-6 Already the storage capacities of magnetic disc files used in peripheral and mass storage have been considerably increased and are expected to improve further. Magnetic core memories for mainframe storage are making way for the considerably faster MOS semiconductor technology. Charge coupled devices, magnetic bubble memories and electron beam accessed memories which offer slightly higher densities with a tradeoff in access time have been demonstrated (Table 1). For future computer systems, optical memories appear to be attractive candidates for several important reasons. Optical data storage is capable of providing extremely high packing densities limited only by the wavelength of light. In principle 108 bits/ cm2 for two -dimensional storage is possible, but in practice this is limited to about 10' bits/cm2. Considerably greater storage and processing capacities are possible in the case of volume storage with a theoretical limit of 1012 bits /cm3. The estimated practical limit for such a system is about 1010 bits /cm3.7 Furthermore, optical techniques permit both fast random access time (μs or less) and data transfer rates on the order of G bits/ sec. The upper limits for access time and throughput are determined by the ultimate capabilities attainable by the various system components. Bit error rates and cost per bit have been speculated to be one error in 108 retrieved bytes and 10 -4 cents /bit respectively. In addition, optical memories offer the advantage of immunity from electromagnetic interference. Work supported in part by the National Aeronautics and Space Administration and the National Science Foundation. This is a revision of a paper presented at the SPIE seminar on Optical Information Processing, August 24, 1976, San Diego, California. Paper 1376 received July 29, 1976 ; revised October 20, 1976. Optical data storage can be accomplished either by bit -by -bit recording or by holographic recording. High storage densities8 and minimum information losses from localized defects can be achieved only by holographic recording techniques. Holographic optical memories were first proposed about a decade ago. Although research has been going on in this area ever since, holographic memories in photorefractive media have so far not proved economically practical in commercial storage and display systems. One important reason for this has been the cost factor of developing optical memories as compared to existing memory technology. We can identify a similar long period of development for magnetic bubble memory technology which is only within the last year or so reaching economic viability as a result of technical advances on a broad front. Current optical memories and display systems use several relatively complex, high cost optical system components which present a serious drawback from a manufacturer's point of view. A typical holographic recording arrangement (Figure 1) consists of: a coherent light source, beam deflector, a data input element or page composer, X -Y BEAM DEFLECTOR MODULATOR LASER FLY'S EYE LENS PAGE HOLOGRAPHIC DETECTOR COMPOSER STORAGE ARRAY MEDIUM Figure 1. Experimental arrangement for a holographic read -write memory. a recording medium and a detector matrix. Considerable research has gone into each of these principal components9 and considerable progress has been made. This has led to the development of several memory and display prototypes but in order to develop the potentials of holographic optical storage on a commercial scale, further progress needs to be made in the areas of low-cost, coherent light sources, fast and effective beam deflectors, high -speed page composers and optical storage mateMarch -April 1977 / Vol. 16 No. 2 / OPTICAL ENGINEERING / 189 i