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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Erasable optical memory using stimulated electronic transition concept

Sacharia Albin, James D. Satira, Linwood Watkins, Thomas A. Shull

Published Aug 1, 1990• 1 citation
Publisher

Abstract

Mass storage optical memory for use aboard unmanned orbital space platforms are currently under development by NASA. These storage systems with large information capacities, high data transfer rates, and high mean time between failures are needed. An optical system using stimulated electronic transition (SET) concept would be ideal for this application. For a single reading and writing beam system, the estimated storage capacity of 66 Gbits per disk, and a data transfer rate of 186 Mbits per second can be achieved. There are several oxide and sulfide materials that can be doped with rare earth elements to produce SET for optical storage. Unlike magneto-optic and phase change processes, the SET concept does not involve localized heating which produces material fatigue. Since electronic transitions are the only processes involved in SET storage, the disk will have an infinite lifetime.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

DOI
10.1117/12.22046

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Mass storage optical memory for use aboard unmanned orbital space platforms are currently under development by NASA. These storage systems with large information capacities, high data transfer rates, and high mean time between failures are needed. An optical system using stimulated electronic transition (SET) concept would be ideal for this application. For a single reading and writing beam system, the estimated storage capacity of 66 Gbits per disk, and a data transfer rate of 186 Mbits per second can be achieved. There are several oxide and sulfide materials that can be doped with rare earth elements to produce SET for optical storage. Unlike magneto-optic and phase change processes, the SET concept does not involve localized heating which produces material fatigue. Since electronic transitions are the only processes involved in SET storage, the disk will have an infinite lifetime.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
A new concept for an erasable optical memory is demonstrated using stimulated electronic transition (SET). Large bandgap semiconductors are suitable materials for the SET medium. We have investigated the properties of MgS:Eu, Sm and SrS:Eu, Sm as possible media for the SET process. Quantum storage is achieved in the form of charges in deep levels in the medium and stimulated radiative recombination is used as the reading process. Unlike magneto-optic (M-O) and phase change (PC) processes, optical writing, reading and erasing are achieved without localized heating. The SET process will have an inherently faster data transfer rate and a higher storage density, and the medium will be more durable than the M-O and PC media. A possible application of the SET process in neural networks is also discussed.
We are developing a high density, high speed optical memory, using rare-earth doped hole- burning materials. These materials are theoretically capable of achieving storage densities of 1000 Gigabits/cm3 at input/output (I/O) rates of several gigabits/sec. One remarkable attribute of this storage concept is that both the temporal and spatial information encoded on a laser beam can be stored. Because both the temporal and spatial information can be stored, digital data can be recorded serially as a data or packet stream as well as holographically using the page format. During the past year we have achieved a breakthrough in demonstrating random access holographic data storage at high frame rates. Five hundred holograms were stored and retrieved with very good fidelity at 30 Hz (video rate). Each holographic image with 512 multiplied by 488 pixels could be randomly accessed during the storage and retrieval process. This frame rate is the highest demonstrated frame rate for any optical technique. This breakthrough was achieved through the invention of a memory architecture that allows multiple holographic frames to be stored without any mechanical beam scanning. In this new architecture multiplexing of 500 holograms was achieved by stepping the laser frequency over a range covered by high speed acousto-optic modulators (AOM).
We describe the first experiments to validate the concept of a high-speed, high-density random access memory. This technique is based on a hybrid of the time-domain stimulated echo and the frequency domain scheme. The basic approach we have developed is the partitioning of the absorption frequency domain into smaller bins, so that each frequency bin stores a smaller portion of information independently. The advantage of the approach is the flexibility it offers in tailoring the memory system to match the computational processor. We have demonstrated storage and retrieval of data for long time in frequency bins separated by 750 MHz. We have achieved data rates of 40 MHz.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
We describe a new approach to develop a high-speed, high-density random access cache memory. This new technique is based on a novel approach using, in essence, a hybrid of the time domain stimulated echo (SE) concept and the frequency domain scheme. It has been demonstrated that the rare-earth doped crystals, can at low temperatures store large amounts of information for up to 24 hours using SE. The basic approach we have developed is the partitioning of the absorption frequency domain into smaller bins, so that each frequency bin stores a smaller portion of information independently. The bins are distinguishable by their different absorption frequencies and they are accessed by changing the laser frequency (color). However, information is still stored and retrieved using the time-domain pulse sequency used in SE. Specifically, with the laser frequency set at the particular absorption of a frequency bin, the laser is externally modulated by an acousto-optic modulator to produce amplitude modulated pulses representing the digital data, the write pulse, and the read pulse, during the write and retrieve operation, respectively. The advantages of this approach are that memory can be stored in space, time, and frequency domain, allowing us to tailor a flexible memory architecture to match those of the computational processor.
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.