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

Optical coherent transient memory systems

William Randall Babbitt

Published Nov 9, 1993• 0 citations
Publisher

Abstract

A performance analysis of an optical coherent transient optical memory system is presented. The simplest configuration of a coherent transient memory is assumed in which a large block of bits are stored at each laser spot by a single data pulse. The reference and recall beams are assumed to be temporally brief and a photon gated storage medium is assumed. Even with this basic system configuration, the performance predictions yield promising results. With the assumed material parameters, data rates of 5 GHz, data block sizes of 4 X 105, and storage densities up to 4.3 X 1011 bits per cm2 are predicted.

DOI
10.1117/12.163603

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A performance analysis of an optical coherent transient optical memory system is presented. The simplest configuration of a coherent transient memory is assumed in which a large block of bits are stored at each laser spot by a single data pulse. The reference and recall beams are assumed to be temporally brief and a photon gated storage medium is assumed. Even with this basic system configuration, the performance predictions yield promising results. With the assumed material parameters, data rates of 5 GHz, data block sizes of 4 X 105, and storage densities up to 4.3 X 1011 bits per cm2 are predicted.
This final report of contract F49620-91-C-0088 describes the development of the analytical tools required to evaluate optical coherent transient memory, triple-product correlator, and continuous signal processor systems. One key tool is a coherent transient simulator based on the optical Bloch equations. The coherent transient output signal that results from an arbitrary sequence of pulses, each having an arbitrary amplitude, duration, center frequency, chirp, phase, and timing can be calculated. The simulator was used to study excitation pulse saturation and it effects on the output signal efficiency and fidelity and to study frequency chirped reference pulse saturation. Analytical tools were developed to assess the effects of local heating and spatial crosstalk. A performance evaluation of a coherent transient continuous optical correlator is reported, along with new implementation techniques. The continuous processor's predicted time-bandwidth product, data bandwidth, and pattern storage density are 10000, 5 GHz, and 100000 patterns per square centimeter, respectively. A proof of concept demonstration of the continuous optical processor was performed in europium doped yttrium silicate in collaboration with IBM Almaden Research Center. Research into divalent ion doped crystals as photon gated materials for coherent transient systems was performed.
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We propose a novel approach to time-domain storage of serial optical data. In this approach, two identical impulse- equivalent sequences are used as write and read pulses to store and later retrieve optical information. These pulse sequences are both amplitude- and phase-modulated in such a way that their autocorrelations resemble, as closely as theoretically possible, the autocorrelation of a single large brief pulse to ensure faithful data retrieval. Such impulse-equivalent sequences are known to exist for all lengths, permitting the storage of large bandwidth data. In a proof-of-concept experiment, we have successfully stored a 42-microsecond long stream containing 420 bits of data in a 40-MHz spectral channel in a Eu3+:Y2SiO5 crystal. The stored data was repeatedly recalled for up to eight times with high signal fidelity, showing no measurable sidelobe-induced noise.
The objective of this research was to quantitatively evaluate the concept of time-domain optical memory (TDOM) using the hole-burning mechanism. The TDOM concept offers both high speed and high memory density. The advantage of TDOM over traditional semiconductor memory is its inherent parallelism, which allows data to be stored and retrieved from the memory at much higher speed. The practical success of the proposed memory, therefore, depends critically on-among other issues-the storage, retrieval, and processing of images. In addition, new data storage techniques are needed to implement a practical memory. The last three years of this research program have focused on (1) development of new phase modulation techniques to enhance the storage, retrieval, and integrity of the stored information; (2) experimental studies of image storage and phase conjugate retrieval; (3) Image processing; and (4) practical holographic storage of images by using the hybrid time-frequency approach. Specifically, SRI demonstrated data encryption by phase modulating the read and write pulses. Image correlation and convolution was demonstrated and its implication for pattern recognition examined. Finally, this laboratory demonstrated high-fidelity random access and retrieval using holographic storage in Pr3+: YA103 with a single-frequency ring laser. These experiments have clearly demonstrated the practical viability ofmore » TDOM with existing technology. In the next stage of the research, the performance Issues such as bit-error-rates, signal-to-noise ratios, and laser stability need to be addressed. Stimulated photon echoes, Optical memory, Image processing, Pattern recognition, Phase conjugation.« less
We have found that highly efficient waveform recall is possible in coherent transient systems in which the storage is optically thick. Coherent transients may be used in a variety of information storage and processing applications with advantages over traditional electronic methods. However, it is believed that a serious problem in application of photon echoes in practical systems is the relatively low efficiency of the process. We show in our numerical studies that waveform recall efficiencies greater than unity can be achieved in absorbing media with appropriate choice of absorption length and brief pulse area, even for very weak data pulses. We also present our preliminary experimental results in Barium vapor in which efficiencies of 50% were obtained for both the stimulated and two-pulse photon echoes.