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Optical Engineering

All-optical associative memory using photorefractive crystals and a saturable absorber

Markus Duelli, Roger Sean Cudney, Claudia Keller, Peter Günter

Published Jul 1, 1995• 8 citations
Publisher

Abstract

We report on the investigation of a new configuration of an all-optical associative memory. The images to be recalled associatively are stored in a LiNbO3 crystal via angular multiplexing. Thresholding of the reconstructed reference beams during associative readout is achieved by using a saturable absorber with an intensity-tunable threshold. We demonstrate associative readout and error correction for 10 strongly overlapping black-and-white images. Associative recall and full reconstruction is performed when only 1/500 of the image stored is entered.

DOI
10.1117/12.204796

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We report on the investigation of a new configuration of an all-optical associative memory. The images to be recalled associatively are stored in a LiNbO3 crystal via angular multiplexing. Thresholding of the reconstructed reference beams during associative readout is achieved by using a saturable absorber with an intensity-tunable threshold. We demonstrate associative readout and error correction for 10 strongly overlapping black-and-white images. Associative recall and full reconstruction is performed when only 1/500 of the image stored is entered.
A new implementation of an all-optical associative memory is presented. It is realized in a nonresonant cavity containing a photorefractive and a nematic liquid crystal. The images, which are stored on slides, modulate the intensities of the signal beams circulating in the cavity. In a photorefractive KNbO3 crystal the signal images are compared with an arbitrary input image on the pump beam by multiple two-beam coupling. In the nematic liquid-crystal cell the signal images are thresholded by a feedback-induced optical bistability. Associative readout, error correction, and image completion are demonstrated for two and three stored black-and-white images. The performance of the new scheme is estimated to be comparable with that of nonlinear holographic associative memories based on thin holograms.
We propose and demonstrate a new long-term readout holographic memory using two photorefractive crystals. Images are stored in the first crystal and reconstructed with low-intensity beams, then amplified in the second crystal operating in a high-gain low-noise configuration. By using combinations of LiNbO(3)/Bi(12)SiO(20) and BaTiO(3)/Bi(12)SiO(20) as storage/amplification crystals, continuous readout of the angularly coded memories for more than 20 h and 40 min, respectively, is experimentally achieved. A lower bound for the index modulation to be allocated to one stored image in LiNbO(3)is estimated.
We propose and demonstrate a long-term readout photorefractive memory. It is made up of two photorefractive crystals: the first where the images are stored and the second where the signal is amplified by use of a low-noise self-bent pump-beam technique, making it possible to use very low illumination of the storage crystal. A LiNbO(3)/BaTiO(3) configuration is used with angular multiplexing; this technique permits continuous readout for more than 60 days. The long-term storage capacity and the maximum number of stored images are estimated.
Lithium niobate crystal has been used for storing information that can be accessed through an associative recall. This fact demonstrates the application of the crystal to the optical memory. specifically In an optical ring resonator. One important factor that determines the recall ability is the diffraction efficiency. Particularly this quantity Is also a function of the polarization of the readout beam. The diffraction efficiency of the crystal Is significant If the sufficient build-up of power in the real-time programmable ring resonator is required. To this end this paper is concentrated on the study of the dependency of diffraction efficiency on the polarization of recall beam. Experimental results are also given to indicate the significance of orientation of c-axis of the crystal in the experimental configuration. 2.
Associative optical memories have attracted increasing interest as they can offer new, all-optical solutions to various problems, for example in pattern recognition. In such memories, stored informations are retrieved from partial or noisy inputs. An optical laser resonator can act as such an associative processor. Using holograms, different images are associated to each of the modes of the optical cavity. Thus a partial input image can force the resonator to oscillate on the closest stored information. Two wave mixing arrangements in photorefractive crystals, which provide very large amplification factors, can be used to overcome the large losses inherent to these resonators.