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Optics Letters

Long-term readout of photorefractive memories by using a storage/amplification two-crystal configuration

Henri J. Rajbenbach, Sambath Bann, Jean‐Pierre Huignard

Published Dec 1, 1992• 11 citations
Publisher

Abstract

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.

DOI
10.1364/ol.17.001712

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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.
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.
We propose and experimentally demonstrate two original methods for suppressing the beam fanning noise of photorefractive amplifiers. The first one involves two wave mixing in a slowly rotating crystal, while the second requires to tilt the crystal with respect to the recording beams. This method applies to materials which exhibit a sharp resonance of the gain versus the pump-probe beam angle. With both methods a large improvement of the S/N ratio is achieved and virtually noise free image amplifiers are demonstrated. In Bi12SiO20, subpicowatt optical signals are detected and amplified. By using combinations of LiNbO3/Bi12SiO20, as storage and amplification crystals respectively a long term readout (more than 20 hours) of a photorefractive memory is achieved.
We propose a dynamic refreshing technique in a photorefractive memory with double mutually pumped phase conjugate mirrors. Long-term continuous readout without any fixing technique can be achieved by the high quality image refreshing that uses the resonance of the phase conjugate beams with image information between phase conjugate mirrors. We perform the experiment on the long-term reading of the images in the photorefractive memory with BaTiO/sub 3/ crystals and achieve the continuous reading time over 10 times longer than the time constant of the crystal without image degradation.
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.