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Ferroelectrics

Self induced coherent light resonators with photorefractive KNbO3crystals

Gilles Pauliat, M. Ingold, Germano Montemezzani, Peter Günter

Published Apr 1, 1989• 0 citations
Publisher

Abstract

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.

DOI
10.1080/00150198908211351

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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.
One can construct a memory having associative characteristics using optical resonators with an internal gain medium. The device operates on the principle that an optical resonator employing a holographic grating can have user prescribed eigenmodes. Information that is to be recalled is contained in the hologram. Each information entity (e.g. an image of a cat) defines an eigenmode of the resonator. The stored information is accessed by injecting partial information (e.g. an image of the cat's ear) into the resonator. The appropriate eigenmode is selected through a competitive process in a gain medium placed inside the resonator. With a net gain greater than one, the gain amplifies the field belonging to the eigenmode that most resembles the injected field; the other eigenmodes are suppressed via the competition for the gain. One can expect this device to display several intriguing features such as recall transitions and creativity. I will discuss some of the general properties of this class of devices and present the results from a series of experiments with a simple holographic resonator employing photorefractive gain.
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.
A programmable optical resonator is one in which a hologram assumes the role of one of the mirrors of an ordinary optical resonator. A holographic recording medium allows the optical electric field eigenmodes of the resonator to be user-defined. Interest in this kind of resonator stems from their potential as associative optical memories. When the holographic medium is a photorefractive or other real-time recording medium, the resonator can learn, recall, and forget programmed information. Recall is achieved by placing a gain medium, such as an externally pumped photorefractive crystal, inside the resonator. The gain medium excites eigenmodes of the resonator. Using LiNbO3 as the recording medium and pumped BaTiO3 as the gain medium we have been able to store and recall complicated images as eigenmodes of a resonator.
A high capacity associative memory would find applications in computing and perhaps lend some insight into the understanding of collective neural networks and human memory. In this work an entirely optical associative memory device is proposed. The device operates on the principle that an optical resonator employing a holographic grating can have user prescribed eigenmodes. Information which is to be recalled is contained in the hologram. Each information entity (e.g., an image of a cat) defines an eigenmode of the resonator. The stored information is accessed by injecting partial information (e.g., an image of the cat’s ear) into the resonator. The appropriate eigenmode is selected through a competitive process in a gain medium placed inside the resonator. With a net gain greater than one, the gain amplifies the field belonging to the eigenmode which most resembles the injected field; the other eigenmodes are suppressed via the competition for the gain. The principles underlying the proposed device are outlined and the issues which must be addressed are delineated.
A coherent optical memory is described. The device operates on the principle that an optical resonator employing a holographic grating can have user-prescribed eigenmodes. Each information entity (e.g., an image of a cat) defines an eigenmode of the resonator. The stored information is accessed by injecting partial information (e.g., an image of the cat’s ear) into the resonator. A gain medium internal to the resonator amplifies the field belonging to the eigenmode that most resembles the injected field; the other eigenmodes are suppressed through a competition for the gain. A simple optical memory exhibiting associative recall is demonstrated.
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.