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Optical Engineering

Holographic associative memory using a coherently induced double phase conjugate mirror

Xiudong Sun, Zhongxiang Zhou, Yan Li, Yongyuan Jiang, Kebin Xu, Jingwen Zhang

Published Aug 1, 1996• 3 citations
Publisher

Abstract

We demonstrate a new method to implement holographic associative memory based on a coherently induced double phase conjugate mirror that has phase-conjugate reflectivity larger than unity for weak reference signal over a large range of pumping-reference intensity ratio. The phase conjugator, operated with a copper-doped KNSBN crystal, serves as a novel nonlinear feedback and gain element for the associative memory system. A vanadium-doped KNSBN crystal as storage medium improves the response speed of the holographic recording process. A stored image has been completely retrieved when a partial input addresses the associative memory system.

DOI
10.1117/1.600796

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We demonstrate a new method to implement holographic associative memory based on a coherently induced double phase conjugate mirror that has phase-conjugate reflectivity larger than unity for weak reference signal over a large range of pumping-reference intensity ratio. The phase conjugator, operated with a copper-doped KNSBN crystal, serves as a novel nonlinear feedback and gain element for the associative memory system. A vanadium-doped KNSBN crystal as storage medium improves the response speed of the holographic recording process. A stored image has been completely retrieved when a partial input addresses the associative memory system.
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.
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.
I derive and analyze coupled-mode equations for a double phase-conjugating resonator that is loaded with a volume-holographic recording medium in which multiplexed images have been stored. If the only nonlinearity present is the saturating gain of the conjugators, there is no multistability: The most deeply imprinted image always wins the competition for saturation. If several images are imprinted with equal depth, any slowly varying superposition of these images is neutrally stable. However, if a nonlinearity, arising from a space-charge-dependent cubic polarization, is present the resonator does exhibit multiple basins of stability that correspond to the faithful reconstruction of each stored image. The momentary injection of an initial image will tip the system into the basin that corresponds to the stored image that it most closely resembles, even if that image is not the most deeply imprinted one.
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.