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

Study of LiNbO 3 in optical associative memory

Xiao L. Yuan, Mohammed R. Sayeh

Published Mar 1, 1991• 1 citation
Publisher

Abstract

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.

DOI
10.1117/12.25810

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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.
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.
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.
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.
Integrated optical quantum memories are a scalable solution to synchronize a large number of quantum nodes. Without compact quantum memories, some astonishing quantum applications such as distributed quantum computing and quantum sensor networks would not be possible. Rather than find a specific material that meets all the requirements of an on-chip quantum memory as other protocols usually do, we propose to assign the memory requirements on coherent storage and controllability to rare-earth ions and a lithium niobate crystal, respectively. Specifically, optical quantum states are stored in an erbium-doped lithium niobate microcavity by utilizing the electro-optic effect of lithium niobate. The cavity frequency can be shifted by an external electric field, thus modifying the resonance condition between the cavity and the collective atomic excitation. This effect is further used to suppress or enhance the emission of photon echoes. Our calculated results show that high efficiency and a low noise performance are achievable.
The speed of modern supercomputers is primarily limited by the relatively slow speed of memory. Research over the past years has focused on developing optical memory systems that, by nature of optics, promise to achieve high speed and high memory capacity. Three- dimensional optical memory systems are well known for their storage capacity and achievable short access time. But the early attempts to exploit 3-dimensional media were limited to photorefractive crystals such as lithium niobate, due to the unavailability of any other suitable medium (alternative media are precluded by their diffraction limited storage densities and non- erasable characteristics). Although volume phase holograms can be recorded in photorefractive crystals, these materials have many obvious flaws such as strong scattering, phase distortion, self diffraction, fast decay, and low refractive index modulation. There have also been continuous efforts to develop new 3-D media (such as photon echo, photorefractive polymers, bacteriorhodusphin, memory access cache, etc., but none can be applied in real world 3-D device because these materials are still in the early stage of development.
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.