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Optical Computing

Sparse-Wavelength Angularly Multiplexed Volume Holographic Memory

Scott P. Campbell, Xianmin Yi, Pochi Yeh

Published Jan 1, 1995• 0 citations

Abstract

Photorefractive-based optical memory systems have received considerable attention in recent years [1-6] In these systems, numerous volume holograms are stored within a single photorefractive crystal, with the writing and recall selectivity generally being performed via angle [1] wavelength [2,3], or phase [4] multiplexing. As well, non-photorefractive-based optical memory systems [7,8] have been proposed. In this paper, we present a hybrid system [6] that utilizes a combination of wavelength and angle multiplexing in a photorefractive medium. Such a hybridization is driven by photonic source generation and manipulation device limitations, as well as the desire to expand information throughput rates in optical memory systems via spectral parallelism [9]. Our design is essentially the summation over wavelength of many angularly multiplexed volume memory systems. We therefore compare our system’s parameters to those of purely wavelength or angle multiplexed ones.

DOI: 10.1364/optcomp.1995.owa4

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One of the most attractive features of an optical memory is its ability to write and read data in a bit-parallel format, giving rise to theoretically very high (in excess of 1 Gbps) data transfer rates. However, such a potential has not been demonstrated experimentally because of various inherent technical difficulties associated with existing optical storage techniques. In a photorefractive or a persistent spectral hole-burning (PSHB) memory, for example, the time required to record one page varies from a fraction of a second to several seconds.1-3 For a page containing 1000 × 1000 bits of data, it translates to a bandwidth of approximately 1 Mbps, substantially slower than any existing semiconductor memories.

One of the unique and perhaps the most attractive features of an optical memory is its ability to store and retrieve information in a bit-parallel format.1-3 Recently, we have proposed a new scheme for parallel data storage in TDOM.4,5 This scheme was proven to be practical and can be implemented with existing technologies. In a proof-of-principle experiment, four wavelength-multiplexed single-page volume spectral holograms, generated with black-and-white transparencies, were successfully stored in a single spatial location at a speed of approximately 23 μs/frame. The experimental results project system's input/output bandwidth to exceed 40 Gbps, two orders of magnitude faster than a semiconductor cache memory.

An encrypted optical memory system that uses a wavelength code as well as input and Fourier-plane random phase codes is proposed. Original data are illuminated by a coherent light source with a specified wavelength and are then encrypted with two random phase codes before being stored holographically in a photorefractive material. Successful decryption requires the use of a readout beam with the same wavelength as that used in the recording, in addition to the correct phase key in the Fourier plane. The wavelength selectivity of the proposed system is evaluated numerically. We show that the number of available wavelength keys depends on the correlation length of the phase key in the Fourier plane. Preliminary experiments of encryption and decryption of optical memory in a LiNbO(3):Fe photorefractive crystal are demonstrated.

The use of holography is attractive for optical data storage due to its potential for implementing page oriented data storage systems. However, multiplexed recording using Bragg selective materials has not provided a large enough storage capability to justify the expense of these systems. Persistent spectral hole burning holography allows multiplexed recording using much smaller wavelength separations between the holograms, and hence dramatically increases the storage capacity possible in holographic memory systems [1]. A comparison between the wavelength selectivity obtained for transmission gratings recorded using hole burning in a dye doped polymer film versus Bragg selectivity in a 1 cm thick iron-doped lithium niobate crystal is shown in Fig. (1).

Summary form only given. One of the unique and perhaps the most attractive features of an optical memory is its ability to store and retrieve information in a bit-parallel format. Recently, we have proposed a new scheme for parallel data storage in TDOM. This scheme was proven to be practical and can be implemented with existing technologies. In a proof-of-principle experiment, four wavelength-multiplexed single-page volume spectral holograms, generated with black-and-white transparencies, were successfully stored in a single spatial location at a speed of approximately 23 /spl mu/s/frame. The experimental results project system's input/output bandwidth to exceed 40 Gbps, two orders of magnitude faster than a semiconductor cache memory. Here, we report on the use of the proposed scheme to demonstrate the storage of a large number of spectral holograms in a Eu/sup 3+/-doped Y/sub 2/SiO/sub 5/ crystal.