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Spectral Hole-Burning and Related Spectroscopies: Science and Applications

Temporal Accessing of Frequency-Domain Optical Storage: Specific Approaches and General Considerations

THOMAS W. MOSSBERG

Published Jan 1, 1991• 0 citations
Publisher

Abstract

The ultimate memory device would be one in which a bit of data is stored in every atom or molecule within a storage material. Such a memory would have an incredible storage capacity of somewhere in the range of 1022 bits/cm3. Traditional optical memories, whether two- or three-dimensional, can never hope to achieve atomic-level storage densities for the simple reason that minimally sized storage cells always have edge dimensions on the order of or larger than the wavelength of light employed. In the case of visible light, cubic wavelength scale storage volumes contain billions of atoms.

DOI
10.1364/pshb.1991.fb1

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The ultimate memory device would be one in which a bit of data is stored in every atom or molecule within a storage material. Such a memory would have an incredible storage capacity of somewhere in the range of 1022 bits/cm3. Traditional optical memories, whether two- or three-dimensional, can never hope to achieve atomic-level storage densities for the simple reason that minimally sized storage cells always have edge dimensions on the order of or larger than the wavelength of light employed. In the case of visible light, cubic wavelength scale storage volumes contain billions of atoms.
The diffraction-limited spot size of a focused laser beam puts an upper boundary on the storage densities in conventional optical data storage. This can be overcome by storing multiple bits of data in each spatial location. In materials with an inhomogeneously broadened absorption line, such as crystals doped with rare-earth ions, frequency can be used as an additional addressing dimension. In some materials the number of spectral channels can be as high as 107. Spectral data storage can be done using coherent transient interactions between light and atoms and can be described in terms of photon echo processes. In this Master's Project the swept-carrier time-domain (SCTD) approach, a hybrid between frequency- and time-domain approaches, is used. An overview of the theory is given, followed by a description ofthe set-up and equipment. A maximum of 330 bits of data have been stored in one spatial location. The necessary conditions for successful storage and retrieval of data have been examined and suggestions are given on how to extend the results further.
We propose and demonstrate a novel type of frequency-selective optical memory that writes and reads the data in both the time domain and the frequency domain. Temporal 16-bit data were stored by accumulated-photon-echo bit-by-bit storage at 103 frequency addresses within the inhomogeneous line of the (7)F(0)-(5)D(0) transition of Eu(3+):Y(2)SiO(5), which yields a total memory capacity of 1.6 kbits in a single spot of 240-microm diameter. The keys to the success of this experiment are this material's long dephasing time and lack of spectral diffusion.
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In photography, information is stored in a medium which is essentially two-dimensional. Three-dimensional optical storage is possible in semitransparent colored materials, like alkali halides with color centers. With the use of coherent light sources, like lasers, large amounts of information can be stored in the volume, and retrieved with little interference. The storage of information is accomplished by the formation of interference patterns between each two plane parallel waves. This paper develops the theory of this form of storage. It turns out that the information storage capacity is as if every little cube with sides equal to the wavelength of light acts as an independent information storage cell, and the essential noise in recovering this information is only the statistical fluctuation in the number of color centers in such a cube. The storage capacity is therefore of the order of 1012–1013 bits per cm3. The main property of this way of information storage is the appearance of a “ghost image,” partly but not completely analogous to the one described previously. This property makes three-dimensional storage very suitable for associative memories. The theory lends support to Beurle’s proposed mechanism of information storage in the brain.