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Optics in Computing

Storage Limits of Two-Photon Based Three-Dimensional Memories

Hakan Ürey, Frederick B. McCormick

Published Jan 1, 1997• 0 citations
Publisher

Abstract

Two-photon absorption 3-D optical memories have the potential for simple media fabrication, parallel access for high data transfer rates, high volumetric storage density, and low raw bit error rates (BER).1-2 The writing or erasure takes place at the intersection of two orthogonally propagating beams where a unique physical location is allocated to each stored bit. The data can be stored and retrieved in several formats in a single operation. By using either orthogonal-propagating beams or counter-propagating beams, it is possible to control the region of overlap of the two beams to be a single point, a line, or a plane. We will refer to those data formats as bit, vector, and image (or page) storage.

DOI
10.1364/oc.1997.owd.3

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Two-photon absorption 3-D optical memories have the potential for simple media fabrication, parallel access for high data transfer rates, high volumetric storage density, and low raw bit error rates (BER).1-2 The writing or erasure takes place at the intersection of two orthogonally propagating beams where a unique physical location is allocated to each stored bit. The data can be stored and retrieved in several formats in a single operation. By using either orthogonal-propagating beams or counter-propagating beams, it is possible to control the region of overlap of the two beams to be a single point, a line, or a plane. We will refer to those data formats as bit, vector, and image (or page) storage.
Data transfer rates of recently proposed 3D two-photon memories are potentially limited by repetition rates of pulsed lasers. Herein we propose a method of spatially and temporally multiplexing pulse from a multi-Watt, 100 MHz repetition rate, sub-picosecond pulse width mode-locked laser to a linear array of parallel data channels in order to increase the two- photon writing rate. Nearby channels are desynchronized so that pulses incident on adjacent regions of the storage medium do not overlap far from the focal plane. This prevents pairs of photons directed to proximate bits from being simultaneously absorbed in regions of beam overlap which would otherwise degrade the optical sectioning property of the two-photon absorption method. This method may be applied to an earlier proposed architecture that uses 2D arrays of channels for massively parallel memories. Our modified writing method obviates the necessity for intersecting beams and thus reduces the diffraction limit to the storage density of this alternative approach.
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.
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.
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.