Radiation imagery chemistry: process – composition – or product th – Holographic process – composition – or product
Reexamination Certificate
1999-07-29
2001-11-27
Angebranndt, Martin (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Holographic process, composition, or product
C430S002000, C430S281100, C430S280100, C369S103000
Reexamination Certificate
active
06322931
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the storage and retrieval of digital data using an optical medium. More specifically, the present invention relates to a method and apparatus for altering a pre-formatted holographic grating in a localized region by thermal disruption of the grating utilizing non-linear optical absorption.
2. Background
Optical data storage technology has tended to follow two complementary lines of development. In one approach, data is encoded as minute variations in the surface of a recording medium, such as a compact disc, or CD. The data are readable using optical means (usually a laser), similar to the way in which data recorded in a magnetic medium are readable with a magnetically-sensitive head, or data recorded in a vinyl medium are readable with a needle. Unlike vinyl recording, however, in optical storage the data are usually stored digitally. For read-only compact discs, data are stored as microscopic pits on the surface of a substrate. In addition, recordable or re-writable bit-based optical systems are readily available. Examples include magneto-optic systems, in which the orientation of a magnetic domain changes the direction of rotation of the polarization of a reflected, focussed light beam; phase-change systems, in which a medium can be locally crystalline or polycrystalline, each of which states have a variance in reflectivity; and, dye-polymer systems, in which the reflectivity of a medium is changed by the high-power illumination.
Each bit of data has specific physical location in the storage medium. The storage density of optical media is limited by physical constraints on the minimum size of a recording spot. Another basic limitation of conventional optical storage is that data are usually stored on the surface of the medium only. Recording throughout the volume of a storage medium would provide an opportunity to increase capacity.
Multi-layer storage is also possible, but usually requires the manufacture of special, heterogeneous, layered recording media, whose complexity increases quickly with the number of layers needed. Most commercially-available multi-layer optical storage media offer no more than two data layers, and come in a pre-recorded format.
An alternative approach to traditional optical storage is based on holographic techniques. In conventional volume holographic recording, laser light from two beams, a reference beam and a signal beam containing encoded data, meet within the volume of a photosensitive holographic medium. The interference pattern from the superposition of the two beams results in a change or modulation of the refractive index of the holographic medium. This modulation within the medium serves to record both the intensity and phase information from the signal. The recorded intensity and phase data are then retrieved by exposing the storage medium exclusively to the reference beam. The reference beam interacts with the stored holographic data and generates a reconstructed signal beam which is proportional to the initial signal beam used to store the holographic image. For information on conventional volume holographic storage, see, for example, U.S. Pat. Nos. 4,920,220, 5,450,218, and 5,440,669.
Typically, volume holographic storage is accomplished by having data written on the holographic medium in parallel, on 2-dimensional arrays or “pages” containing 1×10
6
or more bits. Each bit is generally stored as information extending over a large volume of the holographic storage medium, therefore, it is of no consequence to speak in terms of the spatial “location” of a single bit. Multiple pages can then be stored within the volume by angular, wavelength, phase-code or related multiplexing techniques.
Unfortunately, conventional volume holographic storage techniques generally require complex, specialized components such as amplitude and/or phase spatial light modulators. Ensuring that the reference and signal beams are mutually coherent over the entire volume of the recording medium generally requires a light source with a relatively high coherence length, as well as a relatively stable mechanical system. These requirements have, in part, hindered the development of inexpensive, stable, and robust holographic recording devices and media capable of convenient operation in a typical user environment.
In order for volumetric optical data storage to mature into a viable data storage option the process must be developed so that the operation is relatively simple, inexpensive and robust. Foremost in this development is accomplishing multi-depth bit-wise optical data storage and/or retrieval. As data recording proceeds to a greater number of depths within the storage medium it becomes increasingly more critical to isolate the recorded bit within a specific area within the medium. In multi-depth storage and/or retrieval, it is also important to write data at a given depth without affecting data at other depths. Further, for multi-depth bit-wise optical data storage and/or retrieval, it is important to have separate write and read conditions, so that readout does not negatively affect recorded data.
BRIEF DESCRIPTION OF THE INVENTION
Briefly, and in general terms, the present invention comprises an improved optical data storage medium including a host material and molecules that exhibit non-linear absorption suspended therein. The molecules exhibiting non-linear absorption may be selected from the group of molecules exhibiting two-photon absorption or reverse saturable absorption. Preferably, the host material is a photopolymer
The invention further comprises a method for preparing the optical storage medium. The method comprises dissolving the molecules that exhibit non-linear absorption throughout a photopolymer host material by mixing the dye in the photolymer in concentration levels that approach the solubility limit of the molecules within the host material.
Preferably, the invention also comprises an optical data storage device comprising the optical data storage medium discussed above and having a format hologram stored within the medium. This optical data storage device may take the form of a disk, a tape, a card or the like.
In one embodiment the invention comprises an optical data storage system wherein the storage system uses the optical data storage medium described above when altering a pre-formatted holographic grating in a localized region by non-linear thermal disruption of the grating. Additionally, the invention is embodied in a method for optical data storage wherein the storage medium of the present invention embodies a preformatted holographic grating that is altered in a localized region by non-linear thermal disruption of the holographic grating.
Additionally, another embodiment of the present invention comprises an optical data retrieval system embodying the optical data storage device described above and a method for retrieving data stored on a format hologram written on the optical data storage device of the present invention.
The non-linear absorbing molecules allow for energy in the form of heat to be generated during the optical data storage process. This heat provides for localized alterations to occur in the pre-formatted holographic grating. The non-linear feature of these molecules is instrumental in allowing for alterations to occur at specific depths and locations within the storage medium while limiting the secondary effects on other locations within the storage medium.
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Cumpston Brian H.
Marder Seth R.
Perry Joseph W.
Angebranndt Martin
Sierra Patent Group Ltd.
Siros Technologies, Inc.
Smith Andrew V.
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