Radiation imagery chemistry: process – composition – or product th – Holographic process – composition – or product – Composition or product or process of making the same
Reexamination Certificate
1999-01-12
2001-11-27
Angebranndt, Martin (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Holographic process, composition, or product
Composition or product or process of making the same
C430S001000, C430S945000, C430S321000, C430S394000, C369S024010, C359S107000, C359S107000
Reexamination Certificate
active
06322933
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to storage devices comprising bulk monolithic materials including formatted multiple optical data regions, such as data layers and/or tracks, and methods for forming such optical data regions, data layers and/or tracks in such materials.
2. The Prior Art
In conventional holographic storage, data is stored as holograms resulting from the interference of a signal and reference beam. During storage, both the reference and signal beams are incident on the storage medium. During retrieval, only the reference beam is incident on the medium. The reference beam interacts with a stored hologram, generating a reconstructed signal beam proportional to the original signal beam used to store the hologram.
For information on conventional volume holographic storage see for example U.S. Pat. Nos. 4,920,220, 5,450,218, and 5,440,669. In conventional volume holographic storage, each bit is stored as a hologram extending over a substantial volume of the storage medium. Multiple bits are encoded and decoded together in pages, or two-dimensional arrays of bits. Multiple pages can be stored within the volume by angular, wavelength, phase-code, or related multiplexing techniques. Each page can be independently retrieved using its corresponding reference beam. The parallel nature of the storage approach allows high transfer rates and short access times, since as many as 10
6
bits within one page can be stored and retrieved simultaneously.
Conventional page-based volume holographic storage generally requires complex, specialized components such as amplitude and/or phase spatial light modulators. Moreover, ensuring that the reference and signal beams are mutually coherent over the entire volume of the storage medium generally requires a light source with a relatively high coherence length, as well as a relatively stable mechanical system. Mechanical stability and coherence-length requirements have hindered the development of inexpensive, stable, and rugged holographic storage devices capable of convenient operation in a typical user environment.
In U.S. Pat. No. 4,458,345, Bjorklund et al. describe a bit-wise volume holographic storage method using signal and reference beams incident on a rotating disk in a transmission geometry. The signal and reference beams are incident from the same side of the disk. The angle between the reference and signal beams can be altered to store holograms at various depths within the medium. A separate photodetector is used to retrieve data stored at each depth. The interaction of light with the medium is localized through two-photon recording.
In U.S. Pat. No. 5,659,536, Maillot et al. describe a system in which multiple holograms are stored at each location in a disk through wavelength multiplexing. Each hologram spans the depth of the medium. In U.S. Pat. No. 5,289,407, Strickler et al. describe a multi-layered, non-holographic, index-perturbation optical storage system. Bits are stored as localized perturbations in the index of refraction of a photopolymer, caused by the high intensity at the focus of a single laser beam.
BRIEF DESCRIPTION OF THE INVENTION
Co-pending Ser. No. 09/016,382, filed Jan. 30, 1998, discloses the recording an elementary format grating having one grating vector representing its fringe periodicity, and the recording of a compound format grating, in which two gratings having two different spatial frequencies are superimposed to define different data layers in a bulk, monolithic medium. The present invention employs more complex formatting structures and methods, thus providing for improved data storage.
Briefly, and in general terms, the present invention provides a storage device comprising a photoactive storage medium formatted with a complex format grating hologram to provide multiple data regions, data layers and/or tracks, and to methods for defining such data layers and/or tracks, in such a photoactive storage medium. The storage medium is preferably, but not necessarily, a bulk, monolithic material. The methods of the present invention can be used to record a complex format hologram to define tracks and/or layers for volumetric data storage.
Volumetric storage includes direct recording of data by introducing a volumetric bit as an index or absorption change at spatial locations in a photoactive storage medium. Volumetric storage also includes storage by selective alteration of a format hologram. The present invention provides a method for generating complex format holograms comprising several component gratings, each of which may be characterized by a separate spatial frequency and which can be further configured to define tracks. Data can then be recorded along the tracks. Each component grating is formed by focussing a pair of mutually coherent light beams into the medium.
Data layer and/or track definition in a photoactive medium using a complex format grating according to the present invention can be accomplished by superimposing several holograms so that the nulls resulting from their beating define the boundaries of data layers and/or tracks. The tracks can take several forms, including, without limitation, concentric tracks for use in disk-oriented storage devices and linear volumes defining tracks for use in card- and tape-oriented storage devices. The complex format grating can be configured to define tracks throughout the volume of the material. Tracks stored throughout the volume can be used, for example, to define data layers. In one embodiment, the tracks take the form of concentric generally-toroidal volumes in each layer. The words “tubes” and “tracks” will both be used herein to refer to these linear and toroidal volumes. In one embodiment, the tubes are disposed in an alternately offset arrangement from layer to layer, providing a closely-packed track structure.
According to one aspect of the present invention, a storage device is provided in which the multiple tracks have been defined by formatting according to the methods of the present invention. In one embodiment of the storage device according to the present invention, the tracks are defined in such a manner that strong fringe contrast of the complex format gratings occurs inside the track and weaker fringe contrast occurs at the borders of the track. Generally, when a confocal detector is used for data readout, a focussed readout beam Bragg-matched to a format grating and incident on the center of the track is more strongly reflected than a focussed readout beam Bragg-matched to the format grating and incident on the edge of the track. Such a confocal detector is described in co-pending application Ser. No. 09/016,382 filed Jan. 30, 1998
According to another aspect of the present invention, a system and method are provided for recording and reading data in a device in which a complex format grating has been configured to define multiple tubes or tracks. Data can be recorded, for example, by focussing a recording beam on the center of a track and modulating that beam as the medium moves past the recording beam. The recorded data can be read, for example, by focussing a reading beam on the center of a track and detecting the modulation of the reflected beam as the medium moves past the recording beam.
During recording and readout, a focussed Bragg-matched beam can be used to monitor the relative positions of the beam focus and the center of the track volume. During recording, this beam can be either the same beam as the recording beam or a separate beam superimposed on the recording beam. Monitoring the reflected signal using a detection bandwidth that averages a substantial number signal modulation events allows measurement of an average reflected signal whose strength depends on the position of the beam focus with respect to the center of the track volume.
A formatting method according to the present invention provides the feature of modulating the reflectivity of the medium transversely as well as in depth. This method defines multiple tubes or tracks within a modulated media for
Daiber Andrew J.
Honda Tokuyuki
McLeod Robert R.
Angebranndt Martin
Sierra Patent Group Ltd.
Siros Technologies, Inc.
Smith Andrew V.
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