Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1999-07-29
2001-10-30
Tran, Thang V. (Department: 2651)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
Reexamination Certificate
active
06310850
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 novel optical storage device using a bulk optical recording medium having a spatially-modulated refractive index with data stored thereon as localized alterations in the medium.
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 multidepth 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 provides a system and method for storage and/or retrieval of digital data using a bulk optical recording medium having a spatially-modulated refractive index profile that can be altered locally. Data can be written on multiple layers throughout the volume of the bulk recording medium using relatively simple and inexpensive optical methods.
In a presently preferred embodiment of the invention, the bulk, monolithic medium has a periodic, spatially-modulated refractive index that varies along a single depth axis of the medium, defining a plurality of reflective Bragg fringes. The bulk medium can be produced, for example, by recording a format hologram in a photosensitive medium, preferably a photopolymer. The periodic structure of the bulk medium defines a first Bragg reflection condition, so that the medium is relatively reflective to light of specific wavelengths incident upon the material at specific orientations with respect to a format hologram, and relatively transparent to other sources of light at particular wavelength/orientation combinations.
Generally, the format hologram itself does not represent stored data, and can be produced under controlled conditions in a factory environment. Instead, data are written in the bulk medium by focusing a single write beam onto desired storage locations within the volume of the medium. The write beam stores data locally by modification of the medium. In a preferred embodiment of the present invention, in altered locations, a second Bragg reflection condition is established that is distinct from the first Bragg reflection condition of the bulk medium, so that the alterations can be detected as variations in the reflectivity of the storage locations to a retrieval beam of light. The second Bragg condition may be achieved, e.g., by changing the fringe spacing and/or index of refraction at a data storage location. The retrieval beam may be tuned sufficiently close to either the first or second Bragg reflection conditions to allow substantial reflection to distinguish the two Bragg reflection conditions. Data storage and retrieval can be performed using relatively simple and inexpensive optical and mechanical components in a user environment.
A preferred embodiment comprises a periodic spatially-modulated refractive index, but in general, other index profiles are possible, such as chirped profiles.
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Cumpston Brian H.
Hesselink Lambertus
Honda Tokuyuki
Orlov Sergei
Sochava Sergei
Sierra Patent Group Ltd.
Siros Technologies, Inc.
Smith Andrew V.
Tran Thang V.
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