Radiation imagery chemistry: process – composition – or product th – Holographic process – composition – or product
Reexamination Certificate
1999-03-01
2003-09-30
Angebranndt, Martin (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Holographic process, composition, or product
C430S002000, C359S003000
Reexamination Certificate
active
06627354
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to information storage media, in particular storage media useful with holographic systems.
2. Discussion of the Related Art
Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, so-called page-wise memory systems, in particular holographic systems, have been suggested as alternatives to conventional memory devices. Page-wise systems involve the storage and readout of a representation, e.g., a page, of data. Typically, recording light passes through a two-dimensional array of dark and transparent areas representing data, and the holographic system stores, in three dimensions, holographic representations of the pages as patterns of varying refractive index in a storage medium. Holographic systems are discussed generally in D. Psaltis et al., “Holographic Memories,”
Scientific American
, November 1995, the disclosure of which is hereby incorporated by reference. One method of holographic storage is phase correlation multiplex holography, which is described in U.S. Pat. No. 5,719,691 issued Feb. 17, 1998, the disclosure of which is hereby incorporated by reference.
FIG. 1
illustrates the basic components of a holographic system
10
. System
10
contains a modulating device
12
, a photorecording medium
14
, and a sensor
16
. Modulating device
12
is any device capable of optically representing data in two-dimensions. Device
12
is typically a spatial light modulator that is attached to an encoding unit which encodes data onto the modulator. Based on the encoding, device
12
selectively passes or blocks portions of a signal beam
20
passing through device
12
. In this manner, beam
20
is encoded with a data image. The image is stored by interfering the encoded signal beam
20
with a reference beam
22
at a location on or within photorecording medium
14
. The interference creates an interference pattern (or hologram) that is captured within medium
14
as a pattern of, for example, varying refractive index. It is possible for more than one holographic image to be stored at a single location and/or for holograms to be stored in overlapping positions, by, for example, varying the angle, the wavelength, or the phase of the reference beam
22
, depending on the particular reference beam employed. Signal beam
20
typically passes through lens
30
before being intersected with reference beam
22
in the medium
14
. It is possible for reference beam
22
to pass through lens
32
before this intersection. Once data is stored in medium
14
, it is possible to retrieve the data by intersecting reference beam
22
with medium
14
at the same location and at the same angle, wavelength, or phase (depending on the multiplexing scheme used) at which reference beam
22
was directed during storage of the data. The reconstructed data passes through lens
34
and is detected by sensor
16
. Sensor
16
is, for example, a charged coupled device or an active pixel sensor. Sensor
16
typically is attached to a unit that decodes the data.
The capabilities of such holographic storage systems are limited in part by the storage media. Iron-doped lithium niobate has been used as a storage medium for research purposes for many years. However, lithium niobate is expensive, is poor in sensitivity (1 J/cm
2
), has relatively low index contrast (&Dgr;n of about
10
−4
), and exhibits destructive read-out (i.e., images are destroyed upon reading). Alternatives have therefore been sought, particularly in the area of photosensitive polymer films. See, e.g.,
Selected Papers on Holographic Recording
, H. J. Bjelkagen, ed., SPIE Press, Vol. MS 130 (1996). The materials described in this set of articles generally contain a photoimageable system containing a liquid monomer material (the photoactive monomer) and a photoinitiator (which promotes the polymerization of the monomer upon exposure to light), where the photoimageable material system is located within an organic polymer host matrix that is substantially inert to the exposure light. During writing of information into the material by exposure to radiation in selected areas, the monomer polymerizes in the exposed regions. Due to the lowering of the monomer concentration caused by induced polymerization, monomer from the dark, unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting concentration gradient create a refractive index change, forming the hologram representing the data. Typically, the system is then fixed by a flood cure exposure, which destroys any remaining photosensitivity in the medium. (For further discussion of the recording mechanism, see “Organic Photochemical Refractive Index Image Recording Systems” in
Advances in Photochemistry
, Vol. 12, John Wiley & Sons (1980).) Most holographic systems of this type are based on photopolymerization of free-radical photoactive monomers such as acrylate esters. See, for example, U.S. patent application Ser. No. 08/698,142 (our reference Colvin-Harris-Katz-Schilling 1-2-16-10), the disclosure of which is hereby incorporated by reference.
While such photopolymer systems provide useful results, they exhibit changes in dimension due to shrinkage induced by polymerization of the photactive monomers. Dimensional changes are also caused by thermal expansion. (Typical linear coefficient of thermal expansion values for these systems range from about 100 to about 300 ppm/° C.) These dimensional changes, while small, tend to distort the recorded holographic gratings, degrade the fidelity with which data is capable of being recovered, and thereby limit the density of data which the polymer is able to support. Some attempts to overcome these dimensional changes have led to experimentation with porous glass matrices containing a photoimageable system. See, e.g., U.S. Pat. Nos. 4,842,968 and 4,187,111; V. I. Sukhanov et al., “Sol-Gel Porous Glass as Holographic Medium,”
Journal of Sol
-
Gel Science and Technology
, Vol. 8, 1111 (1997); S. A. Kuchinskii, “Principles of hologram formation in capillary composites,”
Opt. Spectrosc
., Vol. 72, No. 3, 383 (1992); S. A. Kuchinskii, “The Principles of Hologram Formation in Capillary Composites,”
Laser Physics
, Vol. 3, No. 6, 1114 (1993); V. I. Sukhanov, “Heterogeneous recording media,”
Three
-
Dimensional Holography: Science, Culture, Education
, SPIE Vol. 1238, 226 (1989); V. I. Sukhanov, “Porous glass as a storage medium,”
Optica Applicata
, Vol. XXIV, No. 1-2, 13 (1994); and J. E. Ludman et al., “Very thick holographic nonspatial filtering of laser beams,”
Opt. Eng
., Vol. 36, No. 6, 1700 (1997).
U.S. Pat. No. 4,842,968, for example, discloses a process in which a porous glass matrix is immersed in a photoimageable system, such that the photoimageable system diffuses into the open pores of the matrix. After exposure to light, the unexposed, i.e., non-polymerized, portions of the photoimageable system must be removed from the pores with a solvent. Typically, a different material offering desirable refractive index contrast is then introduced into the emptied pores. It is only after these steps that a readable hologram is formed. (While the initial irradiation step tended to form a latent image in these previous matrix-based media, the latent image could not be read non-destructively by the same wavelength of light used for recordation, i.e., the reference beam could not be used for readout. Thus, no hologram was considered to have been formed. As used herein, the term readable hologram indicates a pattern capable of being non-destructively read by the same wavelength of light used for recordation.)
While glass matrices offer desirable rigidity and structural integrity, as well as formation of relatively thick, e.g., greater than 1 mm, media, the '968 patent illustrates several practical drawbacks encountered in such matrix-based recording media. Specifically, complex chemical treatments with solvents are required after exposure to remove reacted or un
Chandross Edwin Arthur
Galvin-Donoghue Mary Ellen
Neenan Thomas Xavier
Patel Sanjay
Angebranndt Martin
Lucent Technologies - Inc.
Thomas Kayden Horstemeyer & Risley LLP
Wilde Peter V. D.
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