Optical article and process for forming article

Details Optical article and process for forming article Optical article and process for forming article

1998-12-09

2002-11-19

Angebranndt, Martin (Department: 2872)

Radiation imagery chemistry: process, composition, or product th

Holographic process, composition, or product

C430S002000, C430S290000, C430S281100, C359S003000

Reexamination Certificate

active

06482551

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to optical articles including holographic recording media, in particular media useful either with holographic storage systems or as components such as optical filters or beam steerers.
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 an entire two-dimensional 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 imprinted into 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. In one embodiment of phase correlation multiplex holography, a reference light beam is passed through a phase mask, and intersected in the recording medium with a signal beam that has passed through an array representing data, thereby forming a hologram in the medium. The spatial relation of the phase mask and the reference beam is adjusted for each successive page of data, thereby modulating the phase of the reference beam and allowing the data to be stored at overlapping areas in the medium. The data is later reconstructed by passing a reference beam through the original storage location with the same phase modulation used during data storage. It is also possible to use volume holograms as passive optical components to control or modify light directed at the medium, e.g., filters or beam steerers. Writing processes that provide refractive index changes are also capable of forming articles such as waveguides.
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, 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 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, exhibits poor sensitivity (1 J/cm
2
), has 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,
1
o particularly in the area of photosensitive polymer films. See, e.g., W. K. Smothers et al., “Photopolymers for Holography,” SPIE OE/Laser Conference, 1212-03, Los Angeles, Calif., 1990. The material described in this article contains 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 system is in an organic polymer host matrix that is substantially inert to the exposure light. During writing of information into the material (by passing recording light through an array representing data), the monomer polymerizes in the exposed regions. Due to the lowering of the monomer concentration caused by the 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. Unfortunately, deposition onto a substrate of the pre-formed matrix material containing the photoimageable system requires use of solvent, and the thickness of the material is therefore limited, e.g., to no more than about 150 &mgr;m, to allow enough evaporation of the solvent to attain a stable material and reduce void formation. In holographic processes such as described above, which utilize three-dimensional space of a medium, the storage capacity is proportional to a medium's thickness. Thus, the need for solvent removal inhibits the storage capacity of a medium. (Holography of this type is typically referred to as volume holography because a Klein-Cook Q parameter greater than 1 is exhibited (see W. Klein and B. Cook, “Unified approach to ultrasonic light diffraction,”
IEEE Transaction on Sonics and Ultrasonics
, SU-14, 1967, at 123-134). In volume holography, the media thickness is generally greater than the fringe spacing,)
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, also relates to a photoimageable system in an organic polymer matrix, but allows fabrication of thicker media. In particular, the application discloses a recording medium formed by polymerizing matrix material in situ from a fluid mixture of organic oligomer matrix precursor and a photoimageable system. A similar type of system, but which does not incorporate oligomers, is discussed in D. J. Lougnot et al.,
Pure and Appl. Optics
, 2, 383 (1993). Because little or no solvent is typically required for deposition of these matrix materials, greater thicknesses are possible, e.g., 200 &mgr;m and above. However, while useful results are obtained by such processes, the possibility exists for reaction between the precursors to the matrix polymer and the photoactive monomer. Such reaction would reduce the refractive index contrast between the matrix and the polymerized photoactive monomer, thereby affecting to an extent the strength of the stored hologram.
Thus, while progress has been made in fabricating photorecording media suitable for use in holographic storage systems, further progress is desirable. In particular, media which are capable of being formed in relatively thick layers, e.g., greater than 200 &mgr;m, which substantially avoid reaction between the matrix material and photomonomer, and which exhibit useful holographic properties, are desired.
SUMMARY OF THE INVENTION
The invention constitutes an

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