Radiation imagery chemistry: process – composition – or product th – Holographic process – composition – or product
Reexamination Certificate
2002-05-16
2004-06-01
Angebranndt, Martin (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Holographic process, composition, or product
C430S002000, C430S290000, C359S003000
Reexamination Certificate
active
06743552
ABSTRACT:
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. In particular, this invention relates to rapid mass production of high performance holographic recording article.
BACKGROUND
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.
A hologram stores data in three dimensions and reads an entire page of data at one time, i.e., page-wise, which is unlike an optical CD disk that stores data in two dimensions and reads a track at a time. 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.
The advantages of recording a hologram are high density (storage of hundreds of billions of bytes of data), high speed (transfer rate of a billion or more bits per second) and ability to select a randomly chosen data element in 100 microseconds or less. These advantages arise from three-dimensional recording and from simultaneous readout of an entire page of data at one time.
A hologram is a pattern, also known as a grating, which is formed when two laser beams interfere with each other in a light-sensitive material (LSM) whose optical properties are altered by the intersecting beams. Before the bits of data can be imprinted in this manner in the LSM, they must be represented as a pattern of clear and opaque squares on a display such as a liquid crystal display (LCD) screen, a miniature version of the ones in laptop computers. Other devices such as reflective LCD's or reflective deformable micromirror devices can also be used to represent the data. A blue-green laser beam, for example, is shined through this crossword-puzzlelike pattern called a page, and focused by lenses to create a beam known as a signal beam. A hologram of the page of data is created when the signal beam meets another beam, called the reference beam, in the LSM. The reference beam could be collimated, which means that all its light waves are synchronized, with crests and troughs passing through a plane in lockstep. Such waves are known as plane waves. The reference beam may also be a spherical beam or may be phase-encoded or structured in other manners well known in the field of holography. The grating created when the signal and reference beams meet is captured as a pattern of varying refractivity in the LSM.
After recording the grating, the page can be holographically reconstructed by for example shining the reference beam into the LSM from the same angle at which it had entered the LSM to create the hologram. As it passes through the grating in the LSM, the reference beam is diffracted in such a way that it recreates the image of the original page and the information contained on it. A reconstructed page is then projected onto a detector such as an array of electrooptical detectors that sense the light-and-dark pattern, thereby reading all the stored information on the page at once. The data can then be electronically stored, accessed or manipulated by any conventional computer.
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.
The capabilities of 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, 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 preformed 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. Pat. No. 6,013,454 and application Ser. No. 08/698,142, the disclosures of which are hereby incorporated by reference, also relate to a photoimageable system in an organic polymer matrix. 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 betw
Schnoes Melinda
Setthachayanon Songvit
Angebranndt Martin
InPhase Technologies Inc.
Morrison & Foerster / LLP
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