Process for holography involving skip-sorted hologram storage

Details Process for holography involving skip-sorted hologram storage Process for holography involving skip-sorted hologram storage

2000-06-07

2003-09-02

Juba, John (Department: 2872)

Optical: systems and elements

Holographic system or element

For producing or reconstructing images from multiple holograms

C359S025000, C365S125000, C365S216000, C369S103000

Reexamination Certificate

active

06614566

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to page-wise storage systems, in particular holographic storage 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, page-wise memory systems, in particular holographic systems, have been suggested as alternatives to conventional memory devices. Holographic systems typically involve the storage and readout of entire pages of information, these pages consisting of arrayed patterns representing information. In general, a holographic system stores, in three dimensions, holographic representations of the pages as patterns of varying refractive index and/or absorption imprinted into a storage medium. Holographic systems are discussed generally in D. Psaltis et al., “Holographic Memories,”
Scientific American
, November 1995.
Holographic systems are characterized by their high density storage potential and the potential speed at which the stored information is randomly accessed and retrieved. In fact, because information is typically manipulated, i.e., stored and retrieved, on a page-by-page basis, the speed of storage and retrieval compares favorably to conventional magnetic disk or compact disk storage systems. A significant advantage of holographic systems, however, is storage capacity. It is possible for each page stored as a holographic image to contain thousands or even millions of elements. Theoretically, it is believed that at the present time, up to 10
14
bits of information are storable in approximately 1.0 cm
3
of holographic storage medium.
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 beam reflecting off or passing through device
12
. In this manner, a signal 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
(generally referred to as angle, wavelength, and phase correlation multiplexing, respectively). 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 CMOS sensor. Sensor
16
typically is attached to a unit that decodes the data.
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. 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 position of the medium relative to the signal and reference beams is changed to allow 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. Other techniques that store data by using motion of the media relative to the beams include aperture multiplexing (see U.S. Pat. No. 5,892,601) and shift multiplexing (see
Optics Letters
, Vol. 20, No. 7, 782-784 (1995)). Phase correlation, aperture, and shift multiplexing all involve storing holograms in different locations, but with some overlap between them.
As individual data pages are laid down in multiplexing space, there is a limit to how close together the holograms can be recorded without encountering cross-talk during read-out. However, even when sufficient space is provided between holograms, it is possible for cross-talk noise to be introduced into a given hologram during the readout and/or recording of neighboring holograms. Techniques have therefore been developed in an effort to reduce or avoid such introduced cross-talk. One technique—sparse recording—is useful for angle, wavelength, and phase code multiplexing techniques, i.e., techniques in which holograms have nearly the same physical location. In sparse recording, holograms that are stored at very close angles or wavelengths to each other are stored in an order distinct from their sequential angular or wavelength displacement. For example, if holograms are to be multiplexed at angles of 1°, 2°, 3°, 4°, 5°, 6°, 7°, 8°, 9°, and 10°, the holograms may be stored in an actual sequence such as 1°, 9°, 4°, 2°, 6°, 3°, 7°, 10°, 5°, and 8°. This lessens the cross-talk between holograms. (See C. Gu and J. Hong, “Noise gratings formed during the multiple exposure schedule in photorefractive media,”
Optics Comm
., Vol. 93, 213-18 (1992).) While sparse recording tends to be useful, it is also relatively complex, particularly during readout. Moreover, it's usefulness, as noted above, is primarily for techniques that involve nearly complete physical overlap of holograms.
In addition to this potential cross-talk problem, degradation of stored holograms can also occur due to local changes in a medium's refractive index and physical dimensions. Specifically, in photopolymer-based media, photoactive monomers and/or oligomers are selectively reacted to form holograms, and this polymerization tends to induce some local shrinkage. Thus, each successive hologram storage induces additional physical changes in the overall medium, e.g., changes in the bulk index and the extent of diffusion. These additive changes can introduce significant distortion when reading out the holograms, i.e., Bragg detuning of the holograms. In addition, photopolymer media tend to have finite dynamic range, i.e., index change. And, for spatial multiplexing techniques, photopolymer media tend to exhibit non-uniform recording across the medium, thereby inducing degradation of the stored data. (Spatial multiplexing, as used herein, indicates that the multiplexing technique involves changes to location of the medium relative to the signal and reference beams, and that the holograms have some overlap between them.) Advantageously, the holographic storage technique is designed to compensate for such changes.
Thus, holographic storage techniques that substantially reduce problems associated with cross-talk and with physical changes in storage media, particular photopolymer media, are desired.
SUMMARY OF THE INVENTION
The invention relates to a skip-sorted spatial multiplexing technique that addresses problems inherent in photopolymer media, including cross-talk and physical change of the medium. Such spatial multiplexing techniques include shift, phase correlation, and aperture multiplexing. Skip-sorted refers to a storage technique in which a uniform background exposure of

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