Optical: systems and elements – Holographic system or element – For producing or reconstructing images from multiple holograms
Reexamination Certificate
2000-04-20
2002-05-14
Henry, Jon (Department: 2872)
Optical: systems and elements
Holographic system or element
For producing or reconstructing images from multiple holograms
C359S024000, C359S012000, C365S125000
Reexamination Certificate
active
06388779
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 potential for both high density storage potential and high retrieval speed. 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 spatial light modulator (SLM)
12
, a photorecording medium
14
, and a sensor
16
. The SLM
12
is any device capable of optically representing data in two-dimensions. The SLM
12
is generally attached to an encoding unit which encodes data onto the modulator. Based on the encoding, the SLM
12
selectively passes or blocks portions of a beam passing through it or reflecting off of it. In this manner, the 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 sensor. Sensor
16
typically is attached to a unit that decodes the data.
Unfortunately, while page-wise storage and readout in a holographic system offers the potential for high speed and capacity, the page-wise nature of the system also introduces potential problems. For example, each pixel generated by the SLM
12
has a corresponding target pixel on the sensor
16
, i.e., the system is designed such that each pixel stored in the medium by directing the signal beam through the SLM will, upon readout, be directed to a particular, corresponding pixel on the sensor. If, at any point in the system, the page of information is offset by a single pixel or the image becomes blurred or distorted, the stored information will be unreadable. Introduction of non-imaging elements into the system, e.g., storage media, tend to induce aberrations in the relayed image, thereby contributing to this problem. In particular, while putting the medium at an angle to the signal beam is generally desirable for photopolymer media and for some multiplexing geometries, such a configuration tends to introduce even more significant aberration.
It is therefore desired to develop techniques to reduce or avoid the introduction of aberrations into holographic systems, particular aberration introduced by non-imaging elements such as storage media.
SUMMARY OF THE INVENTION
The invention provides a process and apparatus for holography in which aberrations introduced by storage media, particularly photopolymer-type media, are substantially reduced. Specifically, as reflected in
FIG. 3
, photopolymer-type media
56
are generally placed at an angle to the optical path
64
in a holographic system, in order to reduce the effects of writing-induced shrinkage. This angular placement, however, introduces aberrations into the relayed image of the system, including astigmatic aberration, i.e., a tilt in the plane of focus
62
. This tilt causes the page of data to match up poorly to the plane of the sensor
60
, thereby affecting the accuracy of read-out.
It has been discovered, however, that it is possible to compensate for at least a portion of such aberration by introducing an orthogonal tilt in the spatial light modulator, as shown in FIG.
4
. The rotation of the SLM
70
creates a similar aberration in the plane orthogonal to the plane tilted by the medium, and thereby makes the data page come into a sharper focus over the entire page. This effect allows presentation of a flatter, more focused, and less distorted image
80
at the sensor
82
, and leads to better matching of each encoded pixel to its target pixel.
The invention therefore involves a holographic system containing (a) a storage medium (
74
) located in the system's optical path (
84
), where the medium is rotated around a first axis (
76
) such that the surface of the medium is in a non-orthogonal relationship with the optical path, and (b) a spatial light modulator (
70
) also located in the optical path, where the modulator is rotated around a second axis (
72
) that is substantially orthogonal to the first axis, and wherein the surface of the modulator is in a non-orthogonal relationship with the optical path. (Optical path indicates a path from the spatial light modulator through the medium and onto the sensor, and is also referred to in the art as the central ray.) The invention is particularly useful for 4F imaging systems.
REFERENCES:
patent: 2896499 (1959-07-01), De Lang
patent: 3697149 (1972-10-01), Heeckeren et al.
patent: 3794407 (1974-02-01), Nishimura
patent: 3911729 (1975-10-01), Collins
patent: 3940204 (1976-02-01), Withrington
patent: 5200857 (1993-04-01), Matsushita
patent: 5490013 (1996-02-01), Shimizu et al.
patent: 5719691 (1998-02-01), Curtis et al.
patent: 0969323 (2000-01-01), None
Psaltis, D. et al., “Holographic Memories,”Scientific American(1995) p. 70-75.
Lundquist, P.M. et al., “Holographic Digital Data Storage in a Photorefractive Polymer”,Optics Letters, vol. 21, No. 12, pp 890-892 (1996).
Zhao, C. et al., “Shrinkage-corrected volume holograms based on photopolymeric phase media for surface-normal optical interconnect”,Applied Physics Letters, vol. 71, No. 11, pp. 1464-1466 (1997).
Anonymous: “Correction of Astigmatism for Off Axis Reconstruction Beam Holographic Deflector”,IBM Technical Disclosure Bulletin, vol. 23, No. 9, pp 4255-4256 (1981).
Curtis Kevin Richard
Tackitt Michael C
Henry Jon
Lucent Technologies - Inc.
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