Optical: systems and elements – Holographic system or element – For reconstructing image
Reexamination Certificate
2000-01-04
2001-12-18
Chang, Audrey (Department: 2872)
Optical: systems and elements
Holographic system or element
For reconstructing image
C359S016000, C359S010000, C359S364000, C359S363000, C365S124000, C365S125000
Reexamination Certificate
active
06331904
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a holographic storage and retrieval system, and especially to an optical system which delivers a reference beam to a holographic storage medium.
BACKGROUND
In holographic storage, data is stored in a hologram resulting from the interference of a signal and a reference beam. During storage, both the reference and the signal beams are incident on the storage medium. During retrieval, only the reference beam is incident on the medium. The reference beam interacts with the stored hologram, generating a reconstructed signal beam proportional to the original signal beam used to store the hologram. Relative to conventional magnetic and optical data storage methods, holographic data storage promises high storage densities, short access times, and fast data transfer rates. The widespread use of holographic data storage has been hindered in part by the relative complexity of the specialized components required for storage and retrieval of data.
For information on conventional volume holographic storage see for example U.S. Pat. Nos. 4,920,220, 5,450,218, and 5,440,669. In conventional volume holographic storage, each bit is stored as a hologram extending over the entire volume of the storage medium. Multiple bits are encoded and decoded together in pages, or two-dimensional arrays of bits. Multiple pages are stored within the volume by angular, wavelength, phase-code, or related multiplexing techniques. Each page can be independently retrieved using its corresponding reference beam. The parallel nature of the storage approach allows high transfer rates and short access times, since as many as
106
bits within one page can be stored and retrieved simultaneously.
In a conventional angular multiplexing scheme, the angle between the signal beam and the reference beam is changed. Such a process is normally achieved by a combination of an angularly tunable mirror and an optical relay system, as is shown in
FIG. 1. A
reference beam
112
is reflected by an angularly tunable mirror
103
, such as a galvanometer, in a first position
100
. The light spot
102
on the angularly tunable mirror is imaged in the center of a holographic storage crystal
104
by a 4F relay imaging system. The reflected beam
116
passes through two lenses
106
and
108
, which have the same focal length F, and interferes with a signal beam
114
in the holographic storage medium
104
. The angularly tunable mirror
103
is placed at the focal plane of the lens
106
. The distance between the two lenses
106
and
108
is 2F. The center plane
110
between the two lenses
106
and
108
is the Fourier plane of the lens
106
. The holographic storage medium
104
is positioned at a distance of F from the lens
108
. When the angularly tunable mirror
103
rotates to a second position
101
, a second reflected reference beam
118
passes through lenses
106
and
108
, enters the holographic storage medium
104
, and interferes with a signal beam
114
at the same position and yet a different angle with respect to the beam
116
. The relay performance of a conventional refractive optical system is inversely proportional to the range of angles it is designed to relay. This angular multiplexing system is usually space demanding. Furthermore, since the lenses
106
and
108
are not monolithic, an optical alignment procedure is required before use.
U.S. Pat. No. 5,671,073 taught a shift multiplexing method. A spherical wave or a fan of plane waves can be used as the reference which interacts with a signal beam in a holographic storage medium at an angle. In fact, different parts of the reference interact with the signal beam at slightly different angles. The holographic storage medium is shifted at predetermined distances with respect to the signal and reference beams in order to record different pages of data. Different parts of the reference contribute to the writing and reading of different holograms at different displacements. Shift multiplexing can be considered as another form of angular multiplexing.
An imaging system using all reflective optics has been disclosed in the U.S. Pat. No. 3,190,171. The prior art teaches the construction of a viewing device using a relay imaging system. This relay imaging system uses concave and convex mirrors. Similar systems have also been taught in U.S. Pat. Nos. 4,796,984, and 4,293,186. The concave-convex-mirror imaging system has excellent off-axis optical performance. Application of this system to lithography technology has been taught in U.S. Pat. No. 3,748,015, and in A. Offner's article: “New Concepts in Projection Mask Aligners”, OPTICAL ENGINEERING, Vol. 14, No. 2, 1975.
SUMMARY
Briefly, and in general terms, the present invention applies the concave-convex mirror imaging system to a holographic storage system. More specifically, the present invention uses concave and convex mirrors to form a reference beam telescope, which delivers a reference beam to a holographic storage medium. This invention makes use of the superior off-axis performance of the concave-convex mirror imaging system to achieve angular multiplexing in holographic storage. Furthermore, the present invention provides a monolithic reference telescope which features high optical performance for collimated reference beam and robustness.
A relay imaging system according to the present invention, which delivers a reference beam to a holographic storage medium, comprises at least one reflective convex mirror and one reflective concave mirror. The concave mirror M
1
and the convex mirror M
2
have the same mechanical axis. The reflective surfaces of the two mirrors are opposite each other. The concave mirror is normally larger than the convex mirror. An incident beam is reflected at least twice on the concave and at least once on the convex mirror.
In the preferred embodiment, by way of example and not necessarily by way of limitation, the concave mirror and the convex mirror are both spherical. The concave mirror has a center C
1
and a radius of curvature R
1
, and the convex mirror has a center C
2
and a radius of curvature R
2
. Furthermore, C
1
and C
2
are substantially close to each other. The convex mirror is substantially close to the Fourier surface of the concave mirror.
In order to describe the path of the reference beam, one may divide the reference beam into several portions. A portion R(
0
) of the reference beam is incident on an area O of an angularly tunable mirror. A portion R(
1
) of the reference beam extends from the area O to the concave mirror M
1
. A portion R(
2
) of the reference beam is reflected by the concave mirror M
1
, and incident on the convex mirror M
2
. A portion R(
3
) of the reference beam is reflected by the convex mirror M
2
and incident on the concave mirror M
1
. A portion R(
4
) is reflected by the concave mirror M
1
, and incident on the storage medium.
A collimated reference beam incident on the concave mirror M
1
is focused on the surface of the convex mirror M
2
, reflected by the convex mirror M
2
and then reflected by the concave mirror for the second time and re-collimated by the concave mirror, and finally incident on the holographic storage medium. The holographic storage medium is positioned so that the area O of the angularly tunable mirror is imaged in the center of the holographic medium. A signal beam interacts with the reference beam R(
4
) in the holographic storage medium to create a hologram. When the angularly tunable mirror is tuned angularly, the position of the image of area O inside the storage medium does not change, yet the incident angle of the reference beam R(
4
) on the storage medium changes. As a consequence, the angle between the reference beam and the signal beam changes, and angular multiplexing is achieved.
In the most preferred embodiment, the concave mirror and the convex mirror are incorporated into a monolithic piece, having the back surface of the piece as the concave mirror, and a part of the front surface as the convex mirror. The entrance surface and the e
Daiber Andrew J.
McDonald Mark E.
Chang Audrey
Lumen IRS, Inc.
Siros Technologies, Inc.
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