Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2001-05-01
2004-02-24
Tran, Thang V. (Department: 2651)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C365S125000, C365S216000
Reexamination Certificate
active
06697316
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of holographic data storage, and more generally, to the problem of correcting errors that arise from the misregistration of detectors (e.g., pixels) and coherent beams of optical radiation.
BACKGROUND
The ever increasing demand for readily accessible information has been enabled by the increasing data storage capacity of computers and the decreasing cost of storing this data. Magnetic storage media have played a prominent role in the Information Age, and more recently, optical data storage technologies have become commonplace. However, both magnetic and conventional optical data storage technologies, in which individual bits are stored as distinct markers on the surface of the recording medium, are approaching physical limits beyond which the storage of even more information is impractical. Storing information throughout the volume of a medium, rather than just on its surface, appears to be a high capacity alternative to currently used information storage technologies. Holographic data storage is one such volumetric approach to data storage.
As shown in
FIG. 1
, with holographic data storage, an entire “page” of information in the form of bit images is recorded at one time as an optical interference pattern within a holographic storage medium
10
. Lithium niobate is one photosensitive optical material commonly used in holographic storage applications. The input data to be stored in the medium
10
is optically prepared by passing an input beam
14
of that light (e.g., from a laser, not shown) through a spatial light modulator (SLM)
18
. The SLM
18
functions as a “pixelated” input device and typically includes a liquid crystal panel similar to those used in laptop computers. Individual pixel elements
22
within the SLM
18
may be turned on or off (although shades of gray are also possible), as illustrated in
FIG. 1
by the SLM's white and dark elements, respectively. In this manner, the input beam
14
is spatially modulated to form an object beam
26
containing a page of information determined by the particular on/off configuration of the pixel elements
22
of the SLM
18
; the object beam
26
may be thought of as containing an array of individual beams (not shown in
FIG. 1
) corresponding to SLM pixel elements
22
. This information-containing object beam
26
may then be directed through one or more optical elements
30
such as a lens and into the holographic storage medium
10
. The information contained in the object beam
26
can be recorded in the storage medium
10
by intersecting the object beam with a second beam
34
of coherent light (e.g., from a laser, not shown) known as the reference beam. The reference beam
34
, which is of the same wavelength as the object beam
26
, is designed to be easy to reproduce, e.g., it may be simply a collimated beam having a planar wavefront. Intersecting the object beam
26
and the reference beam
34
in the storage medium
10
produces an optical interference pattern that results in chemical and/or physical changes in the medium itself, such as a change in absorption or refractive index, thereby producing a grating within the storage medium. This optical interference pattern, recorded as physical changes to the material within the storage medium
10
, contains the information represented by the corresponding on/off (or gray level) configuration of the SLM pixel elements
22
.
Information may be retrieved from the storage medium
10
as illustrated in FIG.
2
. The reference beam
34
is directed into the storage medium
10
which has now been physically altered to contain a grating corresponding to the interference pattern generated by the intersection of the object beam
26
and the reference beam
34
. When the storage medium
10
is illuminated in this manner, some of the reference beam
34
is diffracted by the grating such that a reconstructed object beam
38
emerges from the storage medium
10
. This reconstructed object beam
38
contains the same information (and propagates in the same direction) as the object beam
26
. The reconstructed object beam
38
may then be imaged through one or more optical elements
42
(e.g., lens) and onto a grid or array
44
of discrete detector elements
48
or pixels (e.g., an array of nominally square pixels in a charge coupled device or CCD), so that the page of information (or “data page”) originally modulated onto the object beam
26
by the SLM pixel elements
22
can be retrieved. Alternatively, information may be retrieved by illuminating the storage medium
10
with a reference beam (not shown) that is phase-conjugate to the reference beam
34
(see J. Ashley et al., “Holographic data storage”, IBM J. Res. Develop., vol. 44, no. 3, May 2000, pp. 341-368). This phase conjugate reference beam is diffracted by the grating within the storage medium
10
to construct a phase-conjugate object beam. A beamsplitter and a polarization element maybe used to direct this phase-conjugate object beam onto the detector array
44
. The relationship between a beam and its phase-conjugate is that they have exactly the same spatial wavefront, but propagate in opposite directions (much like a movie played in reverse).
A large number of interference gratings can be formed within a single storage medium
10
by altering the angle between the object beam
26
and the reference beam
34
, or alternatively, by using other wavelengths for the object and reference beams. In this manner, a large number of data pages may be stored in the storage medium, and any particular data page can be read out independently of other data pages. The theoretical limit to this volumetric approach to data storage and retrieval has been estimated to be on the order of tens of terabits per cubic centimeter. (See J. Ashley et al., supra.) Holographic data storage also offers the prospect of faster access times, since laser beams can be moved rapidly (unlike actuators used in disk drives), and the output wavelength of certain kinds of lasers can be rapidly and accurately tuned.
To retrieve data from the storage medium
10
without error, the data page represented by the complex optical wavefront of the reconstructed object beam
38
would ideally be imaged onto the pixels
48
of the detector array
44
such that each of the individual beams (not shown in
FIGS. 1 and 2
) corresponding to an individual SLM pixel element
22
fell onto a single pixel
48
in the detector array
44
. That is, the reconstructed optical beam
38
would be imaged onto the array
44
such that there would be a one-to-one correspondence between the SLM pixel elements
22
and the detector pixels
48
. In practice, such a high degree of correlation is difficult to achieve. Misfocusing of the detector array
44
(e.g., with respect to the focal plane of the optical element
42
) as well as optical aberrations in the imaging system lead to a situation in which energy that was intended for a particular pixel
48
lands on one or more adjacent pixels. This misregistration of the pixels
48
with the individual beams within the reconstructed object beam
38
results in interpixel interference or crosstalk. Interpixel interference may lead to errors in the retrieved data when signals from the pixels
48
are read out to, for example, a processor or computer (as in
FIG. 3
below). However, in current holographic systems, some stressing of the physical components of the holographic system beyond the point of ideal imaging may be acceptable if coding and signal processing techniques can be employed to reduce the bit error rate (BER) to a sufficiently low level.
As discussed above, each portion of the data-bearing optical wavefront intended for an SLM pixel element
22
should ideally land on a single detector pixel
48
. Both the SLM
18
and the output detector array
44
typically contain pixels on a nominally square grid having a grid-spacing or pitch &dgr;. If the pixel pitches of the input and output devices are different, the optical imaging system is ideally designed to cont
International Business Machines - Corporation
Johnson Daniel E.
Tran Thang V.
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