Static information storage and retrieval – Read/write circuit – Optical
Reexamination Certificate
2002-07-03
2003-12-09
Nelms, David (Department: 2818)
Static information storage and retrieval
Read/write circuit
Optical
C365S125000, C341S106000
Reexamination Certificate
active
06661725
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an apparatus for storing/restoring holographic data and a method for coding/decoding holographic data; and, more particularly, to an apparatus for storing/restoring holographic data and a method for coding/decoding holographic data, which improve a BER (bit error rate) by preventing isolated “on” or “off” phenomena from occurring in pixels of holographic data stored to or restored from a storage media.
BACKGROUND OF THE INVENTION
Recently, many researches on technologies for storing volume of holographic data have been under way by the help of a remarkable development of, e.g., a semiconductor laser, a CCD (charge coupled device), an LCD (liquid crystal display) and so on. Results of these researches are now being applied in such a field as a finger print recognition system, as well as any fields that take advantage of a large-scaled storage capability and a high-speed transmission rate.
A conventional holographic data storing/restoring apparatus stores an interference pattern, which is generated when an object beam for a target object interferes with a reference beam, into a storage media sensitive to an amplitude of the interference pattern, the storage media being made of, e.g., crystal. The apparatus stores data on an amplitude and a phase of the object beam by varying an angle of the reference beam, such that a three-dimensional shape of the target object can be represented on the storage media. In this way, the storage media can contain several hundreds or thousands of holograms, each of which is represented in the form of a page including binary digits.
In a recording mode, the conventional holographic data storing/restoring apparatus splits a laser beam from a light source into a reference beam and an object beam. And then, the object beam is modulated into binary data comprising pixels, each of which represents either light or shade of a target object, according to externally inputted data (i.e., input data to be stored). Next, an interference pattern is obtained through the interference of the modulated object beam (i.e., a signal beam) with the reference beam split from the laser beam and reflected from a mirror having a certain inclination. The interference pattern is stored in the storage media as holographic data corresponding to the input data.
In this case, the holographic data are multiplexed before being stored in the storage media. The multiplexing of the holographic data can be performed by using, e.g., an angle multiplexing, a wavelength multiplexing or a phase multiplexing.
In the meanwhile, in a reproducing mode, the conventional holographic data storing/restoring apparatus shuts out an object beam split from a laser beam. On the other hand, the apparatus irradiates a reference beam split from the laser beam to the storage media after the reference beam being reflected from a mirror at a predetermined angle. Thereafter, the irradiated reference beam is diffracted by employing the interference pattern stored in the storage media, such that a page of demodulated binary data can be obtained.
In this case, the reference beam, which is used in restoring the holographic data, has the same reflection angle as that of the reference beam used in storing the holographic data.
However, the conventional holographic data storing/restoring apparatus has a problem that, in general, an amplitude of a restored signal has a non-uniform distribution due to several factors such as a non-uniformity in an intensity of a laser beam, a distortion noise generated through a lens, and a scattering and a diffraction in the apparatus.
Meanwhile, a conventional method for coding/decoding holographic data uses a threshold value to distinguish between two binary digits, i.e., 0 and 1 when restoring holographic data stored in the storage media. The threshold value can be a fixed value, e.g., 0.5 or an average of pixels included in a page representing holographic data, or a local threshold that is locally determined.
In case the fixed value is used in decoding holographic data, a pixel included in the page is considered to be 1 when a level of the pixel is larger than a threshold value, i.e., the average or 0.5. On the other hand, the pixel is considered to be 0 when the level is smaller than the threshold value. The conventional coding/decoding method, which uses a fixed value as the threshold, is advantageous in that a high code rate can be obtained. However, it has a problem that a restoring error rate becomes higher for decoding data on an edge part of the page.
Meanwhile, the local threshold is determined as follows. First, a page representing holographic data is divided into several areas. And then, a different threshold value is set for each of the areas. That is, a higher threshold value is set for an area near the center of the page while a smaller threshold value is set for an area near the edge of the page. A pixel included in the page is considered to be 0 or 1, respectively, depending on a level of the pixel being smaller or larger than the threshold value.
In case the local threshold is used in decoding holographic data, both a high code rate and a low restoring error rate can be obtained. However, the restoring error rate may increase when an identical set of local thresholds is applied to coding/decoding systems, each of which has a different noise pattern depending on characteristics of the system and other environmental factors.
Alternatively, as a holographic data coding/decoding method for reducing the restoring error rate, a binary differential coding/decoding method can be used. The binary differential coding/decoding method encodes a sequence of binary digits by taking advantage of a characteristic that a level of a pixel representing a binary digit “1” is larger than that of a pixel representing a binary digit “0” in a local area. For example, binary digits “0” and “1” are encoded into “01” and “10”, respectively. Further, decoding of the encoded binary digits is performed in reverse order of the coding process.
The conventional binary differential coding/decoding method is advantageous in that a low restoring error rate can be obtained, while it is disadvantageous in that a code rate is considerably low (50%).
Further, when binary digits are encoded by using the conventional binary differential coding/decoding method, as shown in
FIGS. 5A and 5B
, isolated “on” or “off” phenomena may occur in a part of a page. These phenomena may cause noises on the page stored in a storage media, which is due to a fact that the object beam modulated in accordance with the encoded binary digits is diffracted while passing through the spatial light modulator.
In this case, a portion of a laser beam corresponding to the isolated “on” spreads over its neighboring pixels, such that a laser beam intensity corresponding to the neighboring pixels increases while that corresponding to the isolated “on” pixel decreases. Further, a portion of the object beam corresponding to neighboring pixels of the isolated “off” spreads over the page, such that a laser beam intensity corresponding to the neighboring pixels decreases while that corresponding to the isolated “off” pixel increases.
In this situation, the isolated “on” pixel introduces relatively low laser beam intensity compared to a non-isolated “on” pixel. Also, the isolated “off” pixel introduces relatively high laser beam intensity compared to a non-isolated “off” pixel. Thus, a laser beam intensity for the isolated “off” (or “on”) pixel may be considered to be higher (or lower) than those for neighboring “on” (or “off”) pixels, which causes a restoring error rate to be higher.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an apparatus for storing/restoring holographic data and a method for coding/decoding holographic data that improve a restoring error rate by removing isolated “on” or “off” pixels in a page representing the holographic data.
In accordance with one aspect of the present invention, there is provided an apparatus for sto
Anderson Kill & Olick P.C.
Auduong Gene N.
Daewoo Electronics Corporation
Nelms David
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