DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Referring to FIG. 1 , there is shown a block diagram for illustrating a holographic digital data storage system in accordance with a preferred embodiment of the present invention, wherein the system comprises an encoding circuit 110 , a recording and reconstructing circuit 120 and a decoding circuit 130 .

[0031] The encoding circuit 110 has a concentric pattern generator 114 and an optical information arrangement unit 112 . The encoding circuit 110 encodes optical information to generate circular page data to be provided to the recording and reconstructing circuit 120 , wherein the optical information has an 1×m pixel array, 1 and m being positive integers, respectively, and each pixel has a binary value to represent a pixel brightness.

[0032] The concentric pattern generator 114 generates concentric patterns as a servo pattern. Referring to FIG. 2 , there is shown circular page data 20 for illustrating two concentric patterns. Two concentric patterns, i.e., outermost pattern 200 and an innermost pattern 204 , are belt-shaped with predetermined belt widths w ₁ and w ₂ , respectively. Each of two concentric patterns 200 and 204 includes a plurality of discontinuous pixels disposed with a belt-shaped pattern.

[0033] The optical information arrangement unit 132 arranges the optical information of the 1×m pixel array within the outermost pattern 200 to generate the circular page data 20 . The optical information may not be overlapped with two concentric patterns 200 and 204 themselves. Referring back to FIG. 2 , there are shown three spacer belts 201 , 203 and 205 on which no information is recorded and two data regions 202 and 206 on which the optical information is recorded. Three spacer belts 201 , 203 and 205 are preferably inserted along the inner circle of the outermost pattern 200 and the outer and the inner circle of the innermost pattern 204 , respectively, so that the circular page data 20 may be separated into two concentric patterns 200 and 204 and two data regions 202 and 206 . For example, while two concentric patterns 200 and 204 are made of white pixels with a pixel value ‘1’, three spacer belts 201 , 203 and 205 are made of black pixels with a pixel value ‘0’. In another example, address information on the optical information may be arranged within the innermost pattern 204 .

[0034] The recording and reconstruction circuit 120 has a laser 121 , a beam splitter 122 , a first and a second shutter 123 A and 123 B, a first and a second mirror 124 and 127 , an actuator 126 , a spatial light modulator(SLM) 125 , a storage medium 128 and a charge coupled device(CCD) 129 . In the recording and reconstruction circuit 120 , a laser beam emitted from the laser 121 has generally a shape of circle and impinges onto the beam splitter 122 which is capable of partially reflecting the laser beam to thereby obtain a reference and a signal beam, wherein the reference beam is a portion of the laser beam transmitted through the beam splitter 122 and the signal beam is a remaining portion of the laser beam reflected from the beam splitter 122 .

[0035] The reference beam enters into the storage medium 128 through a reference beam path (S 1 ) after being reflected by the first mirror 127 which is controlled by the actuator 126 . In the meantime, after being reflected by the second mirror 124 , the signal beam enters through a signal beam path (S 2 ) into the SLM 125 at which the signal beam may be modulated with the circular page data fed from the optical information arrangement unit 112 of the encoding circuit 110 .

[0036] Then, the signal beam is focused on the storage medium 128 . In the storage medium 128 , the signal beam interferes with the reference beam to thereby generate an optical interference pattern therebetween, wherein the optical interference pattern contains information on the signal beam, i.e., information on the circular page data. The optical interference pattern is converted into an optical index pattern in the storage medium 128 whose refractive index changes in response to the optical interference pattern to thereby record the signal beam into the storage medium 128 . The storage medium 128 is a holographic memory formed of a photo-refractive crystal.

[0037] To read the signal beam recorded in the storage medium 128 , only the first shutter 123 B remains open while the second shutter 123 A turns to be closed so that only the reference beam may be introduced to the storage medium 128 as a reconstruction reference beam. If the reconstruction reference beam is introduced into the storage medium 128 , a diffracted beam, i.e., a diffracted beam corresponding to the optical index pattern, may be generated in the storage medium 128 , wherein the diffracted beam is generated from the optical interference pattern of the storage medium caused by irradiation of the reconstruction reference beam into the storage medium. The diffracted beam enters the CCD 129 which is capable of detecting the power of the diffracted beam to reconstruct the circular page data. The reconstructed circular page data is transmitted to the decoding circuit 130 .

[0038] The decoding circuit 130 has an elliptic pattern extractor 132 , an elliptic pattern estimator 134 and an optical information reconstruction unit 136 . The reconstructed circular page data is transmitted to the elliptic pattern extractor 132 and the optical information reconstruction unit 136 .

[0039] The elliptic pattern extractor 132 extracts two elliptic patterns from the diffracted beam. It is suggested that two concentric patterns in the holographic page data may be deformed into two elliptic patterns in the diffracted beam, respectively.

[0040] The elliptic pattern estimator 134 estimates two elliptic patterns to generate two estimated concentric patterns. According to a projected invariant property of projected concentric circle centers, under any projective transformation, the projected concentric circle center, i.e., the projected concentric pattern center, must be on a line which is constructed with the ellipse centers of the ellipses, i.e., the elliptic patterns, that result from the projection of the concentric circles, i.e., the concentric patterns. Specifically, from each of two elliptic patterns, the ellipse centers are detected so that a line through the ellipse centers may be constructed. After intersection points between the line and the elliptic patterns are detected, the projected concentric circle center may be estimated by using a cross ratio between the intersection points and then two estimated concentric patterns may be generated. Then, a compensated vector between two elliptic patterns and two estimated concentric patterns may be provided to the optical information reconstruction unit 136 .

[0041] The optical information reconstruction unit 136 uses the compensated vector to reconstruct the optical information. The reconstructed optical information is outputted as output data. The compensated vector may be used to generate a servo signal for modifying an irradiation location of the reference beam so that another diffracted beam dislocated by the compensated vector may be obtained. In another example, the compensated vector may be used to compensate the diffracted beam itself.

[0042] Referring to FIG. 3 , there is shown a flow chart for recording the optical information in accordance with the present invention.

[0043] At step S 310 , at least two concentric patterns are generated as servo-pattern information. As shown in FIG. 2 , two concentric patterns, i.e., outermost pattern 200 and an innermost pattern 204 , are belt-shaped with predetermined belt widths w ₁ , and w ₂ , respectively.

[0044] At step S 320 , the optical information of the 1×m pixel array is arranged within the outermost pattern 200 to generate the circular page data 20 . The optical information may not be overlapped with two concentric patterns 200 and 204 themselves.

[0045] At step S 330 , the circular page data 20 is used to modulate the signal beam.

[0046] At step S 340 , the signal beam interferes with the reference beam so that an optical interference pattern between the signal beam and the reference beam may be recorded in the holographic memory.

[0047] Referring to FIG. 4 , there is shown a flow chart for reconstructing holographic page data to restore the optical information from the holographic memory in accordance with the present invention. It is suggested that the holographic page data has at least two concentric patterns with the optical information arranged within an outermost pattern of at least two concentric patterns. It is also suggested that two concentric patterns in the holographic page data are deformed into two elliptic patterns in the diffracted beam.

[0048] At step S 410 , two elliptic patterns are extracted from the diffracted beam.

[0049] At step S 420 , two elliptic patterns are estimated to generate two estimated concentric patterns. Specifically, from each of two elliptic patterns, an ellipse center is detected so that a line through two ellipse centers may be constructed. After intersection points between the line and the elliptic patterns are detected, the projected concentric circle center may be estimated by using a cross ratio between the intersection points and then two estimated concentric patterns may be generated. Then, a compensated vector between two elliptic patterns and two estimated concentric patterns may be generated.

[0050] At step S 430 , the compensated vector is used to reconstruct the optical information. The reconstructed optical information is outputted as output data. The compensated vector may be used to generate a servo signal for modifying an irradiation location of the reference beam so that another diffracted beam dislocated by the compensated vector may be obtained. In another example, the compensated vector may be used to compensate the diffracted beam itself.

[0051] While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.