Method and apparatus for storing and retrieving a sequence of digital page data

In a storing mode of an apparatus for storing and retrieving a sequence of digital page data, a first/second complex reference beam and a modulated signal beam converge on a storing location to generate a sequence of first/second interference patterns to be sequentially stored on a holographic medium. A shift selectivity of the first complex reference beam is larger than that of the second complex reference beam so that the second interference pattern is used as a servo pattern to sequentially determine where the first interference patterns have been stored.

FIELD OF THE INVENTION

The present invention relates to a holographic digital data storage system; and, more particularly, to a method and apparatus for storing and retrieving a sequence of digital page data by using two complex reference beams of a wavelength whose selectivities are different from each other.

BACKGROUND OF THE INVENTION

Responding to ever increasing demands for an optical storage system that can store a large amount of data, such as data for a motion picture film, various types of holographic digital data storage systems incorporating therein a holographic medium made of a photo-refractive crystal such as lithium niobate or the like have been recently developed for realizing high density optical storage capabilities.

The holographic digital data storage system allows a modulated signal beam having information therein to coherently interfere with a reference beam to generate an interference pattern therebetween and, then, controls the interference pattern to be stored as index perturbations (holograms) in a specific recording location of the holographic medium such as a photo-refractive crystal, wherein the photo-refractive crystal is a material which may react differently on interference patterns depending on the respective amplitudes and phases thereof.

To realize high density storage capabilities, many schemes for hologram multiplexing have been suggested, such as angular multiplexing, wavelength multiplexing, shift multiplexing and phase code multiplexing. Recently, a correlation multiplexing has received considerable attention for its sharp spatial shift selectivity, wherein the correlation multiplexing employs a random pattern (RP) referencing scheme, a speckle pattern referencing scheme or a complex referencing scheme in which a quasi-random-phased speckle wave front is used as a reference beam. Large numbers of holograms may therefore be multiplexed in essentially a same volume of the holographic medium through only a micron-size spatial translation of the holographic medium relative to the reference beam.

Referring toFIG. 1, there is shown a block diagram for illustrating a conventional holographic digital data storage system multiplexed by using a correlation multiplexing. The conventional holographic digital data storage system includes a laser100, a beam splitter101, a first and a second mirror102and104, a spatial light modulator (SLM)105, a diffuser108, a holographic medium110, a shutter111, a linear stage112and a charge coupled device (CCD)120.

In a storing mode, a coherent monochromatic beam, e.g., a laser beam emitted from the laser100, impinges onto the beam splitter101. The beam splitter101splits the laser beam into a reference beam R and a signal beam S. The reference beam R is a portion of the laser beam transmitted through the beam splitter101and the signal beam S is a remaining portion of the laser beam reflected from the beam splitter101. After being reflected by the first mirror102, the reference beam enters into the diffuser108. The diffuser108transforms the reference beam into a complex reference beam RDfor a correlation multiplexing.

In the meantime, the signal beam S is reflected by the second mirror104and, then, enters into the SLM105. Since a sequence of digital page data is sequentially provided to the SLM105, the signal beam S is sequentially modulated with the digital page data to generate a modulated signal beam SM.

The modulated signal beam SMand the complex reference beam RDconverge on the holographic medium110to generate a sequence of interference patterns to be sequentially stored in the holographic medium110.

To read out the stored data, a retrieving reference beam with characteristics matching with those of the reference beam used during the storing mode must be illuminated precisely to a specific storing location of the holographic medium and diffracts off the stored index perturbations to reconstruct a reconstructed signal beam corresponding to the modulated signal beam.

Specifically, in a retrieving mode, the shutter111located along a path of the signal beam turns to be closed so that only a retrieving reference beam R may be obtained from the coherent monochromatic beam, wherein the retrieving reference beam R of the retrieving mode is substantially same as the reference beam R of the storing mode.

After being reflected by the first mirror102, the retrieving reference beam enters into the diffuser108. The diffuser108transforms the retrieving reference beam into a complex retrieving reference beam RD. Therefore, the complex retrieving reference beam is substantially same as the complex reference beam in the storing mode.

The complex retrieving reference beams RDis illuminated on the holographic medium110in which the interference patterns have been sequentially stored, to sequentially reconstruct a reconstructed signal beam. The reconstructed signal beam is substantially a diffracted beam which is generated from the interference patterns through the irradiation of the complex retrieving reference beams RDinto the holographic medium110. The reconstructed signal beam is captured with a predetermined interval to sequentially recover the digital page data.

Usually, a high-precise linear stage on which the holographic medium is installed has been precisely controlled with the predetermined interval by a DC servo motor, to determine the specific storing location. In other words, after the storing location is detected by using the DC servo motor, the CCD camera may have captured the reconstructed signal beam to read out the digital page data. Since, therefore, the DC servo motor is controlled to sequentially move the high-precise linear stage by a predetermined interval/distance, the storing locations of the digital page data are not precisely detected.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to provide a method and apparatus for storing and retrieving a sequence of digital page data by using two complex reference beams of a wavelength so that storing locations may be optically detected by using one of two complex reference beams.

In accordance with a preferred embodiment of the invention, there is provided a method for storing a sequence of digital page data into a holographic medium which moves continuously and linearly, the method comprising the steps of:

splitting a coherent monochromatic beam into a reference beam and a signal beam;

sequentially modulating the signal beam with the digital page data to generate a modulated signal beam;

transforming the reference beam into a first and a second complex reference beam; and

converging the modulated signal beam and the first and second complex reference beams on the holographic medium to generate a sequence of first and second interference patterns to be sequentially stored in the holographic medium,

wherein a shift selectivity of the first complex reference beam is larger than that of the second complex reference beam so that the second interference patterns are used as servo patterns to sequentially determine where the first interference patterns have been stored, and

wherein the shift selectivity of the first and second complex reference beams is a minimum movement of the holographic medium relative to the first and second complex reference beams which causes little correlation between every two neighboring first and second interference patterns.

In accordance with another preferred embodiment of the invention, there is provided method for retrieving the digital page data from the first interference patterns of claim1, the method comprising the steps of:

obtaining a retrieving reference beam from the coherent monochromatic beam, wherein the retrieving reference beam is substantially same as the reference beam;

modifying the retrieving reference beam into a first and a second complex retrieving reference beam, wherein the first and the second complex retrieving reference beams are substantially same as the first and the second complex reference beams, respectively;

illuminating the first and the second complex retrieving reference beams on the holographic medium in which the first and the second interference patterns have been stored, to sequentially reconstruct a reconstructed signal beam from the first and the second interference patterns, wherein the reconstructed signal beam corresponds to the modified signal beam;

separating the reconstructed signal beam into a first and a second reconstructed signal beam, wherein the first and second reconstructed signal beams correspond to the first and second interference patterns and the shift selectivity of the first and second reconstructed signal beams is substantially same as that of the first and second complex reference beams so that the shift selectivity of the first reconstructed signal beam is larger than that of the second reconstructed signal beam; and

sequentially capturing the first reconstructed signal beam based on the second reconstructed signal beam to sequentially recover the digital page data therefrom.

In accordance with another aspect of the invention, there is provided an apparatus for storing and retrieving a sequence of digital page data, the apparatus comprising: a holographic medium, which moves continuously and linearly, for storing the digital page data therein and retrieving the digital page data therefrom;

means for splitting a coherent monochromatic beam into a reference beam and a signal beam;

means for, in a storing mode, sequentially modulating the signal beam with the digital page data to generate a modulated signal beam;

means for, in a retrieving mode, preventing the signal beam from being propagated so that only the reference beam is transmitted as a retrieving reference beam, wherein the retrieving reference beam is substantially same as the reference beam;

means for, in the storing mode, transforming the reference beam into a first and a second complex reference beams and, in the retrieving mode, transforming the retrieving reference beam into a first and a second complex retrieving reference beams, wherein the first and the second complex retrieving reference beams are substantially same as the first and the second complex reference beams, respectively;

means for, in the storing mode, converging the modulated signal beam and the first and second complex reference beams on the holographic medium to generate a sequence of first and second interference patterns to be sequentially stored on the holographic medium, wherein a shift selectivity of the first complex reference beam is larger than that of the second complex reference beam so that the second interference pattern is used as a servo pattern to sequentially determine where the first interference patterns has been stored, and wherein the shift selectivity of the first and second complex reference beams is a minimum movement of the holographic medium relative to the first and second complex reference beams which causes little correlation between every two neighboring first and second interference patterns;

means for, in the retrieving mode, illuminating the first and the second complex retrieving reference beams on the holographic medium in which the first and the second interference patterns have been stored, to sequentially reconstruct a reconstructed signal beam from the first and the second interference patterns, wherein the reconstructed signal beam corresponds to the modified signal beam;

means for separating the reconstructed signal beam into a first and a second reconstructed signal beam, wherein the first and second reconstructed signal beams correspond to the first and second interference patterns and the shift selectivity of the first and second reconstructed signal beams is substantially same as that of the first/second complex reference beam so that the shift selectivity of the first reconstructed signal beam is larger than that of the second reconstructed signal beam; and

means for sequentially capturing the first reconstructed signal beam based on the second reconstructed signal beam to sequentially recover the digital page data.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIGS. 2A and 2B, there is shown a block diagram for illustrating a holographic digital data storage system in accordance with a preferred embodiment of the present invention.FIG. 2Aillustrates a storing mode of the holographic digital data storage system whileFIG. 2Billustrates a retrieving mode thereof. The holographic digital data storage system includes a laser200, a first, a second and a third beam splitter210,230and250, a shutter211, a first, a second and a third mirror215,232and234, a spatial light modulator (SLM)220, a polarizer231, a first and a second diffuser236and238, a holographic medium240, a photodetector252, a pulse generator254and a charge coupled device (CCD)260. For illustration, it is assumed that the holographic medium240having a type of a disk is rotated at a rotational speed such that a storing location of the holographic medium240may move continuously and linearly.

In the storing mode shown inFIG. 2A, a coherent monochromatic beam, e.g., a laser beam emitted from the laser200, impinges onto the first beam splitter210. The first beam splitter210splits the laser beam into a reference beam R and a signal beam S. The reference beam R is a portion of the laser beam transmitted through the first beam splitter210and the signal beam S is a remaining portion of the laser beam reflected from the first beam splitter210.

The second beam splitter230divides the reference beam R into a first and a second reference beam. The first reference beam is a portion of the reference beam R transmitted through the second beam splitter230and the second reference beam is a remaining portion of the reference beam R reflected from the second beam splitter230.

After being reflected by the first and the second mirror215and232, the first and the second reference beam enter into the first and the second diffuser236and238, respectively. The first diffuser236transforms the first reference beam into a first complex reference beam R1for a wider correlation multiplexing while the second diffuser238transforms the second reference beam into a second complex reference beam R2for a narrower correlation multiplexing. In other words, a shift selectivity of the first diffuser236is larger than that of the second diffuser238so that the shift selectivity of the first complex reference beam R1is larger than that of the second complex reference beam R2. It is preferable that the shift selectivity of first complex reference beam R1is approximately 2 to 50 times larger than that of the second complex reference beam R2.

Referring toFIG. 3, there is shown a graph for illustrating the shift selectivity of the first complex reference beam R1which is larger than that of a second complex reference beam R2in accordance with the present invention. The shift selectivity of the first and second complex reference beams represents a minimum movement of the holographic medium240relative to the first and second complex reference beams which causes little correlation between every two neighboring first/second interference patterns as will be described, wherein the first and second interference patterns correspond to the first and second complex reference beams. Therefore, the smaller is the shift selectivity, the more holograms, i.e., holographic interference patterns, may be stored.

Referring back toFIG. 2A, the first and the second complex reference beam is introduced into the holographic medium240.

In the meantime, the signal beam S is reflected by the third mirror234and, then, enters into the SLM220. Since a sequence of digital page data is sequentially provided to the SLM220, the signal beam S is sequentially modulated with the digital page data to generate a modulated signal beam SM. The modulated signal beam SMis modulated with a modulation period, that depends on the larger selectivity, i.e., the selectivity of the first complex reference beam R1. Since the holographic medium240with a type of a disk is usually rotated at a predetermined speed v such that the selectivity of the first and second complex reference beams R1and R2may be related with the modulation period of the modulated signal beam SM. Specifically the modulation period of the modulated signal beam SMis same as or larger than an interval that is given by the larger selectivity, i.e., the selectivity of the first complex reference beam R1, divided by the speed v of the holographic medium240.

The modulated signal beam SMand the first and second complex reference beams R1and R2converge on holographic medium240to generate a sequence of first and second interference patterns to be sequentially stored in the holographic medium240. The modulated signal beam SMand the first and the second complex reference beams R1and R2converge on a location on which the holographic medium240is located. The second interference patterns generated by the second complex reference beam R2, the selectivity of which is smaller than that of the first reference beam R1, will be used as servo patterns to sequentially determine where the first interference patterns have been stored. The correlation between the first and the second complex reference beams R1and R2is relatively small to be negligible.

In the retrieving mode shown inFIG. 2B, the shutter211located along a path of the signal beam turns to be closed so that only a retrieving reference beam R may be obtained from the coherent monochromatic beam, wherein the retrieving reference beam R of the retrieving mode is substantially same as the reference beam R of the storing mode.

The second beam splitter230divides the retrieving reference beam R into a first and a second retrieving reference beam, wherein the first and second retrieving reference beams of the retrieving mode are substantially same as the first and second reference beams of the storing mode.

After being reflected by the first and the second mirrors215and232, the first and the second retrieving reference beams enter into the first and the second diffusers236and238, respectively. The first diffuser236transforms the first retrieving reference beam into a first complex retrieving reference beam R1while the second diffuser238transforms the second retrieving reference beam into a second complex retrieving reference beam R2. Therefore, the first and second complex retrieving reference beams are substantially same as the first and second complex reference beams in the storing mode. Since the shift selectivity of the first diffuser236is larger than that of the second diffuser238, the shift selectivity of the first complex retrieving reference beam R1is larger than that of the second complex retrieving reference beam R2.

In the retrieving mode, it is preferable that one of the first and the second retrieving reference beams is vertically polarized so that the first and the second complex retrieving reference beams R1and R2are vertically polarized with each other. For example, inFIG. 2B, the polarizer231is used to vertically polarize the second retrieving reference beam. It is natural that the second complex retrieving reference beam R2is vertically polarized as shown in FIG.2B.

The first and the second complex retrieving reference beams R1and R2are illuminated on the holographic medium240in which the first and the second interference patterns have been sequentially stored, to sequentially reconstruct a reconstructed signal beam. The reconstructed signal beam is substantially a diffracted beam which is generated from the first and the second interference patterns through the irradiation of the first and the second complex retrieving reference beams R1and R2into the holographic medium240.

The reconstructed signal beam enters into the third beam splitter250, which separates the reconstructed signal beam into a first and a second reconstructed signal beam SM1and SM2. The first and second reconstructed signal beams correspond to the first and second interference patterns stored by the first and second complex retrieving reference beams. Since the second complex retrieving reference beam R2is vertically polarized, the first reconstructed signal beam SM1is a portion of the reconstructed signal beam transmitted through the third beam splitter250and the second retrieving signal beam SM2is a remaining portion of the reconstructed signal beam reflected from the third beam splitter250.

Then, the first reconstructed signal beam SM1is captured based on the second reconstructed signal beam SM2such that the digital page data are sequentially recovered, wherein the shift selectivity of the first reconstructed signal beam SM1is larger than that of the second reconstructed signal beam SM2. Specifically, the photodetector252continuously detects the second reconstructed signal beam SM2and the pulse generator254determines whether or not an intensity of the second reconstructed signal beam SM2is larger than a predetermined threshold. If the intensity of the second reconstructed signal beam SM2is larger than the predetermined threshold, the pulse generator254generates a trigger signal to control the CCD260. It is preferable that the trigger signal is a pulse which is periodically activated with the modulation period of the modulated signal beam SM. Whenever the trigger signal is periodically activated, the CCD260sequentially captures the first reconstructed signal beam SM1to sequentially recover the digital page data.