Abstract:
A holographic data recording apparatus includes: a signal beam patterning unit for irradiating a signal beam onto a holographic medium, the signal beam including a data pattern to be recorded; and a cylindrical optical body including a cylindrical reflective surface, by which a reference beam incident thereto is reflected toward the holographic medium at a predetermined angle, wherein the data pattern is recorded on the holographic medium by interfering the signal beam with the reference beam on the holographic medium. The holographic data recording speed is increased and the cost of the holographic data recording apparatus is reduced.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to holographic data recording apparatus and method; and more particularly, to holographic data recording apparatus and method capable of recording a plurality of holographic data in a holographic medium by angular multiplexing using a cylindrical optical body.  
       BACKGROUND OF THE INVENTION  
       [0002]     Conventional holographic memory systems normally employ a page-by-page storage approach. An input device such as SLM (spatial light modulator) presents recording data in the form of a two dimensional array (referred to as a page), while a detector array such as CCD camera is used to retrieve the recorded data page upon readout. Other architectures have also been proposed wherein a bit-by-bit recording is employed in lieu of the page-by-page approach.  
         [0003]     All of these systems, however, suffer from a common drawback that they require the recording of huge quantities of separate holograms in order to fill the memory to its full capacity. A typical page-oriented system using a megabit-sized array would require the recording of hundreds of thousands of hologram pages to reach the capacity of 100 GB or more. Even with the hologram exposure times of millisecond-order, the total recording time required for filling a 100 GB-order memory may easily amount to at least several tens of minutes, if not hours. Thus, another conventional holographic ROM system has been developed, where the time required to produce a 100 GB-order capacity disc may be reduced to under a minute, and potentially to the order of seconds.  
         [0004]      FIG. 1  is a view schematically illustrating a conventional method for recording data on a disc-type holographic medium. As shown in  FIG. 1 , a data mask  48  is placed above a holographic medium  50  which serves as an optical data storage medium, while a conical mirror  32  is placed below the holographic medium  50 . To record data on the holographic medium  50 , a signal beam is irradiated downward onto the upper surface of the holographic medium  50  via a bit pattern of the data mask  48  and at the same time, a reference beam is irradiated onto the lower surface of the holographic medium  50  after the reflection by the conical mirror  32 . The signal beam is interfered with the reference beam at the holographic medium  50 , thereby recording the holographic data on the holographic medium  50  according to the bit pattern of the data mask  48 .  
         [0005]     When conical mirrors having different base angles are used, it is possible to record a plurality of holographic data in the same physical space of the holographic medium  50  by angular multiplexing (see “Holographic disk recording system”, US patent application publication No. U.S. 2003/0161246A1, by Ernest Chuang, et al.).  
         [0006]      FIG. 2  shows a conventional holographic ROM system including a light source  10 ; a shutter  12 ; mirrors  14 ,  28 ,  34 ,  40 ; HWPs (half wave plates)  16 ,  24 ,  36 ; spatial filters  18 ,  30 ,  42 ; lenses  20 ,  44 ; a PBS (polarization polarization beam splitter)  22 ; polarizers  26 ,  38 ; a conical mirror  32 ; a data mask  48 ; and a holographic medium  50 .  
         [0007]     The light source  10  emits a laser beam with a constant wavelength, e.g., a wavelength of 532 nm. The laser beam, which is of only one type of linear polarization, e.g., P-polarization or S-polarization, is provided to the mirror  14  via the shutter  12  which is opened to transmit the laser beam therethrough when recording data on the holographic medium  50 . The mirror  14  reflects the laser beam to the HWP  16 . The HWP  16  rotates the polarization of the laser beam by θ degree (preferably 45θ). And then, the polarization-rotated laser beam is fed to the spatial filter  18  for removing noises included in the polarization-rotated laser beam. And then, the polarization-rotated laser beam is provided to the lens  20  for expanding the beam size of the laser beam up to a predetermined size. Thereafter, the expanded laser beam is provided to the PBS  22 .  
         [0008]     The PBS  22 , which is manufactured by repeatedly depositing at least two kinds of materials each having a different refractive index, serves to transmit, e.g., a horizontally polarized laser beam, i.e., P-polarized beam, along a S 1  path and reflect, e.g., a vertically polarized laser beam, i.e., S-polarized beam, along a S 2  path. Thus the PBS  22  divides the expanded laser beam into a transmitted laser beam (hereinafter called a reference beam) and a reflected laser beam (hereinafter called a signal beam) having different polarizations, respectively.  
         [0009]     The signal beam, e.g., of a S-polarization, is reflected by the mirror  34 . And then the reflected signal beam is provided to the mirror  40  via the HWP  36  and the polarizer  38  sequentially. Since the HWP  36  can rotate the polarization of the signal beam by θ′ degree and the polarizer  38  serves to pass only a P-polarized signal beam, the HWP  36  and the polarizer  38  can regulate the amount of the P-polarized signal beam arriving at the mirror  40  by changing θ′. And then the P-polarized signal beam is reflected by the mirror  40  toward the spatial filter  42  for removing spatial noise of the signal beam and allowing a Gaussian beam thereof to be transmitted therethrough. And then the signal beam which is a perfect Gaussian is provided to the lens  44  for expanding the beam size of the signal beam up to a preset size. Thereafter, the expanded signal beam is projected onto the holographic medium  50  via the data mask  48 . The data mask  48 , presenting data patterns for recording, functions as an input device, e.g., a spatial light modulator (SLM).  
         [0010]     Meanwhile, the reference beam is fed to the mirror  28  via the HWP  24  and the polarizer  26  sequentially. Since the HWP  24  can rotate the polarization of the reference beam by θ″ degree and the polarizer  26  serves to pass only a P-polarized reference beam, the HWP  24  and the polarizer  26  can regulate the amount of the P-polarized reference beam arriving at the mirror  28  by changing θ″. Therefore, the polarization of the reference beam becomes identical to that of the signal beam. And then the mirror  28  reflects the P-polarized reference beam toward the spatial filter  30  which removes spatial noise of the signal beam and allows a Gaussian beam thereof to be transmitted therethrough. And then the reference beam which is a perfect Gaussian beam is projected onto the conical mirror  32  (the conical mirror  32  being of a circular cone having a circular base with a preset base angle between the circular base and the cone), which is fixed by a holder (not shown). The conical mirror  32  reflects the reference beam toward the holographic medium  50 . The incident angle of the reflected reference beam on the holographic medium  50  is determined by the base angle of the conical mirror  32 .  
         [0011]     When conical mirrors having different base angles are used in the above-mentioned holographic data recording apparatus, it is possible to record holographic data in the same physical space of the holographic medium  50  by angular multiplexing. In other words, another conical mirror having a base angle different from the base angle of the conical mirror  32  is used in the holographic data recording apparatus, in place of the conical mirror  32 , the incident angle of the reference beam irradiated onto the holographic medium  50  is changed so that the reference beam and the signal beam provide a new interference pattern. Thus, new holographic data can be recorded on the holographic medium  50  by angular multiplexing.  
         [0012]     However, the conventional holographic data recording apparatus is problematic in that, in order to reflect a reference beam toward a disc-type holographic medium at a desired incident angle, it is necessary to use a conical mirror having a specified base angle capable of providing the desired incident angle. Thus, to record a plurality of holographic data on a holographic medium by angular multiplexing, the required number of conical mirrors must be the same as the desired number of the incident angles of the reference beam, so that the cost of the holographic data recording apparatus is increased.  
         [0013]     Furthermore, the replacement of every conical mirror is a difficult and complex process so that the recording speed is decreased.  
       SUMMARY OF THE INVENTION  
       [0014]     It is, therefore, an object of the present invention to provide holographic data recording apparatus and method capable of recording a plurality of holographic data in the same physical space of a holographic medium by angular multiplexing, the incident angle of a reference beam being changed by using only one cylindrical optical body, thus increasing the holographic data recording speed and reducing the cost thereof.  
         [0015]     In a first aspect of the present invention, there is provided a holographic data recording apparatus including: a signal beam patterning unit for irradiating a signal beam onto a holographic medium, the signal beam including a data pattern to be recorded; and a cylindrical optical body including a cylindrical reflective surface, by which a reference beam incident thereto is reflected toward the holographic medium at a predetermined angle, wherein the data pattern is recorded on the holographic medium by interfering the signal beam with the reference beam on the holographic medium.  
         [0016]     In a second aspect of the present invention, there is provided a holographic data recording apparatus including: a light source for generating a laser beam; a first polarization beam splitting unit for splitting the laser beam into a signal beam and a reference beam; a signal beam patterning unit for irradiating a signal beam onto a holographic medium, the signal beam including a data pattern to be recorded; a cylindrical optical body including a cylindrical reflective surface, by which a reference beam incident thereto is reflected toward the holographic medium at a predetermined angle; and an incident angle control unit for controlling the incident angle of the reference beam irradiated onto the cylindrical optical body, wherein the data pattern is recorded on the holographic medium by interfering the signal beam with the reference beam on the holographic medium.  
         [0017]     In a third aspect of the present invention, there is provided a holographic data recording apparatus including: a signal beam patterning unit for irradiating a signal beam onto a holographic medium, the signal beam including a data pattern to be recorded; and a taper-shaped beam generating unit for converting a reference beam incident thereto into a taper-shaped beam having a circular cross-section at each end thereof and then irradiating the converted reference beam onto the holographic medium, wherein the data pattern is recorded on the holographic medium by interfering the signal beam with the reference beam on the holographic medium.  
         [0018]     In a fourth aspect of the present invention, there is provided a holographic data recording method for recording a data pattern on a holographic medium by interfering a signal pattern including the data pattern thereon with a reference beam on the holographic medium, the method including the steps of: irradiating the reference beam onto a cylindrical optical body, which reflects the reference beam toward the holographic medium, thereby interfering the reference beam with the signal beam on the holographic medium to record the data pattern on the holographic medium; and adjusting the incident angle of the reference beam irradiated onto the cylindrical optical body, thereby changing the incident angle of the reference beam irradiated onto the holographic medium after reflected by the cylindrical optical body in order to record a new data pattern on the holographic medium by angular multiplexing.  
         [0019]     In a fifth aspect of the present invention, there is provided a holographic data recording method including the steps of: splitting a reference beam emitted from a light source into a first reference beam and a second reference beam; irradiating the first and the second reference beams onto an optical body at first symmetrical angles which are symmetrical around a central axis of the optical body; reflecting the first and the second reference beams by the optical body, thereby irradiating the first and the second reference beams onto a holographic medium at second symmetrical angles which are symmetrical around the central axis of the optical body; and interfering the first and the second reference beams with a signal beam including a data pattern thereon on the holographic medium, thereby recording the data pattern on the holographic medium.  
         [0020]     In a sixth aspect of the present invention, there is provided a holographic data recording method including the steps of: splitting a reference beam emitted from a light source into N sub-reference beams; irradiating the sub-reference beams onto an optical body such that the sub-reference beams are directed to a central axis of the optical body; reflecting the sub-reference beams by the optical body, thereby irradiating the sub-reference beams onto a holographic medium at predetermined incident angles which are symmetrical around the central axis of the optical body; and interfering the sub-reference beams with a signal beam including a data pattern thereon on the holographic medium, thereby recording the data pattern on the holographic medium, wherein N is a natural number. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which:  
         [0022]      FIG. 1  is a view schematically illustrating a conventional holographic data recording method;  
         [0023]      FIG. 2  shows a view illustrating the configuration of a conventional holographic data recording apparatus;  
         [0024]      FIG. 3  represents a view schematically illustrating a holographic data recording method in accordance with the present invention;  
         [0025]      FIG. 4  offers a view illustrating optical paths of an incident beam and a reflected beam of a cylindrical mirror in accordance with the present invention; and  
         [0026]      FIG. 5  presents a view illustrating the configuration of a holographic data recording apparatus in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.  
         [0028]      FIG. 3  is a view schematically illustrating a holographic data recording method in accordance with the present invention.  
         [0029]     A data mask  48  is placed above a holographic medium  50  so that a signal beam is irradiated onto the upper surface of the medium  50  via the data mask  48  having a data pattern thereon. Furthermore, a cylindrical mirror  102  having a cylindrical reflective surface is placed below the holographic medium  50 . To record data on the holographic medium  50 , the signal beam is projected onto the upper surface of the holographic medium  50 , after passing through a bit pattern  49  on the data mask  48 . At the same time, a first reference beam and a second reference beam are symmetrically irradiated at the same incident angle onto the cylindrical reflective surface of the cylindrical mirror  102  which is placed below the holographic medium  50 . Thereafter, the first and the second reference beams are symmetrically reflected by the cylindrical reflective surface of the cylindrical mirror  102  thereby being irradiated onto the lower surface of the holographic medium  50  in outward radial directions thereof.  
         [0030]     Each of the first and the second reference beams, which are reflected by the cylindrical reflective surface of the cylindrical mirror  102  toward the holographic medium  50 , has a half taper-shaped cross-section. The first and the second reference beams individually have a semicircular optical cross-section at each end thereof, with the center of the semicircular cross-section located at a central axis of the holographic medium  50 . Therefore, the first and the second reference beams are integrated into a complete taper-shaped reference beam having a circular optical cross-section at each end thereof. The complete taper-shaped reference beam is interfered with the signal beam at the holographic medium  50 , so that holographic data can be recorded on the holographic medium  50  in accordance with the bit pattern of the data mask  48 .  
         [0031]     The reflected angles of the first and the second reference beams which are reflected by the cylindrical mirror  102  may be controlled by changing the incident angles of the first and the second reference beams projected onto the cylindrical mirror  102 . Thus, the incident angles of the first and the second reference beams onto the holographic medium  50  may be controlled by changing the incident angles of the first and the second reference beams onto the cylindrical mirror  102 , resulting in the angular multiplexing.  
         [0032]      FIG. 4  is a view illustrating the optical reflection properties of the cylindrical mirror  102 . The point A in  FIG. 4  denotes the focus of the cylindrical mirror  102 , which is disposed on the central axis of the holographic medium  50 . Due to the optical reflection properties of the cylindrical mirror  102 , parallel incident lights projected onto the cylindrical mirror  102  are reflected by the cylindrical mirror  102  as if the reflected lights are emitted from a virtual point light source which is located at the focus A of the cylindrical mirror  102 .  
         [0033]     Thus, since the focus A is located on the central axis of the holographic medium  50  as described above, the first and the second reference beams which are incident on the cylindrical mirror  102  are always reflected by the cylindrical reflective surface so that, if viewed from the top of the holographic medium  50 , they are emitted from a virtual point light source which is located at the central axis of the holographic medium  50 . To reflect the first and the second reference beams which are incident on the cylindrical mirror  102  toward all of the 360° angular area of the disc-type holographic medium  50 , two semicylindrical mirrors each having a 180° angular reflective surface are assembled into the cylindrical mirror  102  as shown in  FIGS. 3 and 4 . When the first and the second reference beams are symmetrically projected onto the reflective surface of the cylindrical mirror  102 , the first and the second reference beams are reflected toward all of the 360° angular area in the same manner as that described for the conical mirror of  FIG. 1 .  
         [0034]      FIG. 5  is a view illustrating the configuration of a holographic data recording apparatus in accordance with the present invention, wherein like parts appearing in  FIG. 2  are represented by like reference numerals. As shown in  FIG. 5 , the holographic data recording apparatus in accordance with the present invention comprises a light source  10 ; mirrors  14 ,  34 ,  40 ,  106 ,  112 ,  114 ; polarization beam splitters (PBSs)  22 ,  104 ; a cylindrical mirror  102 ; rectangular slots  110 ,  118  for forming rectangular beams; a first incident angle control unit  108 ; a second incident angle control unit  116 ; a data mask  48 ; and a holographic medium  50 . Moreover, the holographic data recording apparatus further comprises a shutter  12 ; Half Wave Plates (HWPs)  16 ,  24 ,  35 ; spatial filters  18 ,  30 ,  42 ; magnifying lenses  20 ,  44 ; and polarizers  26 ,  38 .  
         [0035]     The laser beam emitted from the light source  10  is linear-polarized, e.g., P- or S-polarized. The laser beam emitted from the light source  10  is splitted by the PBS  22  and then propagates along two optical paths S 1  and S 2 . Thereafter, the splitted laser beam propagating along the optical path S 2  is splitted by PBS  104  and then propagates along two optical paths S 21  and S 22 .  
         [0036]     A signal beam splitted by the PBS  22  propagates along the optical path S 1  and then irradiated onto the holographic medium  50  in the same manner as that described for the conventional holographic data recording apparatus of  FIG. 1 .  
         [0037]     A reference beam, splitted by the PBS  22 , propagates along the optical path S 2 , i.e., passes through the HWP  24 , the polarizer  26 , the spatial filter  30 , the PBS  104  in that order. The reference beam is splitted by the PBS  104  into a first reference beam which is propagating along the optical path S 21  and a second reference beam which is propagating along the optical path S 22 .  
         [0038]     On the optical path S 21 , the first reference beam in the form of a circular beam is converted into a first reference beam in the form of a rectangular beam by the rectangular slot  110  and then the first reference beam in the form of the rectangular beam is provided to the mirror  106 . Thereafter, the mirror  106  reflects the first reference beam toward the cylindrical mirror  102 . At the cylindrical mirror  102 , the first reference beam is reflected toward the holographic medium  50 .  
         [0039]     On the optical path S 22 , the second reference beam in the form of a circular beam is reflected by the mirror  112  toward the rectangular slot  118 , and then converted into a rectangular beam by the rectangular slot  118  and then the second reference beam in the form of the rectangular beam is provided to the mirror  114 . Thereafter, the mirror  114  reflects the second reference beam toward the cylindrical mirror  102 . At the cylindrical mirror  102 , the second reference beam is reflected toward the holographic medium  50 .  
         [0040]     Since the signal beam, the first reference beam and the second reference beam which are irradiated onto the holographic medium  50  are controlled to have the same polarization pattern, the signal beam is interfered with the first and the second reference beams on the holographic medium  50 . For example, when the signal beam is S-polarized, the first and the second reference beams must also be S-polarized. Furthermore, the first and the second reference beams are irradiated onto the cylindrical mirror  102  at the same incident angle in symmetrical directions, and then reflected by the cylindrical mirror  102  at the same reflection angle in symmetrical directions toward the holographic medium  50 .  
         [0041]     Considering the plan view of the cylindrical mirror  102  observed at the mirrors  106  and  114  which are respectively placed on the optical paths S 1  and S 2 , the cylindrical mirror  102  looks like a rectangular shape. Thus, when the circular first and second reference beams are converted into the rectangular first and second reference beams by the rectangular slots  110  and  118 , the size, i.e., breadth, of each of the rectangular first and second reference beams must be adjusted to be equal to the size, i.e., diameter, of the cylindrical mirror  102 . In case the first and the second reference beams are not adjusted to the size of the cylindrical mirror  102 , the undesired interference pattern of the signal and the reference beams may be generated on the holographic medium  50 .  
         [0042]     The holographic data recording apparatus in accordance with the present invention can record new holographic data in the same physical space of the holographic medium  50  by angular multiplexing while controlling arrangement angles of the mirrors  106  and  114  as described in  FIG. 3 . In other words, when the arrangement angles of the mirrors  106  and  114  are controlled by the first and the second incident angle control units  108  and  116 , the incident angles of the first and the second reference beams irradiated onto the cylindrical mirror  102  are changed. Thus, the reflection angles of the first and the second reference beams reflected by the cylindrical mirror  102  are changed so that the incident angles of the first and the second reference beams irradiated onto the holographic medium  50  are also changed. Therefore, the first and the second reference beams whose incident angles are changed are interfered with the signal beam on the holographic medium  50 , thereby forming a new interference pattern on the holographic medium  50  by angular multiplexing. In other words, the incident angles of the first and the second reference beams are adjusted every time a new signal beam is irradiated onto the holographic medium  50 . Herein, the arrangement angles of the two mirrors  106  and  114  must be adjusted by the first and the second incident angle control units  108  and  116  to be symmetrical with respect to the central axis of the cylindrical mirror  102 .  
         [0043]     Alternatively, it may be understood that the number of optical paths may be viewed if the optical paths allow a plurality of reference beams to be irradiated onto the entire circumferential surface of the cylindrical mirror. The reference beam can be splitted into N sub-reference beams. Furthermore, it may be understood that the number of mirrors forming the cylindrical mirror may be varied if the integrated reflective surfaces of the cylindrical mirrors form the 360°.  
         [0044]     As described above, the present invention provides the holographic data recording apparatus and method capable of recording a plurality of holographic data in the same physical space of a holographic medium by angular multiplexing using a cylindrical mirror. Unlike the conventional holographic data recording apparatus and method of using a plurality of conical mirrors, the holographic data recording apparatus and method of the present invention do not require the replacement of the conical mirrors while recording a plurality of data on a holographic medium, thereby increasing the recording speed and reducing the cost thereof.  
         [0045]     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.