Abstract:
In a holographic digital data storage system, a light source generates a reference beam, a holographic optical element saves a plurality of interference patterns between the reference beam and a plurality of beams of specific sizes and a beam splitter splits each reproduced beam into a holographic signal beam and a holographic reference beam. A medium records an interference pattern between the holographic reference beam and the holographic signal beam and reflecting the holographic reference beam to generate a reflective information beam and, if only the holographic reference beam is illuminated, a displaying means displays a holographic reproduced beam for the holographic signal beam and detecting the reflective information beam.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a continuation application of U.S. Ser. No. 09/815,046 filed on Mar. 23, 2001, now U.S. Pat. No. 6,999,397, which claims priorities thereon pursuant to 35 USC 120. 

   FIELD OF THE INVENTION 
   The present invention relates to a holographic digital data storage system; and, more particularly, to a holographic digital data storage system compatible with a CD/DVD player. 
   BACKGROUND OF THE INVENTION 
   Recently, there have been reported increasing levels of active researches on holographic digital data storage systems triggered by the development of semiconductor lasers, charge coupled devices (CCDs), liquid crystal displays (LCDs) and the like. Since the holographic digital data storage system normally features a large storage capacity and high data transfer rate, it has already been applied to, e.g., fingerprint recognition systems for storing and reproducing fingerprints, and the scope of its applications keeps expanding. 
   The holographic digital data storage system allows a signal beam transmitted from an object to interfere with a reference beam, and writes interference patterns generated from such interference phenomena on a storage medium such as a crystal or a photopolymer which reacts differently depending on the amplitude and phase of an interference pattern. In the holographic digital data storage system, the phase of the signal beam as well as the amplitude thereof may be recorded by changing an incident angle of the reference beam, so that a three dimensional display of an object can be realized. Further, hundreds to thousands of holographic digital data comprised of binary data on a page-by-page basis can be stored in a single space of the storage medium. 
     FIG. 11  depicts an overall block diagram of a holographic digital data storage system, wherein the holographic digital data storage system comprises a light source  20 , a beam expander  21 , a beam splitter  22 , two reflection mirrors  23  and  24 , a spatial light modulator (SLM)  25 , a medium  26  and a CCD  27 . 
   The light source  20  generates an optical signal, e.g., a laser beam, whose wavelength falls within a specific wavelength band required for the holographic digital data. The beam expander  21  expands the size of the laser beam. 
   The beam splitter  22  separates the expanded laser beam into a reference beam and a signal beam and transfers the reference beam and the signal beam through two different transmission channels, wherein the reference beam and the signal beam correspond to a transmitted beam and a reflected beam, respectively. 
   The reference beam is reflected at the reflection mirror  24  so that the reflected reference beam is transferred to the medium  26 . The signal beam, on the other hand, is reflected at the reflection mirror  23  so that the reflected signal beam is transferred to the SLM  25 . The SLM  25  modulates the reflected signal beam into binary pixel data on a page basis. The modulated signal beam is transferred to the medium  26 . In case the reflected signal beam is, for example, image data provided on a frame basis, the reflected signal beam is preferably modulated on a frame basis and the reflection mirror  24  functions to change the reflection angle of the reflected reference beam by a small amount. 
   The medium  26  stores the interference pattern acquired from an interference phenomenon between the reflected reference beam and the modulated signal beam, wherein the interference pattern depends on the reflected signal beam, i.e., the data inputted to the SLM  25 . In other words, the modulated signal beam irradiated to the medium  26  is modulated on a page basis and the reflected reference beam is reflected in an angle corresponding to the modulated signal beam. The modulated signal beam interferes with the reflected reference beam within the medium  26 . The amplitude and phase of the interference pattern results in a photo-induction within the medium  26  so that the interference pattern may be written on the medium  26 . 
   When only the reference beam is irradiated onto the medium  26  in order to reconstruct the data written thereon, the reference beam is diffracted by the interference pattern within the medium  26  so that a check pattern with original brightness on a pixel basis may be restored. When the check pattern is irradiated on the CCD  27  in turn, the original data may be restored. The reference beam used for reproducing the data written on the medium  26  should be irradiated at the same incident angle as that of the reference beam when recording the data on the medium  26 . 
     FIG. 12  presents a block diagram of a conventional CD or DVD player, wherein the CD/DVD player comprises a high frequency overlap module  10 , two mirrors  11  and  18 , a polarizing prism  12 , a cylindrical lens  13 , a photodiode (PD)  14 , a λ/ 4  plate  15 , a disc medium  16 , an object lens  17  and a collimating lens  19 . A detailed description for the structure and the operational principle of such CD/DVD player will be omitted here since it is well known to a person having ordinary skill in the relevant art. 
   As for the conventional CD/DVD player of  FIG. 12  and the conventional holographic digital data storage system of  FIG. 11 , however, there has been found a drawback in that they cannot be compatible with each other since the positions of their detectors, e.g. optical diodes, are different from each other. To be specific, since the CD/DVD player has its detector along a direction of reflection while the holographic digital data storage system has its detector along a transmission direction, a single detector cannot be used for both systems. Further, the size difference of beams used in the two systems is so great that two different optical instruments are required. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a holographic digital data storage system compatible with a CD/DVD player by using a holographic optical element with a plurality of beam sizes and numerical apertures produced by employing a spatial multiplexing technique or an angular multiplexing technique. 
   In accordance with a preferred embodiment of the present invention, there is provided a holographic digital data storage system comprising: 
   a light source for generating a reference beam; 
   means for saving a plurality of interference patterns between the reference beam and a plurality of beams of specific sizes and, if only the reference beam is illuminated, generating a plurality of reproduced beams corresponding to the plurality of beams of specific sizes; 
   means for splitting each reproduced beam into a reflected beam and a transmitted beam and assigning one of the reflected beam and the transmitted beam as a holographic reference beam; 
   means for modulating the other of the reflected beam and the transmitted beam into a holographic signal beam corresponding to a holographic input signal; 
   means for recording an interference pattern between the holographic reference beam and the holographic signal beam and reflecting the holographic reference beam to generate a reflective information beam, wherein the reflective information beam proceeds along an opposite direction to the holographic reference beam; and 
   means for, if only the holographic reference beam is illuminated, displaying a holographic reproduced beam for the holographic signal beam and detecting the reflective information beam. 
   In accordance with another preferred embodiment of the present invention, there is provided a holographic digital data storage system comprising: 
   a light source for generating a reference beam; 
   means for adjusting a polarization of the reference beam to generate a multiplicity of polarized beams with a multiplicity of polarization components, respectively; 
   means for storing a number of interference patterns between the multiplicity of polarized beams and a number of reflective beams of specific sizes and between the multiplicity of polarized beams and holographic beams of specific sizes, wherein the holographic beams of specific sizes have the multiplicity of polarization components, and, if the multiplicity of polarized beams are illuminated, generating reflective reproduced beams corresponding to the reflective beams of specific sizes and holographic reproduced beams corresponding to the holographic beams of specific sizes, wherein the holographic reproduced beams have a multiplicity of holographic polarization components transferred through separate paths; 
   means for collimating the polarization directions of the holographic reproduced beams to generate a first and a second holographic beam, wherein one of the first and the second beam is used as a holographic reference beam; 
   means for modulating the other of the first and the second beam into a holographic signal beam corresponding to holographic input signals; 
   means for recording an interference pattern between the holographic reference beam and the holographic signal beam and reflecting the reflective reproduced beams to generate reflective information beams, wherein the reflective information beams proceed along an opposite direction of the reflective reproduced beams; 
   means for, if only the holographic reference beam is illuminated, displaying a holographic reproduced beam for the holographic signal beam and detecting the reflective information beams; and 
   means for introducing the reflective reproduced beam into said recording means and transferring the reflective information beams into said displaying. 
   In accordance with still another preferred embodiment of the present invention, there is provided a holographic digital data storage system comprising: 
   a light source for generating a reference beam; 
   means for splitting the reference beam into a first and a second beam to proceed through separate paths; 
   means for modulating the first beam into a holographic signal beam corresponding to holographic input signals; 
   means for storing a number of interference patterns between the second beam and reflective beams of specific sizes and between the second beam and a holographic beam of specific size and, if the second beam is illuminated, generating reflective reproduced beams corresponding to the reflective beams of specific sizes and a holographic reproduced beam corresponding to the holographic beam of specific size, wherein the holographic reproduced beam functions as a holographic reference beam; 
   means for recording an interference pattern between the holographic reference beam and the holographic signal beam and reflecting the reflective reproduced beams to generate reflective information beams, wherein the reflective information beams proceed along an opposite direction of the reflective reproduced beams; 
   means for, if only the holographic reference beam is illuminated, displaying a holographic reproduced beam for the holographic signal beam and detecting the reflective information beam; and 
   means for introducing the reflective reproduced beams into said recording means and turning the reflective information beams into said displaying means. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIG. 1  presents a block diagram of a holographic digital data storage system compatible with a CD/DVD player in accordance with a first embodiment of the present invention; 
       FIG. 2  describes an embodiment of the holographic beam splitter shown in  FIG. 1 ; 
       FIG. 3  illustrates a block diagram of a holographic digital data storage system compatible with a CD/DVD player in accordance with a second embodiment of the present invention; 
       FIG. 4  demonstrates an embodiment of the holographic polarized beam splitter shown in  FIG. 3 ; 
       FIG. 5  represents a block diagram of a holographic digital data storage system compatible with a CD/DVD player in accordance with a third embodiment of the present invention; 
       FIG. 6  explains an embodiment of the holographic polarized beam splitter shown in  FIG. 5 ; 
       FIG. 7  shows a block diagram of a holographic digital data storage system compatible with a CD/DVD player in accordance with a fourth embodiment of the present invention; 
       FIG. 8  sets forth an embodiment of the holographic polarized beam splitter shown in  FIG. 7 ; 
       FIG. 9  provides a block diagram of a holographic digital data storage system compatible with the CD/DVD player in accordance with a fifth embodiment of the present invention; 
       FIG. 10  exhibits an embodiment of the holographic optical element shown in  FIG. 9 ; 
       FIG. 11  displays a block diagram of a conventional holographic digital data storage system; and 
       FIG. 12  depicts a block diagram of a conventional CD/DVD player. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a block diagram of a holographic digital data storage system  100  in accordance with a first embodiment of the present invention which is compatible with a CD/DVD player, wherein the holographic storage system  100  comprises a light source  102 , a holographic beam splitter  104 , a beam splitter  106 , three lenses  108 ,  124 ,  126 , a medium  110 , two mirrors  112 ,  120 , a charge coupled device (CCD)  114 , a photodiode (PD)  116 , a shutter  118  and a spatial light modulator (SLM)  122 . 
   The light source  102  is an essential element for the writing and reconstruction process of the holographic digital data storage system. A laser, for example, can be used as the light source. The light source  102  provides an optimum wavelength band for the medium  110  of the holographic digital data storage system. An available wavelength band depends on a photo-sensitizer and an initiator added to the medium  110 . 
   The holographic beam splitter  104  is made of a same material as used in a holographic memory. The beam from the light source  102  is used as a reference beam. If a beam of a specific size is introduced to the holographic beam splitter  104  with a predetermined angle with respect to the reference beam, an interference pattern between the reference beam and the beam of the specific size is recorded within the holographic beam splitter  104 . 
   Referring to  FIG. 2 , there is illustrated an embodiment of the holographic beam splitter  104  which may be made by employing an angular multiplexing technique. It is assumed that three reference beams B REF   CD , B REF   DVD , B REF   HDDS  are introduced, wherein the three reference beams B REF   CD , B REF   DVD , B REF   HDDS  have different incident angles but have a same wavelength. If the three reference beams B REF   CD , B REF   DVD , B REF   HDDS  and their corresponding beams of specific sizes B CD , B DVD , B HDDS  are introduced with predetermined relative angles, respectively, the interference patterns between three reference beams B REF   CD , B REF   DVD , B REF   HDDS  and their corresponding beams of specific sizes B CD , B DVD , B HDDS  are recorded within the holographic memory by using the angular multiplexing method. The beam sizes and shapes of beams of specific sizes B CD , B DVD , B HDDS  depend on the medium on which they are recorded. If only the three reference beams B REF   CD , B REF   DVD , B REF   HDDS  are introduced at corresponding predetermined respective angles to the holographic memory in which the interference patterns have been recorded, three reproduced beams B CD   RE , B DVD   RE , B HDDS   RE  for three beams of specific sizes B CD , B DVD , B HDDS  are generated. The intensities of the three reproduced beams B CD   RE , B DVD   RE , B HDDS   RE  may be represented as diffraction efficiencies of the interference patterns. The diffraction efficiency in photopolymer may be substantially 100%. 
   A beam factor BF of the CD/DVD player should be constant for the holographic digital data storage system and the CD/DVD player to be compatible. In general, the beam factor B F  of the CD player is 0.57690 □m −1  and the beam factor B F  of the DVD player is 0.9230 □m −1 . The beam factor B F  can be calculated as follows: 
                   B   F     =       N   .   A   .     λ             Eq   .           ⁢   1               
wherein λ and N.A. represent a wavelength of the beam and a numerical aperture, respectively. When a different wavelength is used, the N.A. can be adjusted in such a way that the B F  remains constant and thus the CD/DVD player can be played. The N.A. is calculated as follows:
   N.A.=n ·sin α  Eq. 2 
wherein n represents a refractive index of a material filled behind the lens through which the beam passes and αrepresents a concentration angle with respect to an optical axis, i.e., a central axis, of the lens in case an incident beam vertical to the lens is concentrated on a focus. In other words, sin α is a function of the focal length F of the lens and a beam width W of the beam incident into the lens and is given as follows:
 
   
     
       
         
           
             
               
                 
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                 Eq 
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   Accordingly, the beam width W can be derived from the following equation: 
   
     
       
         
           
             
               
                 W 
                 = 
                 
                   
                     
                       2 
                       ⁢ 
                       
                         FB 
                         F 
                       
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                       λ 
                     
                     n 
                   
                   · 
                   
                     1 
                     
                       
                         1 
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                             ( 
                             
                               
                                 
                                   B 
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                 Eq 
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                 4 
               
             
           
         
       
     
   
   Consequently, the B 1 , can be sustained at a constant value by controlling the beam width W and thereby adjusting the N.A., so that the CD/DVD player can be played. 
   When a laser beam having a wavelength λ of 532 nm is transmitted through the air whose refractive index is 1 and a lens with a focal length F of 1 cm is employed, a beam factor B FCD  for the CD player and a beam factor B FDFD  for the DVD player are 0.5769 μm −1  and 0.9230 μm −1  , respectively. Accordingly, the beam widths W CD  and W DVD  required in the CD/DVD player are calculated as follows, respectively: 
                   W   CD     =         2   ⁢     (     1   ⁢           ⁢   cm     )     ⁢     (     0.5769   ⁢           ⁢     μm     -   1         )     ⁢     (     0.532   ⁢           ⁢   μm     )           1   -         (     0.5769   ⁢           ⁢     μm     -   1         )     2     ⁢       (     0.532   ⁢           ⁢   μm     )     2             =     0.64495   ⁢           ⁢   cm               Eq   .           ⁢   5                 W   DVD     =         2   ⁢     (     1   ⁢           ⁢   cm     )     ⁢     (     0.9230   ⁢           ⁢     μm     -   1         )     ⁢     (     0.532   ⁢           ⁢   μm     )             1   -       ⁢       (     0.9230   ⁢           ⁢     μm     -   1         )     2     ⁢       (     0.532   ⁢           ⁢   μm     )     2         =     1.12734   ⁢           ⁢   cm               Eq   .           ⁢   6               
The beam widths W CD /W DVD  for the beams of specific sizes B CD , B DVD  are 0.64495 cm and 1.12734 cm, respectively.
 
   When an Nd-YAG laser beam having a wavelength λ of 650 nm is transmitted through the air whose refractive index is 1 and a lens with a focal length F of 1 cm is employed, the beam widths W CD  and W DVD  required in the CD/DVD player are calculated as follows, respectively: 
                     W   CD     =         2   ⁢     (     1   ⁢           ⁢   cm     )     ⁢     (     0.5769   ⁢           ⁢     μm     -   1         )     ⁢     (     0.650   ⁢           ⁢   μm     )           1   -         (     0.5769   ⁢           ⁢     μm     -   1         )     2     ⁢       (     0.650   ⁢           ⁢   μm     )     2             =     0.80900   ⁢           ⁢   cm         ⁢     
             Eq   .           ⁢   5                 W   DVD     =         2   ⁢     (     1   ⁢           ⁢   cm     )     ⁢     (     0.9230   ⁢           ⁢     μm     -   1         )     ⁢     (     0.650   ⁢           ⁢   μm     )             1   -       ⁢       (     0.9230   ⁢           ⁢     μm     -   1         )     2     ⁢       (     0.650   ⁢           ⁢   μm     )     2         =     1.49980   ⁢           ⁢   cm               Eq   .           ⁢   6               
The beam widths W CD /W DVD  for the beams of specific sizes B CD , B DVD  are 0.80900 cm and 1.49980 cm, respectively.
 
   The beam size can be adjusted for both a holographic mode and a CD/DVD mode with a same wavelength by using the holographic beam splitter  104 . 
   In the holographic mode, the reference beam B REF   HDDS  is introduced to the holographic beam splitter  104  with a predetermined incident angle. The holographic beam splitter  104  generates a reproduced beam B HDDS   RE  corresponding to the reference beam B REF   HDDS  and the beam of specific size B HDDS ; and the beam splitter  106  splits the reproduced beam B HDDS   RE  into a reflected beam and a transmitted beam. The reflected beam is illuminated into the medium  110  through a path A. The shutter  118  on the path A may operate to transmit the reflected beam only in the recording state of the holographic mode and not in the reproduction state of the holographic mode. After being transmitted through the shutter  118 , the reflected beam is reflected again by the mirror  120  and modulated by the SLM  122  in order to correspond to input signals so that a holographic signal beam is generated. The holographic signal beam is focused to the medium  110  by the lens  124 . In the meantime, the transmitted beam is illuminated through the path B to the medium  110  as a holographic reference beam, wherein the lens  108  functions to concentrate the holographic reference beam. The interference pattern between the holographic reference beam and the holographic signal beam is recorded on the medium  110 . 
   The medium  110  may be movable upwards or downwards so that the beam may be focused in front of or in the rear of the medium  110  by the lenses  124  and  108 . For example, in case a shift multiplexing principle is used, the beam is preferably focused in front of the medium  110  by the lenses  124  and  108  in the holographic mode while the beam may be preferably focused in the rear of the medium  110  in the CD/DVD mode. In case two lenses with two different focal distances are used, a lens with a shorter focal distance may be preferably used for the holographic mode, if necessary, while the other lens with the longer focal distance may be used for the CD/DVD mode. 
   In the reproduction state of the holographic mode, the shutter  118  is shut off so that only the transmitted beam is introduced to the medium through the path B. Since the transmitted beam functions as the holographic reference beam, a holographic reproduced beam is produced in an original direction of the holographic signal beam introduced into the medium  110  in the recording state. The holographic reproduced beam is focused by the lens  126  and displayed on the charge coupled device (CCD)  114 . 
   In the CD/DVD mode, the holographic beam splitter  104  is rotated by a predetermined angle so that the reference beam B REF   CD /B REF   DVD  may be introduced and, therefore, the reproduced beam B CD   RE /B DVD   RE  corresponding to the reference beam B REF   CD /B REF   DVD  is generated by the holographic beam splitter  104 . The beam splitter  106  divides the reproduced beam B CD   RE /B DVD   RE  into a reflected beam and a transmitted beam and the shutter  118  makes the reflected beam shut off. The transmitted beam is introduced into the CD/DVD medium  110  through the lens  108  after passing through the path B. The beam factor B F  of the beam has previously been controlled before the beam is introduced to the medium  110 . The beam is reflected by the medium  110  to generate a CD/DVD reproduced beam and the CD/DVD reproduced beam is transferred through the path B. The CD/DVD reproduced beam is reflected and transmitted again by the beam splitter  106  to generate a reflected reproduced beam and a transmitted reproduced beam. The transmitted reproduced beam is transferred to the holographic beam splitter  104  so that it does not affect the reproduction signal. Accordingly, the reflected reproduced beam proceeds along the path C to be detected by the photodiode  116  or the CCD  114 . 
   Referring to  FIG. 3 , there is illustrated a block diagram of a holographic digital data storage system  300  in accordance with a second embodiment of the present invention which is compatible with a CD/DVD player, wherein the holographic storage system  300  comprises a light source  302 , a λ/ 2  plate  303 , a holographic polarized beam splitter  304 , a beam splitter  306 , three lenses  308 ,  324 ,  326 , a medium  310 , two mirrors  312 ,  320 , a charge coupled device (CCD)  314 , a photodiode (PD)  316 , a shutter  318 , a beam expander  321  and a spatial light modulator (SLM)  322 . In comparison with the first embodiment illustrated in  FIG. 1 , the λ/ 2  plate  303 , the holographic polarized beam splitter  304  and the beam expander  321  are added. 
   The λ/ 2  plate  303  allows the polarization direction of the linearly polarized beam introduced from the light source  302  to be rotated by a predetermined angle. The beam with the rotated polarization direction is introduced to the holographic polarized beam splitter  304 . 
   The holographic polarized beam splitter  304  is made of a higher birefringence material such as LiNbO 3  or BaTiO 3 . Since the refractive index difference between the ordinary beam and the extraordinary beam may be used, the reproduced beams may be selectively generated in accordance with the reference beams with different polarization directions. 
   Referring to  FIG. 4 , there is illustrated an embodiment of the holographic beam splitter  304  made by using the birefringence characteristics. It is assumed that CD/DVD horizontal/vertical polarized beams of specific sizes B CD&amp;DVD  are used to reproduce the CD player and the DVD player. The CD/DVD horizontal/vertical polarized beams of specific sizes B CD&amp;DVD  have the beam sizes and the beam shapes required in the CD player and the DVD player, respectively. The horizontal/vertical polarized reference beams B REF  are introduced from the λ/ 2  plate  303  and the CD/DVD horizontal/vertical polarized beams of specific sizes B CD&amp;DVD  are also introduced with a predetermined angle with respect to the horizontal/vertical polarized reference beams B REF . The interference pattern between the reference beams B REF  and the CD/DVD horizontal/vertical polarized beams of specific sizes B CD&amp;DVD  is recorded on the holographic polarized beam splitter  304 . In the reproduction mode, only the horizontal/vertical polarized beams B REF  are introduced so that the reproduced beams B CD&amp;DVD   RE  corresponding to the CD/DVD horizontal/vertical polarized beams of specific sizes B CD&amp;DVD  are produced along the incident direction of the CD/DVD horizontal/vertical polarized beams of specific sizes B CD&amp;DVD . 
   Since the polarization direction is changed by the λ/ 2  plate  303 , no additional device is required to move or rotate the holographic polarized beam splitter  304 . The beam expander  321  must be added in order that only two horizontal/vertical polarizations are used for changing the beam factor of the CD/DVD beams into that of the holographic beam. 
   In the CD/DVD mode, the λ/ 2  plate  303  is controlled to make the direction of the beam be oriented to be horizontal or vertical. The horizontal/vertical polarizations correspond to the CD and the DVD mode, respectively, and the reproduced beams B CD&amp;DVD   RE  with the corresponding beam sizes are generated to be illuminated to the beam splitter  306 . The beam splitter  306  divides the reproduced beam B CD&amp;DVD   RE  into a reflected beam and a transmitted beam and the shutter  318  makes the reflected beam shut off. The remaining process is the same as that of the CD/DVD mode of the holographic digital data storage system shown in  FIG. 1 . 
   In the holographic mode, the λ/ 2  plate  303  is rotated by a predetermined polarization angle so that the polarization of the beam may be changed. The polarization angle is not limited and the beam with a predetermined polarization angle is introduced into the holographic polarized beam splitter  304  as a reference beam. The holographic polarized beam splitter  304  generates the reproduced beam B CD&amp;DVD   RE  corresponding to the horizontal and the vertical components of the reference beam B RED  and the beam splitter  306  divides the reproduced beam B CD&amp;DVD   RE  into a holographic reference beam and a holographic signal beam. The holographic reference beam proceeds through the path B and the holographic signal beam proceeds through the path A so that an interference pattern is recorded on the medium  310 . The beam expander  321  is added on the path A in order to control the beam size of the holographic signal beam. In the reproduction mode, the shutter  318  is controlled in order that only the holographic reference beam is introduced to the medium  310  and a holographic reproduced beam corresponding to the holographic signal beam is generated. The holographic reproduced beam is displayed on the CCD  314 . 
   Referring to  FIG. 5 , there is illustrated a block diagram of a holographic digital data storage system  500  compatible with a CD/DVD player in accordance with a third embodiment of the present invention, wherein the holographic storage system  500  comprises a light source  502 , two λ/ 2  plates  503  and  532 , a holographic polarized beam splitter  504 , a beam splitter  506 , four lenses  508 ,  524 ,  526  and  528 , a medium  510 , three mirrors  512 ,  520  and  530 , a charge coupled device (CCD)  514 , a photodiode (PD)  516 , a shutter  518  and a spatial light modulator (SLM)  522 . In the holographic digital data storage system  500  shown in  FIG. 5 , the optical path of the CD/DVD mode is different from that of the holographic mode. In comparison with the fist embodiment shown in  FIG. 1 , two λ/ 2  plate  503  and  532 , the holographic polarized beam splitter  504 , the lens  528  and the mirror  530  are added and the shutter  518  is moved. 
   The λ/ 2  plate  503  allows the polarization direction of the linearly polarized beam introduced from the light source  502  to be rotated by a predetermined angle. The beam with the rotated polarization direction is introduced to the holographic polarized beam splitter  504 . 
   The holographic polarized beam splitter  504  is made of a higher birefringence material such as LiNbO 3  or BaTiO 3 . Since the refractive index difference between the ordinary beam and the extraordinary beam may be used, the reproduced beams may be selectively generated in accordance with the reference beams with different polarization directions. 
   Referring to  FIG. 6 , there is illustrated an embodiment of the holographic polarized beam splitter  504  made by using the birefringence characteristics. It is assumed that CD/DVD horizontal/vertical polarized beams of specific sizes B CD&amp;DVD  are used to reproduce the CD player and the DVD player, respectively and holographic horizontal/vertical polarized beams of specific sizes B HDDSH  and B HDDSV  are used to reproduce the holographic signals. The CD/DVD horizontal/vertical polarized beams of specific sizes B CD&amp;DVD  have the beam sizes and the beam shapes required in the CD player and the DVD player, respectively. The horizontal/vertical polarized reference beams B REF  are introduced from the λ/ 2  plate  503  and the CD/DVD horizontal/vertical polarized beams of specific sizes B CD&amp;DVD  are also introduced with a predetermined angle with respect to the horizontal/vertical polarized reference beams B REF . The holographic horizontal/vertical polarized beams of specific sizes B HDDSH  and B HDDSV  are also introduced with a predetermined angle from each other. In other words, if the horizontal polarized reference beam B REF  with a horizontal polarized component is introduced, two horizontal polarized beams of specific sizes B CD&amp;DVD  and B HDDSH  are recorded on the holographic polarized beam splitter  504  with two different incident angles and, if the vertical polarized reference beam B REF  with a vertical polarized component is introduced, two vertical polarized beams of specific sizes B CD&amp;DVD  and B HDDSV  are recorded with two different incident angles. In the reproduction mode, if only the horizontal polarized reference beam B REF  is illuminated, two reproduced beams B CD&amp;DVD   RE  and B HDDSH   RE  are generated along the incident direction of two horizontal polarized beams of specific sizes B CD&amp;DVD  and B HDDSH , respectively. For illustration, it is supposed that the CD/DVD horizontal/vertical polarized beams of specific sizes B CD&amp;DVD  are used to the CD/DVD player, respectively, and the holographic horizontal/vertical polarized beams of specific sizes B HDDSH  and B HDDSV  are used as a holographic reference beam and a holographic signal beam. 
   In the CD mode, the λ/ 2  plate  503  is controlled to make the direction of the beam be horizontally oriented. If only the horizontal polarized beam is introduced into the holographic polarized beam splitter  504 , the horizontally polarized CD reproduced beam B CD   RE  and the horizontally polarized holographic reproduced beam B HDDSH   RE  are provided along the path B and the path C, respectively. The CD reproduced beam B CD   RE  is transferred into the shutter  518 , the beam splitter  506  and the lens  508  along the path B and introduced to the medium  510 . The reflected beam of the CD reproduced beam B CD   RE  reflected by the medium  510  functions as a CD signal beam. The CD signal beam is further reflected by the beam splitter  506 , and proceeds along the path D to be detected by the PD  516  or the CCD  514 . The holographic reproduced beam B HDDSH   RE  is transferred into the λ/ 2  plate  532 , the mirror  530  and the lens  528  and introduced into the medium  510 . Since, however, the medium  510  is of a reflection type in the CD mode, the holographic reproduced beam B HDDSH   RE  is reflected with the same angle as the incident angle so that the CD player may be reproduced with no error. If necessary, a shutter may be added on the path C. 
   In the DVD mode, the λ/ 2  plate  503  is controlled to make the direction of the beam be vertically oriented. If only the vertical polarized beam is introduced into the holographic polarized beam splitter  504 , the vertically polarized DVD reproduced beam B DVD   RE  and the vertically polarized holographic reproduced beam B HDDSV   RE  are provided along the path B and the path A, respectively. The DVD reproduced beam BDVD RE  is transferred through the path B to be used to reproduce the DVD player while the holographic reproduced beam B HDDSV   RE  is reflected by the medium  510  so that it does not influence the production of the DVD signal. 
   In the holographic mode, the λ/ 2  plate  503  is rotated by a predetermined polarization angle so that the reference beam has a horizontal and a vertical components. The holographic polarized beam splitter  504  is used to generate three reproduced beams with three different directions. Since, however, the shutter  518  turns to be shut off, there is no beam proceeding on the path B while there are beams proceeding on the path A and the path C. The beam on the path A is modulated by the SLM  522  as the holographic signal beam corresponding to the input signals and, then, introduced into the medium  510 . The beam on the path C is used as the holographic reference beam whose polarization direction turns by 90 degrees by the λ/ 2  plate  532  so that two beams on the path A and the path C have a same polarization direction. The interference pattern between the holographic reference beam and the holographic signal beam is recorded on the medium  510 . In the holographic reproduction mode, the shutter  518  is controlled to be shut off and the λ/ 2  plate  503  is controlled so that only the horizontally polarized reference beam may be introduced to the holographic polarized beam splitter  504 . The beam on the path C of two horizontally reproduced beams generated by the holographic polarized beam splitter  504  is used as the holographic reference beam whose polarization direction is rotated by the λ/ 2  plate  532  so that the holographic reference beam may be introduced into the medium  510 . Accordingly, the holographic reproduced beam proceeds along the extension direction of the path A to be displayed on the CCD  514 . 
   If necessary, the angular multiplexing technique may be used so that the incident angles in the CD/DVD mode and the holographic mode may be changed to record the beams of specific sizes on the holographic polarized beam splitter  504 . In case the holographic polarized beam splitter  504  is rotated to record the interference patterns between the reference beam and the beams of specific sizes, the λ/ 2  plate  503  may be unnecessary and the shutter  518  may be moved to the path A. It is necessary that the shutter  518  on the path A remains shut off except the holographic recording mode. 
   Referring to  FIG. 7 , there is illustrated a block diagram of a holographic digital data storage system  700  compatible with a CD/DVD player in accordance with a fourth embodiment of the present invention, wherein the holographic storage system  700  comprises a light source  702 , two λ/ 2  plates  703  and  732 , a holographic polarized beam splitter  704 , two beam splitters  706  and  734 , four lenses  708 ,  724 ,  726  and  728 , a medium  710 , three mirrors  712 ,  720  and  730 , a charge coupled device (CCD)  714 , a photodiode (PD)  716 , a shutter  718  and a spatial light modulator (SLM)  722 . In the holographic digital data storage system  700  shown in  FIG. 7 , the optical path of the CD/DVD mode is separate from that of the holographic mode. In comparison with  FIG. 1 , two λ/ 2  plates  703  and  732 , the holographic polarized beam splitter  704 , the lens  728 , the mirror  730  and the polarized beam splitter  734  are added and the shutter  718  is shifted from path A to path B. 
   The λ/ 2  plate  703  allows the polarization direction of the linearly polarized beam introduced from the light source  702  to be rotated by a predetermined angle. The beam with the rotated polarization direction is introduced to the holographic polarized beam splitter  704 . 
   The holographic polarized beam splitter  704  is made of a higher birefringence material such as LiNbO 3  or BaTiO 3 . Referring to  FIG. 8 , there is illustrated an embodiment of the holographic polarized beam splitter  704  made by using the birefringence characteristics. It is assumed that CD/DVD horizontal/vertical polarized beams of specific sizes B CD&amp;DVD  are used to reproduce the CD/DVD players, respectively and a holographic beam of specific size B HDDS  is used to reproduce the holographic signals. The CD/DVD holographic/vertical polarized beams of specific sizes B CD&amp;DVD  and the holographic beam of specific size B HDDS  are introduced with predetermined angles, respectively. The CD/DVD horizontal/vertical polarized beams of specific sizes B CD&amp;DVD  have the beam sizes and the beam shapes required in the CD/DVD players, respectively. It is preferable that the holographic beam of specific size be introduced with a polarization angle of 45 degree. For illustration, it is assumed that the CD/DVD horizontal/vertical polarized beams of specific sizes B CD&amp;DVD  are used to the CD/DVD players, respectively, and the holographic beam of specific size B HDDS  with the polarization angle of 45 degree is divided into a horizontal and a vertical polarized beam, wherein the horizontal polarized beam is transmitted and the vertical polarized beam is reflected. 
   In the CD mode, the λ/ 2  plate  703  is controlled to make the direction of the beam be horizontally oriented. If only the horizontal polarized beam is introduced into the holographic polarized beam splitter  704 , the horizontally polarized CD reproduced beam B CD   RE  and the horizontal component of the holographic reproduced beam B HDDS   RE  are provided through the path B and the path C, respectively. The CD reproduced beam B CD&amp;DVD   RE  is detected by the PD  716  or the CCD  714  after passing through the path B and the path D as illustrated in  FIG. 5 . The horizontal component of the holographic reproduced beam B HDDS   RE  is transmitted by the polarized beam splitter  734  and, then, proceeds through the path C to be reflected by the medium  710  without influencing the reproduction of the CD player. If the intensity of the holographic reproduced beam B HDDS   RE  is so high that the CD player may be abnormally reproduced, a shutter may be added between the holographic polarized beam splitter  704  and the polarized beam splitter  734 . 
   In the DVD mode, the λ/ 2  plate  703  is controlled to make the direction of the beam be vertically oriented. The vertically polarized DVD reproduced beam BDVD RE  proceeds through the path B and the path D to be detected as the CD mode while the vertical component of the holographic reproduced beam B HDDS   RE  is reflected by the polarized beam splitter  734  and proceeds through the path A so that the DVD player may be normally reproduced. 
   In the holographic recording mode, the λ/ 2  plate  703  is rotated by a predetermined polarization angle so that the reference beam has a horizontal and a vertical component. The beam required in the CD/DVD player is shut off by the shutter  718  on the path B and only the holographic beam is divided into a horizontal and a vertical polarized beam by the polarized beam splitter  734 . The horizontal polarized beam is transmitted through the polarized beam splitter  734 , modified to be vertically polarized and introduced through the path C into the medium  710  as the holographic reference beam. In the holographic reproduction mode, the λ/ 2  plate  703  is controlled so that only the horizontally polarized reference beam may be introduced to the holographic polarized beam splitter  704 . The shutter  718  turns to be shut off so that no beam proceeds through the path B. The beam transmitted through the polarized beam splitter  734  and the λ/ 2  plate  732  is introduced through the path C to the medium  710  as the holographic reference beam so that the holographic reproduced beam is displayed through the lens  726  to the CCD  714 . 
   Referring to  FIG. 9 , there is illustrated a block diagram of a holographic digital data storage system  900  compatible with a CD/DVD player in accordance with a first embodiment of the present invention, wherein the holographic storage system  900  comprises a light source  902 , two beam splitters  905  and  906 , three mirrors  907 ,  912  and  920 , a holographic optical element (HOE) lens  909 , a medium  910 , a charge coupled device (CCD)  914 , a photodiode (PD)  916 , a shutter  918 , a beam expander  921 , a spatial light modulator (SLM)  922  and two lenses  924  and  926 . In the holographic digital data storage system  900  shown in  FIG. 9 , the HOE lens  909  records two CD/DVD numerical apertures and a holographic numerical aperture by using a spatial multiplexing method or an angular multiplexing method. The HOE lens  909  is made of photopolymer and obtains a diffraction efficiency as much as nearly 100%. 
   Referring to  FIG. 10 , there is illustrated an embodiment of the HOE lens  909  made by using a spatial multiplexing method. The beams with CD/DVD specific numerical apertures and a holographic specific numerical aperture are introduced sequentially in accordance with the reference beam represented by two solid lines. Three different lenses  911  are preferably used to obtain three different numerical apertures. For illustration, it is supposed that the HOE lens  909  is made in order to have a CD numerical aperture at a first location, a DVD numerical aperture at a second location and a holographic numerical aperture at a third location. 
   The beam generated in the light source  902  is divided into a transmitted beam and a reflected beam by the beam splitter  905 . The transmitted beam is reflected by the mirror  907 , transmitted through the beam splitter  906  and, then, illuminated to the HOE lens  909  as the reference beam. As the HOE lens  909  moves to the locations corresponding to the CD/DVD modes or the holographic mode, three beams with their corresponding specific numerical apertures are illuminated through the path B to the medium, respectively. The reflected beam proceeds through the path A. Specifically, the reflected beam is transmitted through the shutter  918  and the beam expander  921  that expands the beam into the holographic beam, reflected by the mirror  920  and modulated by the SLM  922  to be illuminated through the lens  924  into the medium  910  as the holographic signal beam. 
   In the CD mode, the shutter is controlled to be shut off so that the beam proceeds only through the path B. The HOE lens  909  is shifted to the first location so that the beam with a numerical aperture required to the CD player is introduced to the medium  910 . The beam is reflected by the medium  910  and transmitted through the HOE lens  909 . The HOE lens  909  generates a reproduced beam corresponding to the reflected beam by the medium  910 . The reproduced beam is transmitted to the opposite direction of the original reference beam. The reproduced beam by the HOE lens reflected by the beam splitter  906 . The reflected beam is detected by the PD  916  or by the CCD  914 . In the DVD mode, it is sufficient that the HOE lens  909  is shifted to the second location. 
   In the holographic recording mode, the shutter  918  is open and the HOE lens  909  is shifted to the third location. The beam on the path B functions as the holographic reference beam and the other beam on the path A functions as the holographic signal beam. In the holographic reproduction mode, the shutter  918  is shut off and the HOE lens  909  is shifted to the third location so that only the holographic reference beam is introduced to the medium  910 . Accordingly, the holographic reproduced beam corresponding to the holographic reference beam is displayed on the CCD  914  located at a position along the extension direction of the holographic signal beam. 
   If necessary, a polarizer or a wave plate is used to control the holographic signal beam on behalf of the shutter  918  and a wave plate may be located before or after the HOE lens  909  in order to control the intensity of the light. 
   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.