Abstract:
An optical system capable of selectively processing CD, DVD and holographic digital data is provided. The system includes a beam generation unit providing a linearly polarized light beam; a beam splitter for respectively reflecting and transmitting a first and a second linearly polarized light beam component; an optical sensor; a first beam path through which the first linearly polarized light beam component propagates to serve as a reference beam for the holographic digital data; and a second beam path for accepting the second linearly polarized light beam component, the second light beam component serving as a beam source for reproducing the CD and DVD digital data, wherein a beam carrying digital data reproduced from any one of CD, DVD and holographic storage medium propagates along the second beam path and is reflected by the beam splitter toward the optical sensor.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a continuation-in-part application of U.S. application Ser. No. 09/796,637 filed on Mar. 2, 2001 now abandoned. 

   FIELD OF THE INVENTION 
   The present invention relates to an optical system capable of selectively processing CD, DVD and holographic digital data; and, more particularly, to an optical system capable of selectively recording and reproducing digital data on and from a CD, a DVD and a holographic storage medium. 
   BACKGROUND OF THE INVENTION 
   Recently, there are reported increasing levels of active researches on holographic digital data storage systems as semiconductor lasers, charge coupled devices (CCDs), liquid crystal displays (LCDs) and the like are being developed. 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 an interference pattern generated from such interference phenomena on a storage medium such as a crystal or a photopolymer which reacts differently depending on an amplitude or phase of the 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 incidence angle of the reference beam, so that a three dimensional display of an object can be realized. Further, hundreds to thousands of hologram digital data constituted with binary data on a page-by-page basis can be stored in a single space of the storage medium. 
     FIG. 1  depicts an overall block diagram of a holographic digital data storage system, wherein the holographic digital data storage system includes 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 storage medium  26  and a CCD  27 . 
   The light source  20  generates a light beam, 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 by 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 by 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 according to each frame of the reflected signal beam. 
   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  has been modulated on a page basis and the reflected reference beam has been reflected in an angle corresponding to each page of 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 reflecting 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 incidence angle as that of the reference beam when recording the data on the medium  26 . 
     FIG. 2  presents a block diagram of a conventional CD or DVD player, wherein the CD/DVD player includes a high frequency overlap module  10 , two mirrors  11  and  18 , a polarizing prism  12 , a cylindrical lens  13 , an 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. 2  and the conventional holographic digital data storage system of  FIG. 1 , 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 at a direction of reflection while the holographic digital data storage system has its detector at 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, a primary object of the present invention to provide an optical system capable of selectively recording and reproducing digital data on and from a CD, a DVD and a holographic storage medium by changing optical channels and beam sizes through the use of a spatial light modulator (SLM) such as a micro-mirror array or a transmissive liquid crystal display (transmissive LCD) and the like. 
   In accordance with a first preferred embodiment of the present invention, there is provided an optical system capable of selectively processing CD, DVD and holographic digital data, comprising: a beam generation unit providing a linearly polarized light beam; a beam splitter for respectively reflecting and transmitting a first and a second linearly polarized light beam component from the beam generation unit, the first and the second linearly polarized light beam component being orthogonal to each other; an optical sensor; a first beam path through which the first linearly polarized light beam component reflected by the beam splitter propagates to serve as a reference beam for the holographic digital data; and a second beam path for accepting the second linearly polarized light beam component transmitted through the beam splitter, the second light beam component serving as a beam source for reproducing the CD and DVD digital data, wherein a beam carrying digital data reproduced from any one of CD, DVD and holographic storage medium propagates along the second beam path and is reflected by the beam splitter toward the optical sensor. 

   
     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 with conjunction with the accompanying drawings, in which: 
       FIG. 1  is a block diagram of a conventional holographic digital data storage system; 
       FIG. 2  depicts a block diagram of a conventional CD/DVD player; 
       FIG. 3  presents a block diagram of an optical system capable of selectively processing CD, DVD and holographic digital data in accordance with a first embodiment of the present invention; 
       FIG. 4  describes a principle of compatibility of the optical system shown in  FIG. 3 ; 
       FIGS. 5A to 5B  demonstrate a writing/reconstruction principle of an optical system shown in  FIG. 3 ; 
       FIG. 6A  explains an operational principle of a micro-mirror array shown in  FIG. 3  and an optical path thereby; 
       FIG. 6B  shows an operational principle of a micro-mirror array in the case of using the optical expander shown in  FIG. 3  and an optical path thereby; 
       FIG. 7  sets forth a block diagram of an optical system capable of selectively processing CD, DVD and holographic digital data in accordance with another embodiment of the present invention; 
       FIG. 8A  describes an optical path through a spatial light modulator (SLM) shown in  FIG. 7 ; 
       FIG. 8B  describes an optical path at a time when the SLM shown in  FIG. 7  is shifted; and 
       FIG. 9  provides a block diagram of an optical system capable of selectively processing CD, DVD and holographic digital data in accordance with another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  is a block diagram of an optical system for recording and reproducing digital data on and from a holographic, CD and DVD storage medium in accordance with a first embodiment of the present invention. The optical storage system includes a light source  300 , two waveplates  302  and  310 , an optical expander  304 , a polarization beam splitter  306 , a charge coupled device (CCD)  308 , a mirror  312 , a micro-mirror array  314 , two lenses  316  and  318  and a storage medium  320 . 
   The light source  300  is an essential element for the writing and reconstruction processes for the CD, DVD and holographic storage medium. A laser, for example, can be used as the light source. The light source  300  provides a light beam with an optimum wavelength or wavelength band for the medium  320  which can be a CD disc, a DVD disk or a holographic digital data storage disc in accordance with the present invention. The light source  300  provides, e.g., a horizontally (S) polarized laser beam of 532 nm. 
   The waveplates  302  and  310  are installed between the light source  300  and the optical expander  304  and between the polarization beam splitter  306  and the micro-mirror array  314 , respectively, to rotate a polarization angle of an incident laser beam by a predetermined degree. Herein, the waveplates  302  and  310  are half-waveplates. As well known in the art, the half-waveplate can be used to rotate the plane of polarization of a plane-polarized light. When a plane-polarized wave is normally incident on the half-waveplate, and the plane of polarization of the incident wave is at an angle θ with respect to the fast (or slow) axis, the plane wave will be rotated through an angle 2θ. Therefore, the half-waveplate can be used in rotating the plane of polarization from a polarized laser to any other desired plane. 
   The optical expander  304  expands the beam size of the incident laser beam to generate an expanded beam. The polarization splitter  306  splits the expanded beam into a vertically (P) polarized beam reflected toward the mirror  312  and a horizontally (S) polarized beam transmitted toward the λ/2plate  310 . 
   The mirror  312  reflects the P-polarized beam to the lens  316 . The micro-mirror array  314  including a number of mirror pixels can reflect the incident S-polarized beam into various directions by selectively controlling each of the mirror pixels. The micro-mirror array  314  in accordance with the present invention may be used to adjust a numerical aperture of the laser beam when the optical system is in the CD/DVD mode and also be used as a spatial light modulator (SLM) when the optical system is in the holographic mode. 
   The two lenses  316  and  318  are used to focus the laser beams reflected from the mirror  312  and the micro-mirror array  314 , respectively. 
   The medium  320  for the holographic mode in accordance with the present invention is configured to have a mirror coating  324  provided on a substrate  326  and coated a holographic digital data storage material  322  on the mirror coating  324 . In other words, medium  320  for the holographic mode has a similar structure to the CD and DVD disc. 
     FIG. 4  describes a principle of compatibility between the CD, DVD and holographic modes which can be carried out by the optical system in accordance with the present embodiment. It is assumed that the beam is irradiated through the lens with a focal length F at a predetermined angle α. In the CD and DVD modes, beam factors B F  thereof should be maintained constant, respectively. In general, B F  of the CD player is 0.5769 μm −1  and the B F  of the DVD player is 0.9230 μm −1 . The 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 different wavelength is used, the N.A. is controlled 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 after a beam passes through the lens and α represents a focus angle against an optical axis, i.e., a central axis, of the lens 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 given as follows:
 
   
     
       
         
           
             
               
                 
                   sin 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   α 
                 
                 = 
                 
                   
                     W 
                     
                       2 
                       ⁢ 
                       
                         
                           
                             
                               ( 
                               
                                 W 
                                 2 
                               
                               ) 
                             
                             2 
                           
                           + 
                           
                             F 
                             2 
                           
                         
                       
                     
                   
                   . 
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 3 
               
             
           
         
       
     
   
   Accordingly, the beam width W can be derived from an equation as follows: 
   
     
       
         
           
             
               
                 W 
                 = 
                 
                   
                     
                       2 
                       ⁢ 
                       
                         FB 
                         F 
                       
                       ⁢ 
                       λ 
                     
                     n 
                   
                   · 
                   
                     
                       1 
                       
                         
                           1 
                           - 
                           
                             
                               ( 
                               
                                 
                                   
                                     B 
                                     F 
                                   
                                   ⁢ 
                                   λ 
                                 
                                 n 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                     . 
                   
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 4 
               
             
           
         
       
     
   
   Consequently, the B F  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 an Nd-YAG 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 FDVD  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 and DVD mode 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 width W CD /W DVD  can be adjusted by controlling an on/off operation of each mirror pixel of the micro-mirror array  314 . When the system is used in a CD or DVD mode, only a predetermined range of beam (i.e., with a beam width W cd  or W dvd ) proceeds to the medium  320  by controlling all the mirror pixels in the micro-mirror array  314 . 
     FIGS. 5A and 5B  illustrate a writing/reconstruction principle when the optical system is in the holographic mode in accordance with the present embodiment. In the holographic operational mode, the holographic digital data is recorded in a writing operation by using the interference between the reference beam and the signal beam. Herein, the signal beam should enter the lens through a predetermined portion of the lens in such a manner that the incident beam does not interfere with the reflected beam reflected from a surface of the mirror coating  324 . Otherwise, the interference results in a noise. 
   A reconstructed signal beam in a reconstruction operation of the holographic mode proceeds along the direction of the original signal beam. Then, the reconstructed signal beam is reflected by the mirror coating  324  Accordingly, the reconstructed beam outputted in the reproduction operation is a substantial reflection of the signal beam inputted in the writing mode. 
   The followings are detailed description of the operational principle of the optical system operating in the writing operation of the holographic mode in accordance with the present embodiment. 
   The rotation angle of the λ/2plate  302  (i.e., the angle between the plane of polarization of the plane-polarized beam and the fast axis of the λ/2plate  302 ) changes the polarization direction of a laser beam outputted from a light source  300  to generate a polarization-rotated beam after the laser beam passes through the λ/2plate  302 . The above polarization-rotated beam is composed of a vertically (P) polarized beam component and a horizontally (S) polarized beam component, wherein the intensities of the reference and the signal beam depend on the ratio of the P-and the S-polarized beam. Supposing that the laser beam outputted from the light source  302  is a S-polarized beam, if the angle between the plane of polarization of the incident laser beam and the fast axis of the λ/2plate  302  is π/8, the intensity of the reference beam is equal to that of the signal beam. The beam expander  304  expands the polarization-rotated beam and then the polarization splitter  306  divides the expanded polarization-rotated beam into the P-polarized beam and the S-polarized beam. In the present invention, it is assumed for illustration that the S-polarized beam component is transmitted through the polarization splitter  306  and the P-polarized beam component is reflected at the polarization splitter  306  (a polarization splitter having the reverse constitution may also exist). 
   In this embodiment, the waveplate  310  is a λ/2plate and the angle between the plane of polarization of the laser beam incident upon the λ/2plate  310  (i.e., the transmitted S-polarized beam) and the fast axis of the λ/2plate  310  is π/4. Therefore, the transmitted S-polarized beam passes through the λ/2plate  310  and becomes a P-polarized beam. The P-polarized beam from λ/2plate  310  subjects to the micro-mirror array  314  such as a TMA (thin-film micro-mirror array). Specifically, by controlling the mirror pixels in the holographic mode, the incident beam thereto may be modulated into a modulated signal beam that corresponds to the required signal. The modulated signal beam is irradiated into the medium  320  through the lens  318 .  FIG. 6A  demonstrates an operational principle of the micro-mirror array  314  and an optical channel of the beam. With the central axis of the lens  318  being a datum, all pixels within a half plane (e.g., a left half plane in  FIG. 6A ) of the micro-mirror array  314  goes off. That is, the pixels in the left half plane of the micro-mirror array  314  do not reflect the incident beam thereto toward the mirror  318 . In the meantime, pixels in another half plane (e.g., a right half plane in  FIG. 6A ) of the micro-mirror array  314  goes on and off to provide a modulated signal beam. That is, according to the required signal, some of the pixels in the right half plane reflect light toward the lens  318 , and the others don&#39;t. 
   An optical expander  400  may be added between the micro-mirror array  314  and the lens  318  in order to obtain a higher resolution and to store more data.  FIG. 6B  describes an operational principle of the micro-mirror array  314  and the optical path thereby in case the optical expander  400  is used. When the optical expander  400  is utilized, however, a distortion by an aberration and the like can be generated as the modulated signal beam reaches farther from the optical axis of the lens  318 . 
   The P-polarized beam from the polarization splitter  306 , which is a reference beam, is reflected by the mirror  312  and irradiated into the medium  320  through lens  316 . In the above step, when a shift multiplexing method, as shown in  FIG. 3 , is used, the lens may be adjusted so that the vertically polarized beam is focused before the storage medium  320  and then illuminated to the medium  320  as the reference beam. In case another multiplexing method is adopted, an adequate module therefor should be added. For example, in case an angular multiplexing method is utilized, a unit such as a Galvano mirror may be preferably added. 
   As described above, data can be written on a holographic digital data storage material  322  by using the interference between the reference beam and the signal beam. 
   In the reconstruction operation, a S-polarized beam is generated by the light source  300 . Then, the S-polarized beam is sent toward the λ/2plate  302 . In the reconstruction operation, the rotation angle of the λ/2plate  302  (i.e., the angle between the plane of polarization of the plane-polarized beam and the fast axis of the waveplate  302 ) is π/4. Accordingly, the S-polarized beam is changed into a P-polarized beam after passing the λ/2plate  302 . Then, the beam outputted from the λ/2plate  302  is expanded by the optical expander  304 . The expanded P-polarized beam is reflected by the polarization splitter  306  toward the mirror  312 . Consequently, the P-polarized beam is irradiated onto the storage medium  320  via lens  316 . By irradiating only the reference beam onto the medium  320 , the signal beam is reconstructed from the holographic digital data storage material  322 ; and the reconstructed signal beam is reflected by the mirror coating  324 . The reflected reconstructed beam passes through the lens  318 . The region of the lens  318  where the reconstructed beam passes through is a region of the lens  318  which had not been used in the writing operation. The reconstructed beam passes through the lens  318  and then proceeds to the micro-mirror array  314 . In the reconstruction mode, the micro-mirror array  314  functions as a mirror having a single reflective plane. After the direction of the reconstructed beam is changed at the micro-mirror array  314 , the reconstructed beam proceeds to the λ/2plate  310 . In the reconstruction operation, the λ/2plate  310  is adjusted such that an angle between the plane of polarization of the reconstructed beam incident upon the λ/2plate  310  (, which is a P-polarized beam,) and the fast axis of the λ/2plate  310  is 0 degree. Accordingly, the λ/2plate  310  adjusts the polarization direction of the reconstructed beam such that the reconstructed beam should be reflected at the polarization splitter  306 . That is, the reconstructed beam remains as the P-polarized beam after passing through the λ/2plate  310 . Then, the reflected reconstructed beam is sent to the CCD  308 . 
   When the optical system is operated in the CD or the DVD mode, the λ/2plate  302  may be preferably adjusted in such a way that only the S-polarized beam (the component which transmits the polarization splitter  306 ) exists at the output of the λ/2plate  302 . That is, supposing that the laser beam outputted from the light source  300  is the S-polarized beam, the λ/2plate  302  is modulated to make an angle between a plane of polarization of the laser beam outputted from the light source  302  and the fast axis of the λ/2plate  302  to be 0. The S-polarized beam outputted from  302  is expanded by the beam expander  304  and the expanded beam is totally transmitted to the λ/2plate  310  through the polarization splitter  306 . At this time, the λ/2plate  310  is modulated to make an angle between a plane of polarization of the transmitted beam and the fast axis of the λ/2plate  310  be π/8. Accordingly, the beam which is transmitted through the polarization beam splitter  306  is controlled so that the plane of polarization of the transmitted beam rotates by as much as π/4 by the second plate  310 . The beam is modulated by the micro-mirror array  314  so as to have a beam size adequate for the CD or DVD player. Afterwards, the beam passes through the lens  318  and is irradiated onto the medium  320  on which data are recorded in the CD or DVD format, thereby generating the reflected beam. 
   After the beam irradiated on the medium  320  is reflected, the reflected beam subsequently passes through the lens  318  and the micro-mirror array  314 . Then the plane of polarization of the reflected beam is rotated by as much as π/4 by the λ/2plate  310 . Accordingly, the reflected beam becomes a P-polarized beam, now. This beam is reflected by the polarization beam splitter  306  and finally sent to the CCD  308 . 
   The CD/DVD player can be implemented in the holographic digital data storage system by configuring the structure as cited above and by following the above-described operation. 
     FIG. 7  depicts a block diagram of an optical system for recording and reproducing digital data on and from a holographic, CD and DVD storage medium in accordance with a second embodiment of the present invention. The optical system includes a mirror  313  and a transmissive type SLM  700  such as a transmissive type LCD instead of the micro-mirror array  314  of  FIG. 3 . 
   The beam width can be adjusted by an on-off operation of the transmissive type SLM  700 , and the structure and operational method of the optical system in  FIG. 7  is the same as that illustrated in  FIG. 3  except the transmissive type SLM  700  and the mirror  313 . 
     FIG. 8A  describes an optical path of the optical system with the transmissive type SLM  700  such as the transmissive type LCD. An active region of the transmissive type SLM  700 , a dashed portion in  FIG. 8A , is adjusted so that the optical system of the present invention can record and reproduce digital data on and from a holographic, CD and DVD storage medium. 
     FIG. 8B  explains an optical path when the position of the transmissive type SLM  700  is changed. It is possible to enlarge input data capacity of the holographic digital data storage system by moving the transmissive type SLM  700  into a region of the holographic digital data storage system. 
   When the active region of the transmissive type SLM  700 , a dashed portion in  FIG. 8B , is dislocated from the optical path, as shown in  FIG. 8B , the intensity of the reconstructed beam is more reduced than that of the reconstructed beam in the embodiment shown in  FIG. 8A . In such an event, a larger volume of data may be inputted by adjusting the beam size. In  FIG. 8B , it can be seen that the HDDS beam size is larger than that in  FIG. 8A . 
     FIG. 9  is a block diagram of an optical system capable of selectively processing CD, DVD and holographic digital data in accordance with another embodiment of the present invention. The holographic digital data storage system includes a high frequency overlap module  10 , four mirrors  11 ,  18 ,  905  and  907 , a polarizing prism  12 , a cylindrical lens  13 , a charge coupled device (CCD)  14 , a disc medium  16 , an object lens  17 , a collimating lens  19 , three λ/2plates  900 ,  902  and  903  and a spatial light modulator (SLM)  904 . 
   The first λ/2plate  900 , the second λ/2plate  902  and the third λ/2plate  903  are disposed at an output terminal of the high frequency overlap module  10  and a forward reflection terminal and a transmission terminal of the polarizing prism  12 , respectively. The SLM  904  is located between the object lens  17  and the mirror  18 . The CCD  14  is located at the backward reflection terminal of the polarizing prism  12 . 
   In the CD or DVD mode, a numerical aperture may be adjusted by performing an on-off operation for all the pixels in the SLM  904  during the reproducing process. Further, supposing that the laser beam outputted from the high frequency overlap module  10  is a horizontally (S) polarized beam, the λ/2plate  900  is modulated to make an angle between a plane of polarization of the laser beam outputted from the high frequency overlap module  10  and the fast axis of the λ/2plate  900  be 0. Furthermore, the λ/2plate  903  is modulated to make an angle between a plane of polarization of the laser beam transmitted by the polarizing prism  12  and the fast axis of the λ/2plate  903  be π/8. By way of such a configuration, it is possible that the λ/2plate  900  may not rotate the plane of polarization of the S-polarized laser beam while the λ/2plate  903  functions similar to the λ/4plate  15  shown in  FIG. 3 . Specifically, the λ/2plate  903  is controlled in order that the beam reflected by the disk medium  16  should be reflected by the polarizing prism  12  to proceed to the CCD  14 . This is possible because the plane of polarization of the laser beam is π/4 rotated when it passes λ/2plate  903  once; and further, the laser beam passes the λ/2plate  903  twice before it comes back to the polarizing prism  12 . In a writing operation of the holographic digital data storage system, the λ/2plate  900  is modulated to make an angle between a plane of polarization of the S-polarized laser beam outputted from the high frequency overlap module  10  and the fast axis of the first half-waveplate  900  be π/8. Accordingly, the plane of polarization of the S-polarized laser beam is rotated π/4 by the λ/2plate  900 , and then the beam is divided into two components by the polarizing prism  12 , which are a P-polarized beam and a S-polarized beam. The subsequent writing/reconstruction mechanism is identical with the conventional method. A reconstructed beam at the time of reproducing process proceeds to the opposite direction of the reference beam used for in the writing operation. In a reconstruction operation of the optical system, supposing that the laser beam outputted from the high frequency overlap module  10  is a S-polarized beam, the λ/2plate  900  and the λ/2plate  902  are modulated to make angles between planes of polarization of the laser beams incident thereupon and the fast axes thereof be π/4 and π/8, respectively. The S-polarized laser beam becomes a P-polarized laser beam after passing the λ/2plate  900 . Then, the P-polarized beam goes toward the λ/2plate  902  after being reflected by the polarizing prism  12 . The plane of polarization of the reflected beam is rotated π/4 after passing the λ/2plate  902 . Then, the beam is reflected by mirrors  905  and  907 , respectively, and sent to the medium  16 . In the medium  16 , a reconstructed signal beam is generated along the direction of the original signal beam. The reconstructed signal beam is rotated π/4 again after passing the λ/2plate  902 . Consequently, the reconstructed signal beam becomes a S-polarized beam. Then, the horizontally polarized beam is sent to the cylindrical lens  13  and the charge coupled device (CCD)  14 , sequentially. Herein, the λ/2plate  903  may not function. 
   The disk medium  16  used in the optical system shown in  FIG. 9  should have no reflection coating. Further, the medium  16  should be a photopolymer, not a photorefractive crystal. Since the photorefractive crystal changes refracting angle of an incident beam according to a plane of polarization, it cannot be used in the optical system shown in  FIG. 9 . In the meantime, the photopolymer maintains refracting angle of an incident beam irrespective of a plane of polarization of the incident beam. Therefore, the optical system of the present invention may record and reproduce digital data on and from a holographic, CD and DVD storage medium. 
   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.