Patent Publication Number: US-2005128544-A1

Title: Holographic ROM system

Description:
FIELD OF THE INVENTION  
      The present invention relates to a holographic ROM system; and, more particularly, to a holographic ROM system capable of preventing a noise component from being recorded in a holographic medium when recording data therein, the noise component being produced due to an interference of a reflected reference beam generated by reflecting a reference beam against a data mask with the reference beam and/or a modulated signal beam.  
     BACKGROUND OF THE INVENTION  
       FIG. 1  shows a configuration of a conventional holographic ROM system. As shown in  FIG. 1 , the conventional holographic ROM system includes a light source  10 , HWPs (half wave plates)  12  and  22 , a beam expander  14 , a PBS (polarized beam splitter)  16 , polarizers  18  and  24 , mirrors  20 ,  26  and  28 , a conical mirror  30 , a holographic medium  32  and a data mask  34 .  
      The light source  10  emits a laser beam with a wavelength of, e.g., 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 HWP  12 . The HWP  12  rotates the polarization of the laser beam by θ degree (preferably 45°). And then, the polarization-rotated laser beam is provided to the beam expander  14  for expanding the beam size of the laser beam up to a predetermined size. Thereafter, the expanded laser beam is provided to the PBS  16 .  
      The PBS  16 , 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., S-polarized beam, along a S1 path and reflect, e.g., a vertically polarized laser beam, i.e., P-polarized beam, along a S2 path. Thus the PBS  16  divides the expanded laser beam into a transmitted laser beam (hereinafter, referred to as “reference beam”) and a reflected laser beam (hereinafter, referred to as “signal beam”) having different polarizations, respectively.  
      The signal beam, e.g., of a S-polarization, is provided to the mirror  20  via the polarizer  18 . Herein, the polarizer  18  serves to pass only an S-polarized signal beam. The S-polarized signal beam is reflected by the mirror  20  and then projected onto the data mask  34 . The data mask  34 , presenting a predetermined data pattern to be recorded in the holographic medium  32 , functions as an input device, e.g., a spatial light modulator (SLM). The signal beam is modulated by the data pattern of the data mask  34  so that a modulated signal beam is propagated toward the holographic medium  32 .  
      Meanwhile, the reference beam is fed to the mirror  26  via the HWP  22  and the polarizer  24  sequentially. Since the HWP  22  can rotate the polarization of the reference beam by θ degree and the polarizer  24  serves to pass only an S-polarized reference beam, the HWP  22  and the polarizer  24  can regulate the amount of the S-polarized reference beam arriving at the mirror  26  by changing θ. Therefore, the polarization of the reference beam becomes identical to that of the modulated signal beam. Next, the S-polarized reference beam is reflected by the mirror  28  and then projected onto the conical mirror  30  fixed by a holder (not shown), wherein the conical mirror  30  is of a circular cone having a circular base with a preset base angle between the circular base and the cone. The conical mirror  30  reflects the reference beam conically toward the holographic medium  32 . The incident angle of the reference beam incident on the holographic medium  32  is determined by the base angle of the conical mirror  30 .  
       FIG. 2  shows a holographic medium wherein data patterns are recorded by employing the conventional holographic ROM system. As shown in  FIG. 2 , the holographic medium  32  is a disk-shaped material for recording certain data patterns. The data mask  34  provides the data patterns to be stored in the holographic medium  32 . By illuminating one side of the holographic medium  32  with the modulated signal beam perpendicular to the data mask  34  and the opposite side thereof with the reference beam, an interference pattern generated due to an interference of the reference beam with the modulated signal beam is recorded in the holographic medium  32 . Meanwhile, the holographic ROM system may perform an angular multiplexing by exchanging the conical mirror  30  with another conical mirror having a different reflection angle and then providing the reference beam and the modulated signal beam along the aforementioned paths to thereby record another data pattern in the holographic medium  32 .  
      However, as shown in  FIG. 3 , in the conventional holographic ROM system, a transmitted reference beam generated from a transmission of a portion of the reference beam entering a holographic medium  32  is reflected by the data mask  34  so that a reflected reference beam thereof is generated. The reflected reference beam is reentered into the holographic medium  32 . Accordingly, an interference pattern produced due to an interference of the reflected reference beam with the reference beam and/or the modulated signal beam as well as that produced due to an interference of the reference beam with the modulated signal beam, i.e., a noise component A is recorded in the holographic medium  32 .  
     SUMMARY OF THE INVENTION  
      It is, therefore, an object of the present invention to provide a holographic ROM system capable of preventing a noise component produced due to an interference of a reflected reference beam generated by reflecting a reference beam against a data mask with the reference beam and/or a modulated signal beam from being recorded in a holographic medium by modifying a polarization of the reflected reference beam to be perpendicular to those of the reference beam and/or the modulated signal beam.  
      In accordance with the present invention, there is provided a holographic ROM system for recording data in a holographic medium, the holographic ROM system comprising: 
          a data mask for modulating a signal beam with the data to generate a modulated signal beam to interfere with a reference beam introduced in the holographic medium and reflecting a transmitted reference beam, generated from a transmission of a portion of the reference beam through the holographic medium, to generate a reflected reference beam to be propagated toward the holographic medium; and     a beam polarization changer for changing a polarization of the reflected reference beam so that the polarization thereof is different from those of the reference beam and the modulated signal beam, to thereby prevent the reflected reference beam from interfering with the reference beam and the modulated signal beam.       

    
    
     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  shows a diagram of a conventional holographic ROM system;  
       FIG. 2  illustrates a diagram for explaining a process in which a data pattern is recorded by employing the conventional holographic ROM system;  
       FIG. 3  provides a diagram for describing a process in which a noise component is generated during a data recording process of the conventional holographic ROM system;  
       FIG. 4  depicts a diagram for explaining a process for changing a polarization of a linear polarized beam in accordance with the present invention;  
       FIG. 5  describes a diagram for depicting a data recording process employing a holographic ROM system in accordance with a preferred embodiment of the present invention; and  
       FIG. 6  presents a diagram of a holographic ROM system in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 4  illustrates a process for transforming a horizontally polarized beam (or a vertically polarized beam) into a vertically polarized beam (or a horizontally polarized beam) by using two quarter wave plates (QWPs). As shown in  FIG. 4 , in case a linear polarized beam enters a QWP  410 , the linear polarized beam is transformed into a circular polarized beam. Further, in case a P circular polarized beam and an S circular polarized beam enter another QWP  420 , the P circular polarized beam and the S circular polarized beam are transformed into an S linear polarized beam and a P linear polarized beam, respectively. In the result, the linear polarized beam (P linear-polarized beam) transformed by the QWP  420  is perpendicular to the linear polarized beam (S linear-polarized beam) entering the QWP  410 .  
      The present invention employs such a transforming principal of the linear polarized beam by the QWP. In other words, as illustrated in  FIG. 5 , two QWPs  126  and  128  are installed at both sides of a data mask  124  for generating a data pattern to be recorded into a holographic medium  122 . Accordingly, if a P linear-polarized beam as a reference beam enters a lower surface of the holographic medium  122 , a portion of the reference beam is transmitted through the holographic medium  122  to generate a transmitted reference beam. The transmitted reference beam is transmitted through the QWP  128  so that the transmitted reference beam is changed to a P circular-polarized beam. After the transmitted reference beam is reflected by the data mask  124  to be a reflected reference beam, the reflected reference beam is transmitted again through the QWP  128  so that the reflected reference beam is changed to an S linear-polarized beam. Thus, a polarization of the reflected reference beam becomes an S linear polarization that is perpendicular to that of the reference beam which has a P linear polarization.  
      In the meantime, an S linear-polarized beam as a signal beam enters an upper surface of the QWP  126 . In this case, the signal beam is transmitted through the QWP  126  so that the signal beam is changed to an S circular-polarized beam. The signal beam is modulated with data in the data mask  124  to generate a modulated signal beam. The modulated signal beam is transmitted through the QWP  128  so that the modulated signal beam is changed to a P linear-polarized beam. The modulated signal beam is propagated toward the holographic medium  122 . Since, however, the reflected reference beam (the S linear-polarized beam) do not interfere with the modulated signal beam (the P linear-polarized beam) or the reference beam (the P linear-polarized beam), the reference beam incident on the holographic medium  122  interferes with the modulated signal beam having a polarization identical to that of the reference beam, but not with the reflected reference beam having a polarization perpendicular to that of the reference beam. Accordingly, contrary to the conventional holographic system described with reference to  FIG. 3 , it is possible to avoid a noise generated due to the interference of the reflected reference beam with the reference beam.  
       FIG. 6  provides a structure of a holographic ROM system in accordance with a preferred embodiment of the present invention, the holographic ROM system employing a transforming principal of the polarization direction of a linear polarized beam, which is described with reference to  FIGS. 4 and 5 . As illustrated in  FIG. 6 , the holographic ROM system in accordance with the preferred embodiment of the present invention includes a light source  100 , a HWP (half wave plate)  102 , a beam expander  104 , a PBS (polarized beam splitter)  106 , polarizers  108  and  114 , mirrors  110 ,  116  and  118 , a conical mirror  120 , a holographic medium  122 , a data mask  124  and a first and a second QWPs  126  and  128 .  
      The light source  100  emits a laser beam with a wavelength of, e.g., 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 HWP  102 . The HWP  102  rotates the polarization of the laser beam by θ degree (preferably 45°). And then, the polarization-rotated laser beam is provided to the beam expander  104  for expanding the beam size of the laser beam up to a predetermined size. Thereafter, the expanded laser beam is provided to the PBS  106 .  
      The PBS  106 , 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., S linear-polarized beam, along a S1 path and reflect, e.g., a vertically polarized laser beam, i.e., P linear-polarized beam, along a S2 path. Thus the PBS  106  divides the expanded laser beam into a transmitted laser beam (hereinafter, referred to as “reference beam”) and a reflected laser beam (hereinafter, referred to as “signal beam”) having different polarizations, respectively.  
      The signal beam, e.g., of an S linear-polarization, is provided to the mirror  110  via the polarizer  108 . Herein, the polarizer  108  serves to pass only an S linear-polarized signal beam. The S linear-polarized signal beam is reflected by the mirror  110  and then projected onto the holographic medium  122  via the first and the second QWP  126  and  128  and the data mask  124 . In this case, since the signal beam modulated by the data mask  124  as a modulated signal beam passes the first and the second QWP  126  and  128 , the polarization direction of the modulated signal beam is changed from the S linear polarization to the P linear polarization. Therefore, the P linear-polarized modulated signal beam enters the holographic medium  22 .  
      Meanwhile, the reference beam is fed to the mirror  116  via the polarizer  114 . Herein, a polarization of the reference beam is the P linear-polarized beam that is opposite to that of the signal beam. Thereafter, the P linear-polarized reference beam is reflected by the mirror  118  and then projected onto the conical mirror  120  fixed by a holder (not shown), wherein the conical mirror  120  is of a circular cone having a circular base with a preset base angle between the circular base and the cone. The conical mirror  120  reflects the reference beam toward the holographic medium  122 . The incident angle of the reflected reference beam incident on the holographic medium  122  is determined by the base angle of the conical mirror  120 .  
      The modulated signal beam and the reference beam have the same polarization direction (P linear-polarization), so that an interference pattern generated by the interference therebetween is recorded in the holographic medium  122 . Since, however, the reflected reference beam produced by a reflection of the reference beam against the data mask  124  passes again the second QWP  128 , the polarization direction thereof is changed into the S linear-polarized beam. Accordingly, the reflected reference beam does not interfere with the reference beam that initially entered the holographic medium  122 . As a result, it is possible to prevent a noise generated due to the interference of the reflected reference beam with the reference beam from being recorded therein as in the conventional holographic ROM system.  
      Meanwhile, the holographic ROM system may perform an angular multiplexing by exchanging the conical mirror  120  with another conical mirror having a different reflection angle and then providing the reference beam and the modulated signal beam along the aforementioned paths to thereby record another data pattern in the holographic medium  122 .  
      As described above, in accordance with the present invention, a noise can be prevented from being recorded in a holographic medium by modifying a polarization of a reflected reference beam to be perpendicular to that of a reference beam by using a QWP, the reflected reference beam being produced by a reflection of the reference beam entering the holographic medium against a data mask.  
      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.