Abstract:
A holographic ROM system includes a light source for emitting a laser beam; an expanding unit for expanding the laser beam; and a mask for modulating a certain portion of the expanded laser beam to thereby generate a signal beam which is directly provided to a holographic medium and sifting the remainder portion of the expanded laser beam to thereby generate a reference beam which is provided to a conical mirror reflecting the reference beam toward the holographic medium. The holographic medium is composed of a recording region onto which both the reference beam and the signal beam are projected; and a transparent region which passes through the reference beam sifted by the mask, to thereby provide the reference beam to the conical mirror.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a holographic ROM (read-only memory) system; and, more particularly, to the holographic ROM system capable of achieving an enhanced recording and reading efficiency with a simplified structure. 
     BACKGROUND OF THE INVENTION 
     Conventional holographic memory systems normally employ a page-oriented storage approach. An input device such as a SLM (spatial light modulator) presents recording data in the form of a two dimensional array (referred to as a page), while a detector array such as a CCD camera is used to retrieve the recorded data page upon readout. Other architectures have also been proposed wherein a bit-by-bit recording is employed in lieu of the page-oriented approach. All of these systems, however, suffer from a common drawback in that they require the recording of a huge number of separate holograms in order to fill the memory to capacity. A typical page-oriented system using a megabit-sized array would require the recording of hundreds of thousands of hologram pages to reach the capacity of 100 GB or more. Even with the hologram exposure times of millisecond-order, the total recording time required for filling a 100 GB-order memory may easily amount to at least several tens of minutes, if not hours. Thus, another conventional holographic ROM system such as FIG. 1 has been developed, where the time required to produce a 100 GB-order capacity disc may be reduced to under a minute, and potentially to the order of seconds. 
     The conventional holographic ROM system of FIG. 1 includes a light source  100 , HWPs (half wave plates)  102 ,  112 , an expanding unit  104 , a PBS (polarizer beam splitter)  106 , polarizers  108 ,  114 , mirrors  110 ,  116 , a mask  122 , a hologram medium  120 , and a conical mirror  118 . 
     The light source  100  emits a laser beam with a constant wavelength, e.g., a wavelength of 532 nm. The laser beam, which is of only one type of linear polarization, e.g., P-polarization or S-polarization, is provided to the 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 fed to the expanding unit  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 one type of polarized laser beam, e.g., P-polarized beam, and reflect the other type of polarized laser beam, e.g., S-polarized beam. Thus the PBS  106  divides the expanded laser beam into a transmitted laser beam (hereinafter, a signal beam) and a reflected laser beam (hereinafter, a reference beam) having. different polarizations, respectively. 
     The signal beam, e.g., of a P-polarization, is fed to the polarizer  108 , which removes imperfectly polarized components of the signal beam and allows only the purely P-polarized component thereof to be transmitted therethrough. And then the signal beam with perfect or purified polarization is reflected by the mirror  110 . Thereafter, the reflected signal beam is projected onto the holographic medium  120  via the mask  122 . The mask  122 , presenting data patterns for recording, functions as an input device, e.g., a spatial light modulator (SLM). 
     On the other hand, the reference beam is fed to the HWP  112 . The HWP  112  converts the polarization of the reference beam such that the polarization of the reference beam becomes identical to that of the signal beam. And then the reference beam with converted polarization is provided to the polarizer  114 , wherein the polarization of the reference beam is more purified. And the reference beam with perfect polarization is reflected by the mirror  116 . Thereafter, the reflected reference beam is projected onto the conical mirror  118  (the conical mirror  118  being of a circular cone having a circular base with a preset base angle between the circular base and the cone), which is fixed by a holder (not shown). The reflected reference beam is reflected toward the holographic medium  120  by the conical mirror  118 . The incident angle of the reflected reference beam on the holographic medium  120  is determined by the base angle of the conical mirror  118 . 
     The holder for fixing the conical mirror  118  should be installed on the bottom side of the conical mirror  118 , in order to prevent the reference beam from being blocked by the holder. Since the holder should be placed on the bottom side of the conical mirror  118 , it is usually installed through a center opening  124  of the holographic medium  120 . 
     The holographic medium  120  is a disk-shaped material for recording the data patterns. The disc-sized mask  122  provides the data patterns to be stored therein. By illuminating the mask  122  with a normally incident plane wave, i.e., the signal beam, and by using the reference beam incident from the opposite side to record holograms in the reflection geometry, the diffracted pattern is recorded in the holographic medium  120 . A conical beam shape is chosen to approximate the plane wave reference beam with a constant radial angle at all positions on the disc, such that the hologram can be read locally by a fixed-angle narrow plane wave while the disc is rotating during playback. Furthermore, an angular multiplexing can be realized by using the conical mirror  118  with a different base angle (see “Holographic ROM system for high-speed replication”, 2002 IEEE, by Ernest Chuang, et al.). 
     By using the above-mentioned scheme, the time required to produce a fully recorded 100 GB-order capacity disc may be reduced to less than a minute, and potentially to an order of seconds. 
     However, there are critical problems in the prior art system; 
     First, the conventional holographic ROM system of FIG. 1 has a complicated structure. That is, the conventional holographic ROM system requires numerous components including the PBS  106 , the HWPs  102 ,  112 , the mirrors  110 ,  116 , and the polarizers  108 ,  114  increasing the manufacturing cost thereof. 
     Secondly, replacement of the conical mirror  118  for angular multiplexing can be highly cumbersome. Since the holder is packed into a compact space between the holographic medium  120 , the conical mirror  118  and the mask  122 , the alignment between the holographic medium  120  and the mask  122  can be easily disturbed whenever the conical mirror  118  is replaced for the angular multiplexing, complicating the replacement process of the conical mirror  118 . 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a holographic ROM system with a simplified structure, which also enables a conical mirror to be replaced easily for angular multiplexing. 
     In accordance with the present invention, there is provided a holographic ROM system including: a light source for emitting a laser beam; an expanding unit for expanding the laser beam; and a mask for modulating a certain portion of the expanded laser beam to thereby generate a signal beam which is directly provided to a holographic medium and sifting the remainder portion of the expanded laser beam to thereby generate a reference beam which is provided to a conical mirror reflecting the reference beam toward the holographic medium, wherein the holographic medium includes: a recording region onto which both the reference beam and the signal beam are projected; and a transparent region which passes through the reference beam sifted by the mask, to thereby provide the reference beam to the conical mirror. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which: 
     FIG. 1 shows a conventional holographic ROM system; 
     FIG. 2 describes a holographic ROM system in accordance with a preferred embodiment of the present invention; 
     FIG. 3A represents a plane view of a holographic medium included in the holographic ROM system of FIG. 2; 
     FIG. 3B offers a plane view of a mask included in the holographic ROM system of FIG. 2; 
     FIG. 3C illustrates a recording mechanism in accordance with the preferred embodiment of the present invention; and 
     FIG. 3D explains paths of the reference beam between a conical mirror and the holographic medium in accordance with the preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 shows a holographic ROM system in accordance with a preferred embodiment of the present invention. The holographic ROM system includes a light source  200 , an expanding unit  204 , a mask  206 , a holographic medium  208 , and a conical mirror  210 , disposed in that order along a straight main optical axis, i.e., OPaxis. The holographic medium  208  preferably has a CD-like disc shape and is provided with a transparent region  208   a  at the center thereof for passing therethrough a reference beam toward the conical mirror  210  and an annular-shaped recording region  208   b  therearound as shown in FIG.  3 A. The mask  206  serves to generate a data pattern for recording as in the conventional mask  122  of FIG. 1, but differs therefrom in that the mask  206  is provided with a beam passing region  206   a  and a data pattern region  206   b  as shown in FIG.  3 B. The beam passing region  206   a  is located at the central region of the mask  206  and serves to pass the reference beam therethrough toward the conical mirror  210  via the transparent region  208   a  of the holographic medium  208 , as will be described in more detail hereinafter. The data pattern region  206   b , having an annular shape, is provided around the beam passing region  206   a . The beam passing region  206   a  may be a simple open hole or be made of a transparent material or a combination thereof. The mask  206  can be of a type having the regions  206   a  and  206   b  of fixed sizes to be used only for a specific type of the holographic medium  208  or can be configured to vary the sizes thereof adaptive to various types of the holographic medium  208 . In any case, it is preferable that the sizes of the transparent region  208   a  and the recording region  208   b  be identical to those of the beam passing region  206   a  and the data pattern region  206   b , i.e., the inner and the outer diameters of the recording region  208   b  equaling to those of the data pattern region  206   b.    
     As shown in FIG. 2, the light source  200  emits a laser beam with a certain beam size and a constant wavelength, e.g., of 532 nm. The laser beam is fed to the expanding unit  204  which expands the width of the laser beam to provide a collimated beam of plane wave. The expanded laser beam preferably covers the whole area of the beam passing region  206   a  and the data pattern region  206   b  but with a diameter size close to the outer diameter of the recording region  208   b  and a uniform intensity to maximize the optical and the recording efficiencies. 
     As shown in FIG. 3C, the whole expanded laser beam is directly forwarded to the mask  206  and then provided to the holographic medium  208  through two optical paths. A first path is that the outer portion of the expanded laser beam is fed to the data pattern region  206   b  of the mask  206  and then projected onto the holographic medium  208  through a first surface facing the mask  206 . And a second path is that the inner portion of the expanded laser beam passes through the transparent region  208   a  of the holographic medium  208  via the beam passing region  206   a  of the mask  206  and then reflected by the conical mirror  210  and then projected to the holographic medium  208  through a second surface opposite the mask  206 . The laser beam passing through the beam passing region  206   a  serves as a reference beam and the laser beam passing through the data pattern region  206   b  of the mask  206  serves as a signal beam. 
     The data pattern region  206   b  of the mask  206  provides a data pattern for writing data in the holographic medium  208 . By illuminating the mask  206  with a normally incident plane wave, i.e., the signal beam, and by using the reference beam to record holograms in the reflection geometry, the diffracted pattern of the signal and the reference beam is recorded in the holographic medium  208 . 
     A conical beam shape is chosen to approximate the plane wave reference beam with a constant radial angle at all positions on the recording region  208   b  of the holographic medium  208 . The data can be written on the entire recording region  208   b  at once by a single shot of laser beam without rotating the holographic medium  208 . Such recorded hologram can be read locally by using a reference beam only, which is identical to that employed during the recording process but of a fixed-angle narrow plane wave, while the holographic medium  208  is rotating during playback. That is, the geometry shown in FIG. 2 is used only during the recording operation. 
     The conical mirror  210  has a reflecting surface  210 a of a cone shape facing the holographic medium  208  and is detachably fixed by a holder  212 , which maintains the conical mirror  210  at a fixed position by holding same at the back side thereof opposing the holographic medium  208  in order not to block the reference beam. Further, the main optical axis, Opaxis, of the laser beam is to coincide with the symmetry axis of the conical reflecting surface  210   a  passing through the apex thereof; and the base diameter of the conical reflecting surface  210   a  is preferably not to be smaller than the diameter of the transparent region  208   a  to maximize the optical efficiency. Since the replacement of the conical mirror  210  is executed without disturbing the arrangement between the holographic medium  208  and the mask  206 , the conical mirror  210  can be rather easily replaced with another one having a different base angle for multiplexing. 
     Since the reference beam shares the optical axis of the signal beam (i.e., the main optical axis “Opaxis”) until it reaches the conical mirror  210 , without being split therefrom, the present invention can be implemented by employing only the light source  200 , the expanding unit  204 , the mask  206 , the holographic medium  208 , and the conical mirror  210 , without having to use the HWPs  102 ,  112 , the PBS  106 , the polarizers  108 ,  114 , and the mirrors  110 ,  116  of the prior art shown in FIG.  1 . Therefore, the structure of the holographic ROM system of the present invention is greatly simplified, resulting in a significant reduction of the manufacturing costs. 
     FIG. 3D provides the path of the reference beam between the conical mirror  210  and the holographic medium  208  in accordance with the preferred embodiment of the present invention. If the incident angle between a direction of the incident reference beam and a surface of the holographic medium  208  is set to be ∠ 2   a  and an angle ∠DBC, i.e., the base angle of the conical reflecting surface  210   a  is ∠b, the angles ∠ODA and ∠DAO equal ∠ 2   b  and ∠ 2   a , respectively. Thus, it can be seen that: 
     
       
         ∠ a+∠b =45°  Eq. 1 
       
     
     In case the reference beam with a radius of X 1  is reflected by the conical mirror  210  and illuminated onto the holographic medium  208 , the beam size (i.e., width) X 2  of the reflected reference beam of the conical beam shape on the holographic medium  208  can be defined as: 
     
       
           X   2 = X   1 /sin  2   a   Eq. 2 
       
     
     In case the size of the recording region  208   b  (i.e., the width thereof being equal to one-half of the difference between the outer and the inner diameters thereof) of the holographic medium  208  is X 3 , X 2  needs to be equal to or larger than X 3  in order to write data on the recording region  208   b  at once. Then, the relationship between X 1  and X 3  can be defined as: 
     
       
           X   2 = X   1 /sin  2   a≧X   3   Eq. 3 
       
     
     Thus, the angle ∠a can be defined in terms of X 1  and X 3  as: 
      ∠ 2   a &lt;sin −1  ( X   1 / X   3 )  Eq. 4 
     Therefore, if ∠ 2   a  is determined by Eq. 4, a suitable base angle ∠b of the conical reflecting surface  210   a  can be obtained. 
     The location on the holographic medium  208 , where the incident reference beam is projected, may vary with the distance between the conical mirror  210  and the holographic medium  208 . In order for the size X 2  of the reflected beam on the holographic medium  208  to coincide with that of the recording region  208   b  thereof or in order for the central ray of the reference beam to pass through the inner boundary of the recording region  208   b , the distance ‘Y’ between the apex of the conical reflecting surface  210   a  and the holographic medium  208  can be defined as: 
     
       
           Y=X   1 ·tan  2   a   Eq. 5 
       
     
     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 the scope of the invention as defined in the following claims.