Abstract:
A holographic apparatus includes a mask for modulating a signal beam to generate a modulated signal beam; a conical prism, which has a cone portion and a base portion, for refracting a reference beam to generate a refracted reference beam, wherein the refracted reference beam interferes with the modulated signal beam in a holographic medium to thereby record data thereon, the base portion facing the holographic medium. The holographic apparatus can be miniaturized by a positional relationship between the conical prism and the holographic medium.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a holographic digital data storage system, e.g., a ROM (read-only memory) system; and, more particularly, to an apparatus and a method for recording digital data on a holographic storage system capable of miniaturizing the holographic digital data storage system, e.g., a conical prism. 
     BACKGROUND OF THE INVENTION 
     Conventional holographic digital data storage systems normally employ a page-oriented storage approach. An input device such as an SLM (spatial light modulator) presents recording data in the form of a two dimensional array (referred to as a page). 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 the one shown in  FIG. 1A  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 storage system (see “Holographic disk recording system”, U.S. patent application publication No. US2003/0161246A1, by Ernest Chuang, et al.) shown in  FIG. 1A  includes a light source  100 ; HWPs (half wave plates)  102 ,  112 ; an expanding unit  104 ; a PBS (polarization beam splitter)  106 ; polarizers  108 ,  114 ; mirrors  110 ,  116 ,  117 ; a mask  122 ; a holographic medium  120 ; and a conical prism  118 . 
     The light source  100  emits a laser beam with a constant wavelength, e.g., a wavelength of 532 nm. The laser beam of only one type of linear polarization, e.g., either P- 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 called a signal beam) and a reflected laser beam (hereinafter called 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). 
     Meanwhile, 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 further purified. And the reference beam with perfect polarization is reflected by the mirror  116  and then the mirror  117  sequentially. Thereafter, the reflected reference beam is projected onto the conical prism  118  (the conical prism  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 refracted toward the holographic medium  120  by the conical prism  118 . The angle of incidence of the refracted reference beam on the holographic medium  120  is determined by the base angle of the conical prism  118 . 
     The holographic medium  120  is preferably of a disk-shaped material for recording the data patterns. The mask  122 , also having a disk shape of a similar size to that of the holographic medium  120 , provides the data patterns to be stored in the holographic medium  120 . 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 refraction geometry, the diffracted pattern is recorded in the holographic medium  120 . Furthermore, an angular multiplexing can be realized by using the conical prism  118  with a different base angle. 
       FIG. 1B  depicts optical paths of the reference beam from the conical prism  118  to the holographic medium  120  in the conventional holographic storage system of  FIG. 1A . 
     The circular base of the conical prism  118  is preferably parallel with the holographic medium  120 , and does not face the holographic medium  120 , i.e., a vertex of the conical prism  118  faces the holographic medium  120 . The holographic medium  120  is provided with a hole region  120   b  at the center thereof and an annular-shaped recording region  120   a  therearound. Further, the symmetry axis of the holographic medium  120  is coincident with that of the conical prism  118  passing through the vertex thereof. 
     As shown in  FIG. 1B , the reference beam with a radius of W 1  strikes the circular base of the conical prism  118 . The reference beam propagates in a first propagating direction which is perpendicular to the holographic medium  120 , i.e., to the circular base. The reference beam is not refracted at the circular base, because the first propagating direction is normal to the circular base. Thus, the reference beam also propagates in the medium of the conical prism  118  in the first propagating direction until the reference beam reaches the surface of the cone of the conical prism  118 . At the surface of the cone, the reference beam is refracted, to thereby produce the refracted reference beam which then propagates toward the holographic medium  120  in the medium of the air in a second propagating direction as shown in  FIG. 1B , while obeying Snell&#39;s law. 
     In  FIG. 1B , the surface of the cone includes a first half cone surface  118   a  and a second half cone surface  118   b . Moreover, the holographic medium  120  includes a first half recording region  120   aa  which is located on the same side as that of the first half cone surface  118   a  and a second half recording region  120   ab  which is located on the same side as that of the second half cone surface  118   b . A first half reference beam is transmitted to the first half cone surface  118   a  after passing through the circular base and then refracted toward the second half recording region  120   ab  and a second half reference beam is transmitted to the second half cone surface  118   b  after passing through the circular base and then refracted toward the first half recording region  120   aa.    
     Therefore, as shown in  FIG. 1B , near the holographic medium  120 , the refracted first half reference beam and the refracted second half reference beam form a conical shell beam shape whose center portion is empty so that a cross section of the conical shell beam shape cut by a plane parallel with the holographic medium  120  may be an annular shape. 
     After the reference beam with a radius of W 1  is refracted at the cone surface  118 , the size of the refracted reference beam (i.e., the width thereof being equal to one-half of the difference between the outer and the inner diameters of the annular-shaped cross section thereof) is decreased down to W 2  because a refractive index of the conical prism  118  is larger than 1. 
     In case the size of the recording region  120   a  (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  120  is W 3  as shown in  FIG. 1B , the size W 2  of the refracted reference beam needs to be equal to or larger than W 3  in order to write data on the recording region at once. 
     However, there is a critical problem in the prior art as follows. 
     A radius of the circular base of the conical prism  118  is preferably slightly larger than (approximately equal to) that of the reference beam W 1  such that the entire refracted reference beam with the size of W 2  is irradiated onto the recording region  120   a  with the size of W 3 . However, since the radius of the circular base is larger than W 2  which is equal to or larger than W 3 , the size of the conical prism  118 , i.e., the radius of the circular base, should be larger than that of the recording region  120   a . Thus, the size of the conical prism  118  becomes larger so that the conventional holographic digital data storage system cannot be miniaturized. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide an apparatus and a method for recording digital data on a holographic storage system capable of miniaturizing the holographic storage system, e.g., a conical prism. 
     In accordance with one aspect of the present invention, there is provided a holographic apparatus including: a mask for modulating a signal beam to generate a modulated signal beam; a conical prism, which includes a cone portion and a base portion, for refracting a reference beam to generate a refracted reference beam, wherein the refracted reference beam interferes with the modulated signal beam in a holographic medium to thereby record data thereon, the base portion facing the holographic medium. 
     In accordance with another aspect of the present invention, there is provided a holographic apparatus including: a light source for emitting a laser beam; a beam splitter for dividing the laser beam into a reference beam and a signal beam; a mask for modulating the signal beam to generate a modulated signal beam; and a refractor for refracting the reference beam to generate a refracted reference beam, wherein the refracted reference beam interferes with the modulated signal beam in the holographic medium to thereby record the data thereon. 
     In accordance with a further aspect of the present invention, there is provided a holographic method including the steps of: (a) generating a laser beam; (b) dividing the laser beam into a reference beam and a signal beam; and (c) modulating the signal beam to generate a modulated signal beam and, at the same time, refracting the reference beam to generate a refracted reference beam, wherein the refracted reference beam interferes with the modulated signal beam in the holographic medium to thereby record the data thereon. 
    
    
     
       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 embodiment given in conjunction with the accompanying drawings, in which: 
         FIG. 1A  shows a conventional holographic storage system; 
         FIG. 1B  depicts optical paths of a reference beam from a conical prism to a holographic medium in the conventional holographic storage system of  FIG. 1A ; 
         FIG. 2  describes a holographic storage system in accordance with a preferred embodiment of the present invention; 
         FIG. 3A  offers an optical path of a reference beam passing through a conical prism included in the holographic ROM system of  FIG. 2 ; and 
         FIG. 3B  explains optical paths of the reference beam from the conical prism to a holographic medium in accordance with the preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  shows a holographic storage system, e.g., a holographic ROM system, in accordance with a preferred embodiment of the present invention. The holographic storage system of  FIG. 2  includes a light source  200 ; HWPs (half wave plates)  202 ,  212 ; an expanding unit  204 ; a PBS (polarization beam splitter)  206 ; polarizers  208 ,  214 ; mirrors  210 ,  216 ,  217 ; a mask  222 ; a holographic medium  220 ; and a conical prism  218 . 
     The holographic storage system of the present invention is generally identical to that of the prior art shown in  FIGS. 1A and 1B , excepting the conical prism  218  which substitutes for a conical prism  118  in  FIGS. 1A and 1B . The functions of the other parts of the holographic storage system of the present invention except the conical prism  218  are basically identical to those of the prior art, so that detailed explanation thereabout is abbreviated. 
     As shown in  FIG. 2 , a signal beam is irradiated onto the holographic medium  220  via the polarizer  208 , the mirror  210  and the mask  222 , which are disposed in that order along a signal beam optical path, i.e., Spath, and a reference beam is irradiated onto the holographic medium  220  from the opposite side via the HWP  212 , the polarizer  214 , the mirror  216 , the mirror  217  and the conical prism  218 , which are disposed in that order along a reference beam optical path, i.e., Rpath. 
     The conical prism  218  is of a circular cone having a circular base with a preset base angle between the circular base and the cone. Herein, the circular base faces the holographic medium  220  and is preferably parallel with the holographic medium  220 . The holographic medium  220  preferably has a CD-like disc shape. That is, the holographic medium  220  is provided with a hole region  220   b  at the center thereof and an annular-shaped recording region  220   a  therearound as shown in  FIG. 3B . The diameter of the circular base of the conical prism  218  is preferably not to be smaller than that of the hole region  220   b  to maximize the optical efficiency. Further, the symmetry axis of the holographic medium  220  is coincident with that of the conical prism  218  passing through a vertex thereof. 
       FIG. 3A  offers optical path of the reference beam passing through the conical prism  218  in accordance with the preferred embodiment of the present invention. 
     The reference beam with a radius of X 1 , which propagates in a first propagating direction normal to the circular base, strikes a surface of the cone of the conical prism  218 . Then, the reference beam is refracted at the surface of the cone so that the refracted reference beam propagates in a second propagating direction in the medium of the conical prism  218 , while obeying Snell&#39;s law:
 
sin a=n sin b  Eq. 1
 
where the index of refraction of the medium of the conical prism  218  is defined to be ‘n’, and the index of refraction of the air to be 1. And ‘a’ represents the base angle, which is an acute angle, i.e., an angle less than 90°. Since ‘a’ is defined to be the base angle of the conical prism  218 , a first angle of incidence, i.e., an angle between the first propagating direction of the reference beam and a first vertical direction normal to the surface of the cone, also becomes ‘a’, as shown in  FIG. 3A . Moreover, an acute angle ‘b’ indicates a first angle of refraction, i.e., an angle between the second propagating direction of the refracted reference beam and the first vertical direction normal to the surface of the cone.
 
     Then, the refracted reference beam propagates straightly in the second propagating direction in the medium of the conical prism  218  until the refracted reference beam reaches the circular base of the conical prism  218 . At the circular base, the refracted reference beam is refracted once more, to thereby produce a twice-refracted reference beam which is then provided to the holographic medium  220  in a third propagating direction through the air as shown in  FIG. 3B , while obeying Snell&#39;s law:
 
n sin c=sin d  Eq. 2
 
where the indexes of refraction of the medium of the conical prism  218  and the air are ‘n’ and 1, respectively, as mentioned above. An acute angle ‘c’ is a second angle of incidence, i.e., an angle between the second propagating direction of the refracted reference beam and a second vertical direction normal to the circular base, and an acute angle ‘d’ is a second angle of refraction, i.e., an angle between the third propagating direction of the twice-refracted reference beam and the second vertical direction normal to the circular base.
 
     ‘a’, ‘b’ and ‘c’ are related by a following equation, as shown in FIG.  3 A:
 
 b+c=a   Eq. 3
 
     Eq. 1 and Eq. 3 can be arranged resulting in following equations:
 
 b =sin −1 (sin  a/n )  Eq. 4
 
 c=a−b=a −sin −1 (sin  a/n )  Eq. 5
 
     If Eq. 4 and Eq. 5 are inserted into Eq. 2, a following equation is obtained:
 
 n  sin{ a −sin −1 (sin  a/n )}=sin  d =&gt;sin −1   [n  sin{ a −sin −1 (sin  a/n )}]= d   Eq. 6
 
     Therefore, as shown in Eq. 6, the second angle of refraction ‘d’ can be adjusted by varying the base angle ‘a’ of the conical prism  218 . Since an angle of incidence of the twice-refracted beam on the holographic medium  220 , i.e., an acute angle between the third propagating direction of the twice-refracted reference beam and a third vertical direction normal to the surface of the holographic medium  220 , is identical to the second angle of refraction ‘d’ as shown in  FIG. 3B , the angle of incidence of the twice-refracted reference beam on the holographic medium  220  is also determined by the base angle ‘a’ of the conical prism  218 . In Eq. 6, since ‘a’ is a more dominant factor than ‘sin −1 (sin a/n)’, ‘d’ becomes increased as ‘a’ increases. 
     In  FIG. 3B , the reference beam with a radius of X 1  is irradiated onto the surface of the cone of the conical prism  218 . The surface of the cone includes a first half cone surface  218   a  and a second half cone surface  218   b . Moreover, the holographic medium  220  includes a first half recording region  220   aa  which is located on the same side as that of the first half cone surface  218   a  and a second half recording region  220   ab  which is located on the same side as that of the second half cone surface  218   b . A first half reference beam refracted at the first half cone surface  218   a  is refracted once more at the circular base to be provided to the second half recording region  220   ab  and a second half reference beam refracted at the second half cone surface  218   b  is refracted once more at the circular base to be provided to the first half recording region  220   aa.    
     Considering only the second half reference beam for the convenience of depiction as shown in  FIG. 3B , after the second half reference beam, whose cross section cut by a plane parallel with the holographic medium  220  is a hemicyclic shape with a radius of X 1 , is refracted at the second half cone surface  218   b , the beam size (i.e., width) of the refracted second half reference beam is increased up to X 2  at the circular base of the conical prism  218  because ‘n’ is larger than 1. Then the refracted second half reference beam with the size of X 2  is refracted once more at the circular base to thereby become a twice-refracted second half reference beam which is illuminated onto the first half recording region  220   a  of the holographic medium  220  with the angle of incidence being ‘d’. 
     In case the size of the recording region  220   a  (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  220  is X 3  as shown in  FIG. 3B , the beam size X 2  of the twice-refracted second half reference beam needs to be equal to or larger than X 3  in order to write data on the recording region  220   a  at once. Then, the relationship between X 1 , X 2  and X 3  can be defined as:
 
 X   2 = X   1 /cos  d&gt;X   3 =&gt; d &lt;cos −1 ( X   1 / X   3 )  Eq. 7
 
     If Eq. 7 is inserted into Eq. 6, a following equation is obtained:
 
sin −1   [n  sin{ a −sin −1 (sin  a/n )}]= d &lt;cos −1 ( X   1 / X   3 )  Eq. 8
 
     The location on the holographic medium  220 , where the twice-refracted reference beam is projected, may vary with a distance ‘Y’ between the circular base of the conical prism  218  and the holographic medium  220 . In order for a portion of the twice-refracted reference beam passing through a periphery of the circular base to be irradiated onto a borderline between the recording region  220   a  and the hole region  220   b  of the holographic medium  220 , the distance ‘Y’ can be depicted as:
 
 Y=X   2 /tan  d   Eq. 9
 
     Thus, the angle of incidence ‘d’ of the twice-refracted reference beam is also determined by the distance ‘Y’. 
     A radius of the circular base of the conical prism  218  is preferably slightly larger than (approximately equal to) that of the reference beam X 1  such that the entire reference beam is irradiated onto the recording region  220   a  having the size of X 3 . 
     However, since the radius of the circular base is smaller than X 2  which is not smaller than the size of the recording region  220   a , the size of the conical prism  218 , i.e., the radius of the circular base, need not be larger than that of the recording region  220   a . Thus, the size of the conical prism  218  can be made to be smaller than X 3  so that the holographic storage system can be miniaturized. 
     When the present invention is compared with the prior art shown in  FIG. 1B , a reference beam is irradiated from a conical prism  118  of the prior art onto a holographic medium  120  after only one refraction procedure thereby, but the conical prism  218  of the present invention performs two refraction procedures therethrough. According to the prior art, a size of the reference beam is reduced due to only one refraction procedure so that sizes of the conical prism  118  and the reference beam irradiated onto the conical prism  118  should be larger than those of the present invention. However, in accordance with the present invention, a size of the reference beam is increased due to two refraction procedures so that sizes of the conical prism  218  and the reference beam become smaller than those of the prior art. Therefore, a size of the holographic storage system can be greatly miniaturized in accordance with the present invention. 
     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.