Abstract:
A holographic ROM system includes an alignment apparatus for aligning a holographic medium for storing data and a mask with patterns of the data. The alignment apparatus has a beam irradiating unit for irradiating a light beam to alignment marks of the holographic medium and the mask; an alignment mechanism for moving at least one of the holographic medium and the mask in response to a control signal; a photo detecting unit for detecting the beam passing through the alignment marks of the holographic medium and the mask while the holographic medium and/or the mask being moved; and a control unit for generating the control signal, the control unit controlling the alignment mechanism based on intensity of the beam detected by the photo detecting unit.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a holographic ROM (read-only memory) system; and, more particularly, to a holographic ROM system including an alignment apparatus capable of automatically and precisely aligning a holographic medium for storing data and a mask with patterns of the data. 
   BACKGROUND OF THE INVENTION 
   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 holographic ROM system such as shown in  FIG. 5  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 holographic ROM system in  FIG. 5  includes a light source  1 , HWPs (half wave plates)  2 ,  12 , an expanding unit  4 , a PBS (polarizer beam splitter)  6 , polarizers  8 ,  14 , mirrors  10 ,  16 , a mask  22 , a holographic medium  20 , and a conical mirror  18 . 
   The light source  1  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  2 . The HWP  2  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  4  for expanding the beam size of the laser beam up to a predetermined size. Thereafter, the expanded laser beam is provided to the PBS  6 . 
   The PBS  6 , 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  6  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  8 , 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  10 . Thereafter, the reflected signal beam is projected onto the holographic medium  20  via the mask  22 . The mask  22 , 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  12 . The HWP  12  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  14 , wherein the polarization of the reference beam is more purified. And the reference beam with perfect polarization is reflected by the mirror  16 . Thereafter, the reflected reference beam is projected onto the conical mirror  18  (the conical mirror  18  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  20  by the conical mirror  18 . The incident angle of the reflected reference beam on the holographic medium  20  is determined by the base angle of the conical mirror  18 . 
   The holder for fixing the conical mirror  18  should be installed on the bottom side of the conical mirror  18 , 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  18 , it is usually installed through a center opening  24  of the holographic medium  20 . 
   The holographic medium  20  is a disk-shaped material for recording the data patterns. The mask  22  provides the data patterns to be stored in the holographic medium  20 . By illuminating the mask  22  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  20 . 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  18  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. 
   Meanwhile, in order to record holographic data in the holographic medium  20  (hereinafter, also referred to as “disk”), it is required to precisely align the mask with the disk. In a conventional method of aligning the disk and the mask, an operator directly observes alignment marks formed thereon by using a high multiple microscope and an illuminating device. 
   However, in such a conventional method, the productivity of the holographic ROM system is decreased since the operator should align the disk and the mask while directly observing the alignment marks thereof with his/her eyes through the microscope. Further, if the operator is not a skilled person, there is high likelihood of a misalignment of the disk and the mask. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a holographic ROM system including an alignment apparatus for aligning a holographic medium for storing therein data and a mask with patterns of the data, and a method for aligning the holographic medium and the mask, wherein the holographic medium and the mask are automatically and precisely aligned. 
   In accordance with an aspect of the present invention, there is provided a holographic ROM system including: an alignment apparatus for aligning a holographic medium for storing data and a mask with patterns of the data, the alignment apparatus having a beam irradiating unit for irradiating a light beam to alignment marks of the holographic medium and the mask; an alignment mechanism for moving at least one of the holographic medium and the mask in response to a control signal; a photo detecting unit for detecting the beam passing through the alignment marks of the holographic medium and the mask while the holographic medium and/or the mask being moved; and a control unit for generating the control signal, the control unit controlling the alignment mechanism based on intensity of the beam detected by the photo detecting unit. 
   In accordance with another aspect of the present invention, there is provided a method for aligning a holographic medium for storing data and a mask with patterns of the data in a holographic ROM system, the method comprising the steps of: irradiating a light beam to alignment marks of the holographic medium and the mask while moving the holographic medium and the mask; detecting intensity of the beam passing through the alignment marks of the holographic medium and the mask; and locating the holographic medium and the mask based on the intensity of the beam detected. 

   
     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 schematic diagram of an alignment apparatus in accordance with a preferred embodiment of the present invention; 
       FIG. 2  describes an exploded view of an alignment mechanism of the alignment apparatus; 
       FIG. 3  represents an explanatory view for explaining an operation of the alignment apparatus; 
       FIG. 4A  is a graph showing intensity of a beam detected by a photodetector while a mask support is moved in the X-direction; 
       FIG. 4B  is a graph showing intensity of a beam detected by the photodetector while a disk support is moved in the Y-direction; and 
       FIG. 5  sets forth a schematic view of a holographic ROM system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, a preferred embodiment of the present invention will be described with reference to the accompanying drawings. 
     FIG. 1  shows a schematic diagram of an alignment apparatus for aligning a holographic medium for storing data and a mask with patterns of the data, for use in a holographic ROM system, in accordance with the preferred embodiment of the present invention. The basic constitutions of the holographic ROM system are substantially identical to those described in the Background of the Invention with reference to  FIG. 5 ; therefore, detailed descriptions thereof will be omitted. 
   The alignment apparatus includes a light source  102 , a focusing lens  104 , an optical fiber  106 , an alignment mechanism  108 , a photodetector  110  and a control unit  112 . Reference numeral  108   a  is a disk-shaped holographic medium (hereinafter, also referred to as “disk”) with an alignment mark  108   a   1 , and reference numeral  108   b  is a mask with an alignment mark  108   b   1 . The mask  108   b  has, e.g., slit patterns corresponding to data to be stored in the holographic medium or disk  108   a.  In a recording process of the holographic ROM system, a signal beam is projected onto the disk  108   a  via the mask  108   b  while a reference beam is incident onto the disk  108   a  from the opposite side to record holograms in the reflection geometry so that an interference pattern is recorded in the disk  108   a.    
   Referring to  FIG. 1 , the light source  102  emits a beam that is used to align the disk  108   a  and the mask  108   b,  and the beam is focused through the focusing lens  104  onto the optical fiber  106 . The optical fiber  106  is coupled to, e.g., a center opening serving as the alignment mark  108   a   1  of the disk  106 . 
   The beam transferred by the optical fiber  106  passes through the respective alignment marks  108   a   1 ,  108   b   1  of the disk  108   a  and the mask  108   b,  which are held in parallel and spaced from each other by the alignment mechanism  108 , to be detected by the photodetector  110 . 
   The alignment mechanism  108  in accordance with the preferred embodiment of the present invention will now be described with reference to  FIG. 2 . The disk  108   a  is mounted on a disk support  1083  via a disk damper  1082  and the mask  108   b  is mounted on a mask support  1084 . A stationary plate  1110  is interposed between the disk support  1083  and the mask support  1084 . The mask support  1084  is movably coupled to one side of the stationary plate  1110  through a pair of X-directional guides  1084   a  and can be moved along the X-directional guides  1084   a  by an X-drive unit (e.g., actuator)  1081  within a predetermined range (X-directional beam scan range). The disk support  1083  is movably coupled to the other side of the stationary plate  1110  through a pair of Y-directional guides  1083   a  and can be moved along the Y-directional guides  1083   a  by a Y-drive unit (e.g., actuator)  1085  within a predetermined range (Y-directional beam scan range). At this time, X-directional movement and Y-directional movement can be made simultaneously or sequentially. 
   Referring now to  FIG. 3 , the Y-drive unit  1085  drives the disk support  1083  in the Y direction in response to a control signal CS 2  from the control unit  112 , and the X-drive unit  1081  drives the mask support  1084  (not shown in  FIG. 3 ) in the X direction in response to a control signal CS 1  from the control unit  112 . Specifically, while the light beam is irradiated onto the alignment mark  108   b   1  of the mask  108   b  through the optical fiber  106  coupled to the center opening or the alignment mark  108   a   1  of the disk  108   a,  the mask support  1084  is moved by the X-drive unit  1081  within the X-directional beam scan range and the disk support  1083  is moved by the Y-drive unit  1085  within the Y-directional beam scan range. 
   In this embodiment, the Y and X directional beam scan ranges of the disk  108   a  and the mask  108   b  which are respectively moved in the Y and X directions together with the disk support  1083  and the mask support  1084  are each about 1 mm. The beam scan ranges may be properly selected as desired. 
   Meanwhile, the photodetector  110  detects the beam passing through the alignment marks  108   a   1 ,  108   b   1  when the mask  108   b  and the disk  108   a  are respectively moved in X and Y directions in response to the driving control signals CS 1 , CS 2  from the control unit  112 , and the control unit  112  monitors the intensity of the beam detected by the photodetector  110  and stops the disk  108   a  and the mask  108   b  at a position where the greatest intensity of the beam is observed. The greatest intensity of the beam means that the alignment marks  108   a   1 ,  108   b   1  are most precisely aligned with each other. Although one alignment mark  108   b   1  is provided on the mask  108   b  in this embodiment, two or more alignment marks  108   b   1  may be provided thereon. In case two or more alignment marks  108   b   1  are provided and two or more optical fibers are provided correspondingly, the sum of the intensity of the beam passing through each of the alignment marks  108   b   1  will be increased compared with the case of one alignment mark. 
   Further, the control unit  112  controls the X-drive unit  1081  to move the mask support  1084  in the X direction within the X-directional beam scan region in response to an operational signal of a user (an automatic alignment operation signal for the disk and the mask) and stop the mask support  1084  at a position where the greatest intensity of the beam is observed. For example, as shown in  FIG. 4A , considering the X-directional beam scan region as x 0 –x 1 , the mask support  1084  is stopped at the position x where the greatest beam intensity is detected. 
   Then, the control unit  112  controls the Y-drive unit  1085  to move the disk support  1083  in the Y direction within the Y-directional beam scan region and stop the disk support  1083  at a position where the greatest intensity of the beam is detected. For example, as shown in  FIG. 4B , considering the Y-directional beam scan region as y 0 –y 1 , the disk support  1083  is stopped at the position y where the greatest beam intensity is detected. 
   As described above, in this embodiment, the control unit  112  sequentially controls the mask support  1084  and the disk support  1083  to move the mask  108   b  and the disk  108   a,  respectively. However, the control unit  112  may control first the disk support  1083  to move the disk  108   a,  and then the mask support  1084  to move the mask  108   b . Alternatively, the control unit  112  may simultaneously control the mask support  1084  and the disk support  1083  to move the mask  108   b  and the disk  108   a,  respectively. In addition, although the disk  108   a  and the mask  108   b  are moved in this embodiment, only one of them may be moved for the alignment thereof. In this case, the control unit  112  may controls the mask support  1084  or the disk support  1083  to move. 
   Moreover, a monitor (not shown) may be provided to display the intensity of the beam detected by the photodetector  110  so that a user can manipulate the X-drive unit and Y-drive unit while directly observing the intensity of the beam passing through the alignment marks of the disk and the mask. 
   In accordance with the present invention, an automatic and precise alignment of the disk and the mask can be achieved without a microscope and an illumination device used in the conventional alignment process, thereby increasing the productivity of the holographic ROM system and preventing misalignment between the mask and the disk. 
   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.