Abstract:
A mirror angle measurement and a servo control apparatus for a Holographic Digital Data Storage (HDDS) system is disclosed. The mirror angle measurement apparatus includes a first mirror having a first reflecting surface by which the reference beam is reflected toward the holographic medium, and a second reflecting surface by which a control beam is reflected to adjust the incident angle of the reference beam; a transmission unit for causing control beam to the second reflecting surface; a second mirror for returning the control beam to the second reflecting surface; and a photo detector unit for detecting the amounts of the control beam. The angle of the first mirror is adjusted based on a comparison result between the amounts of the control beam detected by the photo detector unit.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a holographic digital data storage system; and more particularly, to a mirror angle measurement and a servo control apparatus for a holographic digital data storage system. 
   BACKGROUND OF THE INVENTION 
   Recently, technology for storing holographic digital data is being advanced in various fields with the aid of the development of a semiconductor laser, a Charge Coupled Device (CCD), a Liquid Crystal Display (LCD) and the like. For example, a fingerprint recognition system for storing and reproducing fingerprints has been put to practical use. That is, the holographic digital data storage technology is adopted to the various fields in which the high storage capacity and very high data rate are required. In the Holographic Digital Data Storage (HDDS) system for storing large amounts of data, an interference pattern, generated by allowing a signal beam to interfere with a reference beam, is recorded on a holographic medium such as a photorefractive crystal, which is sensitive to the amplitude of the interference pattern. The HDDS system can display the three dimensional (3D) image of an object by using several hundred to several thousand pieces of hologram data, which are stored on the holographic medium in the form of a binary two dimensional array (referred to as a page). 
     FIG. 1  is a diagram showing the configuration of a conventional HDDS system. Referring to  FIG. 1 , the conventional HDDS system includes a light source  10 , a beam expanding lens  12 , a polarization beam splitter (PBS)  14 , a spatial light modulator (SLM)  16 , a condenser lens  18 , a first and a second mirrors  20  and  24 , a delay lens  22 , a holographic medium  26 , an objective lens  28 , and a charge coupled device (CCD)  30 . 
   A laser beam generated by the light source  10  passes through the beam expanding lens  12 , and then is divided into a reference beam and a signal beam by the PBS  14 . The signal beam is inputted to the SLM  16  and then modulated into a page of binary data (bright and dark) by the SLM  16 . Thereafter, the modulated signal beam is provided to the holographic medium  26  through the condenser lens  18 . 
   Meanwhile, the first mirror  20  reflects the reference beam, which is provided by the PBS  14 , toward the second mirror  24  via the delay lens  22 . Thereafter, the second mirror  24  adjustably reflects the reflected reference beam toward the holographic medium  26 . Herein, the angle of the second mirror  24  is adjusted by an actuator (not shown). 
   The interference pattern generated by the interference phenomenon between the signal beam transmitted from the SLM  16  and the reference beam reflected by the second mirror  24  is recorded on the recording layer of the holographic medium  26 . In this case, the interference pattern is recorded on the recording layer of the holographic medium  26  through the light-induced generation of a mobile charge in the holographic medium  26  in response to the amplitude of the interference pattern. 
   Thereafter, the conventional HDDS system reproduces data recorded on the holographic medium  26  in the following manner. In order to reproduce the data recorded on the holographic medium  26 , only the reference beam should be irradiated onto the holographic medium  26  while the signal beam is blocked. For this purpose, the signal beam provided from the PBS  14  may be blocked by closing a shutter (not shown) located between the PBS  14  and the SLM  16 , and the reference beam provided from the PBS  14  is allowed to be incident on the first mirror  20  by opening another shutter (not shown) located between the PBS  14  and the first mirror  20 . The reference beam reflected through the first and the second mirrors  20  and  24  in that order is incident on the holographic medium  26  at an incidence angle identical to that used during the recording operation. Then, the interference pattern recorded on the recording layer of the holographic medium  26  diffracts the reference beam, so that the original data, i.e., the page of binary data (check board-shaped pattern), can be reproduced. The reproduced page of binary data is provided to the CCD  30  via the objective lens  28 , and then the CCD  30  takes the image of the reproduced page to convert the image into electrical data. 
   According to the conventional HDDS system, the angle of the second mirror  24  should be adjusted to allow the reference beam to be incident on the holographic medium  26  during playback. In the conventional HDDS system, the angle of the second mirror  24  may be adjusted to be identical to that during the recording operation. However, a unit for measuring the error in the angle of the second mirror  24  and performing the servo control for compensating the error is not included in the conventional HDDS system. Therefore, the conventional HDDS system is problematic in that it is difficult to reproduce the data accurately. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a mirror angle measurement and a servo control apparatus for an HDDS system, capable of measuring the error in the angle of a mirror for allowing a reference beam to be incident on a holographic medium and performing servo control on the error by mounting a reflecting surface on the back surface of the mirror and by measuring the amount of a control beam by way of using the reflection phenomenon of the control beam between the reflecting surface of the back surface of the mirror and an auxiliary mirror facing the mirror. 
   In accordance with a preferred embodiment of the present invention, there is provided a mirror angle measurement apparatus for a Holographic Digital Data Storage (HDDS) system, the HDDS system causing a reference beam to be incident on a holographic medium to reproduce digital data recorded on the holographic medium, the mirror angle measurement apparatus including: a first mirror having a first reflecting surface on a front surface thereof by which the reference beam is reflected toward the holographic medium and a second reflecting surface on a back surface thereof by which a control beam is reflected to adjust an incident angle of the reference beam while the reference beam is incident on the holographic medium; a transmission unit for providing the control beam to the second reflecting surface; a second mirror, in case the control beam provided from the transmission unit is reflected by the second reflecting surface toward the second mirror, for returning the control beam to the second reflecting surface by reflecting the control beam; and a photo detector unit for detecting the amounts of the control beam that is reflected twice by the second reflecting surface, wherein an angle of the first mirror is adjusted based on data on the amounts of the control beam detected by the photo detector unit. 
   In accordance with another preferred embodiment of the present invention, there is provided a servo control apparatus for an HDDS system, the HDDS system causing a reference beam to be incident on a holographic medium to reproduce recorded digital data recorded on the holographic medium, the servo control apparatus including: a first mirror having a first reflecting surface on a front surface thereof by which the reference beam is reflected toward the holographic medium and a second reflecting surface on a back surface thereof by which a control beam is reflected to adjust an incident angle of the reference beam while the reference beam is incident on the holographic medium; a transmission unit for providing the control beam to the second reflecting surface; a second mirror, in case the control beam provided from the transmission unit is reflected by the second reflecting surface toward the second mirror, for returning the control beam to the second reflecting surface by reflecting the control beam; a photo detector unit for detecting the amounts of the control beam that is reflected twice by the second reflecting surface; an actuator for adjusting an angle of the first mirror; and a servo control part for operating the actuator based on data on the amounts of the control beam detected by the photo detector unit. 

   
     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  is a diagram showing the configuration of a conventional HDDS system; 
       FIG. 2  is a diagram showing the configuration of a mirror angle measurement apparatus for an HDDS system in accordance with a preferred embodiment of the present invention; 
       FIG. 3  is a diagram showing the configuration of a mirror angle measurement apparatus for the HDDS system in accordance with another preferred embodiment of the present invention; and 
       FIG. 4  is a diagram showing the configuration of a mirror angle servo control apparatus for the HDDS system in accordance with still another preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 2  is a diagram showing the configuration of a mirror angle measurement apparatus which is applied to an HDDS system in accordance with a preferred embodiment of the present invention. Referring to  FIG. 2 , the HDDS system includes a light source  100 , a beam expanding lens  102 , a first PBS  104 , an SLM  106 , a condenser lens  108 , a first and a second mirrors  110  and  114 , a delay lens  112 , a holographic medium  116 , an objective lens  118 , and a CCD  120 . Moreover, the mirror angle measurement apparatus includes a third and a fourth mirrors  122  and  128 , a second PBS  124 , a λ/4 plate  126 , a Fourier condenser lens  130 , and a Photo Detector Integrated Circuit (PDIC)  132 . In this case, the fourth mirror  128  is placed parallel to the second mirror  114 . 
   Since the beam expanding lens  102 , the first PBS  104 , the SLM  106 , the condenser lens  108 , the first mirror  110 , the delay lens  112 , the holographic medium  116 , the objective lens  118  and the CCD  120  included in the HDDS system in accordance with the present invention are identical to those of the conventional HDDS system of  FIG. 1 , the description thereof is omitted. The second mirror  114  which can be rotated to adjust a reference beam to be incident on a proper location of the holographic medium  116  is constructed in such a way that the back surface thereof as well as the front surface thereof has a reflecting surface. By the front reflecting surface of the second mirror  114 , the reference beam provided by the first mirror  110  is reflected; and by the back reflecting surface of the second mirror  114 , a control beam provided by the second PBS  124 , which is disposed on a control beam path for measuring the angle of the second mirror  114 , is reflected toward the fourth mirror  128 . 
   The mirror angle measurement apparatus for the HDDS system in accordance with the present invention will be described in more detail. 
   In the mirror angle measurement apparatus for the HDDS system in accordance with the present embodiment, the control beam with a vertical polarization generated by the light source  100  is incident on the third mirror  122  which reflects the control beam toward the second PBS  124 . The second PBS  124  reflects the control beam provided from the third mirror  122  to the λ/4 plate  126  because the control beam is vertically polarized. The λ/4 plate  126  converts the vertically polarized control beam into a circularly polarized control beam by delaying the vertically polarized control beam by λ/4, and then provides the circularly polarized control beam to the back reflecting surface of the second mirror  114 . Thereafter, the circularly polarized control beam is reflected by the back reflecting surface of the second mirror  114  toward the fourth mirror  128 , while the front reflecting surface of the second mirror  114  is used to reflect the reference beam toward the holographic medium  116  thereby recording data on the holographic medium  116 . 
   The fourth mirror  128  is disposed to be perpendicular to the circularly polarized control beam incident thereon in case the data recorded on the holographic medium  116  is reproduced by placing the second mirror  114  at an angle identical to that during the recording operation. Therefore, in case the angle of the second mirror  114  during playback is identical to that during the recording operation, the circularly polarized control beam incident on the fourth mirror  128  provided from the second mirror  114  is so reflected by the fourth mirror  128  as to return through the same beam path to the second mirror  114 . The circularly polarized control beam is reflected by the back reflecting surface of the second mirror  114  and then transmitted to the λ/4 plate  126 , which converts the circularly polarized control beam into a horizontally polarized control beam by λ/4 delay. Since the second PBS  124  has the characteristic of passing the horizontally polarized control beam therethrough, the second PBS  124  transmits the horizontally polarized control beam to the PDIC  132  via the condenser lens  130 . The PDIC  132  detects the horizontally polarized control beam transmitted through the second PBS  124  by using segmented sensor regions A and B. If the amounts of the horizontally polarized control beam detected by the two segmented sensor regions are identical each other, a servo control part (not shown in  FIG. 2 ) determines the current angle of the second mirror  114  to be correct, i.e., stops the adjustment of the angle of the second mirror  114 . In contrast, if the amounts of the horizontally polarized control beam detected by the two sensor regions A and B are not identical each other, the error in the angle of the second mirror  114  is measured to adjust it by the servo control part. 
     FIG. 3  is a diagram showing the configuration of a mirror angle measurement apparatus for the HDDS system in accordance with another preferred embodiment of the present invention. Referring to  FIG. 3 , the fourth mirror  128  of  FIG. 2  is modified into a multi-mirror  128   a  having a plurality of reflecting surfaces. For example, when the inside angles between the reflecting surfaces of the multi-mirror  128   a  are θ 1  and θ 2  (θ 1 =θ 2  or θ 1 ≠θ 2 ), the circularly polarized control beam provided from the second mirror  114  can be reflected at different angles by the reflecting surfaces of the multi-mirror  128   a . Accordingly, the position of the multi-mirror  128   a  may be adjusted in response to the angle of the second mirror  114 , until the circularly polarized control beam reflected by the back reflecting surface of the second mirror  114  toward the multi-mirror  128   a  is perpendicular to the reflecting surface of the multi-mirror  128   a . Thereafter, the amounts of the horizontally polarized control beam transmitted through the second PBS  124  are detected by the two segmented sensor regions in the PDIC  132  to determine whether the angle of the second mirror  114  during playback is identical to that during the recording operation. 
     FIG. 4  is a diagram showing the configuration of a servo control apparatus for the HDDS system in accordance with a still another preferred embodiment of the present invention. Referring to  FIG. 4 , the mirror angle servo control apparatus, including a servo control part  134  and an actuator  136 , is added to the mirror angle measurement apparatus for the HDDS system of  FIG. 2 . 
   In the servo control apparatus, the two pieces of data on the amounts of the horizontally polarized control beam detected by the segmented sensor regions A and B of the PDIC  132  are transmitted to the servo control part  134 . The servo control part  134  compares the amounts of the horizontally polarized control beam with each other, and then performs servo control on the actuator  136  for adjusting the angle of the second mirror  114  in response to a comparison result. That is, if the amounts of the horizontally polarized control beam detected by the two segmented sensor regions of the PDIC  132  are identical each other, the servo control part  134  determines that the angle of the second mirror  114  has been correctly adjusted so that the actuator  136  is not operated. In contrast, if the amounts of the horizontally polarized control beam detected by the two segmented sensor regions of the PDIC  132  are not identical each other, the servo control part  134  determines that the angle of the second mirror  114  has not been correctly adjusted so that the actuator  136  is operated to adjust the angle of the second mirror  114 . 
   Thereafter, to confirm whether or not the angle of the second mirror  114  has been correctly adjusted, the control beam generated by the light source  100  is again incident on the back reflecting surface of the second mirror  114  via the third mirror  122 , the second PBS  124 , and λ/4 plate  126  in that order, and then reflected toward the fourth mirror  128  by the second mirror  114 , and then reflected to the back reflecting surface of the second mirror  114  by the fourth mirror  128 , and then transmitted from the back reflecting surface of the second mirror  114  to the PDIC  132  via the λ/4 plate  126 , the second PBS  124  and the condenser lens  132  in that order. The two pieces of data on the amounts of the horizontally polarized control beam detected by the PDIC  132  are transmitted to the servo control part  134 , and then the servo control part  134  compares the amounts of the horizontally polarized control beam with each other, thereby controlling the operation of the actuator  136 . That is to say, the servo control part  134  adjusts the angle of the second mirror  114  by operating the actuator  136  until the two pieces of data on the amounts of the horizontally polarized control beam become identical each other, thereby allowing the reference beam to be incident on the proper location of the holographic medium  116 . 
   As described above, in accordance with the present invention, the back reflecting surface is disposed on the back surface of the second mirror  114 , the front reflection surface of the second mirror  114  being used to allow the reference beam to be incident on the holographic medium  116 . Moreover, the mirror angle measurement apparatus detects the amounts of the control beam that has experienced the reflection phenomenon between the back reflecting surface of the second mirror  114  and the fourth mirror  128  facing the second mirror  114 , thereby measuring the error in the angle of the second mirror  114 . 
   Furthermore, the present invention is provided with the servo control apparatus for controlling the operation of the actuator  136  to adjust the angle of the second mirror  114 . 
   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.