Patent Publication Number: US-7593304-B2

Title: Apparatus and method for recording and reproducing hologram, and spatial light modulator therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the priority Japanese Patent Application 2004-285491, filed on Sep. 29, 2004; the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical recording/reproducing technique for recording and reproducing information with the use of light, and more particularly to an apparatus and a method of recording and reproduction with the use of holography, and a spatial light modulator therefor. 
   2. Description of the Related Art 
   Conventionally, an optical recording/reproducing apparatus that records information with the use of light is utilized for recording and reproducing a large volume of data such as a high-density image data. Such optical recording/reproducing apparatuses which are already put into practical use are, for example, a magneto-optical recording/reproducing apparatus for a medium such as a Magneto Optical Disk (MO), and a phase-change optical recording/reproducing apparatus for a medium such as a Compact Disk Rewritable (CD-RW), a Digital Versatile Disk Random Access Memory (DVD-RAM). 
   In recent years, among the optical recording/reproducing apparatuses, a hologram recording/reproducing apparatus draws a particular attention as an apparatus capable of providing a further improvement in information recording density. In the hologram recording/reproducing apparatus, in general, information to be recorded is provided to an information beam as a two-dimensional pattern, and the information beam is interfered with a reference beam in an optical recording medium for recording of the information as an interference pattern (hologram). For reproduction, the reference beam alone is applied onto the recorded interference pattern in the same arrangement as at the recording, to retrieve the information as a diffraction image from the hologram. Thus, in the hologram recording/reproducing apparatus, information is recorded and/or reproduced as a two-dimensional pattern, whereby high-speed recording and reproduction of large-volume information can be achieved. 
   In view of further improvement in information recording density, various types of hologram recording/reproducing apparatuses are proposed. One example is a hologram recording/reproducing apparatus of volume hologram type. The volume hologram type hologram recording/reproducing apparatus is provided with an optical recording medium whose thickness is sufficiently larger than a wavelength of light, in order to allow recording of various interference patterns in a thickness direction as well as in a plane direction of the optical recording medium. Hence, the interference pattern can be three-dimensionally recorded in the optical recording medium. In other words, information can be recorded in the same region of the optical recording medium in a multiplexing manner, to increase the storage capacity. 
   Another proposed hologram recording/reproducing apparatus is a shift multiplexing type. In the shift multiplexing type hologram recording/reproducing apparatus, position of irradiation of the reference beam at the information reproduction is slightly shifted from those at the recording. Then, even when the recorded interference pattern is irradiated with the reference beam, due to the lack of phase matching between the reference beam and the interference pattern, the diffraction pattern cannot be obtained. When the reference beam is maintained in the position where the diffraction pattern is not obtained and further recording of interference pattern with another information beam is performed, a plurality of two-dimensional information can be recorded in a multiplex manner in the same recording area of the optical recording medium depending on the arrangement of the reference beam. 
   Still another proposed hologram recording/reproducing apparatus uses a spatially modulated reference beam. In a simple hologram recording/reproducing apparatus, a reference beam with an in-phase plane wave is used. In this hologram recording/reproducing apparatus, however, a spatially modulated reference beam is used. The recorded interference pattern is complicated and the phase matching condition for the reference beam and the interference pattern is strict. Hence, the higher recording multiplicity is achievable (Japanese Patent Laid-Open Publication No. 2002-123949, for example, discloses a hologram recording/reproducing apparatus employing a holography and using a recording reference beam whose phase is spatially modulated.). In a recently-proposed hologram recording technique, one spatial light modulator generates both the information beam and the modulated reference beam for the hologram recording (see, for example, Hideyoshi Horimai and Jun Li, “A novel Collinear optical Setup for Holographic data Storage System,” Technical Digest of Optical Data Storage Topical Meeting 2004, pp. 258-260). 
   In the conventional hologram recording/reproducing apparatuses as described above, however, the arrangement of the optical elements inside the apparatus, and a subtle fluctuation in the position of the optical recording medium with respect to the apparatus, for example, significantly affect the recording/reproduction, and the portability of the optical recording medium, and the compatibility among the apparatuses or the like are difficult to enhance. At insertion and removal of the optical recording medium to and from the hologram recording/reproducing apparatus, for example, sometimes a minute misalignment between the optical recording medium and the hologram recording/reproducing apparatus occurs. Such misalignment may cause reproduction error. Specifically in the spatial modulating type hologram recording/reproducing apparatus, the strict phase matching condition between the reference beam and the interference pattern makes the problem of the misalignment of the optical elements even more notable. 
   The conventional hologram recording/reproducing apparatus in general includes an optical recording medium, a recording/reproducing optical system that irradiates the optical recording medium with an information beam and/or a reference beam, and a spatial light modulator that generates the information beam and the reference beam. Hence, in order to improve the reproducibility at the irradiation of the optical recording medium with the reference beam, the misalignment between the optical recording medium and the spatial light modulator must be corrected. In the conventional technique, however, only a unit for correcting the misalignment between the optical recording medium and the recording/reproducing optical system is proposed and the correction of misalignment between the optical recording medium and the spatial light modulator is not possible. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, a hologram recording/reproducing apparatus includes a recording/reproducing optical system that guides at least one of an information beam and a reference beam to an optical recording medium; a spatial light modulator that is arranged in an optical path of the recording/reproducing optical system, and spatially modulates a beam guided via the recording/reproducing optical system to generate the information beam; a first misalignment detecting unit that detects a first misalignment between the recording/reproducing optical system and the spatial light modulator using a beam for detecting the first misalignment; and a first misalignment correcting unit that corrects the first misalignment based on the first misalignment detected by the first misalignment detecting unit. 
   According to another aspect of the present invention, a spatial light modulator is arranged in an optical path of a recording/reproducing optical system that guides at least one of an information beam and a reference beam to an optical recording medium, and spatially modulates a beam guided via the recording/reproducing optical system to generate the information beam. The spatial light modulator includes a diffracting unit that diffracts a beam for detecting a first misalignment between the spatial light modulator and the recording/reproducing optical system. 
   According to still another aspect of the present invention, a hologram recording/reproducing method is of correcting a first misalignment between a recording/reproducing optical system and a spatial light modulator in a hologram recording/reproducing apparatus that includes the recording/reproducing optical system guiding at least one of an information beam and a reference beam to an optical recording medium, and the spatial light modulator arranged in an optical path of the recording/reproducing optical system and spatially modulates a beam guided via the recording/reproducing optical system to generate the information beam. The hologram recording/reproducing method includes irradiating the spatial light modulator with a beam for detecting a first misalignment via the recording/reproducing optical system; receiving the beam for detecting the first misalignment, the beam being diffracted by the spatial light modulator; detecting the first misalignment based on a state of the received beam for detecting the first misalignment; and correcting the first misalignment based on the detected first misalignment. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an optical recording medium employed in a first embodiment; 
       FIG. 2  is a diagram of an overall structure of a hologram recording/reproducing apparatus according to the first embodiment; 
       FIG. 3  is a diagram of a modulation pattern of a spatial light modulator at information recording; 
       FIG. 4  is a diagram of a modulation pattern of the spatial light modulator at information reproduction; 
       FIG. 5  is a vertical sectional view of a spatial light modulator according to the first embodiment; 
       FIG. 6  is a plane view of the spatial light modulator according to the first embodiment; 
       FIG. 7  is a perspective view of a digital micro mirror device; 
       FIG. 8  is a vertical sectional view of a pixel in an ON state; 
       FIG. 9  is a vertical sectional view of a pixel in an OFF state 
       FIG. 10  is a block diagram of a first correction circuit; 
       FIG. 11  is a block diagram of a second correction circuit; 
       FIG. 12  is a perspective view of an optical recording medium employed in a second embodiment; and 
       FIG. 13  is a diagram of an overall structure of a hologram recording/reproducing apparatus according to the second embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Exemplary embodiments of a hologram recording/reproducing apparatus according to the present invention will be explained in detail below with reference to the accompanying drawings. 
   A first embodiment is explained. In the first embodiment, the idea of the present invention is applied to a hologram recording/reproducing apparatus of a reflective colinear interferometry. Similarly to the conventional apparatus, a hologram recording/reproducing apparatus according to the embodiment includes an optical recording medium, a recording/reproducing optical system that irradiates the optical recording medium with an information beam and/or a reference beam for recording/reproduction of the information on the optical recording medium, a spatial light modulator (SLM) that generates the information beam and the reference beam to be employed in the recording/reproducing optical system, and a second correction optical system that corrects misalignment between the optical recording medium and the recording/reproducing optical system (a second misalignment) (hereinafter referred to as a second correction as necessary). Further, the hologram recording/reproducing apparatus according to the embodiment, as a different feature from the conventional apparatus, includes a first correction optical system that corrects misalignment between the recording/reproducing optical system and the spatial light modulator (a first misalignment) (hereinafter referred to as a first correction as necessary). The hologram recording/reproducing apparatus of the embodiment can correct the misalignment between the optical recording medium and the spatial light modulator based on the recording/reproducing optical system with the use of the first correction optical system and the second correction optical system. 
   First, a structure of an optical recording medium  1  employed in the hologram recording/reproducing apparatus having the above-described features is explained.  FIG. 1  is a perspective view of the optical recording medium  1  (a side part of the optical recording medium  1  is shown to be broken). The optical recording medium  1  may be formed in any planar shape, such as a disk shape, a card shape, or a block shape. The optical recording medium  1  shown in  FIG. 1  is formed in a disk-shape. The optical recording medium  1  includes a protective layer  2 , a recording layer  3 , a gap layer  4 , a dichroic reflective layer  5 , a transparent substrate  6 , and a reflective layer  7  laminated in this order from a side of beam incidence from the hologram recording/reproducing apparatus. The optical recording medium  1  corresponds to an optical recording medium in the appended claims. 
   The protective layer  2  serves to protect a surface of the recording layer  3 . The recording layer  3  serves to record information through reception of irradiated information beam and reproduction beam from the hologram recording/reproducing apparatus. Hereinbelow, the information beam and the reproduction beam are collectively referred to as a recording beam as necessary. The recording layer  3  is formed from a material which optical characteristics such as an absorption and refractive index change according to the intensity of the irradiated recording beam, i.e. intensity of an electromagnetic wave. The gap layer  4  serves to prevent the recording of the hologram in an area where the light intensity of the recording beam is particularly high, and is formed from a material which transmits the recording beam with high intensity and does not mix with the recording material of the recording layer  3 . The dichroic reflective layer  5  is a selective reflective layer which wavelengths of reflection and transmission are determined so as to reflect the recording beam irradiated from a recording/reproducing beam source  21  explained later and to transmit a second correction beam irradiated from a second correction beam source  71  explained later. The dichroic reflective layer  5  is formed from a dichroic multi-layer film composed of, for example, SiO 2 , TiO 2 , NbO 3 , and CaF 2 . 
   The transparent substrate  6  serves to transmit the second correction beam that is incoming via the dichroic reflective layer  5  and is formed from a material such as a glass, and polycarbonate. The reflective layer  7  serves to reflect the second correction beam transmitted through the dichroic reflective layer  5  and the transparent substrate  6  and is formed from a material having a high reflective index for the wavelength of the second correction beam, for example, aluminum. Here, though not shown in the drawing, a surface of the reflective layer  7  on the side of the transparent substrate  6  is formed to be an irregular surface where tracking information and address information are recorded. Here, the tracking can be performed in any manners and a continuous rotation servo system or a sampled servo system can be used. The tracking information can be recorded as a wobble pit, for example. When the optical recording medium  1  having the above-described structure, is irradiated with the recording beam via an objective lens  30  which is a part of the hologram recording/reproducing apparatus, the information beam and the reference beam interfere in the recording layer  3 , thereby forming a hologram  8 . 
   A structure of a hologram recording/reproducing apparatus  10  for recording and reproduction of the information on the optical recording medium  1  having the above-described structure is explained.  FIG. 2  is a diagram of an overall structure of the hologram recording/reproducing apparatus  10  according to the embodiment. The hologram recording/reproducing apparatus  10  serves to perform recording, reproduction or the like of the information on the optical recording medium  1  and includes a recording/reproducing optical system  20 , a spatial light modulator  40 , a first correction optical system  50 , and a second correction optical system  70 . A component which has a known optical function corresponding to the name and can be structured according to a known technique is not specifically explained. 
   The recording/reproducing optical system  20  serves to record and reproduce the information and includes a recording/reproducing beam source  21 , a beam expander  22 , a mirror  23 , an imaging lens  24 , a mirror  25 , an imaging lens  26 , a polarizing beam splitter  27 , a dichroic prism  28 , a waveplate  29  for optical rotation, an objective lens  30 , an imaging lens  31 , and a two-dimensional photodetector  32 . The recording/reproducing optical system  20  corresponds to a recording/reproducing optical system in the appended claims. 
   Here, the recording/reproducing beam source  21  is a light source used for recording and reproduction. Hereinbelow, the light beam which is emitted from the recording/reproducing beam source  21  and not subjected to the modulation by the spatial light modulator  40  yet is referred to as a recording/reproducing beam as necessary. A laser which has a suitable coherence that allows the acquisition of interference pattern can be used as such light source, for example, a linearly-polarized laser such as a laser diode, a He—Ne laser, an Argon ion laser, and a YAG laser can be employed. The beam expander  22  is a shaping unit that shapes the recording/reproducing beam emitted from the recording/reproducing beam source  21  into a collimated beam and is formed with a pair of lenses as shown. The imaging lens  24  serves to focus the recording/reproducing beam on the spatial light modulator  40  and corresponds to “an optical element that is arranged in a closest proximity to the spatial light modulator” in the appended claims. 
   The polarizing beam splitter  27  serves to transmit the recording beam as well as to reflect the reproducing beam. The polarizing beam splitter  27  is formed, for example, from a prism having dielectric layers. The dichroic prism  28  serves to transmit the recording beam and the reproducing beam while reflecting the first correcting beam and the second correcting beam explained later. The dichroic prism  28  is formed, for example, as a prism coated with metal thin layers or dielectric layers that transmits a beam with a certain wavelength. The dichroic prism  28  corresponds to “a common optical element in the recording/reproducing optical system” in the appended claims. The waveplate  29  serves to rotate the polarization plane of the light passing through the waveplate  29 , and can be formed with a ¼ waveplate or a ½ waveplate. The two-dimensional photodetector  32  is a photoelectric converter that converts the reproducing beam into electrical signal and can be formed with, for example, a charge coupled device (CCD) array. 
   A structure of the spatial light modulator  40  is explained next. The spatial light modulator  40  simultaneously generates the information beam to which information is given as a two-dimensional pattern and the spatially modulated reference beam from the recording/reproducing beam irradiated via the recording/reproducing optical system  20 . The spatial light modulator  40  corresponds to a spatial light modulator in the appended claims. 
   The spatial light modulator  40  schematically is formed with a plurality of pixels  41  arranged in a two-dimensional matrix shape as shown in  FIGS. 3 and 4  and generates the information beam and the reference beam by changing the direction of the recording/reproducing beam for every pixel  41 , or by changing the polarizing direction of the recording/reproducing beam for every pixel  41 . In a modulation pattern formed with the pixels  41 , an area near the center of optical axis is an information beam area  42  for the generation of the information beam and an area in periphery is a reference beam area  43  for the generation of the reference beam. Only the reference beam area  43  for the generation of the reference beam is shown in  FIG. 4 . 
   Details of the structure of the spatial light modulator  40  are explained.  FIG. 5  is a vertical section of the spatial light modulator  40  and  FIG. 6  is a plane view of the spatial light modulator  40 . The spatial light modulator  40  is formed with a Digital Micro-mirror Device (DMD)  46  placed in a space surrounded by a housing  44  and a transparent window  45 . The DMD  46 , as shown in a schematic perspective of  FIG. 7 , is formed with multiple pixels (micro-mirror)  41  which are coupled with each other via hinges  47  and arranged two-dimensionally on an upper surface of the substrate  48  of  FIG. 5 . 
   Each pixel  41  can be electrically controlled independent of each other and rotatable around the hinge  47 .  FIG. 8  is a vertical section of the pixel  41  in an ON state and  FIG. 9  is a vertical section of the pixel  41  in an OFF state. The pixel  41 , in a reference state, is horizontally arranged with respect to the substrate  48 . In the ON state shown in  FIG. 8 , the pixel  41  can rotate by +10 degrees, while in the OFF state shown in  FIG. 9 , the pixel  41  can rotate by −10 degrees. Hence, when an incoming beam L IN  is incident on the substrate  48  from the same direction, a reflective beam L OUT1  in the ON state forms an angle of 40 degrees with a reflective beam L OUT2  in the OFF state. In  FIGS. 7 to 9 , the pixel  41  in the OFF state is denoted as a pixel  41   a , whereas the pixel  41  in the ON state is denoted as a pixel  41   b . In  FIGS. 8 and 9  the optical axis L AXIS  of the pixel  41  is denoted by a dotted line. Thus, the electrical switching of the state of the pixel  41  between ON and OFF according to the information to be recorded on the optical recording medium  1  allows the formation of the modulation pattern as shown in  FIGS. 3 and 4 , and through the adjustment of the direction of reflection of the recording/reproducing beam according to the information, the information beam and the reference beam can be spatially modulated. 
   Returning to  FIGS. 5 and 6 , the transparent window  45  is provided with an alignment mark M. The alignment mark M is formed substantially in a shape of a planar cross where two tracks T 1  and T 2  run approximately perpendicular with each other. Tracks T 1  and T 2  may be formed in any manner. For example, the upper surface of the transparent window  45  may be cut to the thickness of approximately 1 μm with a diamond dicer to form the tracks T 1  and T 2 . The alignment mark M corresponds to a diffracting unit in the appended claims. With the alignment mark M in the spatial light modulator  40 , the first correction beam incident on the spatial light modulator  40  can be diffracted at the alignment mark M and the first correction can be performed with the diffracted light beam. 
   The tracks T 1  and T 2  can be formed in any direction as far as the first correction is possible. In the embodiment, the tracks are formed along two directions approximately perpendicular to the optical axis of the spatial light modulator  40  so that these two directions can be defined. Specifically, the transparent window  45  with the alignment mark M is fixed to the housing  44  so that the center of the DMD  46  on the plane corresponds with the center of the alignment mark M (a point where tracks T 1  and T 2  cross), and the directions of arrangement of the pixels  41  in DMD  46  correspond with the directions of tracks T 1  and T 2 . Hence, the misalignment in two directions approximately perpendicular to the optical axis of the spatial light modulator  40  can be detected and the first correction is allowed with the movement of the spatial light modulator  40 , the imaging lens  24 , or the like in the same direction. 
   A structure of the first correction optical system  50  is explained. In  FIG. 2 , again, the first correction optical system  50  is a first correcting unit that serves to correct the position of the spatial light modulator  40  in the recording/reproducing optical system  20 , and includes a first correcting beam source  51 , a collimate lens  52 , a polarizing beam splitter  53 , a waveplate  54  for an optical rotation, a condenser lens  55 , a convex lens  56 , a cylindrical lens  57 , a quarter dividing photodetector  58 , a first correction circuit  59 , a voice coil motor  60 , and a piezoactuator  61 . 
   The first correcting beam source  51  is a light source of a beam for the first correction (hereinafter also referred to as a first correcting beam as necessary), and a linearly-polarized laser may be employed, for example. Specifically, a laser diode, a He—Ne laser, an Argon ion laser, a YAG laser or the like can be used. The first correcting beam desirably has a different wavelength from the wavelength of the recording/reproducing beam emitted from the recording/reproducing beam source  21 . For example, when the recording/reproducing beam source  21  employs a laser of approximately 532 nm in wavelength, the first correcting beam source  51  preferably employs a red laser diode of approximately 650 nm in wavelength. The first correcting beam and the second correcting beam may be of the same wavelength, and the first correcting beam source  51  can employ a laser of a same wavelength as a laser used in the second correcting beam source  71  explained later. 
   The collimate lens  52  is a collimated beam generating unit that converts the first correcting beam into a collimated beam. The polarizing beam splitter  53  serves to transmit the first correcting beam emitted from the first correcting beam source  51  and to reflect the first correcting beam reflected by the spatial light modulator  40 . The waveplate  54  serves to rotate a plane of polarization of a beam passing through the waveplate  54 . The waveplate  54  may be formed with a ¼ waveplate or a ½ waveplate, and the ¼ waveplate is more preferable for its good transmission efficiency. The quarter dividing photodetector  58  is a photoelectric converter having beam receivers  58   a  to  58   d  (not shown in  FIG. 2 ) as four separate parts, receives the first correcting beam reflected by the spatial light modulator  40 , and outputs an output signal according to the misalignment between the recording/reproducing optical system  20  and the spatial light modulator  40  (hereinafter referred to as a misalignment signal as necessary). 
   The first correction circuit  59  is a control circuit for the first correction that receives the misalignment signal output from the quarter dividing photodetector  58  to calculate an amount of movement of the spatial light modulator  40  and the imaging lens  24  necessary for the first correction. Further, the first correction circuit  59  outputs a driving signal to the voice coil motor  60  and the piezoactuator  61  for the movement control corresponding to the calculated amount of movement. The first correction circuit  59  is shown in block form in  FIG. 10 . As shown in  FIG. 10 , the first correction circuit  59  includes an adder  59   a  that adds outputs from beam receivers  58   a  and  58   d  located at one opposing corners of the quarter dividing photodetector  58 , an adder  59   b  that adds outputs from beam receivers  58   b  and  58   c  located at another opposing corners of the quarter dividing photodetector  58 , a subtractor  59   c  that calculates a difference between the output from the adder  59   a  and the output from the adder  59   b  to generate a focus error signal FE 1  by astigmatism method, an adder  59   d  that adds outputs from beam receivers  58   a  and  58   b  located next to each other in a lateral direction of the quarter dividing phototdetector  58 , an adder  59   e  that adds outputs from beam receivers  58   c  and  58   d  that located next to each other in a lateral direction (a Y direction in  FIG. 2 ) of the quarter dividing photodetector  58 , a subtractor  59   f  that calculates a difference between the output from the adder  59   d  and the output from the adder  59   e  to generate a tracking error signal TE 1  by push-pull method, an adder  59   g  that adds outputs from beam receivers  58   b  and  58   d  located next to each other in a longitudinal direction (a direction perpendicular to X direction and Y direction in  FIG. 2 ) of the quarter dividing photodetector  58 , an adder  59   h  that adds outputs from beam receivers  58   a  and  58   c  located next to each other in the longitudinal direction of the quarter dividing photodetector  58 , and a subtractor  59   i  that calculates a difference between the output from the adder  59   g  and the output from the adder  59   h  to generate a tracking error signal TE 2  by push-pull method. 
   Returning to  FIG. 2 , again, the voice coil motor  60  is a moving unit that moves the imaging lens  24  in three directions, i.e. two directions perpendicular to the optical axis direction of the imaging lens  24  (X direction and Y direction in  FIG. 2 ) and the direction of the optical axis (a direction perpendicular to X and Y directions in  FIG. 2 ) for the first correction. The piezoactuator  61  is a moving unit that moves the spatial light modulator  40  in three directions, i.e. two directions perpendicular with each other in a modulation plane of the spatial light modulator  40  (X and Y directions in  FIG. 2 ) and a direction of the perpendicular line of the modulation plane (a direction perpendicular to X and Y directions in  FIG. 2 ) for the first correction. 
   The first correcting beam source  51 , the collimate lens  52 , the polarizing beam splitter  53 , the waveplate  54 , the condenser lens  55 , the convex lens  56 , the cylindrical lens  57 , and the quarter dividing photodetector  58  correspond to a first misalignment detecting unit in the appended claims. More specifically, the first correcting beam source  51  corresponds to a light source, and the quarter dividing photodetector corresponds to a beam receiving unit. The first correction circuit  59 , the voice coil motor  60 , and the piezoactuator  61  correspond to a first misalignment correcting unit in the appended claims. More specifically, the first correction circuit  59  corresponds to a calculating unit, and the voice coil motor  60  and the piezoactuator  61  correspond to a driving unit. 
   A structure of the second correction optical system  70  is explained. The second correction optical system  70  is a second correcting unit that corrects a relative position of the recording/reproducing optical system  20  and the optical recording medium  1 , and includes a second correcting beam source  71 , a collimate lens  72 , a polarizing beam splitter  73 , a waveplate  74  for optical rotation, a convex lens  75 , a cylindrical lens  76 , a quarter dividing photodetector  77 , a second correction circuit  78 , and a voice coil motor  79 . 
   The second correcting beam source  71  serves as a light source of beams for the second correction (hereinafter referred to as a second correcting beam as necessary), and employs a linearly-polarized laser, for example. The second correcting beam is desirably in a different wavelength from the recording/reproducing beam emitted from the recording/reproducing beam source  21 . For example, when the recording/reproducing beam source  21  employs a laser with wavelength of approximately 532 nm, the second correcting beam source  71  preferably employs a red laser diode of approximately 650 nm in wavelength. The collimate lens  72  is a collimated beam generating unit that converts the second correcting beam into a collimated beam. The polarizing beam splitter  73  serves to transmit the second correcting beam emitted from the second correcting beam source  71  and to reflect the second correcting beam reflected by the optical recording medium  1 . The waveplate  74  serves to rotate the plane of polarization of the beams transmitted through the waveplate  74  and can be formed with a ¼ waveplate or a ½ waveplate, for example. The quarter dividing photodetector  77  is a photoelectric converter having beam receivers  77   a  to  77   d  (not shown in  FIG. 2 ) as four separate parts, receives the second correcting beam reflected by the optical recording medium  1 , and outputs an output signal (misalignment signal) corresponding to the misalignment between the optical recording medium  1  and the recording/reproducing optical system  20 . 
   The second correction circuit  78  is a control circuit for the second correction that receives the misalignment signal output from the quarter dividing photodetector  77 , calculates an amount of movement of the objective lens  30  necessary for the second correction and supplies an output to the voice coil motor  79  for the movement control corresponding to the calculated amount of movement. The second correction circuit  78  is shown in a block form in  FIG. 11 . As shown in  FIG. 11 , the second correction circuit  78  includes an adder  78   a  that adds outputs from beam receivers  77   a  and  77   d  located at one opposing corners of the quarter dividing photodetector  77 , an adder  78   b  that adds outputs from beam receivers  77   b  and  77   c  located at another opposing corners of the quarter dividing photodetector  77 , a subtractor  78   c  that calculates a difference between the output from the adder  78   a  and the output from the adder  78   b  to generate a focus error signal FE 2  by stigmatism method, an adder  78   d  that adds outputs from beam receivers  77   a  and  77   b  located next to each other along a track direction of the quarter dividing photodetector  77 , an adder  78   e  that adds outputs from beam receivers  77   c  and  77   d  located next to each other along the track direction of the quarter dividing photodetector  77 , a subtractor  78   f  that calculates a difference between the output from the adder  78   d  and the output from the adder  78   e  to generate a tracking error signal TE 3  by a push-pull method, and an adder  78   g  that adds the output from the adder  78   d  and the output from the adder  78   e  to generate a reproduction signal RF. In the present embodiment, the reproduction signal RF is a signal that reproduces the information previously recorded on the reflective layer  7  of the optical recording medium  1 . 
   Returning to  FIG. 2 , again, the voice coil motor  79  is a driving unit that can move the objective lens  30  in three directions, i.e., two directions perpendicular to the direction of the optical axis of the objective lens  30  (X and Y directions in  FIG. 2 ) and the direction of the optical axis (a direction perpendicular to X and Y directions in  FIG. 2 ) for the second correction. 
   The second correcting beam source  71 , the collimate lens  72 , the polarizing beam splitter  73 , the waveplate  74  for optical rotation, the convex lens  75 , the cylindrical lens  76 , and the quarter dividing phototdetector  77  correspond to a second misalignment detecting unit in the appended claims. The second correction circuit  78  and the voice coil motor  79  correspond to a second misalignment correcting unit in the appended claims. 
   An information recording/reproduction method conducted with the hologram recording/reproducing apparatus  10  having the above-described structure is explained. The recording and reproduction of the information, however, can be conducted basically in the same manner as in the conventional technique and only an outline is explained herein. A method of information recording is explained first. The recording/reproducing beam emitted from the recording/reproducing beam source  21  in  FIG. 2  is expanded and shaped into a collimated beam by the beam expander  22 , to be irradiated onto the spatial light modulator  40  via the mirror  23 . Then, the modulation pattern as shown in  FIG. 3  is displayed on the spatial light modulator  40 . Thus, among the recording/reproducing beams irradiated onto the spatial light modulator  40 , the beams irradiated onto the information beam area  42  is spatially modulated to be the information beam including the information as a two-dimensional pattern, while the beams irradiated onto the reference beam area  43  is spatially modulated to be the reference beam. In other words, the recording beam is generated so as to include the information beam in the central portion of the optical axis and the reference beam in the peripheral portion of the optical axis. The recording beam is incident on the polarizing beam splitter  27  sequentially via the imaging lens  24 , the mirror  25 , and the imaging lens  26 . 
   Here, the direction of polarization of the recording beam (the direction of polarization of the recording/reproducing beam) is adjusted at the time of emission from the recording/reproducing beam source  21  so that the recording beam passes through the polarizing beam splitter  27 . Hence, the recording beam passes through the polarizing beam splitter  27  and enters the dichroic prism  28 . The recording beam transmitted through the dichroic prism  28 , further passes through the waveplate  29 , is irradiated onto the optical recording medium  1  via the objective lens  30 , and condensed so that the beam diameter is minimum on a surface of the reflective layer  7  of the optical recording medium  1  of  FIG. 1 . With the irradiation of the recording beam onto the optical recording medium  1 , the information beam and the reference beam constituting the recording beam interfere with each other in the recording layer  3 , to form the hologram  8  with the interference pattern in the optical recording medium  1 . Thus, the information is recorded in the optical recording medium  1 . 
   The information reproduction is conducted as follows. Returning to  FIG. 2  again, the recording/reproducing beam emitted from the recording/reproducing beam source  21  is irradiated onto the spatial light modulator  40  as at the recording. At the reproduction, the modulation pattern as shown in  FIG. 4  is displayed on the spatial light modulator  40 . The modulation pattern at the reproduction only has the reference beam area  43  same with that in the modulation pattern at the recording shown in  FIG. 3 . Hence, only the beam irradiated onto the reference beam area  43  among the recording/reproducing beam irradiated onto the spatial light modulator  40  is spatially modulated, and only the reference beam same with that at the recording is generated. The reference beam is, similarly to the time of recording, irradiated onto the optical recording medium  1  and a part thereof is diffracted by the hologram  8  of  FIG. 1 , on passing through the optical recording medium  1  to be a reproducing beam. 
   The reproducing beam, after the reflection by the reflective layer  7 , passes through the objective lens  30  shown in  FIG. 2 , and is rotated by the waveplate  29  on passing therethrough. Thus, the reproducing beam comes to include a different polarizing component from the reference beam. Hence, the reproducing beam, after passing through the dichroic prism  28 , is reflected by the polarizing beam splitter  27 . The reflected reproducing beam is focused onto the two-dimensional photodetector  32  via the imaging lens  31  as a reproduction image. A portion of the reference beam which is not diffracted by the hologram  8  of  FIG. 1  is transmitted and focused onto the two-dimensional photodetector  32  of  FIG. 2  similarly to the reproducing beam. In the two-dimensional photodetector  32 , however, the reproducing beam is focused at the center portion while the transmitted beam is focused at the peripheral portion. Hence, the reproducing beam can easily be spatially separated for reproduction of the information recorded on the optical recording medium  1 . 
   A misalignment correction with the first correction optical system  50  and the second correction optical system  70  is explained. Here, a relative position of the recording/reproducing optical system  20  and the spatial light modulator  40  is corrected with the first correction optical system  50 , followed by a correction of a relative position of the recording/reproducing optical system  20  and the optical recording medium  1  with the second correction optical system  70 . The corrections are conducted in this order since the corrected relative position in the first conducted correction either of the recording/reproducing optical system  20  and the spatial light modulator  40  or of the recording/reproducing optical system  20  and the optical recording medium  1  is employed as a reference for the following correction. However, the first correction or the second correction may precede the other. When it is not necessary to perform the corrections in sequence, it is possible to perform the corrections simultaneously. 
   First, the correction with the first correction optical system  50  is explained. The first correction beam emitted from the first correcting beam source  51  and having a different wavelength from the recording/reproducing beam is shaped into a collimated beam by the collimate lens  52  to be incident on the polarizing beam splitter  53 . Here, to allow the transmission of the first correcting beam through the polarizing beam splitter  53 , the direction of polarization of the first correcting beam is adjusted upon emission from the first correcting beam source  51 . Hence, the first correcting beam passes through the polarizing beam splitter  53 . Thus transmitted first correcting beam passes through the waveplate  54  and the condenser lens  55  to be incident on the dichroic prism  28 . Here as mentioned above, the dichroic prism  28  is designed so as to transmit the beam with the wavelength of the recording beam and to reflect the beam with the wavelength of the first correcting beam. Hence, the first correcting beam is reflected by the dichroic prism  28  and enters the polarizing beam splitter  27 . Since the polarizing beam splitter  27  is designed so as to transmit the beam with the wavelength of the first correcting beam as mentioned above, the first correcting beam passes through the polarizing beam splitter  27  and enters the spatial light modulator  40  sequentially via the imaging lens  26 , the mirror  25 , and the imaging lens  24 . The emission of the first correcting beam from the first correcting beam source  51  up to the incidence of the first correcting beam into the spatial light modulator  40  corresponds to the irradiation step in the appended claims. 
   Here, the first correcting beam is condensed to the minimum beam diameter on the surface of the alignment mark M of the spatial light modulator  40  of  FIGS. 5 and 6  by the imaging lens  24 . The alignment mark M is designed so as to transmit the recording/reproducing beam and to reflect the first correcting beam. Thus, the first correcting beam is reflected by the alignment mark M and diffracted by tracks T 1  and T 2  at reflection. 
   The first correcting beam, after the diffraction and reflection by the spatial light modulator  40 , enters the dichroic prism  28  sequentially via the imaging lens  24 , the mirror  25 , the imaging lens  26  and the polarizing beam splitter  27 . Then, the first correcting beam is reflected by the dichroic prism  28  and further passes through the condenser lens  55  and the waveplate  54 . Since the first correcting beam comes to include a different polarizing component from the beam as emitted from the first correcting beam source  51  on passing through the waveplate  54 , the first correcting beam is reflected by the polarizing beam splitter  53 . In view of the reflection efficiency, the rotation angle of the waveplate  54  is desirably adjusted so that the reflectance ratio of the first correcting beam at the polarizing beam splitter  53  is maximum. Thus reflected first correcting beam passes through the convex lens  56  and the cylindrical lens  57 , to be received by the quarter dividing phototdetector  58  (corresponding to the reception of beam step in the appended claims). The cylindrical lens  57  changes the shape of the first correcting beam to adjust the misalignment of focus direction (optical axis direction). 
   Then, as shown in  FIG. 10 , a misalignment signal is output from the respective beam receivers  58   a  to  58   d  of the quarter dividing photodetector  58  corresponding to the misalignment of the recording/reproducing optical system  20  and the spatial light modulator  40  (corresponding to the detection step in the appended claims). The first correction circuit  59  drives the piezoactuator  61  in  FIG. 2  and moves the spatial light modulator  40  toward the direction of perpendicular thereof so that the focus error signal FE 1  obtained based on the misalignment signal attains zero. The first correction circuit  59  also drives the piezoactuator  61  of  FIG. 2  and moves the spatial light modulator  40  in two directions perpendicular with each other in the modulation plane so that the tracking error signals TE 1  and TE 2  obtained from the quarter dividing photodetector  58  in  FIG. 10  both attain zero. Further, when a rapid misalignment component is generated due to the vibration or the like at the recording/reproduction, the first correction circuit  59  drives the voice coil motor  60  of  FIG. 2  and drives the imaging lens  24  in two directions perpendicular to the optical axis direction of the imaging lens  24  and the optical axis direction so that the focus error signal FE 1  and the tracking error signals TE 1  and TE 2  all attain zero, thereby correcting the misalignment (the movement of the spatial light modulator  40  and the imaging lens  24  corresponds to the correction step in the appended claims). 
   Thus, the correction of relatively large amount is conducted by the piezoactuator  61  that moves the spatial light modulator  40  whereas the correction of relatively small amount is conducted by the voice coil motor  60  that moves the imaging lens  24 . The movement of the spatial light modulator  40  is more direct and preferable for the first correction. However, the spatial light modulator  40  is heavier and more difficult to move rapidly compared with the optical element such as the imaging lens  24 . Hence, in the embodiment, rough correction is first performed with the movement of the spatial light modulator  40  followed by minute and rapid correction conducted with the movement of the imaging lens  24 . When correction is performed with the movement of a plurality of elements (spatial light modulator  40  and imaging lens  24 ), the speed of response of the correction can be enhanced. 
   In the first correction, the misalignment of the spatial light modulator  40  can be corrected with the movement of an optical element other than the imaging lens  24 , such as the imaging lens  26 . The imaging lens  26  is, however, arranged farther from the spatial light modulator  40  compared with the imaging lens  24 . Hence, for the correction of the same amount of misalignment, the imaging lens  26  needs to be moved farther than the imaging lens  24 , which is not preferable in view of the speed of response. In other words, it is preferable in view of the speed of response to correct the misalignment of the spatial light modulator  40  with the movement of an optical element in closest proximity to the spatial light modulator  40 , i.e., the imaging lens  24 . 
   The correction of the relative position of the recording/reproducing optical system  20  and the optical recording medium  1  with the second correction optical system  70  is explained. The second correcting beam emitted from the second correcting beam source  71  in  FIG. 2  is shaped into a collimated beam by the collimate lens  72  and enters the polarizing beam splitter  73 . The direction of polarization of the second correcting beam is adjusted upon emission from the second correcting beam source  71  so that the second correcting beam passes through the polarizing beam splitter  73 . Thus, the second correcting beam passes through the polarizing beam splitter  73 . Then, the second correcting beam passes through the waveplate  74  and is reflected by the dichroic prism  28 . Further, the second correcting beam passes through the waveplate  29 , is irradiated onto the optical recording medium  1  via the objective lens  30 , passes through the dichroic reflective layer  5  of the optical recording medium  1  of  FIG. 1 , and is condensed to the minimum beam diameter on the surface of the reflective layer  7 . The second correcting beam is reflected by the reflective layer  7  and modulated at the reflection by pits (not shown) that are formed on the surface of the reflective layer  7 . 
   The second correcting beam modulated and reflected by the optical recording medium  1  is collimated by the objective lens  30 , passes through the waveplate  29 , is reflected by the dichroic prism  28  and further passes through the waveplate  74 . The second correcting beam comes to include a different polarization component from the beam as emitted from the second correcting beam source  71 , on passing through the waveplates  29  and  74 . Hence, the second correcting beam is reflected by the polarizing beam splitter  73 , passes through the convex lens  75  and the cylindrical lens  76 , and is detected by the quarter dividing photodetector  77 . Then, as in  FIG. 11 , the second correction circuit  78  drives the voice coil motor  79  of  FIG. 2  and moves the objective lens  30  so that the focus error signal FE 2  and the tracking error signal TE 3  obtained from the quarter dividing photodetector  77  each attain zero, thereby correcting the misalignment. 
   As explained above, in the present embodiment as in the conventional technique, the relative position of the optical recording medium  1  and the recording/reproducing optical system  20  can be corrected with the second correction optical system  70 , whereas the relative position of the recording/reproducing optical system  20  and the spatial light modulator  40  can be corrected with the first correction optical system  50 . As a result, the relative position of the optical recording medium  1  and the spatial light modulator  40  can be corrected based on the recording/reproducing optical system  20 . Hence, at the information recording, the fluctuation in the hologram  8  caused according to the fluctuation in the arrangement of respective optical elements can be suppressed, while at the information reproduction, the fluctuation in the intensity of reproducting beam caused according to the fluctuation in arrangement of respective optical elements can be suppressed. Thus, the reproducibility of the relative position between the optical recording medium  1  and the spatial light modulator  40  at the information recording and reproduction can be enhanced. Thus, more precise reproduction of information is allowed. Further, even when the optical recording medium  1  is inserted/removed to/from the hologram recording/reproducing apparatus  10 , the proper alignment is possible. Thus, the portability of the optical recording medium  1 , the compatibility of various hologram recording/reproducing apparatus  10  can be enhanced. 
   A second embodiment of the present invention is explained. In the second embodiment, a hologram recording/reproducing apparatus of transmissive colinear interfererometry is employed. A component not specifically explained is same with the component in the optical recording medium explained with reference to the first embodiment. The same component is denoted with same reference character. 
   First, a structure of an optical recording medium  80  to be employed for the recording/reproduction by the hologram recording/reproducing apparatus according to the embodiment is explained.  FIG. 12  is a perspective view of the optical recording medium  80  (a side part of the optical recording medium is shown to be broken). The optical recording medium  80 , different from the first embodiment, needs to transmit the reproducing beam without reflecting the same. Hence, the optical recording medium  80  is of a structure of the optical recording medium  1  without the gap layer  4 , the dichroic reflective layer  5 , and the reflective layer  7 . On the outer side surface of the transparent substrate  6 , pits (not shown) are formed for tracking servo. 
   A structure of a hologram recording/reproducing apparatus  100  for recording/reproducing information on the optical recording medium  80  with the above-described structure is explained.  FIG. 13  is a diagram of an overall structure of the hologram recording/reproducing apparatus  100  according to the second embodiment. A component not specifically mentioned is same as in the hologram recording/reproducing apparatus  100  of the first embodiment and the same component is denoted with the same reference character. The hologram recording/reproducing apparatus  100  includes a recording/reproducing optical system  110 , a spatial light modulator  40 , a first correction optical system  50 , and a second correction optical system  70 . 
   The recording/reproducing optical system  110  transmits the reproducing beam without reflecting the same by the optical recording medium  80  and focuses the reproducing beam on the two-dimensional photodetector  32 . Hence, different from the first embodiment, the imaging lens  33  and the two-dimensional photodetector  32  are arranged as opposing to the spatial light modulator  40  with the optical recording medium arranged therebetween. Further, the polarizing beam splitter  27  shown in  FIG. 2  is not included. Still further, different from the first embodiment, in the second embodiment, the spatial light modulator  40  and the imaging lens  24  are arranged so that the optical axis of the imaging lens  24  matches with the optical axis of the imaging lens  26 . However, the structure using the mirror  25  as in the first embodiment may be employed. The spatial light modulator  40 , the first correction optical system  50 , and the second correction optical system  70  have same structures as in the first embodiment. 
   Next, the information recording/reproducing method with the hologram recording/reproducing apparatus  100  having the above-described structure is explained. The information recording is first explained. The recording/reproducing beam emitted from the recording/reproducing beam source  21  in  FIG. 13 , similarly to the first embodiment, is irradiated onto the optical recording medium  80  via the spatial light modulator  40 , the dichroic prism  28  and the like, whereby the hologram  8  of  FIG. 12  is formed on the optical recording medium  80 . Thus, the information is recorded on the optical recording medium  80 . 
   Next, the information reproduction is explained. Returning to  FIG. 13 , the recording/reproducing beam emitted from the recording/reproducing beam source  21 , similarly to the first embodiment, is modulated by the spatial light modulator  40  to be the reference beam. The resulting reference beam is irradiated onto the optical recording medium  80  via the dichroic prism  28  or the like. A part of the reference beam is diffracted by the hologram  8  to be a reproducing beam on passing through the optical recording medium  80 . The reproducing beam is emitted via the optical recording medium  80  and focused on the two-dimensional photodetector  32  by the imaging lens  33  as a reproduction image. The reference beam not diffracted by the hologram  8  is transmitted and focused on the two-dimensional photodetector  32  similarly to the reproducing beam. Since the focused reference beam has the reproducing beam in the center portion and the transmissive beam in the peripheral portion, the reproducing beam can easily be spatially separated for the reproduction of information recorded on the optical recording medium  80 . 
   The correction of misalignment with the first correction optical system  50  and the second correction optical system  70  is explained. The correction with the first correction optical system  50  is conducted similarly to the first embodiment. The first correcting beam emitted from the first correcting beam source  51  in  FIG. 13  passes through the polarizing beam splitter  53 , is reflected by the dichroic prism  28 , and enters the spatial light modulator  40  as in the first embodiment. The first correcting beam is reflected by the alignment mark M of the spatial light modulator  40  of  FIGS. 5 and 6  and diffracted by the tracks T 1  and T 2  at reflection. The first correcting beam thus diffracted and reflected by the spatial light modulator  40  is reflected by the dichroic prism  28  and comes to include a different polarization component from the beam as emitted from the first correcting beam source  51  on passing through the waveplate  54 . Thus, the first correcting beam is reflected by the polarizing beam splitter  53  to be received by quarter dividing photodetector  58 . The following process of the first correction is same with the first embodiment. 
   The second correction is conducted in the same manner as in the first embodiment except that the second correcting beam passes through the optical recording medium  80 . The second correcting beam emitted from the second correcting beam source  71  in  FIG. 13 , as in the first embodiment, is irradiated onto the optical recording medium  80  via the dichroic prism  28  or the like and condensed to the minimum beam diameter on the outer surface of the transparent substrate  6  of the optical recording medium  80 . A part of the second correcting beam is reflected at the outer surface of the transparent substrate  6  and modulated by the pits formed on the outer surface of the transparent substrate  6 . The second correcting beam after the modulation and reflection by the optical recording medium  80  enters the quarter dividing photodetector  77  via the dichroic prism  28  or the like, similarly to the first embodiment. The following process of the second correction is same with the first embodiment. 
   Thus, also in the transmissive colinear interferometry hologram recording/reproducing apparatus  100  according to the second embodiment, the relative position of the optical recording medium  80  and the recording/reproducing optical system  110  can be corrected with the second correction optical system  70 . Further, the relative position of the recording/reproducing optical system  110  and the spatial light modulator  40  can be corrected with the first correction optical system  50 . As a result, the relative position of the optical recording medium  80  and the spatial light modulator  40  can be corrected based on the recording/reproducing optical system  110 . Hence, at the information recording, the fluctuation of the hologram  8  caused according to the fluctuation of the arrangement of respective optical elements can be suppressed whereas at the information reproduction, the fluctuation in the intensity of reproducing beam caused according to the fluctuation in the arrangement of respective optical element can be suppressed. 
   A result of an evaluation test of recording/reproduction performance of the hologram recording/reproducing apparatus  10  according to the first embodiment is explained. In a first example, the information is recorded onto the optical recording medium  1  with the hologram recording/reproducing apparatus  10 . Then, the reproduction is carried out by the same hologram recording/reproducing apparatus  10  with correction and without correction. The results from the reproduction with correction are compared with the results from the reproduction without correction. 
   Details of the hologram recording/reproducing apparatus  10  used in the first example is explained. A neodymium YAG laser emitting a coherent beam of a second harmonic (wavelength of 532 nm) is used as the recording/reproducing beam source  21  of  FIG. 2 , and a linearly-polarized laser diode (wavelength of 650 nm) is used as the first correcting beam source  51  and the second correcting beam source  71  of  FIG. 2 . A track of 1 μm in width is provided in the form of a cross as the alignment mark M of the spatial light modulator  40  of  FIGS. 5 and 6 . Further, a CCD array is used as the two-dimensional photodetector  32  of  FIG. 2 , a ¼ wavelength plate for 532 nm wavelength is used as the waveplate  29  and a ¼ waveplate for 650 nm wavelength are used as the waveplates  54  and  74 . Further, the orientation of the wavelength plate used as the waveplate  29  is adjusted so that the intensity of the reproducing beam is maximum at the upper surface of the two-dimensional photodetector  32 . Further, the orientations of the waveplates  54  and  74  are adjusted so that the light intensity at the quarter dividing photodetectors  58  and  77  are maximum, respectively. 
   The optical recording medium  1  is mounted onto the hologram recording/reproducing apparatus  10  with the above-described structure and the information recording is conducted. Specifically, the optical recording medium  1  is secured on the spindle motor (not shown) and made to rotate at the speed of 1 rpm. While the first correction and the second correction are conducted, the recording/reproducing beam source  21  is made to flash in synchronization with address signals and the recording of the hologram  8  is performed. Here, the light intensity on the surface of the optical recording medium  1  is 0.1 mW, and the spot size of the laser beam on the upper surface of the recording layer  3  is 500 μm in diameter. An area of ninety thousands pixels (300×300) is used as the pixels  41  of the DMD  46  in the spatial light modulator  40 . An area of 22500 pixels (150×150) in the center is used as the information beam area  42 . Neighboring nine pixels (3×3) are defined as one unit panel and total of 2500 panels are used as the information beam area. For the input of information, 9:5 modulation is used where five panels of the nine panels (3×3) are used as bright panels, i.e. panels transmitting the light. 
   Thus recorded information is reproduced under two different conditions. First, the optical recording medium  1  is removed from the hologram recording/reproducing apparatus  10 . Then the optical recording medium  1  is secured on a spindle motor (not shown) of the same hologram recording/reproducing apparatus  10  again. Then, the optical recording medium  1  is made to rotate at the speed of 1 rpm. While the first correction and the second correction are conducted, the recording/reproducing beam source  21  is made to flash in synchronization with address signals and the reproduction of the hologram  8  is performed with the two-dimensional photodetector  32 . At the reproduction, only the reference beam area  43  as shown in  FIG. 4  is displayed on the DMD  46  of the spatial light modulator  40  to generate the reference beam. The light intensity on the surface of the optical recording medium  1  is 0.05 mW. 
   Then, the information is reproduced under the same condition as the first reproduction, but without the first correction and the second correction. The results of reproductions under two different conditions are compared. Specifically, a particular threshold is set with respect to each pixel  41  in the information beam area  42  (150×150) obtained by the two-dimensional photodetector  32 . Based on the threshold, the determination of bright panel and dark panel at the reproduction under each condition is performed to provide an output pattern. The output pattern is compared with an input pattern that is provided to the DMD  46  of the spatial light modulator  40 . As a result, while in the reproduction with the first and the second corrections, no panel is determined to be erroneous among 2500 panels, in the reproduction without the first and the second corrections, ten panels are determined to be erroneous among 2500 panels. Thus, it is confirmed that the first and the second correction reduce the number of panels determined to be erroneous, thus enhancing the precision of information reproduction. 
   In the second example, information is recorded in the optical recording medium  1  with the hologram recording/reproducing apparatus  10 . After the recording, the reproduction is performed with a different hologram recording/reproducing apparatus  10  from the apparatus used for the recording, first with correction and then without correction. The conditions and results not specifically mentioned are same with the first example. The hologram recording/reproducing apparatus used for the recording (hereinafter referred to as a hologram recording/reproducing apparatus  10 A) and a hologram recording/reproducing apparatus used for the reproduction (hereinafter referred to as a hologram recording/reproducing apparatus  10 B) are different apparatuses with the same structure. The positioning error between the tracks T 1  and T 2  provided as the alignment mark M in the spatial light modulator  40  of the hologram recording/reproducing apparatus  10 A and the tracks T 1  and T 2  provided as the alignment mark M in the spatial light modulator  40  of the hologram recording/reproducing apparatus  10 B is equal to or less than 1 μm. 
   First, the optical recording medium  1  where the information is recorded with the hologram recording/reproducing apparatus  10 A is removed from the hologram recording/reproducing apparatus  10 A and is placed in another hologram recording/reproducing apparatus  10 B and secured onto a spindle motor (not shown). Then the information is reproduced while the first and the second corrections are performed. 
   Then, with the hologram recording/reproducing apparatus  10 B, the information is reproduced under the same condition but without the first and the second corrections. Then the results of reproduction under two different conditions are compared. As a result, while in the reproduction with the first and the second corrections, the number of panels determined to be erroneous is zero among 2500 panels, in the reproduction without the first and the second corrections, the number of panels determined to be erroneous is 50 among 2500 panels. Hence, it is confirmed that even when the different hologram recording/reproducing apparatus  10 B is used for the reproduction, the number of the panels determined to be erroneous can be reduced with the first and the second corrections, whereby the precision of information reproduction can be enhanced. 
   In a third example, information recorded without the first correction is reproduced in the same hologram recording/reproducing apparatus  10  with the apparatus at the time of recording, first with correction and then without correction. The results are compared and explained. The conditions and results not specifically mentioned are same with the first example. 
   First, the optical recording medium  1  is mounted onto the hologram recording/reproducing apparatus  10  for the information recording. At the recording the first correction is not performed and only the second correction is performed. 
   Then, the optical recording medium  1  is removed from the hologram recording/reproducing apparatus  10  and placed and secured again in the same hologram recording/reproducing apparatus  10 . The reproduction is performed with the first and the second corrections. 
   Then, the reproduction of information is performed again, under the same condition, though without the first correction and only with the second correction. The results of reproduction under two different conditions are compared. As a result, in the reproduction with the first and the second corrections the number of panels determined to be erroneous is eighty among 2500 panels, whereas in the reproduction only with the second correction the number of panels determined to be erroneous is 150 panels among 2500 panels. Thus, it is confirmed that the number of panels determined to be erroneous can be further reduced when both first and second corrections are performed compared with the reproduction only with the second correction, whereby the precision in information reproduction can be enhanced. In comparison with the results of the first example, it is confirmed that the precision in information reproduction can be enhanced when the first correction is performed also at the time of recording. 
   In the foregoing, the exemplary embodiments and examples are explained. Specific structure and method of the present embodiments can optionally changed or improved in the scope of technical spirit of the invention as defined by the appended claims. Modifications of the embodiments are explained below. 
   The optical recording medium  1  or  80  can be structured in a different known form than in the above-described form. For example, the focus positions of the recording beam and the second correcting beam can be made different, with the use of an objective lens having aberration chromatica, with a wider distance between the second correcting beam source  71  and the collimate lens  72  in  FIG. 2 , with the insertion of a concave lens for correction between the waveplate  74  and the dichroic prism  28  of  FIG. 2 , or the like. 
   In addition, any structure can be adopted as the basic structure of the hologram recording/reproducing apparatus to which the present invention is applied. For example, other than the reflective colinear interferometry or the transmissive colinear interferometry described above, the hologram recording/reproducing apparatus may use reflective two beam interferometry or transmissive two beam interferometry. 
   In addition, the spatial light modulator  40  shown in  FIGS. 5 and 6  can be formed with another known structure other than the structure with the DMD  46  as described above. For example, the spatial light modulator  40  may be a reflective liquid crystal spatial light modulator, the transmissive liquid crystal spatial light modulator, or the like as far as it is provided with the plurality of pixels  41  and the transparent window  45 . The shape of the alignment mark M can also be changed to forms other than the cross-shape described above. For example, more tracks can be provided to form a shape of beams radiating from the center. In addition, with respect to the position of formation of the alignment mark M in the direction of the optical axis of the spatial light modulator  40 , the mark M is preferably formed in proximity to the pixel  41  for the strict focusing in the optical axis direction. In addition, a protective layer  2  may be further formed upon the transparent window  45  to suppress the scattering of light by the alignment mark M. Further, a dichroic reflective layer that transmits the recording beam but reflects the first correcting beam may be formed upon the surface where the alignment mark M is formed, to achieve an efficient reflection of the first correcting beam. 
   Further, the structure of the first correction optical system  50  can be optionally changed as far as the above-described effects can be realized. In particular, a specific structure of the optical elements can be varied. The driving unit may be formed so that only the spatial light modulator  40  is movable, only the imaging lens  24  is movable, the imaging lens  26  instead of the imaging lens  24  is movable, the imaging lens  26  in addition to the spatial light modulator  40  and the imaging lens  24  is movable, for example. Since the focusing in the optical axis direction of the spatial light modulator  40  has relatively large allowable value, the spatial light modulator  40  or the like may be formed to be movable only in X direction or Y direction in  FIG. 2  and the movement in the optical axis direction may be omitted. 
   In the embodiments, the position of the optical recording medium  1  or  80  and the spatial light modulator  40  is corrected with both the first correction optical system  50  and the second correction optical system  70 . When only the correction of relative position of the recording/reproducing optical system  20 ,  110  and the spatial light modulator  40  is desirable, however, the second correction optical system may be omitted. In addition, when both the first correction optical system  50  and the second correction optical system  70  are provided, the structure of the recording/reproducing optical system  20  can be simplified as shown in the embodiments if the first correcting beam and the second correcting beam are led to the recording/reproducing optical system  20  via the dichroic prism  28  which is a common optical element for both systems. In other words, compared with a conventional apparatus where only the second correction optical system  70  is provided, the apparatus of the embodiment allows the introduction of the first correcting beam into the recording/reproducing optical system  20  without the increase in the number of optical elements. However, if such benefit is not specifically necessary, the first correcting beam may be led to the recording/reproducing optical system  20  via any optical element. For example, in  FIG. 2  another dichroic prism may be provided between the polarizing beam splitter  27  and the dichroic prism  28  so that the first correcting beam is led through the new dichroic prism to the recording/reproducing optical system  20 . 
   In addition, the problems to be solved by the invention and the effects of the invention are not limited to those mentioned above. The problems not specifically described may be solved and the effects not specifically described may be exerted. A part of the problems described above may be solved or a part of the effects described above may be exerted. For example, even when the reproducibility at the irradiation of the optical recording medium with the reference beam is not hundred percent, as far as the reproducibility is enhanced compared with the conventional technique, it should be said that the problems are solved. 
   The drawings are merely exemplary and the dimension and the ratio of the components are not limited to those described in the drawings. 
   Further, all or a part of the control which is explained to be automatically performed in the embodiments may be manually performed. On the other hand, all or a part of the control which is explained to be manually performed may be automated according to the known technique or the idea of the present invention. In addition, the control process of the first correction circuit  59  and the second correction circuit  78  described with reference to the embodiments may be constituted as a Central Processing Unit (CPU) and a computer program which is read out and executed by the CPU. 
   Thus, the present invention is useful for the recording and reproduction utilizing the holography and particularly suitable for the enhancement of the reproducibility at the irradiation of the optical recording medium with the reference beam in the hologram recording/reproducing apparatus. 
   Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.