Abstract:
An optical information recording method includes the following steps. A converting step is converting a light beam into an information beam carrying information by using a spatial beam modulator. A focusing step is focusing the information beam on an optical information recording medium including an information recording layer. An irradiating step is irradiating the optical information recording medium with a reference beam and the information beam so that the reference beam and the information beam intersect with each other on the information recording layer by using an optical component. A rotating step is rotating the optical information recording medium or the optical component by using a drive unit for performing angle-multiplex recording. A management information recording step is recording management information at two or more relative angles having an angle interval smaller than twice at least a first null angle.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2009-072778, filed on Mar. 24, 2009, the entire contents of which are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to an optical information recording method and an optical information recording apparatus to record information as a hologram. 
       DESCRIPTION OF THE BACKGROUND 
       [0003]    An optical information recording medium typified by CD (Compact Disk), DVD (Digital Versatile Disk), Blue-ray Disc, HD DVD (Hi-Definition Digital Versatile Disk), etc. has contributed to an increase in a recording density. For example, the recipes for the increase include shortening the wavelength of laser light and increasing a numerical aperture (NA) of an object lens. However, both the recipes are considered to approach the technical limit thereof. A novel method or principle is required to increase the recording density. 
         [0004]    Recently, a high-density recording method of volumetrically recording type (referred to as a “holographic memory” hereinafter) and a holographic recording/reproducing apparatus have been developed for practical use. In the holographic memory, a spatial beam modulator (or a spatial light modulator) modulates laser light spatially to generate an information beam carrying information. Furthermore, a reference beam is generated from the light source of the information beam. The reference beam has the same wavelength as the information beam. The information beam and the reference beam are directed to the same spot inside an optical information recording medium to record interference fringes in the spot of the optical information recording medium. The interference fringes are generated by the interference between the information beam and the reference beam. 
         [0005]    When reproducing in the holographic memory, only the reference beam is directed to the spot to reproduce the information beam at the time of recording, thereby allowing it to obtain the information modulated at the time of recording. DVD, etc. employs a surface recording mode where recording marks are recorded on the surface of DVD, whereas a holographic optical disk employs volumetric recording which allows it to record in a thickness direction of an information recording layer. Thereby, the holographic optical disk acquires a higher recording density than DVD. 
         [0006]    DVD records marks as bit data of ON/OFF, while the information beam is recorded as interference fringes to which relatively many pieces of information are modulated collectively in the holographic memory. A set of pieces of information is a modulated pattern of the information beam stored in the optical information recording medium, i.e., a two-dimensional bar-code unit for recording/reproducing which includes black-and-white dots and is called “page data”. 
         [0007]    One of the methods to increase the recording density of the holographic memory is multiplex recording. This multiplex recording is a mode which records two or more page data on the same spot of the holographic recording medium. Examples of the multiplex recording proposed include angle-multiplex recording with shifting the incident angle of a laser beam, shift multiplex recording with shifting a beam position slightly, and wavelength multiplex recording with shifting the wavelength of a laser beam. 
         [0008]    In the angle-multiplex recording or the shift multiplex recording, changing a relative position or angle of the laser beam to the optical information recording medium enables multiplex recording. The angle-multiplex recording system is a novel mode which has been never employed in conventional CD and DVD recorders, and is essential to so-called a dual beam interference mode which records interference fringes generated between an information beam and a reference beam onto a recording layer. 
         [0009]    The angle-multiplex recording modes include two technologies. One rotates the recording medium without changing the position of a laser beam to perform multiplex recording. The other rotates the position of the laser beam around the recording medium to perform multiplex recording. The rotation axis thereof is generally set to pass through an axis (y-axis) perpendicular to an incident plane (x-z plane) of the information beam and the reference beam. The rotations of the recording medium and the position of the laser beam are called a θ y  rotation. The multiplicity due to the rotation is called a θ y  multiplicity. 
         [0010]    Moreover, materials of the recording medium for the holographic memory have been recently developed using a photopolymer, and some are close to practical use. The photopolymer is an organic material which induces a change in the refractive index thereof in response to light irradiation, thereby showing various reaction mechanisms depending on organic material employed. The photopolymer is cured to become soft and gumlike, and it is, therefore, difficult to hold the simple body thereof. The photopolymer is poured into a space formed between a pair of glass substrates sandwiching a properly thick spacer, thereby forming a recording layer held by the pair of glass substrates. 
         [0011]    The recording medium using a photopolymer is easy to manufacture, while the recording medium is vulnerable to a temperature change. The vulnerability arises from two points. One is that the thermal expansion coefficient of the photopolymer differs from that of the glass substrate by two orders of magnitude. The other is that the refractive index of the photopolymer changes greatly with temperature. When the recording temperature to record information on the recording medium differs from the reproducing temperature to read out the information, a diffracting grating recorded deforms. The deformation of the diffracting grating arises from the fact that the deformation thereof parallel to the glass substrate depends dominantly on the thermal expansion of the glass substrate, and the deformation thereof vertical to the glass substrate depends dominantly on the thermal expansion of the photopolymer. This also causes a problem that only a fraction of the recorded information is reproduced as a result of the different reproducing conditions in the same page data. 
         [0012]    When recording page data at an angle θ y  is followed by recording different page data, the angle θ y  is changed so that a crosstalk does not arise from adjacent pages when reproducing. When reproducing page data, a clear reproduction image is obtained all over as shown in  FIG. 11-1 , if there is no difference between the temperatures for reproducing and recording the page data. If there is a difference between the temperatures for reproducing and recording the page data, the diffraction grating in the recording medium deforms to shift the reproducing condition. Accordingly, only a fraction of the reproduction image is obtained as shown in  FIG. 11-2 . The larger the temperature difference is, the narrower the fraction is. When the angle θ y  is multiplexed, the deformed diffraction grating causes partial overlapping of reproducing conditions on adjacent pages. In such a case, fractions of page data on several adjacent pages are reproduced alongside of a fraction of page data on a desired page as shown in  FIG. 11-3 . As mentioned above, the temperature difference makes it difficult to reproduce when the angle θ y  is multiplexed. 
         [0013]    In order to solve the problem due to the temperature difference, a method is proposed. In the method, the temperature of a recording medium is recorded when recording and the wavelength of light is tuned with reference to the data of the recorded temperature when reproducing (JP-A 2006-267554 (KOKAI)). 
         [0014]    In the above-mentioned method, it is necessary to firstly read the temperature data recorded. However, a wavelength or angle most suitable for reproducing is unknown in an initial condition. Therefore, it takes time to reproduce in the initial condition. When the temperature difference is larger, it is impossible to determine the most suitable wavelength or angle, thereby making it impossible to reproduce. 
       SUMMARY OF THE INVENTION 
       [0015]    According to a first aspect of the invention, an optical information recording method includes a converting step, a focusing step, an irradiating step, a driving step, and a management information recording step. The converting step is converting a light beam emitted from a light source into an information beam carrying information by using a spatial beam modulator. The focusing step is focusing the information beam on an optical information recording medium including an information recording layer. The irradiating step is irradiating the optical information recording medium with a reference beam and the information beam so that the reference beam and the information beam intersect with each other on the information recording layer by using an optical component. The information recording layer is capable of recording information as a hologram due to interference fringes generated by interference between the information beam and the reference beam. The rotating step is rotating the optical information recording medium or the optical component by using a drive unit to change relative angles among the information beam, the reference beam and the optical information recording medium for performing angle-multiplex recording of the information onto the information recording layer. The management information recording step is recording management information at two or more relative angles having an angle interval smaller than twice at least a first null angle, the management information being used for reading out user information recorded on the optical information recording medium. 
         [0016]    According to a second aspect of the invention, an optical information recording apparatus includes a spatial beam modulator, an optical component, a drive unit, and a control unit. The spatial beam modulator modulates laser light spatially to generate an information beam carrying information. The laser light is emitted from a light source. The optical component focuses the information beam on an optical information recording medium including an information recording layer in order to irradiate the optical information recording medium with a reference beam and the information beam so that the reference beam and the information beam intersect with each other on the information recording layer. The information recording layer is capable of recording information as a hologram due to interference fringes generated by interference between the information beam and the reference beam. The drive unit rotates the optical information recording medium or the optical component. The control unit performs angle-multiplex recording of the information on the optical information recording medium by controlling the light source to emit the beam while rotating the optical information recording medium or the optical component so that relative angles among the information beam, the reference beam and the optical information recording medium are changed. In addition, the control unit records the same pieces of management information at two or more relative angles having an angle interval smaller than twice at least a first null angle. The management information is used when reading out user information. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention. 
           [0018]      FIG. 1  is a view showing a composition of a holographic memory recording/reproducing apparatus. 
           [0019]      FIG. 2  is a flow chart showing the procedure of management information recording process. 
           [0020]      FIG. 3  is a view showing a crosstalk between adjacent pages. 
           [0021]      FIG. 4  is a view showing a relationship between an angle interval of adjacent pages and diffraction efficiency. 
           [0022]      FIG. 5  is a view schematically showing management information recorded by management information recording processing. 
           [0023]      FIG. 6  is a flow chart showing steps of the management information reproduction processing. 
           [0024]      FIG. 7  is a view to explain a θ y  angle when reproducing the management information. 
           [0025]      FIG. 8  is a view schematically showing page data of the management information read by the management information reproduction processing. 
           [0026]      FIG. 9  is a view to explain a bit display of a spatial beam modulator. 
           [0027]      FIG. 10  is a view to explain another bit display of the spatial beam modulator. 
           [0028]      FIG. 11-1  is a view showing a reproduction image of information recorded by a conventional recording mode. 
           [0029]      FIG. 11-2  is a view showing another reproduction image of information recorded by a conventional recording mode. 
           [0030]      FIG. 11-3  is a view showing another reproduction image of information recorded by a conventional recording mode. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0031]    Embodiments of the invention are explained with reference to the accompanying drawings below. 
       First Embodiment 
       [0032]      FIG. 1  is a view showing a holographic memory recording/reproducing apparatus  100  which is provided with main optical components. The holographic memory recording/reproducing apparatus  100  records information on an information recording layer of an optical information recording medium  111 , and reads out the information recorded on the information recording layer. The information recording layer is referred to simply as the “recording layer” hereinafter. In the optical information recording medium  111 , two glass substrates sandwich a photopolymer with a thickness of 0.2 mm to 2.0 mm therebetween, for example, thereby forming the recording layer in the optical information recording medium  111 . When the recording layer is exposed to a light beam, the refractive index of an area exposed to the light beam changes. Thereby, when the intensity distribution of light is resulted from a two-beam interference, the diffraction grating due to the interference fringes will be written in. Information is recorded on the optical information recording medium  111  as the diffraction grating. 
         [0033]    The operation for recording information in the holographic memory recording/reproducing apparatus  100  is explained with reference to  FIG. 1 . A parallel pencil emitted from a laser light source  101  passes through a shutter  102  to be incident on PBS (polarized beam splitter)  103 . An s-polarized beam is reflected by PBS  103  to be employed as an information beam. A p-polarized beam passes through PBS  103  to be employed as a reference beam. The s-polarized beam reflected passes through the shutter  104  to be incident on a relay lens  105 . The beam diameter of the s-polarized beam is expanded by the relay lens  105  to make the s-polarized beam a parallel pencil so that the s-polarized beam is incident on a spatial beam modulator  107  with making the beam-intensity thereof uniform to some extent. Then, the parallel pencil is reflected by a reflection mirror  106  to be amplitude-modulated into page data of two-dimensional bar-codes by the spatial beam modulator  107 . As the spatial beam modulator  107 , a transmissive LCD (LCD: Liquid Crystal Display), a reflective FLCOS (FLCOS: Ferroelectric Liquid Crystal On Silicon), DMD (Digital Micromirror Device), etc. can be employed. 
         [0034]    The beam diameter of the information beam which was amplitude-modulated is changed into a size fit to the incidence diameter of an object lens  110  by the relay lens  108 . A Nyquist aperture  109  is disposed at the position of a beam waist to be formed by the relay lens  108 . The position of the beam waist has a plane for Fourier transform of the intensity distribution of the information beam which is amplitude-modulated by the spatial beam modulator  107 . A Fourier-transform image of the information beam is conjugate to the information beam near the optical information recording-medium  111 . The size of the Nyquist aperture  109  defines a beam diameter of the information beam near the optical information recording-medium  111 . When recording information, the optical information recording-medium  111  is scanned parallel to the surface thereof, thereby allowing it to record with a small step as the size of the information beam is small near the optical information recording-medium  111 . That is, the recording density increases as the size of information beam is small. Therefore, it is desirable to minimize the Nyquist aperture  109  as much as possible. 
         [0035]    The information beam having been incident on the object lens  110  is given a lens power to be focused near the optical information recording-medium  111 . A focusing position may be not only inside the optical information recording-medium  111 , but outside the optical information recording medium  111 . 
         [0036]    On the other hand, a reference beam which passes through PBS  103  is converted into the s-polarized beam by a λ/2 plate  112 . The reflection mirrors  113  and  114  reflect the reference beam which is changed by the relay lens  115  into a beam with the diameter mostly same as that of the information beam on the optical information recording medium  111 . Then, the reference beam is allowed to be incident on the optical information recording medium  111  as a parallel pencil. More minutely, the reference beam is incident on the same position of the optical information recording medium  111  as the information beam. Then, the information beam and the reference beam interfere with each other, and interference fringes generated are recorded onto the recording layer of the optical information recording medium  111  as a change in the refractive index of the recording layer. 
         [0037]    In angle-multiplex recording, the optical information recording medium  111  is irradiated simultaneously with the information beam and the reference beam as mentioned above while rotating the optical information recording medium  111  around a rotation axis lying in the recording layer thereof and passing through the irradiated position thereof. 
         [0038]    Here, a xyz orthogonal coordinate system is introduced to be fixed to the recording layer of the optical information recording medium  111 . A recording spot irradiated with the information and reference beams is chosen as an origin. A z-axis is taken in a thickness direction, i.e., perpendicularly to the surface of the optical information recording medium  111 . Then, an x-axis and a y-axis are taken in directions perpendicular to the z-axis, i.e., in in-plane directions of the recording layer of the optical information recording medium  111  to be normal to each other. 
         [0039]    The holographic memory recording/reproducing apparatus  100  performs θ y  angle-multiplex recording around the y-axis (the in-plane axis). Specifically, a system controller  140  controls an actuator to record two or more pieces of information with rotating the optical information recording medium  111  around the y-axis by each θ y -angle step (Δθ y ) and irradiating the medium  111  with both the information beam and the reference beam at each θ y  angle. The θ y  rotation changes a relative angle among the information beam, the reference beam and the optical information recording medium  111 , thereby allowing it to record different page data. Here, the θ y -angle step is a unit angle to perform the θ y  rotation of the optical information recording medium  111  for the θ y  multiplex recording. 
         [0040]    In this embodiment, the optical information recording medium  111  is rotated to change the relative angle among the information beam, the reference beam and the optical information recording medium  111 . Alternatively, the relative angle can be changed by controlling optical components such as mirrors in order to control the emission direction of the reference beam. In this case, the reflection mirror  114  is replaced with a galvanometer mirror, and the relay lens  115  is arranged so that the reflection plane of the galvanometer mirror is conjugate to the recording spot of the optical information recording medium  111 . The reflection mirror  114  is rotated to change the relative angle. 
         [0041]    Moreover, although the θ y  multiplex recording is performed in this embodiment, embodiments of the invention are not limited to this. For example, 0, multiplex recording may be performed so that information is recorded with rotating the optical information recording medium  111  around the z-axis by each θ z -angle step (Åθ z ). Moreover, a recording principle using the θ y  multiplex recording in combination with the θ z  multiplex recording may be employed. 
         [0042]    There are user information and management information as information to be recorded on the optical information recording medium  111 . Here, the user information (referred to as the “user data” hereinafter) is information recorded by instructions from a user. Management information is used by the holographic memory recording/reproducing apparatus  100  when reading out the user data recorded on the optical information recording medium  111 . 
         [0000]    The management information includes followings:
 
a temperature when recording user data;
 
an angle interval to change the relative angle among the information beam, the reference beam and the optical information recording medium  111  when recording the user data; and
 
a wavelength of the laser light source  101  when recording user data, etc.
 
         [0043]    For example, when reproducing the user data, reading out the temperature for recording the user data allows it to identify reproducing conditions such as an optimal angle pitch for reproducing, a wavelength of the laser light source  101  based on the temperature. Recording the angle pitch or the wavelength of the laser light source  101  preliminarily at the time of recording allows it to identify optimal reproducing conditions. 
         [0044]    Moreover, when user data has been already recorded, address information may be recorded as management information. The address information specifies a physical position where user data is recorded in the optical information recording medium  111 . Reading out the address information for reproducing allows it to identify the recorded position where the user data was recorded. 
         [0045]    Moreover, a standard image may be recorded as management information. The standard image is recorded in order to maintain the compatibility of a recording drive. For example, when adjusting according to the characteristic of the recording drive, the standard image to display a certain regular pattern is preliminarily recorded, thereby allowing it to adjust image quality, etc. Moreover, when synthesizing two or more reproduction images for reproducing, a needed read-out angle interval may be recorded as a piece of management information. 
         [0046]    As mentioned above, management information is to be used when reading user data. For this reason, the management information is desirably read out as rapidly as possible, and as certainly as possible. 
         [0047]    A performance for reproducing information in the holographic memory recording/reproducing apparatus  100  is explained with reference to  FIG. 1 . A beam emitted from the laser light source  101  passes the shutter  102 , and PBS  103  makes a p-wave and an s-wave serve as the information beam and the reference beam, respectively, as well as in recording. However, the shutter  104  is shut in reproducing. Thereby, the information beam is shut off here. The reference beam serves as a parallel pencil, and is directed to the optical information recording medium  111 , as well as in recording. The system controller  140  controls an actuator  130  so as to set the θ y  angle of the optical information recording medium  111  to a suitable angle. Thereby, a reproduction beam which fulfills reproduction conditions comes out of the optical information recording medium  111 . This reproduction light beam passes through the lens  116  to be allowed to enter into a two-dimensional array sensor  117 . The two-dimensional array sensor  117  acquires image data to be amplitude-modulated by the spatial beam modulator  107 . The image data are processed by the signal-processing circuit (not shown), and read out as data. As the two-dimensional array sensor  117 , CCD and CMOS can be employed, for example. 
         [0048]    Next, management information record processing is explained. In the processing, the holographic memory recording/reproducing apparatus  100  constituted as mentioned above records management information. In the management information record processing, as shown in  FIG. 2 , the system controller  140  sets an origin of coordinates to a desired position of the optical information recording medium  111  firstly (Step S 100 ). Furthermore, the θ y  angle is set to a desired angle (Step S 102 ). The spatial beam modulator  107  displays the modulated management information (Step S 104 ). Here, the system controller  140  sets “1” to the variable n (Step S 106 ). The system controller  140  controls to irradiate the optical information recording medium  111  simultaneously with the information beam and the reference beam for a constant time (Step S 108 ), thereby writing management information on a predetermined page of the optical information recording medium Hi. 
         [0049]    The system controller  140  controls the actuator  130  to rotate the optical information recording medium  111 , thereby changing the θ y  angle by only Δθ y1  (Step S 110 ), without changing the management information displayed on the spatial beam modulator  107  in order to continuously record the same pieces of the management information. The system controller  140  adds 1 to the variable n (Step S 112 ). Processing from Step S 108  to Step S 112  is repeated until the variable n reaches N. The same pieces of the management information are recorded over the recording angle range of Δθ y1 ×N through the above processing. 
         [0050]    Optimal reproducing conditions cannot be acquired as a result of a temperature change, etc. in some cases. Even so, the same pieces of the management information are recorded as two or more continuous page data to allow it to read out a piece of the management information in case that the piece of the management information coincides with any one of reproducing conditions for page data within the recording angle range. 
         [0051]    It is desirable that continuous page data of the management information fully generates a crosstalk. That is, Δθ y1  is set as a value within an angle range where the crosstalk takes place as a result of a difference between temperatures for the recording and reproducing of the optical information recording medium  111 . The same pieces of the management information are recorded in the angle range where the crosstalk takes place, thereby allowing it to acquire the management information from reproduction images even when fractions of two or more adjacent page data are reproduced simultaneously. 
         [0052]      FIG. 3  is a view showing a crosstalk of adjacent pages. The horizontal axis of the graph in  FIG. 3  shows a multiplexed angle. The vertical axis thereof shows diffraction efficiencies. When generally making θ y  serve as a variable, the brightness of a reproduction beam from a recorded page is proportional to the square of a sine function. When two page data are recorded at intervals of an angle (first null angle) at which the diffraction efficiency firstly becomes 0, the sum of the diffraction efficiencies in the middle of the two adjacent peaks is 80% of the diffraction efficiency at each peak, as shown in  FIG. 3 , thereby showing that a sufficient crosstalk takes place. 
         [0053]    Furthermore, when changing the angle interval of the adjacent page data, i.e., the adjacent page interval, the diffraction efficiency just in the middle changes as shown in  FIG. 4 . The horizontal axis of the graph in  FIG. 4  represents an angle interval of the adjacent page data. In addition, the first null angle is normalized to 1. The vertical axis represents the sum of the diffraction efficiencies of the page data in the middle angle of each angle interval. In addition, the value at the peak of the diffraction efficiency corresponding to 1-page data is normalized to 1. As shown in  FIG. 4 , at the angle twice the angle of the first null, the diffraction efficiency is mostly zero. That is, the crosstalk does not take place. Therefore, Δθ y1  is desirably smaller than an angle twice the first null angle. 
         [0054]    As mentioned above, in order to surely reproduce in spite of the crosstalk, it is desirable to record the same pieces of management information over the recording angle range where the sufficient crosstalk takes place. Therefore, N is desirably a value at which the value of the recording angle range (Δθ y1 ×N) becomes larger than the first null angle, for example. 
         [0055]    After repeating the processing from Step S 108  to Step S 112  to record predetermined management information, the system controller  140  instructs the actuator  130  to rotate the θ y  angle by only Δθ y2  (Step S 122 ) in order to further record other management information (“No” at Step S 120 ). Here, Δθ y2  is desirably an angle which does not generate a crosstalk with the adjacent page data. That is, it is desirable that Δθ y2  is larger than the first null angle at a minimum. 
         [0056]    The system controller  140  changes the management information displayed on the spatial beam modulator  107  (Step S 124 ). And, “1” is again set to the variable n (Step S 106 ), and the processing from Step S 108  to the step S 112  is repeated until the variable n reaches N. Thereby, the changed management information is recorded over a recording angle range of Aθ y1 ×N. Through the above-mentioned process, all the management information is recorded to end management information recording process (“Yes” at Step S 120 ). 
         [0057]      FIG. 5  is a view showing a relationship of θ y  angles among the management information A, the management information B, and the management information C each recorded via the above processing. The horizontal axis of the graph in  FIG. 5  represents the θ y  angle, and the vertical axis represents diffraction intensity. As shown in  FIG. 5 , two or more data of the management information A are recorded as page data which are continuous over the recording angle range (Δθ y1 ×N). The management information A and the management information B are separated by only the angle Δθ y2  at which no crosstalk takes place. That is, the angle Δθ y2  is a difference between the initial angle of the management information B and the terminal angle of the management information A. 
         [0058]    The same pieces of management information are continuously recorded over the recording angle range, thereby allowing it to read out desired management information in case that a reproducing angle coincides with any angle within the recording angle range even if the most suitable condition is not acquired when reproducing. The respective page data are recorded every angle interval generating a crosstalk. Therefore, even simultaneously reproducing fractions of two or more adjacent page data allows it to exactly acquire management information from this image simultaneously reproduced, as the adjacent page data are the same as the management information. Furthermore, the angle intervals to record different management information are separated by the angle which generates no crosstalk, thereby allowing it to exactly read out the different management information when reproducing. Therefore, the different management information can be read out rapidly and certainly. 
         [0059]    Reproduction processing for the holographic memory recording/reproducing apparatus  100  to reproduce management information is explained with reference to  FIG. 6 . In the reproduction processing of management information, as shown in  FIG. 6 , the system controller  140  first sets a predetermined position of the optical information recording medium  111  as an origin of coordinates (Step S 200 ). The system controller  140  instructs the actuator  130  to set the θ y  angle of the optical information recording medium  111  to a desired angle (Step S 202 ). Here, the θ y  angle is desirably an angle which lies in the middle of the recording angle range within which management information is recorded. 
         [0060]    The optical information recording medium  111  is irradiated with a reference beam (Step S 204 ). A reproduction image is acquired by the two-dimensional array sensor  117  and processed by the signal-processing circuit (Step S 206 ). 
         [0061]    Furthermore, for reading out another piece of management information (“Yes” at Step S 208 ), the actuator  130  rotates the optical information recording medium  111  to change the θ y  angle by a predetermined angle (Δθ y3 ), thereby setting the θ y  angle approximately in the middle of the angle range in which the subsequent management information is recorded (Step S 210 ). And, the processing returns to Step S 204 . Reading out all the management information (“No” at Step S 208 ) ends the reproduction processing of management information. 
         [0062]    Here, the θ y  angle for reproducing management information is explained with reference to  FIG. 7 . The horizontal axis of the graph in  FIG. 7  represents the θ y  angle, and the vertical axis represents the diffraction intensity. For example, when reproducing the management information A, θ yA  is set as the θ y  angle approximately in the middle of the recording angle range for the management information A at Step S 202 . The management information A is read out by irradiating the optical information recording medium  111  with a reference beam at an angle of θ yA . For subsequently reading out the management information B, the θ y  angle is changed by a predetermined angle Δθ y3  to set the θ y  angle approximately in the middle of the recording angle range for the management information B at Step S 210 . And, the management information B is read out by irradiating the optical information recording medium  111  with a reference beam at an angle of θ yB . Thereby, the management information A and the management information B can be reproduced quickly and exactly. 
         [0063]    In addition, as mentioned above, the same pieces of management information are recorded over the recording angle range, and the θ y  angle, therefore, does not need to be set strictly in the middle angle of the recording angle range. Furthermore, it is not necessary to fit to the reproduction conditions of specific page data. Even if brightness of reproduction images varies as a result of a disagreement with reproduction conditions, it is surely possible to read out page data recorded on some pages close to each other. 
         [0064]    In case that a large difference between temperatures for recording and reproducing generates a crosstalk, fractions of two or more pages could be displayed side by side. However, the same management information recorded connects fragmentary reproduction images from two or more pages, thereby yielding complete management information. For this reason, predetermined management information can be reproduced without a special adjustment of wavelength or angle. Therefore, management information can be reproduced rapidly and certainly. 
         [0065]    As mentioned above, according to the first embodiment of the invention, it is not necessary to exactly adjust reproducing conditions, and it is possible to acquire management information as an exact reproduction image even if a crosstalk is generated. Therefore, it is possible to reduce time to reproduce user data after setting the optical information-recording medium  111  into the recording/reproducing apparatus  100 , i.e., to rapidly access user data soon after starting up the holographic memory recording/reproducing apparatus  100 . 
         [0066]    Alternatively, the spatial beam modulator  107  may express 1-bit information with 2 pixels as a modified example of the holographic memory recording/reproducing apparatus  100  according to the first embodiment, as shown in  FIG. 9 . Furthermore, as another example, the spatial beam modulator  107  may express 1-bit information with 4 pixels of two-row by two-column, as shown in  FIG. 10 . 
         [0067]    When fractions of adjacent pages are reproduced side by side so that the relative angle among the reference beam, the information beam and the optical information recording medium  111  for recording is quite different from that for reproducing, the position of the reproduction image could shift in some cases. In such a case, a bit error rate increases. Then, a bit error rate (bER) can be reduced by expressing 1 bit with two or more pixels as mentioned above. Thereby, although the amount of information decreases, the amount of relative position shifts to 1 bit can be reduced. Thereby, a reproduction image can be stably read out. 
       Second Embodiment 
       [0068]    A holographic memory recording/reproducing apparatus according to a second embodiment records management information continuously in the predetermined θ y  angle range. The holographic memory recording/reproducing apparatus according to the first embodiment changes the θ y  angle, and sets the θ y  angle to a predetermined angle. Then, a beam is emitted from the laser light source thereof. And, the emission of the beam is once stopped. The θ y  angle is changed again, and set to a predetermined angle. Then, the beam is emitted from the laser light source thereof. Thus, management information is recorded at intervals of Δθ y1 . 
         [0069]    On the other hand, the holographic memory recording/reproducing apparatus according to the second embodiment changes the θ y  angle from a predetermined θ y  angle in the recording angle range of Δθ y1 ×N while continuing to emit a beam from the laser light source thereof. Thereby, continuous management information is recorded in the recording angle range of Δθ y1 ×N as in analog recording. In addition, when the actuator  130  changes the θ y  angle by a minimum angle unit which the actuator  130  can control or by a predetermined unit of Δθ y1  or less as in digital recording, a beam is emitted continuously from the laser light source while changing the θ y  angle within the recording angle range. 
         [0070]    Thus, when a variation in brightness of reproduction images arises from the disagreement with the reproducing conditions, continuously recording management information leads to a reduction in the variation. Therefore, management information can be reproduced with more accuracy. 
         [0071]    In addition, the composition and operation of the holographic memory recording/reproducing apparatus according to the second embodiment are the same as those of the holographic memory recording/reproducing apparatus according to the first embodiment except the way of recording management information.