Abstract:
An optical device for holographic recording or reading is provided and includes: a light emission unit that emits light to be irradiated on an optical recording medium, the light being recording or reading light; a first prism unit that moves in accordance with a moving speed of the optical recording medium and refracts the light to move the light irradiated on the optical recording medium over a distance in a moving direction of the optical recording medium so that the light is irradiated on substantially the same position on the optical recording medium for a period of time; and a second prism unit that moves in accordance with the moving speed of the first prism unit to compensate for an optical path length of the light.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 USC §119 from Japanese Patent Application No. 2007-119136 filed Apr. 27, 2007. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to an optical device for holographic recording or reading and a method for controlling the same. 
     (ii) Related Art 
     A holographic recording and reading method is a method for recording or reading digital data by means of recording or reading, as a hologram, signal light in which binary digital data “0 and 1” are digitally imaged as “bright and dark”, for example. In this case, the signal light is subjected to Fourier transform by means of a lens, and a Fourier-transformed image is irradiated on an optical recording medium. Simultaneously, reference light which interferes with the signal light to generate interference fringes is also irradiated on the optical recording medium. 
     In such a holographic recording-and-reproduction method, an inexpensive semiconductor laser is often used as a source of recording light or reproduction light. Since output light of the semiconductor laser is weak, the recording light or reproduction light must be irradiated on the same position on an optical recording medium for a given period of time in order to enhance the accuracy of recording or reading operation. 
     SUMMARY 
     According to an aspect of the invention, there is provided an optical device for holographic recording or reading, comprising: 
     a light emission unit that emits light to be irradiated on an optical recording medium, the light being recording or reading light; 
     a first prism unit that moves in accordance with a moving speed of the optical recording medium and refracts the light to move the light irradiated on the optical recording medium over a distance in a moving direction of the optical recording medium so that the light is irradiated on substantially the same position on the optical recording medium for a period of time; and 
     a second prism unit that moves in accordance with the moving speed of the first prism unit to compensate for an optical path length of the light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a view showing a configuration of an optical device for holographic recording or reading according to an exemplary embodiment of the present invention; 
         FIG. 2  is a view showing a configuration of a compensation prism unit and an irradiation position-movement prism unit; 
         FIG. 3  is a descriptive view of an optical path along which light travels; 
         FIG. 4  is a descriptive view of the principle of movement of a light-incident position on an optical recording medium by the irradiation position-movement prism unit; 
         FIG. 5  is a view showing another configuration of a compensation prism unit and an irradiation position-movement prism unit; and 
         FIG. 6  is a view showing an arrangement of disks constituting a compensation prism unit and an irradiation position-movement prism unit. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present invention will be described below with reference to the accompanying drawings. 
       FIG. 1  shows a configuration of a holographic recording or reading optical device according an exemplary embodiment to the present invention. In  FIG. 1 , when signal light is recorded as a hologram, coherent light from a light source  10  is converted into collimated light of a large diameter through lenses  12 ,  14 , and the collimated light enters into a spatial light modulator  16 . 
     The spatial light modulator  16  includes, for example, a liquid-crystal panel, and produces a digital image (a binary image, but the present invention is not limited thereto), in which binary digital data “0, 1” are taken as “bright, dark”, by means of a computer (not illustrated). As a result, the intensity of the light passed through the spatial light modulator  16  is modulated according to values of respective pixels of the binary image, to thus become signal light S. The signal light S passes through a prism unit  20  for compensation (compensation prism unit) and a prism unit  22  for moving an irradiation position (irradiation position-movement prism unit) and enters into a Fourier transform lens  26  through a relay lens  24 . The signal light S is subjected to Fourier transform by means of the Fourier transform lens  26  and irradiated on an optical recording medium  28 . 
     The irradiation position-movement prism unit  22  is provided for periodically moving recording light or reproduction light (signal light S or reference light R) to be irradiated on the optical recording medium  28 , over a distance in the direction of movement of the optical recording medium  28  which is rotating, so that the recording light or reproduction light is irradiated on substantially the same position on the optical recording medium  28  for a period of time. The compensation prism unit  20  is provided for compensating for an optical path length of the recording light or reproduction light passing through the irradiation position-movement prism unit  22  in such a way a focal position is fixed in a thicknesswise direction of the optical recording medium. The compensation prism unit  20  and the irradiation position movement prism unit  22  will be described later. 
     The reference light R shares an optical axis with the signal light S and is irradiated on the optical recording medium  28  from the outside of the signal light S. The coherent light from the light source  10  is converted into collimated light through the lenses  12  and  14 , and the collimated light enters, as the reference light R, into an outer peripheral region of the spatial light modulator  16 . The reference light R passed through the outer peripheral region of the spatial light modulator  16  is irradiated on the optical recording medium  28  through the relay lens  18 , the compensation prism unit  20 , the irradiation position-movement prism unit  22 , the relay lens  24 , and the Fourier transform lens  26 . 
     Through the foregoing processes, the signal light S and the reference light R, which have been subjected to Fourier transform, interfere with each other in the optical recording medium, and the signal light S is recorded as a hologram in the optical recording medium  28 . 
     The spatial light modulator  16  is described as a transmission type but can also be embodied as a reflection type. The reference light R may not share the optical axis with the signal light S. In that case, the reference light R passes through another optical path differing from the optical path for the signal light S by means of an appropriate beam splitter and an appropriate reflection mirror, and the compensation prism unit  20  and the irradiation position-movement prism unit  22  are placed in the optical path of the reference light R. 
     In  FIG. 1 , when information is read from diffracted light of the hologram, coherent light from the light source  10  is converted into only the reference light R by means of the spatial light modulator  16 , and the reference light R passes through the compensation prism unit  20  and the irradiation position-movement prism unit  22  through the relay lens  18 , to thus enter the Fourier transform lens  26  through the relay lens  24 . The reference light R is irradiated on the optical recording medium  28  through the Fourier transform lens  26 , and thus-generated light which is diffracted from the hologram is converted into collimated light through an inverse Fourier transform lens  26 . The collimated light is received by an appropriate photosensor, thereby acquiring information included in the hologram. 
       FIG. 2  shows a configuration of the compensation prism unit  20  and the irradiation position-movement prism unit  22 . In  FIG. 2 , the compensation prism unit  20  includes a fixed prism  34   a  and a disk  32   a  having a given number of rotary prisms  30   a  along a periphery of the disk  32   a . Moreover, the irradiation position-movement prism unit  22  includes a fixed prism  34   b  and a disk  32   b  having a given number of rotary prisms  30   b  along a periphery of the disk  32   b . The compensation prism unit  20  and the irradiation position-movement prism unit  22  are arranged in such a way that a rotation axis  36   a  of the compensation prism unit  20  forms an angle with a rotation axis  36   b  of the irradiation position-movement unit  22 . In the embodiment shown in  FIG. 2 , the disk  32   a  forming the compensation prism unit  20  and the disk  32   b  forming the irradiation position-movement prism unit  22  are configured so as to be rotated around the respective rotation axes  36   a  and  36   b  in direction of arrow A and direction of arrow B (in the same direction when viewed in a light-incident direction) by an unillustrated appropriate rotation controller and an unillustrated appropriate motor. 
     In the compensation prism unit  20 , light (the signal light S or the reference light R) enters into an incident plane of the rotary prism  30   a  at right angle and passes through the fixed prism  34   a  to enter into the irradiation position-movement prism unit  22 . In the irradiation position-movement prism unit  22 , the light enters into the rotary prism  30   b  and passes through the fixed prism  34   b  to exit toward the optical recording medium  28 . In  FIG. 2 , after passing the compensation prism unit  20 , light passes through the irradiation position-movement prism unit  22 . However, the present invention is not limited to this sequence, and the sequence may also be inverted. 
       FIG. 3  shows a descriptive view of the optical path through which light travels. In  FIG. 3 , after entering into an incident plane I of the rotary prism  30   a , the light through the fixed prism  34   a  to enter into an oblique surface V of the rotary prism  30   b . After entering into the oblique surface V of the rotary prism  30   b , the light enters into the fixed prism  34   b . At this time, the light exits an exit plane VIII of the fixed prism  34   b  at an exit position β displaced leftwardly from an entrance position α on the oblique surface V of the rotary prism  30   b . A difference between the entrance position α and the exit position β is dependent on the thickness of the rotary prism  30   b . In conjunction with rotation of the disk  32   b  constituting the irradiation position-movement prism unit  22 , the rotary prism  30   b  also moves in the direction of arrow B in  FIG. 3 , and hence the exit position β moves leftwardly in the drawing in conjunction with movement of the rotary prism  30   b . As a result, the direction of light entering into the relay lens  24  shown in  FIG. 1  is changed, and the position of the light entering into the optical recording medium  28  through the Fourier transform lens  26  is also moved. The direction of movement of the entrance position (an incident position of light on the optical recording medium) can coincide with the direction of movement of the optical recording medium  28  by means of setting the direction of rotational movement of the rotary prism  30   b  and the layout between the rotary prism  30   b  and the fixed prism  34   b.    
     The irradiation position-movement prism unit  22  is provided with the plurality of rotary prisms  30   b  and rotates in the direction of arrow B in  FIG. 2 . Accordingly, the position of the light entering into the optical recording medium  28  is periodically moved for each of the rotary prisms  30   b . So long as the moving speed of the rotary prisms  30   b  is set in accordance with the rotating speed of the optical recording medium  28 , substantially the same position on the optical recording medium  28  can be irradiated with the light for a given period of time by means of periodic movement of the incident position of the light. 
     The optical path length of the light passing through the rotary prisms  30   b  and the fixed prism  34   b  is changed in conjunction with movement of the rotary prisms  30   b  in the direction of arrow B. Thereby, the focal position of the Fourier transform lens  26  is also displaced in the traveling direction of the light, and hence forming a hologram in the optical recording medium  28  may become difficult. Accordingly, the optical path length is corrected by the rotary prisms  30   a  and the fixed prism  34   a  serving as the compensation prism unit  20  in such a way that the focal point is not deflected in the thicknesswise direction of the medium. Specifically, the incident planes of the rotary prisms  20   a  and the incident planes of the rotary prisms  20   b , where light enters, are arranged so as to spatially overlap each other in the traveling direction of light, and the disk  32   a  and the disk  32   b  are rotated at the same speed. At this time, as shown in  FIG. 3 , the thickness of the rotary prism  30   a  through which light passes becomes smaller in conjunction with movement of the rotary prism  30   a  in the direction of arrow A, and the thickness of the rotary prism  30   b  through which light passes becomes greater in conjunction with movement of the rotary prism  30   b  in the direction of arrow B. As a result, the optical path length of light passing through the rotary prisms  30   a , the fixed prism  34   a , the rotary prisms  30   b , and the fixed prism  34   b  can be made constant at all times. Therefore, position displacement of the focal point of the Fourier transform lens  26  can be avoided. 
     The compensation prism unit  20  is configured in such a way that the incident plane I of the rotary prism  30   a  and the exit plane IV of the fixed prism  34   a  become parallel to each other. Since light is arranged so as to enter into the incident plane I of the rotary prism  30   a  at right angle, the light incident on the rotary prism  30   a  and the light exiting from the fixed prism  34   a  become parallel to each other. Further, the compensation prism unit  20  is also configured in such a way that the incident plane II of the rotary prism  30   a  and the exit plane III of the fixed prism  34   a  become parallel to the direction A of movement of the rotary prism  30   a . Thereby, even when the rotary prisms  30   a  moves in the direction of arrow A, light enters into a constant position on the incident plane III of the fixed prism  34   a . Accordingly, the light exits from a constant position on the exit plane IV of the fixed prism  34   a , and an optical path length of light passing through the rotary prism  30   a  and the fixed prism  34   a  is determined by only the position of the rotary prism  30   a  where light enters. Therefore, a change in the optical path length due to movement of the rotary prisms  30   b  of the irradiation position-movement prism unit  22  can be readily corrected by controlling only the moving speed of the rotary prisms  30   a.    
       FIG. 4  shows a descriptive view of the principle of the light-incident position on the optical recording medium  28  being moved by the irradiation position-movement prism unit  22 . In  FIG. 4 , light focused by the relay lens  18  shown in  FIG. 1  at a position in front of the relay lens  24  is collimated by the relay lens  24 , and the thus-collimated light is focused on the optical recording medium  28  through the Fourier transform lens  26 . In general, when the collimated light enters the Fourier transform lens  26 , the incident position of light on the optical recording medium  28  is not moved even when the collimated light is parallel-shifted in a direction perpendicular to the optical axis, the light-incident position on the optical recording medium  28  is not moved. However, when the focal point focused in front of the relay lens  24  is moved in the direction perpendicular to the optical axis, the light-incident position on the optical recording medium  28  is also moved. 
     In  FIG. 4 , when the position of the focal point (the focus made by the relay lens  18  shown in  FIG. 1 ) in front of the relay lens  24  is displaced over merely a distance L 1  by means of the rotary prism  30   b  and the fixed prism  34   b  serving as the irradiation position-movement prism unit  22 , the light-incident position on the optical recording medium  28  is displaced by L 2 . At this time, provided that the focal length of the relay lens  24  is taken as f 1  and that the focal length of the Fourier transform lens  26  is taken as f 2 , a relationship of f 1 /f 2 =L 1 /L 2  stands. Accordingly, the amount of displacement of the light-incident position on the optical recording medium  28  can be set according to the equation. 
       FIG. 5  shows another configuration of the compensation prism unit  20  and the irradiation position-movement prism unit  22 . In  FIG. 5 , the oblique surfaces of the respective rotary prisms  30   a  formed on the compensation prism unit  20  are opposite in direction to the oblique surfaces shown in  FIG. 2 . The disk  32   a  and the disk  32   b  are arranged so as to become offset from each other in a direction interconnecting the rotation centers of the disks, as shown in  FIG. 6B . Moreover, as indicated by an arrow C, the rotating direction of the disk  32   a  is opposite to the rotating direction of the same shown in  FIG. 2 . Therefore, in the embodiment shown in  FIG. 5 , the disk  32   a  and the disk  32   b  rotate in opposite directions (when viewed in a light-incident direction). In such a configuration, the position of light entering the optical recording medium  28  can be moved by the principle shown in  FIG. 3 . 
       FIGS. 6A and 6B  show an arrangement of the disk  32   a  and disk  32   b  constituting the compensation prism unit  20  and the irradiation position-movement prism unit  22  shown in  FIGS. 2 and 6 .  FIG. 6A  shows an arrangement of  FIG. 2 , in which the disk  32   a  and the disk  32   b  spatially overlap each other with respect to the vertical direction (i.e., the incident direction of light). 
     In contrast,  FIG. 6B  shows an arrangement of  FIG. 5 , in which the disk  32   a  and the disk  32   b  are offset from each other with respect to the direction of the line interconnecting the rotation centers of the disks  32   a  and  32   b . Therefore, of the rotary prisms  30   a  and  30   b , only a prism into which light enters and prisms adjacent thereto spatially overlap each other. 
     The disk  32   a  and the disk  32   b , which are shown in  FIG. 2 , may also be arranged as shown in  FIG. 6B  and rotated in a single direction (the direction of arrows A and B). Alternatively, the disk  32   a  and the disk  32   b , which are shown in  FIG. 5 , may also be arranged as shown in  FIG. 6A  and rotated in an opposite direction (the direction of arrows B and C).