Abstract:
An optical apparatus includes: a focusing lens that focuses light which is one of recording light being irradiated to an optical recording medium with reference light for recording a hologram and reference light being irradiated to an optical recording medium for reading out a hologram; and a focal position-shifting unit that refracts the light while moving in accordance with a moving speed of the optical recording medium, so as to shift a focal position of the light in the optical recording medium a distance in a thickness direction of the optical recording medium.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on and claims priority under 35 USC §119 from Japanese Patent Application No. 2007-219587 filed Aug. 27, 2007. 
       BACKGROUND 
       [0002]    (i) Technical Field 
         [0003]    This invention relates to an optical apparatus. 
         [0004]    (ii) Related Art 
         [0005]    A hologram recording and reproducing apparatus records and reproduces digital data by means of recording and reproducing, as a hologram, signal light in which binary digital data “0 and 1” etc., is digitally imaged as, for example, “bright and dark”. In the case of a Fourier transform hologram, signal light is subjected to Fourier transform by means of a lens and the Fourier-transformed light is irradiated to an optical recording medium together with reference light interfering with the recording light to generate an interference fringe, and the generated interference fringe is recorded in the optical recording medium as a hologram. To reproduce data, reference light is irradiated to the optical recording medium, diffracted light is received at a photo detector, and a digital image is reproduced to obtain digital data. 
         [0006]    To enhance the recording capacity of an optical recording medium, a method of multiply recording a hologram in the thickness direction of the optical recording medium has been proposed. In this case, it is necessary to shift the focal position of recording light and reference light at high speed and precisely in the thickness direction of the optical recording medium. 
       SUMMARY 
       [0007]    According to an aspect of the invention, there is provided an optical apparatus including: 
         [0008]    a focus lens that focuses light which is one of recording light being irradiated to an optical recording medium for recording a hologram and reference light being irradiated to an optical recording medium for reproducing (reading out) a hologram; and 
         [0009]    a focal position-shifting unit that refracts the light while moving in accordance with a moving speed of the optical recording medium, so as to shift a focal position of the light in the optical recording medium a distance in a thickness direction of the optical recording medium. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Embodiments of the present invention will be described in detail based on the following figures, wherein: 
           [0011]      FIG. 1  shows a hologram recording and reproducing apparatus including an optical apparatus according to an exemplary embodiment of the invention; 
           [0012]      FIG. 2  shows a focal position-shifting plate and an optical path length-compensation plate; 
           [0013]      FIG. 3  is a schematic drawing of the light focal position-shifting function of the focal position-shifting plate; 
           [0014]      FIG. 4  is a schematic drawing of a principle of shifting a light focal position; and 
           [0015]      FIG. 5  shows an explanation drawing for calculating the shifting distance of the light focal position. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    An exemplary embodiment will be discussed with reference to the accompanying drawings. 
         [0017]      FIG. 1  shows a hologram recording and reproducing apparatus including an optical apparatus according to an exemplary embodiment of the invention. In  FIG. 1 , to record signal light as a hologram, coherent light from a light source  10  is converted into collimated light having a wide diameter by means of lenses  12  and  14 , and the collimated light is made incident on a spatial light modulator  16 . 
         [0018]    The spatial light modulator  16  include a liquid crystal panel for example and produces a digital image (binary image; however, not limited thereto) in which binary digital data “0 and 1” is taken as “bright and dark” by means of a computer (not shown). Accordingly, the light passed through the spatial light modulator  16  is intensity-modulated in accordance with the value of each pixel of the binary image to produce signal light S. The signal light S is Fourier-transformed (focused) through a Fourier transform lens  18  and is irradiated to an optical recording medium  22  through a focal position-shifting plate  20 . 
         [0019]    Reference light R is made to have a common optical axis to the signal light S and is irradiated to the optical recording medium  22  from the outside of the signal light S. As the reference light R, coherent light from the light source  10  is converted into collimated light by means of the lenses  12  and  14  and the collimated light is made incident on the outer peripheral area of the spatial light modulator  16 . The reference light R passed through the outer peripheral area of the spatial light modulator  16  is irradiated to the optical recording medium  22  through the Fourier transform lens  18  and the focal position-shifting plate  20  like as in the signal light S. 
         [0020]    As a result of the above process, the signal light S and the reference light R after Fourier-transformed interfere with each other in the optical recording medium  22  and the signal light S is recorded in the optical recording medium  22  as a hologram. 
         [0021]    The spatial light modulator  16  is described as transmission type, but can also be of reflection type. The reference light R is not limited to the above case in which it passes through the outer peripheral area of the spatial light modulator  16 . For example, the reference light R may be passed through a different optical path from the signal light S by using an appropriate beam splitter, an appropriate reflecting mirror and the like, to have the optical axis common to the signal light S at the stage at which the reference light R is incident on the Fourier transform lens  18 . 
         [0022]    The focal position-shifting plate  20  is adapted to shift the focal position of recording light (the signal light S and the reference light R at the recording time) and the reference light R (at the reproducing time) to be irradiated to the optical recording medium  22  a distance in the thickness direction of the optical recording medium  22 . The focal position-shifting plate  20  is described later. 
         [0023]    Next, in  FIG. 1 , to reproduce information from diffracted light of a hologram, coherent light from the light source  10  is converted into collimated light having a wide diameter by means of the lenses  12  and  14 , and only reference light R is allowed to be incident on the Fourier transform lens  18  through the spatial light modulator  16  and is irradiated to the optical recording medium  22  while the light focal position is adjusted with the focal position-shifting plate  20 . The thus-generated diffracted light from the hologram passes through an optical path length-compensation plate  24  and is converted into collimated light through an inverse Fourier transform lens  26  and the collimated light is received by an appropriate photo detector  28  to obtain information contained in the hologram. 
         [0024]    Since the image forming position at the time of the reproducing shifts a distance as long as the shifting length of the light focal position by the focal position-shifting plate  20 , the image detected by the photo detector  28  blurs. Then, the light path length is compensated by the optical path length-compensation plate  24  to avoid the blur of the image. The optical path length-compensation plate  24  is described later. 
         [0025]      FIGS. 2A and 2B  show illustrative configurations of the focal position-shifting plate  20  and the optical path length-compensation plate  24 . The focal position-shifting plate  20  and the optical path length-compensation plate  24  each is formed of an optically transparent material such as glass, transparent resin, etc. In  FIG. 2A , the optical recording medium  22  existing between the focal position-shifting plate  20  and the optical path length-compensation plate  24  is not shown.  FIG. 2B  is a plan view of the focal position-shifting plate  20  and the optical path length-compensation plate  24 . 
         [0026]    In  FIG. 2A , the focal position-shifting plate  20  and the optical path length-compensation plate  24  each is formed like a disk. Planes orthogonal to the optical axis of recording light or reference light are formed on the circumference of each of the focal position-shifting plate  20  and the optical path length-compensation plate  24 , and the planes are divided areas whose thicknesses in the optical axis direction are periodically different. In an embodiment op  FIG. 2A , two kinds of thickness are provided so that an alternating pattern of a raised plane α and a recess plane β is formed on the periphery of the disk, but the embodiment is not limited thereto. Three or more kinds of areas different in the thickness may be formed in accordance with a multiple recording in the thickness direction of the optical recording medium  22 . 
         [0027]    In each of the focal position-shifting plate  20  and the optical path length-compensation plate  24  according to the embodiment of  FIG. 2A , portions of the raised planes α have the same thickness in a direction of the optical axis and also portions of the recess planes β have the same thickness in the direction of the optical axis. As shown in  FIG. 2B , the raised planes α formed on the focal position-shifting plate  20  and the optical path length-compensation plate  24  are formed as the same plane shape, and the recess planes β are also formed as the same plane shape. The raised plane α and the recess plane β are planes of the same size. Further, the focal position-shifting plate  20  and the optical path length-compensation plate  24  are placed so that light passed through the raised plane α of the focal position-shifting plate  20  passes through the recess plane β of the optical path length-compensation plate  24  downstream of the optical path, and light passed through the recess plane β of the focal position-shifting plate  20  passes through the raised plane α of the optical path length-compensation plate  24  downstream of the optical path, as indicated by the dashed lines in  FIG. 2A . In this state, the focal position-shifting plate  20  and the optical path length-compensation plate  24  rotate at the same speed in the same direction. Consequently, the light passed through the focal position-shifting plate  20  and the optical path length-compensation plate  24  always travels the same distance through the optically transparent material such as glass. Accordingly, blurring of the image detected by the photo detector  28  can be avoided. 
         [0028]    The thickness difference between the raised plane α and the recess plane β can be determined by the fluctuation amount of the necessary focal length (focal position) and the value of the thickness difference can be obtained by a calculation expression described later. The Fourier transform lens  18  and the inverse Fourier transform lens  26  should be designed so as to lessen the effect of aberration caused by the sum of the thicknesses of the focal position-shifting plate  20  and the optical path length-compensation plate  24 , which is a constant. 
         [0029]      FIGS. 3A and 3B  are schematic drawings of the light focal position-shifting function of the focal position-shifting plate  20 . In  FIGS. 3A and 3B , the focal position-shifting plate  20  has the raised planes α and the recess planes β described above in the periphery portion. The focal position-shifting plate  20  rotates in the arrow A direction at a speed so that the raised planes α and the recess planes β through which recording light or reference light passes change in accordance with the moving speed of the optical recording medium  22 . The rotation speed of the focal position-shifting plate  20  is determined by the moving speed of the optical recording medium  22 , the surface concentration of information multiple-recorded in the optical recording medium  22 , the length (pitch) in the arrow A direction of the raised planes α and the recess planes β, and the like. It is not desirable to irradiate light so that recording light and reference light are put on the steps between the raised plane α and the recess plane β in each of the focal position-shifting plate  20  and the optical path length-compensation plate  24 . It is desirable that control should be performed so that a laser is on only when the optical path passes through the flat portions of the raised plane α and the recess plane β. Thus, it is desirable that the beam spot on the surfaces of the steps should be small as much as possible and that the corresponding planes of the focal position-shifting plate  20  and the optical path length-compensation plate  24  face to the optical recording medium  22  and are placed at an equal distance from the optical recording medium  22 . 
         [0030]    As shown in  FIGS. 3A and 3B , when emission light from the Fourier transform lens  18  is incident on the raised plane α or the recess plane β of the focal position-shifting plate  20 , the light focal position changes in the thickness direction of the optical recording medium  22  because of refraction of light. For example, if light passes through the raised plane α, the light focal position becomes the back of the optical recording medium  22 , namely, a distant position from the incidence surface of the optical recording medium  22 , and if light passes through the recess plane β, the light focal position becomes the front of the optical recording medium  22 , namely, a near position to the incidence surface of the optical recording medium  22 . 
         [0031]      FIG. 4  is a schematic drawing of a principle of shifting the light focal position. In  FIG. 4 , emission light from the Fourier transform lens  18  passes through the focal position-shifting plate  20  and is focused at a position of the optical recording medium  22 . When light passes through the focal position-shifting plate  20 , because of refraction of light, the light focal position shifts to the back of the optical recording medium  22  from the light focal position applied when the focal position-shifting plate  20  does not exist (indicated by the dashed line). Thus, the thickness of the focal position-shifting plate  20  is appropriately adjusted, whereby the light focal position can be controlled to a plurality of positions (two positions in  FIGS. 3A and 3B ) in the thickness direction of the optical recording medium  22  as shown in  FIGS. 3A and 3B . 
         [0032]      FIGS. 5A ,  5 B, and  5 C are drawings for calculation of the shifting distance of the light focal position. In  FIG. 5A , when the focal position-shifting plate  20  does not exist, emission light of the Fourier transform lens  18  is incident on the optical recording medium  22  at an incidence angle Θ 1 , propagates through the optical recording medium  22  at a refraction angle (angle relative to the normal raised on the incidence surface) Θ 2 , and is focused at a position of distance f 1  from the incidence surface. At this time, it is assumed that shift between the incidence position and the light focal position in the direction along the incidence surface is n 1 . It is also assumed that the refractive index of air is n 1  (=1) and that of the optical recording medium  22  is n 2 . 
         [0033]    Next, in  FIG. 5B , the case where the focal position-shifting plate  20  having thickness displaced between the Fourier transform lens  18  and the optical recording medium  22  is considered. However, for convenience of calculation, it is assumed that the focal position-shifting plate  20  and the optical recording medium  22  are placed in intimate contact with each other and the refractive indexes of the focal position-shifting plate  20  and the optical recording medium  22  are the same value (n 2 ). In this case, emission light from the Fourier transform lens  18  is refracted on the incidence face of the focal position-shifting plate  20  and propagates through the focal position-shifting plate  20  and the optical recording medium  22  at the refraction angle Θ 2  as shown in  FIG. 5C . 
         [0034]    Assuming that shift between the incidence position on the incidence surface of the optical recording medium  22  when the focal position-shifting plate  20  does not exist and the incidence position on the focal position-shifting plate  20  in the direction of the incidence surface is a 2 , if the focal position-shifting plate  20  having thickness d is placed, the incidence position shifts a 1 +a 2  in the direction of the incidence surface relative to the light focal position. Assuming that light is not refracted on the incidence surface of the focal position-shifting plate  20 , light arrives at the emission surface of the focal position-shifting plate  20  at a position of shift a 2  in the direction of the incidence surface from the incidence position. 
         [0035]    The relationship between each angle and length in  FIG. 5C  is arranged to: 
         [0000]        a 1= f 1×tan Θ2 
         [0000]        a 2= d ×tanΘ1 
         [0036]    When the distance between the emission surface of the focal position-shifting plate  20  and the light focal position is f 2 , 
         [0000]    
       
         
           
             
               
                 
                   
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         [0037]    When the numerical aperture of the Fourier transform lens  18  us NA, 
         [0000]      Θ1=arc sin  NA, Θ 2=arc sin  NA/n 2 
         [0038]    Therefore, 
         [0000]      Δ f=d ×{tan(arc sin  NA )−tan(arc sin  NA/n 2)}/{tan(arc sin  NA/n 1)} 
         [0039]    The invention can be applied without departing from the scope of the description. For example, in the Specification, the depth of the light focal position is changed at two positions using two surfaces of raised plane α and recess plane β, but the number can be adjusted as required. The raised planes α and the recess planes β are arranged like a disk and are rotated in the perpendicular direction to the optical axis of the recording light or the reference light, but the invention is not limited thereto. Any moving of the planes can be made in accordance with the moving speed of the optical recording medium  22 .