Abstract:
A hologram reading apparatus includes: a unit holding a hologram recording medium in which data page is recorded by irradiating as a single beam both reference light and signal light modulated by a spatial light modulator including a first pixel area for modulating the reference light and a second pixel area for modulating the signal light, a pitch of pixels in the first pixel area being different from that in the second pixel area; a Fourier transform lens subjecting reproduction light to a Fourier transformation; a filter disposed on a Fourier transform plane of the reproduction light by the Fourier transform lens, the filter shielding the reference light at a first spatial frequency band and transmitting the signal light at a second spatial frequency band; and a reading unit receiving the reproduction light transmitted through the filter and reading the data page modulated to the signal light included in the reproduction light.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on and claims priority under 35 USC §119 from Japanese Patent Application No. 2007-224489 filed Aug. 30, 2007. 
       BACKGROUND 
       [0002]    (i) Technical Field 
         [0003]    The present invention relates to a hologram reading apparatus, a hologram reading method, a hologram recording apparatus and a hologram recording method. 
         [0004]    (ii) Related Art 
         [0005]    Some of hologram recording techniques employ the so-called coaxial-type recording technique in which a hologram recording medium is irradiated with reference light and signal light as a single beam to thereby record a hologram formed by the interference between the reference light and the signal light in the hologram recording medium. One of the advantages resulted from the employment of the coaxial-type recording technique is that the hologram recording/reading apparatus can be miniaturized. 
         [0006]      FIG. 6  shows a part of an optical system constituting a hologram recording/reading apparatus  2  in a related art. As shown in  FIG. 5 , the hologram recording/reading apparatus  2  is configured in a manner that a spatial light modulator  25  irradiates a hologram recording medium  100  with spatial-modulated reference light and spatial-modulated signal light as the same beam to thereby record date therein. On the other hand, in the case of reading data, only the reference light serving as a reading beam is irradiated to the hologram recording medium  100  to thereby reproduce a reproduction beam, then an iris  45  shields a reference light portion of the reproduction beam and a filter  47  disposed at the focal plane of a Fourier transform lens  46  extracts a desired spatial frequency band of the signal light. A light receiving element  50  reads data based on the extracted desired spatial frequency band of the signal light. 
       SUMMARY 
       [0007]    According to an aspect of the invention, there is provided a hologram reading apparatus including: 
         [0008]    a holding unit that holds a hologram recording medium in which data page is recorded by irradiating as a single beam both reference light and signal light which are modulated by a spatial light modulator, wherein the spatial light modulator includes a first pixel area for modulating the reference light and a second pixel area for modulating the signal light based on data page to be recorded, and a pitch of pixels in the first pixel area is different from that in the second pixel area; 
         [0009]    a Fourier transform lens that subjects reproduction light, which is reproduced by irradiating the reference light to the hologram recording medium, to a Fourier transformation; 
         [0010]    a filter disposed on a Fourier transform plane of the reproduction light by the Fourier transform lens, wherein the filter shields the reference light at a first spatial frequency band and transmits the signal light at a second spatial frequency band, based on that the reference light and the signal light which are included in the reproduction light differ from each other in distance of image formation positions of a spatial frequency component thereof; and 
         [0011]    a reading unit that receives the reproduction light transmitted through the filter and reads the data page modulated to the signal light included in the reproduction light. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Embodiments of the present invention will be described in detail based on the following figures, wherein: 
           [0013]      FIG. 1  is a diagram showing a hologram recording/reading apparatus according to an exemplary embodiment; 
           [0014]      FIG. 2  is a diagram showing a spatial light modulator; 
           [0015]      FIG. 3  is a diagram showing the focal plane of a lens and a filter; 
           [0016]      FIG. 4  is a diagram showing the focal plane of a lens and a filter; 
           [0017]      FIGS. 5A-5B  are diagrams showing the focal plane of a lens and a filter; and 
           [0018]      FIG. 6  is a diagram showing a part of a hologram recording/reading apparatus in a related art, 
       
    
    
       [0019]    wherein description of some reference numerals and signs are set forth below.
     1  hologram recording/reading apparatus     2  hologram recording/reading apparatus (related art)     10  light source     12  shutter     14  half wave plate     16  polarizing plate     18  enlarging/collimating optical system     20  mirror     22  polarization beam splitter     24  spatial light modulator     25  spatial light modulator (related art)     26 ,  28  lens     30 ,  32  Fourier transform lens     34  filter     36 ,  38  Fourier transform lens     40  medium holding portion     42 ,  44  Fourier transform lens     45  iris     47  filter     50  light receiving element     60  filter     61  band-pass filter     100  hologram recording medium     200  reference light pixel area     300  signal light pixel area     400  0-order DC component     410  primary-order DC components of signal light     510  primary-order DC components of reference light   
 
       DETAILED DESCRIPTION 
       [0048]    Hereinafter, an exemplary embodiment of the invention will be explained with reference to drawings. 
         [0049]      FIG. 1  is a diagram showing a hologram recording/reading (reproducing) apparatus  1  according to an exemplary embodiment. As shown in  FIG. 1 , the hologram recording/reading apparatus  1  includes a light source  10 , a shutter  12 , a half wave plate  14 , a polarizing plate  16 , an enlarging/collimating optical system  18 , a mirror  20 , a polarization beam splitter  22 , a spatial light modulator  24 , lenses  26 ,  28  and Fourier transform lenses  30 ,  32  constituting a relay lens system, a filter  34 , a Fourier transform lens  36  for focusing reference light or both the reference light and signal light in a hologram recording medium  100 , a Fourier transform lens  38  for relaying transmitted light (reproduction light) transmitted through the hologram recording medium, a medium holding portion  40  for holding the hologram recording medium  100 , Fourier transform lenses  42 ,  44  constituting the relay lens system, a filter  60  and a light receiving element  50 . 
         [0050]    The light source  10  irradiates coherent light acting as a light source of the signal light and the reference light for recording hologram. As the coherent light, a light source such as a laser beam having been known conventionally may be employed. As the laser beam, a laser beam of a waveform (for example, a green laser etc. having a wavelength of 532 nm) having the sensitivity at the optical recording layer of the hologram recording medium  100  may be employed. 
         [0051]    The shutter  12  is provided on an optical path of the laser beam irradiated from the light source  10 . The laser beam is interrupted in accordance with the opening/closing of the shutter  12 . The laser beam passed through the shutter  12  further passes the half wave plate  14  and the polarizing plate  16  and so is adjusted in its light intensity and polarization direction. 
         [0052]    The laser beam passed through the polarizing plate  16  is converted into collimated light with a diameter by the enlarging/collimating optical system  18 . The laser beam thus converted into the collimated light by the enlarging/collimating optical system  18  enters into the polarization beam splitter  22 . 
         [0053]    The polarization beam splitter  22  transmits p-polarized light of the incident laser beam and reflects s-polarized light thereof. The laser beam reflected by the polarization beam splitter  22  enters into the spatial light modulator  24 . 
         [0054]    The spatial light modulator  24  polarizes and modulates the laser beam entered from the polarization beam splitter  22  in accordance with a pattern according to recording information. The recording information is represented by a pattern image of bright and dark in which digital data “0”, “1” is made correspond to “bright”, “dark”, respectively. The laser beam having alight intensity modulation pattern subjected to the light intensity modulation enters again into the polarization beam splitter  22 . In this case, since the polarization beam splitter  22  transmits the p-polarized light, the light modulated by the spatial light modulator  24  transmits the polarization beam splitter  22 . 
         [0055]      FIG. 2  shows an example of the configuration of the spatial light modulator  24 . As shown in  FIG. 2 , the spatial light modulator  24  according to the embodiment is arranged to include reference light pixel area  200  for modulating the reference light and signal light pixel area  300  for modulating the signal light in a manner that the signal light pixel area  300  is disposed at the center portion and the reference light pixel area  200  is disposed at the outer periphery of the signal light pixel area  300 . 
         [0056]    Each of the reference light pixel area  200  and the signal light pixel area  300  is configured by a plurality of pixels and each of the pixels is intensity-modulated into bright or dark in accordance with two-dimensional image data for modulating the reference light and the signal light. In  FIG. 2 , the painted pixels in each of the reference light pixel area  200  and the signal light pixel area  300  represent “dark” pixels. In  FIG. 2 , the painted pattern representing the “dark” pixels is differentiated between the reference light pixel area  200  and the signal light pixel area  300  merely for the sake of the explanation, and actually each of the color and pattern of the “dark” pixels is not differentiated therebetween. 
         [0057]    The pixels contained in the signal light pixel area  300  generate a two-dimensional image obtained by coding data page to be recorded and subject the signal light to the spatial modulation. Also, the reference light pixel area  200  may generate a two-dimensional image obtained by coding a random pattern and subject the reference light to the spatial modulation. The reference light is not necessarily modulated. However, when the reference light is subjected to the random modulation with a period almost same as that of the data page pattern, the reference light can be irradiated uniformly at the time of data page recording, whereby the overlapping of the signal light and the reference light is made large at the hologram recording area and hence data page can be recorded with good accuracy. 
         [0058]    The spatial light modulator  24  according to the embodiment is characterized in that the pitch of the pixels contained in the reference light pixel area  200  differs from the pitch of the pixels contained in the signal light pixel area  300 . That is, supposing that the pitch of the pixels contained in the reference light pixel area  200  is d 1  and the pitch of the pixels contained in the signal light pixel area  300  is d 2 , d 1  is smaller than d 2  in this embodiment. The pitch of the pixels means a distance between the adjacent pixels contained in each of the pixel areas of the spatial light modulator  24 . In this manner, since the pitch of the pixels for modulating the reference light and the pitch of the pixels for modulating the signal light in the spatial light modulator  24  are differentiated, the distance of each of the 0-order component and the primary-order component is differentiated between the reference light and the signal light. That is, the distance between the image formation positions of bright spots on the Fourier transform plane is differentiated between the reference light and the signal light. The embodiment simultaneously performs the cutting of the reference light and the extraction of a desired frequency band of the signal light by utilizing the difference of the distance between the image formation positions. This process will be explained later. 
         [0059]    Recording light including the signal light and the reference light each subjected to the spatial modulation by the spatial light modulator  24  is relayed by the lenses  26 ,  28  constituting the relay lens system and entered into the Fourier transform lens  30 . The recording light is focused by the Fourier transform lens  30  so as to pass the filter  34 . A frequency band of the recording light is cut when passing through the filter  34 . Since the frequency band of the recording light is cut by the filter  34 , the recording more effectively utilizing the hologram recording medium  100  can be realized. The filter  34  may be constituted by a low pass filter for passing the DC component of the primary or less-order of the spatial frequency component of the reference light and the signal light. In this case, the radius of the transmission portion of the filter  34  is set to be fλ/d 1  (the pitch of the pixels of the reference light) or more. 
         [0060]    The recording light passed through the filter  34  is converted into collimated light again by the Fourier transform lens  32  and entered into the Fourier transform lens  36  for focusing the recoding beam in the hologram recording medium  100 . 
         [0061]    The Fourier transform lens  36  focuses the reference light and the signal light in the hologram recording medium  100  which is held by the medium holding portion  40 . Then, hologram (interference fringe) formed by the interference between the reference light and the signal light at the position where the reference light and the signal light are focused is recorded in an optical recording layer of the hologram recording medium  100 . The aforesaid explanation is a recording process for recording data page in the hologram recording medium  100 . 
         [0062]    Next, the explanation will be made as to a process of reading (reproducing) the data page recorded in the hologram recording medium  100  by the hologram recording/reading apparatus  1 . 
         [0063]    First, in the hologram recording/reading apparatus  1 , only the reference light is irradiated to the hologram recording medium  100 . The irradiated reference light is diffracted by the hologram formed in the hologram recording medium  100  and so reproduction light is obtained. The reproduction light thus obtained includes the reference light and the signal light irradiated at the time of forming the hologram. 
         [0064]    Since the hologram recording medium  100  is a recording medium of a transmission type, the reproduction light transmits the hologram recording medium  100 , then is relayed by the Fourier transform lens  38  and enters into the Fourier transform lens  42 . The filter  60  is disposed on the focal plane of the Fourier transform lens  42 . 
         [0065]      FIG. 3  shows an example of the focal plane of the Fourier transform lens  42  (hereinafter called a Fourier transform plane) disposed at the filter  60 . As shown in  FIG. 3 , a 0-order DC component  400  of the signal light is located at the center of the Fourier transform plane, and primary-order DC components  410  of the signal light are located around the 0-order DC component. Further, primary-order DC components  510  of the reference light are located on the outside of the primary-order DC component of the signal light. In this embodiment, a spot distance L 1  of the reference light can be represented by the following expression (1) and a spot distance L 2  of the signal light can be represented by the following expression (2), where d 1  represents the pitch of the pixels of the reference light pixel area  200  and d 2  represents the pitch of the pixels of the signal light pixel area  300  in the spatial light modulator  24 , f represents the focal distance of the lens, and λ represents the wavelength of the coherent light. The spot distance means a distance between the 0-order component and the primary-order component. 
         [0000]        L 1= fλ/d 1   (1) 
         [0000]        L 2= fλ/d 2   (2) 
         [0066]    When the filter  60  is configured as a low pass filter having a transmission portion with a radius r satisfying the relation of L 2 &lt;r&lt;L 1  and is disposed at the Fourier transform plane, as shown in  FIG. 3 , the reference light can be cut from the reproduction light and only the signal light having the desired frequency band (the primary or less-order DC component) can be transmitted and extracted. When the wavelength λ of the coherent light is 532 nm, the focal distance f of the lens is 100 mm, the pitch d 1  of the pixels of the reference light is 20 μm, the pitch d 2  of the pixels of the signal light is 24 μm, L 1  becomes 2.217 mm and L 2  becomes 2.66 mm. When the filter is configured as a low pass filter having a radius r of 2.5 mm, the reference light can be cut from the reproduction light and the spatial frequency band of the primary or less-order DC component of the signal light can be extracted. 
         [0067]    Since the filter  34  does not remove the reference light, the radius of the filter  34  is is set to be larger than L 1 . That is, in the aforesaid numerical example, the primary-order DC component of each of the reference light and the signal light can be transmitted by setting the radius of the filter  34  to 2.7 mm 
         [0068]    The reproduction light (signal light) transmitted through the filter  60  is relayed by the Fourier transform lens  44  and focused on the light receiving element  50 . The light receiving element  50  reads the recorded data page modulated in the signal light based on the light intensity modulation pattern of the signal light. 
         [0069]    In the hologram recording/reading apparatus  1  according to the embodiment described above, since filter  60  simultaneously performs both the removal of the reference light from the reproduction light and the extraction of the desired frequency band of the signal light, as compared with the hologram recording/reading apparatus  2  of the related art shown in  FIG. 6 , the iris  45  and the Fourier transform lenses  42 ,  44  constituting the relay lens system can be eliminated and so the further miniaturization of the optical system can be realized. 
         [0070]    The invention is not limited to the aforesaid embodiment. 
         [0071]    For example, the filter disposed at the focal plane of the Fourier transform lens  42  is not limited to the low pass filter shown in  FIG. 3 . The filter may be configured as a band-pass filter, like a filter  61  shown in  FIG. 4 , which cuts the 0-order DC component of the signal light but transmits the frequency band including the primary-order DC component of the signal light. In this case, supposing that the transmission portion of the filter  61  is configured as a square shape in a manner that the size of each side of the outer periphery thereof is S 1  and the size of each side of the inner periphery thereof is S 2 , the following expressions are satisfied. 
         [0000]        L 2&lt; S 1/2&lt; L 1,  R spot&lt; S 2/2&lt; L 2 
         [0000]    In this case, Rspot represents the radius of the spot. Of course, the filter disposed at the Fourier transform plane of the Fourier transform lens  42  is not limited to the configuration shown in the drawings and may be configured to have various shapes accorded to the desired spatial frequency band of the signal light to be transmitted. 
         [0072]    Further, although in the aforesaid embodiment, the pitch (d 1 ) of the pixels of the reference light pixel area is set to be smaller than the pitch (d 2 ) of the pixels of the signal light pixel area, the pitch (d 1 ) of the pixels of the reference light pixel area is set to be larger than the pitch (d 2 ) of the pixels of the signal light pixel area. In this case, according to the aforesaid expressions (1) and (2), the spot distance L 2  of the signal light becomes longer than the spot distance L 1  of the reference light on the Fourier transform plane.  FIG. 5A  shows the Fourier transform plane in this case. As also clear from  FIG. 5A , the primary-order components  410  of the signal light locate at the outside of the primary-order components  510  of the reference light. In this case, when a filter  62  configured as shown in  FIG. 5B  is disposed at the focal plane of the Fourier transform lens  42 , the reference light can be cut and only the spatial frequency band including the primary-order components  410  of the signal light can be transmitted. Supposing that the size of each side of the outer periphery of the transmission portion of the filter  62  is S 3  and the size of each side of the inner periphery thereof is S 4 , S 3  and S 4  satisfy the relation of 2 L 1 &lt;S 3 &lt;2L 2 &lt;S 4 &lt;4L 1 . For example, when the wavelength λ of the coherent light is 532 nm, the focal distance f of the lens is 100 mm, the pitch d 1  of the pixels of the reference light is 24 μm, the pitch d 2  of the pixels of the signal light is 20 μm, L 1  becomes 2.66 mm and L 2  becomes 2.217 mm. In this case, when the filter  62  is configured as a band pass filter having S 3  of 6.7 mm and S 4  of 5 mm, the reference light can be eliminated from the reproduction light and the spatial frequency band including the primary-order DC component of the signal light can be extracted. 
         [0073]    Further, although the aforesaid embodiment is arranged in a manner that the hologram recording/reading apparatus  1  performs both the recording and reading of hologram, a hologram recording apparatus for recording hologram and a hologram reading apparatus for reading hologram may be provided separately, of course.