Abstract:
A hologram reading apparatus includes: a unit for 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 direction of an arrangement period 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 shielding the reference light at a first spatial frequency band and transmitting the signal light at a second spatial frequency band; and a unit for receiving the reproduction light and reading the data page modulated to the signal light included in the reproduction light.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 USC §119 from Japanese Patent Application No. 2007-224488 filed Aug. 30, 2007. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to a hologram reading apparatus, a hologram reading method, a hologram recording apparatus and a hologram recording method. 
     (ii) Related Art 
     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 apparatus can be miniaturized. 
       FIG. 5  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 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 
     According to an aspect of the invention, there is provided a hologram reading apparatus including: 
     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 direction of an arrangement period of pixels in the first pixel area is different from that in the second pixel area; 
     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; 
     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 direction of a period of image formation positions of a spatial frequency component thereof; and 
     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 
       Embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a diagram showing a hologram recording/reading apparatus according to an exemplary embodiment; 
         FIG. 2  is a diagram showing a spatial light modulator; 
         FIG. 3  is a diagram showing the focal plane of a lens and a filter; 
         FIG. 4  is a diagram showing the focal plane of a lens and a filter; and 
         FIG. 5  is a diagram showing a part of a hologram recording/reading apparatus in a related art, 
     
    
    
     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 
     Hereinafter, an exemplary embodiment of the invention will be explained with reference to drawings. 
       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, and a light receiving element  50 . 
     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. 
     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. 
     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 splitter  12 . 
     The polarization beam splitter  22  transmits a p-polarized light of the incident laser beam and reflects an s-polarized light thereof. The laser beam reflected by the polarization beam splitter  22  enters into the spatial light modulator  24 . 
     The spatial light modulator  24  polarizes and modulates the laser beam entered from the polarization beam splitter  22  so as to have a pattern according to recording information. The recording information is represented by a pattern image of bright and dark in which digital data “0” and “1” is made correspond to “bright” and “dark”, respectively. The laser beam having a light 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 beam, the beam modulated by the spatial light modulator  24  transmits the polarization beam splitter  22 . 
       FIG. 2  shows a 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 a reference light pixel area  200  for modulating the reference light and a signal light pixel area  300  for modulating the signal light in a manner that the signal beam light 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 . 
     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 to 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” pixel 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” pixel is not differentiated therebetween. 
     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. 
     The spatial light modulator  24  according to the embodiment is characterized in that the direction of the arrangement period of the pixels contained in the reference light pixel area  200  differs from the direction of the arrangement period of the pixels contained in the signal light pixel area  300 . The direction of the arrangement period of the pixels means the disposing direction of the adjacent pixels. In this embodiment, it is supposed that the direction of the arrangement period of the pixels in the reference light pixel area  200  inclines by about 45 degrees or 45 degrees with respect to the direction of the arrangement period of the pixels in the signal light pixel area  300 . In this manner, in the spatial light modulator  24 , since the direction of the arrangement period of the pixels for modulating the reference light differs from the direction of the arrangement period of the pixels for modulating the signal light, the direction of the period at the focusing position of the spatial frequency component of the signal light in a Fourier transform plane differs from that of the reference light. The embodiment simultaneously performs the shielding of the reference light and the extraction of a desired frequency band of the signal light by utilizing the difference between the position of a bright spot of the reference light and the position of a bright spot of the signal light on the Fourier transform plane. This process will be explained later. 
     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 shielded when passing through the filter  34 . Since the frequency band of the recording light is shielded 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 or more, where f represents a focal length of the Fourier transform lens, λ represents a wavelength of the coherent light and d represents a pixel pitch of each of the reference light pixel area and the signal light pixel area. 
     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 light in the hologram recording medium  100 . 
     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 . 
     Next, the explanation will be made as to a process of reading data page recorded in the hologram recording medium  100  by the hologram recording/reading apparatus  1 . 
     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. 
     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 . A filter  60  is disposed on the focal plane of the Fourier transform lens  42 . 
       FIG. 3  shows a focal plane (hereinafter called a Fourier transform plane) of the Fourier transform lens  42  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 respectively located in the horizontal and vertical directions from the 0-order DC component  400 . Further, primary-order DC components  510  of the reference light are located in the directions inclined by +45 degree and −45 degree from the 0-order DC component  400 , respectively. In this embodiment, a spot distance L between the reference light and the signal light can be represented by the following expression (2), where d represents a pixel pitch of each of the reference light pixel area  200  and the signal light pixel area  300 , f represents a focal distance of the lens, and λ represents a wavelength of the coherent light.
 
 L=fλ/d    (1)
 
     As shown in  FIG. 3 , the spot positions of the reference light and the signal light locate at different positions each inclined by 45 degree from the 0-order DC component  400  supposed to be the origin. For example, as shown in  FIG. 3 , when a low pass filter having a transmission portion of a square shape each side having a length S satisfying the following expression (2) is disposed on the Fourier transform plane in a manner that the corner portions of the transmission portion locate respectively in the horizontal and vertical directions from the center position thereof, the reference light is cut from the reproduction beam. Further, only the signal light having the desired frequency band (frequencies equal to or lower than that of the primary-order DC component) is transmitted and extracted.
 
√{square root over (2)} L&lt;S&lt; 2 L    (2)
 
     Further, in the aforesaid example, when it is concretely supposed that the wavelength λ of the coherent light is 532 nm, the focal distance of the lens is 100 mm and the pixel pitch d is 20 μm, L is obtained as 2.66 mm. In this case, when the filter is configured as a low pass filter having sides each with a length S of 4 mm, S can satisfy the aforesaid expression (2) and the reference light can be eliminated from the reproduction light. Further, the signal light having frequencies equal to or lower than that of the primary-order DC component can be extracted. 
     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 (reproduces) the recorded data page modulated in the signal light based on the light intensity modulation pattern of the signal light. 
     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 signal light with the desired frequency band, as compared with the hologram recording/reading apparatus  2  in the related art shown in  FIG. 5 , 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. 
     The invention is not limited to the aforesaid embodiment. 
     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 to have a shape shown in  FIG. 4  and configured as a band-pass filter  61  which shields the 0-order DC component of the signal light but transmits the signal light with a frequency band containing the primary-order DC components  410 . In this case, supposing that the transmission portion of the band-pass filter  61  is configured as a square shape in a manner that the size of each side of the outer periphery thereof is S1 and the size of each side of the inner periphery thereof is S2, the following expressions are satisfied.
 
√{square root over (2)} L&lt;S 1&lt;2 L,  2 R   spot   &lt;S 2&lt;√{square root over (2)} L  
 
In this case, R spot  represents a radius of the spot. Of course, the filter disposed at the Fourier transform plane 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 beam to be transmitted.
 
     Further, the shape of the transmission portion of the filter such as the band-pass filter  61  disposed at the focal plane of the Fourier transform lens  42  is not limited to the aforesaid square shape, and may be various shapes such as a polygon, ellipse, of course. 
     Further, although the aforesaid embodiment is arranged in a manner that the hologram recording/reading apparatus  1  performs both the recording and reproduction of hologram, a hologram recording apparatus for recording hologram and a hologram reproducing apparatus for reproducing hologram may be provided separately, of course.