Patent Publication Number: US-2007120042-A1

Title: Optical information recording-reproduction apparatus

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an optical information recording-reproduction apparatus. In particular, the present invention relates to an optical information recording-reproduction apparatus which utilizes holography for recording information on a recording medium and reproducing the recorded information from the recording medium.  
      2. Description of the Related Art  
      In information recording by holography on a recording medium, a light beam carrying image information (information light) and a light beam for reference (reference light) are superposed in a recording medium and the formed interference fringes (hologram) are written in the recording medium. In reproducing the information, a reference light beam is projected onto the recording medium to reproduce the image information by diffraction caused by the interference fringes. In recent years, a holographic memory is attracting attention for practical use as an ultra-high-density data storage. In particular, an optical disk memory is attracting attention which records image information or the like developed two-dimensionally, by utilizing holography on a disk-shaped recording medium like a CD and a DVD, and reproducing the information from the recording medium.  
      For example, recording-reproduction apparatuses employing a collinear type holographic memory which utilize the above technologies are disclosed in the following two documents: Proceedings of 35 th  Meeting on Light Wave Sensing Technology, June, 2005, pp.75-82 “Holographic Memory/Measurement &amp; Nano Control Technologies for Blostering HVD™”; and NIKKEI ELECTRONICS, 2005.1.17., pp.105-114 “Holographic Medium Will Achieve 200G Bytes in 2006”.  
      In this system characteristically, the information light and the reference light are generated by one and the same spatial light modulator, and the two light fluxes are allowed to travel along the same optical axis, and are focused on a recording medium by an object lens to record the information as a hologram. In reproduction of the information in this system, only the reference light flux generated by the spatial light modulator is focused on the recording medium carrying the information, and the information light is reproduced by diffraction caused by the hologram.  
      The spatial light modulation pattern for generating the information light and the reference light has the center region for generating the information light and the peripheral region for generating the reference light. According to the document: OPTICAL REVIEW vol. 12 No. 2(2005) pp. 90-92 “Advanced Collinear Holography”, the spatial light modulation pattern has roughly a constitution shown in  FIGS. 2A and 2B .  
       FIG. 2A  illustrates schematically information pattern area  21  and reference pattern area  22  of the modulation pattern within the effective light flux diameter (corresponding to the incident light flux diameter). Actually, the reference pattern area in the spatial light modulator is made larger than the area of radius r 3  in consideration of the tolerable positional deviation. In  FIG. 2B , information pattern area  21  is a circular area of radius r 1 , and reference pattern area  22  is an annular area between a circle of a radius r 2  and a circle of a radius r 3 . The ratio of the radiuses is approximately as below: r 1 :r 2 :r 3 ≈60:70:100.  
      The above system has disadvantages that, when the light source such as a semiconductor laser having Gauss-distributed light intensity is employed for the recording-reproduction, the light including the tailing portion of the light intensity distribution needs to be utilized necessarily for securing the intensity for the recording.  
       FIG. 3  shows schematically distribution  23  of the intensity of a light flux introduced into spatial light modulator  4 . The broken lines show diameter D of the light flux on the spatial light modulator corresponding to light flux introduced into the object lens. As understood from  FIG. 3 , in a collinear system, the information light derived from central portion of the spatial light modulator has a higher intensity, whereas the reference light derived from the peripheral portion of the spatial light modulator has a lower intensity. Therefore, the intensity difference can be caused between the information light and the reference light.  
      Generally, the brightness/darkness modulation degree of light interference is maximal when the interfering light beams have an equal intensity. Therefore, the brightness/darkness modulation degree in the recorded hologram is lower when the intensity is different between the interfering light beams. This can lower the S/N ratio of the signals reproduced from the hologram undesirably.  
     SUMMARY OF THE INVENTION  
      To solve the above problem, the present invention intends to obtain satisfactory modulation degree of the interference with an inexpensive laser light source as the recording-reproduction light source.  
      According to an aspect of the present invention, there is provided a collinear system optical information recording-reproduction apparatus, comprising a laser light source, a spatial light modulator having an information pattern area for generating information light and a reference pattern area for generating reference light, an object lens for introducing the information light and the reference light to a recording medium for recording information on the recording medium by holography, and a light-receiving element for receiving reflected light obtained by projecting only the reference light to the recording medium, for reproducing the information, wherein average intensity of the information light and average intensity of the reference light introduced to the object lens are equal to each other.  
      The laser light source is preferably a semiconductor laser.  
      The spatial light modulator preferably comprises liquid crystal devices.  
      In the optical information recording-reproduction apparatus, polarization directions in the information pattern area and the reference pattern area are preferably rotated by adjusting an applied voltage between electrodes in the information pattern area and the reference light area. Further, the ratio of the area of the information pattern area to the area of the reference pattern area is preferably designed to make equal the quantity toward the object lens of the information light to that of the reference light.  
      The spatial light modulator is preferably a deformable or digital mirror device.  
      According to the present invention, with a laser light source, the average intensities of the information light and of the reference light derived from a spatial light modulator can be made equal. This enables sufficient modulation degree of interference even with an inexpensive light source like a semiconductor laser to provide an optical information recording-reproduction apparatus at a low cost with a high performance.  
      Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates optical paths in the optical information recording-reproduction apparatus of the first embodiment.  
       FIGS. 2A and 2B  are schematic drawings of a spatial light modulator.  
       FIG. 3  is a schematic drawing of an intensity distribution of an introduced light flux in the spatial light modulator. 
    
    
     DESCRIPTION OF THE EMBODIMENTS  
      Embodiments of the present invention are described in detail by reference to drawings.  
     Embodiment 1  
       FIG. 1  illustrates optical paths in the optical information recording-reproduction apparatus of the first embodiment of the present invention.  FIGS. 2A and 2B  are schematic drawings of a spatial light modulator.  FIG. 3  is a schematic drawing of an intensity distribution of an introduced light flux in the spatial light modulator.  
      This embodiment provides an optical information recording-reproduction apparatus employing a collinear type holographic memory.  
      Firstly, the optical paths for information recording are explained. The light flux emitted from violet LD (laser diode)  1 , a semiconductor laser, as the recording-reproduction light source, is converted by collimator  2  into a parallel light flux, enlarged in the direction of minor axis of the ellipsoid to be circular by beam-shaping prism  3 , and introduced into spatial light modulator  4 .  
      The light emission pattern of violet LD  1  has a full angle at the half maximum of θ //  of 8° in the direction parallel to the paper sheet face direction of  FIG. 1 , and θ 13  of 20° in the direction perpendicular to the paper face direction of  FIG. 1 . Beam-shaping prism  3  is capable of magnifying the angle θ //  by a factor of 2.5. Thereby the intensity distribution in the light flux is approximated by an isotropic Gauss distribution: the distribution is as shown in  FIG. 3  in any direction in the cross-section.  
      Spatial light modulator  4  comprises liquid crystal devices. The respective picture elements input information to the introduced light flux by changing selectively the polarization direction by a predetermined angle by utilizing the optical rotatory power of the liquid crystal for selective reflection of the light by polarization beam splitter  5 . Spatial light modulator  4  has information pattern area  21  and reference pattern area  22  as shown schematically in  FIG. 2A . The areas are designed to have the parameters of r 1 , r 2 , and r 3  as shown in  FIG. 2B  in the ratio of r 1 :r 2 :r 3 ≈60:70:100, as in conventional modulators. The two areas generate simultaneously the information light and the reference light.  
      The recording light flux composed of the information light and the reference light is allowed to pass through polarization beam splitter  5  and a pair of relay lenses (first relay lens  7  and second relay lens  9 ), and is converted from linearly polarized light to circularly polarized light by ¼-wavelength plate  10 . In the recording, the recording light flux passes through dichroic beam splitter  8  and is deflected by mirror  11  to be projected through object lens  12  onto hologram disk  13 , a recording medium. In hologram disk  13 , the information light and the reference light are allowed to interfere and the resulting hologram is recorded.  
      Hologram disk  13  is constructed of a transparent substrate, a recording layer which absorbs violet light and transmits red light, and a reflection layer, the layers being arranged in the named order from the light introduction side, although not shown in the drawing. The above-mentioned hologram is recorded in the recording layer. Hologram disk  13  is rotated on the disk rotation axis by a driving means.  
      The light flux has the intensity in nearly isotropic distribution  23  after adjustment by the above beam-shaping prism  3 . The light flux is adjusted to have its diameter at a position of 1/e 2  of the maximum intensity in superposition on the pupil of object lens  12 .  
      In the information reproduction process, the reproducing light flux behaves basically in the same manner as in the information recording process, except that, in the information reproduction, only the pattern of the reference light is formed by spatial light modulator  4 . In the information reproduction, information pattern area  21  may be masked. The reference light projected onto hologram disk  13 , is diffracted by the recorded hologram to generate reproduction light carrying the information of the hologram.  
      This reproduction light is converted to a parallel light flux by object lens  12 , and further converted by ¼-wavelength plate  10  to linearly polarized light directed perpendicular to the light projected to hologram disk  13 . Thereafter, the reproduction light travels reversely along the optical path of the light projection through polarization beam splitter  5  to CMOS sensor  6  to reproduce the information. In this process, of the reproduction light, peripheral portion of the light which has not contributed the diffraction is intercepted not to enter CMOS sensor  6 .  
      For reading a servo signal or an addressing signal, a light flux is emitted from a red LD  14 , a light source, for reading the servo signal or the addressing signal. The light flux passes through polarization beam splitter  15  and coupling lens  16 , and is reflected by dichroic beam splitter  8 , and is then allowed to pass through relay lens  9 . The transmitted light flux becomes a nearly parallel light flux. The light flux passes through ¼-wavelength  10  to be deflected by mirror  11 , and is projected through object lens  12  to hologram disk  13 .  
      The light flux reflected by the reflection layer of hologram disk  13  carries information for reading the servo signal or the addressing signal. This light flux travels reversely along the same optical path, and is reflected by polarization beam splitter  15 . The reflected light flux travels through sensor lens  17  to PD (photodiode)  18 , a light element for receiving a servo signal or an addressing signal to reproduce the servo signal or the addressing signal.  
      Next, the information light intensity and the reference light intensity are considered. The intensity distribution in the light flux having passed through beam-shaping prism  3  is regarded as a Gauss distribution as shown in  FIG. 3 . The radius of the pupil of object lens  12  is regarded to be equal to the radius r 3  shown in  FIG. 2B . Then, in spatial light modulator  4 , the intensity (Ii) of the light flux at information pattern area  21  and the intensity (Ir) at reference pattern area  22  introduced from beam-shaping prism  3  are at a ratio of Ii:Ir≈1.0:0.47 or thereabout.  
      Therefore, information pattern area  21  and reference pattern area  22  have respectively a prescribed pattern, and not all the light beams from all of the picture elements travel through spatial light modulator  4  to reach object lens  12 . However, in average, the light from the respective area can be regarded to reach the object lens  12  at a light quantity ratio of about 1.0:0.47. In consideration as one light beam, the brightness/darkness ratio is about 3.2, the modulation degree of interference being low. In this state, the S/N ratio of the reproduced signal is low which is derived from the hologram having formed by interference in hologram disk  13 .  
      Therefore, in this Embodiment, the apparent transmittance of information pattern area  21  is lowered to about half to make equal the quantity of light transmitted through information pattern area  21  to objective lens  12  and the quantity of light transmitted through reference pattern area  22  to objective lens  12 . (Herein, the term “equal” signifies that a relative quantity is within the range of ±20%. The same is true in the description below.)  
      In this Embodiment, the light flux from beam-shaping prism  3  is P-polarized. Polarization beam splitter  5  reflects S-polarized light component toward object lens  12 . Therefore, the interelectrode voltage applied to the liquid crystal devices is changed for the areas to rotate the polarization direction by 90° in reference pattern area  22  to introduce the light to object lens  12 , whereas, in information pattern area  21 , the polarization direction is rotated by about 45°.  
      Otherwise, to decrease the apparent transmittance of information pattern area to about ½, an ND filter (neutral density filter) may be placed on the liquid crystal devices, or the ND filter may be placed in the optical path of the parallel light flux before polarization beam splitter  5 . However, it can cause cost-up, so that the aforementioned system is employed in this embodiment.  
      In the aforementioned system, regarding the S-polarization component, in the information pattern area  21 , the efficiency of reflection at polarization beam splitter  5  is about half of that in reference pattern area  22 . In such a manner, the transmittance of the information light from information pattern area  21  to object lens  12  and the transmittance of the reference light from the reference pattern area  22  to object lens  12  can be adjusted.  
      By this adjustment, the quantity of the light transmitted from information pattern area  21  to object lens  12  and the quantity of the light transmitted from reference pattern area  22  to object lens  12  can be made equal. Therefore the ratio of the brightness to the darkness can be made comparable to (Bright)/(Dark)≈∞, and the modulation degree of the interference can be raised. Thereby, the S/N ratio of reproduction signal derived from the hologram formed by interference in hologram disk  13  can be increased.  
      In the embodiment illustrated in  FIG. 1 , spatial light modulator  4  comprises transmission type liquid crystal devices. Otherwise, the spatial light modulator may be of a reflection type in which a mirror is additionally used to reflect the light flux introduced from beam-shaping prism  3 . The reflective type spatial light modulator may be a DMD (deformable mirror device, or digital micro-mirror device). In this DMD, without use of an ND filter, the rotation of the polarized light cannot be utilized. In such a case, the reflectivity of the DMD is changed positively between information pattern area  21  and reference pattern area  22 .  
      As described above, according to this embodiment, sufficient interference modulation degree can be achieved by adjusting the information light intensity and the reference light intensity to be nearly equal even with an inexpensive recording-reproduction light source like a semiconductor laser.  
     Embodiment 2  
      In this embodiment, the optical information recording-reproduction apparatus has the same optical paths as that shown in  FIG. 1 .  
      In this Embodiment, the area ratio of the information pattern area  21  and reference pattern area  22  in the effective light flux is made different from that of Embodiment 1. Specifically, the ratio of radiuses r 1 , r 2 , and r 3  is changed to r 1 :r 2 :r 3 =47:57:100.  
      Thereby, in spatial light modulator  4 , the intensity (Ii) of the light flux at information pattern area  21  and the intensity (Ir) at reference pattern area  22  introduced from beam-shaping prism  3  are at a ratio of Ii:Ir≈1.0:1.0.  
      As the results, the quantity of the light transmitted through information pattern area  21  to objective lens  12  and the quantity of the light transmitted through reference pattern area  22  to object lens  12  are made equal to each other. Therefore, ratio of the intensities of the brightness to the darkness can be made comparable to (Bright)/(Dark)≈∞, and the modulation degree of the interference can be raised. Naturally, in this Embodiment, the efficiency of the light transmission from spatial light modulator  4  to objective lens  12  is uniform within the light flux.  
      Practically, the ratio of the parameters shown in  FIG. 2B  is in the range of r 1 :r 2 :r 3 =(45 to 50):(55 to 60):100.  
      Specifically, at r 1 :r 2 :r 3 =45:55:100, (Bright):Dark)≈9.5; at r 1 :r 2 :r 3 =50:60:100, (Bright):(Dark)≈17.2. Thus the interference modulation degree can be sufficiently high.  
      From the above, with a semiconductor laser as the light source, the radius of the information pattern area is preferably about half the radius of the effective light flux at the spatial light modulator.  
      Embodiment 2 can be conducted more readily than Embodiment 1, since only the range of the information pattern area is to be adjusted. As described above, according to this Embodiment, sufficiently high interference modulation degree can be achieved even with inexpensive semiconductor laser as the recording-reproduction light source by making approximately equal the intensities of the information light and the reference light.  
      While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.  
      This application claims priority from Japanese Patent Application No. 2005-343880 filed on Nov. 29, 2005, which is hereby incorporated by reference herein.