Patent Publication Number: US-2007121469-A1

Title: Holographic optical pickup apparatus

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a holographic optical pickup apparatus for recording or reproducing information to or from a recording medium by a holographic optical memory system.  
      2. Description of the Related Art  
      In recent years, for super-high density optical recording, volume holography, in particular, digital volume holography has been practically developed and attention has been focused thereon. The term “volume holography” refers to a system for three-dimensionally writing an interference pattern on a recording medium by utilizing a thickness direction thereof as well.  
      The system is advantageous in that the diffraction efficiency can be improved by an increase in thickness and the recording capacity can be increased using multiple recording. Further, the term “digital volume holography” refers to a holographic recording system in which image information to be recorded is limited to a binarized digital pattern, while using the same recording medium and recording system as those in the case of the volume holography.  
      In the digital volume holography, for example, even image information such as an analog picture is temporarily developed to two-dimensional digital pattern information, which is then recorded as image information. At the time of reproduction, the digital pattern information is read out and decoded, so that the original image information is obtained and displayed.  
      Examples of the technique of realizing the digital volume holography which have been proposed include: a two-beam interference method of allowing information light and reference light to separately enter a recording medium at different irradiation angles and recording an interference pattern therebetween; and a collinear system for allowing information light and reference light to coaxially enter a recording medium.  
      Further, as a light source for realizing such optical systems as described above, it is desirable to use a semiconductor laser from the viewpoint of reduction of cost and ease of handling.  
      However, since light emitted from a semiconductor laser has an elliptical cross section, the short axis of the emitted light having the elliptical cross section has been hitherto extended by a beam shaping means such as a beam shaping prism to form the emitted light in a circular shape.  
      A technique using the beam shaping means is disclosed in, for example, ISOM/ODS 2005 which is an international conference.  FIG. 7  shows a collinear optical system as shown in  FIG. 1  in ThE 3  “Optical Collinear Holographic Recording System Using a Blue Laser and a Random Phase Mask”.  
       FIG. 8  shows an optical system based on a two-beam interference method as shown in  FIG. 2  in ThE 5  “Temperature Tolerance Improvement with Wavelength Tuning Laser Source in Holographic Data Storage”. As can be seen from  FIGS. 7 and 8 , the beam shaping element is disposed in the vicinity of the light source (beam shaping element being a prism in each of  FIGS. 7 and 8 ).  
      Next, a collinear optical system according to a conventional example will be described in detail with reference to  FIG. 9 . First, the case where recording is performed on a hologram medium  216  which is a recording medium will be described. A light beam emitted from a green laser  201  serving as a light source is converted into a parallel light beam by a collimator  202 . Then, the parallel light beam is incident on a beam shaping prism  301  serving as a beam shaping means, whereby the short-axis of an exiting light beam having an elliptical cross section is extended.  
      After that, the light beam is reflected by a mirror  203  to illuminate a spatial light modulator (SLM)  204 . In the example shown in  FIG. 9 , a deformable mirror device (DMD) is used as the SLM  204 . A light beam reflected by a pixel indicating information of “1” on the SLM  204  is reflected toward the hologram medium  216 , while a light beam reflected by a pixel indicating information of “0” is not reflected toward the hologram medium  216 . On the collinear-system SLM  204 , there are provided a portion for modulating information light  206  and another portion for modulating reference light  205  which circularly surrounds the portion for modulating information light  206 .  
      The reference light  205  and the information light  206  reflected by the pixel indicating the information of “1” on the SLM  204  pass through a polarization beam splitter (PBS)  207  in p-polarization and travel toward the hologram medium  216  through a relay lens- 1   208 , a mirror  209 , a relay lens- 2   210 , and a dichroic beam splitter (DBS)  211 .  
      Further, at that time, the reference light  205  and the information light  206 , which have been converted into circular polarizing lights (for example, right-hand circular polarizing lights) by passing through a quarter-wave plate (QWP)  212 , are reflected by a mirror  213  and then incident on an objective lens  214  having a focal length F. A pattern displayed on the SLM  204  passes through the two relay lenses  208  and  210  to form an intermediate image at a distance of F before the objective lens  214 . Thereby, an pattern image (not shown) of the SLM  204 , the objective lens  214  and the hologram medium  216  are disposed distant from one another by the distance of F, thereby constructing a so-called  4 F optical system.  
      The hologram medium  216  has a disk shape and is held by a spindle motor  215  so as to be rotatable. The reference light  205  and the information light  206  are condensed on the hologram medium  216  by the objective lens  214  to produce an interference fringe by interference therebetween. An interference fringe pattern at the time of recording is recorded as a refractive index distribution in a polymer material of the hologram medium to form a digital volume hologram. Further, the hologram medium has a reflective film provided therein.  
      In addition to the green laser  201  for performing hologram recording/reproduction, a red laser  220  for emitting light to which the hologram medium is non-photosensitive is provided, whereby a displacement of the hologram medium  216  relative to the reflective film set as a reference surface can be detected with high precision. Thereby, even when the hologram medium  216  is subjected to axial deflection or radial runout, it is possible to cause a recording spot to dynamically follow a medium surface using an optical servo technique, so that the interference fringe pattern can be recorded with high precision. Hereinafter, a brief description will be given.  
      A linear polarizing light beam emitted from the red laser  220  passes through a beam splitter (BS)  221  and is then converted into a parallel light beam by a lens  222 . The light beam is reflected by a mirror  223  and the dichroic beam splitter (DBS)  211  to travel toward the hologram medium  216 . Further, the light beam which has been converted into circular polarizing light (for example, right-hand circular polarizing light) by passing through the quarter-wave plate (QWP)  212  is reflected by the mirror  213  and is then incident on the objective lens  214 . The incident light beam is condensed as a very small light spot on the reflective surface of the hologram medium  216 .  
      The reflected light beam becomes circular polarizing light of the opposite rotation (for example, left-hand circular polarizing light) and is incident on the objective lens  214  again to be converted into a parallel light beam. The light beam is reflected by the mirror  213  and passes through the quarter-wave plate (QWP)  212  to be converted into a linear polarizing light beam which is perpendicular to the light beam traveling on the approach path to the hologram medium  216 . The light beam reflected by the dichroic beam splitter (DBS)  211  passes through the mirror  223  and the lens  222  as in the case of the approach path. Then, the light beam is reflected by the beam splitter (BS)  221  and guided to a servo photodetector  224 . The servo photodetector  224  has a plurality of light receiving surfaces (not shown) and detects position information on the reflective surface using a known method, based on which the focus control and tracking control of the objective lens  214  can be performed.  
      Next, an operation in the case where recording information is reproduced from the hologram medium  216  serving as the recording medium by use of the above-mentioned optical system will be described. A light beam emitted from the green laser  201  serving as the light source illuminates the spatial light modulator (SLM)  204  as is the case with recording. At the time of the reproduction, only the portion for modulating the reference light  205  on the SLM  204  displays the information of “1” and all the portion for modulating the information light  206  displays the information of “0”. Therefore, only light reflected by pixels corresponding to the portion for the reference light is reflected toward the hologram medium  216 , while the information light is not reflected toward the hologram medium  216 .  
      As is the case with the recording, the reference light  205  is converted into circular polarizing light (for example, right-hand circular polarizing light) and condensed on a recording medium on a disk (not shown) to reproduce information light, which is reproduced light, from the recorded interference fringe pattern. The information light which has been reflected by the reflective film of the recording medium becomes circular polarizing light of the opposite rotation (for example, left-hand circular polarizing light) and is incident on the objective lens  214  again to be converted into a parallel light beam. Then, the light beam is reflected by the mirror  213  and passes through the quarter-wave plate (QWP)  212  to be converted into a linear polarizing light beam (S-polarized light) which is perpendicular to the light beam traveling on the approach path to the hologram medium  216 . At this time, an intermediate image of the SLM display pattern as reproduced is formed at the distance of F from the objective lens  214 .  
      The light beam which passed through the dichroic beam splitter (DBS)  211  travels to the polarization beam splitter (PBS)  207  through the relay lens- 2   210 , the mirror  209 , and the relay lens- 1   208 . The light beam reflected by the polarization beam splitter (PBS)  207  again forms an image as an intermediate image of the SLM display pattern at a conjugate position of the spatial light modulator (SLM)  204  by the relay lens- 2   210  and the relay lens- 1   208 .  
      An aperture  217  is provided in advance at the conjugate position to shield unnecessary reference light existing at the periphery of the information light. An intermediate image formed again by a lens  218  forms the SLM display pattern consisting of only the information light portion on a CMOS sensor  219  serving as a photodetector. Therefore, unnecessary reference light is not incident on the CMOS sensor  219 , so that a reproduced signal having a high S/N ratio can be obtained.  
      Next, an optical system based on the two-beam interference method according to a conventional example will be described in detail with reference to  FIG. 10 . A light beam emitted from a green laser  201  serving as a light source is converted into a parallel light beam by a collimator  202 . Then, the parallel light beam is incident on a beam shaping prism  301  serving as a beam shaping means to extend a short-axis of an exiting light beam having an elliptical cross section. After that, the light beam is split into a reference light  205  and an information light  206  by a beam splitter (BS)  227 .  
      At that time, the reference light  205  passes through an objective lens- 2   225  and is incident on a hologram medium  216 . On the other hand, the information light  206  is incident on a spatial light modulator (SLM)  204 . In the example shown in  FIG. 10 , a liquid crystal device having a plurality of pixels is used as the SLM  204 . The information light  206  passes through the spatial light modulator (SLM)  204  and then is reflected by a mirror  203  to be projected to the hologram medium  216  through an objective lens- 1   214 . As a result, an interference fringe pattern formed by interference between the reference light  205  and the information light  206  is recorded in the hologram medium  216 .  
      Here, by setting the light transmitting/shielding patterns of the respective pixels of the liquid crystal device which is the spatial light modulator (SLM)  204 , desirable data can be recorded in the hologram medium  216 .  
      When the hologram medium  216  in which the data is recorded is irradiated with only the reference light  205 , the reference light  205  is diffracted by the interference fringe in the hologram medium  216 . As a result, diffraction light corresponding to the pattern displayed on the liquid crystal device which is the spatial light modulator (SLM)  204  at the time of recording is generated. Therefore, when the diffraction light is condensed by an objective lens- 3   226  and received by, for example, an image pickup apparatus  219  such as a CCD, the recorded data can be reproduced.  
      Examples of the above-mentioned holography technique include “Measurement and Nano Control Technology for supporting Holographic Memory/HVD™,” (Proceedings of 35th Meeting on Lightwave Sensing Technology, June 2005, pp. 75-82) and “Holographic Media will soon take off and 200 Gbyte will be realized in 2006” (Horigome et al, Nikkei Electronics, 2005, 1.17, pp. 105 to 114).  
      In the above-mentioned conventional techniques, a beam shaping means is disposed between a light source and a spatial light modulator, and light incident on the spatial light modulator has a circular shape formed by extending the short-axis of light beam with an elliptical cross section emitted from the light source. Therefore, the spatial light modulator and other optical parts which are disposed subsequent to the beam shaping means are increased in size with the increase in the light beam diameter. Further, there is also posed a problem that the spatial light modulator, the CMOS sensor, or the like is increased in cost with the increase in size thereof.  
     SUMMARY OF THE INVENTION  
      It is, therefore, an object of the present invention to provide a holographic optical pickup apparatus which can be reduced in thickness and production cost while having a beam shaping means.  
      To be specific, the present invention provides a holographic optical pickup apparatus comprising: a laser light source; a spatial light modulator for separating light emitted from the laser light source into information light and reference light; an objective lens for condensing the information light and the reference light on a recording medium; a photodetector for detecting reproduced light from the recording medium; and a beam shaping element disposed between the spatial light modulator and the objective lens, for extending a short axis of light with an elliptical cross section emitted from the laser light source.  
      With such structure, a light beam incident on the spatial light modulator has an elliptical cross section which is smaller in diameter, so that the spatial light modulator can be reduced in size and cost.  
      Further, by adopting such disposition that the short-axis of the emitted light having the elliptical cross section is perpendicular to a surface of a medium, a spatial light modulator having a rectangular shape can be disposed such that a short side thereof is perpendicular to the surface of the medium, so that it is possible to reduce the thickness of a portion between the laser light source and the beam shaping means of the optical pickup apparatus.  
      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  is a developed view showing an optical system of a holographic optical pickup apparatus according to Embodiment 1 of the present invention.  
       FIG. 2  is a perspective view showing the optical system of the optical pickup apparatus as shown in  FIG. 1  which is actually disposed.  
       FIG. 3  is a view showing a spatial light modulator of the optical system shown in  FIG. 1 .  
       FIG. 4  is a view showing Embodiment 2 of the present invention.  
       FIG. 5  is a view showing Embodiment 3 of the present invention.  
       FIG. 6  is a view showing Embodiment 4 of the present invention.  
       FIG. 7  is a view showing a conventional collinear optical system.  
       FIG. 8  is a view showing a conventional two-beam interference optical system.  
       FIG. 9  is a developed view showing a collinear optical system of a conventional optical pickup apparatus.  
       FIG. 10  is a developed view showing a two-beam interference optical system of a conventional optical pickup apparatus. 
    
    
     DESCRIPTION OF THE EMBODIMENTS  
      Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the attached drawings.  
     Embodiment 1  
       FIGS. 1 and 2  show a holographic optical pickup apparatus according to Embodiment 1 of the present invention.  FIG. 1  is a developed view showing an optical system of the holographic optical pickup apparatus.  FIG. 2  is a perspective view showing the optical system shown in  FIG. 1 , which is actually disposed in the optical pickup apparatus.  
      The fundamental structure shown in  FIGS. 1 and 2  is identical to that of the optical system according to the conventional example as shown in  FIG. 9 . In  FIGS. 1 and 2 , the elements which are the same as those shown in  FIG. 9  are identified by like numerals or symbols, and therefore detailed description thereof is omitted here. Incidentally, a hologram medium  216  having a disk shape as described later and the optical parts for performing the optical servo technique using the red laser described with reference to the conventional example are omitted in  FIG. 2 . In the developed view of  FIG. 1 , attention is focused on a short-axis of emitted light having an elliptical cross section which is a feature of the present invention. Therefore, respective optical parts in  FIGS. 1 and 2  are depicted such that angles of orientation relative to optical axes thereof are different from each other. Incidentally, the term “cross section of light” herein employed refers to a cross section of light taken in a direction perpendicular to the traveling direction of the light.  
      In this embodiment, a beam shaping prism  301  is disposed between a spatial light modulator (SLM)  204  and a polarization beam splitter (PBS)  207  for guiding information light  206  as reproduced light to a CMOS sensor  219 . A green laser  201  serving as a light source is disposed such that the short-axis of emitted light having an elliptical cross section is perpendicular (direction indicated by an arrow A in  FIG. 2 ) to a disk surface of the hologram medium  216  having a disk shape.  
      Further, as shown in  FIG. 3 , the spatial light modulator (SLM)  204  has a rectangular outer shape and is constituted by rectangular pixels arranged in a grid pattern. The spatial light modulator (SLM)  204  is disposed such that the direction of the short sides of the pixels and the spatial light modulator (SLM)  204  (direction indicated by an arrow B) is perpendicular to the disk surface. The outer shape (contour) of the spatial light modulator (SLM)  204  is designed so as to substantially circumscribe the light with the elliptical cross section emitted from the light source. Further, the outer shape (contour) of each of the pixels is designed so as to be similar to that of the spatial light modulator (SLM)  204 .  
      In this embodiment, a light beam with the elliptical cross section emitted from the green laser  201  serving as the light source is converted into a parallel light beam by a collimator  202  to illuminate the spatial light modulator (SLM)  204  after reflection by a mirror  203 . After that, at the time of recording, reference light  205  and the information light  206  which have been reflected by pixels indicating information of “1” on the SLM  204  are incident on the beam shaping prism  301  serving as a beam shaping means. On the other hand, at the time of reproduction, only the reference light  205  is incident on the beam shaping prism  301 . Thereafter, the short axis of the emitted light having the elliptical cross sectional shape is extended. Then, the light from the beam shaping prism  301  passes through the polarization beam splitter (PBS)  207  in p-polarization and is incident on a relay lens- 1   208 . Incidentally, the optical system provided subsequent to the relay lens- 1   208 , the servo system, and the like are identical to those described with reference to  FIG. 9 .  
      With the above-mentioned structure, unlike the structure shown in  FIG. 9 , the shape of the light beam incident on the spatial light modulator (SLM)  204  has an elliptical cross section and is smaller in diameter, so that the spatial light modulator (SLM)  204  can be reduced in size and production cost. Further, since the light source is disposed such that the short-axis of the emitted light having the elliptical cross sectional shape is perpendicular to the disk surface, the spatial light modulator (SLM)  204  having the rectangular shape can be disposed such that the short side thereof is perpendicular to the disk surface. Therefore, as shown in  FIG. 2 , it is possible to reduce the thickness of at least a portion between the green laser  201  and the beam shaping prism  301  of the apparatus.  
      Further, by designing the shape of the pixels of the spatial light modulator (SLM)  204  so as to be similar to the outer shape of the spatial light modulator (SLM)  204 , the diameter of the light beam which has been reflected by each of the pixels and passed through the beam shaping prism  301  can be made equal to that in the conventional example. Therefore, it is possible to ensure the compatibility of recording information between the apparatus according to the conventional example and the optical pickup apparatus according to the present invention.  
     Embodiment 2  
       FIG. 4  is a view showing Embodiment 2 of the present invention.  FIG. 4  is a developed view showing an optical system of a holographic optical pickup apparatus according to this embodiment. The fundamental structure is identical to that shown in  FIGS. 1 and 2 . In  FIG. 4 , the elements which are the same as those shown in  FIGS. 1 and 2  are identified by like numerals or symbols, and therefore detailed description thereof is omitted here.  
      In this embodiment, a beam shaping prism  301  is disposed between a PBS  207  located subsequent to the spatial light modulator (SLM)  204 , for guiding information light  206  as reproduced light to a CMOS sensor  219 , and a relay lens- 1   208 .  
      Next, a feature of the present embodiment will be described. At the time of reproduction, each of the information light  206  and the reference light  205  which are reproduced from the disk-shaped hologram medium  216  becomes circular polarizing light (for example, left-hand circular polarizing light) with a rotation opposite to that of the light traveling on the approach path to the hologram medium  216 .  
      Then, the light is incident on the objective lens  214  again to be converted into a parallel light beam. The light beam is reflected by a mirror  213  and passes through a quarter-wave plate (QWP)  212  to be converted into a linear polarizing light beam (s-polarized light) which is perpendicular to the light beam traveling on the approach path to the hologram medium  216 . At this time, an intermediate image of the SLM display pattern as reproduced is formed at the distance of F from the objective lens  214 .  
      The light beam passing through the dichroic beam splitter (DBS)  211  is incident on the beam shaping prism  301  again through the relay lens- 2   210 , the mirror  209 , and the relay lens- 1   208 . Thereby, contrary to the case of the approach path, the light beam incident on the beam shaping prism  301  comes to have an elliptical cross sectional shape because of a size reduction in one axial direction thereof, and the light beam having the elliptical cross section travels toward the polarization beam splitter (PBS)  207 . After that, the light beam reflected by the PBS  207  again forms an image as an intermediate image of the SLM display pattern at a conjugate position of the SLM  204  by the relay lens- 2   210  and the relay lens- 1   208 .  
      An aperture  217  is provided in advance at the conjugate position to shield unnecessary reference light existing at the periphery of the information light. Thereby, an intermediate image formed again by a lens  218  forms the SLM display pattern consisting of only the information light portion on a CMOS sensor  219  serving as a photodetector.  
      With the above-mentioned structure, in addition to the effects described in Embodiment 1, each of the PBS  207 , the aperture  217 , and the CMOS sensor  219  can be reduced in size and thickness. In particular, since the CMOS sensor  219  is a high-cost optical element, the cost reduction thereof resulting from the size reduction is very advantageous.  
     Embodiment 3  
       FIG. 5  is a view showing Embodiment 3 of the present invention.  FIG. 5  is a developed view showing an optical system of an optical pickup apparatus according to this embodiment. The fundamental structure is identical to that of the optical system in Embodiment 1. In  FIG. 5 , the elements which are the same as those shown in  FIGS. 1 and 2  are identified by like numerals or symbols, and therefore detailed description thereof is omitted here.  
      In this embodiment, a beam shaping mirror  302  is used as the beams shaping means and disposed between an objective lens  214  and a QWP  212 . Therefore, the light beam emitted from a red laser  220  for optical servo, which is normally a semiconductor laser, can also be shaped therewith, so that the quality of red laser light for servo can be improved. Thus, in this embodiment, the red laser  220  is desirably disposed such that a short-axis of light having an elliptical sectional shape emitted therefrom is extended by the beam shaping mirror  302 .  
      Further, with the above-mentioned structure, a conventional mirror  213  can be removed, so that the production cost of the apparatus can be reduced corresponding thereto. Moreover, as compared with Embodiment 2, all the optical elements located between a relay lens- 1   208  and the QWP  212  can be reduced in size, so that the apparatus can be further reduced in size, thickness, and cost.  
      In this embodiment, the beam shaping mirror  302  is used as the beams shaping means. However, the present invention is not limited thereto. Therefore, for example, even when a reflection type diffraction grating is used, the same effects can be obtained.  
     Embodiment 4  
       FIG. 6  is a view showing Embodiment 4 of the present invention.  FIG. 6  is a developed view showing an optical system of an optical pickup apparatus according to this embodiment. The fundamental structure is identical to that of the optical system according to the conventional example as shown in  FIG. 10 . In  FIG. 6 , the elements which are the same as those shown in  FIG. 10  are identified by like numerals or symbols, and therefore detailed description thereof is omitted here.  
      In this embodiment, a beam shaping prism  301  is disposed between a liquid crystal device which is a spatial light modulator (SLM)  204  and a mirror  203 . With such a configuration, as compared with the case shown in  FIG. 10 , not only a reduction in effective light beam diameter of an optical system between a collimator  202  and the spatial light modulator (SLM)  204  but also a reduction in effective light beam diameter of an optical system between a beam splitter (BS)  227  and an objective lens- 2   225  for condensing a reference light  205  on the hologram medium  216  can be attained, whereby reduction in cost can be realized.  
      Further, as is the case with Embodiment 1, by adopting such disposition that the short-axis of emitted light having an elliptical cross sectional shape is perpendicular to a recording medium surface, the spatial light modulator (SLM)  204  having the rectangular shape can be disposed such that the short side thereof is perpendicular to the medium surface. Therefore, the thickness of the entire apparatus can also be reduced. Moreover, by disposing the beam shaping prism  301  between the objective lens- 1   214  and the mirror  203 , the mirror  203  can also be reduced in size and cost.  
      The present invention is not limited to only the above-mentioned embodiments. For example, other than the green laser as mentioned above, a blue-violet semiconductor laser which has been put to practical use in recent years can be used as the holographic light source. Further, it is to be understood that not only a disk-shaped medium but also a card-shaped medium or the like can be used as the hologram medium.  
      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 the benefit of Japanese Patent Application No. 2005-343883, filed Nov. 29, 2005, which is hereby incorporated by reference herein in its entirety.