Patent Publication Number: US-2006002278-A1

Title: Spherical aberration corrector, optical pickup unit, and optical disk unit

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to spherical aberration correctors, optical pickup units, and optical disk units, and more particularly to a spherical aberration corrector correcting, in support of multiple standards, spherical aberration resulting from the thickness of the surface resin layer of an optical disk, an optical pickup unit including such a spherical aberration corrector, and an optical disk unit including such an optical pickup unit.  
      2. Description of the Related Art  
      Conventionally, an optical pickup unit is known that includes multiple optical systems in order to support optical disks of different standards using different laser wavelengths and different objective lens numerical apertures. In such an optical pickup unit including multiple optical systems, spherical aberration correction corresponding to each type of optical disk is required if each type of optical disk has multiple recording layers.  
      Japanese Laid-Open Patent Application No. 2003-173547 discloses an optical pickup unit including lens switching means for switching spherical aberration correction lenses in accordance with a difference in optical disk standards. However, this optical pickup unit does not support the case where each of optical disks of different standards has multiple recording layers.  
      Japanese Laid-Open Patent Application No. 2002-334475 discloses a technique for controlling an axis offset in the case of moving a lens by supporting the lens with a folded spring. However, it is difficult to make the folded spring.  
      Japanese Laid-Open Patent Application No. 09-022539 discloses a technique concerning a method of placing spherical aberration correction means in and removing it from a common optical path in order to compatibly play back optical disks different in substrate thickness with a single optical pickup. However, this conventional technique also fails to support the case where each of the optical disks of different standards has multiple recording layers.  
      Japanese Patent No. 3223074 discloses a method that disposes a spherical aberration correction lens in an optical path and places it into and out of the optical path in an optical pickup unit including a beam shaping prism.  
      Japanese Laid-Open Patent Application No. 05-266511 discloses a beam expander as means for correcting spherical aberration, the beam expander being disposed after a beam shaping prism and adjusting the convergence angle and the divergence angle of light entering an objective lens by switching the distance between lenses. In this case, the divergence angle and the convergence angle of a light beam entering the objective lens are adjusted by changing the distance between lenses so as to prevent spherical aberration from occurring on a recording surface to be subjected to recording and reproduction.  
      In order to perform spherical aberration correction in correspondence to each type of optical disk in an optical pickup unit including multiple optical systems as described above, drive means for moving the optical components of each optical system is required. However, there is a disadvantage such that the optical pickup unit is increased in size if the drive means is provided individually for each optical system.  
      An increase in the size of the optical pickup unit itself leads to an increase in the size of the optical disk drive unit. Accordingly, it is desired to prevent an increase in size in the optical pickup unit including multiple optical systems.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is a general object of the present invention to provide an optical pickup unit in which the above-described disadvantage is eliminated.  
      A more specific object of the present invention is to provide a spherical aberration corrector correcting spherical aberration in support of multiple standards without an increase in size, the spherical aberration resulting from the thickness of the surface resin layer of an optical disk, and an optical pickup unit including such a spherical aberration corrector.  
      Another more specific object of the present invention is to provide an optical disk unit including such an optical pickup unit.  
      One or more of the above objects of the present invention are achieved by a spherical aberration corrector including a drive part configured to drive an optical element provided in each of a plurality of optical paths so that the optical elements move in conjunction with each other, wherein the spherical aberration corrector corrects spherical aberration by moving a position of each optical element.  
      According to one aspect of the present invention, optical elements provided in multiple optical paths, respectively, are driven in conjunction with each other by a drive part. This makes it possible to correct spherical aberration in multiple standards, and to reduce the number of components.  
      One or more of the above objects of the present invention are also achieved by a spherical aberration corrector including: laser light sources of different wavelengths; a light guiding part configured to guide light beams emitted from the laser light sources to a same optical path; a beam expander including a first lens and a second lens disposed so that the light guiding part is placed between the first and second lenses, the first lens being disposed in the same optical path, the second lens being disposed in each of optical paths of the light beams before being guided to the same optical path; and a drive part configured to drive the first lens disposed in the same optical path.  
      According to one aspect of the present invention, a part of the two lens groups of each beam expander is shared. As a result, it is possible to reduce a load on a driving force and to reduce the number of components.  
      One or more of the above objects of the present invention are also achieved by an optical pickup unit including a spherical aberration corrector according to the present invention.  
      According to one aspect of the present invention, the spherical aberration corrector of an optical pickup unit supporting multiple standards can be reduced in size. Accordingly, it is possible to prevent the optical pickup unit from increasing in size.  
      One or more of the above objects of the present invention are also achieved by an optical disk unit including an optical pickup unit including a spherical aberration corrector according to the present invention.  
      According to one aspect of the present invention, the spherical aberration corrector of an optical pickup unit supporting multiple standards can be reduced in size. Accordingly, it is possible to prevent an optical disk unit including such an optical pickup unit from increasing in size.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a diagram showing the basic configuration of the optical elements of an optical system block in an optical pickup unit including multiple optical systems;  
       FIG. 2  is a diagram showing the basic configuration of the optical elements of an optical system block in another optical pickup unit including multiple optical systems;  
       FIG. 3  is a diagram showing a method of correcting spherical aberration in an optical pickup unit;  
       FIG. 4  is a diagram showing a configuration of the optical elements of the optical system block of an optical pickup unit according to a first embodiment of the present invention;  
       FIG. 5  is a diagram showing a method of correcting spherical aberration in an optical pickup unit;  
       FIGS. 6A and 6B  are diagrams showing other configurations of the optical block of the optical pickup unit according to the first embodiment of the present invention;  
       FIGS. 7 and 8  are diagrams showing configurations of a lens fixing member according to the first embodiment of the present invention;  
       FIG. 9  is a diagram showing the basic configuration of the optical system block of an optical pickup unit;  
       FIG. 10  is a diagram showing a method of correcting spherical aberration in the optical pickup unit;  
       FIGS. 11 and 12  are diagrams showing a configuration of optical elements in the optical system block of an optical pickup unit according to a second embodiment of the present invention;  
       FIG. 13  is a diagram showing sliding of a spherical aberration correction lens frame according to the second embodiment of the present invention;  
       FIG. 14  is a diagram showing other sliding of the spherical aberration correction lens frame according to the second embodiment of the present invention;  
       FIGS. 15 and 16  are diagrams showing another configuration of the optical system block of the optical pickup unit according to the second embodiment of the present invention;  
       FIGS. 17 through 19  are diagrams showing a configuration of a lens frame for spherical aberration correction lenses applicable to the optical elements of the optical system block of an optical pickup unit according to a third embodiment of the present invention;  
       FIG. 20  is a diagram showing another configuration of the lens frame for spherical aberration correction lenses applicable to the optical elements of the optical system block of the optical pickup unit according to the third embodiment of the present invention;  
       FIG. 21  is a diagram showing a configuration of the optical system block of an optical pickup unit according to a fourth embodiment of the present invention;  
       FIG. 22  is a diagram showing a configuration of the optical system block of an optical pickup unit according to a sixth embodiment of the present invention;  
       FIG. 23  is a diagram showing a configuration of the optical system block of an optical pickup unit according to a seventh embodiment of the present invention;  
       FIGS. 24 and 25  are diagrams showing a configuration of a movable lens frame of an optical pickup unit according to an eighth embodiment of the present invention;  
       FIG. 26  is a diagram showing a configuration of a known movable lens frame;  
       FIG. 27  is a graph showing the relationship between the movement of a lens in an optical axis direction and the offset of the lens in its longitudinal direction;  
       FIG. 28  is a graph showing axis offsets for different lengths of a spring member;  
       FIG. 29  is a diagram showing a configuration of a movable lens frame of an optical pickup unit according to a ninth embodiment of the present invention;  
       FIG. 30  is a diagram showing a configuration of an optical pickup unit according to a tenth embodiment of the present invention; and  
       FIG. 31  is a block diagram showing a disk drive according to an 11 th  embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A description is given below, with reference to the accompanying drawings, of embodiments of the present invention. In the following embodiments, a description is given, taking as an example the case of applying a spherical aberration correction of the present invention to an optical pickup unit.  
     First Embodiment  
      First, a description is given, with reference to  FIGS. 1 through 8 , of a structure of an optical pickup unit  30  according to a first embodiment of the present invention. Prior to this, a description is given, with reference to  FIGS. 1 and 2 , of basic configurations of the optical system block of an optical pickup unit including multiple optical systems.  
       FIG. 1  is a diagram showing the basic configuration of the optical elements of an optical system block in an optical pickup unit  1  including multiple optical systems.  
      In the optical pickup unit  1  shown in  FIG. 1 , laser light (beam) emitted from a laser diode  11  of one optical system passes through a coupling lens  12 , a beam splitter  13 , and an objective lens  14  to be focused into a spot on the recording surface of a disk  10 . Reflected light from the recording surface of the disk  10  has its optical path changed by 90° in the beam splitter  13  so as to reach a photodetector  16  through a condenser lens  15 .  
      Laser light (beam) emitted from a laser diode  21  of the other optical system passes through a coupling lens  22 , a beam splitter  23 , and an objective lens  24  to be focused into a spot on the recording surface of a disk  20 . Reflected light from the recording surface of the disk  20  has its optical path changed by 90° in the beam splitter  23  so as to reach a photodetector  26  through a condenser lens  25 . In this case, the laser diodes  11  and  21  emit respective laser beams of different wavelengths.  
       FIG. 2  is a diagram showing the basic configuration of the optical elements of an optical system block in an optical pickup unit  2  including multiple optical systems. In  FIG. 2 , the same elements as those of  FIG. 1  are referred to by the same numerals, and a description thereof is omitted.  
      The optical pickup unit  2  shown in  FIG. 2  is configured to share the objective lens  14  by merging two optical paths into a single optical path in the middle of the two optical paths.  
      In this case, laser light (beam) emitted from the laser diode  11  of one optical system passes through the coupling lens  12  and the beam splitter  13  to be reflected by a dichroic prism  17  and then deflected by a deflection mirror  18 . Then, the laser light passes through the objective lens  14  to be focused into a spot on the recording surface of the disk  10 . Reflected light from the recording surface of the disk  10  travels via the deflection mirror  18  and the dichroic prism  17  to the beam splitter  13 . The reflected light has its optical path changed by 90° in the beam splitter  13  so as to reach the photodetector  16  through the condenser lens  15 .  
      Laser light (beam) emitted from the laser diode  21  of the other optical system passes through the coupling lens  22  and the beam splitter  23  to be reflected by a prism  27 . Then, the laser light passes through the dichroic prism  17  and then is deflected by the deflection mirror  18  so as to be focused into a spot on the recording surface of a disk  20  through the objective lens  14 . Reflected light from the recording surface of the disk  10  travels via the deflection mirror  18 , the dichroic prism  17 , and the prism  27  to the beam splitter  23 . The reflected light has its optical path changed by 90° in the beam splitter  23  so as to reach the photodetector  26  through the condenser lens  25 . In the optical pickup units  1  and  2  configured as shown in  FIGS. 1 and 2 , respectively, a spherical aberration correction part to correct spherical aberration is required if the disk has multiple recording layers or the disk includes a spherical aberration more than or equal to an allowable value.  
       FIG. 3  is a diagram showing a method of correcting spherical aberration in an optical pickup unit. According to the spherical aberration correction method shown in  FIG. 3 , the divergence angle and the convergence angle of a light beam entering the objective lens  14  are adjusted by moving the laser diode  11  in the optical axis directions (directions indicated by the double-headed arrow), thereby preventing spherical aberration from being caused on the target recording surface.  
      In the case of applying the spherical aberration correction method shown in  FIG. 3  to the optical pickup units  1  and  2  shown in  FIGS. 1 and 2 , respectively, a drive part to drive a laser diode is required for each of the laser diodes  11  and  12 , thus increasing the size of the optical pickup units  11  and  12 .  
      Accordingly, in the first embodiment of the present invention, the optical pickup unit may be configured as follows.  
       FIG. 4  is a diagram showing a configuration of the optical elements of the optical system block of an optical pickup unit  30  according to the first embodiment of the present invention. In  FIG. 4 , the same elements as those of  FIG. 1  are referred to by the same numerals, and a description thereof is omitted.  
      As shown in  FIG. 4 , in the optical pickup unit  30  according to the first embodiment, the two laser diodes  11  and  21  are fixed by a diode fixing member  31 , and the diode fixing member  31  is driven by a single drive part. That is, the laser diodes  11  and  21  are moved (forward and backward) along the optical axis directions by the single drive part, so that these two laser diodes  11  and  21  are moved together, that is, in conjunction with each other. In other words, in this case, the laser diodes  11  and  21  are moved together to a position where it is possible to properly correct the aberration of one of two optical systems provided in the optical pickup unit  30  which one is being used for recording or reproduction. For instance, in the case of  FIG. 4 , the diode fixing member  31  may be driven as indicated by  31   a , so that the laser diodes  11  and  21  are moved as indicated by  11   a  and  21   a . Accordingly, this configuration reduces two drive parts conventionally required for the laser diodes  11  and  21 , respectively, to the single drive part. Accordingly, it is possible to prevent an increase in the size of an optical pickup unit by reducing the number of components of the optical pickup unit.  
      As a method of correcting spherical aberration in the optical pickup unit, a method shown in  FIG. 5  is also known, where spherical aberration is corrected by moving the coupling lens  22  in the optical axis directions (indicated by the double-headed arrow). For instance, in the case of  FIG. 5 , the coupling lens  22  may be moved as indicated by  22   a . In this case, the optical pickup unit also increases in size as in the spherical aberration correction method shown in  FIG. 3  because a drive part to drive a coupling lens is required for each of the coupling lenses  12  and  22 .  
      Accordingly, in this embodiment, the optical pickup unit may be configured as follows.  FIGS. 6A and 6B  are diagrams showing other configurations of the optical block of the optical pickup unit  30  according to the first embodiment.  
      According to the configuration shown in  FIG. 6A , the two coupling lenses  12  and  22  are fixed by a single coupling lens fixing member  32 , and the coupling lens fixing member  32  is driven by a single drive part. This configuration also makes it possible to reduce the number of components, thus preventing the optical pickup unit  30  from increasing in size, because two drive parts conventionally required for the coupling lenses  12  and  22 , respectively, are reduced to the single drive part.  
      According to the configuration shown in  FIG. 6B , the coupling lens  12  of one optical system and the laser diode  21  of the other optical system are fixed by a fixing member  33 . This configuration also includes only a single drive part. Accordingly, the number of components can be reduced, so that it is possible to prevent the optical pickup unit  30  from increasing in size.  
      The laser diodes or coupling lenses of the two optical systems of an optical pickup unit can be fixed with a fixing member and driven by a single drive part. This is because when information reading or recording is performed in one of the optical systems, information reading or recording is not performed in the other optical system, so that the position of the laser diode or coupling lens of the optical system in which no information reading or recording is being performed does not matter.  
      There is no particular limitation to the drive part to drive each of the fixing members  31  through  33 . For instance, a motor, a plunger, etc., may be employed as the drive part.  
       FIGS. 7 and 8  are diagrams showing a lens fixing member  40  as an example of the above-described fixing members  31  through  33 . The lens fixing member  40  includes a transmission member  41  and two lens frames  42   a  and  42   b  attached thereon. Guide poles  43   a  and  44   a  are provided on top and at the bottom, respectively, of the lens frame  42   a . Guide poles  43   b  and  44   b  are provided on top and at the bottom, respectively, of the lens frame  42   b . When the optical axes of light beams passing through the lens frames  42   a  and  42   b , respectively, are not parallel as shown in  FIG. 7 , the transmission member  41  is moved forward or backward along the optical axis directions (directions indicated by the double-headed arrow). Meanwhile, as shown in  FIG. 8 , a support part  45  may be provided in the center of the transmission member  41  so that the direction of the optical axis may be changed to a rectilinear direction by turning the transmission member  41  with the support part  45  serving as a supporting point.  
      In  FIGS. 7 and 8 , the lens fixing member  40  that moves lenses is shown as a fixing member. The same applies to the case of moving laser diodes by fixing the laser diodes with a fixing member.  
      Further, in the first embodiment, a description is given of the case where the optical system block of the optical pickup unit  1  shown in  FIG. 1  is employed. Alternatively, it is also possible to realize an optical pickup unit according to the first embodiment using the optical system block of the optical pickup unit  2  configured as shown in  FIG. 2 .  
      Further, in the first embodiment, a description is given of the case where two optical systems are provided in an optical pickup unit so as to support optical disks of two different standards. Alternatively, three or more optical systems may be driven by a single drive part in order to support three or more standards.  
     Second Embodiment  
      Next, a description is given, with reference to  FIGS. 9 through 16 , of a structure of an optical pickup unit according to a second embodiment of the present invention. First, a description is given, with reference to  FIG. 9 , of the basic configuration of the optical elements of an optical system block to be applied to the optical pickup unit of this embodiment.  
      An optical pickup unit  50  shown in  FIG. 9  includes a laser diode  51  emitting laser light (beam) a collimator lens  52 , a beam shaping prism (beam splitter)  53 , an objective lens  54 , a condenser lens  55 , and a photodetector  56 . Beam shaping is performed by the beam shaping prism  53 . In the optical system block thus configured, a light beam entering the beam shaping prism  53  should be a parallel beam. Accordingly, it is impossible to correct spherical aberration by displacing the laser diode  51  or the collimator lens  52 . Therefore, in an optical pickup unit of such a configuration, a spherical aberration correction lens  57  correcting spherical aberration is disposed in a parallel beam path and is placed into and out of the optical path as shown in  FIG. 10 , which method is disclosed in Japanese Patent No. 3223074 as described above.  
      However, in the case of configuring an optical pickup unit including multiple optical systems in order to support optical disks of multiple standards using the optical system of the optical pickup unit as shown in  FIG. 10 , a drive part is required for each optical system in order to place its spherical aberration correction lens  57  into and out of its optical path. Accordingly, the optical pickup unit increases in size.  
      Accordingly, in the second embodiment of the present invention, the optical pickup unit may be configured as follows.  
       FIGS. 11 and 12  are diagrams showing a configuration of optical elements in the optical system block of an optical pickup unit according to the second embodiment of the present invention. In  FIGS. 11 and 12 , the same elements as those of  FIGS. 9 and 10  are referred to by the same numerals, and a description thereof is omitted.  
      Referring to  FIG. 11 , in the optical pickup unit according to the second embodiment, laser light (beam) emitted from a laser diode  61  of one optical system passes through a collimator lens  62 , a beam splitter  63 , a spherical aberration correction lens  67 , and an objective lens  64  to be focused into a spot on the recording surface of the disk  10 . Reflected light from the recording surface of the disk  10  has its optical path changed by 90° in the beam splitter  63  so as to reach a photodetector  66  through a condenser lens  65 .  
      Laser light (beam) emitted from the laser diode  51  of the other optical system passes through the collimator lens  52 , the beam shaping prism (splitter)  53 , a spherical aberration correction lens  57 , and the objective lens  54  to be focused into a spot on the recording surface of the disk  20 . Reflected light from the recording surface of the disk  20  has its optical path changed by 90° in the beam splitter  53  so as to reach the photodetector  56  through the condenser lens  55 . The laser diodes  51  and  61  emit laser beams of different wavelengths also in this case.  
      According to this embodiment, the two spherical aberration correction lenses  67  and  57  are held by a single lens frame (lens holding part)  68  for spherical aberration correction lenses. This lens frame  68  is driven by a single drive part so as to move in directions perpendicular to the optical path of each optical system as shown in  FIGS. 11 and 12 . Thereby, each of the spherical aberration correction lenses  67  and  57  is placed into and out of the corresponding optical path.  
      This configuration has only a single drive part. Accordingly, the number of components can be reduced, so that it is possible to prevent the optical pickup unit from increasing in size.  
      Further, also in this case, while information reading or recording is performed in one optical system, no information reading or recording is performed in the other optical system. Accordingly, in the optical system that is not in use, the presence or absence of the corresponding spherical aberration correction lens  67  or  57  does not matter.  
      Further, in the optical pickup unit shown in  FIGS. 11 and 12 , the lens frame  68  to which the spherical aberration correction lenses  67  and  57  are fixed is caused to slide in directions perpendicular to the optical axis, so that each of the spherical aberration correction lenses  67  and  57  is placed into and out of the corresponding optical path. Alternatively, for instance, each of the spherical aberration correction lenses  67  and  57  may be placed into and out of the corresponding optical path by rotating the lens frame  68  about a common rotation axis as shown in  FIGS. 13 and 14 .  
       FIGS. 15 and 16  are diagrams showing another configuration of the optical system block of the optical pickup unit according to the second embodiment. As shown in  FIGS. 15 and 16 , a single spherical aberration correction lens  71  is held by a single lens frame (lens holding part)  72  for spherical aberration correction lenses. It is possible to use each other&#39;s optical path as a space to escape to by driving the lens frame  72  with a drive part. In this case, it is possible to reduce the required space.  
      According to the second embodiment, it is possible to share placement and displacement of a correction lens for switching between the recording layers of optical disks of different standards to be subjected to reading and writing.  
     Third Embodiment  
      In the optical pickup units shown in  FIGS. 9 through 16 , a single spherical aberration lens is placed into and out of each optical path in accordance with the standard of each disk. In this case, however, switching is limited to between two stages.  
      Accordingly, a description is given, with reference to  FIGS. 17 through 20 , of configurations of a lens frame for spherical aberration correction lenses applicable to the optical elements of the optical system block of an optical pickup unit according to a third embodiment of the present invention.  
      In this case, a lens frame  81  for spherical aberration correction lenses having three holes  81   a ,  81   b , and  81   c  as shown in  FIG. 17  is prepared. Spherical aberration correction lenses  82  and  83  are attached to the lens hole  81   a  on the left side and the lens hole  81   c  on the right side, respectively. No lens is attached to the hole  81   b  in the center. A tension spring  84  is attached to the intermediate position of the lens frame  81 . Further, stoppers  85  and  86  are provided on both sides of the lens frame  81 . In this lens frame  81 , the intermediate position between the stoppers  85  and  86  (a neutral position) matches the optical axis of laser light.  
      According to this configuration, when the lens frame  81  is pulled by an electromagnetic part to be in contact with the stopper  85  as shown in  FIG. 18 , the spherical aberration correction lens  83  is placed into the optical axis (optical path) of laser light. On the other hand, when the lens frame  81  is pulled by the electromagnetic part to be in contact with the stopper  86  as shown in  FIG. 19 , the spherical aberration correction lens  82  is placed into the optical axis of laser light. Accordingly, by thus configuring the lens frame  81  for spherical aberration correction lenses, it is possible to perform switching among three stages of the two lenses  82  and  83  and no lens. Further, when the lens frame  81  is in the position where the spherical aberration correction lens  82  or  83  is placed into the optical path of laser light, the lens frame  81  is pulled to be pressed and held against the stopper  86  or  85 . Accordingly, it is possible to retain each of the spherical aberration correction lenses  82  and  83  in a correct lens position.  
      When the lens frame  81  is in the center position (neutral position), the lens frame  81  is held in the center position by the tension spring  84 . However, it is difficult to completely stabilize the lens frame  81  in this state. However, no lens is attached to the hole  81   b  provided in the center of the lens frame  81 . Accordingly, even if the lens frame  81  is offset to some extent, the lens frame  81  can be held without being affected substantially by the offset if the offset is not so much as to block a light beam.  
      Next, a description is given, with reference to  FIG. 20 , of another configuration of the lens frame for spherical aberration correction lenses applicable to the optical elements of the optical system block of the optical pickup unit according to the third embodiment. In  FIG. 20 , the same elements as those of  FIG. 19  are referred to by the same numerals, and a description thereof is omitted.  
      A lens frame  90  shown in  FIG. 20  is configured so that spherical aberration correction can be performed among three stages with respect to each of two optical systems using multiple spherical aberration lenses for each optical system. A torsion coil spring  91  is used instead of the tension spring  84  as the intermediate (neutral) position retaining part of the lens frame  90 . In this case, spherical aberration correction lenses  92 ,  93 ,  94 , and  95  are attached to four lens holes  90   a ,  90   c ,  90   d , and  90   f , respectively, provided in both end parts of the lens frame  90 , and no lens is attached to each of two center holes  90   b  and  90   e . Projections  96  through  99  are provided in order to hold the coil spring of the torsion coil spring  91 .  
      According to this configuration, when the lens frame  90  comes into contact with the stopper  85 , each of the spherical aberration correction lenses  93  and  95  is placed into the corresponding optical axis (optical path) of laser light. On the other hand, when the lens frame  90  comes into contact with the stopper  86 , each of the spherical aberration correction lenses  92  and  94  is placed into the corresponding optical axis of laser light. Accordingly, it is possible to perform three-stage switching in the case of including two optical systems.  
      According to the third embodiment, spherical aberration correction lenses of two types are placed into and out of an optical path by a drive part moving a correction lens frame in a direction perpendicular to a laser optical axis, so that three-stage spherical aberration correction can be performed. Accordingly, it is possible to perform three-stage spherical aberration correction with a simple drive part.  
     Fourth Embodiment  
      Next, a description is given, with reference to  FIG. 21 , of a structure of an optical pickup unit according to a fourth embodiment of the present invention. In  FIG. 21 , the same elements as those of  FIG. 11  are referred to by the same numerals, and a description thereof is omitted.  
      As described above, Japanese Laid-Open Patent Application No. 05-266511 discloses a beam expander as means for correcting spherical aberration, the beam expander being disposed after a beam shaping prism and adjusting the convergence angle and the divergence angle of light entering an objective lens by switching the distance between lenses. In this case, the divergence angle and the convergence angle of a light beam entering the objective lens are adjusted by changing the distance between lenses so as to prevent spherical aberration from occurring on a recording surface to be subjected to recording and reproduction.  
      In this case, however, if an optical pickup unit including multiple optical systems in order to support disks of multiple standards is formed, the optical pickup unit also increases in size because a drive part to drive the position of a lens of the beam expander is required for each optical system.  
      Accordingly, in the optical pickup unit according to the fourth embodiment of the present invention, of lenses  101  and  102  of a beam expander provided in one optical system and lenses  103  and  104  of a beam expander provided in the other optical system, the lenses  102  and  104  are housed in a movable lens frame  105  to be integrated, so that the two lenses  102  and  104  are moved simultaneously along the directions of an optical axis (directions indicated by the double-headed arrow) with a single drive part.  
      Thus, according to this configuration, when information reading or recording is performed in an optical system, the lens position is adjusted for the optical system since no information reading or recording is performed in the other optical system. Accordingly, it is possible to adjust the two optical systems with the single drive part. This makes it possible to reduce the number of components of the optical pickup unit, so that it is possible to prevent the optical pickup unit from increasing in size. If the optical paths of the optical systems are not parallel, each of the lenses  102  and  104  may be moved through a transmission member as shown in  FIG. 7  or  8 .  
      According to the fourth embodiment, a beam expander is provided in each optical path as an optical element, and spherical aberration correction is performed by a drive part driving the movable lenses of the expanders in conjunction with each other. Accordingly, a spherical aberration corrector can be formed in a beam shaping system. Further, since the movable lenses are guided so as to move in an optical axis direction, there is an advantage in that an axis offset is less likely to occur.  
     Fifth Embodiment  
      Next, a description is given of a structure of an optical pickup unit according to a fifth embodiment of the present invention.  
      In the optical pickup unit shown in  FIG. 21 , the position of a movable part may be changed in a multistage manner using a motor, so that the lens distance may be set individually for each beam expander. However, a large space is required in order to provide the motor and a deceleration part.  
      Therefore, if the numerical aperture (NA) of an objective lens is not so high, or if it is possible to reduce variations in substrate thickness, it may be possible to control spherical aberration to allowable values only by performing two-stage switching (of spherical aberration correction) with a plunger on an optical disk having two different recording layers.  
      Accordingly, if each of optical disks of different standards has multiple recording layers, the amount of driving of the lens of each expander may be set to the same value. Thereby, even if the optical disks have different standards, it is possible to prevent a spherical aberration more than specified from occurring in each recording layer with a simple two-stage-switching-type actuator. In this case, the glass material and the curvature of each component lens may be determined so that the movable lens of each beam expander is driven by the same amount.  
      According to the fifth embodiment, there is an advantage in that no complicated drive part is necessary for switching target recording layers.  
     Sixth Embodiment  
      Next, a description is given, with reference to  FIG. 22 , of a structure of an optical pickup unit according to a sixth embodiment of the present invention. In  FIG. 22 , the same elements as those of  FIG. 21  are referred to by the same numerals, and a description thereof is omitted.  
      In an optical pickup unit having a beam expander as shown in  FIG. 21 , an appropriate lens distance of the beam expander is subject to change because of variations in the wavelength of the laser diode  51  or  61  and components. Accordingly, it may be necessary to adjust the lens distance.  
      Accordingly, in this case, lens frames (a position adjustment part)  106   a  and  106   b  that can move the fixed lenses  101  and  103  of the expanders, respectively, in the optical axis directions (directions indicated by the double-headed arrows) are provided as shown in  FIG. 22 . The lens distance of one optical system can be adjusted without affecting the other optical system by moving the corresponding one of the lens frames  106   a  and  106   b  independently at the time of assembly.  
     Seventh Embodiment  
      Next, a description is given, with reference to  FIG. 23 , of a structure of an optical pickup unit according to a seventh embodiment of the present invention. In  FIG. 23 , the same elements as those of  FIG. 21  are referred to by the same numerals, and a description thereof is omitted.  
      In the above-described optical pickup unit shown in  FIG. 21 , with one of the lenses of each beam expander ( 101  or  103 ) being fixed, the other one of the lenses ( 102  or  104 ) is moved in the optical axis directions, thereby varying the lens distance of each beam expander. However, if the drive part is switchable between only two stages as in the case of a plunger, there are only two combinations of lens distances.  
      Accordingly, as shown in  FIG. 23 , of the lenses  101  through  104  of the beam expanders, the two front-side lenses  102  and  104  or a front-side lens group and the two rear-side lenses  101  and  103  or a rear-side lens group are housed in the movable lens frame  105  and a movable lens frame  107 , respectively, and a drive part that can switch the lens frame between two stages is provided for each of the lens frames  105  and  107 . Each of the front-side and rear-side lens groups is driven by the corresponding drive part.  
      According to this configuration, the lens distance of each beam expander can be adjusted with four stages of  a , a+b, a+c, and a+b+c, where  a  is the lens distance at the stage of attachment when the lens distance is smallest (narrowest),  b  is the amount of driving of one of the lens frames  105  and  107 , and  c  is the amount of driving of the other one of the lens frames  105  and  107 .  
     Eighth Embodiment  
      Next, a description is given, with reference to  FIGS. 24 and 25 , of a structure of an optical pickup unit according to an eighth embodiment of the present invention.  
       FIGS. 24 and 25  are diagrams showing a structure of the movable lens frames  105  and  107 . As shown in  FIG. 24 , a main pole controlling the movement of lenses and a sub pole preventing rotation around the main pole in one of the lens frames  105  and  107  interchange their functions with each other in the other one of the lens frames  105  and  107 . That is, a pole  111  serves as the main pole and a pole  112  serves as the sub pole in lens frame  105 . On the other hand, the pole  111  serves as the sub pole and the pole  112  serves as the main pole in the lens frame  107 .  
      As shown in  FIG. 25 , lens driving parts  113  and  114  for the lens frames  105  and  107  are disposed in the vicinity of their respective main poles  111  and  112 . This facilitates disposition of the lens driving parts  113  and  114 .  
     Ninth Embodiment  
      Next, a description is given, with reference to  FIGS. 26 through 29 , of a structure of an optical pickup unit according to a ninth embodiment of the present invention.  
      A method is known where a lens frame  120  of a lens  121  is held with a spring member  122  and is supported so that deflection of the spring member  122  allows a movable part to move as shown in  FIG. 26 .  
      This configuration is advantageous in that it is possible to perform driving with a small force compared with supporting with poles as shown in  FIG. 24  because there is no effect of friction of a sliding part. However, there is a defect in that when the lens frame  120  is moved in an optical axis direction (Δx), the lens  121  is offset in the longitudinal direction of the spring member  122  (Δy) as shown in  FIG. 27 . In order to reduce this effect, it is necessary that the spring member  122  have a great length for a movement in the optical axis direction.  FIG. 28  shows axis offsets for different lengths L 1  and L 2  of the spring member  122 . The length L 2  is twice the length L 1 . If the amount of driving in the optical axis direction is the same, the axis offset is inversely proportional to the length of the spring member  122 . However, if the spring member  122  is increased in length, the space for the spring member  122  should be increased accordingly.  
      Therefore, according to the ninth embodiment, the direction of arrangement of lenses  131  and  132  and the longitudinal direction of a spring  133  are aligned in a lens frame  130 . This makes it possible to increase a spring member in length without wasting space. The spring  133  may be a leaf spring.  
      According to the ninth embodiment, in the case of supporting a lens frame holding lenses with a (leaf) spring member, the lenses may be arranged in the longitudinal direction of the spring member using an increase in the size of a movable part. As a result, it is possible to control an axis offset.  
     Tenth Embodiment  
      Next, a description is given, with reference to  FIG. 30 , of a structure of an optical pickup unit according to a tenth embodiment of the present invention. In  FIG. 30 , the same elements as those of  FIG. 2  are referred to by the same numerals, and a description thereof is omitted.  
      In the optical pickup unit shown in  FIG. 30 , the objective lens  14  is shared between disks of different standards by providing lenses  141  and  142  and a lens  140  before and after merger of optical paths, respectively, so that a beam expander is formed for each optical system, and moving the single common lens after merger of the optical paths. In this case, by adjusting the position of the lens  140  in the beam expander formed by the lenses  140  and  141  and the prism  27  provided therebetween, it is possible to guide a light beam emitted from the laser diode  21  and passing through the coupling lens  22  to the objective lens  14  with its convergence angle or divergence angle being controlled so that no spherical aberration is caused on the recording surface of the disk  10 . Further, by adjusting the position of the lens  140  in the beam expander formed by the lenses  140  and  142  and the prism  17  provided therebetween, it is also possible to guide a light beam emitted from the laser diode  11  and passing through the coupling lens  12  to the objective lens  14  with its convergence angle or divergence angle being controlled so that no spherical aberration is caused on the recording surface of the disk  10 . This configuration makes it possible to save space corresponding to the range of movement of a lens.  
      Accordingly, by providing an optical pickup unit including any of the spherical aberration correctors according to the above-described first through tenth embodiments in an optical disk drive, it is possible to prevent the optical disk drive from increasing in size because the optical pickup unit is prevented from increasing in size since a drive part for correcting spherical aberration can be shared between multiple optical systems.  
      According to the tenth embodiment, a part of the two lens groups of each beam expander is shared. As a result, it is possible to reduce a load on a driving force and to reduce the number of components.  
     11 th  Embodiment  
       FIG. 31  is a basic block diagram showing a disk drive (disk unit) according to an 11 th  embodiment of the present invention. The disk drive shown in  FIG. 31  includes an optical pickup unit  201 , an RF signal processing circuit  202 , a modulation and demodulation circuit  203 , a recording compensation circuit  205 , a CPU  206 , a servo part  207 , and a disk motor  208 . The disk drive shown in  FIG. 31  is of a recording and reproduction type. Alternatively, the disk drive may be of a reproduction type omitting the recording compensation circuit  205 .  
      An audio circuit, an image compression and decompression circuit, and/or an interface for connection to a computer are connected to a signal input and a signal output depending on the purpose of a signal. The recording compensation circuit  205  performs laser modulation with a recording signal. The RF signal processing circuit  202  includes a circuit shaping the waveform of a read signal. The servo part  207  detects error components such as a tracking error signal and a focus error signal from the read signal, and controls the optical pickup unit  201  including a spherical aberration corrector according to the present invention and the disk motor  208  by performing feedback. This servo part  207  performs focus servo, tracking servo, and pickup feed servo. A feed screw system, a rack pinion system, and a linear motor system are known as pickup feed mechanisms.  
      In reproducing information, an information signal recorded on an optical disk  200  is read out by the optical pickup unit  201 , and the read-out signal is input to the RF signal processing circuit  202 . The RF signal processing circuit  202  shapes the waveform of the input signal, and thereafter, inputs the signal to the modulation and demodulation circuit  203 . The modulation and demodulation circuit  203  demodulates the input signal, and thereafter, outputs the signal to, for instance, a host computer (not graphically illustrated).  
      In recording information, when a signal to be recorded is input, the modulation and demodulation circuit  203  modulates the input signal into a signal that is easily recordable on the optical disk  200 . Next, the modulated signal is input to the recording compensation circuit  205 , where laser modulation is performed so that a laser driving current (signal) corresponding to the signal is supplied to the optical pickup unit  201 . In general, a current supplied at the time of information recording is larger than a current supplied at the time of information reproduction. In the optical pickup unit  201 , a semiconductor laser emits light based on the input signal, so that the laser light is emitted onto the recording surface of the optical disk  200  from the optical pickup unit  201 , thereby recording information. During this operation, servo control is constantly performed. The CPU controls, for instance, the servo part  207  and the modulation and demodulation circuit  203 .  
      The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.  
      The configurations of a spherical aberration corrector and an optical pickup unit according to the present invention may be, but are not limited to, those described in the above embodiments. Further, in the above-described embodiments, a spherical aberration corrector according to the present invention is applied to an optical pickup unit. Alternatively, a spherical aberration corrector according to the present invention is also applicable to apparatuses or devices other than the optical pickup unit.  
      The present application is based on Japanese Priority Patent Application No. 2004-197055, filed on Jul. 2, 2004, the entire contents of which are hereby incorporated by reference.