Patent Publication Number: US-2005122857-A1

Title: Optical pickup

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to an optical pickup for recording/reading information data onto/from a recording medium.  
      2. Description of the Related Art  
      At present various optical discs such as a CD, a DVD, and a Blu-ray disc (registered trademark) are commercialized as optical recording media for recording such information data as audio, video, and computer data. Information recording/reproducing apparatus capable of recording/reproducing information data onto/from all types of optical discs which have different recording densities are also put on the market. Generally, such an information recording/reproducing apparatus includes an optical pickup, and a plurality of light sources are mounted on the optical pickup. The light sources generate read (or write) light beams having wavelengths corresponding to the respective types of the optical discs, and the number of light sources are the same as the number of types of the optical disc.  
       FIG. 1  of the attached drawings shows an example of the internal structure of the above-described optical pickup. (For example, see Japanese Patent Kokai No. 2000-298871.)  
      In  FIG. 1 , a first light source  111  generates a first light I having a wavelength of approximately 650 nm which is used for reading information from a DVD. The first light I is directed to an objective lens  131  via an optical path of a beam splitter  113 , a collimator lens  135 , and a beam splitter  133 . A second light source  121  generates a second light II having a wavelength of approximately  780 nm which is used for recording/reading information data onto/from a recording medium such as a CD or a CD-RW. The second light II is guided to the objective lens  131  via an optical path of a beam splitter  123 , a collimator lens  125 , and the beam splitter  133 . The objective lens  131  condenses the first light I or the second light II supplied via the optical path, and irradiates a recording surface of an optical disc  100  with this light. The reflection light from the optical disc  100  is guided to a light receiving lens  137  which functions as an astigmatic element via an optical path of the objective lens  131 , the beam splitter  133 , the collimator lens  135 , and the beam splitter  113 . The light receiving lens  137  gives astigmatism to the reflected light from the optical disc  100 , and irradiates a light receiving surface of a photodetector  140  with this light. The photodetector  140  outputs as a read signal an electric signal obtained by applying photoelectric conversion to the received light. In other words, the reflection light from the optical disc  100  of the first light I generated by the first light source  111  and the reflection light from the optical disc  100  of the second light II generated by the second light source  121  are both converted into read signals by the single photodetector  140 .  
      In assembling an optical pickup which has a plurality of light sources sharing a single photodetector, first, the light receiving lens  137  and the photodetector  140  are placed at appropriate positions such that a beam spot made by the reflected light from the optical disc  100  of the first light I correctly falls on the light receiving surface of the photodetector  140 . Next, the second light source  121  is placed at an appropriate position such that a beam spot made by the reflected light from the optical disc  100  of the second light II correctly falls on the light receiving surface of the photodetector  140 .  
      In order to decide the position of the second light source  121 , the second light source  121  itself needs to be moved not only in the direction of the optical axis, but also in the X-axis and the Y-axis directions on the plane perpendicular to the optical axis, i.e., in total three axis directions. Furthermore, spherical aberration caused by the objective lens and optical disc should be considered when placing various optical components mentioned above. This makes the fabrication difficult.  
     SUMMARY OF THE INVENTION  
      One object of the present invention is to provide an optical pickup which is easy to fabricate.  
      According to a first aspect of the present invention, there is provided an improved optical pickup for recording/reading information data onto/from an optical recording medium. The optical pickup includes a photodetector for obtaining a read signal by applying photoelectric conversion to a light beam incident on a light receiving surface of the photodetector. The optical pickup also includes a first light source for generating a first light beam, and a second light source for generating a second light beam which has a wavelength different from that of the first light beam. The optical pickup also includes a first optical path for directing the first light beam to the recording medium, condensing the reflected light from the recording medium, and guiding the reflected light to the light receiving surface of the photodetector. The optical pickup also includes a second optical path for guiding the second light beam to the recording medium. The optical pickup also includes an optical system correction module provided on the first optical path for making spherical aberration correction when the first light beam is guided to the recording medium, and for making focus correction in condensing the reflected light from the recording medium on the light receiving surface of the photodetector when the second light beam is guided to the recording medium.  
      According to a second aspect of the present invention, there is provided another optical pickup for recording/ reading information data onto/from an optical recording medium. The optical pickup includes a photodetector for obtaining a read signal by applying photoelectric conversion to a light incident on a light receiving surface of the photodetector. The optical pickup also includes a first light source for generating a first light beam, a second light source for generating a second light beam which has a wavelength different from that of the first light beam, and a third light source for generating a third light beam which has a wavelength different from those of the first light beam and the second light beam. The optical pickup also includes a first optical path for directing the first light beam to the recording medium, condensing the reflected light from the recording medium, and guiding the reflected light to the light receiving surface of the photodetector. The optical pickup also includes a second optical path for guiding the second light beam to the recording medium. The optical pickup also includes a third optical path for guiding the third light beam to the recording medium. The optical pickup also includes an optical system correction module provided in the first optical path for making spherical aberration correction when the first light beam is guided to the recording medium, and for making focus correction in condensing the reflected light from the recording medium on the light receiving surface of the photodetector when the second or third light beam is guided to the recording medium.  
      According to a third aspect of the present invention, there is provided an information recording/reproducing apparatus for recording/reproducing information data onto/from an optical recording medium. The apparatus includes an optical pickup, and the optical pickup includes a photodetector for obtaining a read signal by applying photoelectric conversion to a light beam incident on the light receiving surf ace of the photodetector. The optical pickup also includes a first light source for generating a first light beam, and a second light source for generating a second light beam which has a wavelength different from that of the first light beam. The optical pickup also includes a first optical path for directing the first light beam to the recording medium, condensing the reflected light from the recording medium, and guiding the reflected light to the light receiving surface of the photodetector. The optical pickup also includes a second optical path for guiding the second light beam to the recording medium. The optical pickup also includes an optical system correction module provided in the first optical path for making spherical aberration correction when the first light beam is guided to the recording medium, and for making focus correction in condensing the reflected light from the recording medium on the light receiving surface of the photodetector when the second light beam is guided to the recording medium.  
      According to a fourth aspect of the present invention, there is provided another information recording/reproducing apparatus for recording/reproducing information data onto/from an optical recording medium. The apparatus includes an optical pickup, and the optical pickup includes a photodetector for obtaining a read signal by applying photoelectric conversion to the light beam incident on a light receiving surface of the photodetector. The optical pickup also includes a first light source for generating a first light beam, a second light source for generating a second light beam which has a wavelength different from that of the first light beam, and a third light source for generating a third light beam which has a wavelength different from those of the first light beam and the second light beam. The optical pickup also includes a first optical path for guiding the first light beam to the recording medium, condensing the reflected light from the recording medium, and guiding the reflected light to the light receiving surface of the photodetector, a second optical path for guiding the second light beam to the recording medium, and a third optical path for guiding the third light beam to the recording medium. The optical pickup also includes an optical system correction module provided in the first optical path for making spherical aberration correction when the first light beam is guided to the recording medium, and for making focus correction in condensing the reflected light from the recording medium on the light receiving surface of the photodetector when the second or third light beam is guided to the recording medium. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates a configuration of an optical pickup having a plurality of light sources.  
       FIG. 2  is a schematic block diagram showing an information recording/reproducing apparatus on which a pickup having a plurality of light sources is mounted.  
       FIG. 3  is a flowchart of an optical system correction subroutine performed by a controller.  
       FIG. 4  is a block diagram showing a modification of the optical pickup shown in  FIG. 2 .  
       FIG. 5  is a block diagram showing a modification of the optical pickup shown in  FIG. 4 .  
       FIG. 6  is a block diagram showing an information recording/reproducing apparatus equipped with an optical pickup that adopts an expander lens as an optical element for optical system correction.  
       FIG. 7  is a block diagram showing an information recording/reproducing apparatus equipped with a pickup having three light sources.  
       FIG. 8  illustrates a flowchart of an optical system correction subroutine performed by a controller shown in  FIG. 7 .  
       FIG. 9  illustrates a block diagram of an information recording/reproducing apparatus equipped with an optical pickup that adopts a liquid crystal lens as an optical system correction element. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     FIRST EMBODIMENT  
      Referring to  FIG. 2 , an information recording/reproducing apparatus capable of recording/reproducing information data onto/from both a DVD (Digital Versatile Disc) and a Blu-ray Disc will be described.  
      In  FIG. 2 , when an optical disc  2  onto/from which information data is to be recorded/reproduced is a DVD, a controller  1  supplies a recording start signal or a reading start signal to a light source driver  4 . On the other hand, when the optical disc  2  is a Blu-ray disc, the controller  1  supplies a recording start signal or a reading start signal to another light source driver  3 .  
      The light source driver  3  supplies a drive voltage (or a drive current) for recording information onto a Blu-ray disc to a first light source  51  mounted on an optical pickup  5  when the driver  3  receives a recording start signal. When the light source driver  3  receives the reading start signal, the light source driver  3  supplies a drive voltage (or a drive current) to the first light source  51  to read information from a Blu-ray disc. The light source driver  4  supplies a drive voltage (or a drive current) for recording information onto a DVD to a second light source  52  mounted on the optical pickup  5  when the driver  4  receives a recording start signal. When the light source driver  4  receives a reading start signal, the light source driver  4  supplies a drive voltage (or a drive current) for reading information from a DVD to the second light source  52 .  
      The first light source  51  generates a light beam having a wavelength of  405 nm which has optical power corresponding to the drive voltage (or the drive current) supplied from the light source driver  3 . The light beam is guided to a collimator lens  54  via a beam splitter  53 . The collimator lens  54  converts the supplied light beam into a parallel light. The collimator lens  54  is movable in the direction of the optical axis by an actuator  70 . The parallel light obtained via the collimator lens  54  is guided to an objective lens  57  via a beam splitter  55  and a mirror  56 .  
      The second light source  52  generates a light beam having a wavelength of  650 nm which has optical power corresponding to the drive voltage (or the drive current) supplied from the light source driver  4 . The light beam is converted into a parallel light by a collimator lens  58 , and is guided to the objective lens  57  via the beam splitter  55  and the mirror  56 .  
      The objective lens  57  condenses the parallel light supplied via the mirror  56 , and supplies this light to the recording surface of the optical disc  2 . The light-detecting surface of a photodetector  60  is irradiated with the reflected light from the recording surface of the optical disc  2  which passes through an optical path of the mirror  56 , the beam splitter  55 , the collimator lens  54 , the beam splitter  53 , and a detecting lens  59 . The photodetector  60  supplies the electric signal obtained by applying photoelectric conversion to the light incident on the light detecting surface of the photodetector  60  to a read signal processing circuit  6  as a read signal.  
      The read signal processing circuit  6  generates various servo signals such as a focus servo signal, a tracking servo signal, and a slider servo signal based on the read signal. A focus actuator (not shown in the figure) moves the objective lens  57  in the direction perpendicular to the recording surface of the optical disc  2  in response to the focus servo signal. A tracking actuator (not shown in the figure) moves the objective lens  57  in the radial direction of the optical disc  2  in response to the tracking servo signal. A slider mechanism (not shown in the figure) moves the optical pickup  5  in the radial direction of the optical disc in response to the slider servo signal. The read signal processing circuit  6  reproduces the information data stored in the optical disc  2  based on a read sample value obtained by sampling the read signal, and outputs this as reproduced information data. Furthermore, the read signal processing circuit  6  supplies the read-sample value to the controller  1 . In a memory  8 , optimum position information OP which shows an optimum position of the collimator lens  54  on the optical axis when the type of the optical disc  2  is a DVD is stored in advance. In other words, the optimum position information OP shows a position where the reflected light from the optical disc  2  in response to the light beam emitted by the second light source  52  can be correctly condensed on the light receiving surface of the photodetector  60  when the collimator lens  54  is set at a position on the optical axis indicated by the optimum position information OP. The memory  8  has a storage area for storing an amplitude of a read sample value together with a position of the collimator lens  54  at that point in time (referred to as “current position” of the collimator lens  54 ) when performing a spherical aberration correction subroutine (will be described).  
      The controller  1  makes an adjustment to the optical pickup  5  according to an optical system correction subroutine as shown in  FIG. 3  just before starting the recording or reproducing operation to the optical disc  2 .  
      First, the controller  1  determines whether the optical disc  2  onto/from which information data is to be recorded/reproduced is a Blu-ray disc or not (step S 1 ). When it is determined in the step S 1  that the optical disc  2  is a Blu-ray disc, the controller  1  goes on to perform a spherical aberration correction subroutine (step S 2 ).  
      In the spherical aberration correction subroutine, the controller  1  first supplies a lens movement command to a lens driver  7 . The lens driver  7  supplies to the actuator  70  a drive voltage (or a drive current) for gradually moving the collimator lens  54  from a position A to a position B on the optical axis as shown in  FIG. 2  in response to the lens movement command. The actuator  70  gradually moves the collimator lens  54  in the direction of the optical axis from the position A to the position B as shown in  FIG. 2  in response to the drive voltage. While the collimator lens  54  is moving, the controller  1  repeatedly reads a read sample value from the read signal processing circuit  6 , relates position information indicating the current position of the collimator lens  54  on the optical axis (a position between the position A and the position B) to an amplitude value of the current read sample value, and sequentially stores the amplitude values and the position information in the memory  8 . When the collimator lens  54  reaches the position B, the controller  1  retrieves the maximum value out of the amplitude values stored in the memory  8 . Then, the controller  1  reads the stored position information together with the amplitude value from the memory  8 , and supplies to the lens driver  7  a lens position set signal for setting the collimator lens  54  at the position indicated by the position information. This makes the actuator  70  move the collimator lens  54  to the position corresponding to the lens position set signal, and fix the collimator lens  54  at the position.  
      In other words, in the spherical aberration correction subroutine, first, the amount of spherical aberration is gradually changed by moving the collimator lens  54  in the direction of the optical axis from the position A to the position B. Here, the position of the collimator lens  54  when an amplitude value of the read signal is at the maximum is considered as being the position where the amount of spherical aberration is at the minimum, and the collimator lens  54  is fixed at this position. In short, spherical aberration is corrected by moving the collimator lens  54  in the optical axis direction. The collimator lens  54  is used in both a go-route of the optical path of a light beam emitted by the first light source  51  and guided to the optical disc  2 , and a return-route of the optical path of a reflected light reflected from the optical disc  2  and guided to the photodetector  60 . Therefore, even though the collimator lens  54  is moved in the direction of the optical axis for the sake of spherical aberration correction as described above, the beam spot of the reflected light from the optical disc  2  can correctly be formed on the light receiving surface of the photodetector  60 .  
      On the other hand, when it is determined in the step S 1  shown in  FIG. 3  that the optical disc  2  is not a Blu-ray disc, the controller  1  determines that this optical disc  2  is a DVD, and goes on to perform a photodetector focus correction subroutine (step S 3 ).  
      In the photodetector focus correction subroutine, the controller  1  first reads optimum position information OP from the memory  8 , and supplies to the lens driver  7  a lens position set signal for setting the collimator lens  54  at a position on the optical axis indicated by the optimum position information OP. This causes the actuator  70  to move the collimator lens  54  to the position corresponding to the lens position set signal, and fix the collimator lens  54  at this position.  
      In other words, when the type of the optical disc  2  is a DVD, the collimator lens  54  is moved to the position on the optical axis indicated by the predetermined optimum position information OP so that the reflected light from the optical disc  2  in response to a light beam emitted by the second light source  52  can be correctly collected on the light receiving surface of the photodetector  60 .  
      After executing the step S 2  or S 3 , the controller  1  exits the optical system correction subroutine of  FIG. 3 , and goes on to control the recording and reproducing operations to the information data (not shown in the figure).  
      As described above, in the information recording/reproducing apparatus shown in  FIG. 2 , an optical system adjustment is made to the optical pickup  5  corresponding to the type of the optical disc  2  by moving the collimator lens  54  just before the recording/reproducing of the information data. Specifically, when the optical disc  2  is a Blu-ray disc, spherical aberration correction is automatically made by moving the collimator lens  54 . This eliminates the need for placing optical parts at precise positions determined in consideration of spherical aberration in assembling the optical pickup  5 . On the other hand, when the optical disc  2  is a DVD, focus correction for correctly condensing a beam spot of the reflected light from the optical disc  2  on the light receiving surface of the photodetector  60  is automatically made by moving the collimator lens  54 . This eliminates the need for precisely positioning the second light source  52  in the direction of the optical axis in assembling the optical pickup  5 , and makes the fabrication easier by just as much.  
     SECOND EMBODIMENT  
      In the above-described embodiment the collimator lens  54  is a single lens. However, the collimator lens may include a plurality of lenses.  
       FIG. 4  illustrates an information recording/reproducing apparatus having an optical pickup according to another embodiment of the present invention conceived in view of this point.  
      The information recording/reproducing apparatus shown in  FIG. 4  is the same as that shown in  FIG. 2  except for a collimator lens  64 .  
      The collimator lens  64  shown in  FIG. 4  includes a cemented lens of a convex lens L 2  and a meniscus lens L 3 , and a plane-concave lens L 1 . The actuator  70  moves the whole collimator lens  64  in the direction of the optical axis in the same manner as the actuator  70  moves the collimator lens  54  as described above. It should be noted that the actuator  70  may be designed to move only the plane-concave lens L 1  of the collimator lens  64 , or only the cemented lens of the convex lens L 2  and the meniscus lens L 3  in the direction of the optical axis in the same manner as it moves the collimator lens  54 .  
     THIRD EMBODIMENT  
      The present invention may be modified to separate the plane-concave lens L 1  from the collimator lens  64 , and have it disposed on the optical axis between the first light source  51  and the beam splitter  53 , as shown in  FIG. 5 . According to the configuration shown in  FIG. 5 , the reflected light (from the optical disc  2 ) of a light beam emitted by the second light source  52  is guided to the photodetector  60  only via the convex lens L 2  and the meniscus lens L 3  of the collimator lens  64 . The actuator  70  moves the cemented lens including the convex lens L 2  and the meniscus lens L 3  in the direction of the optical axis in the same manner as it moves the collimator lens  54  as described above.  
     FOURTH EMBODIMENT  
      In the above-described configuration, spherical aberration correction and focus correction are made by moving the collimator lens  54  (or the collimator lens  64 ) provided between the beam splitters  53  and  55 . However, an expander lens may be used in stead of the collimator lens.  
       FIG. 6  illustrates an information recording/reproducing apparatus (or the pickup) having the expander lens.  
      In the information recording/reproducing apparatus shown in  FIG. 6 , the configuration is the same as that shown in  FIG. 2  except for a collimator lens  61  and an expander lens  62 .  
      In  FIG. 6  the collimator lens  61  converts a light beam emitted by the first light source  51  into a parallel light. The parallel light is directed to the expander lens  62  via the beam splitter  53 . The expander lens  62  includes a double-concave lens L 1  and a convex lens L 2  which are disposed with a predetermined distance from each other. The expander lens  62  magnifies the beam spot diameter of the parallel light supplied via the beam splitter  53 . The parallel light whose spot diameter has been magnified by the expander lens  62  is guided to the objective lens  57  via the beam splitter  55  and the mirror  56 . The objective lens  57  condenses the parallel light supplied via the mirror  56 , and irradiates the recording surface of an optical disc  2  with this light. The reflected light from the recording surface of the optical disc  2  is guided to the expander lens  62  via the mirror  56  and the beam splitter  55 . The expander lens  62  reduces the beam spot diameter of the reflected light supplied via the beam splitter  55 . The reflected light whose spot diameter has been reduced by the expander lens  62  impinges on the light detecting surface of the photodetector  60  via an optical path of the beam splitter  53  and a detecting lens  59 . The photodetector  60  supplies to the read signal processing circuit  6  an electric signal obtained by applying photoelectric conversion to the light incident on the light detecting surface of the photodetector  60  as a read signal. The read signal processing circuit  6  generates various servo signals such as a focus servo signal, a tracking servo signal, and a slider servo signal based on the read signal. The read signal processing circuit  6  reproduces information data stored in the optical disc  2  based on a read sample value obtained by sampling the read signal, and outputs this as reproduced information data. Further, the read signal processing circuit  6  supplies the read sample value to the controller  1 . In the memory  8 , optimum position information OP which indicates an optimum position of the expander lens  62  on the optical axis when the type of the optical disc  2  is a DVD is stored in advance. The memory  8  has a storage area for storing an amplitude of a read sample value together with a current position of the expander lens  62  at the time of performing a spherical aberration correction subroutine (will be described).  
      In the same manner as in the first embodiment ( FIG. 2 ), the controller  1  shown in  FIG. 6  makes an optical system adjustment to the optical pickup  5  according to the optical system correction subroutine as shown in  FIG. 3  just before starting the recording and reproducing operation to the optical disc  2 . In the spherical aberration correction subroutine of the step S 2 , the controller  1  first supplies a lens distance extension command to the lens driver  7 . The lens driver  7  supplies to the actuator  70  a drive voltage (or a drive current) for gradually extending the distance between the double-concave lens L 1  and the convex lens L 2  within the expander lens  62  in response to the lens distance extension command. In response to the drive voltage, the actuator  70  gradually moves the concave lens L 1  (or the convex lens L 2 , or both of the concave lens L 1  and the convex lens L 2 ) in the direction of the optical axis in such a manner that the distance between the concave lens L 1  and the convex lens L 2  within the expander lens  62  becomes longer. While the lens is moving, the controller  1  repeatedly reads a read sample value from the read signal processing circuit  6 , relates position information indicating the current position of each of the concave lens L 1  and the convex lens L 2  on the optical axis to an amplitude value of the current read sample value, and sequentially stores the amplitude values and the position information in the memory  8 . The controller  1  retrieves the maximum value out of the amplitude values stored in the memory  8 , and reads the stored position information together with the amplitude value from the memory  8 . Then, the controller  1  supplies a lens position set signal for setting the concave lens L 1  and the convex lens L 2  within the expander lens  62  at positions indicated by the position information.  
      In other words, in the spherical aberration correction subroutine, first, the amount of spherical aberration is gradually changed by extending the distance between the concave lens L 1  and the convex lens L 2  of the expander lens  62  more than the predetermined distance. The position of each of the concave lens L 1  and the convex lens L 2  when an amplitude value of the read signal is at the maximum is judged as being the position where the amount of spherical aberration is at the minimum, and the concave lens L 1  and the convex lens L 2  are fixed at these positions. In short, spherical aberration is corrected by adjusting the distance between the concave lens L 1  and the convex lens L 2  of the expander lens  62 .  
      In the photodetector focus correction subroutine of the step S 3  ( FIG. 3 ), the controller  1  first reads the optimum position information OP from the memory  8 , and supplies to the lens driver  7  a lens position set signal for setting the expander lens  62  at the position on the optical axis indicated by the optimum position information OP. This makes the actuator  70  move the expander lens  62  to the position corresponding to the lens position set signal, and fix the expander lens  62  at this position. In other words, when the optical disc  2  is a DVD, the expander lens  62  is moved to the position on the optical axis indicated by the optimum position information OP so that the reflected light of a light beam emitted by the second light source  52  can be correctly condensed on the light receiving surface of the photodetector  60 .  
     FIFTH EMBODIMENT  
      In the above-described embodiments an optical pickup having two light sources  51 ,  52  for a DVD and a Blu-ray disc is described. The present invention can also be applied to an optical pickup having three light sources for a CD, a DVD, and a Blu-ray disc.  
       FIG. 7  is a block diagram showing a configuration of an information recording/reproducing apparatus which is capable of recording/reproducing information data onto/from any of a CD, a DVD, or a Blue-ray disc.  
      In  FIG. 7 , when the optical disc  2  is a DVD, the controller  1  supplies a recording start signal or a reading start signal to the light source driver  4 . When the optical disc  2  is a Blu-ray disc, the controller  1  supplies a recording start signal or a reading start signal to the light source driver  3 . When the optical disc  2  is a CD, the controller  1  supplies a recording start signal or a reading start signal to a light source driver  9 .  
      The light source driver  3  supplies a drive voltage (or a drive current) for recording information onto a Blu-ray disc to the first light source  51  mounted on the optical pickup  5  when the driver  3  receives a recording start signal. When the light source driver  3  receives a reading start signal, the light source driver  3  supplies a drive voltage (or a drive current) for reading information from a Blu-ray disc to the first light source  51 . The light source driver  4  supplies a drive voltage (or a drive current) for recording information onto a DVD disc to the second light source  52  mounted on the optical pickup  5  when the driver  4  receives a recording start signal. When the light source driver  4  receives a reading start signal, the light source driver  4  supplies a drive voltage (or a drive current) for reading information from a DVD to the second light source  52 . The light source driver  9  supplies a drive voltage (or a drive current) for recording information onto a CD to a third light source  66  mounted on the optical pickup  5  when the driver  9  receives a recording start signal. When the light source driver  9  receives a reading start signal, the light source driver  9  supplies a drive voltage (or a drive current) for reading information from a CD to the third light source  66 .  
      The first light source  51  generates a light beam having a wavelength of 405 nm which has optical power corresponding to a drive voltage (or a drive current) supplied from the light source driver  3 . The light beam is directed to the collimator lens  54  via the beam splitter  53 . The collimator lens  54  converts the supplied light beam into a parallel light. The collimator lens  54  is movable in the direction of the optical axis by the actuator  70 . The parallel light obtained via the collimator lens  54  is directed to the objective lens  57  via the beam splitter  55  and the mirror  56 .  
      The second light source  52  generates a light beam having a wavelength of  650 nm which has optical power corresponding to a drive voltage (or a drive current) supplied from the light source driver  4 . The light beam is converted into a parallel light by the collimator lens  58 , and is directed to the objective lens  57  via a beam splitter  65 , the beam splitter  55 , and the mirror  56 .  
      The third light source  66  generates a light beam having a wavelength of 780 nm which has optical power corresponding to a drive voltage (or a drive current) supplied from the light source driver  4 . The light beam is converted into a parallel light by a collimator lens  67 , and is directed to the objective lens  57  via the beam splitters  65 ,  55  and the mirror  56 .  
      The objective lens  57  condenses the parallel light supplied via the mirror  56 , and irradiates the recording surface of the optical disc  2  with this light. The light-detecting surface of the photodetector  60  is irradiated with the reflected light from the recording surface of the optical disc  2  which has passed through an optical path of the mirror  56 , the beam splitter  55 , the collimator lens  54 , the beam splitter  53 , and the detecting lens  59 . The photodetector  60  supplies the electric signal obtained by applying photoelectric conversion to the light incident on the light detecting surface of the photodetector  60  to the read signal processing circuit  6  as a read signal. The read signal processing circuit  6  generates various servo signals such as a focus servo signal, a tracking servo signal, and a slider servo signal based on the read signal. A focus actuator (not shown in the figure) moves the objective lens  57  in the direction perpendicular to the recording surface of the optical disc  2  in response to the focus servo signal. A tracking actuator (not shown in the figure) moves the objective lens  57  in the radial direction of the optical disc  2  in response to the tracking servo signal. Slider mechanism (not shown in the figure) moves the optical pickup  5  in the radial direction of the optical disc in response to the slider servo signal. The read signal processing circuit  6  reproduces the information data stored in the optical disc  2  based on a read sample value obtained by sampling the read signal, and outputs this as reproduced information data. The read signal processing circuit  6  supplies the read sample value to the controller  1 .  
      The memory  8  stores, in advance, optimum position information OP DVD  which shows an optimum position of the collimator lens  54  on the optical axis when the type of the optical disc  2  is a DVD, and optimum position information OP CD  which shows an optimum position of the collimator lens  54  on the optical axis when the type of the optical disc  2  is a CD. In the memory  8  there is provided a storage area for storing an amplitude value of a read sample value together with a current position of the collimator lens  54  when performing a spherical aberration correction subroutine (will be described).  
      The controller  1  makes an adjustment to the optical pickup  5  according to an optical system correction subroutine as shown in  FIG. 8  just before starting recordation/reproduction to/from the optical disc  2 .  
      First, the controller  1  determines whether the optical disc  2  is a Blu-ray disc or not (step S 11 ). When it is determined in the step S 11  that the optical disc  2  is a Blu-ray disc, the controller  1  goes on to perform a spherical aberration correction subroutine (step S 12 ).  
      In the spherical aberration correction subroutine, the controller  1  first supplies a lens replacement signal to the lens driver  7 . The lens driver  7  supplies to the actuator  70  a drive voltage (or a drive current) for gradually moving the collimator lens  54  from a position A to a position B on the optical axis as shown in  FIG. 7  in response to the lens replacement signal. The actuator  70  gradually moves the collimator lens  54  in the direction of the optical axis from the position A to the position B as shown in  FIG. 7  in response to the drive voltage. While the collimator lens  54  is moving, the controller  1  repeatedly reads a read sample value from the read signal processing circuit  6 , relates position information indicating the current position of the collimator lens  54  on the optical axis (a position between the position A and the position B) to an amplitude value of the read sampled value concerned, and sequentially stores the amplitude values and the position information in the memory  8 . When the collimator lens  54  reaches the position B, the controller  1  retrieves the maximum value out of the amplitude values stored in the memory  8 . Then, the controller  1  reads the stored position information together with the amplitude value from the memory  8 , and supplies to the lens driver  7  a lens position set signal for setting the collimator lens  54  at the position indicated by the position information. This makes the actuator  70  move the collimator lens  54  to the position corresponding to the lens position set signal, and fix the collimator lens  54  at this position.  
      In other words, in the spherical aberration correction subroutine of the step S 12 , spherical aberration is corrected by moving the collimator lens  54  in the direction of the optical axis. The collimator lens  54  is used in both a go-route of the optical path of a light beam emitted by the first light source  51  and guided to the optical disc  2 , and a return-route of the optical path of a light reflected from the optical disc  2  and guided to the photodetector  60 . Therefore, even though the collimator lens  54  is moved in the direction of the optical axis for the sake of spherical aberration correction as described above, the beam spot of the reflected light from the optical disc  2  can correctly be formed on the light receiving surface of the photodetector  60 .  
      On the other hand, when it is determined in the step S 11  that the optical disc  2  is not a Blu-ray disc, the controller  1  goes on to determine whether or not this optical disc  2  is a DVD (step S 13 ). When it is determined in the step S 13  that the optical disc  2  is a DVD, the controller  1  goes on to perform a photodetector focus correction subroutine for a DVD (step S 14 ).  
      In the photodetector focus correction subroutine for a DVD, the controller  1  first reads optimum position information OP DVD  from the memory  8 , and supplies to the lens driver  7  a lens position set signal for setting the collimator lens  54  at the position on the optical axis indicated by the optimum position information OP DVD . This makes the actuator  70  move the collimator lens  54  to the position corresponding to the lens position set signal, and fix the collimator lens  54  at this position.  
      In other words, when the optical disc  2  is a DVD, the collimator lens  54  is moved to the position on the optical axis indicated by the predetermined optimum position information OP DVD  so that the reflected light from the optical disc  2  in response to a light beam emitted by the second light source  52  can be correctly condensed on the light receiving surface of the photodetector  60 .  
      On the other hand, when it is determined in the step S 13  that the optical disc  2  is not a DVD, the controller  1  determines that this optical disc  2  is a CD, and goes on to perform a photodetector focus correction subroutine for a CD (step S 15 ).  
      In the photodetector focus correction subroutine for a CD, the controller  1  first reads optimum position information OP CD  from the memory  8 , and supplies to the lens driver  7  a lens position set signal for setting the collimator lens  54  at the position on the optical axis indicated by the optimum position information OP CD . This makes the actuator  70  move the collimator lens  54  to the position corresponding to the lens position set signal, and fix the collimator lens  54  at this position.  
      In other words, when the optical disc  2  is a CD, the collimator lens  54  is moved to the position on the optical axis indicated by the optimum position information OP CD  so that the reflected light from the optical disc  2  in response to a light beam emitted by the third light source  66  can be correctly condensed on the light receiving surface of the photodetector  60 .  
      After executing the step S 12 , S 14 , or S 15 , the controller  1  leaves the optical system correction subroutine ( FIG. 8 ), and goes on to control the recording or reproducing of information data (not shown).  
      As described above, in the information recording/reproducing apparatus shown in  FIG. 7 , an optical adjustment is made to the optical pickup  5  in accordance with the type of the optical disc  2  by moving the collimator lens  54  just before recording/reproducing information data. Specifically, when the optical disc  2  is a Blu-ray disc, spherical aberration correction is automatically made by moving the collimator lens  54 . This eliminates the need for placing optical parts at respective precise positions determined in consideration of spherical aberration in assembling the optical pickup  5 . On the other hand, when the optical disc  2  is a DVD or a CD, focus correction for correctly forming a beam spot of the reflected light from the optical disc  2  on the light receiving surface of the photodetector  60  is automatically made by moving the collimator lens  54 . This eliminates the need for precisely positioning the second light source  52  or the third light source  66  in the direction of the optical axis in assembling the optical pickup  5 , and makes the fabrication easier by just as much.  
     SIXTH EMBODIMENT  
      In the above-described embodiments, the focus correction for the photodetector and spherical aberration correction are made by moving the collimator lens  54  or  64 , or the expander lens  64  in the direction of the optical axis. However, a liquid crystal lens having a lens function whose focal length can be changed arbitrarily may be employed in place of the collimator lens or the expander lens which is movable in the direction of the optical axis.  
       FIG. 9  illustrates an information recording/reproducing apparatus conceived in view of this point.  
      In  FIG. 9 , the configuration is the same as that shown in  FIG. 6  except for a liquid crystal lens  80  adopted in place of the expander lens  62  and a liquid crystal driver  10  adopted in place of the lens driver  7 .  
      In  FIG. 9 , the liquid crystal lens  80  is disposed on the optical axis between the beam splitters  53  and  55 . The liquid crystal lens  80  includes a liquid crystal layer  81  which is filled with liquid crystal in the shape of a double-convex lens, a pair of transparent electrode plates  82   a  and  82   b  which contact the two convex surfaces of the liquid crystal layer  81  respectively, and a pair of glass substrates  83  having a refractive index n 1  which sandwich the transparent electrode plates  82   a  and  82   b.    
      The liquid crystal driver  10  applies a drive voltage between the transparent electrode plates  82   a  and  82   b  of the liquid crystal lens  80  in response to a refractive index set signal supplied from the controller  1 . The liquid crystal layer  81  within the liquid crystal lens  80  has a refractive index n 2  corresponding to the drive voltage applied between the transparent electrode plates  82   a  and  82   b . In other words, the liquid crystal layer  81  is an optical element whose refractive index n 2  changes in accordance with the refractive index set signal supplied from the controller  1 . Therefore, when the refractive index n 2  of the liquid crystal layer  81  is greater than the refractive index n 1  of the glass substrate, the liquid crystal lens  80  functions as a positive lens (convex lens), and when the refractive index n 2  is smaller than the refractive index n 1 , the liquid crystal lens  80  functions as a negative lens (concave lens). In short, the liquid crystal lens  80  functions as a positive lens or a negative lens which has a focal length corresponding to a drive voltage based on the refractive index set signal supplied from the controller  1 . When the refractive index n 2  is equal to the refractive index n 1 , the liquid crystal lens  80  functions as a parallel plate.  
      Thus, by making the liquid crystal lens  80  function as a positive lens or a negative lens in response to the drive voltage applied between the transparent electrode plates  82   a  and  82   b , spherical aberration is corrected just as by moving the collimator lens  54  or  64 , or the expander lens  64  in the direction of optical axis. Further, by changing the focal length of the liquid crystal lens  80  in response to the drive voltage applied between the transparent electrode plates  82   a  and  82   b , focus is corrected just as by moving the collimator lens  54  or  64 , or the expander lens  64  in the direction of optical axis.  
      This application is based on Japanese Patent Application No. 2003-397128 filed on Nov. 27, 2003, and the entire disclosure thereof is incorporated herein by reference.