Patent Publication Number: US-2007104071-A1

Title: Optical pickup and disc drive apparatus

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
      The present invention contains subject matter related to Japanese Patent Application JP 2005-049562 filed in the Japanese Patent Office on Feb. 24, 2005, the entire contents of which being incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to an optical pickup and a disc drive apparatus that make it possible to record information signals to and/or reproduce information signals from three different disc-shaped recording mediums by means of a single objective lens.  
      2. Description of the Related Art  
      Disc drive apparatus for recording information signals to and/or reproducing information signals from a disc-shaped recording medium are known. Such disc drive apparatus includes an optical pickup for irradiating a laser beam to the disc-shaped recording medium by way of an objective lens in order to record or reproduce information signals.  
      In recent years, disc-shaped recording mediums of various different types have been developed. The difference among the different types of recording medium lies in the recording density, the cover thickness and so on. There is a demand for disc-shaped recording mediums having a recording capacity greater than CDs (compact discs) that utilizes a laser beam with a wavelength of about 780 nm and DVDs (digital versatile discs) that utilizes a laser beam with a wavelength of about 660 nm. High recording density optical discs that utilizes a laser beam with a wavelength of about 405 nm have been attracting attention as disc-shaped recording mediums of the next generation that realize a much greater recording capacity.  
      Such high density recording optical discs include blue-ray discs that utilizes a laser beam with a wavelength of about 405 nm and AODs (advance optical discs) that also utilizes a laser beam with a wavelength of about 405 nm. HD-DVDs (high definition DVDs) that conform to a standard similar to the AOD Standard are also known. In the following description, HD-DVDs are included in AODs.  
      Disc drive apparatus that can be used to record information signals to and reproduce information signals from disc-shaped recording mediums of different types, using laser beams of different wavelengths, and are equipped with a plurality of objective lenses for disc-shaped recording mediums of different types are known (see, inter alia, Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 2001-110086).  
      The disc drive apparatus described in Patent Document 1 includes a biaxial actuator provided with a pair of objective lenses typically including one that corresponds to a disc-shaped recording medium to be used with a laser beam of a wavelength of about 405 nm and one that corresponds to a disc-shaped recording medium to be used with a laser beam of a wavelength of about 660 nm so that the laser beam of a wavelength of about 405 nm is converged on the disc-shaped recording medium of one of the two different types by one of the objective lenses, whereas the laser beam of a wavelength of about 660 nm is converged on the disc-shaped recording medium of the other type by the other objective lens for the purpose of recording information signals to or reproducing information signals from the disc-shaped recording mediums of the different types.  
     SUMMARY OF THE INVENTION  
      However, known optical pickups including a plurality of objective lenses for recording information signals to and reproducing information signals from a plurality of disc-shaped recording mediums of different types includes a large number of components to make the disc drive apparatus dimensionally large and consequently raise the manufacturing cost.  
      Additionally, the weight of the movable part of the biaxial actuator is raised because of the plurality of objective lenses so that the responsiveness of the movable part can be degraded in the focusing control operation and the tracking control operation.  
      Therefore, it is desirable to provide an optical pickup and a disc drive apparatus that make it possible to record information signals to and reproduce information signals from disc-shaped recording mediums of three different types by means of a single objective lens.  
      According to the present invention, there is provided an optical pickup including: a first light emitting element that emits a laser beam of a first wavelength; a second light emitting element that emits a laser beam of a second wavelength; a third light emitting element that emits a laser beam of a third wavelength; an objective lens that focuses the laser beams emitted from the first through third light emitting elements onto the signal recording surface of a disc-shaped recording medium; first and second diffracting sections arranged on the light paths of the first through third laser beams and adapted to diffract at least one of the laser beams of the first through third wavelengths; and a light receiving element that receives the return light reflected by the disc-shaped recording medium; the first diffracting section being adapted to substantially transmit one or two of the laser beams of the first through third wavelengths.  
      According to the present invention, there is provided a disc drive apparatus including a disc table that receives a disc-shaped recording medium of one of different types and drives it to rotate, and an optical pickup that records information to or reproduces information from the disc-shaped recording medium received on the disc table, the optical pickup including: a first light emitting element that emits a laser beam of a first wavelength; a second light emitting element that emits a laser beam of a second wavelength; a third light emitting element that emits a laser beam of a third wavelength; an objective lens that focuses the laser beams emitted from the first through third light emitting elements onto the signal recording surface of a disc-shaped recording medium; first and second diffracting sections arranged on the light paths of the first through third laser beams and adapted to diffract at least one of the laser beams of the first through third wavelengths; and a light receiving element that receives the return light reflected by the disc-shaped recording medium; the first diffracting section being adapted to substantially transmit one or two of the laser beams of the first through third wavelengths.  
      Thus, in an optical pickup according to the invention, either the first or second diffracting section is adapted to substantially transmit one or two of the laser beams of the first through third wavelengths so that it is possible to record information signals to and reproduce information signals from disc-shaped recording mediums of three different types to be used with laser beams of different wavelengths by means of a single objective lens.  
      Additionally, an optical pickup according to the invention can record information signals to and reproduce information signals from disc-shaped recording mediums of three different types to be used with laser beams of different wavelengths by means of a single objective lens so that the optical pickup can be produced with a reduced number of components to make it possible to reduce both the dimensions of the optical pickup and the manufacturing cost. Furthermore, since an optical pickup according to the invention is adapted to use a single objective lens, the weight of the movable part of objective lens drive unit is reduced to secure an excellent responsiveness of the movable part in the focusing control operation and the tracking control operation.  
      Still additionally, a disc drive apparatus according to the invention includes a disc table that receives a disc-shaped recording medium of one of different types and drives it to rotate and an optical pickup that records information to or reproduces information from the disc-shaped recording medium received on the disc table and, in the optical pickup, either the first or second diffracting section is adapted to substantially transmit one or two of the laser beams of the first through third wavelengths so that it is possible to record information signals to and reproduce information signals from disc-shaped recording mediums of three different types to be used with laser beams of different wavelengths by means of a single objective lens.  
      Still additionally, a disc drive apparatus according to the invention can record information signals to and reproduce information signals from disc-shaped recording mediums of three different types to be used with laser beams of different wavelengths by means of a single objective lens so that the disc drive apparatus can be produced with a reduced number of components to make it possible to reduce both the dimensions of the disc drive apparatus and the manufacturing cost. Furthermore, since a disc drive apparatus according to the invention is adapted to use a single objective lens, the weight of the movable part of objective lens drive unit is reduced to secure an excellent responsiveness of the movable part in the focusing control operation and the tracking control operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic perspective view of an embodiment of disc drive apparatus according to the invention;  
       FIG. 2  is a schematic illustration of the optical system of an embodiment of optical pickup according to the invention, showing the light paths thereof;  
       FIG. 3  is an enlarged schematic cross sectional view of part of the diffraction element of an embodiment of optical pickup according to the invention;  
       FIG. 4  is a graph illustrating the relationship between the wavelength and the refractive index of a material that can be used for forming the first diffracting section of the diffraction element of an embodiment of optical pickup according to the invention;  
       FIG. 5  is a schematic illustration of an objective lens and a diffraction. element that can be employed when an optical disc  100   c  of the third type is used as disc-shaped recording medium corresponding to a laser beam of the third wavelength  
       FIG. 6  is a schematic illustration of an objective lens and a diffraction element that can be employed when an optical disc  100   d  of the fourth type is used as disc-shaped recording medium corresponding to a laser beam of the third wavelength  
       FIG. 7  is a schematic illustration of another objective lens and another diffraction element that can be employed when an optical disc  100   d  of the fourth type is used as disc-shaped recording medium corresponding to a laser beam of the third wavelength;  
       FIG. 8  is a schematic illustration of the optical system of another embodiment of optical pickup according to the invention, showing the light paths thereof;  
       FIG. 9  is a schematic illustration of an embodiment of optical pickup according to the invention, where a polarizing hologram element is used for diffraction;  
       FIG. 10  is a schematic illustration of the optical system of still another embodiment of optical pickup according to the invention, showing the light paths thereof;  
       FIG. 11  is a schematic illustration of an embodiment of optical pickup according to the invention, where a stepped diffraction element is used for the optical pickup;  
       FIG. 12  is an enlarged schematic cross sectional view of the first diffracting section of an embodiment of optical pickup according to the invention, where a stepped diffraction element is used for the optical pickup;  
       FIG. 13  is a graph illustrating the diffraction efficiencies of laser beams of different wavelengths that varies as a function of the change in the groove depth of the first diffracting section of an embodiment of optical pickup according to the invention, where a stepped diffraction element is used for the optical pickup;  
       FIG. 14  is a graph illustrating the diffraction efficiencies of laser beams of different wavelengths that varies as a function of the change in the groove depth of the second diffracting section of an embodiment of optical pickup according to the invention, where a stepped diffraction element is used for the optical pickup; and  
       FIG. 15  is a schematic illustration of the diffracting section of an embodiment of optical pickup according to the invention, where an anti-stray-light section is provided. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Now, the present invention will be described in greater detail by referring to the accompanying drawings that illustrate preferred embodiments of optical pickup and disc drive apparatus according to the invention.  
      Referring firstly to  FIG. 1  that illustrates an embodiment of disc drive apparatus  1  according to the invention, its members and mechanisms are arranged in an outer cabinet  2  having a disc insertion port (not shown).  
      A chassis (not shown) is arranged in the outer cabinet  2  and a disc table 3 is rigidly secured to the motor shaft of the spindle motor that is fitted to the chassis.  
      A pair of guide shafts  4  are fitted to the chassis and arranged in parallel with each other and a lead screw  5  that is driven to rotate by a feed motor (not shown) is supported by the chassis.  
      The optical pickup  6  includes a movable base  7 , certain optical parts arranged on the movable base  7  and an objective lens drive unit  8  also arranged on the movable base  7 . Bearing sections  7   a ,  7   b  arranged at opposite ends of the movable base  7  are supported by the respective guide shafts  4  so as to be able to slide along the latter. The objective lens drive unit 8 includes a movable section  8   a  and a fixed section  8   b , of which the movable section  8   a  is movably supported by the fixed section  8   b  by way of a suspension (not shown). A nut member (not shown) arranged on the movable base  7  is engaged with the lead screw  5  so that, as the lead screw  5  is driven to rotate, the nut member is fed in the direction that corresponds to the sense of rotation of the lead screw  5  and the optical pickup  6  is driven to move in a radial direction of the disc-shaped recording medium  100  mounted on the disc table 3.  
      Disc-shaped recording mediums  100  that can be mounted on the disc table 3 for use include optical discs such as CDs (compact discs), DVDs (digital versatile discs), CD-Rs (recordable) that are write once type compact discs, DVD-Rs (recordable), CD-RWs (rewritable) that are rewritable type compact discs, DVD-RWs (rewritable) and DVD+RWs (rewritable), high density recording optical discs where information can be recorded at a high density by means of a semiconductor laser for emitting a laser beam with a short wavelength of about 405 nm (violet) and magneto-optical discs.  
      Particularly, this embodiment is designed to record information on and reproduce information from optical discs  100  of three different types by means of the optical pickup  1 . They include optical discs  100   a  of the first type that typically include CDs (compact discs) having a cover thickness of 1.2 mm and adapted to be used with a light beam of a wavelength of about 780 nm for signal recording/reproduction, optical discs  100   b  of the second type that typically include DVDs (digital versatile discs) having a cover thickness of 0.6 mm and adapted to be used with a light beam of a wavelength of about 660 nm, optical discs  100   c  of the third type that typically include blue-ray discs having a cover thickness of 0.1 mm and adapted to be used with a light beam of a wavelength of about 405 nm for high density recording and optical discs  100   d  of the fourth type that typically include AODs (advanced optical discs) having a cover thickness of 0.6 mm and adapted to be used with a light beam of a wavelength of about 405 nm for high density recording.  
      In view of the wavelengths of light beams and the cover thicknesses of the disc-shaped recording mediums  100  to be used with a disc drive apparatus according to the invention, it is desirable that the numerical aperture of the objective lens arranged in the objective lens drive unit  8 , which will be described in greater detail hereinafter, is about 0.65 for optical discs  100   a  of the first type, optical discs  100   b  of the second type and optical discs  100   d  of the fourth type and about 0.85 for optical discs  100   c  of the third type.  
      In the disc drive apparatus  1  having the above described configuration, as the disc table 3 is driven to rotate by the rotary motion of the spindle motor, the disc-shaped recording medium  100 , which is selected from optical discs  100   a ,  100   b ,  100   c ,  100   d  of the first through fourth type and mounted on the disc table 3, is also driven to rotate and, at the same time, the optical pickup  6  is moved in a radial direction of the disc-shaped recording medium  100  to record information on or reproduce information from the disc-shaped recording medium  100 . At this time, the movable section  8   a  of the objective lens drive unit  8  is moved relative to the fixed section  8   b  and a focusing adjusting operation and a tracking adjusting operation are conducted for the objective lens that is arranged in the movable section  8   a  as will be described in greater detail hereinafter.  
      As shown in  FIG. 2 , the optical pickup  6  typically includes a first light source  9 , a second light source  10 , a coupling lens  11 , a light path synthesizing element  12 , a beam splitter  13 , a collimator lens  14 , an upturn mirror  15 , a ¼ wave plate  16 , a diffraction element  17 , an objective lens  18 , a conversion lens  19 , an optical axis synthesizing element  20  and a light receiving element  21 , all of which are arranged on the movable base  7  except the objective lens  18  that is arranged in the movable section  8   a  of the objective lens drive unit  8 .  
      A first light emitting element  9   a  and a second light emitting element  9   b  are arranged in the inside of the first light source  9 . The first light emitting element  9   a  is adapted to emit a laser beam of the first wavelength of about 780 nm for the optical disc  100   a  of the first type and the second light emitting element  9   b  is adapted to emit a laser beam of the second wavelength of about 660 nm for the optical disc  100   b  of the second type.  
      A third light emitting element  10   a  is arranged in the inside of the second light source  10 . The third light emitting element  10   a  is adapted to emit a laser beam of the third wavelength of about 405 nm for the optical disc  100   c  of the third type or the optical disc  100   d  of the fourth type.  
      The coupling lens  11  operates to change the optical magnification of the laser beam emitted from the first light source  9  on the forward route thereof.  
      The light path synthesizing element  12  is typically a beam splitter having a mirror surface  12   a  that has wavelength selectivity. The laser beam of the first or second wavelength that is emitted from the first light emitting element  9   a  or the second light emitting element  9   b , whichever appropriate, of the first light source  9  is reflected by the mirror surface  12   a , whereas the laser beam of the third wavelength that is emitted from the third light emitting element  10   a  of the second light source  10  is transmitted through the mirror surface  12   a.    
      The beam splitter  13  is a polarization beam splitter having a function of either transmitting or reflecting the incident laser beam depending on the direction of polarization of the laser beam. The laser beam is transmitted through the splitting surface  13   a  and directed toward the collimator lens  14  on the forward route thereof but reflected by the splitting surface  13   a  and directed toward the light receiving element  21  on the backward route thereof.  
      The collimator lens  14  has a function of collimating the flux of light of the incident laser beam.  
      The upturn mirror  15  has a function of reflecting the incident laser beam with an angle of about 90°.  
      The ¼ wave plate  16  has a function of giving a phase difference of the ¼ wavelength to the laser beam that passes through it and transforming linearly polarized light into circularly polarized light or vice versa.  
      The diffraction element  17  has a first diffracting section  22  and a second diffracting section  23  formed on the opposite surfaces thereof when an optical disc  100   c  of the third type is used as disc-shaped recording medium  100  that matches a laser beam of the third wavelength.  
      The first diffracting section  22  that is arranged at the side facing the upturn mirror  15  is adapted to diffract a laser beam of the third wavelength but substantially transmit a laser beam of the first wavelength and a laser beam of the second wavelength. Thus, the first diffracting section  22  operates as controlling diffracting section for controlling the extent of diffraction as a function of the wavelength of the incident laser beam.  
      The first diffracting section  22  may be formed typically by bonding two materials A and B that show respective refractive indexes with different frequency characteristics and hence differ from each other in terms of dispersion characteristics to sandwich the diffraction surface  22   a  where a grating is formed as shown in  FIG. 3 . The material A and the material B are so selected that their refractive indexes do not differ significantly between the first wavelength (about 70 nm) and the second wavelength (about 660 nm) but come to differ remarkably at and near the third wavelength (about 405 nm).  
      Thus, it is possible to form a first diffracting section  22  that shows a desired diffraction efficiency by selecting an optimal combination of materials to be bonded together that differ from each other in terms of dispersion characteristics.  
      Alternatively, the first diffracting section  22  may be formed by laying a plurality of thin films having different refractive indexes to form a multilayer structure that diffracts a laser beam of the third wavelength but substantially transmits a laser beam of the first or second wavelength.  
      The second diffracting section  23  of the diffraction element  17  is formed so as to diffract a laser beam of any of the first through third wavelength.  
      The objective lens  18  has a function of focusing the incident laser beam to the recording surface of the disc-shaped recording medium  100 . As shown in  FIG. 5 , the objective lens  18  has an outer peripheral section that is formed as an annular band  18   a  for adjusting the aperture of the lens that corresponds to a laser beam of the third wavelength. In other words, the part  18   b  of the objective lens  18  that is located inside the annular band  18   a  shows a numerical aperture of about 0.65 to correspond to a laser beam of the first or second wavelength and the combined part of the annular band  18   a  and the part  18   b  shows a numerical aperture of about 0.85 to correspond to a laser beam of the third wavelength.  
      The conversion lens  19  has a function of changing the optical magnification of the laser beam emitted from the first light source  9  or the second light source  10  on the forward route thereof.  
      The optical axis synthesizing element  20  has a function of synthesizing the optical axis of the laser beam emitted from the first light source  9  and that of the laser beam emitted from the second light source  10  and converging each of the laser beams to the light receiving element  21 .  
      As a laser beam of the first wavelength or the second wavelength is emitted from the first light emitting element  9   a  or the second light emitting element  9   b , whichever appropriate, of the first light source  9  of the optical pickup  6  having the above described configuration, its optical magnification is changed on the forward route thereof by the coupling lens  11  and then the laser beam is made to enter the light path synthesizing element  12 . The laser beam that enters the light path synthesizing element  12  is reflected by the mirror surface  12   a , subsequently transmitted through the splitting surface  13   a  of the beam splitter  13  and turned into a collimated flux of light by the collimator lens  14  and then reflected by the upturn mirror  15  to enter the ¼ wave plate  16 . A phase difference of π/2 is added to the laser beam that enters the ¼ wave plate  16 . The laser beam entering the ¼ wave plate  16  is a linearly polarized (P-polarized) laser beam but then circularly polarized and made to enter the diffraction element  17 . The laser beam that enters the diffraction element  17  is substantially transmitted through the first diffracting section  22  and diffracted by the second diffracting section  23  and made to enter the part  18   b  of the objective lens  18  so that it is focused onto the recording surface of the optical disc  100   a  of the first type or the optical disc  100   b  of the second type that is mounted on the disc table 3.  
      The laser beam that is focused on the recording surface of the optical disc  100   a  of the first type or the optical disc  100   b  of the second type is reflected by the recording surface and enters again the ¼ wave plate  16  as return light by way of the objective lens  18  and the diffraction element  17 . A phase difference of π/2 is added to the laser beam that enters the ¼ wave plate  16 . The laser beam entering the ¼ wave plate  16  is a circularly polarized laser beam but then linearly polarized (S-polarized) and made to enter the beam splitter  13  by way of the upturn mirror  15  and the collimator lens  14 . The return light that enters the beam splitter  13  is reflected by the splitting surface  13   a  of the beam splitter  13  with its optical magnification changed by the conversion lens  19  on the backward route thereof and made to enter the light receiving element  21  by way of the optical axis synthesizing element  20 . The laser beam that enters the light receiving element  21  is subjected to photoelectric conversion. Thus, information signals are recorded on or reproduced from the optical disc  100   a  of the first type or the optical disc  100   b  of the second type.  
      On the other hand, as a laser beam of the third wavelength is emitted from the third light emitting element  10   a  of the second light source  10 , it is made to enter the light path synthesizing element  12 . The laser beam that enters the light path synthesizing element  12  is transmitted through the mirror surface  12   a  and subsequently through the splitting surface  13   a  of the beam splitter  13 , collimated by the collimator lens  14  and then reflected by the upturn mirror  15  to enter the ¼ wave plate  16 . A phase difference of π/2 is added to the laser beam that enters the ¼ wave plate  16 . The laser beam entering the ¼ wave plate  16  is a linearly polarized (P-polarized) laser beam but then circularly polarized and made to enter the diffraction element  17 . The laser beam that enters the diffraction element  17  is diffracted by the first diffracting section  22  and the second diffracting section  23  and made to enter the annular band  18   a  of the objective lens  18  so that it is focused onto the recording surface of the optical disc  100   c  of the third type that is mounted on the disc table 3.  
      The laser beam that is focused on the recording surface of the optical disc  100   c  of the third type is reflected by the recording surface and enters again the ¼ wave plate  16  as return light by way of the objective lens  18  and the diffraction element  17 . A phase difference of π/2 is added to the laser beam that enters the ¼ wave plate  16 . The laser beam entering the ¼ wave plate  16  is a circularly polarized laser beam but then linearly polarized (S-polarized) and made to enter the beam splitter  13  by way of the upturn mirror  15  and the collimator lens  14 . The return light that enters the beam splitter  13  is reflected by the splitting surface,  13   a  of the beam splitter  13  with its optical magnification changed by the conversion lens  19  on the backward route thereof and made to enter the light receiving element  21  by way of the optical axis synthesizing element  20 . The laser beam that enters the light receiving element  21  is subjected to photoelectric conversion. Thus, information signals are recorded on or reproduced from the optical disc  100   c  of the third type.  
      While the laser beam of the third wavelength is diffracted by the first diffracting section  22  of the diffraction element  17  whereas the laser beam of the first or second wavelength is substantially transmitted through first diffracting section  22  of the diffraction element  17  in the above description, it may alternatively be so arranged that the laser beam of the first or second wavelength is diffracted by the first diffracting section  22  of the diffraction element  17  whereas the laser beam of the third wavelength is substantially transmitted through the first diffracting section  22  of the diffraction element  17  so that the laser beam of the first or second wavelength enters the part  18   b  of the objective lens  18  while the laser beam of the third wavelength enters the annular band  18   a  of the objective lens  18 .  
      With the above described arrangement of using an objective lens  18  having an annular band  18   a  for adjusting the aperture of the lens and a diffraction element  17  having a first diffracting section  22  that operates as controlling diffracting section and a second diffracting section  23  that diffracts a laser beam of any wavelength formed on the opposite surfaces thereof, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums  100  of the three different types including optical discs  100   a ,  100   b ,  100   c  of the first through third type by means of a single objective lens  18 .  
      In this way, a laser beam of any of the three different wavelengths is focused on the recording surface of the optical disc of the corresponding type that is mounted on the disc table 3 by means of the single objective lens of the optical pickup  6  according to the present invention.  
      As either the first diffracting section  22  or the second diffracting section  23  is arranged to operate as controlling diffracting section and the laser beam of one of the first through third wavelengths or the laser beams of two of the first through third wavelengths are substantially transmitted through the controlling diffracting section in the optical pickup  6  according to the invention, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums of the three different types, which correspond respectively to laser beams of three different wavelengths, by means of the single objective lens  18  of the optical pickup  6  according to the invention.  
      Additionally, as information signals can be recorded on and reproduced from disc-shaped recording mediums of the three different types, which correspond respectively to laser beams of different wavelengths, by means of the single objective lens  18  of the optical pickup  6  according to the invention, it is possible to reduce the number of components and hence both the dimensions of the optical pickup and the manufacturing cost. Furthermore, since the optical pickup  6  according to the invention is adapted to use a single objective lens  18 , the weight of the movable part of objective lens drive unit is reduced to secure an excellent the responsiveness of the movable part in the focusing control operation and the tracking control operation.  
      Still additionally, as the objective lens  18  of the optical pickup  6  according to the invention is provided with an annular band  18   a  for adjusting the aperture of the lens and two diffracting sections  22  and  23  are arranged respectively on the surface of incidence and the light emitting surface of the diffraction element  17  so as to use one of the diffracting sections, or the diffracting section  22 , as controlling diffracting section, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums  100  of the three different types including optical discs  100   a ,  100   b ,  100   c  of the first through third types by means of a single objective lens  18 .  
      Still additionally, as the controlling diffracting section of the optical pickup  6  according to the invention is formed by bonding two materials A and B that show different dispersion characteristics, it is possible to form the diffracting section so as to make it show a desired diffraction efficiency by selecting optimal materials for the combination of materials.  
      While the embodiment of disc drive apparatus is described above in terms of an optical disc  100   c  of the third type that is a disc-shaped recording medium  100  on which information signals are recorded and from which information signals are reproduced by means of a laser beam of the third wavelength, an optical pickup  6 A having a diffraction element  17 A and an objective lens  18 A as shown in  FIG. 6  is typically employed when an optical disc  100   d  of the fourth type is used as disc-shaped recording medium  100  so as to record information signals on and reproduce information signals from it by means of a laser beam of the third wavelength (about 405 nm).  
      Now, the optical pickup  6 A to be used for recording information signals on and reproducing information signals from an optical disc  100   d  of the fourth type will be described below in detail. The optical pickup  6 A to be used for an optical disc  100   d  of the fourth type differs from the optical pickup  6  to be used for an optical disc  100   c  of the third type only in the configuration of the diffraction element and that of the objective lens. In the following description, the components that are common to the optical pickup  6 A and the above described optical pickup  6  are denoted respectively by the same reference symbols and will not be described any further.  
      As shown in  FIG. 2 , the optical pickup  6 A typically includes a first light source  9 , a second light source  10 , a coupling lens  11 , a light path synthesizing element  12 , a beam splitter  13 , a collimator lens  14 , an upturn mirror  15 , a ¼ wave plate  16 , a diffraction element  17 A, an objective lens  18 A, a conversion lens  19 , an optical axis synthesizing element  20  and a light receiving element  21 , all of which are arranged on the movable base  7  except the objective lens  18 A that is arranged in the movable section  8   a  of the objective lens drive unit  8 .  
      As shown in  FIG. 6 , the diffraction element  17 A has a first diffracting section  24  and a second diffracting section  25  arranged respectively on the opposite surfaces thereof. The diffraction element  17 A has a configuration similar to the above described diffraction element  17 .  
      The first diffracting section  24  that is arranged at the side facing the upturn mirror  15  is adapted to diffract a laser beam of the third wavelength but substantially transmit a laser beam of the first or second wavelength. Thus, the first diffracting section  24  operates as controlling diffracting section for controlling the extent of diffraction as a function of the wavelength of the incident laser beam.  
      The second diffracting section  25  of the diffraction element  17 A is formed so as to diffract a laser beam of any of the first through third wavelength.  
      Unlike the above described objective lens  18 , the objective lens  18 A is not provided with an annular band and the numerical aperture of the objective lens  18 A is about 0.65.  
      As a laser beam of the first wavelength or the second wavelength is emitted from the first light emitting element  9   a  or the second light emitting element  9   b , whichever appropriate, of the first light source  9  of the optical pickup  6 A having the above described configuration, it is made to enter the diffraction element  17 A by way of the light path similar to that of the above described optical pickup  6 . The laser beam that enters the diffraction element  17 A is substantially transmitted through the first diffracting section  24  of the diffraction element  17 A and diffracted by the second diffracting section  25  and made to enter the objective lens  18 A so that it is focused onto the recording surface of the optical disc  100   a  of the first type or the optical disc  100   b  of the second type that is mounted on the disc table 3 to record information signals on or reproduce information signals from the optical disc  100   a  of the first type or the optical disc  100   b  of the second type. The light path of the return light reflected by the recording surface of the optical disc  100   a  of the first type or the optical disc  100   b  of the second type is same as that of the above described optical pickup  6  and hence will not be described here any further.  
      On the other hand, as a laser beam of the third wavelength is emitted from the third light emitting element  10   a  of the second light source  10 , it is made to enter the diffraction element  17 A by way of the light path similar to that of the above described optical pickup  6  and diffracted by the first diffracting section  24  and the second diffracting section  25  of the diffraction element  17 A. Then, it is made to enter the objective lens  18 A so that it is focused onto the recording surface of the optical disc  100   d  of the fourth type that is mounted on the disc table 3 to record information signals on or reproduce information signals from the optical disc  100   d  of the fourth type. The light path of the return light reflected by the recording surface of the optical disc  100   d  of the fourth type is same as that of the above described optical pickup  6  and hence will not be described here any further.  
      With the above described arrangement of using a diffraction element  17 A having a first diffracting section  24  that operates as controlling diffracting section and a second diffracting section  25  that diffracts a laser beam of any wavelength formed on the opposite surfaces of thereof, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums  100  of the three different types including optical discs  100   a  of the first type, optical discs  100   b  of the second type and optical discs  100   d  of the fourth type by means of a single objective lens  18 A.  
      In this way, a laser beam of any of the three different wavelengths is focused on the recording surface of the optical disc of the corresponding type that is mounted on the disc table 3 by means of the single objective lens of the optical pickup  6 A according to the present invention.  
      As either the first diffracting section  24  or the second diffracting section  25  is arranged to operate as controlling diffracting section and the laser beam of one of the first through third wavelengths or the laser beams of two of the first through third wavelengths are substantially transmitted through the controlling diffracting section in the optical pickup  6 A according to the invention, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums of the three different types, which correspond respectively to laser beams of three different wavelengths, by means of the single objective lens  18 A.  
      Additionally, as information signals can be recorded on and reproduced from disc-shaped recording mediums of the three different types, which correspond respectively to laser beams of different wavelengths, by means of the single objective lens  18 A of the optical pickup  6 A according to the invention, it is possible to reduce the number of components and hence both the dimensions of the optical pickup and the manufacturing cost. Furthermore, since the optical pickup  6 A according to the invention is adapted to use a single objective lens  18 A, the weight of the movable part of objective lens drive unit is reduced to secure an excellent responsiveness of the movable part in the focusing control operation and the tracking control operation.  
      An optical pickup  6 B having a diffraction element and an objective lens as shown in  FIG. 7  may alternatively be employed when an optical disc  100   d  of the fourth type is used as disc-shaped recording medium  100  so as to record information signals on and reproduce information signals from it.  
      Now, the optical pickup  6 B to be used for recording information signals on and reproducing information signals from an optical disc  100   d  of the fourth type will be described below in detail. If compared with the optical pickup  6 , the optical pickup  6 B to be used for an optical disc  100   d  of the fourth type differs from the optical pickup  6  to be used for an optical disc  100   c  of the third type only in the configuration of the diffraction element and that of the objective lens. In the following description, the components that are common to the optical pickup  6 B and the above described optical pickup  6  are denoted respectively by the same reference symbols and will not be described any further.  
      As shown in  FIG. 2 , the optical pickup  6 B typically includes a first light source  9 , a second light source  10 , a coupling lens  11 , a light path synthesizing element  12 , a beam splitter  13 , a collimator lens  14 , an upturn mirror  15 , a ¼ wave plate  16 , a diffraction element  17 B, an objective lens  18 B, a conversion lens  19 , an optical axis synthesizing element  20  and a light receiving element  21 , all of which are arranged on the movable base  7  except the objective lens  18 B that is arranged in the movable section  8   a  of the objective lens drive unit  8 .  
      The diffraction element  17 B only has a first diffracting section  26  arranged on one of the opposite surfaces thereof. The first diffracting section  26  is adapted to diffract a laser beam of the third wavelength but substantially transmit a laser beam of the first or second wavelength. Thus, the first diffracting section  26  operates as controlling diffracting section for controlling the extent of diffraction as a function of the wavelength of the incident laser beam. The first diffracting section  26  of the diffraction element  17 B may be formed like the first diffracting section  22  of the above described diffraction element  17 . In other words, it may be formed by bonding two materials that show respective refractive indexes with different frequency characteristics.  
      Unlike the above-described objective lens  18 , the objective lens  18 B is not provided with an annular band and the numerical aperture of the objective lens  18 B is about 0.65. A second diffracting section  27  is formed on the surface of the objective lens  18 B that is located vis-à-vis the diffraction element  17 B. The second diffracting section  27  is adapted to diffract a laser beam of any of the first through third wavelength.  
      As a laser beam of the first wavelength or the second wavelength is emitted from the first light emitting element  9   a  or the second light emitting element  9   b , whichever appropriate, of the first light source  9  of the optical pickup  6 B having the above described configuration, it is made to enter the diffraction element  17 B by way of the light path similar to that of the above described optical pickup  6 . The laser beam that enters the diffraction element  17 B is substantially transmitted through the first diffracting section  26 , made to enter the objective lens  18 B and diffracted by the second diffracting section  27  so that it is focused onto the recording surface of the optical disc  100   a  of the first type or the optical disc  100   b  of the second type that is mounted on the disc table 3 to record information signals on or reproduce information signals from the optical disc  100   a  of the first type or the optical disc  100   b  of the second type. The light path of the return light reflected by the recording surface of the optical disc  100   a  of the first type or the optical disc  100   b  of the second type is same as that of the above described optical pickup  6  and hence will not be described here any further.  
      On the other hand, as a laser beam of the third wavelength is emitted from the third light emitting element  10   a  of the second light source  10  of the optical pickup  6 B, it is made to enter the diffraction element  17 B by way of the light path similar to that of the above described optical pickup  6  and diffracted by the first diffracting section  26  of the diffraction element  17 B and the second diffracting section  27  of the objective lens  18 B. Then, it is focused onto the recording surface of the optical disc  100   d  of the fourth type that is mounted on the disc table 3 to record information signals on or reproduce information signals from the optical disc  100   d  of the fourth type. The light path of the return light reflected by the recording surface of the optical disc  100   d  of the fourth type is same as that of the above described optical pickup  6  and hence will not be described here any further.  
      With the above described arrangement of using a diffraction element  17 B having a first diffracting section  26  that operates as controlling diffracting section and an objective lens  18 B having a second diffracting section  27  that diffracts a laser beam of any wavelength, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums  100  of the three different types including optical discs  100   a  of the first type, optical discs  100   b  of the second type and optical discs  100   d  of the fourth type by means of a single objective lens  18 B.  
      In this way, a laser beam of any of the three different wavelengths is focused on the recording surface of the optical disc of the corresponding type that is mounted on the disc table 3 by means of the single objective lens of the optical pickup  6 B according to the present invention.  
      As either the first diffracting section  26  or the second diffracting section  27  is arranged to operate as controlling diffracting section and the laser beam of one of the first through third wavelengths or the laser beams of two of the first through third wavelengths are substantially transmitted through the controlling diffracting section in the optical pickup  6 B according to the invention, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums of the three different types, which correspond respectively to laser beams of three different wavelengths, by means of the single objective lens  18 B.  
      Additionally, as information signals can be recorded on and reproduced from disc-shaped recording mediums of the three different types, which correspond respectively to laser beams of different wavelengths, by means of the single objective lens  18 B of the optical pickup  6 B according to the invention, it is possible to reduce the number of components and hence both the dimensions of the optical pickup and the manufacturing cost. Furthermore, since the optical pickup  6 B according to the invention is adapted to use a single objective lens  18 B, the weight of the movable part of objective lens drive unit is reduced to secure an excellent responsiveness of the movable part in the focusing control operation and the tracking control operation.  
      As diffracting sections  26 ,  27  are formed respectively on one of the opposite surfaces of the objective lens  18 B and on one of the opposite surfaces of the diffraction element  17 B and the diffracting section  26  of the diffraction element  17 B is made to operate as controlling diffracting section, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums of the three different types including optical discs  100   a  of the first type, optical discs  100   b  of the second type and optical discs  100   d  of the fourth type by means of the single objective lens  18 B of the optical pickup  6 B according to the invention.  
      Like the optical pickup  6  adapted to record information signals on and reproduce information signals from optical discs  100   c  of the third type, the optical pickups  6 A,  6 B adapted to record information signals on and reproduce information signals from optical discs  100   d  of the fourth type may be so arranged that the first diffracting section  24  of the diffraction element  17 A and the first diffracting section  26  of the diffraction element  17 B diffract laser beams of the first and second wavelengths but substantially transmit a laser beam of the third wavelength.  
      While the above described optical pickups  6 ,  6 A,  6 B respectively have diffracting sections formed on the diffraction elements  17 ,  17 A,  17 B and the objective lens  18 B in the above description, a polarization hologram element may alternatively be used to diffract a laser beam as in the case of the optical pickup  6 C illustrated in  FIGS. 8 and 9 .  
      Now, the optical pickup  6 C including a polarization hologram element will be described below. The optical pickup  6 C adapted to diffract a laser beam by means of a polarization hologram element differs from the optical pickups  6 ,  6 A,  6 B only in that the diffraction elements of those optical pickups are replaced by a polarization hologram element. In the following description, the components that are common to the optical pickup  6 C and the above described optical pickup  6  are denoted respectively by the same reference symbols and will not be described any further.  
      As shown in  FIGS. 8 and 9 , the optical pickup  6 C typically includes a first light source  9 , a second light source  10 , a coupling lens  11 , a light path synthesizing element  12 , a beam splitter  13 , a collimator lens  14 , an upturn mirror  15 , a polarization hologram element  28 , an objective lens  18  ( 18 A), a conversion lens  19 , an optical axis synthesizing element  20  and a light receiving element  21 , all of which are arranged on the movable base  7  except the objective lens  18  ( 18 A) that is arranged in the movable section  8   a  of the objective lens drive unit  8 .  
      The disc-shaped recording medium  100  on which information signals are recorded and from which information signals are reproduced by means of a laser beam of the third wavelength in the optical pickup  6 C adapted to diffract laser beams by means of a polarization hologram element may be an optical disc  100   c  of the third type or an optical disc  100   d  of the fourth type.  
      The polarization hologram element  28  has a first diffracting section  29  and a second diffracting section  30  formed respectively on the opposite surfaces thereof.  
      The first diffracting section  29  is formed on the surface of the polarization hologram element  28  located vis-à-vis the upturn mirror  15  is typically adapted to diffract P-polarized light and transmit S-polarized light, whereas the second diffracting section  30  arranged on the opposite surface of the polarization hologram element  28  is typically adapted to diffract S-polarized light and transmit P-polarized light. Therefore, both the first diffracting section  29  and the second diffracting section  30  are controlling diffracting sections that diffract the incident laser beam depending on the sense of polarization of the beam.  
      The objective lens  18  is used when the disc-shaped recording medium  100  on which information signals are recorded and from which information signals are reproduced by means of a laser beam of the third wavelength is an optical disc  100   c  of the third type, whereas the objective lens  18 A is used when the disc-shaped recording medium  100  is an optical disc  100   d  of the fourth type.  
      Since linearly polarized light is focused onto the recording surface of the disc-shaped recording medium  100  by the optical pickup  6 C adapted to diffract a laser beam by means of a polarization hologram element  28 , no ¼ wave plate is used unlike the above described optical pickups  6 ,  6 A,  6 B.  
      In the optical pickup  6 C having the above described configuration, the laser beam emitted from the first light emitting element  9   a  or the second light emitting element  9   b  of the first light source  9  may be S-polarized light and, as emitted S-polarized light enters the polarization hologram element  28 , it is diffracted only by the second diffracting section  30 . The laser beam of diffracted S-polarized light is focused on the optical disc  100   a  of the first type or the optical disc  100   b  of the second type that is mounted on the disc table 3 by the objective lens  18  or the objective lens  18 A to record information signals on or reproduce information signals from the optical disc  100   a  of the first type or the optical disc  100   b  of the second type, whichever appropriate.  
      On the other hand, the laser beam of a wavelength of about 405 nm emitted from the third light emitting element  10   a  of the second light source  10  may be P-polarized light and, as emitted P-polarized light enters the polarization hologram element  28 , it is diffracted only by the first diffracting section  29 . The laser beam of diffracted P-polarized light is focused on the recording surface of the optical disc  100   c  of the third type or the optical disc  100   d  of the fourth type that is mounted on the disc table 3 by the objective lens  18  or the objective lens  18 A to record information signals on or reproduce information signals from the optical disc  100   c  of the third type or the optical disc  100   d  of the fourth type, whichever appropriate.  
      While the sense of polarization of the laser beams of the first wavelength and the second wavelength and that of the laser beam of the third wavelength are orthogonal relative to each other in the above described example, it may alternatively be so arranged that the sense of polarization of the laser beam of the first wavelength and that of the laser beams of the second wavelength and the third wavelength are orthogonal relative to each other.  
      Thus, it is possible to record information signals on and reproduce information signals from disc-shaped recording mediums  100  of three different types that are used respectively with laser beams of different wavelengths by means of a single objective lens  18  or objective lens  18 A when a laser beam is diffracted by means of a polarization hologram element  28 . Thus, it is possible to simplify the overall configuration of the optical pickup.  
      In this way, a laser beam of any of the three different wavelengths is focused on the recording surface of the optical disc of the corresponding type that is mounted on the disc table 3 by means of the single objective lens of the optical pickup  6 C according to the present invention.  
      As either the first diffracting section  29  or the second diffracting section  30  is arranged to operate as controlling diffracting section and the laser beam of one of the first through third wavelengths or the laser beams of two of the first through third wavelengths are substantially transmitted through the controlling diffracting section in the optical pickup  6 C according to the invention, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums of the three different types, which correspond respectively to laser beams of three different wavelengths, by means of the single objective lens of the optical pickup  6 C according to the invention.  
      Additionally, as information signals can be recorded on and reproduced from disc-shaped recording mediums of the three different types, which correspond respectively to laser beams of different wavelengths, by means of the single objective lens of the optical pickup  6 C according to the invention, it is possible to reduce the number of components and hence both the dimensions of the optical pickup and the manufacturing cost. Furthermore, since the optical pickup  6 C according to the invention is adapted to use a single objective lens, the weight of the movable part of objective lens drive unit is reduced to secure an excellent responsiveness of the movable part in the focusing control operation and the tracking control operation.  
      As the optical pickup  6 C according to the invention includes a polarization hologram element  28  that is adapted to operate as diffraction element and diffracting sections are formed respectively on the surface of incidence and on the light emitting surface thereof so that a laser beam selected from laser beams of polarized light whose senses of polarization are orthogonal relative to each other enters the polarization hologram element and the diffracting sections  29 ,  30  of the polarization hologram element operate as controlling diffracting sections, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums of the three different types by means of the single objective lens of the optical pickup  6 C according to the invention.  
      While the above described optical pickups  6 ,  6 A,  6 B,  6 C have first and second diffracting sections that are so-called diffraction grating type two-stepped diffracting sections, the present invention is by no means limited thereto and the diffracting sections may alternatively be formed by holograms showing a staircase-like profile as in the case of the optical pickup  6 D illustrated in  FIGS. 10 and 11 .  
      Now, the optical pickup  6 D including staircase-like diffracting sections will be described below. The optical pickup  6 D differs from the above-described optical pickup  6  having the diffraction element  17  and the objective lens  18  only in terms of the configuration of the diffraction element and that of the objective lens. In the following description, the components that are common to the optical pickup  6 D and the above described optical pickup  6  are denoted respectively by the same reference symbols and will not be described any further.  
      As shown in  FIGS. 10 and 11 , the optical pickup  6 D typically includes a first light source  9 , a second light source  10 , a coupling lens  11 , a light path synthesizing element  12 , a beam splitter  13 , a collimator lens  14 , an upturn mirror  15 , a ¼ wave plate  16 , a diffraction element  17 D, an objective lens  18  ( 18 A), a conversion lens  19 , an optical axis synthesizing element  20  and a light receiving element  21 , all of which are arranged on the movable base  7  except the objective lens  18  ( 18 A) that is arranged in the movable section  8   a  of the objective lens drive unit  8 .  
      As shown in  FIGS. 11 and 12 , the diffraction element  17 D has a first diffracting section  31  and a second diffracting section  32  arranged respectively on the opposite surfaces thereof. Holograms are formed respectively in the first diffracting section  31  and the second diffracting section  32  of the diffraction element  17 D. The holograms show a profile like that of staircases having five steps respectively including the first through fourth steps  33   a  through  33   d ,  34   a  through  34   d  of a same depth. In other words, the holograms have respective diffraction planes including the first through fifth diffraction planes  33   e  through  33   i ,  34   e  through  34   i  that are arranged at regular intervals. While the number of steps of the staircase-like profile that defines the number of diffraction planes is five in this embodiment, the number of steps is by no means limited to five.  
      The first diffracting section  31  arranged at the side located vis-à-vis the upturn mirror  15  is formed by bonding two different members to be bonded that show respective refractive indexes, more specifically two different members to be bonded that show respective refractive indexes whose frequency characteristics are different from each other. A desired diffraction efficiency can be achieved when the difference between the refractive index of the member that operates as the substrate of the first diffracting section  31  and that of the member to be bonded is not smaller than 0.05. A desired diffraction efficiency can be achieved when the refractive index of the member that operates as the substrate is not higher than 1.55 and that of the member to be bonded is not lower than 1.60. Both the member that operates as the substrate and the member to be bonded are typically made of synthetic resin. As shown in  FIG. 12 , the first diffracting section  31  is formed by bonding material C and material D that show different refractive indexes so as to sandwich the diffraction surface  31   a  showing a profile like that of staircases having five steps. Table 1 below shows the refractive index of the material C and that of the material D for the different wavelengths to be used.  
                       TABLE 1                                      wavelength (nm)                                 first   second   third           wavelength 780   wavelength 660   wavelength 405                                         material C   1.50482   1.50678   1.52433       material D   1.62   1.6355   1.68396                  
 
       FIG. 13  shows the diffraction efficiency of the first diffracting section  31  having a profile like that of a staircase having five steps and made of the material C and the material D listed in Table 1 that varies as a function of the change in the total groove depth. The expression of total groove depth as used herein refers to the total of the depths of the above-described first through fourth steps  33   a  through  33   d . The steps are made to show a substantially same depth (the gap separating any two adjacently located ones of the first through fifth diffraction planes). In  FIG. 13 , each of the curves indicates the ratio of the total quantity of incident light of the laser beam to the quantity of diffracted light of the laser beam, which is the diffraction efficiency. In  FIG. 13 , the curves L 10 , L 11  and L 1−1  are respectively for diffracted (transmitted) light of the 0-th degree, diffracted light of the 1st degree and that of the −1 st degree of a laser beam with a wavelength of 780 nm (the first wavelength) and the curves L 20 , L 21  and L 2−1  are respectively for diffracted (transmitted) light of the 0-th degree, diffracted light of the 1st degree and that of the −1st degree of a laser beam with a wavelength of 660 nm (the second wavelength), whereas the curves L 30 , L 31  and L 3−1  are respectively for diffracted (transmitted) light of the 0-th degree, diffracted light of the 1st degree and that of the −1st degree of a laser beam with a wavelength of 405 nm (the third wavelength).  
      Table 2 below shows the diffraction efficiency of diffracted light of the 1st degree of a laser beam with a wavelength of 780 nm (the first wavelength), the diffraction efficiency (transmittance) of diffracted light of the 0-th degree of a laser beam with a wavelength of 660 nm (the second wavelength) and the diffraction efficiency (transmittance) of diffracted light of the 0-th degree of a laser beam with a wavelength of 405 nm (the third wavelength) when the total groove depth of the first diffracting section  31  of the optical pickup  6 D is made equal to 15.4 μm (15,400 nm).  
                       TABLE 2                                      wavelength (nm)                                 first   second   third           wavelength 780   wavelength 660   wavelength 405           (1st degree)   (0-th degree)   (0-th degree)                                         transmittance/   81%   100%   100%       diffraction       efficiency,       1st diffracting       section       31 (depth =       15.4 μm)                  
 
      The second diffracting section  32  arranged at the side located vis-à-vis the objective lens  18  ( 18 A) has its diffraction surface  32   a  showing a profile like that of a staircase having five steps exposed to air, as shown in  FIG. 12 , and is made of material C listed in Table 1 above.  
       FIG. 14  shows the diffraction efficiency of the second diffracting section  32  having a profile like that of a staircase having five steps and made of the material C listed in Table 1 that varies as a function of the change in the total groove depth. The expression of total groove depth as used herein refers to the total of the depths of the above described first through fourth steps  34   a  through  34   d . The steps are made to show a substantially same depth (the gap separating any two adjacently located ones of the first through fifth diffraction planes). In  FIG. 14 , each of the curves indicates the ratio of the total quantity of incident light of the laser beam to the quantity of diffracted light of the laser beam, which is the diffraction efficiency. In  FIG. 14 , the curves L 10 , L 11  and L 1−1  are respectively for diffracted (transmitted) light of the 0-th degree, diffracted light of the 1st degree and that of the −1st degree of a laser beam with a wavelength of 780 nm (the first wavelength) and the curves L 20 , L 21  and L 2−1  are respectively for diffracted (transmitted) light of the 0-th degree, diffracted light of the 1st degree and that of the −1st degree of a laser beam with a wavelength of 660 nm (the second wavelength), whereas the curves L 30 , L 31  and L 3−1  are respectively for diffracted (transmitted) light of the 0-th degree, diffracted light of the 1st degree and that of the −1st degree of a laser beam with a wavelength of 405 nm (the third wavelength).  
      Table 3 below shows the diffraction efficiency (transmittance) of diffracted light of the 0-th degree of a laser beam with a wavelength of 780 nm (the first, wavelength), the diffraction efficiency of diffracted light of the 1st degree of a laser beam with a wavelength of 660 nm (the second wavelength) and the diffraction efficiency (transmittance) of diffracted light of the 0-th degree of a laser beam with a wavelength of 405 nm (the third wavelength) when the total groove depth of the second diffracting section  32  of the optical pickup  6 D is made equal to 6.2 μm (6200 nm).  
                       TABLE 3                                      wavelength (nm)                                 first   second   third           wavelength 780   wavelength 660   wavelength 405           (0-th degree)   (1st degree)   (0-th degree)                                         transmittance/   100%   88%   100%       diffraction       efficiency,       2nd diffracting       section       32 (depth =       6.2 μm)                  
 
      As pointed out earlier, the first diffracting section  31  is adapted to diffract a laser beam of the first wavelength but substantially transmits a laser beam of the second wavelength and a laser beam of the third wavelength. In other words, the first diffracting section  31  operates as first controlling diffracting section for controlling the extent of diffraction as a function of the wavelength of the incident laser beam.  
      The second diffracting section  32  of the diffraction element  17 D is adapted to diffract a laser beam of the second wavelength but substantially transmit a laser beam of the first wavelength and a laser beam of the third wavelength. Thus, the second diffracting section  32  operates as controlling diffracting section for controlling the extent of diffraction as a function of the wavelength of the incident laser beam.  
      Unlike the first diffracting section  31 , the second diffracting section  32  is not formed by bonding members to be bonded but can be made to be adapted to diffract a laser beam of the second wavelength but substantially transmit a laser beam of the first wavelength and a laser beam of the third wavelength by adjusting the depth and the width of the steps  34   a  through  34   d . In other words, it is formed by bonding an air layer that shows a refractive index equal to 1.  
      However, alternatively, the second diffracting section  32  may be formed by bonding members to be bonded that show respective refractive indexes with different frequency characteristics. When the second diffracting section  32  is formed by bonding members to be bonded that show respective refractive indexes with different frequency characteristics, it is possible to make it diffract a laser beam of the third wavelength but substantially transmits a laser beam of the first wavelength and a laser beam of the second wavelength. Thus, it is possible to raise the degree of design freedom without making the configuration of the pickup a complex one.  
      The objective lens  18  is used when the disc-shaped recording medium  100  on which information signals are recorded and from which information signals are reproduced by means of a laser beam of the third wavelength is an optical disc  100   c  of the third type, whereas the objective lens  18 A is used when the disc-shaped recording medium  100  is an optical disc  100   d  of the fourth type.  
      As a laser beam of the first wavelength is emitted from the first light emitting element  9   a  of the first light source  9  of the optical pickup  6 D having the above described configuration, it is made to enter the diffraction element  17 D by way of the light path similar to that of the above described optical pickups. The laser beam that enters the diffraction element  17 D is diffracted by the first diffracting section  31  but substantially transmitted through the second diffracting section  32  and made to enter the objective lens  18  or the objective lens  18 A so that it is focused onto the recording surface of the optical disc  100   a  of the first type that is mounted on the disc table 3 to record information signals on or reproduce information signals from the optical disc  100   a  of the first type. The light path of the return light reflected by the recording surface of the optical disc  100   a  of the first type is same as that of any of the above described optical pickups and hence will not be described here any further.  
      Similarly, a laser beam of the second wavelength is emitted from the second light emitting element  9   b  of the first light source  9 , it is made to enter the diffraction element  17 D by way of the light path similar to that of the above described optical pickups. The laser beam that enters the diffraction element  17 D is substantially transmitted through the first diffracting section  31  but diffracted by the second diffracting section  32  and made to enter the objective lens  18  or the objective lens  18 A so that it is focused onto the recording surface of the optical disc  100   b  of the second type that is mounted on the disc table 3 to record information signals on or reproduce information signals from the optical disc  100   b  of the second type. The light path of the return light reflected by the recording surface of the optical disc  100   b  of the second type is same as that of any of the above described optical pickups and hence will not be described here any further.  
      On the other hand, as a laser beam of the third wavelength is emitted from the third light emitting element  10   a  of the second light source  10  of the optical pickup  6 D, it is made to enter the diffraction element  17 D by way of the light path similar to that of the above described optical pickups and transmitted through the first diffracting section  31  and the second diffracting section  32  of the diffraction element  17 D. Then, it is made to enter the objective lens  18  or the objective lens  18 A so that it is focused onto the recording surface of the optical disc  100   c  of the third type or the optical disc  100   d  of the fourth type that is mounted on the disc table 3 to record information signals on or reproduce information signals from the third optical disc  100   c  of the third type or the fourth optical disc  100   d  of the fourth type, whichever appropriate. The light path of the return light reflected by the recording surface of the optical disc  100   c  of the third type or that of the optical disc  100   d  of the fourth type is same as that of the above described optical pickups and hence will not be described here any further.  
      With the above-described arrangement of using an objective lens  18  having an annular band  18   a  for adjusting the aperture of the lens and a diffraction element  17 D having a first diffracting section  31  that operates as first controlling diffracting section and a second diffracting section  32  that operates as second controlling diffracting section formed on the opposite surfaces thereof, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums  100  of the three different types including optical discs  100   a ,  100   b ,  100   c  of the first through third type by means of a single objective lens  18 .  
      When the objective lens  18 A is used and a first diffracting section  31  that operates as first controlling diffracting section and a second diffracting section  32  that operates as second controlling diffracting section are formed on the opposite surfaces of the diffraction element  17 D, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums  100  of the three different types including optical discs  100   a ,  100   b ,  100   d  of the first, second and fourth types by means of a single objective lens  18 A.  
      In this way, a laser beam of any of the three different wavelengths is focused on the recording surface of the optical disc of the corresponding type that is mounted on the disc table 3 by means of the single objective lens of the optical pickup  6 D according to the present invention.  
      As either the first diffracting section  31  or the second diffracting section  32  is arranged to operate as controlling diffracting section and the laser beam of one of the first through third wavelengths or the laser beams of two of the first through third wavelengths are substantially transmitted through the controlling diffracting section in the optical pickup  6 D according to the invention, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums of the three different types, which correspond respectively to laser beams of three different wavelengths, by means of the single objective lens.  
      Additionally, as information signals can be recorded on and reproduced from disc-shaped recording mediums of the three different types, which correspond respectively to laser beams of different wavelengths, by means of the single objective lens of the optical pickup  6 D according to the invention, it is possible to reduce the number of components and hence both the dimensions of the optical pickup and the manufacturing cost. Furthermore, since the optical pickup  6 D according to the invention is adapted to use a single objective lens, the weight of the movable part of objective lens drive unit is reduced to secure an excellent responsiveness of the movable part in the focusing control operation and the tracking control operation.  
      As the optical pickup  6 D according to the invention includes diffracting sections  31 ,  32  formed respectively on the surface of incidence and the light emitting surface of the diffraction element  17 D and one of the diffracting sections of the diffraction element  17 D, or the diffracting section  31 , is formed by bonding members to be bonded having different refractive indexes so as to make it operate as controlling diffracting section, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums of the three different types including optical discs  100   a  of the first type, optical discs  100   b  of the second type, optical discs  100   c  of the third type or optical discs  100   d  of the fourth type by means of the single objective lens of the optical pickup  6 D according to the invention.  
      As holograms are formed in the controlling diffracting sections of the optical pickup  6 D according to the invention and made to show a profile like that of staircases having five steps, it is possible to provide diffracting sections showing a desired diffraction efficiency.  
      As the optical pickup  6 D according to the invention includes diffracting sections  31 ,  32  formed respectively on the surface of incidence and the light emitting surface of the diffraction element  17 D and one of the diffracting sections of the diffraction element  17 D, or the diffracting section  31 , is made to operate as first controlling diffracting section, while the other diffracting section, or the diffracting section  32 , is made to operate as second controlling diffracting section so that the first controlling diffracting section diffracts a laser beam of the first wavelength but transmits a laser beam of the second wavelength and a laser beam of the third wavelength and the second controlling diffracting section diffracts a laser beam of the second wavelength but transmits a laser beam of the first wavelength and a laser beam of the third wavelength, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums of the three different types including optical discs  100   a  of the first type, optical discs  100   b  of the second type, optical discs  100   c  of the third type or optical discs  100   d  of the fourth type by means of the single objective lens of the optical pickup  6 D according to the invention.  
      As the optical pickup  6 D according to the invention includes diffracting sections  31 ,  32  formed respectively on the surface of incidence and the light emitting surface of the diffraction element  17 D and one of the diffracting sections of the diffraction element  17 D, or the diffracting section  31 , is formed as first controlling diffracting section by bonding members to be bonded that show different refractive indexes, while the other diffracting section of the diffraction element  17 D, is formed as second controlling diffracting section by bonding members to be bonded that show different refractive indexes, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums of the three different types including optical discs  100   a  of the first type, optical discs  100   b  of the second type, optical discs  100   c  of the third type or optical discs  100   d  of the fourth type by means of the single objective lens of the optical pickup  6 D according to the invention.  
      While the first through third light emitting elements  9   a ,  9   b ,  10   a  of the above described optical pickup  6 D are arranged in the first light source  9  and the second light source  10 , the optical pickup  6 D may alternatively be provided with a single light source where the first light emitting element for emitting a laser beam of the first wavelength, the second light emitting element for emitting a laser beam of the second wavelength and the third light emitting element for emitting a laser beam of the third wavelength are arranged so that information signals may be recorded on and reproduced from a disc-shaped recording medium of any of the three different types by means of a laser beam of the corresponding wavelength emitted from the single light source section, the above described diffraction element  17 D and the above-described objective lens.  
      The above-described optical pickup  6 D including diffracting sections where holograms showing a staircase-like profile are arranged may be replaced by an optical pickup  6 E including first and second diffracting sections and an anti-stray-light section as illustrated in FIG  15 .  
      Now, the optical pickup  6 E including an anti-stray-light section will be described below. The optical pickup  6 E having an anti-stray-light section differs from the above described optical pickup  6 D having holograms that show a staircase-like profile only in terms of the configuration of the diffraction element and that of the objective lens. In the following description, the components that are common to the optical pickup  6 E and the above described optical pickup  6 D are denoted respectively by the same reference symbols and will not be described any further.  
      As shown in  FIGS. 10 and 15 , the optical pickup  6 E typically includes a first light source  9 , a second light source  10 , a coupling lens  11 , a light path synthesizing element  12 , a beam splitter  13 , a collimator lens  14 , an upturn mirror  15 , a ¼ wave plate  16 , a diffraction element  17 E, an objective lens  18  ( 18 A), a conversion lens  19 , an optical axis synthesizing element  20  and a light receiving element  21 , all of which are arranged on the movable base  7  except the objective lens  18  ( 18 A) that is arranged in the movable section  8   a  of the objective lens drive unit  8 .  
      As shown in  FIGS. 15 , the diffraction element  17 E has a first diffracting section  35  and a second diffracting section  36  arranged respectively on the opposite surfaces thereof. The diffraction element  17 E additionally has an anti-stray-light section  37  that is formed in the region of the surface thereof located vis-a-vis the upturn mirror  15  (the surface of incidence) where the first diffracting section  35  is arranged other than the region occupied by the hologram. The anti-stray-light section  37  may typically be produced by forming a convex surface having a large radius of curvature at the light receiving side of the diffraction element  17 E. In other words, as a convex surface having a large radius of curvature is formed, the region of one of the surfaces of the diffraction element  17 E other than the region occupied by the hologram operates as anti-stray-light section  37 . A convex surface may also be formed on the light emitting side of the diffraction element  17 E so as to make it show a profile corresponding to that of the light receiving side.  
      Holograms are formed respectively in the first diffracting section  35  and the second diffracting section  36  of the diffraction element  17 E like those of the first and second diffracting sections  31 ,  32  of the above described diffraction element  17 D. Note, however, that holograms are formed in the above described diffraction element  17 D by using planar surfaces as reference surfaces whereas holograms are formed in the diffraction element  17 E by using convex surfaces as reference surfaces. The holograms show a profile like that of staircases having five steps including the first through fourth steps of a substantially same depth. In other words, the holograms have respective diffraction surfaces including the first through fifth diffraction surfaces that are arranged at regular intervals. While the number of steps of the staircase-like profile that defines the number of diffraction surfaces is five in this embodiment, the number of steps is by no means limited to five. Additionally, while the first diffracting section  35  is formed by bonding different members to be bonded that show respective refractive indexes whose frequency characteristics are different from each other and the second diffracting section  36  is exposed to air, the arrangement of the first and second diffracting sections is by no means limited thereto. Each of the first and second diffracting sections may be formed by bonding different members to be bonded that show respective refractive indexes whose frequency characteristics are different from each other.  
      The anti-stray-light section  37  can effectively prevent problems such as a problem that incident light is reflected by a planar part other than a hologram of an element where the hologram is formed in part of an aperture thereof and reflected light enters the photoelectric transducer of a photo-detector, for example, as stray light to degrade the characteristics of the transducer.  
      As the anti-stray-light section  37  is made to show a curved profile as described above, the incoming laser beam of collimated light is reflected and diffused by it to reduce stray light and hence the adverse effect of stray light.  
      When first and second curved surfaces are formed respectively at the opposite sides of the diffraction element, the incoming laser beam of collimated light can be emitted as a laser beam of collimated light to prevent it from exerting any adverse effect on the downstream optical components where it passes. When the hologram is formed only in part of the aperture of the diffraction element, another hologram may be formed with a different profile or a different depth in a planar area not occupied by the former hologram.  
      The objective lens  18  is used when the disc-shaped recording medium  100  on which information signals are recorded and from which information signals are reproduced by means of a laser beam of the third wavelength is an optical disc  100   c  of the third type, whereas the objective lens  18 A is used when the disc-shaped recording medium  100  is an optical disc  100   d  of the fourth type.  
      As a laser beam of the first wavelength is emitted from the first light emitting element  9   a  of the first light source  9  of the optical pickup  6 E having the above described configuration, it is made to enter the diffraction element  17 D by way of the light path similar to that of the above described optical pickups. The laser beam that enters the diffraction element  17 D is diffracted by the first diffracting section  35  but substantially transmitted through the second diffracting section  36  and made to enter the objective lens  18  or the objective lens  18 A so that it is focused onto the recording surface of the optical disc  100   a  of the first type that is mounted on the disc table 3 to record information signals on or reproduce information signals from the optical disc  100   a  of the first type. The light path of the return light reflected by the recording surface of the optical disc  100   a  of the first type is same as that of any of the above described optical pickups and hence will not be described here any further.  
      Similarly, a laser beam of the second wavelength is emitted from the second light emitting element  9   b  of the first light source  9 , it is made to enter the diffraction element  17 D by way of the light path similar to that of the above described optical pickups. The laser beam that enters the diffraction element  17 D is substantially transmitted through the first diffracting section  35  but diffracted by the second diffracting section  36  and made to enter the objective lens  18  or the objective lens  18 A so that it is focused onto the recording surface of the optical disc  100   b  of the second type that is mounted on the disc table 3 to record information signals on or reproduce information signals from the optical disc  100   b  of the second type. The light path of the return light reflected by the recording surface of the optical disc  100   b  of the second type is same as that of any of the above described optical pickups and hence will not be described here any further.  
      On the other hand, as a laser beam of the third wavelength is emitted from the third light emitting element  10   a  of the second light source  10 , it is made to enter the diffraction element  17 D by way of the light path similar to that of the above-described optical pickups and transmitted through the first diffracting section  35  and the second diffracting section  36  of the diffraction element  17 D. Then,.it is made to enter the objective lens  18  or the objective lens  18 A so that it is focused onto the recording surface of the optical disc  100   c  of the third type or the optical disc  100   d  of the fourth type that is mounted on the disc table 3 to record information signals on or reproduce information signals from the optical disc  100   c  of the third type or the optical disc  100   d  of the fourth type, whichever appropriate. The light path of the return light reflected by the recording surface of the optical disc  100   c  of the third type or that of the optical disc  100   d  of the fourth type is same as that of the above described optical pickups and hence will not be described here any further.  
      Regardless of the light emitting element that is selected out of the first through third light emitting elements  9   a ,  9   b ,  10   a  to emit a laser beam, the anti-stray-light section  37  of the diffraction element  17 E diffuses the laser beam that enters the diffraction element  17 D in the region other than the hologram on the forward route thereof due to the curved profile of the anti-stray-light section  37  so that it can effectively prevent so called stray light from taking place when the reflected laser beam enters the light receiving element  21 .  
      With the above described arrangement of using an objective lens  18  having an annular band  18   a  for adjusting the aperture of the lens and a diffraction element  17 E having a first diffracting section  35  that operates as first controlling diffracting section and a second diffracting section  36  that operates as second controlling diffracting section formed on the opposite&#39;surfaces thereof, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums  100  of the three different types including optical discs  100   a ,  100   b ,  100   c  of the first through third type by means of a single objective lens  18 .  
      When the objective lens  18 A is used and having a first diffracting section  35  that operates as first controlling diffracting section and a second diffracting section  36  that operates as second controlling diffracting section are formed on the opposite surfaces of the diffraction element  17 E, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums  100  of the three different types including optical discs  100   a ,  100   b ,  100   d  of the first, second and fourth types by means of a single objective lens  18 A.  
      In this way, a laser beam of any of the three different wavelengths is focused on the recording surface of the optical disc of the corresponding type that is mounted on the disc table 3 by means of the single objective lens of the optical pickup  6 E according to the present invention.  
      As either the first diffracting section  35  or the second diffracting section  36  is arranged to operate as controlling diffracting section and the laser beam of one of the first through third wavelengths or the laser beams of two of the first through third wavelengths are substantially transmitted through the controlling diffracting section in the optical pickup  6 E according to the invention, it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums of the three different types, which correspond respectively to laser beams of three different wavelengths, by means of the single objective lens and prevent degradation of characteristics due to stray light from taking place by preventing stray light.  
      Additionally, as information signals can be recorded on and reproduced from disc-shaped recording mediums of the three different types, which correspond respectively to laser beams of different wavelengths, by means of the single objective lens of the optical pickup  6 E according to the invention, it is possible to reduce the number of components and hence both the dimensions of the optical pickup and the manufacturing cost. Furthermore, since the optical pickup  6 E according to the invention is adapted to use a single objective lens, the weight of the movable part of objective lens drive unit is reduced to secure an excellent responsiveness of the movable part in the focusing control operation and the tracking control operation.  
      Thus, as described above in detail, a disc drive apparatus  1  according to the invention can record information signals to and reproduce information signals from disc-shaped recording mediums  100  of three different types to be used with laser beams of different wavelengths by means of a single objective lens  18 ,  18 A or  18 B so that it can be produced with a reduced number of components to make it possible to reduce both the dimensions of the disc drive apparatus  1  and the manufacturing cost.  
      Furthermore, since a disc drive apparatus  1  according to the invention is adapted to use a single objective lens  18 ,  18 A or  18 B, the weight of the movable part  8   a  of objective lens drive unit  8  is reduced to secure an excellent responsiveness of the movable part  8   a  in the focusing control operation and the tracking control operation.  
      Still additionally, a disc drive apparatus  1  according to the invention includes a disc table for receiving a disc-shaped recording medium of one of different types and driving it to rotate and an optical pickup  6 ,  6 A,  6 B,  6 C,  6 D or  6 E for recording information to or reproducing information from the disc-shaped recording medium received on the disc table and, in the optical pickup, either the first or second diffracting section is adapted to substantially transmit one or two of the laser beams of the first through third wavelengths so that it is possible to record information signals to and reproduce information signals from disc-shaped recording mediums of three different types to be used with laser beams of different wavelengths by means of a single objective lens.  
      Still additionally, a disc drive apparatus  1  according to the invention can record information signals to and reproduce information signals from disc-shaped recording mediums of three different types to be used with laser beams of different wavelengths by means of a single objective lens so that it can be produced with a reduced number of components to make it possible to reduce both the dimensions of the disc drive apparatus and the manufacturing cost. Furthermore, since a disc drive apparatus  1  according to the invention is adapted to use a single objective lens, the weight of the movable part of objective lens drive unit is reduced to secure an excellent responsiveness of the movable part in the focusing control operation and the tracking control operation.  
      Still additionally, a disc drive apparatus  1  according to the invention includes an annular band formed on the objective lens to adjust the aperture of the objective lens and diffracting sections formed respectively on the surface of incidence and the light emitting surface of the diffraction element, one of the diffracting sections of the diffraction element being adapted to operate as controlling diffracting section so that it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums of the three different types including optical discs  100   a  of the first type, optical discs  100   b  of the second type, optical discs  100   c  of the third type by means of the single objective lens.  
      Alternatively, a disc drive apparatus  1  according to the invention includes diffracting sections formed respectively on one of the opposite surfaces of the objective lens and on one of the opposite surfaces of the diffraction element and the diffracting section of the diffraction element is made to operate as controlling diffracting section so that it is possible to properly record information signals on and reproduce information signals from disc-shaped recording mediums of the three different types including optical discs  100   a  of the first type, optical discs  100   b  of the second type, optical discs  100   d  of the fourth type by means of the single objective lens.  
      Still additionally, in a disc drive apparatus  1  according to the invention, the controlling diffracting section is formed by bonding materials that perform differently for dispersion of light so that it is possible to form a diffracting section that shows a desired diffraction efficiency by selecting an optimal combination of materials to be bonded together that differ from each other in terms of dispersion characteristics.  
      Furthermore, a disc drive apparatus  1  according to the invention may include a polarization hologram element as the diffraction element that is realized by forming diffracting sections respectively on the surface of incidence and on the light emitting surface thereof in such a way that laser beams whose senses of polarization are orthogonal relative to each other can be made to enter the polarization hologram element and the diffracting section of the polarization hologram element is made to operate as the controlling diffracting section so that it is possible to record information signals on and reproduce information signals from disc-shaped recording mediums of the three different types by means of the single objective lens with a simple configuration.  
      While the present invention is described above by way of the best modes of carrying out the invention in terms of the profile and the structure of each component, the present invention is by no means limited thereto and the above described embodiments do not limit the technological scope of the present invention.  
      It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.