Patent Publication Number: US-2006007840-A1

Title: Objective lens and optical head device provided with the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims priority to Japanese Application No. 2004-203431 filed Jul. 9, 2004, which is incorporated herein by reference.  
     FIELD OF THE INVENTION  
      The present invention relates to an objective lens for use in an optical head device for recording and reproducing data with respect to optical recording mediums such as a CD and a DVD having different thicknesses of transparent substrates, using laser light having different wavelengths. The present invention also relates to an optical head device provided with the objective lens.  
     BACKGROUND OF THE INVENTION  
      An optical head device that records and reproduces data with respect to optical recording mediums having different thicknesses of transparent substrates for protecting recording surfaces, and different recording densities, using laser light having different wavelengths has been known in the art. Examples of the optical recording medium include a CD, a DVD and the like. Here, in the CD, the thickness of the transparent substrate which protects the recording surface is 1.2 mm. In the DVD, the thickness of the transparent substrate is 0.6 mm, which is smaller than that of the CD, and a recording density is higher than that of the CD. Therefore, for example, the laser light having a wavelength of 790 nm is used in the recording/reproducing with respect to the CD, whereas laser light having a wavelength of 660 mm is used in the recording/reproducing with respect to the DVD.  
      In this type of optical head device, in order to achieve miniaturization and thickness reduction of the device, a certain constitution has been proposed in which laser light having a wavelength adapted to each optical recording medium is condensed on the recording surface of the optical recording medium using a single optical condensing system (see, e.g., Japanese Patent Application Laid-Open No. 2000-81566).  
      In the optical head device described in Japanese Patent Application Laid-Open No. 2000-81566, an objective lens is used in which diffraction grating is formed in a refraction surface. In this optical head device, diffracted laser light having a short wavelength is condensed on the recording surface of the DVD, and the diffracted laser light having a long wavelength is condensed on the recording surface of the CD. However, since unnecessary diffracted light is generated in the objective lens including the diffraction grating formed in the refraction surface, there is a problem of a loss of quantity of light, indicating a drop of transmittance of the laser light.  
      Therefore, a certain optical head device has been proposed in which the problem of the loss of quantity of light is solved while using the single optical condensing system (e.g., Japanese Patent Application Laid-Open No. 10-55564)  
      In the optical head device described in Japanese Patent Application Laid-Open No. 10-55564, an objective lens is used which has a middle region centering on an optical axis and a peripheral region concentrically formed on an outer peripheral side of the middle region. In the refraction surface of the objective lens, the middle region and the peripheral region are formed into one aspherical shape, and there is a boundary between the middle region and the peripheral region in a position whose numerical aperture substantially agrees with a numerical aperture NA 1  required in the recording/reproducing with respect to the CD.  
      In the optical head device described in Japanese Patent Application Laid-Open No. 10-55564, the middle region of the objective lens is formed into one aspherical shape. Therefore, for example, as shown in  FIG. 8A , a spherical aberration with respect to the DVD has a distribution in which the spherical aberration rapidly and discontinuously changes from a so-called insufficiently corrected state (under) to an excessively corrected state (over) in an outer peripheral portion (boundary between the middle region and the peripheral region, NA 1 ) of the middle region. Alternatively, in the distribution, as shown in  FIG. 8B , excessive correction (over) is seen in the vicinity of a middle between the optical axis and the outer peripheral portion of the middle region in a direction crossing the optical axis at right angles. It is to be noted that NA 2  in  FIG. 8  denotes a numerical aperture required in the recording/reproducing with respect to the DVD.  
      For example, since the recording density of the DVD is higher than that of the CD as described above, a beam spot having a small diameter needs to be formed on the recording surface of the DVD. An effective value of the spherical aberration needs to be reduced in order to form the beam spot having the small diameter. For example, in the optical head device described in Japanese Patent Application Laid-Open No. 10-55564, it is preferable to use the objective lens having the spherical aberration distribution shown in  FIG. 8B  rather than the spherical aberration distribution shown in  FIG. 8A . It is to be noted that as far as  FIG. 8  is referred to, it seems that the effective value of the spherical aberration can be reduced in the spherical aberration distribution shown in  FIG. 8A . However, in actuality,  FIG. 8A  is different from  FIG. 8B  in display scale, and the spherical aberration shown in  FIG. 8A  has very large values as compared with that shown in  FIG. 8B .  
      However, in the spherical aberration distribution shown in  FIG. 8B , there exists a region where the spherical aberration increases in a plus (over) direction in the vicinity of the middle between the optical axis and the outer peripheral portion of the middle region in the direction crossing the optical axis at right angles, and the effective value of the spherical aberration cannot be sufficiently reduced. This is an obstruction in forming the beam spot having a small diameter on the recording surface of the DVD.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide an objective lens by which an effective value of spherical aberration with respect to an optical recording medium is reduced, so that a beam spot having a small diameter can be formed on the recording surface of the optical recording medium. Another object is to provide an optical head device provided with the objective lens.  
      To solve the above-described problem, according to the present invention, there is provided an objective lens for an optical head device which condenses two or more laser beams having wavelengths adapted to two or more optical recording mediums having different thicknesses of transparent substrates on recording surfaces of the optical recording mediums via one optical condensing system to record information on the recording surfaces and/or reproduce the information on the recording surfaces, the objective lens comprising a refraction surface including: a middle region which is formed in a middle portion of the refraction surface centering on an optical axis of the objective lens and which is used with respect to all of the optical recording mediums; and a peripheral region which is concentrically formed adjacent to an outer peripheral side of the middle region, wherein assuming that a distance from the optical axis to an outer peripheral portion of the middle region in a direction crossing the optical axis at right angles is R, there exist, in a range of R/3 to 2R/3 from the optical axis, a zero region where spherical aberration with respect to at least one of the optical recording mediums is zero and/or an increase/decrease region where the spherical aberration with respect to at least one of the optical recording mediums increases or decreases toward zero.  
      In the present invention, in the range of R/3 to 2R/3 from the optical axis, in which the spherical aberration has heretofore increased, there exist the zero region where the spherical aberration with respect to at least one optical recording medium is zero and/or the increase/decrease region where the spherical aberration with respect to at least one of the optical recording mediums increases or decreases toward zero. Therefore, it is possible to sufficiently reduce the spherical aberration with respect to at least one optical recording medium. Since the effective value of the spherical aberration can be sufficiently reduced, it is also possible to reduce a wave front aberration with respect to at least one optical recording medium.  
      Here, in the present specification, the zero region means a region where the spherical aberration with respect to one optical recording medium turns from plus (over) to minus (under) or from minus to plus. The increase/decrease region is a region, in which the spherical aberration with respect to one optical recording medium does not turn to zero, but increases or decreases toward zero and which includes a minimum value or a maximum value.  
      In the present invention, the middle region comprises a plurality of annular refraction surfaces having aspherical shapes which are mutually different in refractive force, and stepped portions are preferably formed toward an optical axis direction in boundaries among the plurality of annular refraction surfaces. By this constitution, the zero region or the increase/decrease region can exist by a simple constitution in the vicinity of a middle between the optical axis and the outer peripheral portion of the middle region in the direction crossing the optical axis at right angles, in which the spherical aberration has heretofore increased because the middle region is formed into one aspherical shape. Therefore, the effective value of the spherical aberration can be sufficiently reduced.  
      In the present invention, for example, the optical recording mediums include a first optical recording medium on which a first laser beam having a first wavelength is condensed, and a second optical recording medium on which a second laser beam having a second wavelength shorter than the first wavelength is condensed, and the zero region and/or the increase/decrease region exist with respect to the second optical recording medium. In this case, it is possible to reduce the effective value of the spherical aberration with respect to the second optical recording medium on which a beam spot having a smaller diameter needs to be formed, and the small beam spot can be formed on the recording surface of the second optical recording medium.  
      Moreover, in the present invention, the optical recording mediums include a first optical recording medium on which a first laser beam having a first wavelength is condensed, and a second optical recording medium on which a second laser beam having a second wavelength shorter than the first wavelength is condensed, and the zero region and/or the increase/decrease region may exist with respect to the first optical recording medium. In this case, it is possible to reduce the effective value of the spherical aberration with respect to the first optical recording medium, and the small beam spot can be formed on the recording surface of the first optical recording medium. As a result, a recording/reproducing performance of the first optical recording medium can be enhanced.  
      Furthermore, according to the present invention, there is provided an objective lens for an optical head device which condenses two or more laser beams having wavelengths adapted to two or more optical recording mediums having different thicknesses of transparent substrates on recording surfaces of the optical recording mediums via one optical condensing system to record information on the recording surfaces and/or reproduce the information on the recording surfaces, the objective lens comprising a refraction surface including: a middle region which is formed in a middle portion of the refraction surface centering on an optical axis of the objective lens and which is used with respect to all of the optical recording mediums; and a peripheral region which is concentrically formed adjacent to an outer peripheral side of the middle region, wherein an optimum region where spherical aberration with respect to one optical recording medium is corrected to be optimum exists in at least one position in a range from the optical axis to an outer peripheral portion of the middle region in a direction crossing the optical axis at right angles.  
      In the present invention, there exists the optimum region where the spherical aberration with respect to one optical recording medium continues to indicate zero. In this case, a limited region, corresponding to this optimum region, sometimes exists in which any laser light is not condensed on the recording surface in the spherical aberration distribution with respect to another optical recording medium. However, when there is an extra power of the laser light with respect to the other optical recording medium, a recording/reproducing performance of the other optical recording medium is not much influenced even in the existence of the limited region with respect to the other optical recording medium, and the recording/reproducing performance of one optical recording medium can be effectively enhanced. That is, since any light is not condensed in the limited region, use efficiency (transmittance) of the light drops with respect to the other optical recording medium. However, when there is an extra power of the laser light, the recording/reproducing performance is not much influenced even with the drop of the use efficiency of the light to a certain degree. Here, the one optical recording medium is, for example, a CD, a DVD, or a blue ray disc (BD).  
      In the present invention, assuming that a distance from the optical axis to the outer peripheral portion of the middle region in the direction crossing the optical axis at right angles is R, the optimum region preferably exists in a range of R/3 to 2R/3 from the optical axis. In this case, even when the use efficiency (transmittance) of the light with respect to the other optical recording medium is sacrificed to a certain degree, the effective value of the spherical aberration with respect to the one optical recording medium is further reduced, and the beam spot having a smaller diameter can be formed by the existence of the optimum region with respect to the one optical recording medium in the vicinity of the middle between the optical axis and the outer peripheral portion of the middle region in a direction crossing the optical axis at right angles, in which the spherical aberration has heretofore increased. As a result, the recording/reproducing performance of the one optical recording medium can be more effectively enhanced. It is possible to form the beam spot having the smaller diameter, via which the light passed through the region having the large spherical aberration is not condensed even with respect to the other optical recording mediums.  
      The objective lens of the present invention can be used in an optical head device comprising: an optical condensing system having the objective lens; and a laser light source which emits the laser beam, wherein information is recorded on the recording surface and/or information on the recording surface is reproduced.  
      When the objective lens of the present invention is used as described above, there exist a zero region where spherical aberration with respect to at least one optical recording medium is zero and/or an increase/decrease region where spherical aberration with respect to at least one optical recording medium increases or decreases toward zero in a range of R/3 to 2R/3 from an optical axis, in which the spherical aberration has heretofore increased. Therefore, an effective value of the spherical aberration with respect to at least one optical recording medium can be sufficiently reduced, and a wave front aberration can also be reduced. Therefore, it is possible to form a beam spot having a smaller diameter on the recording surface of at least one optical recording medium. As a result, a recording/reproducing performance can be enhanced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram showing a constitution of an optical head device according to an embodiment of the present invention;  
       FIG. 2  is a sectional view schematically showing an objective lens of the optical head device shown in  FIG. 1 ;  
       FIG. 3  is a graph showing one example of a spherical aberration distribution at a time when the objective lens shown in  FIG. 2  is used, both  FIG. 3A  and  FIG. 3B  are graphs showing the spherical aberration distribution with respect to a second optical recording medium in the embodiment, and  FIG. 3C  is a graph showing the spherical aberration distribution with respect to a second optical recording medium in a comparative mode;  
       FIG. 4  is a graph showing another example of the spherical aberration distribution at the time when the objective lens shown in  FIG. 2  is used, and  FIG. 4A  and  FIG. 4B  are graphs showing the spherical aberration distributions with respect to a first optical recording medium in the embodiment and that in the comparative mode, respectively;  
       FIG. 5  is a graph showing another example of the spherical aberration distribution at the time when the objective lens shown in  FIG. 2  is used, and  FIG. 5A  and  FIG. 5B  are graphs showing the spherical aberration distributions with respect to the second optical recording medium in the embodiment and the first optical recording medium in the embodiment, respectively;  
       FIG. 6  is a graph showing another example of the spherical aberration distribution at the time when the objective lens shown in  FIG. 2  is used, and  FIG. 6A  and  FIG. 6B  are graphs showing the spherical aberration distributions with respect to the second optical recording medium in the embodiment and the first optical recording medium in the embodiment, respectively;  
       FIG. 7  shows the spherical aberration distribution at the time when the objective lens of the embodiment is used, and  FIG. 7A  and  FIG. 7B  are graphs showing the spherical aberration distributions with respect to a DVD and a CD, respectively; and  
       FIGS. 8A and 8B  are graphs showing a spherical aberration distribution at a time when an objective lens of a conventional technique is used.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      An embodiment of the present invention will be described hereinafter with reference to the drawings.  
      Schematic Constitution of Optical Head Device  
       FIG. 1  is a schematic diagram showing a constitution of an optical head device according to an embodiment of the present invention.  
      In  FIG. 1 , an optical head device  1  condenses a plurality of laser beams having wavelengths adapted to a plurality of types of optical recording mediums  4  having different thicknesses of transparent substrates or recording densities, such as a CD and a DVD, on recording surfaces of the optical recording mediums  4  via one optical condensing system Lo to reproduce or record information with respect to the optical recording mediums  4 . The optical recording mediums  4  in the present embodiment include a CD  41  which is a first optical recording medium, and a DVD  42  which is a second optical recording medium. The optical head device  1  includes: a first laser light source  11  for emitting a first laser beam L 1 , for example, having a first wavelength of 790 nm for use in reproducing the information from the CD  41 ; and a second laser light source  12  for emitting a second laser beam L 2 , for example, having a second wavelength of 660 nm for use in reproducing the information from the DVD  42 . The respective laser beams L 1 , L 2  are guided into the optical recording mediums  4  via the common optical condensing system Lo, and return beams of the respective laser beams L 1 , L 2  reflected by the optical recording mediums  4  are guided into a common light receiving element  25 .  
      The optical condensing system Lo includes: a first beam splitter  21  which allows the first laser beam L 1  to propagate rectilinearly and which reflects the second laser beam L 2  to align both of the beams with a system optical axis L (optical axis of an objective lens, hereinafter referred to as the optical axis L); a second beam splitter  22  which passes the laser beams L 1 , L 2  traveling along the optical axis L; a collimating lens  23  for converting the laser beams L 1 , L 2  passed through the second beam splitter  22  into parallel beams; and an objective lens  3  for forming beam spots of the laser beams L 1 , L 2  emitted from the collimating lens  23  on recording surfaces of the optical recording mediums  4 . In the present embodiment, as to a recording surface  42   a  of the DVD  42  which is one of the optical recording mediums  4 , and a recording surface  41   a  of the CD  41 , the beam spot of the first laser beam L 1  is formed on the recording surface  41   a  of the CD  41 , and the beam spot of the second laser beam L 2  is formed on the recording surface  42   a  of the DVD  42  by the objective lens  3 .  
      Moreover, the optical condensing system Lo includes a grating  24  for guiding into the common light receiving element  25  the return beams of the first and second laser beams L 1 , L 2  reflected by the optical recording mediums  4  and then the second beam splitter  22 .  
      To record/reproduce the information with respect to the CD  41  which is the optical recording medium  4  in the optical head device  1  constituted in this manner, the first laser light source  11  emits the first laser beam L 1  having a wavelength of 790 nm. The first laser beam L 1  is guided into the optical condensing system Lo to form a beam spot B ( 41 ) on the recording surface  41   a  of the CD  41  via the objective lens  3 . The return beam of the first laser beam L 1  reflected by the recording surface  41 a of the CD  41  is condensed onto the common light receiving element  25  via the second beam splitter  22 . The information of the CD  41  is reproduced by a signal detected by the common light receiving element  25 .  
      On the other hand, to reproduce the information of the DVD  42  which is the optical recording medium  4 , the second laser light source  12  emits the second laser beam L 2  having a wavelength of 660 nm. The second laser beam L 2  is also guided into the optical condensing system Lo to form a beam spot B ( 42 ) on the recording surface  42   a  of the DVD  42  via the objective lens  3 . The return beam of the second laser beam L 2  reflected by the recording surface  42   a  of the DVD  42  is condensed onto the common light receiving element  25  via the second beam splitter  22 . The information of the DVD  42  is reproduced by a signal detected by the common light receiving element  25 .  
      Constitution of Objective Lens  
       FIG. 2  is a sectional view schematically showing an objective lens of the optical head device shown in  FIG. 1 .  FIG. 3  is a graph showing one example of a spherical aberration distribution at a time when the objective lens shown in  FIG. 2  is used. Both  FIG. 3A  and  FIG. 3B  are graphs showing the spherical aberration distribution with respect to the second optical recording medium in the embodiment, and  FIG. 3C  is a graph showing the spherical aberration distribution with respect to the second optical recording medium in a comparative mode.  FIG. 4  is a graph showing another example of the spherical aberration distribution at the time when the objective lens shown in  FIG. 2  is used.  FIG. 4A  and  FIG. 4B  are graphs showing the spherical aberration distributions with respect to the first optical recording medium in the embodiment and that in the comparative mode, respectively.  FIG. 5  is a graph showing another example of the spherical aberration distribution at the time when the objective lens shown in  FIG. 2  is used.  FIG. 5A  and  FIG. 5B  are graphs showing the spherical aberration distributions with respect to the second optical recording medium in the embodiment and the first optical recording medium in the embodiment, respectively.  FIG. 6  is a graph showing another example of the spherical aberration distribution at the time when the objective lens shown in  FIG. 2  is used.  FIG. 6A  and  FIG. 6B  are graphs showing the spherical aberration distributions with respect to the second optical recording medium in the embodiment and the first optical recording medium in the embodiment, respectively.  
      As shown in  FIG. 2 , the objective lens  3  is a convex lens whose opposite surfaces are formed into convex shapes. On the objective lens  3 , a refraction surface  30  is formed, and the refraction surface includes: an emission surface  31  formed into a single aspherical shape on an optical recording medium  4  side; and an incidence surface  32  formed on the side of the laser light sources  11 ,  12 . The incidence surface  32  includes: a middle region  33  formed in a middle portion of the incidence surface  32  centering on the optical axis L; and a peripheral region  34  concentrically formed adjacent to an outer peripheral side of the middle region  33 . The middle region  33  is used with respect to the CD  41  and the DVD  42 , and the peripheral region  34  is used only with respect to the DVD  42 .  
      The middle region  33  has a plurality of annular refraction surfaces  33   a  (hereinafter referred to as the annular refraction surfaces  33   a ) which are mutually different in refractive force and which have aspherical shapes. Moreover, stepped portions  33   b  (hereinafter referred to as the stepped portions  33   b ) are formed toward an optical axis L direction in boundaries among the plurality of annular refraction surfaces  33   a.  A numerical aperture of an outer peripheral portion  33   c  of the middle region  33  substantially agrees with a numerical aperture NA 1  required in recording/reproducing the information with respect to the CD  41 . It is to be noted that in  FIG. 2 , only some of the annular refraction surfaces and the stepped portions are denoted with reference numerals.  
      In the present configuration, the peripheral region  34  is formed into one aspherical shape, and this aspherical shape is such an aspherical surface that corrects the spherical aberration with respect to the DVD  42  to be optimum. It is to be noted that in the present embodiment, as shown in  FIG. 2 , the numerical aperture of the outer peripheral portion of the peripheral region  34  substantially agrees with a numerical aperture NA 2  required in recording/reproducing the information with respect to the DVD  42 .  
      Radial-direction positions (positions in a direction crossing the optical axis L at right angles) and heights of the stepped portions  33   b  formed in the middle region  33  are set, and the aspherical shapes of the annular refraction surfaces  33   a  are also set in such a manner as to satisfy predetermined conditions. Specifically, assuming that a distance from the optical axis L to the outer peripheral portion  33   c  of the middle region  33  in the direction crossing the optical axis L at right angles is R, there exist, in a range of R/3 to 2R/3 from the optical axis L, a zero region where spherical aberration with respect to at least one of the CD  41  and the DVD  42  is zero and/or an increase/decrease region where the spherical aberration with respect to at least one of the CD  41  and the DVD  42  increases or decreases toward zero. Alternatively, the radial direction positions and the heights of the stepped portions  33   b,  and the aspherical shapes of the annular refraction surfaces  33   a  are set in such a manner that there exists in at least one position an optimum region where the spherical aberration with respect to either of the CD  41  and the DVD  42  is corrected to be optimum.  
      For example, as shown in  FIG. 3A , the radial direction positions and the heights of the stepped portions  33   b,  and the aspherical shapes of the annular refraction surfaces  33   a  (hereinafter referred to as “the shapes of the stepped portions  33   b  and the like”) are set in such a manner that zero regions a 1  and a 2  exist where the spherical aberration with respect to the DVD  42  is zero in the range of R/3 to 2R/3 from the optical axis L. In this case, the zero region a 1  is a region where the spherical aberration with respect to the DVD  42  turns from plus (over) to minus (under), and the zero region a 2  is a region where the spherical aberration with respect to the DVD  42  turns from minus to plus.  
      Alternatively, as shown in  FIG. 3B , the shapes of the stepped portions  33   b  and the like are set in such a manner that there exists in the range of R/3 to 2R/3 from the optical axis L an increase/decrease region a 3  where the spherical aberration with respect to the DVD  42  increases or decreases toward zero. In this case, in the increase/decrease region a 3 , the spherical aberration does not reach zero, but increases or decreases toward zero, and the region includes a minimum value m 1 . It is to be noted that the shapes of the stepped portions  33   b  and the like may be set in such a manner that both of the zero region and the increase/decrease region exist in the range of R/3 to 2R/3 from the optical axis L.  
      Thus, the shapes of the stepped portions  33   b  and the like are set in such a manner that there exist in the range of R/3 to 2R/3 from the optical axis L the zero region where the spherical aberration with respect to the DVD  42  turns to zero and/or the increase/decrease region where the spherical aberration with respect to the DVD  42  increases or decreases toward zero. In this case, it is seen that an effective value of the spherical aberration with respect to the DVD  42  is reduced as compared with the use of an objective lens in a comparative mode having a region where, as shown in  FIG. 3C , the spherical aberration increases in the vicinity of a middle between the optical axis L and the outer peripheral portion  33   c  in the same manner as in a conventional mode.  
      Moreover, for example, as shown in  FIG. 4A , the shapes of the stepped portions  33   b  and the like are set in such a manner that there exist zero regions a 4  and a 5  where the spherical aberration with respect to the CD  41  turns to zero in the range of R/3 to 2R/3 from the optical axis L. It is to be noted that as described above, the shapes of the stepped portions  33   b  and the like may be set in such a manner that the increase/decrease region exists instead of the zero region, or both of the zero region and the increase/decrease region exist in the range of R/3 to 2R/3 from the optical axis L.  
      Thus, the shapes of the stepped portions  33   b  and the like are set in such a manner that there exist in the range of R/3 to 2R/3 from the optical axis L the zero region where the spherical aberration with respect to the CD  41  turns to zero and/or the increase/decrease region where the spherical aberration with respect to the CD  41  increases or decreases toward zero. In this case, it is seen that an effective value of the spherical aberration with respect to the CD  41  is reduced as compared with the use of an objective lens in a comparative mode in which a spherical aberration distribution is generated in the same manner as in a conventional mode as shown in  FIG. 4B .  
      Alternatively, the shapes of the stepped portions  33   b  and the like are set in such a manner that there exists an optimum region a 6  where the spherical aberration with respect to the DVD  42  is corrected to be optimum as shown in  FIG. 5A . Specifically, the shapes of the stepped portions  33   b  and the like are set in such a manner that the optimum region a 6  exists in the range of R/3 to 2R/3 from the optical axis L. Here, in the optimum region a 6 , the spherical aberration with respect to the DVD  42  continues to indicate 0. In  FIG. 5A , the shapes of the stepped portions  33   b  and the like are set in such a manner that the optimum region a 6  exists in one position, but the shapes of the stepped portions  33   b  and the like may be set in such a manner that a plurality of optimum regions exist or the optimum region exists together with the above-described zero region or the increase/decrease region.  
      It is to be noted that in this case, as shown in  FIG. 5B , there appears a limited region a 61 , corresponding to the optimum region a 6 , where the first laser beam L 1  is not condensed on the recording surface  41  a of the CD  41  on the spherical aberration distribution with respect to the CD  41 .  
      Moreover, the shapes of the stepped portions  33   b  and the like are set in such a manner that there exists an optimum region a 7  where the spherical aberration with respect to the CD  41  is corrected to be optimum as shown in  FIG. 6B . Specifically, the shapes of the stepped portions  33   b  and the like are set in such a manner that the optimum region a 7  exists in the range of R/3 to 2R/3 from the optical axis L. Also in this case, as described above, the shapes of the stepped portions  33   b  and the like may be set in such a manner that a plurality of optimum regions exist or the optimum region exists together with the above-described zero region or the increase/decrease region.  
      It is to be noted that in this case, as shown in  FIG. 6A , there appears a limited region a 71 , corresponding to the optimum region a 7 , where the second laser beam L 2  is not condensed on the recording surface  42   a  of the DVD  42  on the spherical aberration distribution with respect to the DVD  42 .  
      The above-described middle region  33  of the incidence surface  32  is designed, for example, in the following procedure. First, the radial-direction positions and the heights of the stepped portions  33   b  and the aspherical shapes of the annular refraction surfaces  33   a  are set in such a manner that the zero region and/or the increase/decrease region exist with respect to at least one of the CD  41  and the DVD  42  in the range of R/3 to 2R/3 from the optical axis L. Thereafter, as to portions other than portions corresponding to the zero region and the increase/decrease region, the radial-direction positions and the heights of the stepped portions  33   b  and the aspherical shapes of the annular refraction surfaces  33   a  are set in such a manner as to correct the spherical aberrations with respect to both of the CD  41  and the DVD  42  with a good balance.  
      Alternatively, the radial-direction positions and the heights of the stepped portions  33   b  and the aspherical shapes of the annular refraction surfaces  33   a  are set in such a manner that the optimum region exists with respect to the CD  41  or the DVD  42  in a range from the optical axis L to the outer peripheral portion  33   c  of the middle region  33 . Thereafter, as to a portion other than a portion corresponding to the optimum region, the radial-direction positions and the heights of the stepped portions  33   b  and the aspherical shapes of the annular refraction surfaces  33   a  are set in such a manner as to correct the spherical aberrations with respect to both of the CD  41  and the DVD  42  with the good balance.  
      Main Effect of the Present Embodiment  
      As described above, when the objective lens  3  of the present embodiment is used, there exist the zero regions a 1  and a 2  where the spherical aberration with respect to the DVD  42  is zero and/or the increase/decrease region a 3  where the spherical aberration with respect to the DVD  42  increases or decreases toward zero in the range of R/3 to 2R/3 from the optical axis L in which the spherical aberration has heretofore increased. Therefore, as apparent from  FIG. 3 , the effective value of the spherical aberration with respect to the DVD  42  can be sufficiently reduced. Since the effective value of the spherical aberration can be sufficiently reduced, a wave front aberration with respect to the DVD  42  can also be reduced. That is, a small beam spot can be formed on the recording surface  42   a  of the DVD  42  on which the beam spot having a smaller diameter needs to be formed, and a recording/reproducing performance of the DVD  42  can be enhanced.  
      Moreover, when the objective lens  3  of the present embodiment is used, there exist the zero regions a 4  and a 5  where the spherical aberration with respect to the CD  41  turns to zero and/or the increase/decrease region in the range of R/3 to 2R/3 from the optical axis L. Therefore, for example, as apparent from  FIG. 4 , the effective value of the spherical aberration with respect to the CD  41  can be sufficiently reduced. As a result, the small beam can be formed on the recording surface  41   a  of the CD  41 , and the recording/reproducing performance of the CD  41  can be enhanced.  
      In the present embodiment, the middle region  33  comprises a plurality of annular refraction surfaces  33   a  having aspherical shapes having mutually different refractive forces, and the stepped portions are formed toward the optical axis L direction in the boundaries among the plurality of annular refraction surfaces  33   a.  Moreover, the radial-direction positions and the heights of the stepped portions  33   b  and the aspherical shapes of the annular refraction surfaces  33   a  are set in such a manner as to satisfy the preferable conditions. Therefore, with a simple constitution, the zero region or the increase/decrease region can exist in the range of R/3 to 2R/3 from the optical axis L in which the spherical aberration has heretofore increased. As a result, it is possible to sufficiently reduce the effective value of the spherical aberration of the CD  41  or the DVD  42 .  
      Moreover, in the present embodiment, the optimum region a 6  or a 7  exists where the spherical aberration with respect to the CD  41  or the DVD  42  is corrected to be optimum. See  FIG. 5A  and  FIG. 6B . In this case, on the distribution of the spherical aberration with respect to the CD  41  or the DVD  42 , there appears the limited region a 61  or a 71  (see  FIG. 5B  and FIG.  6 A), corresponding to the optimum region a 6  or a 7 , where the laser beam L 1  or L 2  is not condensed on the recording surface  41   a  or  42   a  of the CD  41  or the DVD  42 . However, in a case where there is an extra power in the laser light with respect to the optical recording medium  4  where the limited region a 61  or a 71  appears, even when the optimum region exists with respect to one optical recording medium  4 , the recording/reproducing performance of the other optical recording medium  4  can be enhanced, while the recording/reproducing performance of the one optical recording medium  4  can be effectively enhanced.  
      In the present embodiment, the optimum region a 6  or a 7  exists in the range of R/3 to 2R/3 from the optical axis L. That is, the optimum region a 6 , a 7  exists in this range in which the spherical aberration has heretofore increased. Therefore, the effective value of the spherical aberration with respect to one optical recording medium  4  can be further reduced, and the beam spot having a smaller diameter can be formed.  
      Furthermore, in the present embodiment, since the optical head device  1  comprises the optical condensing system Lo having the objective lens  3 , the recording/reproducing performance can be enhanced with respect to at least one of the CD  41  and the DVD  42 .  
      Another Embodiment  
      The above-described embodiment is an example of a preferable embodiment of the present invention, but the present invention is not limited to this embodiment, and can be variously modified within the scope of the present invention.  
      For example, shapes of stepped portions  33   b  and the like may be set in such a manner that zero regions and/or increase/decrease regions exist with respect to a CD  41  and a DVD  42  in a range of R/3 to 2R/3 from an optical axis L. In this case, recording/reproducing performances of both of the CD  41  and the DVD  42  can be enhanced.  
      It is to be noted that  FIGS. 3 and 4  are conceptual, and a distribution of spherical aberration is not limited to that described with reference to  FIGS. 3 and 4 . For example, plus and minus of the spherical aberration may be reversed in the spherical aberration distribution.  FIGS. 5 and 6  are similarly conceptual, and the spherical aberration distribution is not limited to that described with reference to  FIGS. 5 and 6 .  
      Moreover, the optical recording mediums  4  are not limited to the CD  41  and the DVD  42 , and an objective lens  3  may be used with respect to a BD. That is, the objective lens of the present invention is applicable to an optical head device which serves as both of the CD and the BD, or an optical head device which serves as both of the DVD and the BD.  
      Furthermore, the objective lens of the present invention is not limited to the optical head device using two types of optical recording mediums having different thicknesses of transparent substrates, and the lens is also applicable to an optical head device using three or more types of optical recording mediums having different thicknesses of transparent substrates. In this case, in the objective lens, another peripheral region is concentrically formed on an outer peripheral side of a peripheral region in the above-described embodiment. The constitution of the present embodiment is applicable not only to the objective lens but also to a lens for another optical head device, such as a collimator lens.  
     EXAMPLE  
      An example of an objective lens  3  will be described, to which the present invention is applied. Optical recording mediums in this example are a CD and a DVD.  
      Lens Design Data  
      Wavelengths λ 1 , λ 2 , numerical apertures NA 1 , NA 2 , and lens refractive indexes n 1 , n 2  of the CD and the DVD, which are assumptions of lens design, are as follows. 
          CD     λ 1 =790 nm     NA 1 =0.47     n 1 =1.537     DVD     k 2 =660 nm     NA 2 =0.6     n 2 =1.540        

      Lens design data of the example will be described hereinafter. In the following data, a surface interval is an interval between an incidence surface and an emission surface in an optical axis. A stepped portion of the incidence surface corresponds to a distance from an intersection between the incidence surface and the optical axis to an inner peripheral end of each annular refraction surface. Furthermore, an aspherical shape Z(r) of each annular refraction surface is rotationally symmetric, and is represented by the following equation with respect to a radius coordinate r:  
           Z   ⁡     (   r   )       =         cr   2     /     [     1   +       {     1   -       (     1   +   k     )     ⁢     c   2     ⁢     r   2         }       1   /   2         ]       +     
     ⁢           ⁢       A   2     ·     r   2       +       A   4     ·     r   4       +       A   6     ·     r   6       +   …       ⁢           ,       
 
      where c denotes an inverse number of a radius curvature R, k denotes a conical constant, and A 2   
      , A 4 , A 6 , . . . are secondary, quartic, sextic . . . aspherical coefficients. It is to be noted that in indications of the aspherical coefficients or the like, E and the subsequent number n means 1/10 n . The data of the respective annular refraction surfaces are described in order from an innermost periphery toward an outer periphery. 
          Surface interval 1.75000     Incidence surface     Annular region=0 to 0.40000     Stepped portion=0.00000     R=1.94109     k=0.000000E+00     A 4 =−0.893898E−02     Annular region=0.4 to 0.60000     Stepped portion=0.00864     R=1.94128     k=0.000000E+00     A 4 =−0.676508E−02     Annular region=0.6 to 0.80000     Stepped portion=0.02474     R=1.93794     k=0.000000E+00     A 4 =−0.752818E−02     Annular region=0.8 to 0.95000     Stepped portion=0.02786     R=1.81480     k=0.452784E−01     A 4 =−0.264601E−01     Annular region=0.95 to 1.35000     Stepped portion=0.01464     R=1.99361     k=0.413001E−01     A 4 =0.238894E−02     A 6 =−0.504509E−02     Annular region=1.35 to 1.43900     Stepped portion=0.04010     R=2.29433     k=0.479248E−03     A 4 =0.191457E−01     A 6 =−0.358908E−02     Annular region=1.439 to 1.83000     Stepped portion=0.03016     R=2.00971     k=−0.650893E+00     A 4 =0.109546E−01     A 6 =−0.575933E−03     Emission surface     R=−7.46182     k=0.23703E+01     A 4 =0.255157E−01     A 6 =−0.627395E−02     A 8 =0.665314E−03 
 
 Spherical Aberration Distribution 
       

       FIG. 7  shows a spherical aberration distribution at a time when the objective lens of the example is used.  FIG. 7A  and  FIG. 7B  are graphs showing the spherical aberration distributions with respect to a DVD and a CD, respectively  
      As shown in  FIG. 7A , when the objective lens of the example is used, zero regions a 1  and a 12  exist where spherical aberration with respect to the DVD is zero in a range of R/3 to 2R/3 from an optical axis L (i.e., NA is in a range of 0.47/3 to 2×0.47/3). An increase/decrease region a  13  including a minimum value m 2  also exists. Therefore, an effective value of the spherical aberration with respect to the DVD can be reduced to 0.01×λ 2  or less. As a result, an effective value of a wave front aberration with respect to the DVD can be reduced to 0.04×λ 2  or less.  
      Moreover, as shown in  FIG. 7B , when the objective lens of the example is used, zero regions a 14  and a 15  exist where spherical aberration with respect to the CD is zero in a range of R/3 to 2R/3 from an optical axis L. An increase/decrease region a 16  including a maximum value m 3  also exists. Therefore, an effective value of the spherical aberration with respect to the CD can be reduced to 0.01×λ 1  or less. As a result, an effective value of a wave front aberration with respect to the CD can be reduced to 0.04×λ 1  or less.  
      When the objective lens of the example is used, the effective values of the spherical aberrations and the wave front aberrations of both of the CD and the DVD can be reduced and recording/reproducing functions with respect to both of the CD and the DVD can be enhanced.  
      While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention.  
      The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.