Abstract:
Since the working distance of an objective lens for a high-density optical disk is short, in the case where an optical pick-up device in which a plurality of lenses including an objective lens for the high-density optical disk are installed in the same lens holder and an optical disk apparatus that uses the optical pick-up device are used to drive an objective lens to be used for the optical disk, there is high possibility that when one of the objective lenses is focus controlled, the other objective lens may collide with the optical disk, which presents a problem. To resolve the problem, in the optical pick-up device in which a plurality of objective lenses including the objective lens for the high-density optical disk are installed in the same lens holder and in an optical disk apparatus that uses it, relative positions of the objective lenses in the said lens holder in an optical-axis direction are set to predetermined positions.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an optical pick-up device capable of reproducing or recording a plurality of optical disks each of which has a mutually different recording density, such as DVD, CD, and a high-density optical disk that realizes a capacity several times that of the existing DVD, and to an optical disk apparatus on which the said optical pick-up device is mounted.  
           [0002]    Currently, optical disks includes a disk whose transparent substrate has a thickness of 1.2 mm, such as CD-ROM, CD-R, and CD-RW, and that is read or written by a semiconductor laser of a wavelength in a band of approximately 780 nm, and a disk whose transparent substrate has a thickness of 0.6 mm, such as DVD-ROM, DVD-R, and DVD-RW, and that is read or written by a semiconductor laser of a wavelength in a band of approximately 650 nm. Since the substrate thickness and corresponding wavelength are different in this way depending on the kind of the optical disk, in order to make the same optical pick-up device compatible with the DVD and the CD, it is necessary to install objective lenses each of which is compatible with each optical disk on the optical pick-up device. Conventionally, either an optical pick-up device on which a single lens having plural numerical apertures and plural focal lengths is mounted (for example, see a patent reference 1) or an optical pick-up device in which at least two different lenses are installed in the same lens holder thereof (for example, see a patent reference 2) was used.  
           [0003]    [Patent Reference 1] 
           [0004]    JP-A No. 331362/2000  
           [0005]    [Patent Reference 2] 
           [0006]    JP-A No. 297927/1997  
           [0007]    At present, there is being standardized a high-density optical disk that realizes a capacity several times that of the existing DVD by using a blue-violet semiconductor laser in a wavelength band of 400 nm as a recording/reproducing semiconductor laser and reducing the thickness of its transparent substrate to 0.1 mm. However, the objective lens that is compatible with the CD, the DVD, and the high-density optical disk only by itself has not been disclosed or published yet. Therefore, in order to make the optical pick-up device compatible with these optical disks, it is considered possible to mount on the optical pick-up device at least two kinds of objective lenses, that is, the DVD/CD-compatible special lens and such an objective lens for the high-density optical disk as is described in Technical Digest of Optical Data Storage Topical Meeting held in April 2001 (pp. 100-102). Further, since the optical pick-up device is required to fulfill miniaturization, simplification, and low pricing, it is preferable that the above-mentioned objective lenses are driven by the same actuator.  
           [0008]    Here, because the working distance of the objective lens for the high-density optical disk is extremely short as compared to the working distance of the objective lens for the DVD/CD, in the case where a plurality of objective lenses are driven by the single actuator, there gives rise to an important problem that when one of the objective lenses is brought into focusing or in case the one lens goes out of focus, collision of the other objective lens with the optical disk must be prevented.  
           [0009]    The patent application described in the above-mentioned JP-A No. 297927/1997 is such that a difference in the mount height between a plurality of objective lenses is set equal to a difference between both working distances in order to reduce power consumption in the actuator and nothing is considered for prevention of the disk collision.  
           [0010]    On the other hand, a technology for preventing collision between the objective lenses and the optical disk in the case where a plurality of objective lenses including the objective lens for the high-density optical disk are driven by the same actuator is disclosed in, for example, JP-A No. 67700/2001. However, since a technological idea that is disclosed there is a method of preventing direct collision between the objective lenses and the optical disk by providing a buffer part in a lens holder, it can prevent the optical disk from being damaged or hurt but cannot prevent direct collision between the buffer part and the optical disk, consequently having high possibility that the optical disk may be damaged.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention aims to provide a technology of preventing direct collision between the objective lenses and the optical disk without providing the buffer part and therefore preventing the optical disk and the objective lenses from being damaged in the case where a plurality of objective lenses including the objective lens for the high-density optical disk are driven by the same actuator.  
           [0012]    In order to resolve the problem, in one preferred aspect, this invention utilizes an optical disk apparatus comprises: a turntable on which either a first optical disk or a second optical disk is placed and held; an optical pick-up device in which a first objective lens for focusing a light beam on the first optical disk and a second objective lens for focusing a light beam on the second optical disk are installed in the same lens holder; and controlling means for controlling a position of the lens holder so that a distance between the lens holder and the lens-holder-side surface of the second optical disk when the second optical disk is being reproduced is longer than a distance between the lens holder and the lens-holder-side surface of the first optical disk when the first optical disk is being reproduced. Here, the controlling means, for example, means a control circuit.  
           [0013]    In another aspect, this invention utilizes the optical disk apparatus comprising: the turntable on which either the first optical disk or the second optical disk is placed and held; the optical pick-up device in which the first objective lens for focusing a light beam on the first optical disk and the second optical disk for focusing a light beam on the second optical disk are installed in the same lens holder; controlling means for controlling a position of the lens holder so that a distance between the lens holder and the lens-holder-side surface of the second optical disk when the second optical disk is being reproduced is longer than a distance between the lens holder and the lens-holder-side surface of the first optical disk when the first optical disk is being reproduced, characterized in that denoting the working distance of the first objective lens installed at the near side from the optical disk as WD s , the working distance of the second objective lens installed at the far side from the optical disk as WD 1 , and a maximum value of the deviation of recording layer positions of the second optical disk in an optical-axis direction that is generated by rotation of the second optical disk as δD 1 , and further denoting a difference between a distance from the lens holder to the lens-holder-side surface of the second optical disk when the light beam passes through the second objective lens and is in focus on the second optical disk and a distance from the lens holder to the lens-holder-side surface of the first optical disk when the light beam passes through the first objective lens and is in focus on the first optical disk as α, the α satisfies α&gt;δD 1 −WD s . Here, denoting the working distance of the first objective lens as A, the working distance of the second objective lens as B, and a maximum value of the deviation of recording layer positions of the second optical disk as δB; WD s  and WD 1  means A and B when the first objective lens is placed at a nearer side from the lens-side surface of the optical disk than the second objective lens is, respectively, and δD 1  means a maximum value δB of the deviation of recording layer positions of the second optical disk.  
           [0014]    In still another aspect, this invention utilizes the optical pick-up device in which the first objective lens for focusing a light beam on the first optical disk and the second objective lens for focusing a light beam on the second optical disk are installed in the same lens holder, characterized in that denoting the working distance of the first objective lens installed at the near side from the optical disk as WD s  and the working distance of the second objective lens installed at the far side from the optical disk as WD 1 , the first objective lens and the second objective lens are installed in the lens holder so that a difference X between a distance from a vertex position of the optical-disk-side surface of the first objective lens to the optical disk in the optical-axis direction and a distance from a vertex position of the optical-disk-side surface of the second objective lens X to the optical disk satisfies 0≦X&lt;WD 1 −WD s  (where WD 1 &gt;WD s ).  
           [0015]    In yet another aspect, this invention utilizes an optical disk apparatus comprising: the optical pick-up device in which the first objective lens for focusing a light beam on the first optical disk and the second objective lens for focusing a light beam on a second optical disk are installed in the same lens holder, the turntable on which either the first optical disk or the second optical disk is placed and held; and a control circuit for switching the objective lens according to a kind of the optical disk that is placed and held on the turntable, characterized in that denoting the working distance of the first objective lens installed at the near side from an optical disk placed and held on the turntable as WD s  and the working distance of the second objective lens installed at the far side from the optical disk placed and held on the turntable as WD 1 , the first objective lens and the second objective lens are installed in the lens holder so that a difference X between a distance from a vertex position of the optical-disk-side lens surface of the first objective lens to the optical disk in the optical-axis direction and a distance from a vertex position of the optical-disk-side lens surface of the second objective lens to the optical disk satisfies 0≦X&lt;WD 1 −WD s  (where WD 1 &gt;WD s ).  
           [0016]    Note that also in the optical pick-up device described in the conventional example that is compatible with plural kinds of optical disks with the use of a single lens, the lens holder is moved vertically when a different kind of optical disk is being reproduced or recorded, but this operation is done simply in order to keep the distance between the optical disk and the lens holder so that the distance becomes different by the amount of a difference of the working distances for the respective optical disks and is not done in view of such prevention of collision between the objective lenses and the optical disk as can be achieved by the present invention.  
           [0017]    This invention can provide an optical pick-up device in which an actuator capable of preventing the collision between the objective lenses and the optical disk and causing no damages on the disk, and an optical disk apparatus that uses the optical pick-up device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWININGS  
       [0018]    [0018]FIG. 1 shows an embodiment regarding the actuator according to this invention.  
         [0019]    FIGS.  2 (A) and (B) are views each showing the working distance. FIG. 2(A) shows the working distance of the objective lens for the high-density optical disk. FIG. 2(B) shows the working distance of an objective lens for the existing DVD.  
         [0020]    FIGS.  3 (A) and (B) are views each showing positions of the objective lens for the high-density optical disk and of the objective lens for the existing DVD. FIG. 3(A) is a view for the case where an objective lens  1  is placed at the near side from the optical disk. FIG. 3(B) is a view for the case where an objective lens  2  is placed at the near side from the optical disk.  
         [0021]    FIGS.  4 (A) and (B) are views each showing the space between the optical disk and the objective lens in the presence of disk surface deflection. FIG. 4(A) shows the space between the objective lens for the high-density optical disk. FIG. 4(B) shows the space between the objective lens for the existing DVD and the high-density optical disk.  
         [0022]    FIGS.  5 (A) and (B) are views each showing a difference in the lens holder position between when the high-density optical disk is being reproduced and when the existing DVD is being reproduced. FIG. 5(A) shows the difference in the lens holder position in the case where the objective lens  1  is placed at the near side from the optical disk. FIG. 5(B) shows the difference of the lens holder positions in the case where the objective lens  2  is placed at the near side from the optical disk.  
         [0023]    [0023]FIG. 6 shows the working distance of the DVD/CD-compatible special objective lens when being used for the CD.  
         [0024]    FIGS.  7 (A) and (B) are views each showing positions of the objective lens for the high-density optical disk and of the DVD/CD-compatible special objective lens. FIG. 7(A) shows the positions in the case where the objective lens  1  is placed at the near side from the optical disk. FIG. 7(B) shows the positions in the case where the objective lens  2  is placed at the near side from the optical disk.  
         [0025]    FIGS.  8 (A) and (B) are views each showing the space between the optical disk and the objective lens in the presence of disk surface deflection. FIG. 8(A) shows the space between the objective lens for the high-density optical disk and the CD disk. FIG. 8(B) shows the space between the DVD/CD-compatible special objective lens and the high-density optical disk.  
         [0026]    [0026]FIG. 9 shows an actuator on which the objective lens for the high-density optical disk of a single-lens configuration.  
         [0027]    [0027]FIG. 10 shows an axial sliding type actuator.  
         [0028]    [0028]FIG. 11 shows a first embodiment regarding the optical pick-up device according to this invention.  
         [0029]    [0029]FIG. 12 shows an embodiment regarding the optical disk apparatus according to this invention.  
         [0030]    FIGS.  13 (A) and (B) are views each showing an embodiment regarding a lens holder position according to this invention. FIG. 13(A) shows a position of the lens holder when the high-density optical disk  110  is being reproduced or recorded. FIG. 13(B) shows a position of the lens holder when the existing DVD disk  111  is being reproduced or recorded.  
         [0031]    [0031]FIG. 14 shows an embodiment regarding movement of the lens holder in the optical-axis direction according to this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    FIGS.  13 (A) and (B) are views each showing an embodiment of the optical disk apparatus regarding the lens holder position according to this invention. In FIGS.  13 (A) and (B), the numeral  10  indicates a lens holder, and FIGS.  13 (A) and (B) are views each showing a position of the lens holder  10  when the optical disk is being reproduced or recorded.  
         [0033]    In this embodiment, the lens holder  10  is provided with the objective lens  1  and the objective lens  2  that are compatible with the optical disks each having a mutually different recording density, respectively, wherein the objective lens  1  is specified to be the objective lens for the high-density optical disk that is compatible with a disk of a transparent substrate thickness of, for example, 0.1 mm, more specifically, to be an objective lens composed of a lens  1   a  and a lens  1   b  constituting a two-lens combination just as the above-mentioned objective lens for the high-density optical disk as described above.  
         [0034]    Further, the objective lens  2  is specified to be the objective lens for the existing DVD that is compatible with a disk of a transparent substrate thickness of, for example, 0.6 mm. Note that the objective lens  2  is not limited to the objective lens exclusive for DVD that is compatible with the existing DVD disk, but it is perfectly all right that it is an objective lens that is compatible with a plurality of optical disks each having a different recording density such as the DVD/CD-compatible special objective lens that is also compatible with the CD disk. The objective lens  2  may be the DVD/CD-compatible special objective lens as described above.  
         [0035]    Note here that the lens holder  10  moves in the optical-axis direction for focal adjustment so that the light beam emitted from either the objective lens  1  or the objective lens  2  becomes in focus on the information recording surface of the optical disk that is intended to be reproduced or recorded therewith.  
         [0036]    [0036]FIG. 13(A) shows a position of the lens holder  10  when the high-density optical disk  110  is being reproduced or recorded, that is, when the light beam emitted from the objective lens  1  becomes in focus on the information recording surface of the optical disk that is intended to be reproduced or recorded therewith. FIG. 13(B) shows a position of the lens holder  10  when the existing DVD disk  111  is being reproduced or recorded, that is, when the light beam emitted from the objective lens  2  becomes in focus on the information recording surface of the optical disk that is intended to be reproduced or recorded therewith.  
         [0037]    In this embodiment, denoting a space between the objective lens  1  and the high-density optical disk  110  when the high-density optical disk  110  is being reproduced or recorded as δ1 and a space between the objective lens  1  and the existing DVD disk  111  when the existing DVD disk  111  is being reproduced or recorded as δ2, the objective lens  1  and the objective  2  are installed in the lens holder  10  so that a position of the lens holder satisfies δ1&lt;δ2.  
         [0038]    Thus, when the existing DVD disk  111  is being reproduced or recorded, the space between the objective lens  1  and the existing DVD disk  111  is opened out, which can reduce the possibility that the objective lens  1  may collide with the existing DVD disk  111  in case the objective lens  2  gets out of the control of a focusing servo or when it is brought into focusing.  
         [0039]    As shown in FIG. 14, a coil  72  is assembled in the lens holder  10 . Feeding a current through the coil  72  causes the lens holder  10  to move in the optical-axis direction by interaction with a magnetic circuit part indicated by the numeral  70 , enabling focus adjustment.  
         [0040]    Denoting a space between the optical disk and the objective lens  1  when the coil  72  is not fed with the current as δ3, the lens holder  10  is installed so that δ3, for example, satisfies δ1≦δ3 in this invention.  
         [0041]    Therefore, when the high-density optical disk  110  is being reproduced or recorded in the case of δ1&lt;δ3, it is necessary to bring a position of the lens holder  10  close to the optical disk by (δ3−δ1) as compared to a state of no current being fed through the coil  72 . Therefore, in this case, when the high-density optical disk  110  is being reproduced or recorded, it follows that a direct current (offset) equivalent to lifting up the lens holder  10  by (δ3−δ1) is passing through the coil  72 .  
         [0042]    Note that satisfying δ1≦δ3 makes the following possible: to widen a space between the optical disk and the objective lens  1  in a state of no current being fed through the coil  72 ; and to reduce the risk of collision between the optical disk and the objective lens  1  that is generated by surface deflection of the high-density optical disk  110 , the existing DVD disk  111 , or the CD disk  111  when the high-density optical disk  110 , the existing DVD disk  111 , or the CD disk  111  is rotated in a state of no current being fed through the coil  72 .  
         [0043]    Further, in the embodiment of FIG. 13, there is a difference in direct current (offset) corresponding to (δ2−δ1) between when the high-density optical disk  110  is being reproduced or recorded and when the existing DVD disk  111  is being reproduced or recorded.  
         [0044]    Further FIG. 1 shows an embodiment regarding the optical pick-up device according to this invention. In FIG. 1, the numeral  10  indicates a lens holder, the numeral  70  is a magnetic circuit part, and the numeral  72  is a coil.  
         [0045]    In this embodiment, the lens holder  10  is provided with the objective lens  1  and the objective lens  2  each of which is compatible with an optical disk having a mutually different recording density, and more specifically, the objective lens  1  is specified to be the objective lens for the high-density optical disk that is compatible with a disk of a transparent substrate thickness of 0.1 mm and is composed of the lens  1   a  and the lens  1   b  constituting a two-lens combination just as the objective lens for the high-density optical disk as described above. Further, the objective lens  2 , for example, is specified to be the objective lens for the existing DVD that is compatible with a disk of a transparent substrate thickness of 0.6 mm.  
         [0046]    Here, denoting the working distance of the objective lens  1  for a light beam used in recording or reproducing the high-density optical disk  110  (hereinafter referred to as the optical disk  110  for convenience) as A, the working distance of the objective lens  2  for a light beam used in recording or reproducing the existing DVD disk  111  (hereinafter referred to as the optical disk  111 ) as B, as shown in FIG. 2, (incidentally, the numerical aperture of the objective lens  1 , for example, is specified to approximately 0.75 to 0.90, and the numerical aperture of the objective lens  2 , for example, is specified to approximately 0.60 to 0.67) and further denoting an absolute value of a difference between a distance from a vertex position of the optical-disk-side lens surface of the objective lens  1  to the optical disk in the optical-axis direction and a distance from a vertex position of the optical-disk-side lens surface of the objective lens  2  to the optical disk as X, in this embodiment, the objective lens  1  and the objective lens  2  are installed so that in the case where the objective lens  1  is installed at a nearer side from the optical disk than the objective lens  2  is, X satisfies 0≦X≦B−δB as shown in FIG. 3(A), and in the case where the objective lens  2  is installed at a nearer side from the optical disk than the objective lens  1  is, X satisfies 0≦X≦A−δA as shown in FIG. 3(B).  
         [0047]    Note that δA in 0≦X≦A−δA is a maximum of the deviation of recording layer positions of the optical disk  110  in the optical-axis direction that is generated when the optical disk  110  is being reproduced or recorded (hereinafter referred to as the amount of surface deflection of the optical disk  110  for convenience), and δB in 0≦X≦B−δB is a maximum of the deviation of recording layer positions of the optical disk  111  in the optical-axis direction that is generated when the optical disk  111  is being reproduced or recorded (hereinafter referred to as the amount of surface deflection of the optical disk  111  for convenience).  
         [0048]    Arranging the objective lens  1  and the objective lens  2  in this way makes it possible to reduce the possibility that an objective lens at the near side from the optical disk may collide with the optical disk in case an objective lens at the far side from the optical disk, namely the objective lens  2  in FIG. 3(A) and the objective lens  1  in FIG. 3(B), gets out of the control of the focusing servo.  
         [0049]    For example, as shown in FIG. 4(A), in case the objective lens  2  gets out of the control of the focusing servo when the optical disk  111  is being reproduced or recorded, a space between the objective lens  2  and the optical disk  111  immediately thereafter is open by the working distance B of the objective lens  2 . Therefore, with the arrangement of the objective lenses shown in FIG. 3(A) that satisfies 0≦X≦B−δB, it is possible to keep a space Y equal to or more than δB that is a maximum value of the amount of surface deflection of the optical disk  111  between the objective lens  1  and the optical disk  111  in a state of zero surface deflection, as shown in FIG. 4(A).  
         [0050]    Moreover, as shown in FIG. 4(B), in case the objective lens  1  gets out of the control of the focusing servo when the optical disk  110  is being reproduced or recorded, a space between the objective lens  1  and the optical disk  110  immediately thereafter is open by the working distance A of the objective lens  1 . Therefore, with the arrangement of the objective lenses shown in FIG. 3(B) that satisfies 0≦X≦A−δA, it is possible to keep a space Y equal to or more than δA that is a maximum value of the amount of surface deflection of the optical disk  110  between the objective lens  2  and the optical disk  110  in the state of zero surface deflection, as shown in FIG. 4(B).  
         [0051]    That is, even if the surface deflection of the disk occurs immediately after the objective lens at the far side from the optical disk got out of the control of the focusing servo, this arrangement can prevent direct collision between the objective lens at the near side from the optical disk and the optical disk. Here, the surface deflection (the deviation of recording layer positions) is measured on conditions described in JIS X 6243 (120-mm DVD rewritable disk), p.4, Item 8.1.1 “Test environment conditions,” ibid., p.64, Attachment A (Stipulations) “Measurement of angle deviation α,” etc. using, for example, DVD Mechanical Characteristics Measurement System LM-1200 (DVD) of ONO SOKKI CO., LTD. or the like. Further, the above-mentioned maximum of the deviation may be a maximum of deviation of 0.3 mm described in the above-mentioned JIS X 6243, p. 11, Item 11.5.1 “Amount of deflection in axial direction.” 
         [0052]    Here, in the case where objective lenses having different working distances are driven by the same actuator, it is advantageous that an objective lens of a longer working distance is installed at the far side from the optical disk in consideration of the power consumption in the actuator and a movable range that is required. For example, assuming that the working distance B of the objective lens  2  is longer than the working distance A of the objective lens  1 , pay attention to a difference between a position of the lens holder when the optical disk  110  is being reproduced and a position of the lens holder when the optical disk  111  is being reproduced. The difference W can be made small in the case where the objective lens  2  of a longer working distance is installed at the far side from the optical disk as shown in FIG. 5(A), as compared to a case where the objective lens  2  is installed at the near side from the optical disk as shown in FIG. 5(B). If this difference W arises, a direct current is always needed in order to lift up the lens holder when the disk is being reproduced or recorded, and as the difference W becomes larger, the power consumption increases accordingly. Moreover, a movable range that is required for the actuator also becomes wide.  
         [0053]    Therefore, in this embodiment, to reduce the power consumption in the actuator and narrow the movable range that is required for the actuator as well as to prevent the collision between the optical disk and the objective lens, it is more preferable to install the objective lens of a longer working distance at the far side from the optical disk. As the difference X of the objective lenses becomes closer to the absolute value of the difference in working distance |A−B|, this installation can reduce the power consumption and narrow the movable range further. Therefore, by installing the objective lens of a longer working distance at the far side from the optical disk and additionally by installing the objective lens  1  and the objective lens  2  so that in the case where the working distance B of the objective lens  2  is longer than the working distance A of the objective lens  1 , A and B satisfy (B−A)/2≦X≦B−δB, and in the case where the working distance B of the objective lens  2  is shorter than the working distance A of the objective lens  1 , A and B satisfy (A−B)/2≦X≦A−δA, it is possible to further reduce the power consumption and also narrow the movable range that is required for the actuator as compared to those of such an arrangement of the objective lenses as satisfies 0≦X≦B−δB or 0≦X≦A−δA.  
         [0054]    Here, note that the working distance of the objective lens for the high-density optical disk is generally shorter than the working distance of the objective lens for the existing DVD. Because of this fact, suppose that the objective lens  1  and the objective lens  2  are installed so that A and B satisfy 0≦X≦B−δB or (B−A)/2≦X≦B δB−δB to reduce the power consumption in the actuator and narrow the movable range that is required for the actuator as well as to prevent the collision between the optical disk and the objective lens. In this case, as a concrete numerical value of δB in 0≦X≦B−δB or (B−A)/2≦X≦B−δB, δB in this embodiment, for example, becomes approximately 0.3 mm since the deviation of recording layer positions from a nominal position in a direction perpendicular to the disk reference plane is stipulated to be 0.3 mm or less in the existing DVD Disk Standard Book. Therefore, for example, if the working distance A is set to approximately 0.1 mm and the working distance B is set to approximately 1.7 mm, a range of X as defined by 0≦X≦B−δB becomes 0 mm≦X≦1.4 mm and a range of X as defined by (B−A)/2≦X≦B−δB becomes 0.8 mm≦X&lt;1.4 mm.  
         [0055]    Incidentally, in this embodiment, the objective lens  2  is not limited to the objective lens exclusive for the DVD that is compatible with the existing DVD disk but it is perfectly all right that the objective lens is an objective lens that is compatible with a plurality of optical disks each having different recording density, such as the DVD/CD-compatible special objective lens that is also compatible with the CD disk. The objective lens  2  may be the DVD/CD-compatible special objective lens as described above. In this case, denoting the working distance of the objective lens  2  for a light beam that is used in recording or reproducing the CD disk  112  (hereinafter referred to as the optical disk  112  for convenience) as shown in FIG. 6 as C, the objective lens  1  and the objective lens  2  are installed so that in the case where the objective lens  1  is installed at a nearer side from the optical disk than the objective lens  2  as shown in FIG. 7(A), X satisfies  0 ≦X≦C−δC, and in the case where the objective lens  2  is installed at a nearer side from the optical disk than the objective lens  1  as shown in FIG. 7(B), X satisfies 0≦X≦A−δA.  
         [0056]    Note that δA in 0≦X≦A−δA is a maximum value of the amount of surface deflection of the optical disk  110 , as described above, and δC in 0≦X≦C−δC is a maximum value of the deviation of recording layer positions of the optical disk  112  in the optical-axis direction that is generated by disk rotation (hereinafter referred to as the amount of surface deflection of the optical disk  112  for convenience).  
         [0057]    Arranging the objective lens  1  and the objective lens  2  in this way makes it possible to decrease the possibility that the objective lens at the near side from the optical disk may collide with the optical disk in case the objective lens at the far side from the optical disk gets out of the control of the focusing servo. For example, in case the objective lens  2  gets out of control of the focusing servo when the optical disk  112  is being reproduced or recorded, the space between the objective lens  2  and the optical disk  112  immediately thereafter is open by the working distance C of the objective lens  2 . Therefore, with the arrangement of the objective lenses shown in FIG. 7(A) that satisfies 0≦X≦C−δC, it is possible to keep a space Y equal to or more than δC that is a maximum value of the amount of surface deflection of the optical disk  112  between the objective lens  1  and the optical disk  112  in the state of zero surface deflection, as shown in FIG. 8(A).  
         [0058]    Further, in case the objective lens  1  gets out of control of the focusing servo when the optical disk  110  is being reproduced or recorded, the space between the objective lens  1  and the optical disk  110  immediately thereafter is open by the working distance A of the objective lens  1 . Therefore, with the arrangement of the objective lenses shown in FIG. 7(B) that satisfies 0≦X≦A−δA, it is possible to keep the space Y equal to or more than δA that is a maximum value of the amount of surface deflection of the optical disk  110  between the objective lens  2  and the optical disk  110  that is in the state of zero surface deflection, as shown in FIG. 8(B).  
         [0059]    That is, even if the surface deflection of the disk occurs immediately after the objective lens at the far side from the optical disk got out of the control of the focusing servo, this arrangement can prevent direct collision between the objective lens at the near side from the optical disk and the optical disk.  
         [0060]    In addition, as described above, considering the power consumption in the actuator and the movable range that is required, in the case where the objective lenses having different working distances are driven by the same actuator, it becomes advantageous that the objective lens of a longer working distance is installed at the far side from the optical disk. Therefore, to reduce the power consumption in the actuator and narrow the movable range that is required for the actuator, it is more desirable to install the objective lens of a longer working distance, of the working distance A of the objective lens  1  and the working distance C of the objective lens  2 , at the far side from the optical disk. As the difference X of the objective lenses comes close to the absolute value of the difference in the working distance |A−C|, in the case where the high-density optical disk and the CD disk are being reproduced or recorded, this installation can reduce the power consumption in the actuator and narrow the movable range further. Therefore, by installing the objective lens of a longer working distance at the far side from the optical disk and additionally by installing the objective lens  1  and the objective lens  2  so that in the case where the working distance C of the objective lens  2  is longer than the working distance A of the objective lens  1 , A and C satisfy (C−A)/2≦X≦C−δC, and in the case where the working distance C of the objective lens  2  is shorter than the working distance A of the objective lens  1 , A and C satisfy (A−C)/2≦X≦A−δA, it is possible to reduce the power consumption and also narrow the movable range that is required for the actuator as compared to those of such an arrangement of the objective lenses as satisfies 0≦X≦A−δA or 0≦X≦C−δC.  
         [0061]    Here, note that the working distance of the objective lens for the high-density optical disk is generally shorter than the working distance of the DVD/CD-compatible special objective lens. Because of this fact, suppose that the objective lens  1  and the objective lens  2  are installed so that A and C satisfy 0≦X≦C−δC or (C−A)/2≦X≦C−δC to reduce the power consumption in the actuator and narrow the movable range that is required for the actuator as well as to prevent the collision between the optical disk and the objective lens. In this case, as a concrete numerical value of δC in 0≦X≦C−δC or (C−A)/2≦X≦C−δC in this embodiment, for example, becomes approximately 0.3 mm since the deviation of recording layer positions from a nominal position in a direction perpendicular to the disk reference plane is stipulated to be 0.3 mm or less in the CD Disk Standard Book. Therefore, for example, assuming that the working distance A is set to approximately 0.1 mm and the working distance B is set to approximately 1.3 mm, a range of X as defined by 0≦X≦C−δC becomes 0 mm≦X≦1.0 mm and a range of X as defined by (C−A)/2≦X≦C−δC becomes 0.6 mm≦X≦1.0 mm.  
         [0062]    Incidentally, in this embodiment, the objective lens for the high-density optical disk  1 , for example, is an objective lens composed of the lens  1   a  and the lens  1   b  constituting a two-lens combination, but the objective lens according to this invention is not limited to this. For example, the objective lens for the high-density optical disk  1  may be a single lens as shown in FIG. 9.  
         [0063]    Further, as shown in FIG. 10, the actuator in this embodiment, for example, is the axial sliding type actuator. In FIG. 10, the objective lens  1  and the objective lens  2  are installed in a cylindrical lens holder  10 , which has a structure that allows switching of the objective lens  1  and the objective lens  2  rotationally about an axis  71  in accordance with the optical disk to be recorded or reproduced. Further, by feeding a current through the coil  72 , the objective lens is driven to change its position in the focusing direction and in the tracking direction. Incidentally, the numeral  70  indicates a magnetic circuit part.  
         [0064]    Next, FIG. 11 shows first and second embodiments regarding the optical pick-up device according to this invention. In this embodiment, an actuator to be mounted therein is the actuator that was described in the embodiment regarding the actuator according to this invention. In FIG. 11, a laser light source  22  is a two-wavelength multi-laser light source in which, for example, a semiconductor laser light source  22   a  of an oscillation wavelength in a 650 nm band and a semiconductor laser light source  22   b  of an oscillation wavelength in a 780 nm band are mounted on the same package, and the laser light source  21 , for example, is a semiconductor laser light source of an oscillation wavelength in a 400 nm band. These light sources are made to emit light selectively according to an optical disk to be reproduced or recorded.  
         [0065]    Further, in FIG. 11, the numerals  30  and  31  indicate diffraction gratings, the numeral  32  is a riser mirror, the numerals  33  and  34  are beam splitters, the numeral  35  is a detection lens, the numeral  36  is a photodetector, and the numeral  37  is a collimator lens. Note that description is omitted for a roll of each optical component, a light-detecting plane pattern in the photodetector  36 , and a servo signal detection system that is used in recording or reproducing the optical disks  110 ,  111 , and  112 .  
         [0066]    Although this embodiment relates to an optical pick-up device that is compatible with the DVD, the CD, and the high-density optical disk, in the case of the second embodiment where the compatibility with the CD is not considered, the pick-up device can be modified to match such a purpose, for example, by specifying the laser light source  22  of FIG. 11 to have only a semiconductor laser source  22   a  of an oscillation wavelength in the 650 nm band.  
         [0067]    Next, FIG. 12 is view showing an embodiment regarding an optical disk apparatus according to this invention. In FIG. 12, an optical pick-up device  50  has a constitution, for example, as shown in FIG. 11.  
         [0068]    Various detection signals detected by the optical pick-up device  50  are sent to a servo-signal generating circuit  54  and an information-signal reproducing circuit  55  in a signal processing circuit. From these detection signals, the servo-signal generating circuit  54  generates a focusing error signal and a tracking error signal suitable for each optical disk, and based on these signals an objective lens actuator in the optical pick-up device  50  is driven through the actuator driving circuit  53  to perform position control of the objective lens.  
         [0069]    Further, the information-signal reproducing circuit  55  reproduces an information signal that is recorded in the optical disk  100  from the above-mentioned detection signal. Further, a part of the signals obtained by the above-mentioned servo signal reproducing circuit  54  and information-signal reproducing circuit  55  is sent to a control circuit  56 . The control circuit  56  has functions of: judging a kind of the optical disk  100  that is intended to be reproduced at that time using these various signals; driving one of a laser lighting circuit for high-density optical disk  57 , a laser lighting circuit for DVD  58 , and a laser lighting circuit for CD  59  according to a judgment result; and further switching a circuit configuration of the servo-signal generating circuit  54  so as to select a servo signal detection system that matches the kind of the optical disk, as described in the foregoing. Moreover, the control circuit  56  performs control of feeding a direct current (offset) corresponding to (δ2−δ1) and (δ3−δ1) that was described in the embodiments illustrated in FIG. 13 and FIG. 14 through the coil of the actuator. Here, in the case where the optical pick-up device  50  is a device that is not compatible with, for example, the CD, the laser lighting circuit for CD  59  is unnecessary.  
         [0070]    Further, an access controlling circuit  52  and a spindle motor driving circuit  51  are connected to the control circuit  56 , and these circuits perform access-direction and position control of the optical pick-up device  50  and rotation control of a spindle motor  60  for the optical disk  100 , respectively.