Abstract:
An optical pickup apparatus includes a lens unit which is composed of a first lens group having at least one lens and a second lens group having at least one lens. The first lens group and the second lens group are supported by support members which are engaged with each other. The first lens group and the second lens group are slid in an optical axis direction to change a relative positional relationship between the lens groups, thereby correcting spherical aberration caused in a recording surface of an optical recording medium.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an optical system of an optical pickup apparatus, and more particularly to a spherical aberration correcting optical system.  
         [0003]     2. Related Background Art  
         [0004]     In recent years, in order to increase a recording density of an optical disk apparatus, techniques for shortening a wavelength of a light source and techniques for improving an NA of an objective lens have been actively studied.  
         [0005]     According to some techniques, an optical pickup apparatus including a 405 nm band semiconductor laser and an objective lens with an NA of 0.85 is beginning to be commercially available.  
         [0006]     When a short-wavelength light source and a high-NA objective lens are to be employed for the optical disk apparatus, there are the following. fundamental problems.  
         [0007]     (1) It is easily affected by a tilt of a disk.  
         [0008]     (2) It is easily affected by a thickness error of a transparent substrate.  
         [0009]     (3) It is easily affected by a wavelength hop of the light source.  
         [0010]     In order to solve the problem (1), a thickness of the transparent substrate is reduced. For example, the thickness of the transparent substrate is set to about 100 μm.  
         [0011]     In order to solve the problems (2) and (3), an improved optical system has been designed.  
         [0012]     For example, in order to solve the problem (3) with respect to the mode hop of the light source, a chromatic aberration correcting lens for producing chromatic aberration for canceling chromatic aberration caused by the objective lens has been further provided in an optical system.  
         [0013]     In order to solve the problem (2) with respect to spherical aberration caused when there is a thickness error of the transparent substrate, for example, the following methods have been studied.  
         [0014]     (a) A beam expander is further used. Therefore, a lens interval is changed to produce spherical aberration, thereby canceling spherical aberration caused by the thickness error of the transparent substrate.  
         [0015]     (b) A position of a collimator lens is changed in an optical axis direction to produce spherical aberration, thereby canceling spherical aberration caused by the thickness error of the transparent substrate.  
         [0016]     Such techniques are disclosed by, for example, Japanese Patent Application Laid-Open No. 2002-236252.  
         [0017]     To explain it simply, when the beam expander is to be used, as shown in  FIG. 6A , an expander  33  which is composed of a lens  31  having negative power and a lens  32  having positive power is provided on an optical path of a parallel light flux which is located on a light incident side of an objective lens  35 . An interval between the lens  31  and the lens  32  is changed according to the thickness error of the transparent substrate, thereby producing spherical aberration.  
         [0018]     When the position of a collimator lens  34  is to be changed, an optical system as shown in  FIG. 6B  is used and the collimator lens  34  is moved along an optical axis according to the thickness error of the transparent substrate, thereby producing spherical aberration.  
         [0019]     A mechanism for realizing such an optical system is disclosed by, for example, Japanese Patent Application Laid-Open No. 2003-091847.  
         [0020]     The mechanism will be described with reference to FIG. 1 (corresponding to FIG. 7 in this specification) and FIG. 3 (corresponding to FIG. 8 in this specification) in Japanese Patent Application Laid-Open No. 2003-091847. An expander lens  16  is composed of a concave lens  16   a  and a convex lens  16   b  which are supported by a lens support member  25   a  and a lens support member  25   b , respectively. In  FIG. 7 , reference numeral  10  denotes a light-emitting device,  11  denotes a collimator lens,  12  denotes an optical system,  13  denotes a beam splitter,  14  denotes a lens,  15  denotes a detector,  17  denotes a mirror,  18  denotes a wavelength plate,  19  denotes an objective lens,  20  denotes an optical system,  21  denotes a detector,  22  denotes a stepping motor,  23  denotes a lead screw mechanism,  26  denotes an angle, and  28  denotes a photosensor.  
         [0021]     A guide shaft  27  is inserted into a driving rack  24  integrally engaged with the lens support member  25 , so the concave lens  16   a  can be slid along an optical axis.  
         [0022]     The concave lens  16   a , the convex lens  16   b , and the expander lens  16  composed of the concave lens  16   a  and the convex lens  16   b  in Japanese Patent Application Laid-Open No. 2003-091847 are regarded as “a first lens group”, “a second lens group”, and “a lens unit” in this specification, respectively.  
         [0023]     When the concave lens  16   a  is slid by using the guide shaft  27  as described above, another guide shaft (sub-shaft) is generally provided parallel to the guide shaft  27  in order to prevent the lens support member  25   a  from rotating about the guide shaft  27  and to maintain coaxiality between the optical axis and the concave lens  16   a.    
         [0024]     In particular, the above-mentioned structure is used for, for example, a Blu-ray Disc recorder which is currently manufactured as a product.  
         [0025]     However, the above-mentioned conventional techniques have the following problems.  
         [0026]     In general, allowable variations in coaxiality precision of optical elements including the lens unit and in tilt precision of the optical elements are determined in proportion to an effective light flux diameter.  
         [0027]     Therefore, the allowable variations become smaller as a reduction in thickness of the optical pickup apparatus progresses to reduce the effective light flux diameter.  
         [0028]     More specifically, an optical axis of a general optical pickup apparatus between a laser and a flip-up mirror is parallel to a disk surface.  
         [0029]     Therefore, the effective light flux diameter becomes a parameter for limiting a thickness of the optical pickup apparatus (surface perpendicular to the disk surface) In other words, when a reduction in thickness of the optical pickup apparatus is to be achieved, it is essential to reduce the effective light flux diameter. Therefore, a high-precision lens unit is necessary.  
         [0030]     Not only an external diameter but also coaxiality precisions of the movable concave lens  16   a  and the convex lens  16   b  and tilt precisions thereof relative to the optical axis are required with high precision for the expander mechanism of the expander lens which is the lens unit.  
         [0031]     However, with respect to the coaxialities in the expander mechanism, there are at least nine variations in tolerances such as  
         [0032]     (a-1) coaxiality between the concave lens  16   a  and a concave lens engaging portion of the lens support member  25   a,    
         [0033]     (a-2) a size tolerance between the concave lens engaging portion of the lens support member  25   a  and the center of an insertion hole of the guide shaft  27 ,  
         [0034]     (a-3) engaging backlash between the insertion hole of the guide shaft  27  of the lens support member  25   a  and the guide shaft  27 ,  
         [0035]     (a-4) coaxiality of the guide shaft  27 ,  
         [0036]     (a-5) a size tolerance between the concave lens engaging portion of the lens support member  25   a  and a sub-shaft insertion portion thereof,  
         [0037]     (a-6) engaging backlash between the sub-shaft insertion portion of the lens support member  25   a  and the sub-shaft,  
         [0038]     (a-7) coaxiality of the sub-shaft,  
         [0039]     (a-8) coaxiality between the convex lens  16   b  and a convex lens engaging portion of the lens support member  25   b , and  
         [0040]     (a-9) a size tolerance of the lens support member  25   b  (such as a size from the convex lens engaging portion to an optical base attaching portion).  
         [0041]     With respect to the tilts, there are five variations in tilts such as  
         [0042]     (b-1) a tilt of the concave lens  16   a  and the lens support member  25   a,    
         [0043]     (b-2) a tilt caused by engaging between the insertion hole of the guide shaft  27  of the lens support member  25   a  and the guide shaft  27 ,  
         [0044]     (b-3) a tilt of the guide shaft  27  relative to an optical base (not shown),  
         [0045]     (b-4) a tilt of the convex lens  16   b  and the lens support member  25   b , and  
         [0046]     (b-5) a tilt of the lens support member  25   b  relative to the optical base.  
         [0047]     Therefore, the optical base of the optical pickup apparatus including the expander mechanism requires ensuring of a clearance including the variations, so there is a problem in that a size of the optical pickup apparatus is increased.  
         [0048]     The expander mechanism according to the conventional technique requires ensuring of a parallelism between the guide shaft  27  and the sub-shaft in addition to the above-mentioned variations because the concave lens  16   a  is slid.  
         [0049]     Therefore, an adjusting mechanism for adjusting a tilt of a sliding shaft (guide shaft  27 ) and correcting an eccentricity of a lens is generally provided in the conventional techniques.  
         [0050]     This is because the coaxiality precision and the tilt precision of the expander lens relative to the optical axis cannot be ensured without adjusting the above-mentioned variations.  
         [0051]     An adjusting mechanism will be described in detail with reference to  FIG. 9 .  
         [0052]      FIG. 9  is a schematic perspective view showing an adjusting mechanism corresponding to the conventional adjusting mechanism as shown in  FIGS. 7 and 8 . But, some parts of  FIG. 9  are excerpted from  FIGS. 7 and 8 , additional parts are given to  FIG. 9 , and the shape of some portions are simplified.  
         [0053]     Parts in  FIG. 9  indicated by the same reference characters as in  FIGS. 7 and 8  have the same functions as those of the parts shown by the same reference characters of  FIGS. 7 and 8 .  
         [0054]     In general, heights of both ends of the guide shaft  27  and a height of an end of a sub-shaft  41  are adjusted by using three screws  40  to adjust a parallelism relative to the optical axis, thereby ensuring the coaxiality of the movable concave lens  16   a  during sliding relative to the optical axis.  
         [0055]     At this time, for example, a support surface  42   b  for supporting the guide shaft  27  is formed on an optical base  42  (in which only a part thereof required for this description is shown) with high precision and the guide shaft  27  is pressed to the support surface  42   b  of the optical base  42  to determine a position in a direction, indicated by an arrow A, orthogonal to an adjustment direction.  
         [0056]     When higher precision is required in the direction indicated by the arrow A, which is orthogonal to the adjustment direction, it is necessary that the guide shaft  27  be not pressed to the support surface  42   b  but additional adjustment be required for other portions. Therefore, there is a problem in that the number of parts increases.  
         [0057]     The held convex lens  16   b  is made adjustable in two axis directions (arrows A and B) orthogonal to the optical axis to adjust the coaxiality between the optical axis and the movable concave lens  16   a.    
         [0058]     For example, the lens support member  25   b  is moved for adjustment by a tool in the two axis directions orthogonal to the optical axis while the lens support member  25   b  is urged to a held lens side end surface  42   a  of the optical base  42  by an urging member  43 .  
         [0059]     This is because the movable concave lens and the held convex lens are separately disposed as in the conventional techniques, thereby making it necessary to perform the adjustment so as to align the movable concave lens and the held convex lens with the optical axis of a light-emitting source.  
         [0060]     In particular, an optical element composed of such a lens unit generally has a problem that not only the tilt precision and the coaxiality precision relative to an ideal optical axis but also the tilt precision between two lens groups and the coaxiality precision therebetween are very hard to maintain.  
       SUMMARY OF THE INVENTION  
       [0061]     An object of the present invention is to provide a high-precision optical pickup apparatus without increasing a size thereof.  
         [0062]     The optical pickup apparatus of the present invention is an optical pickup apparatus for condensing light emitted from a light source to a recording surface of an optical recording medium by an objective lens to perform one of information recording and information reproduction, including:  
         [0063]     a lens unit for spherical aberration correction, including a first lens group having at least one lens and a second lens group having at least one lens;  
         [0064]     a first support member for supporting the first lens group; and  
         [0065]     a second support member for supporting the second lens group,  
         [0066]     wherein the first support member and the second support member are engaged with each other and provided slidably in an optical axis direction. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0067]      FIG. 1  is a structural view showing an optical pickup apparatus according to a first embodiment of the present invention;  
         [0068]      FIG. 2  is a graph showing a lens-groups interval relationship in the case where a first lens group  11  is moved;  
         [0069]      FIG. 3  is a graph showing a lens-groups distance relationship in the case where a second lens group  12  is moved;  
         [0070]      FIG. 4  is a schematic perspective view showing a spherical aberration correcting mechanism in the present embodiment;  
         [0071]      FIG. 5  is a cross-sectional view taken along the line  5 - 5  of  FIG. 4 ;  
         [0072]      FIGS. 6A and 6B  are optical views showing conventional adjusting mechanisms;  
         [0073]      FIG. 7  is a perspective view showing a conventional adjusting mechanism;  
         [0074]      FIG. 8  is a structural view showing the conventional adjusting mechanism; and  
         [0075]      FIG. 9  is a schematic perspective view showing the conventional adjusting mechanism. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0076]     Hereinafter, a best mode for embodying the present invention will be described in detail with reference to the drawings.  
       First Embodiment  
       [0077]      FIG. 1  is a structural view showing an optical pickup apparatus according to a first embodiment of the present invention.  
         [0078]     In  FIG. 1 , reference numeral  1  denotes a semiconductor laser,  2  denotes a diffractive grating,  3  denotes a polarization beam splitter (PBS),  4  denotes a condensing lens,  5  denotes a monitor photo diode (PD),  6  denotes a λ/ 4 -plate,  7  to  10  denote lenses,  11  denotes a first lens group,  12  denotes a second lens group,  13  denotes a collimator lens serving as a lens unit,  14  denotes an objective lens,  15  denotes an optical disk,  16  denotes a sensor lens, and  17  denotes a radio frequency (RF) and servo PD.  
         [0079]     A light beam emitted from the semiconductor laser I is separated into a main beam and two sub-beams by the diffractive grating  2 .  
         [0080]     The sub-beams are used for generation of a servo signal for differential push-pull (DPP).  
         [0081]     A part of the light beams from the diffractive grating  2  is reflected on the PBS  3  and condensed to the monitor PD  5  through the condensing lens  4 .  
         [0082]     An output of the monitor PD  5  is used for control of light emission power of the semiconductor laser  1 .  
         [0083]     The light beam passing through the PBS  3  passes through the λ/ 4 -plate  6  and is converted to a parallel light beam by the collimator lens  13  serving as the lens unit. Then, the light beam is imaged by the objective lens  14  onto an information recording surface through a transparent substrate.  
         [0084]     The optical disk  15  is composed of the transparent substrate and the information recording surface.  
         [0085]     The light beam which is reflected on the optical disk  15  is condensed by the objective lens  14  and reflected on the PBS  3  through the collimator lens  13  and the λ/ 4 -plate  6 . The reflected light beam is condensed onto the RF and servo PD  17  by the sensor lens  16 .  
         [0086]     The collimator lens  13  includes two lens groups, that is, the first lens group  11  composed of the spherical lenses  7  and  8  and the second lens group  12  composed of the spherical lenses  9  and  10 .  
         [0087]     An information signal and a serve signal are generated based on an output of the RF and servo PD  17 .  
         [0088]     The case where the transparent substrate of the optical disk  15  has a thickness error will be described below.  
         [0089]     When the transparent substrate has a thickness error, spherical aberration is caused, as is well known. When a short-wavelength light source and a high-NA objective lens are used, the influence of the thickness error is large.  
         [0090]     Therefore, in this embodiment, an interval between the first lens group  11  composed of the spherical lenses  7  and  8  and the second lens group  12  composed of the spherical lenses  9  and  10  in the collimator lens  13  serving as the lens unit is changed to correct the caused spherical-aberration.  
         [0091]     Next, a relationship between the thickness error of the transparent substrate and a lens groups interval for correcting the thickness error will be described.  
         [0092]     Here, a wavelength of the semiconductor laser  1  is about 407 nm in information reproduction, the NA of the objective lens  14  is 0.85, and a focal distance thereof is 1.1765 mm.  
         [0093]     Table 1 shows design values of a projection system in this embodiment. In Table 1, N ( 407 ) indicates a refractive index at a wavelength of 407 nm, ΔN indicates a change in refractive index when the wavelength is increased by 1 nm and corresponds to the dispersion of the refractive index at the vicinity of the wavelength of 407 nm. When a distance in the optical axis direction is X, a height from the optical axis in a direction perpendicular to the optical axis is h, and a conic coefficient is k, an aspherical shape is expressed by the following Equation and shown in Table 2.  
       X   =           h   2     /   r       1   +       1   -       (     1   +   k     )     ⁢       h   2     /     r   2                 +     Bh   4     +     Ch   6     +     Dh   8     +     Eh   10     +     Fh   12     +     Gh   14           
 
         [0094]      FIGS. 2 and 3  show a relationship of lens groups intervals obtained in the case where parameters shown in Tables 1 and 2.  
                                                                 TABLE 1                                   Remarks   r   d   N(407)   ΔN                                    1   LD   ∞   0.78               2       ∞   0.25   1.52947   −0.00008       3       ∞   1.19       4   Diffractive   ∞   1   1.52947   −0.00008       5   grating   ∞   1.6       6   PBS   ∞   2.6   1.72840   −0.00042       7   λ/4-plate   ∞   1.15   1.56020   −0.00020       8       ∞   1.46       9   Collimator   ∞   1.29   1.58345   −0.00014       10   lens   −2.28   0.71   1.80480   −0.00053       11       ∞   0.8       12       13.49   0.74   1.80480   −0.00053       13       4.967   1.26   1.58345   −0.00014       14       −4.411   6.5       15   Objective   0.89427   1.57   1.70930   −0.00021       (Aspherical   lens       surface 1)       16       −3.38795   0.27       (Aspherical       surface 2)       17   Transparent   ∞   0.08   1.62068   −0.00038       18   substrate   ∞   0                    
         [0095]    
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
               
               
                 Aspherical 
                 Aspherical 
                 Aspherical 
               
               
                 coefficient 
                 surface 1 
                 surface 2 
               
               
                   
               
             
             
               
                 k 
                 −4.71569E−01 
                 −8.03122E+02 
               
               
                 B 
                  1.85624E−02 
                  6.93685E−01 
               
               
                 C 
                 −3.32437E−03 
                 −7.23881E−01 
               
               
                 D 
                  2.00843E−02 
                 −1.08028E+01 
               
               
                 E 
                 −2.46799E−02 
                  5.02375E+01 
               
               
                 F 
                  3.71610E−02 
                 −6.88792E+01 
               
               
                 G 
                 −2.00730E−02 
                  0 
               
               
                   
               
             
          
         
       
     
         [0096]      FIG. 2  shows the case where the first lens group  11  was moved (the second lens group  12  was held). The amount of movement per 1 μm of the transparent substrate thickness error is about 28 μm.  
         [0097]      FIG. 3  shows the case where the second lens group  12  was moved. In this case, the amount of movement per 1 μm of the transparent substrate thickness error is about 20 μm.  
         [0098]     For example, when the entire collimator lens  13  is moved, the amount of movement per 1 μm of the transparent substrate thickness error is about 50 μm. When the first lens group  11  is to be moved, an entire length of the optical system does not change. Even when the second lens group  12  is moved, an amount required for the movement is about a half of the movement amount required in the case where the entire collimator lens  13  is moved, that is, it is sufficiently small. Therefore, the optical system can be made compact.  
         [0099]      FIG. 4  is a schematic perspective view showing a spherical aberration correcting mechanism in this embodiment and  FIG. 5  is a cross-sectional view taken along the line  5 - 5  of  FIG. 4 .  
         [0100]     In  FIGS. 4 and 5 , reference numeral  18  denotes a first support member  18  for supporting the first lens group  11  and a second support member  19  for supporting the second lens group  12 .  
         [0101]     The first and second lens groups  11  and  12  are fixed to the first and second lens members  18  and  19 , respectively, by press-fitting or the like while preferable coaxiality is obtained.  
         [0102]     In this embodiment, the first lens group  11  is used as a held lens group and the second lens group  12  is used as a movable lens group.  
         [0103]     In this embodiment, each of the support members is formed in a substantially cylindrical shape concentric to the lens members.  
         [0104]     An external slide portion  19   a  (indicated by a broken line in  FIG. 5 ) of the second support member  19  is engaged with an internal slide portion  18   a  (indicated by an alternate long and short dash line in  FIG. 5 ) of the first support member  18  so as to slide the second support member  19 .  
         [0105]     A convex portion  19   b  is integrally provided on a part of the second support member  19 . Although not shown in the drawings, for example, when a rack member, a stepping motor, and a lead screw are provided on the convex portion  19   b  in the same manner as in the expander mechanism described in Japanese Patent Application Laid-Open No. 2003-091847, the second support member  19  can be driven in the optical axis direction.  
         [0106]     When the above-mentioned structure is used, it is possible to omit the guide shaft and the sub-shaft, unlike the conventional technique. Therefore, the number of parts can be reduced by two.  
         [0107]     Factors in tolerance variations will be described below in detail.  
         [0108]     Factors in coaxiality variations can be reduced to five factors such as  
         [0109]     (c-1) coaxiality between the first lens group  11  and a lens engaging portion of the first support member  18 ,  
         [0110]     (c-2) coaxiality between the lens engaging portion of the first support member  18  and the internal slide portion  18   a  thereof,  
         [0111]     (c-3) coaxiality between the second lens group  12  and a lens engaging portion of the second support member  19 ,  
         [0112]     (c-4) coaxiality between the lens engaging portion of the second support member  19  and the external slide portion  19   a  thereof, and  
         [0113]     (c-5) engaging backlash of the internal slide portion  18   a  of the first support member  18  and the external slide portion  19   a  of the second support member  19 .  
         [0114]     In particular, with respect to the five factors in coaxiality variations, it can be assumed that (c-1)=(a-1), (c-3)=(a-8), and (c-5)=(a-3).  
         [0115]     With respect to (c-2), the lens engaging portion of the first support member  18  and the internal slide portion  18   a  thereof are simultaneously processed by continuous turning so that the portions with the high coaxiality can be realized. With respect to (c-4), the same is expected.  
         [0116]     In contrast to this, with respect to (a-2) and (a-5) in the conventional technique, even in the case of the same parts, they are not formed in a concentric shape, so it is necessary to change a processing location after the lens engaging portion is processed. Therefore, there is a variation in feed precision of a processing apparatus.  
         [0117]     Thus, according to the structure of the present invention, it is possible to reduce the variation by which the coaxiality is affected, unlike the conventional technique.  
         [0118]     Unlike the conventional technique, factors in tilt variations can be reduced to four factors such as  
         [0119]     (d-1) a tilt of the first lens group  11  relative to the internal slide portion  18   a  of the first support member  18 ,  
         [0120]     (d-2) a tilt of the second lens group  12  relative to the external slide portion  19   a  of the second support member  19 ,  
         [0121]     (d-3) a tilt caused by engaging between the internal slide portion  18   a  of the first support member  18  and the external slide portion  19   a  of the second support member  19 , and  
         [0122]     (d-4) a tilt of the first support member  18  located on a holding side relative to the optical base.  
         [0123]     In particular, with respect to the four factors in tilt variations, it can be assumed that (d-1) (b-1), (d-2)=(b-4), (d-3)=(b-2), and (d-4)=(b-3).  
         [0124]     According to the structure of the present invention, the first support member  18  located on the holding side and the second support member  19  are engaged with each other to carry out direct sliding. Therefore, it is unnecessary to adjust the tilts of the first lens group  11  and the second lens group  12  and the coaxiality therebetween.  
         [0125]     The present invention is not restricted to only the above-mentioned embodiment and thus can be applied to, for example, the expander mechanism as described above in the conventional technique.  
         [0126]     This application claims priority from Japanese Patent Application No. 2004-309678 filed on Oct. 25, 2004, which is hereby incorporated be reference herein.